Geology of Australian and Papua New Guinean mineral deposits (Monograph Australasian Institute of Mining and Metallurgy)

GEOLOGY OF AUSTRALIAN AND PAPUA NEW GUINEAN MINERAL DEPOSITS Monograph 22

i

Cover photograph: Aerial view of Main-Barton open pit, Jundee gold mine, WA, viewed from the east, June 1997. The pit is approximately 1.4 km long and 50 m deep. Other Jundee and Nimary pits in the background. Courtesy of Great Central Mines Limited.

ii

GEOLOGY OF AUSTRALIAN AND PAPUA NEW GUINEAN MINERAL DEPOSITS Monograph 22

Edited by D A Berkman and D H Mackenzie

Published by THE AUSTRALASIAN INSTITUTE OF MINING AND METALLURGY Level 3, 15-31 Pelham Street, Carlton Victoria Australia 3053

iii

© The Australasian Institute of Mining and Metallurgy 1998

The Institute is not responsible as a body for the facts and opinions advanced in any of its publications.

ISBN 1 875776 53 2

Desktop published by: Katrina Fogg, Penelope Griffiths and Angela Spry for The Australasian Institute of Mining and Metallurgy

Printed by: RossCo Print Factory 4/188 Plenty Road Preston South Vic 3072

iv

Dedication This volume is dedicated to geologists, past and present, who have contributed so much to the knowledge and understanding of the mineral wealth of Australia and Papua New Guinea.

v

vi

Committees MONOGRAPH 22 COMMITTEE S E Close (Chairman) D A Berkman P F Griffiths D H Mackenzie H M Tutt

EXPERT COMMITTEE S E Close (Chairman) D E Clarke K E Fletcher I G Gould L de Graaf G R T Hudson W R H Ramsay L C Ranford

vii

Acknowledgements The assistance rendered by the following organisations and their staffs in the production of this monograph is gratefully acknowledged: The advice and assistance of the following individuals are also acknowledged with thanks:

Australian Geological Survey Organisation

Northern Territory Department of Mines and Energy

Australian Mineral Industries Research Association Limited

Papua New Guinea - Department of Mining and Petroleum

Bureau of Resource Sciences

Queensland Department of Minerals and Energy

Mineral Resources Tasmania

Tenement Administration Services

Mines and Energy SA

Victorian Department of Natural Resources and Environment

New South Wales Department of Mineral Resources

Western Australian Department of Minerals and Energy

David Blake

Russell Harris

Cathy Brown

Ian Hodgson

Tom Dickson

Kerry O’Sullivan

Brian Elliott

Sandy Paine

Stewart Girvan

Ric Rogerson

Graham Hancock

Len Skotsch

Colour Plates Colour plates have been subscribed for individually and these contributions are acknowledged with thanks. The subscribers and page numbers are listed below: Eagle Mining Corporation Placer Granny Smith Pty Limited Delta Gold NL WMC Resources Limited Outokumpu Mining Australia Hamersley Iron Pty Limited Hancock Prospecting Pty Limited Acacia Resources Limited Otter Gold Mines Limited Nord Pacific Limited Peak Gold Mines

93 180, 183 and 185 203, 204 and 205 234 368 and 369 375, 376, 377 and 378 381, 382, 383 and 384 420 and 421 443 593, 597 and 598 609, 610 and 611

Battle Mountain (Australia) Inc

686 and 687

MIM Holdings Limited

702 and 704

Ross Mining NL

712

Rio Tinto Exploration Pty Limited

827, 828, 829 and 830

viii

Sponsorship This monograph was made possible by the provision of loans from the companies and from Branches of The AusIMM listed below. The Australasian Institute of Mining and Metallurgy sincerely appreciates their support.

Sydney Branch, The AusIMM Newcrest Mining Limited North Limited Acacia Resources Limited Delta Gold NL Homestake Gold of Australia Limited Pasminco Limited Placer Pacific Limited Plutonic Resources Limited North Queensland Branch, The AusIMM Southern Queensland Branch, The AusIMM Aberfoyle Limited Australian Resources Limited Great Central Mines Limited Melbourne Branch, The AusIMM Kalgoorlie Branch, The AusIMM Nuigini Mining Limited Central Victorian Branch, The AusIMM

ix

x

Foreword It is a great pleasure to introduce this monograph. For the fifth time in almost 50 years the geology of the mineral deposits of the region is described in detail, under the auspices of The Australasian Institute of Mining and Metallurgy (The AusIMM). While the papers in this volume present the latest information, together with the previous volumes they continue a worthy tradition and provide a superb historical record, for the benefit of present and future exploration and mining professionals. This monograph spans the last decade and focusses on the discoveries and major changes in geological knowledge and interpretation in that time. Due to the large number of discoveries, the volume includes most of the more significant deposits of minerals, but excludes coal and petroleum. It clearly demonstrates the continuing achievements and dedication of geologists and their colleagues, as well as the success of companies in developing and operating mines. The coverage of a wide range of commodities and the geographic spread throughout Australia and Papua New Guinea well illustrates the variety and adaptability of the industry. The number of papers on gold deposits, especially in Western Australia, emphasises the major exploration and mining focus in that region. In the last decade Australian gold production has lifted from about 110 t in 1987 to an estimated 310 t in 1997, with three-quarters of current Australian gold production mined in Western Australia. The volume is primarily concerned with geological aspects, rather than financial and economic impacts. However, the overview papers clearly demonstrate the mineral industry’s importance and its significant contribution to the economies of both Australia and Papua New Guinea. The industry’s role as a major exporter and its ranking in world terms are highlighted. Also briefly described are the many changes which have occurred in the last decade and the wide range of issues that have been, and still are being, addressed. It is clear that a continuing and vibrant exploration and mining industry is essential for the well being of the region and its peoples. The monograph marks a major change in organisation and production. It is the product of a small, dedicated and efficient team. I pay particular tribute to the Editors, Don Berkman and David Mackenzie, who drew on their experience of Monograph 14, in undertaking their daunting task. Helen Tutt was invaluable as Project Coordinator, with the assistance of the staff of The AusIMM Publications Department. I take responsibility for the planning and direction of the project. We appreciated the support of the Expert Committee and were pleased that we did not need to call unduly upon their expertise. This is the first AusIMM publication to be issued simultaneously in both hardcopy and CD ROM format, which it is believed will be welcomed by many users. Funding for the project has been provided by loans from companies and individual AusIMM Branches. The loans will be repaid from sales proceeds. The generosity of these sponsors is much appreciated, especially that of The AusIMM Sydney Branch, the major lender. This monograph, as with its predecessors, would not have been possible without the enthusiasm and goodwill of the authors and the companies they represent. The support of many in the geological community is gratefully acknowledged. S E Close Chairman of Committees

xi

xii

Editors’ Preface This is the fifth comprehensive compilation of ore deposit geology produced by The Institute, and the first wholly prepared for publication by The Institute employees and consultants. The series began in 1953, with ‘Geology of Australian Ore Deposits’ launched at the Fifth Empire Mining and Metallurgical Congress. The objective of that volume, as stated in the editor’s preface, was ‘. . . to present a geological account of mineralisation in Australia, with emphasis on the factors that have controlled ore deposition . . . The book includes descriptions of all the chief mines now working, and many of the lesser mines of geological interest’. The same objective was applied to the monographs published in 1965, 1975 and 1990, and to this publication.

D A Berkman.

The editors were fortunate in learning their early geological practice under C L Knight (the editor of the 1975 four-volume set), and in assisting F E Hughes in the preparation of the fourth member of the series, the successful two-volume Monograph 14. The present volume is a companion to Monograph 14 and closely follows its style and format. Monograph 22 records the geological setting of the large number of ore deposits found since 1989, when Monograph 14 was completed. Unlike the earlier volumes it contains a minimum of information on regional geology, as we consider that this is adequately covered in Monograph 14. It begins with two review papers, on the mineral industries of Australia and Papua New Guinea, which itemise the economic, legal and technical climate in the context of the deposits described. Another general paper details the Code of public reporting of resources and reserves of the Joint Ore Reserves Committee (JORC), which has been used throughout the volume and is becoming internationally accepted as a standard. The fourth general paper examines current research on exploration techniques and ore deposit geology. Papers on deposits were selected on the basis that they described a deposit, or group of deposits, with a total resource value of $50 million or more, equivalent to at least 100 000 ounces of contained gold, discovered and/or developed since about 1988. Smaller deposits with unique or outstanding geological features were also included as were papers which significantly updated the geological knowledge of older mines and earlier known deposits.

D H Mackenzie.

The descriptions of the deposits are arranged by location, starting in WA, and then across each State of Australia, ending in Papua New Guinea. They are grouped by principal commodity in each State, and then in sequence from north to south. The deposit sites are shown in the end paper maps

The type of deposits described provides an indication of the current focus of the Australian mineral industry. As 69 of the 120 ‘deposit’ papers document orebodies or mineral fields in which gold is the principal or only commercial component, we may conclude that gold mining continues its 20 year dominance of the industry. Several new gold deposits are described, in the Plutonic and Yandal greenstone belts of WA, at Lake Cowal in NSW and at Dead Bullock Soak, NT, and discoveries are also documented in old fields as at Cadia, NSW, Kanowna Belle, WA and Hamata, PNG. Major copper and lead-zinc deposits described include discoveries in the Mount Isa-Cloncurry region such as the blind deposits at Ernest Henry and Cannington and the largely concealed orebody at Century. Additions to resources at old mines at Rosebery and Renison, Tas, Mount Morgan and Gunpowder, Qld and at Norseman and Bulletin, WA, are discussed. New topics include some recently discovered and some revitalised nickel sulphide deposits in the Norseman–Wiluna belt of WA, some lateritic nickel deposits made viable by changes in ore processing methods, and silver-rich deposits at Nimbus, WA, and Bowdens, NSW. The geology of the exciting discovery of coarser-grained heavy mineral deposits in the Murray Basin is documented. The long list of discoveries, often of blind or concealed orebodies, demonstrates that industry growth by exploration is still within our grasp.

xiii

Editorial changes to the papers submitted have, as far as possible, been limited to matters of clarity, seeking to retain the individual author’s style. The editors have, in line with current practice, listed references in full, and not in the timeconsuming abbreviated CASSI system. Much of the editing has been involved with achieving the format and punctuation standards set by The Institute, which generally conform with the AGPS ‘Style Manual’. The editors are conscious that the contract editing system used for the monograph has required authors to make many minor ‘format type’ changes to several drafts of their papers. We thank to authors for their forbearance in accommodating these ‘strictures of the editors’, as one author saw them, and hope they are pleased with the result. We acknowledge assistance from the Australian Geological Survey Organisation (AGSO), who checked that the stratigraphic terms in the monograph conform to the Australian Register of Stratigraphic Nomenclature, and from the Bureau of Resource Sciences, who checked the locations of deposits for the end paper location maps. Most importantly, we extend our gratitude to the hundreds of geoscientists who have found time to record the geology of the ore deposits on which they work, and to whom this publication is dedicated. Monograph 22 owes much to the commitment of Sandra Close who gave it her support from initiation to completion. D A Berkman and D H Mackenzie Joint Editors

xiv

Project Coordinator’s Preface Geology of Australian and Papua New Guinean Mineral Deposits heralds a new era in the publication of The AusIMM Monograph Series. The new methodology involves a small Committee, including a Project Coordinator. It has proved most effective and economical. The Monograph 22 Committee was established early in 1996, under the direction of Sandra Close, (Chairman of The AusIMM Publications Committee) with the Joint Editors, The AusIMM Publications Manager and the Project Coordinator. Overall parameters and criteria for papers were determined and a detailed Guide to Authors and Standard Operating Procedures were developed, to ensure a streamlined process. Over 300 companies were then invited to submit papers if their deposits met the criteria. The invitation was repeated in The AusIMM Bulletin and The Australian Geologist, to ensure that the volume’s coverage was comprehensive. The experience and knowledge the Joint Editors, Don Berkman and David Mackenzie, brought to the project was extremely valuable. Their commitment during the three stage editorial process has enhanced the quality and consistency of the monograph. Penelope Griffiths, The AusIMM Publications Manager provided expertise on publishing and related matters throughout the project. Pamela Bell’s methodical approach established clear and effective procedures for the control and flow of papers through the editorial process. Katrina Fogg and Angie Spry assisted with desktop publishing. I would like to express my appreciation to all those who contributed to the production of this volume, especially the authors of the individual papers whose enthusiasm and cooperation made my task much easier. I have valued Sandra Close’s patient guidance. My role has been both challenging and rewarding and I have enjoyed the opportunity to contribute to this monograph. H M Tutt Project Coordinator

xv

xvi

Contents REVIEW PAPERS The Australian mineral industry, 1986–1996

L C Ranford, D J Perkin and W A Preston

3

Papua New Guinea’s mineral industry, 1986–1996

R Rogerson

33

The JORC Code, 1987–1997

P R Stephenson and N Miskelly

45

Australian mineral exploration research

J Cucuzza and A D T Goode

53

WESTERN AUSTRALIA Mineral deposits of the Padbury, Bryah and Yerrida basins

F Pirajno and W A Preston

63

Plutonic gold deposit

N M Vickery, P M Buckley and R J Kellett

71

Gold deposits of the Peak Hill area

M A Harper, M G Hills, J I Renton and S E Thornett

81

Nimary gold deposits

A P Byass and D R Maclean

89

Jundee gold deposit

G N Phillips, J R Vearncombe and R Murphy

97

Bulletin gold deposit

S C Chanter, P Eilu, M E Erickson, G F P Jones and E Mikucki

105

Some gold deposits of the Bluebird, Nannine and Cuddingwarra goldfields, Murchison district

N J Winnall, T J Hibberd, D S Thynne and E Wahdan

111

Omega gold deposit, Gidgee

D I Ross and D W Smith

119

Kingfisher gold deposit, Gidgee

N J Hazard

123

Bronzewing gold deposit

G N Phillips, J R Vearncombe, I Blucher and D Rak

127

Mount McClure gold deposits

J L Harris

137

Tuckabianna gold deposits

M E Smith

149

New Holland, New Holland South and Genesis gold deposits, Lawlers

N A Inwood

155

Agnew gold deposits

J Broome, T Journeaux, C Simpson, N Dodunski, J Hosken, C De-Vitry and L Pilapil

161

xvii

Deflector gold-copper deposit

P Hayden and G Steemson

167

Tarmoola gold deposit

M C Fairclough and J C Brown

173

Sunrise-Cleo gold deposit

P G N Newton, D Gibbs, A Grove, C M Jones and A W Ryall

179

Lights of Israel gold deposit, Davyhurst

R M Joyce, W K Woodhouse and C H Young

187

Mount Dimer gold deposits

J R McIntyre and A Czerw

191

Broads Dam gold deposits

M J Glasson, R G Henderson and M Tin

197

Kanowna Belle gold deposit

T S Beckett, G J Fahey, P W Sage and G M Wilson

201

Kundana gold deposits

J R Lea

207

Geko gold deposit

G R Hemming

211

Centurion gold deposit, Binduli

M E Ivey, M J Fowler, P G Gent and A J Barker

215

Jubilee gold deposit, Kambalda

I K Copeland and the Geological Staff of New Hampton Goldfields NL

219

Randalls gold deposits

P G N Newton, B Smith, C Bolger and R Holmes

225

Revenge gold deposit, Kambalda

P T Nguyen, J S Donaldson and S G Ellery

233

Nelson’s Fleet gold deposit, St Ives

M Kriewaldt

239

Kambalda-St Ives gold deposits

R B Watchorn

243

Yilgarn Star gold deposit

R A Crookes and D Dunnet

255

Two Boys gold deposit, Higginsville

S H Shedden

261

Norseman gold deposits

N R Archer and B J Turner

265

Nimbus silver-zinc deposit

I R Mulholland, A Cowden, I P Hay, J C Ion and A L Greenaway

273

Weld Range platinum group element deposit

J Parks

279

Panorama zinc-copper deposits

P Morant

287

Magellan lead deposit

B M McQuitty and D J Pascoe

293

Honeymoon Well nickel deposits

M J Gole, D L Andrews, G J Drew and M Woodhouse

297

Mount Keith nickel deposit

S Hopf and D L Head

307

xviii

Rocky’s Reward nickel deposit

C De-Vitry, J W Libby and P J Langworthy

315

Perseverance nickel deposit

J W Libby, P R Stockman, K M Cervoj, M R K Muir, M Whittle and P J Langworthy

321

Murrin Murrin nickel-cobalt deposits

V W Fazakerley and R Monti

329

Cawse nickel-cobalt deposit

K Hellsten, C R Lewis and S Denn

335

Silver Swan, Cygnet and Black Swan nickel deposits

J D Hicks and G D Balfe

339

Kambalda nickel deposits

W E Stone and E E Masterman

347

Maggie Hays nickel deposit

P S Buck, S A Vallance, C S Perring, R E Hill and S J Barnes

357

Forrestania nickel deposits

K M Frost, M Woodhouse and J T Pitkäjärvi

365

The Y2–3 and Y10 iron ore deposits, Yarrie

P J Waters

371

Brockman No. 2 detritals (B2D) iron ore deposit

D M McKenna and R A Harmsworth

375

Hope Downs iron ore deposits

R D Paquay and P K Ness

381

Speewah fluorite deposit

K A Rogers

387

SOUTH AUSTRALIA Perseverance gold deposit, Tarcoola

F J Hughes

395

Iron ore deposits of the northern Gawler Craton

B J Morris, M B Davies and A W Newton

401

NORTHERN TERRITORY Brocks Creek gold deposits, Pine Creek

G C Miller, C M Kirk, G Hamilton and J R Horsburgh

409

Union Reefs gold deposit

P G N Newton, C Switzer, J Hill, G Tangney and R Belcher

417

Mount Todd gold deposits

W R Ormsby, K L Olzard, D J Whitworth, T A Fuller and J E Orton

427

Gold Creek gold deposit

I J Morrison and J A Treacy

433

White Devil gold deposit

C A Bosel and G P Caia

439

Gold deposits of the Tanami Corridor

A Tunks and S Marsh

443

Dead Bullock Soak gold deposits

M E H Smith, D R Lovett, P I Pring and B G Sando

449

Merlin diamondiferous kimberlite pipes

D C Lee, T H Reddicliffe, B H Scott Smith, W R Taylor and L M Ward

461

xix

TASMANIA Tasmania gold deposit, Beaconsfield

P B Hills

467

Henty gold deposit

T Callaghan, S Dunham and W Edgar

473

Rosebery lead-zinc-gold-silver-copper deposit

M V Berry, P W Edwards, H T Georgi, C C Graves, C W A Carnie, R J Fare, C T Hale, S W Helm, D J Hobby and R D Willis

481

Renison Bell tin deposit

B M McQuitty, R H Roberts, P A Kitto and C J Cannard

487

VICTORIA Victorian gold province

G N Phillips and M J Hughes

495

Fosterville gold deposits

N Zurkic

507

Williams United gold deposit, Bendigo

G J McDermott and P W Quigley

511

Bailieston gold deposit

R S Sebek

517

Deborah line of reef gold deposits, Bendigo

D G Turnbull and G J McDermott

521

Eaglehawk-Linscotts reef gold deposits, Maldon

G B Ebsworth, J de Vickerod Krokowski and J Fothergill

527

Stawell gold deposits

D C Fredericksen and M Gane

535

Ballarat gold deposits

D H Taylor

543

NEW SOUTH WALES Timbarra gold deposits

R Mustard, R Nielsen and P A Ruxton

551

Mount Terrible gold deposits

G S Teale

561

McKinnons gold deposit, Cobar

S M Elliott, A Bywater and C Johnston

567

Browns Creek gold-copper deposit

C Wilkins and G Smart

575

Endeavour 42 (E42) gold deposit, Lake Cowal

P McInnes, I Miles, D Radclyffe and M Brooker

581

Elura zinc-lead-silver deposit, Cobar

A E Webster and C Lutherborrow

587

Girilambone district copper deposits

J M Fogarty

593

CSA copper-lead-zinc deposit, Cobar

B L Shi and G C Reed

601

Peak gold-copper-lead-zinc-silver deposit, Cobar

W G Cook, J A Pocock and C L Stegman

609

xx

Potosi zinc-lead-silver deposit, Broken Hill

R Morland and P R Leevers

615

Broken Hill lead-zinc-silver deposit

R Morland and A E Webster

619

Bowdens silver-lead-zinc deposit, Mudgee

I J Pringle and J Elliot

627

Lewis Ponds gold-silver-copper-lead-zinc deposits

R I Valliant and R M D Meares

635

Cadia gold-copper deposit

Newcrest Mining Staff

641

Heavy mineral sand deposits, central Murray Basin

A J Mason, M Teakle and P A Blampain

647

Hillview vermiculite deposit

A R Martin

651

Thuddungra magnesite deposits

V A Diemar

655

QUEENSLAND Atric gold deposit

J S Birch

663

Anastasia gold deposit

J E Nethery

669

Mount Wright gold deposit

K J Harvey

675

Ravenswood gold deposits

D Collett, C Green, D McIntosh and I Stockton

679

Vera North and Nancy gold deposits, Pajingo

D R Richards, G J Elliott and B H Jones

685

Wirralie gold deposit

M J Seed and P A Ruxton

691

Yandan gold deposit

P A Ruxton and M J Seed

695

Tick Hill gold deposit

P J Forrestal, P J Pearson, T Coughlin and C J Schubert

699

Belyando gold deposit

R Mustard

707

Mount Morgan gold-copper deposits

P R Messenger, A Taube, S D Golding and J S Hartley

715

Red Dome and Mungana gold-silver-copper-lead-zinc deposits

J E Nethery and M J Barr

723

Century zinc-lead-silver deposit

G C Broadbent and A E Waltho

729

Surveyor 1 copper-lead-zinc-silver-gold deposit

P S Rea and R J Close

737

Gunpowder copper deposits

S M Richardson and A D Moy

743

Grevillea zinc-lead-silver deposit

D R Jenkins, J P Laurie and S D Beams

753

Ernest Henry copper-gold deposit

A J Ryan

759

xxi

Greenmount copper-cobalt-gold deposit

G D Hodgson

769

Mount Elliott copper-gold deposit

D B Fortowski and S J A McCracken

775

Cannington silver-lead-zinc deposit

A Bailey

783

Osborne copper-gold deposit

N D Adshead, P Voulgaris and V N Muscio

793

Brolga nickel-cobalt deposit

J M Parianos, N F Morwood and J Cook

801

Westmoreland uranium deposits

G M Rheinberger, C Hallenstein and C L Stegman

807

Kunwarara magnesite deposit

D Milburn and S Wilcock

815

PAPUA NEW GUINEA Mount Sinivit gold deposits

I D Lindley

821

Wafi copper-gold deposit

D Tau-Loi and R L Andrew

827

Hamata gold deposit

K P Denwer and B A Mowat

833

Tolukuma gold-silver deposit

D G Semple, G J Corbett and T M Leach

837

Mount Bini copper-gold deposit

M A Dugmore and P W Leaman

843

Gameta gold deposit

K G Chapple and S Ibil

849

Nena copper-gold deposit

A L Bainbridge, S P Hitchman and G J DeRoss

855

xxii

Ranford, L C, Perkin, D J and Preston, W A, 1998. The Australian mineral industry, 1986–1996, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 3–32 (The Australasian Institute of Mining and Metallurgy: Melbourne).

The Australian mineral industry, 1986–1996 1

2

by L C Ranford , D J Perkin and W A Preston INTRODUCTION This paper provides a broad overview of the Australian mineral industry for the decade July 1986 to June 1996. It describes the context in which it has operated, a summary of the more significant technological developments, an overview of the mineral discoveries and some thoughts on the future issues, opportunities and challenges for the industry during the next decade. The first section of the paper, entitled The Australian Mineral Industry: 1.

reflects the importance of the mineral industry to the Australian economy;

2.

provides an overview of Australia’s mineral production and trade;

3.

identifies the trends in mineral exploration expenditure in Australia and the level of overseas exploration by Australian companies; and

4.

provides a broad overview of the changes that have occurred to Australia’s mineral resource inventory over the decade and the costs of additions to resources.

The second section, Issues and Trends, 1986–1996, examines the national and international economic, social and political issues and developments that have impacted on the Australian mineral industry during the last decade. It also considers the developments and trends that have occurred within the mineral industry as a consequence of external factors and technological developments. The third section, Significant Mineral Discoveries and Developments, focuses on the mineral deposits that are described in this publication. It considers the evidence for new mineral deposit styles and provinces, the methods of discovery of the deposits and their geological and geographical spread. The final section, Future Issues, Opportunities and Challenges, briefly considers the various international and national issues expected to influence developments in the mineral industry over the next decade. It summarises the authors’ views on the scientific and technical needs and opportunities to be addressed if the industry is to maintain and increase its contribution to the welfare of all Australians.

1.

Director General, Western Australian Department of Minerals and Energy, 100 Plain Street, East Perth WA 6004.

2.

Principal Geologist, Bureau of Resource Sciences, PO Box E11, Kingston ACT 2604.

3.

Project Manager, Western Australian Department of Resources Development, 170 St George’s Terrace, Perth WA 6000.

Geology of Australian and Papua New Guinean Mineral Deposits

3

The information compiled to prepare this paper indicates that Australia has a vibrant, internationally competitive mineral industry which is attracting about 19% of the funds allocated internationally for mineral exploration. A very high proportion of these funds has been directed towards the search for gold over the last decade, and this seems likely to continue if the gold price and discovery costs remain at the levels experienced over the last few years. There seems little doubt that the world demand for minerals will increase, particularly in the Asian region. However, Australian producers will face strong international competition, especially from the developing countries eager to gain the economic benefits that flow from resource development. It appears that unless there is a satisfactory resolution of the land access problems associated with native title and environmental issues, many Australian companies will continue to move their activities offshore and some foreign investors are likely to go elsewhere. Australia has the technological skills, the prospectivity and the legal and administrative framework necessary to maintain and improve its international position in the world mineral industry. The major hurdles over the next decade are likely to be: 1.

national land access issues related to native title and environmental concerns which could seriously discourage greenfield exploration investment and hence constrain the future growth of the mineral industry; and

2.

international decisions on greenhouse gas emissions which could have serious impacts on the coal sector and on growth in energy intensive processing of some minerals mined in Australia.

THE AUSTRALIAN MINERAL INDUSTRY IMPORTANCE The mineral industry continues to be a major pillar of the Australian economy. In 1995–1996 mining, smelting and refining, including oil and gas, contributed 6% of GDP, compared with rural 4%, manufacturing 14%, with ‘other and services’ making up the remaining 76%. In 1996 the industry employed around 84 000 people directly, and indirectly another 300 000. The sector provided about 60% of Australia’s commodity exports, worth $A34 billion, 45% of all merchandise exports and 35% of all exports, ie total of all goods, including services, exported (ABARE, 1996, 1997). In 1996 the Australian mineral industry produced and exported a wider range of mineral commodities, and in a number of cases in greater quantities, than any other country in the world. Today Australia is as much dependent for its high standard of living on minerals as it was in the latter part of the

3

L C RANFORD, D J PERKIN and W A PRESTON

1800s when Australia was the envy of the world with its high per capita income based on gold production.

Mineral production

TABLE 1 Production of selected mineral commodities, Australia - 1986 and 1996.

The value of Australia’s mineral production has increased by 71% from 1986 to 1996, from $19.7 billion to $33.8 billion, which translates into a 6% annual increase in constant dollar terms over the decade (Fig 1).

Units

The quantity of Australian mineral production increased by an average of 56% on a non-weighted average basis of 20 principal commodities since 1986 (Table 1). The reason for the less than commensurate increase in the value of mineral production in real terms over the decade was the significant decline in metal and mineral prices in constant dollar terms. The commodities whose prices declined most in real Australian dollar terms were coking coal which fell by 38%, steaming coal 31%, gold 43%, nickel 30%, iron ore 28%, zinc 27%, rutile 25% and alumina 24%. The decreases in the real price of zircon concentrates by 13%, lead 9%, and copper 6% were relatively small by comparison. There were production increases for every commodity except tungsten during the decade. The commodities to show the largest increases over the period were gold whose output almost quadrupled, copper which more than doubled, ilmenite which increased by 64%, iron ore 57%, zinc 50%, raw black coal 48%, nickel 47%, diamond 44%, bauxite 33%, manganese ore 28%, uranium 19% and lead 17%. Production of tin, zircon concentrates and silver increased by less than 5% over the decade.

Mineral exports Australia is one of the worlds’ leading mineral exporting nations. Many of the emerging nations as well as Japan and Europe have come to rely on Australia for raw and partly processed mineral products. From 1985–1986 to 1995–1996

Commodity

1986

1996

% Increase(b)

32.384

43.063

33

0.170

0.252

48

37.607

53.600

43

0.525

111

Mt

Bauxite

Bt

Coal, black, raw

Mt

Coal, brown

Mt

Copper (a)

0.248

Mc

Diamonds

29.232

41.993

44

t

Gold (a)

75.079

288.880

285

Bt

Iron Ore

0.094

0.147

57

Mt

Lead (a)

0.448

0.522

17

Mt

Manganese ore

1.649

2.109

28

kt

Nickel (a)

0.077

0.113

47

Petroleum ML

Crude oil and condensate

29.764

31.579

6

Mm3

Natural gas (sales)

14.869

29.799

100

ML

LPG

3.929

3.718

-5

kt

Silver

1.023

1.020

0

kt

Tin (a)

8.515

8.828

4

Mt

Ilemite conc

1.238

2.028

64

Mt

Rutile conc, syn Rutile

0.230

0.634

176

Kt

Uranium (U3O8)

4.899

5.831

19

Mt

Zinc (a)

0.712

1.071

50

Mt

Zircon conc

0.452

0.462

2

(a) Total metallic content of minerals produced (b) Average percentage increase of the 20 commodities considered on a non-weight basis equal to 56% Source: ABARE (1997)

FIG 1 - Value of Australian mineral and petroleum production, investment and exports - 1985–86 to 1995–96 (in constant 1995–96 dollars). Source: ABARE (1997).

4

Geology of Australian and Papua New Guinean Mineral Deposits

THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996

the value of Australia’s mineral exports increased from $16.3 billion to $34.1 billion. Some of the largest increases were recorded by gold, which jumped by five times to a value of $5.6 billion, iron and steel which went up more than ten times to $1.5 billion, copper which increased almost four times to $928 million and zircon which trebled to $223 million (Table 2). The value of exports of coking coal rose by 51% to $4.7 billion, steaming coal increased by 35% to $3.0 billion, and bauxite, alumina and aluminium ingot metal exports combined almost doubled to $5.2 billion. Overall, the value of iron ore, iron and steel and ferroalloys exports more than doubled to $4.5 billion. The value of nickel exports almost trebled to $1.2 billion, as did crude oil exports which reached $1.7 billion in 1995–1996. Zinc exports increased in value by 67% to $816 million over the decade. In quantity terms, mineral and energy exports increased by over 50% for the important bulk commodities like iron ore, coal, and alumina, aluminium ingots and bauxite. However, iron and steel exports increased sixfold to 3.1 Mt and exports of refined gold increased by over six times to reach 341 t in 1995–1996. Crude oil exports more than doubled over the period, and tonnages of tin, uranium and zinc exports increased by around a third. Mineral sands was the only major commodity which did not show significant growth over the decade, reflecting in part the switch to further processing for some of the feedstock previously exported. However, synthetic rutile and titanium dioxide pigment exports, not listed in Table 2, rose more than threefold since 1988–1989 to 304 000 t and 127 000 t respectively in 1995–1996.

Investment in the mining sector Although commanding a high profile in the perceptions of the public, over the decade new capital expenditure on mineral (including petroleum) projects has not attracted more than 20% of total Australian investment in any year. In fact it is only in the last five years that this 20% level has been approached and maintained. In the last half of the 1980s it was 13–15% (Fig 2).

FIG 2 - Mining as a percentage of capital investment in Australia 1986 to 1996.

The Australian Bureau of Agricultural and Resource Economics reported that investment in the mining and petroleum sector was around $7.2 billion in 1995–1996

TABLE 2 Exports of selected commodities from Australia - 1985–86 and 1995–96. Commodity

Quantity Units

1985–1986

Value ($M)

1995–1996

% Increase

1985–1986**

1995–1996

% Increase

4(e)

2(e)

-63

242(e)

78

kt

7 687

10 984

43

1 427

2 717

90

Aluminium (ingot metal)

kt

579

1 043

80

975

2 381

144

Black coal - coking - steaming

Mt Mt

49 43

77 61

58 41

3133(e) 2234(e)

4 746 3 014

51 35

Bauxite

Mt

Alumina

-68

Copper

kt

125

250

100

248

928

274

Gold, refined

t

56

341

509

972

5 607

477

Iron and steel - Iron ore - Iron and steel - Ferroalloys

Mt kt kt

79.5 522 85

126 3 130 156

58 500 84

1 939 129 36

2 863 1 510 104

48 1 071 189

Lead

kt

412

460

11.7

361

456

26

Mineral sands - Ilmenite concentrate - Rutile concentrate - Zircon concentrate

kt kt kt

1 034 230 446

1 192 203 450

15 -12 1

48 116 74

110 137 223

129 18 201

Nickel

kt

na

na

na

438

1 157

164

Petroleum - Crude oil - LPG

ML ML

4 402 1 495

10 899 1 513

148 1.2

667 298

1 675 196

151 -34

Tin

t

7 512

10 812

44

58

72

24

Uranium

t

3 533

4 483

27

373

242

-35

Zinc

kt

662

965

46

489

816

67

**Calendar 1986 (e) BRS estimates Abbreviations t = tonne; L = litre; kt = 103t; Mt = 106t; ML = 106L na = not applicable Source: ABARE (1997)

Geology of Australian and Papua New Guinean Mineral Deposits

5

L C RANFORD, D J PERKIN and W A PRESTON

(ABARE, 1996) and the industry survey for the Minerals Council of Australia indicated that this could approach $8 billion in 1996–1997 (Minerals Council of Australia, 1996). Whereas increases in mining investment were above 30% per year in the mid 1980s, growth ceased by the late 1980s, followed by a major but short-lived resurgence at the start of the 1990s. Since then the annual increase in capital expenditure has averaged around 12%, in line with growth of total new capital investment in Australia (Fig 3). The Minerals Council survey suggests that smelting and refining investment is a particularly strong area of growth at present. FIG 4 - Funds employed and profit pre-tax and before interest for mining in Australia - 1986 to 1996.

smelting and refining end of the industry are significantly higher than in mining (which includes the cost of exploration).

FIG 3 - New capital investment in Australia - 1986 to 1996.

The proliferation of, and emphasis on, gold projects has possibly given an over-inflated perception of investment in the mining sector over the decade. Gold projects tend to be relatively low cost in overall investment terms and only very large gold projects may individually require as much as $100 million investment. However, coal and base metal projects, new iron ore mines, a number of nickel laterite projects in Western Australia, aluminium at Boyne Island and alumina expansions and various gas projects and expansions each require investment in the range $400 to $2000 million. The number of projects currently under construction, plus many at an advanced stage of evaluation, suggests that investment will continue to be at a high level in Australia, barring a dramatic drop in world commodity prices.

Profitability of the Australian mining industry Profitability can be analysed in a number of ways, and results in different sectors of the industry may be markedly different from the aspect of a shareholder, a lending institution or government. To gauge the performance of, or return from, the minerals industry in total, it is appropriate to assess the net profits on funds employed, before taxes and interest charges on borrowings are deducted. In this respect overall industry profits have been relatively low for the last five years, compared to the late 1980s and early 1990s (Fig 4). Return averaged between 12 and 13% over the decade and in 1994–1995 was less than 9%. Such figures are significantly lower than returns in most other sectors of the economy. Despite low profitability, industry balance sheets have generally improved, with large increases in sales volumes and continued reductions in real unit production costs as productivity has improved due to industrial reform. Although company returns vary greatly, as do returns in the different commodity sectors, it is generally the case that returns in the

6

Over the decade the return on investor funds has averaged just under 12%. This, however, is carried by the last three years of the 1980s in which net profit return on average shareholders funds climbed rapidly to reach 23% in 1989–1990. In the two subsequent years there was a massive drop to 12% and then 8%, before levelling out and then plummeting again in 1994–1995 to 5.3%, before reviving to 8.8% in the last year (1995–1996). The results of the last six years are far from satisfactory and are unlikely to attract new investors to mining. Total borrowings in mid 1996 were nearly $8.6 billion which, on assets valued at over $52 billion, is small compared to historical percentages. Debt to equity ratios have progressively decreased over the last five years from 33:67 to 22:78 for the last two years (Fig 5). This contrasts with the 55% debt recorded ten years ago (1986–1987) when borrowings were around $13 billion.

Government revenue from mining The various levels of government - Commonwealth, State and local - obtain significant revenue from the mining industry. Some is collected for services provided by governments and some is by taxes and royalties. Annual industry surveys for the Mining Council of Australia suggest that the average annual government revenue from mining has been over $5 billion for the decade (Fig 6). Royalty figures collected by the Australian Bureau of Statistics, which include petroleum, suggest that these figures may in fact be $1.5 billion higher. About half of the $5 billion is provided by company taxes and royalties and the other half is more or less evenly split between employee income tax and government-provided services. Some charges for services are considered to be excessive by industry and are viewed as de facto taxes. The combination of direct taxes such as company income tax, resources taxes, licence fees and royalties, and indirect taxes like land charges or rates, payroll tax, fringe benefits tax and fuel excise have fluctuated between 40 and 60% of the total industry pre-tax profits in recent years, although 70% of profits was collected five to ten years ago (Fig 7). In more recent years, the figures have been within the range 42–50% and represent a very significant impost on an industry whose returns to shareholders are low. In terms of the distribution of industry revenue, direct and indirect charges to government range between 8 and 12% and

Geology of Australian and Papua New Guinean Mineral Deposits

THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996

FIG 5 - Debt to equity ratio for mining investment in Australia, 1987 to 1996. Source: Minerals Council of Australia (1997).

FIG 6 - Government revenue from mining in Australia, 1987 to 1996. Source: Minerals Council of Australia (1997).

Geology of Australian and Papua New Guinean Mineral Deposits

7

L C RANFORD, D J PERKIN and W A PRESTON

FIG 7 - Government taxes and royalties as a percentage of profit: mining sector in Australia, 1987 to 1996. Source: Minerals Council of Australia (1997).

have averaged around 10% over the decade. The most recently available figure, for 1995–1996, gives a tax and royalty take of $2.4 billion, split 52% as company income tax, 27% as resource taxes, licence fees and royalties and 21% in indirect taxes. Although the income tax figure fluctuates widely, being based on profits, the aggregate of the other taxes and royalties is more consistent as there is a significant proportion of gross revenuebased and specific rate assessments.

Employment and investment multipliers Mining is a capital, rather than labour intensive industry. Direct employment in the industry is estimated by the Australian Bureau of Statistics to be 84 000 in 1997 or only 1% of the total workforce. Approximately three-quarters of the workforce is engaged in mining and exploration and one quarter in smelting and refining. Over the last decade direct employment in mining has dropped from over 100 000 or 1.4% of the Australian workforce as significant productivity improvements have been achieved and large scale, highly mechanised bulk mining activities have become more prevalent. The distribution of employment in the different sectors is 38% or 32 000 persons in metalliferous mining, 25% or 21 000 in coal, 21% or 17 000 in services to mining, 12% or 10 000 in other mining and only 5% or 4000 in oil and gas. Such figures indicate the variation in automation and mechanisation in the different sectors.

Multiplier Mining

Metallic minerals

4.1

Coal, oil and gas

3.6

Services to mining

3.2

Mineral Processing Basic metal products

4.7

Chemical, petroleum and coal products 4.3 Significant multipliers in mining have also been identified for output and income figures by the same study. The study suggests that, in Western Australia, every $1 of output from mining generates approximately the same in output in the nonmining industries, ie, a multiplier of 2. In the service area a multiplier of 3 has been estimated. With respect to income, the multipliers are even higher with around 3 in metallic mining and services to mining, and around 3.5 in mineral processing. In coal, oil and gas the figure is slightly lower at 2.2. Therefore, although overall employment in mining is low, the very significant flow-on effects to other sectors of industry in terms of employment, income and output are much higher than in most other industry sectors.

MINERAL PRODUCTION AND TRADE General

Despite low direct employment figures, multipliers related to mining provide very significant increases in employment. A study by the Economic Research Centre of the University of Western Australia indicates that multipliers in mining are in the range 3.2–4.1 and in mineral processing 4.3–4.7 (Clements and Qiang, 1996).

Australia is a world class producer of many mineral commodities. It is self sufficient in most primary metallic products and many industrial minerals. Its production surplus has enabled it to grow into a major world source of many mineral commodities and it has developed a reputation, particularly in more recent times, as a reliable supplier of high quality products.

This compares with an average estimate of 2.6 across all industries. Individual sector figures are shown below:

Much of the industry is well established and has taken advantage of market expansion opportunities in most sectors

8

Geology of Australian and Papua New Guinean Mineral Deposits

THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996

over the last decade. There has been growth in many of its world ranking mineral outputs, including alumina, zinc, copper, coal, diamonds, titanium mineral products, iron ore, salt, tantalum and lithium, and most spectacularly gold, and to a lesser degree nickel. Levels of smelting and/or refining vary for different commodities, but a significant amount of primary product is exported as ore or concentrate. Major process developments are evident in such sectors as nickel and titanium minerals, and further processing projects for iron ore are either committed or at an advanced stage of assessment.

Some alumina is smelted to aluminium in Qld, Vic, Tas and NSW. With 1.3 Mtpa aluminium output Australia ranks fourth in the world (6% in total) and with almost 1 Mtpa exported Australia is the third largest trader behind Russia and Canada. Aluminium production in Australia has remained relatively constant for about eight years.

Base metals (copper, lead, zinc)

Alumina and aluminium

Australia holds the premier world position in mine production of lead with 17% of world total, is second in zinc at 14% and ninth in copper at 4%. Whereas NW Qld, centred on Mount Isa, provides the long term production base for zinc and copper and Broken Hill in NSW provides the base for zinc and lead, there are a number of other centres in the NT, NSW, WA, Qld, SA and Tas which contribute to the overall output. The development and expansion of the polymetallic Olympic Dam mine in SA has substantially increased copper output and the Carpentaria region in the NW Qld–NT border area is progressively increasing its output. This area will undoubtedly provide the long term focus of base metal production on the basis of discoveries discussed later.

About 10% of Australian bauxite production is exported, but the rest is converted to alumina in integrated operations, in WA, Qld and the NT. Australia is the world’s largest alumina producer and exporter, with one-third of total world output, and around 80% of Australian production is exported.

Although output fluctuates considerably from year to year with variations in the market, the general trend has been growth in mine copper production of the order of 50% and zinc of 30% over the decade. Lead production has increased only marginally over the period and has in fact tended to decline through the 1990s after peaking at the start of the decade.

Over the decade growth in Australian alumina capacity has been around, but slightly below, that of expansion in world production (45%). There are plans for further growth, for which the timing is dependent on market demand.

A moderate proportion of base metal output is processed to either smelted or refined product. About a third of the contained lead, half of the contained copper and nearly threequarters of the zinc are exported as concentrates.

The list of mineral imports to Australia is relatively short, with significant members being potassium fertilisers, phosphate rock, sulphur and the platinum group metals. Imports of processed products such as ferroalloys, speciality steel product and cut-gem diamonds highlight some of the downstream processing deficiencies in Australia. Comments on the individual commodities are set out below and percentages of world production are shown in Fig 8.

FIG 8 - Australia’s mineral production in a world context, at 1 January 1996. Source: ABARE (1997).

Geology of Australian and Papua New Guinean Mineral Deposits

9

L C RANFORD, D J PERKIN and W A PRESTON

Most Australian production is exported and Australia plays a significant role in primary concentrate and refined products trade. In terms of primary concentrates Australia ranks first in the world in zinc and third in lead. Australia is the leading exporter of smelted and/or refined lead products and is the second largest exporter of smelted and/or refined products of zinc.

Coal Australia’s coal production from mines in Qld and NSW makes it one of the world’s major sources of high quality coking coal and the world’s largest seaborne exporter of black coal. Between 65 and 70% of production of black coal is exported and the ratio of coking to steaming coal exports is about 55:45. Black coal production has grown around 30% over the decade, with expansion particularly noticeable over the last two years. Production is currently nearly 200 Mtpa of saleable product of which about 140 Mt is exported. In addition to the domestically consumed black coal, mainly steaming coal, a further 50 Mt of brown coal is produced for domestic power generation.

Diamonds Australia’s diamond output of around 40 M carats per year is based on the single operation at Argyle in the Kimberley region of WA. It provides around one-third of the world’s natural diamond output, although a large portion is of industrial quality, resulting in only 4 to 5% of the world total in value terms. Although Australia supplies over one-third of the world’s gem and cheap gem output and almost double that of any other producer, an overwhelming proportion is at the cheaper gem end of the scale. The continuance of diamond production from Australia beyond the middle of the next decade is, at this stage, dependent on the decision to develop the mine at Argyle at depth, possibly by underground mining. There are no commitments to development of other Australian diamond prospects as yet.

Gold Australia has experienced exponential growth in gold production through the 1980s, mainly centred on WA, but also in Qld. During the decade output increased by a factor of 3.5 times, although plateauing at around 250 t of fine gold in the first half of the 1990s. Australia has improved its ranking from fifth to third largest world primary gold producer during the decade, and now supplies about 11% of world mine output. All gold, apart from that contained in base metal concentrate shipments, is refined in Australia and over 80% of refined output is exported. Australian refineries also refine a small quantity of imported bullion. It appears that the current high production level can be sustained, but major discoveries will be necessary to cause any significant increase in output.

Heavy mineral sands The heavy mineral sands industry, now commonly described as the titanium minerals industry, has experienced fluctuating market conditions over the decade. Long overdue improvements from the mid 1980s during a three year period provided significant growth in the industry and, most particularly, the establishment and expansion of capacity for production of synthetic rutile and titanium dioxide pigment in

10

WA. A further, shortlived boom was experienced at the beginning of the 1990s leading to further mine expansion and process development for added value product. The industry continued to expand subsequently, despite less favourable conditions. Whereas ilmenite production has increased by 40% over the decade, concentrate exports have only grown marginally as most of the expanded output has been converted to synthetic rutile in WA. Through the 1990s synthetic rutile production has doubled and although a significant amount of this is exported, some is used as a feedstock to a modernised and expanded pigment sector. Pigment output has grown by 50% since the beginning of the decade and plans are in place for significant further expansion. Around 70% of pigment production is exported. Australia remains the world’s leading supplier of titanium minerals (ilmenite 25% and rutile 50%), although major expansion of titanium slag production in South Africa is providing strong competition in the market place. There has been little growth in natural rutile output in Australia over the last decade as a result of minimal growth in eastern Australian production. The ilmenite based output, including synthetic rutile from WA, has been the growth area throughout the period. Zircon is a by- or co-product of titanium minerals production and Australia has accounted for about 50% of world output over a long period. Production declined significantly in the early 1990s, but has recovered to 450 000–500 000 tpa in recent years. Most Australian zircon production is exported and extreme volatility in price has been a feature of the market. Monazite sales have progressively declined from around 18 000 t in the mid 1980s to zero by 1995. Competition from other rare earth minerals, especially from China, has been partly instrumental in the decline. However, the cessation of shipments to the French company, Rhone Poulenc, because of waste disposal problems in France, finally led to a complete stop of production. A second attempt to establish a monazite cracking plant in WA is at an advanced stage of approval and commitment.

Iron ore Australia is the world’s leading exporter of iron ore, providing 30% of traded product. Brazil and Australia have dominated the world market for nearly 30 years, supplying about 60% of world trade. Current Australian iron ore shipments of 130 Mtpa are mainly from the Pilbara region of WA, with a small amount from Tas. Approximately 90% of production is exported. Some domestic production is shipped from the Pilbara to eastern states’ iron and steelmaking facilities and locally produced ore is also used for iron and steel making in SA. There has been about 20% growth in iron ore production since the beginning of the 1990s, and all of this growth has come from the Pilbara. In addition, over the last four years there has been increased interest in iron and steel making projects in WA because of the availability of gas supplies at significantly lower prices. The level of processing of iron ore in Australia has been low to date, at about 8.5 Mtpa of steel production, but the potential for iron making developments, through direct reduction technology, is high.

Geology of Australian and Papua New Guinean Mineral Deposits

THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996

Australia is expected to maintain and has the potential to increase its importance in the world iron ore industry, and could become a significant supplier of direct reduced iron to the world market in the medium term.

Manganese ore Australia ranks fifth in the world as a manganese ore producer. Production is mainly from Groote Eylandt in the NT, although in recent years this has been supplemented by production from smaller producers from the East Pilbara region of WA. With production of 2.0–2.2 Mtpa Australia supplies about 10% of world demand. Most ore is exported, although small quantities of manganese-based ferroalloys have been produced in Australia.

Nickel Nickel production, centred in WA, has expanded significantly over the last four years, based on expansions of existing mines and new sulphide ore developments. These expansions have overshadowed the demise of the Greenvale lateritic mine in central Qld. The net nickel output from mining operations has grown by 30% over the last decade, and has exceeded 100 000 tpa of contained nickel since 1995. Lateritic nickel projects at an advanced stage of evaluation or committed to development could result in substantial growth in nickel output in the next few years, and advance Australia’s position from third at present to second ranking in terms of world production. Australia currently produces 10% of world mine output and most is destined for the world market either as concentrate, matte or metal. About 75% of concentrate output is processed to matte or other intermediate product and a large and increasing proportion of this is taken through to refined nickel product.

Salt Salt production is largely from solar evaporation projects on the NW coast of WA. Growth in output has been about 30% over the last decade and is currently 8.7 Mtpa. In terms of world production this is relatively low, at 4% of total. However, it is significant in terms of seaborne trade, where, along with Mexico, Australia dominates the Pacific-based trade where much of the world’s petrochemical industry growth is focussed.

Uranium Despite limitations on production as a result of the Commonwealth Government’s ‘three mines’ policy between 1983 and 1996, Australia has retained its position as the second-ranked world producer, behind Canada, with about 16% of world output. This ranking is more a result of a decline in world production by about 30% throughout the 1990s, than any increase in Australian production. Through the early 1990s Australian output dropped to 2200 t of U308 compared with 4300–4500 tpa through the 1980s. In the last couple of years production has increased to over 5000 tpa U308, due to expansion of the Olympic Dam mine in SA. Future uranium production from Australia could increase significantly, given market opportunities. Expansion at Olympic Dam, the development of Jabiluka in the NT, and of Kintyre in WA plus a number of other projects could provide the increased output.

Geology of Australian and Papua New Guinean Mineral Deposits

Other minerals Australia is a world ranking producer of a number of other minerals. It is the world’s leading producer of tantalite and the lithium-based mineral spodumene, although in terms of lithium content Australia produces considerably less than China. A new plant to produce lithium carbonate in WA will provide a more desirable and competitive lithium product. Aggressive and innovative marketing of tantalite and spodumene over the last decade has resulted in Australia achieving its current position in world markets. Australia has long been a significant tin producer. However the long depression in the market has resulted in little expansion, and tin is now merely a by-product of tantalite production in WA. Australia ranks seventh in world tin production with just over 4% of world output. Australia is a major supplier of silver with 7–9% of world output. Most is recovered as by-product and co-product from base metal mines and as a by-product of gold operations. Despite the exponential growth in gold production there has been no corresponding growth in silver production over the last ten years. Of the industrial minerals, Australia is now a significant producer of magnesite and silica sand. In gemstones Australian opal and sapphire are of world significance and repute.

MINERAL EXPLORATION During the last decade private expenditure on exploration for minerals other than petroleum remained at a relatively high level, in real terms, although it fluctuated in a band between $700 million and $1 billion, as measured in 1996 constant dollars (Fig 9, Table 3). The peak achieved in 1988 was largely due to an increase in risk capital raised prior to the October 1987 crash, as a response to a wave of optimism about commodity growth worldwide which flowed through to investment in exploration and mining companies in Australia. In 1988, >72% of exploration expenditure was for gold. Gold has remained the dominant commodity sought with exploration expenditure consistently accounting for more than 50% of total mineral exploration expenditure. Gold exploration declined in 1996 for the first time since 1992 but was still about 57% of the total. Base metal exploration expenditure, the next largest component overall, has shown a fairly consistent increase in its share from 10 to 20% in the period 1986 to 1988 to around 25 to 30% of the total in the last few years. Diamond exploration expenditure and coal exploration expenditure have remained at around $30 to $50 million per year in 1996 $A over the decade and each represents about 5% of the total. Although diamond exploration has been focussed on frontier, greenfields activity, coal exploration has been concentrated in defined production areas. Iron ore exploration expenditure peaked at about $40 million in 1996 $A terms in 1992 but has fallen substantially since. Similarly, exploration expenditure for heavy mineral sands has fallen from $23 million in 1990, in 1996 A$, to about $9 million in 1996. The largest falls in exploration expenditure over the decade have been for uranium and tin-tungsten, decreasing from about $81 million and $12 million to $7 million and less than $1 million respectively in 1996 dollars. In the case of uranium,

11

L C RANFORD, D J PERKIN and W A PRESTON

FIG 9 - Exploration expenditure by commodity in Australia, 1986 to 1996 (in 1995–96 dollars). Source: ABS, adjusted to 1995-96 using ABARE data.

TABLE 3 Mineral exploration expenditure in Australia, 1986 to 1996 (constant 1996 dollars). Year

Gold

1986

346.3

Copper, lead, zinc, silver, nickel, cobalt 127.7

Diamond

37.3

Coal

Iron ore

52.0

18.7

Mineral sands 9.5

Tin, tungsten 12.4

Uranium Construction materials 81.1

2.7

Other

Total

26.0

713.5

1987

527.4

113.5

25.4

54.0

17.1

10.8

4.4

32.4

4.9

31.9

821.6

1988

799.0

115.4

33.4

37.8

16.2

17.2

3.9

31.9

1.2

46.9

1 103.1

1989

574.9

128.7

41.8

37.2

9.0

21.1

1.8

36.3

1.4

41.9

894.1

1990

405.0

140.0

44.1

39.6

13.4

22.9

3.3

22.3

3.3

27.1

720.9

1991

339.1

193.7

44.0

26.0

12.6

21.6

2.7

14.4

3.0

20.9

678.3

1992

336.9

160.2

39.5

30.4

40.8

15.4

1.9

14.2

1.8

26.9

667.8

1993

350.6

200.9

41.7

26.5

26.3

10.7

0.5

9.6

1.2

24.0

691.9

1994

488.0

206.1

63.1

29.8

20.2

9.1

1.2

8.2

1.4

25.3

852.6

1995

578.1

209.6

50.5

39.6

12.6

6.0

1.1

8.1

0.9

24.5

931.3

1996

547.1

251.8

52.9

52.7

14.1

9.3

0.6

7.2

0.7

23.8

960.2

Source: ABS (1997)

this represents a decrease from 11% of total expenditure in 1986 to less than 1% in 1996. This is attributed to lower spot prices for uranium and the former Commonwealth Government’s three mine policy for uranium. The decline in exploration expenditure for tin and tungsten is closely linked to the fall in tin and tungsten prices in real terms. This decline is considered to be due in part to the collapse of the International Tin Council in the early 1980s and to high production, particularly from China and Brazil, despite subdued demand for both metals. The last several years have seen a noticeable slowing down in the rate of growth of Australian exploration expenditure in real terms and this is a cause for concern. Unless a high and growing level of investment in exploration is maintained a decrease in the rate of mineral deposit discoveries and resource development can be expected to follow over the next 5 to 10 years.

12

OVERSEAS EXPLORATION BY AUSTRALIAN COMPANIES As part of the annual survey of the minerals industry, the Minerals Council of Australia (MCA) has provided information on overseas exploration activity since 1986. This has been compiled by Coopers and Lybrand, based on information supplied to them in confidence by the respondents (Minerals Council of Australia, 1996). The respondent companies constitute the major portion of the industry by aggregate size, ranging from the largest companies through medium to some small exploration ventures although numerically speaking, the companies responding probably constitute less than 10% of all companies exploring in Australia. Omissions from the survey include most of the smaller mining and exploration companies, some overseas controlled companies and a proportion of some joint venture operations.

Geology of Australian and Papua New Guinean Mineral Deposits

THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996

FIG 10 - Estimates of exploration expenditure by Australian companies in various overseas countries or regions (in constant 1996 $A). Source: Minerals Council of Australia (1997).

To enable a comparison with previous surveys on exploration expenditure by the MCA, returns from those respondents which have participated in the survey over a period of years are separately reported as a ‘constant group’, and these data are discussed below. The increase in overseas exploration activities has been particularly marked since 1993 (Fig 10), when expenditure rose from the 30% of constant group total level it had broadly maintained since 1986, rising steeply over three years to reach 41% of total group at $319 million in 1996. In terms of trends in commodity sought over the period, overseas spending on gold and platinum exploration accounted for just over 40% of the total overseas spending by respondents in 1996, well down on the 79% recorded in 1989. In 1988 base metal exploration expenditure overseas was around 10% of the total, rising to 35% by 1993 and remaining at about 30% in 1996. Base metals are the major commodities sought worldwide. Although exploration expenditure for diamonds was the third largest at about 16% of the overseas total in 1996 and represented a fall from its peak of 28% in 1995, its proportion up to and including 1993 was less than 10% of the total spent by the constant group of exploration and mining companies. Australian exploration expenditure in Canada and USA has remained at around 30% of the MCA overseas total since 1988. In 1996 North America accounted for around $100 million, having grown rapidly in 1993–1994. The proportion of exploration spending in Asia and South America has doubled over the last six years and now each represents about 25% of the total. A significant proportion of this is in ‘grassroots’ exploration. The share of expenditure in Papua New Guinea fell from 16% in 1992 to 4% of the total in 1996, a pattern also followed by expenditure in Africa which fell from a peak of around 14% in 1994 and 1995 to 7% in 1996. To date there has been limited spending in countries formerly part of the Commonwealth of Independent States (CIS) and Eastern Europe, even though these areas have attracted significant interest by Australian companies.

Geology of Australian and Papua New Guinean Mineral Deposits

RESOURCES General Based on current knowledge, Australia is undoubtedly one of the world’s richest nations in terms of mineral resources. However, this may change in the future as countries with a range of under-explored geological environments come under the scrutiny of the international exploration industry in the face of growing world demand for minerals. Some of the reasons for Australia’s large stocks of known mineral resources include a large land mass, a stable political system and a history of sustained modern mineral exploration since 1950. This is built on gold and base metal mines and mineral provinces discovered in the preceding 100 years, particularly during the period 1851 to 1900. It is also based on a natural endowment comprising an enviable and complete, but only partially exposed, geological framework comprising rocks from every period of the earth’s history. The rocks host representatives of each period’s characteristic mineral deposit types. Australia’s geological setting, beginning with the Archaean of WA and evolving eastward in a series of successively younger Proterozoic and Phanerozoic basins, fold belts and metamorphosed cratons or blocks, provides the foundation for Australia’s petroleum and mineral resources.

World context In terms of world ranking, Australia has the world’s largest known economic demonstrated resources (EDR) for eight major tradeable mineral commodities - lead, zinc, silver, ilmenite, rutile, zircon, uranium and gem or near gem diamond (Table 4). Australia is ranked in the top three countries in the world for resources of bauxite, copper, gold, iron ore, manganese ore, tantalum and industrial diamonds. In addition, Australia’s stocks of EDR are within the top six countries worldwide for an additional 13 vitally important commodities bauxite, black coal, brown coal, cobalt, copper, gold, iron ore, lithium, manganese ore, nickel, rare earths, tantalum and industrial diamonds. Australia also has almost all of the world’s opal and a significant share of the sapphire resources. Australia’s only apparent mineral deficiencies are

13

L C RANFORD, D J PERKIN and W A PRESTON

TABLE 4 Australia’s identified resources and world economic demonstrated resources of major minerals, 1995. AUSTRALIA 1995 IDENTIFIED RESOURCES

DEMONSTRATED

WORLD 1995

INFERRED

Commodity

Units

Economic

Bauxite

(Mt)

2540

5245

2134

Black coal (recoverable)

(Gt)

49

6

Brown coal (recoverable)

(Gt)

41

(kt Cr)

-

Cobalt

(kt Co)

274

Copper

(Mt Cu)

24

Diamonds - gem and cheap gem - industrial

(106c) (106c)

101 128

Gold

(t Au) (Gt)

Lead Lithium

Chromium

Iron ore

Magnesite

AUSTRALIA’S SHARE OF WORLD ECONOMIC RESOURCES

Subeconomic Undifferentiated

Economic* demonstrated resources

%

Ranking

20 000 (a)

13

3

very large

708

7

6

3

166

313

2000

Pillowed to massive tholeiitic basalt

Lunnon Basalt

Bartram, 1971) to south of St Ives. Its lithology varies from pyritic graphitic slate to magnetite-bearing laminated chert. It separates the dominantly low silica lavas of the underlying formations from the high silica, high magnesium lavas of the overlying formations, and has been dated at 2692±4 Myr (Claoue-Long, Compston and Cowden, 1988). Conformably overlying the Kapai Slate is the Paringa Basalt, a l000–1500 m thick, siliceous high-magnesium basalt. It consists of variolitic pillowed flows and minor dolerite sills

Geology of Australian and Papua New Guinean Mineral Deposits

Siliceous high-Mg basalt, minor interflow sediment

with numerous bands of laminated and 2 to 5 m thick cherty interflow sediment, and has been dated at 2690±5 Myr (J M F Clout, unpublished data, 1991).

Black Flag Group The Black Flag Group consists of a felsic volcanic and sedimentary succession more than 1 km thick, conformably overlying the Kalgoorlie Group. These have been dated at 2676±4 Myr (J C Claoue-Long, I H Campbell and R Hill,

245

R B WATCHORN

unpublished data, 1991). Two formations were defined in the Group in the Kambalda area by Gemuts and Theron (1975); a lower dominantly felsic volcanic unit (Newtown Felsic Member) and an upper dominantly sedimentary unit (Morgan’s Island Epiclastic).

Merougil beds The Merougil beds comprise conglomerate, terrigenous arenite, pebbly arenite and arenite and correspond to the Kurrawang beds west of Kalgoorlie (Griffin, Hunter and Keats, 1983). The unit is estimated to be more than 2000 m thick in the region. The Merougil beds are the youngest unit in the Kambalda area and unconformably overly the Black Flag Group. The beds lack a distinct penetrative fabric, and this, in conjunction with discordant shallow dips, indicates syn- to post-tectonic deposition.

Kambalda intrusive rocks Doleritic to gabbroic dykes and sills, including the Defiance, Condenser and Junction dolerites, intrude the Kalgoorlie and Black Flag groups. The stratiform but discontinuous Defiance Dolerite is believed to have formed by in situ fractionation of the base of the Paringa Basalt. It is up to 300 m thick and is dated at 2693±50 Myr (Compston et al, 1986b). The 500 m thick Condenser Dolerite is stratigraphically and chemically equivalent to the Golden Mile Dolerite at Kalgoorlie. It was intruded transgressively, but is essentially conformable with the contact between the Paringa Basalt and the Black Flag Group. The Junction Dolerite is stratigraphically equivalent to the Condenser Dolerite. It is highly differentiated and has been subdivided into four zones of which Zone 4, a coarse-grained, magnetite-rich, quartz granophyric zone is the favoured host for high grade ore. The regional succession is intruded by at least four distinct episodes of igneous intrusions. The first comprises thin mafic to intermediate, aphyric, fine grained, shear- and layer-bounded sills and dykes which have been affected by all major deformations. The second comprises large (to 300 m thick) subconcordant lamprophyric xenolith-bearing sills with felsic differentiates. A kersantite of this suite has been dated at 2684±6 Myr (Perring, 1988). They intrude the Kambalda Komatiite and the Kapai Slate, and are present as sills or dykes at the Paringa Basalt–Black Flag Group contact in the Lake Lefroy area. The Kambalda Granodiorite and a set of essentially upright, felsic dykes of at least two generations comprise the third episode. These intrusions are dated at 2662±6 Myr (Compston et al, 1986a) These rocks have only weakly developed fabrics which are discordant to earlier (D1 and D2) structures, and intrude along and are deformed by later (D3) structures (Clark, Archibald and Hodgson, 1986). The fourth intrusive event resulted in emplacement of numerous Proterozoic dolerite dykes trending east and ENE. Some have a strong positive magnetic signature and others have a reversed magnetic response suggesting several emplacement regimes. These are dated at 2420±30 Myr (Turek, 1966) and 2042±45 Myr (Compston, 1980).

246

METAMORPHIC AND TECTONIC FRAMEWORK The Kambalda–St Ives region is structurally complex with polyphase deformation accompanying and post-dating regional metamorphism. The regional metamorphic grade reached lower amphibolite facies at 520–550oC and 2–3 kb (Donaldson, 1983; Wong, 1986). The earliest recognised deformation in the Kambalda area (D1) comprises major thrusts with a mylonitic fabric, forming a SE to NW thrust-repeated succession. Major thrusts have been identified, spaced 5–10 km apart, at Foster, St Ives, Tramways and Republican Hill (Fig 3). Recumbent open to tight folds formed during D1. These are commonly dislocated by thrusts which are subparallel to their axial surfaces. Subsequent NNW-trending deformation formed upright open folds (D2). The regional structure is dominated by a broad south-plunging antiform. D2 was synchronous with, but slightly later than, peak metamorphism (Gresham and LoftusHills, 1981). The D3 deformation is characterised by major NNW- and north-trending anastomosing shear zones such as the Boulder–Lefroy and Zulieka shear zones. There are several generations of shear zones and their fabrics have chloritic retrograde metamorphic assemblages. Structures associated with ore deposits are generally late stage D3 shears and fractures, and usually represent third and fourth order splays off the major NNW shear zones. These late stage shears often reactivate earlier D1 or D2 shears, which may also be mineralised. The last generation of major faults (D4) trend NNE, generally with fairly minor (50–200 m) dextral movement.

ORE DEPOSIT FEATURES The locations of the deposits are shown in Figs 1 and 2 and their characteristics are listed in Table 3.

DEPOSIT HOST ROCKS Ore deposits formed in nearly all stratigraphic units. The hosts, in order from the earliest units, and the contained deposits are: • Lunnon Basalt: Hunt; • Kambalda Komatiite: Redoutable, Victory–Repulse and Red Hill; • Devon Consols Basalt: Revenge (W45) and Britannia; • Kapai Slate: Victory, Clifton, Blue Lode, Delta South and Repulse footwall lode; • Defiance Dolerite: Revenge N01 and N22, Defiance, Thunderer, Orchin and North Orchin; • Paringa Basalt: Defiance, Sirius, Apollo and Santa Ana; • Condenser Dolerite: Argo and Junction, and Cave Rocks in an analogous dolerite; and • Intermediate and felsic intrusive rocks: Intrepide and Victory Flames.

BASALT HOSTED DEPOSITS Sirius The Sirius orebody was discovered in 1988 and the top 90 m was worked in an open pit in 1989–90. The depth of weathering reaches 40 m with considerable supergene enrichment of gold.

Geology of Australian and Papua New Guinean Mineral Deposits

KAMBALDA––ST IVES GOLD DEPOSITS

TABLE 3 Characteristics of the Kambalda–St Ives gold deposits, modified after Roberts and Elias (1990). Deposit

Host rocks

Main structure

Lode type

Regional metamorphic assemblage

Lunnon Basalt

Shear, NNW, 7oW

Q vns, bx

hb, ac, cl, pl, Q cl, bt, ank, ca ank, bt, ab, py

Devon Consols Basalt/Kambalda Komatite

Shear, NNW, 70o E

Q vns, mylon, bx

tc, cl, ank, tm

cl, dl, tc, ab

Kambalda Komatite

Shear

Q-cb vns

sp, tc,cl,tm

Orion

Devon Consols Basalt/inflow sediment

Shear, NNW, 60oW

Q vns, mylon

Orchin

Defiance Dolerite/Paringa Basalt

Shear, NE, 45oSE

North Orchin

Defiance Dolerite/Kapal Slate

Victory Defiance

Alteration zoning

Premining reserve ( t gold)

Mining method

3-15

U/G

dl, cl, ab, tc, py, po

31

Open pit, U/G

Q,cl,py

cl

bt,ab,py

25

Open pit, U/G

Q vn, py

q, cl

cl,bt,mt

ab,bt,py,po

10

Open pit, U/G

Shear NNW 10oE

Q vn, py shear

Q,ab,cl

cl,bt,mt

ab,bt,py

15

U/G

Paringa Basalt/ trondhjemite

Q vn, NNW shear, 70oE

Q vn, shear

cl

cl,dl

bt,ab,py

9

Open pit

Argo/Apollo

Condenser Dolerite/Paringa Basalt

N-S shear, 40oW

Q vn,py,shear

cl,hb

cl,dl

ab,bt,py,as

10

Open pit, U/G

Cave Rocks

Dolerite

Shear, NNW, 85oE

Q vns

cl,hb,ac,pl

cl,dl

Bt,ab,Q,po, as

9

Open pit, U/G

Felsic/ intermediate intrusions

NNW Q vn stockwork

Q

pl,hb

cl

py,ab,bt

12

Open pit

Kambalda Komatite intermediate/ felsic intrusions

NNW shear, 60oE

Q vns

tc,dl

cl

bt,py,ab

4

Open pit

Intrepide

Redoutable

Q - quartz, ac - actinolite, cl - chlorite, hb - hornblende, pl - plagioclase, tc - talc, ank, ankerite, tm - tremolite, ca, - calcite, sp - serpentine, mt magnetite, ab - albite, py - pyrite, bt - biotite, dl - dolomite, po - pyrrhotite, ms - muscovite, ep - epiodite, sn - sphene, bx - breccia, mylon mylonite, vns - veins, as - arsenopyrite.

In the last six years the orebody has been accessed and intensively drilled from underground. The Sirius orebody is hosted by the Paringa Basalt on the strongly sheared east limb of the major D2 fold in the Victory area. In this area the Repulse shear zone acted as a sole thrust, with the Victory, Britannia and Sirius shears being associated hanging wall splays (Fig 4).

Geology of Australian and Papua New Guinean Mineral Deposits

The 100–200 m wide Sirius shear dips 30–60o ENE, and has a strong, late, flat, west-dipping crenulation cleavage overprinting a strong chlorite-biotite foliation (D Barrett, unpublished data, 1993). The 400 m long, 100 m thick by 300 m deep orebody is localised where the Sirius shear changes up dip from a 35o east dip to subvertical (Fig 5). Multiple generations of quartz veins have been emplaced, thrust folded

247

383 500 E

382 000 E

R B WATCHORN

shear zones. The shear zones contain 0.3–2 g/t gold and are associated with sparse quartz veining. Most of the gold is associated with biotitisation of the wall rock. Except for a high grade, laminated, crack-seal quartz vein at the north of the orebody, where gold is associated with the laminations in the vein, most of the quartz veins have low gold grades.

THUNDERER

534 500 N

The orebody is open to the north, south and down dip. The current plan is to mine the top 250 m as an open pit, and the deeper parts and the thinner ore zones to the north and south from underground.

NORTH ORCHIN BRITANNIA SIRIUS Cross section on Fig 5

LIFEBOAT

Britannia REPULSE

The Britannia lodes are 200 m west of Sirius in the footwall of a major listric shear zone (Figs 4 and 5) and were discovered in 1987. Initially (1987–1990) worked in a pit, the orebody was drilled and accessed by underground development in 1993.

VICTORY DEFIANCE 533 000 N

FO ST

FLAMES E R

N31 ST RU TH

CONQUEROR 0

750 metres

LEGEND Proterozoic dyke Defiance Dolerite, Zone 1 Defiance Dolerite, Zones 2-5

Lamprophyre

Kapai Slate

Felsic intrusive

Devon Consols Basalt Tripod Hil Komatiite

Black Flag Group Paringa Basalt

Dolerite

Victory-Defiance premined ore reserve boundary Kambalda local grid

Footwall basalt

FIG 4 - Geological plan of the Victory area, showing gold deposits.

and brecciated. The final set of flat, SW dipping veins was emplaced after the major movement. Higher grade (2–5 g/t) gold mineralisation occurs as lenses of intense biotite-quartz veining, with 2–10% pyrite, within

DE IANCE

The orebody is hosted by the 100 m wide, major early chloritic Britannia shear, which is subvertical at the surface and flattens to 45o at depth. The shear marks the contact between the Devon Consols Basalt and the Paringa Basalt. It is one of the earlier (D3) shears in the area and is intruded by sheared, late stage, felsic dykes. Elsewhere the intrusive rocks crosscut and thus post-date the major NNW folding (D2). The orebodies are focussed on the felsic intrusive contacts (Fig 6) and the ore zones have a gentle north plunge, in sympathy with late flat NNW-dipping veins. Steeply plunging, en echelon, high grade shoots within this zone are associated with magnetite and massive euhedral sulphides, including 0.5–1 cm pyrite grains with 1 g/t over economic widths were identified in 24 of the 32 sections drilled (D J Porter, unpublished data, 1989).

255

R A CROOKES and D DUNNET

In late December 1989 Orion Resources NL entered into a subscription agreement with Gasgoyne to fund reverse circulation percussion (RC) drilling at 40 m and subsequently 20 m centres. The results of the drilling were used to delineate a resource of 1.163 Mt at 5.1 g/t gold (C C Schaus, unpublished data, 1992). Orion acquired a direct 45% share of the Yilgarn Star Project by September 1990 and assumed management control of the production joint venture in January 1991. Additional RC drilling in 1991 outlined Proved and Probable Reserves of 1.536 Mt at 4.17 g/t gold. Mining by open cut methods to 130 m vertical depth was completed in October 1995. Ore production from underground mining commenced in March 1995, with the full production rate of 0.55 Mtpa achieved in mid 1996. The resource potential is estimated to be high as the entire deposit remains open at depth and along strike in both directions. Drilling data are available to 740 m vertical depth, highlighting the strong downdip continuity of gold mineralisation, the mine sequence and the associated alteration system.

REGIONAL GEOLOGY The Marvel Loch–Yilgarn Star area is in the central-southern part of the Archaean Yilgarn Craton and within the Southern Cross Province. The mineralisation and regional geology of the Southern Cross greenstone belt have been well documented (Keats, 1991). The Marvel Loch–Yilgarn Star area falls into three structural domains shown in Fig 1 (R Marston, unpublished data, 1993): 1.

2.

3.

The Northern domain is an area between Marvel Loch and the Yilgarn Star haul road and comprises an arcuate belt of rocks striking around the southern perimeter of the Ghooli Dome granitoid batholith. The Central domain is a latitudinal zone about 5 km wide, north to south, extending from Great Victoria in the west to the Yilgarn Star mine in the east, which contains a prominent east-trending Proterozoic mafic dyke near its northern boundary, close to the Yilgarn Star haul road. The Southern domain is a narrow belt of rocks striking around the northern perimeter of the Parker granitoid dome.

It is likely that the present form of the area is due to progressive ENE to WSW compressional deformation involving polyphase folding and major shear coupling, directed by forces associated with the granite emplacement. The regional stratigraphic sequence is dominated by a range of metasedimentary and metavolcanic rocks, with a complex history of structural deformation, metamorphism and metasomatism. The metasedimentary sequence comprises a package of alternating schistose rocks, originally shale, siltstone, sandstone, wacke and conglomerate, and many have the graded and thinly bedded appearance of turbidites. Semipelitic quartz-mica schist is the most common outcrop. Iron-rich cherty and carbonaceous chemical sediments are typically developed at the base of this sequence, with layered calc-silicate amphibolites locally referred to as ‘banded amphibolites’. These rocks are interlayered with more siliceous metasediment and may represent tuffaceous mafic rocks, metamorphosed marly sediment or hydrothermal alteration zones.

256

A local structural unconformity with pronounced faulting and folding is often apparent between the metasediment and a mafic-ultramafic rock sequence. These metavolcanic rocks comprise a pile of tholeiitic to magnesian mafic and ultramafic rocks of extrusive and intrusive origins containing interflow pelitic metasediments. Several horizons of oxide-type banded iron formation and chert occur in the lower half of the sequence. Dark green homogenous amphibolites are the most common outcrops (Barnes and Schaus, 1993). The major contrast in ductility between the two sequences, coupled with the prevalence of pelitic, carbonaceous and sulphidic rocks at the base of the metasedimentary sequence, has resulted in the focus of folding and faulting at this major rock contact. This in turn has influenced the movement of metamorphic hydrothermal fluids and the formation of mineral deposits (R Marston, unpublished data, 1993).

DEPOSIT GEOLOGY LITHOLOGY The deposit lies along the NW-trending Yilgarn Star shear zone (YSSZ), which is bound to the stratigraphic contact of a thick unit of altered metagreywacke with ultramafic amphibolechlorite rocks. A wedge of skarn-banded gneiss occurs along this contact in the central and southern sectors of the orebody. The distribution of these rock types is shown in Fig 2, which highlights the summary geology as mapped in the open pit at 2320 m RL (mine datum) and an interpretation from drill holes projected in cross section to 500 m depth. The mine sequence is shown in Fig 3. The uppermost part of the exposed sequence, units A to F, is a series of interbedded altered amphibolites and graphitic schists, which are generally unmineralised. Units G to L comprise 120 m of altered metagreywacke overlying and often incorporating unit M. Units M to S are of economic importance and are detailed below. Unit M is a knotted mica schist, which forms the hanging wall to much of the gold mineralisation. It is a fine grained quartz-andalusite-biotite-muscovite-tourmaline schist, interlayered with quartz-muscovite schist and minor quartzplagioclase gneiss. Minor quartz-actinolite lenses are common in the lower parts of this unit. Pyrrhotite stringers 2–3 mm thick are found throughout and constitute between 1 and 3% of the rock volume. Unit N, the contact skarn, comprises massive pyrrhotite in bands averaging 1 m thick, but varying from 0.1 to 8.0 m. It contains bands of garnet-cummingtonite and olivine-calcite skarn and garnet-rich quartz-actinolite schist. Minor constituents are calcite, magnetite and rare arsenopyrite. Unit O is a skarn-banded gneiss comprising massive bluegrey, indurated quartz-actinolite schist, with individual beds to 3 cm thick. Minor fuchsite bands and quartz veins occur throughout. This unit displays crosscutting brittle fractures which have been subsequently transgressed by the anastomosing YSSZ. Interlayered with quartz-actinolite schist are bands of brown to black quartz-biotite schist with minor groundmass diopside. Quartz and diopside veins to 1 m thick are common. Crosscutting faults host intense biotite alteration and quartz veining. Pale green, layer-parallel diopside bands, with abundant pyrrhotite and minor groundmass carbonate are common throughout, and contain gold, nickel, zinc, lead, bismuth and silver mineralisation.

Geology of Australian and Papua New Guinean Mineral Deposits

YILGARN STAR GOLD DEPOSIT

FIG 2 - Yilgarn star geological plan at 2320 m RL, showing simplified geology and ore blocks, and cross section on line 10 900 m N, looking north.

At the base of the skarn-banded gneiss sequence is a 3 to 5 m thick zone of fuchsite alteration and quartz veining in a foliated quartz-diopside and quartz-biotite groundmass. Minor grey to white quartz veins are also present, generally as 5 to 10 cm stringers with minor pyrrhotite. Grey-green quartz-diopside and brown quartz-biotite constitute the groundmass in this subunit and occur as 10 to 30 cm thick laminae. The fuchsite subunits are thought to represent the southward continuation of

Geology of Australian and Papua New Guinean Mineral Deposits

the sericite-rich shears that dominate the shear zones in the northern half of the pit. Unit P is the Yilgarn Star shear zone (YSSZ). It transgresses several units and is characterised by an alteration assemblage of potassium-iron-magnesium-chromium phyllosilicates, calcsilicates, iron oxides and iron-arsenic sulphides. This assemblage overprints the metamorphic rocks at the main contact, and subsidiary alteration zones occur on parallel and

257

R A CROOKES and D DUNNET

regional folding event disclosed in premetamorphic structures within the ultramafic rocks, followed by sinistral brittle-ductile transcurrent shearing (D1 as mapped in the pit, but D2 regionally), then dextral ductile shearing occurring during preto post-peak metamorphism (D3), and a later period of brittle fracturing. The dominant structural style in the metasediment is tight, shear related NW-trending isoclinal folding, with a strong, penetrative WSW-dipping axial planar foliation. The foliation contains a steeply north-pitching penetrative mineral stretching lineation. The mean bedding orientation swings between 240o/76o and 275o/76o (all bearings refer to mine grid, where GN = 315o true) and the dominant north-trending schistose foliation has an approximate mean orientation of 267 o/74o. Fig 4 highlights the significant structural form surfaces. The footwall ultramafic rocks are massive to weakly foliated, and where present, the millimetre scale foliation is defined by penetrative layering of tabular chlorite and elongate tremolite. A strong mineral lineation also pitches steeply NW. The YSSZ is an oblique-slip WNW-trending ductile-brittle shear system that is the focal point of extensive metasomatism, veining and mineralisation. The alteration assemblages indicate a long history of progressive deformation and fluid flow, influenced by both D2 and D3 shearing events. Shallowly plunging boudins and the growth of fibrous minerals on fractures that dip gently SE in the YSSZ suggest an extensional regime. The shear zone margins appear to dip west less steeply than the internal foliation, indicating west-block-up movement (with an accompanying sinistral component). Most of the long axes of the boudins are concordant with the steep NW lineation, as are the axes of deformed quartz veins.

FIG 3 - Stratigraphic section for the mine sequence at Yilgarn Star.

horse-tailing structures in the footwall and hanging wall. Deformed hydrothermal veining is an integral component of the YSSZ and its gold lodes (Barnes and Schaus, 1993). Unit Q, the quartz-diopside-carbonate alteration zone, is a narrow band of fine grained, pale green diopside-quartzcarbonate alteration associated with the YSSZ at the top of the ultramafic unit. Unit R typically comprises a tension vein array of calciteolivine-magnetite±quartz veins, with a granoblastic texture and irregular wall rock contacts. Unit S is the predominant host to these veins. Unit S comprises a tremolite-chlorite schist, a weakly foliated amphibole-chlorite metakomatiite with widely spaced magnesian skarn bands. This basal footwall ultramafic sequence is known to extend for 2 km NW of the Yilgarn Star pit and for over 0.5 km to the SE. It has a total apparent thickness of 600 to 800 m and is progressively thinner towards the north. The unit is believed to be a metamorphosed magnesian basalt and contains chlorite and tremolite as the main constituents with minor amounts of calcite, diopside, feldspar, ankerite, phlogopite, magnetite and quartz.

STRUCTURE The complex history of structural deformation is well preserved, with four possible events having occurred: an early

258

Late structural elements (ie post-dating the YSSZ) are all brittle in character and occur predominantly in the ultramafic rocks. Subordinate transverse trending foliation and fractures are present, spatially limited to within about 100 m of the contact zone. Laterally persistent fractures typically oriented at 222o/78o occur immediately below the YSSZ. A significant proportion have subhorizontal slickensides which may be mineral lineations. Faults rarely pass completely through the YSSZ, implying that the YSSZ is the site of mechanical decoupling between the strong footwall rocks and the weaker hanging wall rocks. Within near–hanging wall mineralised and altered metasediments, transverse structures are evident as a schistose foliation and quartz veins, typically oriented at 237o/76o. Sinistral shearing is invoked as being crucial for their development. Fractures and veins are predominantly oriented normal to one of the foliation planes and tend to occur as steep and flat dipping groups within an overall dispersion predominantly oriented normal to the principal north trending foliation. A high proportion of the flat dipping structures include quartz and/or carbonate fill and locally have a 30o SE or NW dip. These fractures and veins are interpreted as extensional features, preferentially developed normal to the locally dominant horizontal stress field. Their density and style similarly reflect the relative brittleness of their host rock.

MINERALISATION Gold is associated with structurally controlled dilatant sites in the veined, ductile-brittle YSSZ. The level of gold mineralisation is closely linked to the intensity of shearing and concentration of veining. The deposit is broadly 1.2 km long

Geology of Australian and Papua New Guinean Mineral Deposits

YILGARN STAR GOLD DEPOSIT

FIG 4 - Structural element form surfaces at Yilgarn Star.

and dips 76o towards 268o parallel to S0 to a vertical depth of at least 740 m. It varies in width from less than 1 m to a series of individual mineralised structures that are combined as mining blocks up to 30 m wide. The orebody is divided into three primary domains along strike: 1.

Northern or Premier lode mineralisation transgresses the YSSZ in quartz-diopside-carbonate altered ultramafic rock and may extend to 10 m below the main shear zone. Intense diopside alteration and carbonate-olivinemagnetite veining correlate with the highest gold grades. The YSSZ and immediate hanging wall sediment comprise variably sheared sericite-andalusite-biotitefuchsite schist containing elongate andalusite porphyroblasts and biotite-rich laminae. Pyrrhotite to 5% occurs as 1 to 3 mm disseminations. The presence of quartz-pyrrhotite veins, generally to 10 cm wide, often with hydrothermal biotite selvages, signifies gold mineralisation, with grades reaching 50 g/t.

2.

The Central lodes are a series of mineralised (axial planar?) shears in muscovite-andalusite-biotite schist, up to 30 m above the main YSSZ, which form the bulk of the lode system. Pelitic beds have preferentially deformed, with intense shearing anastomosing around more competent psammitic beds, creating a focus for potassic alteration and gold mineralising fluids. Quartz-pyrrhotite veins parallel the shear surfaces and can occur in brittle fractures within rafts of indurated quartz-actinolite schist. Quartz-andalusite-biotite schist beds also host gold, confined to small brittle faults from 2 to 40 m long. These faults are characterised by zones of sinuous grey and milky white quartz veins and stringers which are between 5 and 50 cm wide.

Geology of Australian and Papua New Guinean Mineral Deposits

3.

The complex Southern lode system comprises two main lodes along the upper and lower contacts of units N and O (Fig 2). In addition to the mineralisation styles of the Premier lode, gold occurs in carbonate-olivine-magnetite veins, as vein packages paralleling steep 290o to 320o trending faults. This ore type is concentrated within 15 m of the metasediment–ultramafic rock contact and occurs as distinctive black and white spotted, lenticular granular veins, with traces of pyrrhotite. The carbonate content varies from 10 to 70% and is commonly about 30%. Gold content also varies greatly, and south of 10 300 N this rock becomes the dominant ore type with individual veins containing coarse particulate gold of 1–3 mm diameter and grades in excess of 50 g/t. North of 10 300 N gold grades range between 0.5 and 15 g/t, corresponding to increased carbonate content and reduced magnetite. In unit O, the skarn-banded gneiss, gold also occurs in sheared sericite-andalusite-biotite-fuchsite schist and in lenses of randomly distributed biotite-quartz schist.

The prime control of gold mineralisation is the sheared metasediment–ultramafic rock contact zone. All economic resources occur within 50 m of this contact, and all substantial lenses and pods of gold mineralisation appear to be physically connected to this contact.

ACKNOWLEDGEMENTS Permission to publish by Orion Resources NL, Gasgoyne Gold Mines NL, and Gemini Mining Pty Ltd is gratefully acknowledged. The authors wish specially to thank past and present mine geologists for their contributions to the geological development of the project.

259

R A CROOKES and D DUNNET

REFERENCES Barnes, J F H and Schaus, C C, 1993. Exploration and resource implications of going underground, in Open Pit to Underground: Making the Transition (Eds: W J Shaw and S E Ho), pp 1–15, AIG Bulletin 14. Keats, W, 1991. Geology and gold mines of the Bullfinch-Parker Range region, Southern Cross province, Western Australia, Geological Survey of Western Australia Report 28.

260

Geology of Australian and Papua New Guinean Mineral Deposits

Shedden, S H, 1998. Two Boys gold deposit, Higginsville, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 261–264 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Two Boys gold deposit, Higginsville by S H Shedden 1 INTRODUCTION The deposit is within a 19.2 ha mining lease at the Higginsville mining centre, 112 km south of Kalgoorlie and 45 km north of Norseman, WA, at AMG coordinates 379 200 m E, 6 487 300 m N and lat 31o44′S, long 121o43′E on the Widgiemooltha (SH 51–14) 1:250 000 scale and the Cowan (3234) 1:100 000 scale map sheets (Fig 1). Mining lease M15/231 is owned by Gindalbie Gold NL, is operated under a 50:50 production joint venture between Gindalbie Gold NL and Barminco Pty Ltd (the Two Boys Joint Venture) and is 1.5 km SE of the Resolute Ltd gold treatment plant at Higginsville (Fig 2).

FIG 2 - Local geological map and mine site plan, Higginsville (after Resolute Ltd, 1996).

EXPLORATION AND MINING HISTORY Mining at Higginsville commenced about 1900 and has continued sporadically to the present. Major open pit mining commenced in 1988, following the discovery of the Poseidon and Poseidon South gold deposits by Samantha Exploration NL (now Resolute Ltd).

FIG 1 - Location and regional geological map of the Higginsville area (after Resolute Ltd, 1996).

Mine development by the Two Boys Joint Venture commenced on 18 January 1997. Concurrently, underground operations were being developed at the neighbouring Poseidon South and Chalice gold mines, both operated by Resolute Ltd. These events marked a new era of undergound mining in the Higginsville district. 1.

Managing Director, Gindalbie Gold NL, PO Box 10400, Kalgoorlie WA 6430.

Geology of Australian and Papua New Guinean Mineral Deposits

Two Boys was discovered by prospectors in 1933, and produced a reported 4714 oz of gold between 1933 and 1964 from the oxidised portion of a shallow dipping reef structure. The average grade of mined ore during that period has been calculated at 19.77 g/t gold (Gindalbie Gold NL, 1994). In 1966 the Two Boys mining lease was purchased by prospector W T Trythall who worked the deposit via a vertical shaft to about 50 m depth and explored it by several diamond drill holes during the following years. In 1983 Samantha Exploration NL consolidated ownership of most of the Higginsville mining centre, including the Two Boys mining lease, and commenced systematic exploration. Two Boys was purchased from Samantha and Trythall by Gindalbie Gold NL (GBG) in 1994. To that time, three successive drilling programs had resulted in an Inferred and Indicated Resource estimate of 109 000 t averaging 4.99 g/t

261

S H SHEDDEN

gold (uncut), based on 100 reverse circulation (RC), diamond and open-hole percussion drill holes totalling 5047 m. The occasional high grade intercept did not encourage an early resumption of mining. Drilling around the old workings was difficult because of mine openings and the presence of clay.

by dolerite and gabbro. The greenstones are bounded by contemporaneous basaltic volcanic rocks and younger Archaean clastic sediment of the Black Flag beds, and the sequence has been subjected to upper greenschist to lower amphibolite facies regional metamorphism (Griffin, 1989).

During 1994 GBG drilled a further 57 RC drill holes, testing the mineralised zone to a maximum vertical depth of about 100 m. The Inferred and Identified Resource was then estimated to be 330 000 t averaging 4.38 g/t gold (uncut) or 3.14 g/t gold after a top cut to 16 g/t gold and a 1 g/t lower cutoff. This was generally based on a nominal drill intercept spacing of 20 by 25 m. Drilling to the end of 1994 totalled over 10 000 m in 157 holes (S H Shedden, unpublished data, 1995).

The Higginsville belt is bounded to the east by the NNWtrending Zuleika Shear Zone, which may be traced for more than 100 km. The greenstones have been subjected to at least three phases of deformation. The earliest phase (D1) is defined by north–south regional shortening which generated south over north low angle thrusting. The second phase (D2) consists of a ENE-trending regional shortening event which resulted in open upright folding about a NNW-trending fold axis and low to high angle reverse thrusts along fold limbs. Second order splay faults were also generated as low angle thrusts. The third phase (D3) consists of ESE–WNW regional shortening which generated sinistral strike-slip shear zones along NNW-oriented thrusts and along the fold limbs formed during D2. Second and third order splay faults off major NNW-oriented structures are common and generally occur as low angle thrusts during D3 (D Goodwin, personal communication, 1997).

As the bulk of the resource was only accessible by underground mining, attention then focussed on a high grade zone in the eastern sector of M 15/231. Drilling had initially suggested that this zone was limited to around 15 000 t at an average grade of about 20 g/t gold. To facilitate planned mining of the high grade zone, GBG entered into a production joint venture with underground mining contractor Barminco Pty Ltd. Barminco had the right to conduct check drilling, which commenced in November 1995. The first nine holes formed a pattern around the high grade zone at a spacing of 12.5 by 10 m. These holes confirmed the presence of the high grade zone and drilling continued, testing the mineralised body to about 187 m vertical depth, beyond which it remains untested. By June 1996, 64 RC drill holes totalling 7572 m had been completed in this program. An Inferred and Indicated Resource for the high grade zone was then estimated to be 220 000 t at an average uncut grade of 19.9 g/t gold (for 141 000 contained oz) or 15.6 g/t gold after a top cut to 80 g/t gold, within a total Inferred and Indicated Resource of 570 000 t at an average uncut grade of 9.2 g/t gold, equal to 169 000 contained oz (R G Colville, unpublished data, 1997). Mine development commenced in January 1997 based on a Proved and Probable Ore Reserve estimated to be 230 000 t at 12.29 g/t gold after a top cut of 80.0 g/t gold and a 4 g/t lower cutoff (R G Colville, unpublished data, 1997). The expected recovery of 95% should enable 86 000 oz to be produced. As the main mineralised body and subsidiary mineralised zones have yet to be fully explored and diamond drilling has intersected a favourable structure at depth below the mineralised shear, significant extensions to known mineral resources are anticipated. Drilling and development by Barminco have resulted in the third and perhaps ultimate Two Boys mining operation. Mine access will be by a 5.5 by 5.5 m decline with a gradient of 1 in 7. The deposit is planned to be mined by a combination of mechanical long hole overhand retreat and conventional airleg stoping commencing in about June 1997. The ore will be treated under a custom milling arrangement at Coolgardie, 110 km from the mine site.

REGIONAL GEOLOGY Two Boys is within the Archaean Yilgarn Block and the Kalgoorlie Terrane of the Norseman–Wiluna greenstone belt (Fig 1). The Higginsville area is underlain by a fault bounded, thrust repeated, NNW-trending 5 km wide sector of the greenstone belt. The Archaean greenstones dominantly comprise metamorphosed high magnesium basalt, minor komatiite flows and minor interflow clastic sediment, intruded

262

ORE DEPOSIT FEATURES REGOLITH Two Boys is covered by a lateritic weathering profile to about 50 m thick. The upper 2 to 3 m consists of intensely ferruginised and calcareous clays which overlie a mottled zone to 40 m thick. A narrow, poorly developed pallid zone 2 to 3 m thick is commonly observed in most drill holes, and overlies a weathered bed rock zone of variable thickness (S H Shedden, unpublished data, 1995).

LITHOLOGY The Two Boys lease (Fig 3) is underlain by a sequence of high magnesium basalt, gabbro and minor sediment. The sequence includes a quartz gabbro unit, the Fairplay gabbro, which cuts the SW corner of the tenement (Figs 2 and 3). A NW strike and subvertical dip have been interpreted from drill intercepts of a narrow metasedimentary unit. High magnesium basalt is the dominant rock type. It varies texturally from a fine grained, variolitic rock to a doleritic granophyre, with the strongest gold mineralisation developed within the granophyric phases.

STRUCTURE The sequence is cut by a low angle shear zone formed during D2 as a low angle thrust, known as the Two Boys shear zone (TBSZ). The TBSZ strikes east and has an overall dip of about 27 to 30o to the NNE. The TBSZ hosts the Two Boys gold deposit, which occurs as lenses of quartz-vein reef of variable width and extent with sheared, altered and mineralised selvages (Fig 4). The TBSZ crops out within the southern boundary of the mining lease and is marked by a prominent quartz reef to 4 m thick (Fig 3). A second significant reef structure 100 m north of the TBSZ outcrop strikes NNW with a shallow easterly dip. Drilling of this structure to-date has intersected alteration and subeconomic gold mineralisation.

Geology of Australian and Papua New Guinean Mineral Deposits

TWO BOYS GOLD DEPOSIT, HIGGINSVILLE

is incomplete. The TBSZ extends beyond the limits of the Two Boys mining lease and is being tested elsewhere by drilling (Resolute Ltd, 1996, 1997). Adjacent to the TBSZ foliation of the host high-magnesium basalt is intense over several metres into the wall rock, and is associated with intense biotite-chlorite-carbonate-sericitepyrite alteration. The alteration is the key identifier of mineralisation in drill holes. Mineralisation occurs in quartz veins and sheared selvages and is often visible as coarse free gold grains to several millimetres in diameter. Carbonate, arsenopyrite and pyrite are essential accessories to gold. Carbonate is widely and irregularly distributed as coarse grained intergrowths with quartz and as disseminated fine grained masses throughout the altered zone. Arsenopyrite occurs as euhedral crystals to several millimetres wide. Fine grained euhedral pyrite has a much wider distribution than arsenopyrite, occurring throughout the alteration zone. Minor scheelite, as discrete crystals within quartz veins, is also frequently associated with the mineralisation.

FIG 3 - Surface plan and projection of mineralisation, Two Boys gold mine.

Subsidiary zones of gold mineralised quartz-carbonate veining have been intersected above and below the Two Boys reef. Little is known of the lower zones due to the lack of drill penetration past the main shear. The upper subsidiary mineralised zones do not display the same continuity, thickness or grade as that within the TBSZ ore zone, but economic mineralisation is evident and remains to be fully tested.

ORE CONTROLS The primary gold mineralisation at Two Boys was emplaced in dilatant lenses within the TBSZ which has been demonstrated to persist for several hundred metres down dip to the north. The TBSZ is interpreted as a D2 low angle thrust. During D3, the structure was reactivated, forming dilational sites and allowing subsequent gold deposition post-D3. The development of gold mineralisation within the TBSZ is enhanced in upward flexures and appears also to favour doleritic phases of the host high-magnesium basalt. Gold mineralisation is notably weak where the TBSZ extends through the Fairplay gabbro to the west.

FIG 4 - Cross section between holes BTB004 and BTB030, looking NW.

MINERALISATION Modelling of the surface of the gold-bearing quartz vein within the TBSZ has indicated that vein thickness and gold grade increase consistently with upward flexures in the quartz vein. Whereas the TBSZ and the associated quartz veining, wall rock alteration and gold mineralisation are laterally persistent throughout the lease area, higher grade gold mineralisation is confined to two main areas in the east and west of the lease. The full extent of higher grade mineralisation is unknown as drilling

Geology of Australian and Papua New Guinean Mineral Deposits

Primary gold mineralisation at Higginsville, although persistently associated with intense wall rock alteration and ubiquitous arsenopyrite-pyrite-scheelite, occurs in a range of rock types and structures. Hosts include quartz gabbro at Poseidon South, Poseidon and Fairplay and coarse grained tremolitic ultramafic rock at Erin (Fig 2). The epigenetic deposition of gold mineralisation at Higginsville was therefore largely structurally controlled, with the mineralising fluids derived from a common source. Mineralising fluids were channelled along second or third order structures related to the regional scale Zuleika Shear Zone, with higher grade ore formed in low pressure dilatant zones within granophyric host rocks where compression resulted in brittle failure. The spatial and timing relationships between the brittle failure zones and the enclosing ductile-deformed shear zone have not yet been studied. Although earlier mining exploited oxidised ore zones at Two Boys, there is little evidence in drill holes of any significant secondary enrichment of gold relative to the grade of the primary zone.

263

S H SHEDDEN

ACKNOWLEDGEMENTS This paper is published with the permission of Gindalbie Gold NL. The Two Boys story is one of persistent, if sporadic, exploration and mining over 64 years. The effort and commitment of the management of Barminco Pty Ltd, in particular R G Colville, W T Trythall’s long term faith in the ability of Two Boys to yield a mineable gold deposit, and the ready assistance of Resolute Ltd during the exploration and development phase are acknowledged.

264

REFERENCES Gindalbie Gold NL, 1994. Prospectus (Gindalbie Gold NL: Perth) Griffin, T J, 1989. Widgiemooltha, Western Australia - 1:250 000 geological series (2nd edition), Geological Survey of Western Australia, Record 1989/4. Resolute Ltd, 1996, 1997. Quarterly reports to the Australian Stock Exchange.

Geology of Australian and Papua New Guinean Mineral Deposits

Archer, N R and Turner, B J, 1998. Norseman gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 265–272 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Norseman gold deposits 1

by N R Archer and B J Turner

2

INTRODUCTION

Harlequin, Bullen and OK underground mines (Fig 1), the Scotia operation having been temporarily suspended in 1995.

The deposits are on the Norseman (SI 51–20) 1:250 000 scale and Norseman (3233) 1:100 000 scale map sheets at lat 31o12′S, long 121o47′E, AMG coordinates 385 000 E and 6 436 000 N, at the southern end of the Eastern Goldfields Province of Western Australia (Fig 1). Total production from the field to June 1996 is nearly 5 Moz of gold, and Central Norseman Gold Corporation (CNGC) has produced more than 80% of this since it commenced production in 1935. CNGC production in 1995–96 was 119 603 oz from 306 440 t of ore at an average recovered grade of 12.1 g/t gold, predominantly from the

Published resources for CNGC at June 1996 are shown in Table 1.

EXPLORATION AND MINING HISTORY GENERAL The Norseman Goldfield was discovered in 1894 and most mining, and 85% of gold production, has taken place on the Mararoa and Crown reefs in the main field, and at the North Royal and Princess Royal reefs, about 10 km to the north (Fig 1). Although some regional exploration was carried out before 1983, no significant mineralisation was discovered outside the main field or Royal areas. Early exploration at the major reefs had been successful by using the concepts of a ‘favourable’ stratigraphic sequence and down plunge repetitions (Thomas, Johnson and MacGeehan, 1990). This ‘favourable bed’ hypothesis continued to heavily influence exploration thinking by CNGC and, with the inaccessibility of the surrounding salt lake environment, tended to restrict most exploration to the lower portion of the Woolyeenyer Formation. Exploration was heavily biased towards looking for north striking and easterly dipping, high grade quartz veins.

BULLEN MINE

FIG 1 - Location and general geological map, Norseman area.

1.

2.

Formerly Manager, Geology and Exploration, Central Norseman Gold Corporation, PO Box 56, Norseman WA 6443. Now Consulting Geologist, Longbow Geological Services, 33 Highbridge Way, Karringup WA 6018. Mine Geologist, Central Norseman Gold Corporation, PO Box 56, Norseman WA 6443.

Geology of Australian and Papua New Guinean Mineral Deposits

The long history of the field and the high grade, nuggety nature of the orebodies resulted in much exploration being carried out by underground driving along the known reefs, with only a few drill holes. This means that even now in the centre of the main field the drilling coverage is sparse. Easterly striking veins in the main field were known from the earliest days but their importance as ore sources has only recently been recognised. These ‘cross links’ quite commonly pinch out before they intersect the main north striking reefs and would not have been found by driving on the main reefs. The Bullen deposit was found in 1990 and the Viking ‘re-discovered’ in 1986 by correlating ‘spurious’ intersections in diamond drill holes drilled to test targets on the main reefs. Mining has now taken place on these cross links at the St Patrick’s mine, the Alimak stope in the Regent mine, the Viking mine (Royal Standard reef) and at the Bullen mine (Bluebird link). High gold grades make these very profitable operations and attractive exploration targets. The lack of effective drilling in the main field means that many such opportunities remain.

OK MINE About 2 km to the south of the main field, in the OK and Cumberland areas, the reefs predominantly strike east and are subvertical. The OK mine was originally worked in the 1930s, but lay idle until 1980 when the shaft was re-opened by CNGC

265

N R ARCHER and B J TURNER

TABLE 1 CNGC June 1995 and June 1996 Ore Reserves and Mineral Resources. June 1995 Ore (‘000 t)

Gold grade (g/t)

June 1996 Contained gold (‘000 oz)

Ore (‘000 t)

Gold grade Contained gold (g/t) (‘000 oz)

RESERVES UNDERGROUND

Proved Probable

SUBTOTAL OPENCUT

Proved Probable

SUBTOTAL TOTAL PROVED AND PROBABLE

220 610

16.9 14.7

119 288

303 547

15.4 12.4

150 220

830

15.3

409

852

13.5

370

10 70

5.0 7.6

2 17

0 106

0.0 14.4

0 49

80

7.3

19

106

14.4

49

910

14.6

428

958

13.6

419

830 1430 1600

7.1 6.0 6.9

190 277 355

1040 1410 1540

6.4 5.9 6.6

213 266 326

3860

6.6

822

3990

6.4

805

3910 1160 670

0.7 3.5 1.5

93 132 32

3910 1110 440

0.7 2.8 3.2

93 99 45

5740

1.4

257

5460

1.3

237

9600

3.5

1079

9450

3.5

1042

10 510

4.5

1507

10 408

4.4

1461

RESOURCES UNDERGROUND

Measured Indicated Inferred

SUBTOTAL OPEN CUT

Measured Indicated Inferred

SUBTOTAL TOTAL RESOURCES GRAND TOTAL

to mine remnant ore from the OK Main reef. Underground drilling of the east striking tensional Main reef led to the discovery of the 300o striking O2 reef.

SCOTIA MINE Rotary air blast drilling beneath Tertiary cover, approximately 30 km south of the main field, on the southern margin of a WNW-striking magnetic zone identified by E S T O’Driscoll, resulted in a strong arsenic anomaly (M W Nevill, unpublished data, 1983). Further work, including drilling under old workings, led to the discovery of the Scotia deposits. This complex vein system occurs within the lower Woolyeenyer Formation to the south of the NE-striking, Proterozoic Dambo fault.

Follow up aircore and diamond drilling defined a reserve by the end of September 1993 and underground production commenced in June 1995. Sailfish, Harlequin, and another low grade resource at Cobbler were originally defined as high priority targets by CNGC geologists.

PRODUCTION AND MINING Recent production data from CNGC operations is shown in Table 2 and the contribution of each of the major orebodies to CNGC’s production history is shown in Fig 2.

HARLEQUIN MINE The projection of known mineralised trends has targeted potential for gold orebodies in salt lake covered greenstone to the west and NW of Norseman. In addition gold-bearing quartz veins, similar in style to those at Norseman, occur at Higginsville, 30 km NNW of the North Royal mine across Lake Cowan. In 1990, a dedicated Hagglund mounted drill rig and air compressor commenced aircore testing on the salt lake beneath Tertiary sediment and Recent mud. This was the first successful drill rig of its type used on salt lakes in Australia. Holes were drilled on regional traverses at a spacing of 2 km by 400 m, and at closer spacings, to test targets defined by interpretation of magnetic data to confirm CNGC’s regional geological understanding. Within six months of drilling the first aircore hole a resource had been identified at Sailfish. In December 1992, the first aircore hole on a ground magnetic target at Harlequin intersected 10 m grading 8.1 g/t gold.

266

FIG 2 - CNGC production record, 1937–1996.

Prior to the opening of the Harlequin mine in 1995 the Bullen mine (Bluebird link) was the most important ore source for CNGC (Fig 2) during some of the more difficult years in the

Geology of Australian and Papua New Guinean Mineral Deposits

NORSEMAN GOLD DEPOSITS

TABLE 2 OK, Scotia, Bullen and Harlequin production from 1984 to 1996. OK MINE

SCOTIA MINE

Gold Contained gold grade (oz) (g/t)

BULLEN MINE

Tonnes (‘000)

Gold grade (g/t)

Contained gold (oz)

HARLEQUIN MINE

Tonnes (‘000)

Gold grade (g/t)

Contained gold (oz)

Tonnes (‘000)

Gold Contained gold grade (oz) (g/t)

Year

Tonnes (‘000)

1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996

7203 8060 4973 13 139 20 478 51 758 47 801 54 665 43 231 60 673 43 305 44 189 31 443

11.9 11.5 14.4 12.9 11.4 8.7 8.4 8.2 8.5 8.8 8.3 12.8 8.1

2758 2974 2301 5467 7524 14 512 12 958 14 413 11 713 17 128 11 571 17 788 8179

1477 76 912 164 397 238 298 89 294 56 090 49 785 48 669 67 211 18 656

5.1 9.4 5.6 4.6 5.7 8.2 6.1 6.1 6.8 3.4

244 23 185 29 571 35 511 16 344 14 861 9717 9514 14 041 2052

14 146 52 693 72 157 54 414

21.2 20.6 19.4 17.6

9 638 34 869 44 316 30 706

7346 178 082

19.5 12.6

4538 72 034

Totals

430 918

9.3

129 286

810 789

5.9

155 040

193 410

19.2

119 529

185 428

12.8

76 572

history of the operation. The deposit has a 40o dip and is mined by airleg, room and pillar methods. At the OK mine the very narrow quartz widths (average 0.3 m), highly variable grades and slow mining rate mean that mining is only marginally profitable. Longhole mining methods are used. A small underground resource remains at Scotia. Two open cut mines operated in the late 1980s (Fig 2) and an underground mine was accessed via a decline from the base of Pit 3 from 1989 and exploited a complex vein system (Fig 3).

The Harlequin orebody is wider than the average Norseman vein and most stopes are mined by long hole methods. The main ore shoot on the HV1 vein is commonly between 150 and 200 m long. Structural complexity, wide ore zones (commonly more than 4.5 m) and multiple reefs in some areas increase the amount of gold which can be recovered per vertical metre. Other recent gold producers include Australis NL, which produced from low grade (1 to 3 g/t) deposits in the Noganyer Formation during the 1980s, and more recently Australasian Gold Mines at the Red, White and Blue deposit, also in the Noganyer Formation. A number of small prospector-scale mines have also been worked.

PREVIOUS DESCRIPTIONS The Norseman mines have been the subject of many geological studies over their 100 year history. Thomas, Johnson and MacGeehan (1990) highlighted the important prior work. Since then significant advances in our understanding have been made. Perring and McNaughton (1990) used lead isotope studies to show that significant remobilisation of oreassociated metals (and possibly gold) occurred during the Proterozoic. Age dating constraints were imposed as a result of work by Hill, Campbell and Compston (1992), Kent (1994) and McCuaig (1996). The concept of a crustal continuum of deposit types from Scotia in the south to the North Royal in the north was proposed by McCuaig et al (1993).

REGIONAL GEOLOGY Thomas, Johnson and MacGeehan (1990) gave a good description of the regional geology. Doepel (1973) completed mapping of the Norseman 1:250 000 scale sheet (SI 51–2) while more recently McGoldrick (1993) completed mapping of the Norseman 1:100 000 scale sheet (3233). Significant new data have become available through aircore drilling by CNGC in areas covered by lakes and Tertiary and Recent sediment.

FIG 3 - Schematic cross section on 6 406 650 m N, Scotia orebodies (‘A’ to ‘F’ lodes), looking north.

Geology of Australian and Papua New Guinean Mineral Deposits

Studies of regional scale, publicly available, aeromagnetic data together with closer spaced company surveys, have advanced the regional understanding. S G Peters (unpublished data, 1991) and others, including D W Haynes (unpublished data, 1991), and L A Offe, S G Peters, J S Chapman and N W

267

N R ARCHER and B J TURNER

Brand (unpublished data, 1991) have recognised that the folding of the Noganyer Formation described by Keele (1984) is actually the core of a regional antiform, and that rocks previously assigned to the Mount Kirk, Buldania and Killaloe formations (Fig 4) are probably able to be correlated. A number of CNGC geologists and P McGoldrick (personal communication, 1993) have proposed thrust repetitions of stratigraphy, but these ideas are yet to be tested. Another significant advance in the regional geological understanding has been the recognition that the isolated outcrop of ultramafic rock on a small island near Jimberlana Station is part of a regionally extensive, probably extrusive unit, which has been informally named the Talbot Island

ultramafic (L A Offe, S G Peters, J S Chapman and N W Brand, unpublished data, 1991). The unit can be traced on images of magnetic data as a magnetic high, and it forms a marker horizon of the antiformal structure. The Talbot Island ultramafic is known to contain disseminated nickel sulphide mineralisation (predominantly millerite), as first noted during regional mapping by P McGoldrick (personal communication, 1990).

ORE DEPOSIT FEATURES The most common gold mineralisation style consists of quartz veins hosted within metamorphosed Archaean mafic rocks. The Mararoa and Crown reefs are near north striking and easterly dipping quartz veins which occur over 3.5 km strike length. Higher grade ore shoots occur where the veins intersect coarser grained, mafic (E Cameron, unpublished data, 1968) or ultramafic dykes (J S Chapman and L A Offe, unpublished data, 1993) which have intruded relatively fine grained, metamorphosed (upper greenschist to lower amphibolite facies) basalt. The Royal orebodies are also predominantly north striking, but are structurally more complex quartz veins and shears which occur over a similar strike extent to the veins in the main field. Higher grade sections are hosted within coarser grained mafic intrusive rocks (‘gabbros’) as in the main field.

BULLEN MINE The mine accesses the Bluebird link orebody which is a near 070o striking and 40o SE dipping, predominantly tensional, laminated quartz vein within the favourable sequence. The vein is hosted within a similarly oriented medium to coarse grained gabbro, and is bounded to the east by the Mararoa reef and is either bounded, or offset, at its western end by the Bluebird shear (Fig 5). The vein does not outcrop and feathers out near 100 m vertical depth. In the upper two levels the orebody has a strike length of nearly 200 m but by the 19 level (405 m below surface) this has increased to nearly 400 m, reflecting the divergence of the bounding structures (Fig 6). Despite the greater strike length the ounces per vertical metre have remained constant with depth. Although the geometry of the ore body is relatively simple, significant variations in dip occur.

FIG 4 - Norseman ‘stratigraphy’ (thrust repetitions are likely to occur), with positions of mineralisation. Modified after Doepel (1973).

268

FIG 5 - Schematic relationship between Bluebird link and bounding shears, Bullen mine (not to scale).

Geology of Australian and Papua New Guinean Mineral Deposits

NORSEMAN GOLD DEPOSITS

The quartz lenses at Scotia are small tabular bodies with strike lengths of less than 100 m and down dip extents of less than 40 m, plunging to the north at approximately 20o. The quartz averages 1.1 m wide and is massive and dark coloured, a feature which has been attributed to strain (J Skeet, unpublished data, 1988; R S Waugh, unpublished data, 1991). The grade distribution is atypical of Norseman orebodies. The quartz has a lower grade than other Norseman deposits but a much lower coefficient of variation. The gold is free milling, but is very fine grained and has a much lower silver content than in the main field (J Skeet, unpublished data, 1988; S B Luitjens, unpublished data, 1991). Free gold has only very rarely been observed. The predominantly biotite-hornblende and plagioclase (McCuaig et al, 1993) alteration halo, which is generally 0.7 m wide, carries grades of 1 to 3 g/t gold. Other alteration minerals include epidote, ilmenite, actinolite, clinopyroxene (diopside), calcite, microcline, zoisite and garnet. Common accessory minerals in the ore veins are carbonate, scheelite, pyrite, pyrrhotite and chalcopyrite, with trace amounts of galena and arsenopyrite. FIG 6 - Plan view of Bullen mine showing high, medium and low grade zones.

The vein averages 0.7 m in width and has very little associated shearing, reflecting its extensional character. Like other Norseman orebodies the gold is free milling and commonly visible and usually concentrated within laminations in the vein. Minor biotite alteration occurs intermittently around the vein, extending for no more than a metre into the country rock. Common accessory vein minerals include carbonate, scheelite, pyrite, pyrrhotite, galena and sphalerite.

OK MINE The OK mine exploits the O2 reef, a shear hosted vein near the top of the favourable sequence. The reef pinches out above 2 level at about 55 m below surface, but continues to the 21 level, 460 m below surface. The Main reef is barren below the 5 level, 150 m below the surface. The gold in the O2 reef is free milling and hosted by a very narrow (0.3 m average width) laminated quartz vein which is commonly surrounded by a selvage to 2 m wide of predominantly biotite alteration. The veins are most commonly hosted by fine grained metamorphosed basalt or relatively fine grained intrusive rocks. Accessory minerals include carbonate, scheelite, pyrite, chalcopyrite and arsenopyrite. The O2 and Main reefs are among the most nuggety at Norseman and definition of ore blocks is extremely difficult, so that the assigning of grades from drill holes to sections of reef is almost impossible. Dilution during mining tends to lessen the effects of this variation due to the incorporation of low grade material from the alteration halo.

Two major brittle fault sets occur in this area. These are the FN series which strike 010 o, dipping 50o to the west, with reverse movements varying from 1 to 40 m. These commonly host pegmatite intrusions, which have been dated at 2621±98 Myr (Kent, 1994). The EW series are, as the name implies, an east-striking subvertical fault set with dextral movement to 200 m, with the largest of these intruded by a Proterozoic dyke.

HARLEQUIN MINE The most important ore bearing quartz vein so far known at Harlequin is HV1, a 070o striking and 50o SE dipping and relatively wide (average 4 m width, maximum 11 m) quartz vein. A generalised cross section through this orebody is shown in Fig 7. The vein orientation is similar to that of the cross links (including the Bluebird link) in the main field but

SCOTIA MINE Gold mineralisation at Scotia is significantly different to that in the main Norseman field. The ore has a similar stratigraphic setting in coarse grained mafic intrusive rocks of the lower Woolyeenyer Formation, but has been subject to midamphibolite facies metamorphism (McCuaig et al, 1993). The Scotia orebodies are hosted by a weak to moderate, 1 to 20 m wide north striking, easterly dipping (average 50 o) shear zone, which in the mine area is hosted by a subparallel coarse grained mafic intrusive unit (Fig 3).

Geology of Australian and Papua New Guinean Mineral Deposits

FIG 7 - Cross section on 385 125 m E at Harlequin HV1 prospect, looking west.

269

N R ARCHER and B J TURNER

HV1 is more structurally complex and has more associated shearing. Intense biotite and arsenopyrite alteration persists for several metres around the orebody, and commonly contains high gold values. As is common elsewhere at Norseman the ore grade portions of the HV1 vein are hosted by coarser grained mafic intrusions. Structural investigations suggest that the geometry of the Harlequin vein sets may not be the same as in the main field. However, a north striking and east dipping vein, HV6, has been located to the west of HV1, in the Harlequin West area, and an Indicated Resource and Probable Reserve have been defined at depth. Quartz veins at HV6 are hosted by zones of strong shearing which are commonly 10 m wide and can be as much as 30 m wide. The host rock for HV6, even in the ore grade zones, is basalt. The ore shoot lies close and subparallel to the intersection of HV6 and a northerly striking body of microgranodiorite. The first ore-hosting vein located at Harlequin has a similar orientation to the veins which have been mined last in the main field. The Harlequin orebody occurs in the Desirable pillow lavas, a part of the stratigraphic succession long thought to be unfavourable for gold mineralisation. This highlights the fact that although the ‘favourable bed’ hypothesis has been useful as a guide to ore, the position in the stratigraphy should be viewed as less important than structurally prepared zones.

ORE GENESIS, MODELS AND CONTROLS ON MINERALISATION BULLEN MINE D N Kelly (unpublished data, 1992) suggested that this vein is emplaced in a tensional opening formed by movement on the Mararoa reef and Bluebird shear. The location of this opening has been influenced by the presence of a favourably oriented, relatively coarse-grained mafic intrusive. The relationship of the Bluebird link to the Mararoa reef and Bluebird shear is shown in Fig 5.

OK MINE The steep, thin, near east striking veins at the OK are hosted within fine grained basalt. The block of country around the OK and Cumberland mines, and south of a WNW-striking fault known as E-fault, predominantly hosts east oriented reefs. Efault terminates the north striking reefs in the main field to the north.

COMMON FEATURES All the Norseman reefs share common features which give clues to their genesis: 1.

2.

270

Most of the high grade ore zones occur where veins intersect ‘gabbro’ intrusions, and specific oriented contacts are particularly favourable. This is most likely a result of competency contrasts which allow preferential propagation of cracks and other openings within the coarser grained rocks, and the amount of veining is controlled by the orientation of the contact relative to stress directions. Zones where NNE- and west-dipping felsic, dacitic porphyries are intersected by the reefs tend to be zones of intense structural complexity and gold grades are even

more variable than usual. In some reefs these can be zones of high grades, and in others, low grades. This reflects the geometry relative to the local direction of maximum compression, and therefore whether the structures are tight or open. 3.

Most reefs have only very narrow (a few metres at most) alteration selvages. In some cases these selvages host high gold grades but in all cases the grade drops off very quickly away from the quartz vein. The northern deposits usually have wider alteration haloes caused by more reaction of ore fluids with host rocks (McCuaig et al, 1993).

The orebodies are almost completely structurally controlled. Ore bearing fluids with fluid inclusion compositions which suggest a mixed source (R T Bills, unpublished data, 1990) deposited gold and other minerals when sudden pressure release occurred, either due to seismic events (Sibson, 1990) or when fluids gained access to pre-existing openings. Minor wall rock reaction also took place and the alteration assemblage (including the abundance of gold and other sulphides) was determined by the wall rock composition (McCuaig, 1996). The gold is not uniformly distributed and the gold in the orebodies is normally nuggety. The erratic distribution of gold on the Bluebird link, for example, is shown in Fig 6. The orebodies are now believed to have been emplaced close to the time of peak metamorphism as described by McCuaig (1996). The V0 and V2 veins described by Thomas, Johnson and MacGeehan (1990) are now believed to be of the same age, resulting from the same stress field and are tensional and shear veins respectively. The strike of the main regional compressive stress direction is thought to have been between NNE and ENE (N R Archer, unpublished data, 1991) but locally blocks of country have vein sets which give evidence of different orientations. East–west compression is suggested at the OK mine (P Bird, unpublished data, 1990). Complex reef shapes which include buckling, faulting, ramping and multiple crosscutting vein sets are perhaps due to rotation of blocks of country during the vein-forming deformation event. Major reorientations of stress directions may have resulted from failure of regional scale faults which may be existing, or new, blockbounding structures. Proterozoic orogenic activity associated with the Albany–Fraser Orogen has caused later faulting and redistribution of ore-associated minerals and possibly gold (Perring and McNaughton, 1990; Kent, 1994; McCuaig, 1996). Archaean shear zones have provided preferential pathways for movement of Proterozoic fluids associated with the Jimberlana Dyke suite. Diopside- and microcline-bearing assemblages have been noted in shears near the Jimberlana Dyke (N R Archer, unpublished data, 1990) and at Scotia near a Proterozoic intrusive (B J Turner, personal communication, 1996). Local gold enrichment, not due to weathering, is also known near the edge of the Jimberlana Dyke and is further evidence of remobilisation of gold during the Proterozoic.

MINE GEOLOGICAL METHODS Traditional mine geological methods, such as back mapping, plan and longitudinal projections, classical and other varieties of polygonal ore reserve estimation methods are used. Inherent problems of grade prediction persist and driving along the reefs remains the only way to assign reliable grades to ore blocks.

Geology of Australian and Papua New Guinean Mineral Deposits

NORSEMAN GOLD DEPOSITS

Plans are in place to apply computer-based three dimensional modelling techniques, with the greatest benefit expected to be in communicating the complex geometry of reefs, such as in the Harlequin West area.

and mine geologists have added to the understanding of Norseman geology. More recently K Johnson, D N Kelly, S G Peters and L A Offe have made significant contributions. C Stephens is thanked for his technical review of the paper.

CONCLUSIONS

REFERENCES

Each of the more recently exploited Norseman orebodies is different to some extent from the veins which have been the big producers in the past. At the Bullen mine cross links are 070o striking and SE dipping tensional veins which are part of the same shear-vein system as the north striking producers, but their importance was not recognised in the past.

Doepel, J J G, 1973. Norseman, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SI 51–2.

At the OK mine the major producing vein is the 300o striking, subvertical, shear hosted O2 reef, in contrast to the east striking subvertical tensional reefs (Main reef) which have been historical producers. At Scotia, south of the Proterozoic Dambo fault, the structurally complex orebodies contain a number of small lensoidal ore shoots affected by localised Proterozoic alteration. Mining at Harlequin has so far been on the 070° striking and SE dipping HV1 vein, but continuing exploration has defined mineralised veins with other orientations. Although the Harlequin area has many similarities in geometry to the main field, the differences include more shearing associated with veins in the HV1 orientation. The Scotia, Bullen, OK and Harlequin mines have so far produced only about 500 000 oz, or 12%, of the gold mined from the Norseman Goldfield. However each is contributing to a new phase in the history of the field. Not only are different styles of mineralisation proving to be important but a much larger area of fertile country is now known to exist and many other promising prospects are being tested. Technological advances have made it possible to explore even the most hostile of environments.

ACKNOWLEDGEMENTS Central Norseman Gold Corporation and WMC Resources are thanked for permission to publish the paper. Many exploration

Geology of Australian and Papua New Guinean Mineral Deposits

Hill, R I, Campbell, I H and Compston, W, 1989. Age and origin of granitic rocks in the Kalgoorlie-Norseman region of Western Australia: Implications for the origin of Archaean crust, Geochimica et Cosmochimica Acta, 53:1259–1275. Keele, R A, 1984. Emplacement and deformation of Archaean goldbearing quartz veins, Norseman, Western Australia, PhD thesis (unpublished), University of Leeds. Kent, A J R, 1994. Geochronological constraints on the timing of Archean gold mineralization in the Yilgarn Craton, Western Australia, PhD thesis (unpublished), Australian National University, Canberra. McCuaig, T C, 1996. The genesis and evolution of lode gold mineralization and mafic host lithologies in the late-Archaean Norseman Terrane, Yilgarn Block, Western Australia, PhD thesis (unpublished), University of Saskatchewan, Saskatoon. McCuaig, T C, Kerrich, R, Groves, D I and Archer, N, 1993. The nature and dimension of regional and local gold-related hydrothermal alteration in tholeiitic metabasalts in the Norseman Goldfields: the missing link in crustal continuum of gold deposits, Mineralium Deposita, 28: 420–435. McGoldrick, P, 1993. Geology of the Norseman 1:100 000 sheet (Geological Survey of Western Australia: Perth). Perring, C S and McNaughton, N J, 1990. Proterozoic remobilization of ore metals within Archaean gold deposits: lead-isotope evidence from Norseman, Western Australia, Australian Journal of Earth Sciences, 37: 369–372. Sibson, R H, 1990. Faulting and fluid flow, in Tectonically Active Regimes of the Continental Crust (Ed: B E Nesbit), Mineralogical Association of Canada Short Course 18, pp 93–132. Thomas, A, Johnson, K and MacGeehan, P J, 1990. Norseman gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 493–504 (The Australasian Institute of Mining and Metallurgy: Melbourne).

271

272

Geology of Australian and Papua New Guinean Mineral Deposits

Mulholland, I R, Cowden, A, Hay, I P, Ion, J C and Greenaway, A L, 1998. Ninbus silver-zinc deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 273–278 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Nimbus silver-zinc deposit 1

2

3

4

by I R Mulholland , A Cowden , I P Hay , J C Ion and A L Greenaway INTRODUCTION The deposit is 17 km ESE of Kalgoorlie, WA, at about lat 30o47′S, long 121o39′E or AMG coordinates 370 500 E, 6 592 500 N (Fig 1), on the Kurnalpi (SH 51–10) 1:250 000 scale and the Kanowna (3236) 1:100 000 scale map sheets. It is about 2 km NE of the historic Boorara mining centre and straddles the old Boorara–Bulong water pipeline track. The deposit is owned by Archaean Gold NL.

5

An Identified Mineral Resource totalling 929 000 t at 270 g/t silver and 0.3 g/t gold has been estimated for the oxide and transition zone portions of the deposit at the Discovery, Western and Eastern zones (Table 1, Fig 2). This represents about 8.14 Moz of contained silver and 9900 oz of contained gold, or 118 400 oz of gold equivalent where gold equivalent = gold + silver/75. No resource estimate has yet been made for the sulphide mineralisation, exploration of which is continuing. TABLE 1 Nimbus resource summary. Resource category

Ore type

Ore (’000 t)

Ag Au Ag (g/t) (g/t) (’000 oz) 370

0.3

5078

Au (oz)

Oxide Transition

70.0

340

0.4

754

820

Indicared

Oxide

89.5

110

0.2

318

590

305.5

190

0.4

1892

4010

Inferred

Oxide

15.0

40

0.6

21

280

Transition

23.0

100

0.4

76

290

929.0

270

0.3

8139

9900

Total oxide and transition resources

FIG 1 - Location map and regional geology (from Ahmat, 1995).

Contained metal

Measured

Transition

426.0

Grade

3910

Notes: 1. Based on an undiluted block model 2. Ag grade is cut to 3000 g/t in the Eastern zone and 2200 g/t in the Discovery zone 3. Lower cutoff is 0.5g/t gold equivalent, where gold equivalent = gold + silver/75 4. There may be discrepancies in totals due to rounding.

Nimbus is the first example of high-grade silver-zinc mineralisation found in the Kalgoorlie area, and it may epitomise a new province of volcanogenic massive sulphide (VMS) deposits. Within the oxide zone, to 90 m depth, outcropping silver and gold mineralisation is underlain by a blanket of transition zone silver-gold mineralisation at the base of complete oxidation. In fresh rock, high grade silver-zinclead sulphide mineralisation has been intersected. 1.

Formerly Development Manager, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.

2.

Formerly Managing Director, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.

3.

Formerly Project Geologist, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.

4.

Formerly Chief Geologist, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.

5.

Formerly Exploration Geologist, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.

Geology of Australian and Papua New Guinean Mineral Deposits

FIG 2 - Schematic drilling plan, Nimbus deposit, showing mineralised zones projected to surface.

273

I R MULHOLLAND et al

EXPLORATION HISTORY The Nimbus area was previously explored for precious and base metals, and minor anomalous silver and arsenic values in soil had been identified before the involvement of Archaean Gold NL. The current leases were pegged by a prospector, C J Hake, in 1991. Mr Hake and partners recovered over 160 oz of gold from a quartz outcrop some 300 m south of the outcrop of the Discovery zone. In 1993 an Archaean Gold geologist, Patrick Cheetham, while reviewing the prospect with Mr Hake, recognised a heavy grey-green mineral present in panned cuttings of a drill hole as possibly being silver chloride. Analysis of cuttings and nearby outcrops provided silver values to 390 ppm and an option to purchase was negotiated. Archaean Gold commenced exploration at Nimbus in July 1994 using 200 by 40 m spaced soil sampling followed by infill soil sampling. By December 1994 widespread gold and/or silver anomalies in soil with peak values of 119 ppb gold (BCL) and 13 000 ppb silver (AAS) had been outlined in the vicinity of the discovery outcrop. In March 1995, rotary air blast (RAB) drilling at the soil anomalies in the Discovery zone intersected silver values of >1000 g/t over wide downhole intervals (>40 m), accompanied by highly anomalous zinc, gold and lead values in completely oxidised rock. In June 1995 additional drilling of silver anomalies resulted in the discovery of the Eastern zone. Gossan outcrop at the Eastern zone was mildly anomalous, with silver values to about 7 ppm. Since discovery, 32 538 m of RAB, 18 449 m of reverse circulation (RC) and 3214 m of diamond core drilling have been used to delineate the three areas of oxide and transition zone mineralisation at the Discovery, Western and Eastern zones and to test deeper sulphide mineralisation at Nimbus Deeps (Fig 2). A prefeasibility study into mining of the oxide and transition zone mineralisation is in progress.

REGIONAL GEOLOGY Nimbus is within the uppermost felsic units of the Boorara Domain of the Archaean Kalgoorlie Terrane (Swager et al, 1990) (Fig 1), adjacent to the Boorara Shear (Ahmat, 1995). Stratigraphy is poorly understood and it is difficult to distinguish between intrusive and extrusive felsic igneous rocks and between flows, tuffs and volcaniclastic sediment because of the effects of metamorphism, weathering and the lack of critical exposures (Ahmat, 1995). No formal stratigraphic names have yet been allocated to the Nimbus host rocks.

Nimbus is hosted by feldsparphyric felsic rocks of rhyolitic to dacitic composition, including flow-banded lava, breccia (hyaloclastite and autobreccia) and fine grained felsic volcaniclastic sandstone. Siliceous layered chert, black shale and sulphide-rich variants of these sediments are also present. Primary flow brecciation and cooling fractures and/or intrusive flow brecciation occur along the glassy contacts of some volcanic units. Glassy chilled margins to fragments are common. This assemblage of host rocks, although unclear in its geometry, is typical of that hosting VMS deposits. Regional greenschist facies metamorphism is represented by the development of sericite, chlorite and quartz. All volcaniclastic rocks and most sulphide-rich rocks are often schistose with an upright penetrative fabric, defined by sericite and/or chlorite, which dips at 65o towards 225o magnetic. In more competent chert and felsic rock this fabric is weakly developed or absent. A strong stretching lineation dipping at 30o towards 135o magnetic lies in the plane of the fabric. The orientation of unit boundaries is generally unclear but most readily definable units at surface, such as chert outcrops and gossan float trails, are parallel to this regional fabric.

MINERALISATION Secondary silver mineralisation occurs in two zones - within the completely oxidised weathering zone (termed oxide mineralisation), and at or near the base of oxidation (termed transition zone mineralisation). Sulphide zone mineralisation occurs within fresh rock below the transition zone.

Oxide zone Oxide mineralisation at the Discovery, Western and Eastern zones (Figs 2, 3 and 5) is a maximum of 80 m wide, 50 m deep and 120 m long. There is a higher grade core of silver-gold mineralisation where values of silver in the range 500–3000 g/t are common, possibly representing relict primary mineralisation. For example hole BOC011 returned 23 m at 960 g/t silver, BOD028 returned 39 m at 890 g/t silver, BOD059 returned 42 m at 1170 g/t and BOM002 returned 34 m at 1464 g/t silver. Hydromorphic dispersion is probably responsible for the gradual decrease of silver and gold values in weathered rock around this higher grade core.

ORE DEPOSIT FEATURES LOCAL GEOLOGY The base of complete oxidation is at about 80 to 100 m below surface with all rocks above this completely weathered to kaolinite, iron oxide and quartz. Primary rock type in the weathered zone is difficult to determine with certainty. A 10 to 20 m thick zone of partially weathered material lies below the base of oxidation and above fresh rock. In this zone rock textures are better preserved and rock types are more recognisable.

274

FIG 3 - Schematic cross section A – A′ looking north. Location on Fig 2.

Geology of Australian and Papua New Guinean Mineral Deposits

NIMBUS SILVER-ZINC DEPOSIT

TABLE 2 Nimbus Deeps representative drilling results. Hole No

Interval (m)

Zn (%)

Pb (%)

Ag (g/t)

BOD022

18

1.6

0.7

276

BOD036

41

4.3

0.7

353

BOD044

25

3.6

0.2

148

BOC0682

11

6.8

0.6

254

BOC076

8

2.3

0.4

99

BOD100

50

6.8

1.1

219

BOD103

31

1.8

0.2

275

BOD167

1

6.6

1.0

350

BOD168

5

17.9

2.0

943

BOD168

11

5.7

0.9

106

BOD169

36

7.2

1.6

399

BOD174

5

1.0

-

24

BOD174

9

2.3

0.5

102

BOD175

9.6

10.1

7.4

907

BOD175

8

2.3

0.2

128

BOD175

11.3

1.2

0.1

80

BOD177

5.7

12.4

4.1

1200

1

1. BOD prefix indicates diamond drill hole 2. BOC prefix indicates RC drill hole.

FIG 4 - Schematic cross section B – B′ looking NW. Location on Fig 2.

5). The soft clays and earthy gossans developed at this level contain native silver and secondary copper minerals, principally chalcocite.

Sulphide zone

Transition zone

Sulphide mineralisation occurs beneath and down plunge from the oxide and transition zone mineralisation at Discovery, Eastern and Western zones. Individual sulphide bodies vary in true thickness from 2 to 25 m (Fig 4), have a horizontal extent of >100 m (Fig 2) and a vertical extent of at least 50 to 80 m (Figs 4 and 5). Representative assay values from drilling are given in Table 2.

Transition zone mineralisation occurs at or near the base of oxidation and there is some local continuity with oxide mineralisation above and sulphide mineralisation below (Fig

Weak alteration forms a halo up to 700 m wide centred on the mineralisation. Strong sericitisation of feldspar plus disseminated euhedral pyrite and yellow tourmaline, albite and carbonate are typical.

The principal silver-bearing mineral within the oxide zone has been identified as cerargyrite [Ag(Cl,Br,I)]. A complex lead-iron-antimony-copper sulpho-arsenate, electrum, and iron and manganese oxides also occur.

FIG 5 - Schematic longitudinal projection C – C′ looking NE. Location on Fig 2.

Geology of Australian and Papua New Guinean Mineral Deposits

275

I R MULHOLLAND et al

Intense alteration occurs within 50 to 100 m of sulphide-rich mineralisation. Carbonate and sulphides are abundant and they replace and destroy pre-existing textures. Green and black chlorite and either fuchsite or biotite develop as vein microselvages and may define the schistose fabric. Two styles of sulphide mineralisation have been observed; massive sulphide and sulphide breccia.

Massive sulphide Massive sulphide occurs as either massive banded sphaleritegalena or as massive pyrite, in layers of 2 to 10 m true thickness. The massive banded sphalerite-galena may contain high silver values in addition to high base metal values, for example hole BOD175 intersected 9.6 m at 10.1% zinc, 7.4% lead and 907 g/t silver, and hole BOD177 intersected 5.7 m at 12.4% zinc, 4.1% lead and 1200 g/t silver (Table 2). Sphalerite is the dominant mineral, forming an even grained mosaic of 0.2–0.3 mm diameter equant crystals and is the host to other sulphides, principally pyrite, galena and boulangerite, as intergrowths in the sphalerite. Silver-rich tetrahedrite and pyrargyrite are sometimes present. Arsenopyrite is consistently present as fine idiomorphic rhombs in the other sulphides, and up to 10% quartz is common as gangue. In most mineralised zones there are cataclastic microstructures and pressure shadows associated with more competent primary relic features such as crystals and clasts. Massive pyrite is usually barren of zinc, lead and silver.

Sulphide breccia Sulphide breccia represents widespread sulphide replacement of a pre-existing breccia zone, probably a hyaloclastite or a stringer zone. Mineralisation is usually thick (25 to 30 m true thickness) but of lower grade than the massive sulphide type, for example hole BOD 100 intersected 50 m at 6.8% zinc, 1.1% lead and 219 g/t silver (Table 2). Sulphide breccia originally consisted of fragments of felsic porphyritic host rocks in a matrix of quartz and chlorite. The breccia matrix and then the rock fragments have been altered and progressively replaced by pyrite and at a later stage by sphalerite. At the lower margins of the breccia, alteration intensity is low and angular rock fragments are easily identifiable. Alteration intensity increases upwards and rock fragments and original textures are progressively destroyed. Towards the top of each sulphide breccia zone the intensity of sphalerite replacement increases such that within 2 to 10 m of the top of the zone (Fig 6) there is near total replacement of matrix and rock fragments by sphalerite and pyrite. There are at least two generations of sphalerite, a brownish (higher iron) variety being later than honey coloured (lower iron) sphalerite. Galena, boulangerite, arsenopyrite, pyrite and pyrargyrite occur in minor to trace quantities. Minerals identified are pyrite, sphalerite, galena, silverbearing tetrahedrite [(Cu,Fe,Zn,Ag)12Sb 4S13], boulangerite (Pb5Sb4S11), pyrargyrite (Ag3SbS3), arsenopyrite (FeAsS), jalpaite [(Ag,Cu)2S], native silver, amalgam (Ag,Hg), bournonite (PbCuSbS3), chalcopyrite, electrum and cinnabar.

FIG 6 - Schematic section through sulphide mineralisation.

276

Geology of Australian and Papua New Guinean Mineral Deposits

NIMBUS SILVER-ZINC DEPOSIT

ORE GENESIS Nimbus displays many features of a VMS type deposit. The two mineralised components of the deposit, massive sulphide and sulphide breccia, have a different dip, but similar strike and plunge. At Discovery zone the association of flat-lying banded massive sphalerite and galena with overlying chert and argillite may indicate a classic VMS exhalative position. The underlying (subvertical) sulphide breccia either represents sulphide replacement of a hyaloclastite (quench breccia) which acted as a fluid conduit, or it may simply be a sphalerite-rich stringer feeder zone beneath the exhalative position now represented by massive sphalerite-galena and pyrite. Oxide mineralisation represents hydromorphic redistribution and possible enrichment of relatively immobile elements such as silver, lead and gold from weathered massive sulphide and sulphide breccia. Copper and zinc are almost totally removed from the oxide zone by weathering of the original primary sulphide mineralisation. Transition zone mineralisation has been formed by supergene weathering at or about the water table.

PLANNED MINING OPERATIONS Metallurgical studies for prefeasibility purposes indicate that the oxide mineralisation is amenable to a standard cyanide leach, similar to most gold ores, with recoveries of 95% silver and 80% gold. Recovery of silver and gold from the leach

Geology of Australian and Papua New Guinean Mineral Deposits

solution will probably be by zinc precipitation using a Merill Crowe circuit. Other process options such as flotation and direct reduction are also being investigated.

ACKNOWLEDGEMENTS The authors acknowledge the permission of Archaean Gold NL to publish this paper. Nimbus was not discovered and developed by one individual, but rather a team of people, working together with a common discovery goal. In addition to the authors, this team included P L Cheetham and J Stockley. Mineragraphic examinations by R Townend and petrographic work by M Maxwell aided geological interpretations, as did discussions with visiting geologists. Thanks are also extended to Genalysis Laboratory Services who developed a new silver fire assay technique, P Benjamin who drafted the figures and K Spiers who typed the manuscript.

REFERENCES Ahmat, A L, 1995. Kanowna, Western Australia - 1:100 000 geological series, Western Australia Geological Survey, Explanatory Notes 3236. Swager, C P, Griffin, T J, Witt, W K, Wyche, S, Ahmat, A L, Hunter, W M and McGoldrick, P J, 1990. Geology of the Archaean Kalgoorlie Terrane - an explanatory note, Western Australia Geological Survey Record 1990/12.

277

278

Geology of Australian and Papua New Guinean Mineral Deposits

Parks, J, 1998. Weld Range platinum group element deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 279–286 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Weld Range platinum group element deposit 1

by J Parks

INTRODUCTION The Archaean Weld Range ultramafic-mafic complex is about 700 km NNE of Perth, WA, in the Murchison Province, some 70 km NNW of the township of Cue and about 1 km north of the banded iron formation (BIF) ridges of the Weld Range. The complex is at lat 26o50′S, long 117o45′E on the Belele (SG 50–11) 1:250 000 scale and the Madoonga (2444) 1:100 000 scale map sheets (Fig 1). Platinum group element (PGE) mineralisation has been identified within a laterally persistent olivine-clinopyroxenite unit with a strike length of 15 km and in the overlying regolith. An Inferred Resource has been estimated for the supergene 1.

Senior Geologist, Battle Mountain (Australia) Inc, 106 Dalrymple Road, Currajong Qld 4814.

deposit which is the first potentially economic discovery of supergene PGE in Australia (Table 1). Studies including metallurgical testing are continuing. Sparse drill testing of mineralisation in fresh rock shows apparent continuity which is typical of this style of mineralisation, but the testing is presently insufficient to allow resource estimation. The hard rock mineralisation remains open at depth and for a further 4 km of strike length. The Weld Range complex is enclosed by tenements subject to a joint venture between Sons of Gwalia Ltd (SOG) with 65% equity who manage the joint venture, and Dragon Mining NL (Dragon) with 35%. The original joint venture from 1991 was between Dragon and Austmin Gold NL (Austmin), which became a subsidiary of Burmine Ltd in 1993. SOG assumed control following a merger with Burmine Ltd in May 1996.

FIG 1 - Location and simplified geological map, showing location of drill hole traverses and cross section lines on Figs 3 and 5.

Geology of Australian and Papua New Guinean Mineral Deposits

279

J PARKS

TABLE 1 Inferred Resources at different cutoff grades, Weld Range supergene PGE deposit. Cutoff Pt+Pd+Au Pt (g/t)

Grade Pd (g/t)

Au

Mt

Combined Contained oz Pt+Pd+Au Pt+Pd+Au (g/t)

0.50

0.6

0.5

-

14.76

1.1

522 000

0.75

0.8

0.6

-

9.53

1.4

429 000

1.00

0.9

0.7

-

6.30

1.6

324 000

2.00

1.5

1.1

0.1 1.38

2.7

120 000

EXPLORATION HISTORY The ultramafic rocks of the Weld Range complex have been explored for nickel sulphides, initially by International Nickel Australia Ltd between 1969 and 1971 and then by Australian Consolidated Minerals NL (ACM) in 1971, subsequently in joint venture with Broken Hill Proprietary Ltd (BHP) from 1972 to 1973. Only rare grains of sulphide (in part supergene) were identified, although significant nickel mineralisation was discovered in laterite in some localities, but was considered too discontinuous to be economically viable. Petrography by BHP on fresh drill hole samples showed the rocks to be olivine and olivine-chromite cumulates with clinopyroxene (diopside)olivine cumulates along the southern margin near the interpreted top of an ultramafic lopolithic intrusion (C J Palethorpe, unpublished data, 1972). Traces of chalcopyrite were noted in two samples from the olivine-clinopyroxene unit, with up to 1000 ppm copper obtained from drill hole samples of ultramafic clays developed above the same unit. No significance was attached to these weak copper enrichments and no samples were analysed for PGE. Potential for chromite mineralisation was noted but not tested due to the thickness of laterite cover. Additional exploration in the area by the ACM–BHP joint venture was directed at volcanogenic copper-zinc deposits in rocks interpreted as felsic volcanic breccias intruded by mafic to intermediate rocks flanking the ultramafic intrusion to the SW. No mineralisation was located, however petrography of selected samples showed that the rocks including the polymict breccias are mafic. Rocks described include basalt with crescumulate textures (bladed pyroxenes indicative of supercooling) and uralitised feldspathic pyroxenite. Regional mapping showed that these rocks are part of the greenstone sequence (Elias, 1982; Watkins and Hickman, 1990). Potential for PGE mineralisation in the area was not recognised until the late 1980s when Dragon and Austmin acquired adjacent tenements enclosing the ultramafic complex. Dragon initially focussed on the chromium potential of the laterites in an area investigated by CRA Limited in 1976–1977. CRA had concluded that most of the chromium was associated with iron oxides and not recoverable as chromite. Dragon has now identified an Inferred Resource of 31 Mt at 3.6% chromium at a 2% chromium cutoff to 12 m depth within an area of 5.4 km2 excised from the joint venture (Fig 1). Laterite samples collected from the area of the chromium resource were assayed for gold and the full suite of PGE. No anomalous PGE values were obtained, but it was recognised that other PGE targets within the intrusion remained untested (Morgan, 1987).

280

However, conventional models for PGE mineralisation based on stratiform layered mafic-ultramafic complexes such as the Bushveld Complex were not thought applicable, due to the apparent absence of gabbroic rocks. Importantly the gabbroic top to the ultramafic cumulates was not recognised until field reconnaissance in 1990 by Geological Consultants International on behalf of Austmin identified rare outcrops of gabbro near the SW margin of the intrusion. At this time the polymict breccias and associated rocks in the SW corner of the intrusion were re-interpreted as a marginal facies to the ultramafic-mafic complex rather than part of the greenstone sequence. This meant that conventional PGE models could be applied and gave significance to the weak copper sulphide mineralisation previously identified at the top of the ultramafic sequence. The exploration concept was subsequently validated when drilling by Austmin, as managers of the joint venture with Dragon, intersected PGE mineralisation in the regolith and in fresh rock immediately below the gabbro contact. To date, 24 diamond drill holes for 3740 m, 41 reverse circulation holes for 3690 m and 367 rotary air blast/aircore holes for 15 428 m have been drilled by the joint venture partners, mostly directed at the known mineralisation.

REGIONAL GEOLOGY The age and contact relationships between the Weld Range complex and the country rocks are not precisely known, due to the paucity of outcrop. The complex is bounded to the south by apparently concordant dolerites and banded iron formation units which constitute the Weld Range. Granite flanks the complex to the north and west (Fig 1). Regional geological mapping and interpretation places the Weld Range complex in the Gabanintha Formation of the Luke Creek Group, the lowermost of the two greenstone sequences in the Murchison Supergroup (Watkins and Hickman, 1990). The Gabanintha Formation is overlain by the Windaning Formation, which in this locality constitutes the jaspilitic BIF and dolerite forming the core of the Weld Range greenstone belt. A U-Pb age of 3000 Myr has been obtained from rocks in the upper part of the Luke Creek Group (Watkins and Hickman, 1990). The Gabanintha Formation in the greenstone belts up to 150 km south of Meekatharra is characterised at or near its base by a laterally extensive volcanic-intrusive ultramafic complex up to 1400 m thick. Weld Range is a discrete ultramafic-mafic intrusive complex to 8 km thick. Complexes of such thickness have not previously been recognised in the Luke Creek Group.

DEPOSIT GEOLOGY The geology of the complex is poorly understood due to sparse exposure and lateritic weathering to more than 80 m in the centre of the intrusion. BHP aeromagnetic data show that the complex is triangular in plan with the widest point at the stratigraphic top with a strike length of about 15 km. Available data suggest it is a recumbent lopolith such that the layering now dips at 60 to 85o south. Greenschist facies metamorphism has mostly destroyed primary silicate minerals although olivine and clinopyroxene grains commonly have fresh cores, and primary textures are typically well preserved. In some places the rocks have been intensely carbonate-talc altered with almost total destruction of minerals and texture. Shearing has been noted locally.

Geology of Australian and Papua New Guinean Mineral Deposits

WELD RANGE PLATINUM GROUP ELEMENT DEPOSIT

Outcrop within the complex, with the exception of the marginal rocks, is restricted to a hill of lateritic duricrust. Silicified ultramafic caprock showing relict igneous textures is exposed in places on the southern flanks of this hill. The laterite profile developed over the complex has been variably stripped, but generally to the level of the upper saprolite (Harrison, 1993). Lateritic gravel, assorted locally derived alluvium and colluvium, calcrete and aeolian sand overlie this truncated surface to depths of over 20 m. Depths of both weathering and transported overburden thin to less than 2 m over the SW of the intrusion.

LITHOLOGY AND STRATIGRAPHY Layering is evident in aeromagnetic data, but the drill hole data do not allow a reliable characterisation of these layers, which appear to reflect in part varying degrees of serpentinisation. The complex can be subdivided into a lower Ultramafic series to 5 km thick and an overlying Mafic series to 3.5 km thick (Fig 1). A Marginal series, consisting of breccia, fine grained mafic rocks and feldspathic pyroxenite, has only been observed on the SW margin of the intrusion, but may be more extensive. The Ultramafic series is rhythmically layered, whereas the Mafic series appears to be mostly massive. The upper part of the Ultramafic series and the entire gabbro sequence have been extensively intruded by a variety of mostly aplite dykes or sills. These sills are broadly parallel to the sequence but in detail pinch, swell, anastomose and bifurcate, but do not appear to be associated with any stoping or loss of sequence. Aplite units to 34 m thick disrupt the PGE mineralised zone. The relationship between the aplite and the granitic country rocks is not known. The contact between the Ultramafic and Mafic series is locally intruded by aplite and typically displays greater alteration than the adjacent sequence, obscuring primary contact relationships. However, the contact appears to be sharp and faulted at least in part. Difficulties in correlation between drill holes further indicate that it may be unconformable.

Ultramafic series The Ultramafic series can be subdivided into a lower olivinechromite zone and an overlying wehrlite zone. The olivinechromite zone forms the greater part of the sequence. The first appearance of cumulus clinopyroxene marks the base of the wehrlite zone, which is a maximum of 500 m thick.

Olivine-chromite zone BHP petrographic data show the olivine-chromite zone consists mostly of olivine-chromite cumulates and subordinate chromite-olivine cumulates and olivine adcumulates. No discrete chromitite layers or chromite adcumulates have been observed. Textures are typically mesocumulate to adcumulate with original interstitial material consisting of orthopyroxene, clinopyroxene and plagioclase, with rare biotite.

Wehrlite zone The wehrlite zone, which hosts the PGE mineralisation, consists of fine to medium grained (1–2 mm) olivine and clinopyroxene cumulates. Textures are mesocumulate to adcumulate. Clinopyroxene adcumulates and olivine adcumulates (dunite) are a minor component and where present appear to lack lateral continuity. Orthopyroxene is locally a

Geology of Australian and Papua New Guinean Mineral Deposits

minor (to 15%) cumulus phase. In some places it forms anhedral poikilitic grains 1 cm across. Rare chromite has been observed in only two samples: intergrown with sulphide in a mineralised sample from near the western end of the complex, and as very fine grained inclusions within clinopyroxene in a mineralised sample from the central part of the intrusion. Interstitial minerals are orthopyroxene, hornblende and possibly rare plagioclase. Green-brown hornblende and subordinate red-brown hornblende (kaersutite?), are a common although minor (12

12

Clunes

47

37

>10

10

Chiltern

46

1

>45

45

Maryborough

32

3

>29

29

Berringa

30

18

>12

Egerton

27

16

11

Harrietville

24

12

>12

12 11

Avoca

23

23

18

5

Ararat

20

3

Tarnagulla

>13

>13

major

St Arnaud

12

12

3

Beaufort

>10

major

8

Dunolly-Moliagul

>10

4

major

3

Taradale-Lauriston

>10

6

major

2480

980

1490

>27

>27

Total Victorian Province5 Fosterville 1. 2. 3. 4. 5. 6.

6

300

At several goldfields a meaningful subdivision of alluvial into placer and palaeoplacer is not possible. Placer and palaeoplacer gold production are assigned to the adjoining major ‘primary’ goldfields from which that gold is thought to be derived (Phillips and Hughes, 1995). Substantial placer production is poorly accounted for (ie over 500 t of ‘total alluvial’ is unassigned). Creswick figure has been substantially revised upwards with the recent addition of placer production not included in Phillips and Hughes (1996). The nature of source information means that totals are rounded. Fosterville figure is predominantly a resource, rather than production.

A very important source of gold was placer and palaeoplacer deposits, again with most production concentrated in central Victoria, and related to adjacent primary deposits (Fig 2).

GOLD MINING AND EXPLORATION DURING THE 1980s AND 1990s The developments in exploration and mining in the province over the last decade provide a particularly interesting study of the interplay of government, community and industry perceptions and attitudes as they impact upon what should be viewed as a highly prospective gold province. The results of a few positive changes can already be seen in the early 1990s through increased mining and renewed exploration expenditure.

PERCEPTIONS OF PROSPECTIVITY One of the reasons that Victoria did not attract the exploration attention during the 1980s that was afforded many other gold provinces was the perception that gold in Victoria was essentially exhausted (Bowen and Whiting, 1975, p 648;

496

1190

Thomas, 1988), and the belief that the types of deposits to be found in Victoria were likely to be uneconomic. Such a viewpoint is not totally disproved until major discoveries are made, but on the basis of the minimal exploration effort in Victoria since 1910 and the observation that all other major gold provinces worldwide yielded further deposits during intensive 1980s style exploration, Phillips and Hughes (1995) argued strongly against the perception of gold exhaustion in Victoria. Recent additions at Stawell (Fletcher, 1995; Fredericksen and Gane, this publication) and new mineralisation styles at Fosterville (Zurkic, this publication) and Bailieston (Sebek, this publication) may be the start of a new period of discovery.

ATTITUDES TO MINING The closure of most of Victoria's gold mines by 1915 meant that the strong mining culture of the 19th century was all but gone by the 1980s. With little community or government support for mining and some significant opposition, exploration was at a very low level through the 1980s and discoveries were negligible, having the effect of reinforcing the negative

Geology of Australian and Papua New Guinean Mineral Deposits

VICTORIAN GOLD PROVINCE

sentiment towards Victorian gold prospectivity. The Victorian Minister for Energy and Minerals referred to the 1980s decline in Victorian mining activity by noting: ‘This decline is due to several factors including government indifference, a plethora of regulatory approvals and an anti-mining ethic’ (Plowman, 1994). His department went on to rapidly address some of these shortcomings.

province in which economic gold deposits form by the same fundamental genetic process (Phillips and Hughes, 1995, 1996), rather than as a geographic region in which these deposits have formed by multiple, unrelated or vaguely related processes.

NEW INITIATIVES

Gold production in Victoria during the period 1970–1985 never exceeded 1 tpa. By 1995 production had reached 5 t. This increase can be attributed to the production from the Stawell and Fosterville mines, and some intermittent mining at Heathcote, Benambra, Nagambie, Maldon, Avoca and St Arnaud, as well as recovery from old tailings dumps.

Initiatives since the 1980s that have changed the role of exploration in Victoria can be divided into areas of policy, geological data and ideas. Policy primarily involves government decisions and action that impact on exploration and mining, geological data include geophysics and relate to new information to assist exploration, and ideas refer to conceptual inputs to exploration models.

Policy Dealing with regulatory bodies related to mining and exploration in Victoria has been greatly facilitated by reducing the number of individual inquiry points, and by speeding up the approval process for granting of tenements. In this regard Victoria is now competitive with other Australian States.

Data Areas where new data are particularly useful, such as airborne geophysics, have been addressed through a major initiative by the Victorian Government (Victorian Initiative for Minerals and Petroleum which commenced in 1994), which includes new aeromagnetic, gravity and radiometric surveys. Enormous potential exists to reprocess and re-evaluate the extensive, well documented observations of Mines Department personnel made in the past during the active period of each mining operation, and these are now becoming available in digital form.

Ideas Until recently, very little synergy had been achieved from the relationship between descriptive models and genetic models as they relate to Victorian gold deposits, compared to the achievements, say, with Archaean gold deposits. This situation has arisen from the lack of major synthesis studies of Victorian gold deposits in the 1980s, the perception that Victorian gold geology was unlike the setting in other gold provinces, and the large gap between some genetic models which had been suggested from Victoria (eg gold in granites, exhalite gold), and what can be gleaned from past mining records of all significant producers (gold in metasedimentary rocks and mafic dykes). The metamorphic model being adopted here for Victorian gold deposits represents a significant advance on this, and has arisen from greenstone gold work (Phillips and Groves, 1983), from other slate belts (Goldfarb, Snee and Pickthorn, 1993), from theoretical studies on gold deposit formation, and from work in Victoria including the Castlemaine district (Cox et al, 1995 and earlier references; Hughes, Ho and Hughes, 1996) and Stawell (Wilson et al, 1992). As a genetic model, it provides the basis for prioritising data collection (much being already available in the literature), testing ideas, revising the genetic model, and collecting further data in an iterative process capable of generating considerable synergy (Phillips, Eshuys and Hellsten, 1996). It involves viewing Victoria as a

Geology of Australian and Papua New Guinean Mineral Deposits

MINING AND EXPLORATION ACTIVITY

With new regulations and some mining successes, exploration activity has increased dramatically in Victoria from a very low base. From an expenditure on gold exploration of $8M/yr in 1990, this increased to $26M/yr in 1995, and in 1996 the non-petroleum exploration expenditure (mostly on gold exploration) was approximately $50M/yr. Most exploration has focussed on significant past producers (eg Tarnagulla, Maldon, Ballarat, Bendigo, Clunes, Creswick and Woods Point) and prospective ground of the Ballarat zone. However, there has been additional exploration in previously unproductive areas such as far NW Victoria under deep alluvial cover where major companies have taken out large tenement holdings, and in far eastern Victoria.

ADVANCES IN VICTORIAN GOLD GEOLOGY In the 1980s, substantial advances were being made in understanding the geology of Archaean greenstone gold provinces, the Witwatersrand goldfields, the Alaskan slate belt provinces, and of some gold-only epithermal systems, but no similar advances occurred for the Victorian gold province. Such synthesis has recently been achieved for the Victorian province by an integration of detailed and regional gold metallogenic studies that open up a field of possibilities for new exploration and models (Phillips and Hughes, 1995, 1996; Hughes, Phillips and Gregory, 1997). Two dimensions of integration have provided the platforms for advance over the last decade. The first is integration of all aspects of geology to provide the best possible understanding of a single goldfield. This was pioneered at Castlemaine–Chewton (several papers summarised in Cox et al, 1995), and this approach has also been effective at Stawell (Wilson et al, 1992) and Maldon (Hughes, Phillips and Gregory, 1996, 1997). By using all available geological data, these long-term projects have been able to tackle some of the key issues influencing Victorian gold distribution. Three of these integrated studies are described where the results have widespread application to Victorian gold deposits. The second dimension of integration relates to scale, and has involved considering gold mineralisation, its chemistry, and structural and tectonic setting, at several scales. This includes the scales of ore shoots, orebodies, goldfields, geological zones and the province, and the Victorian gold province in terms of other major global gold provinces. This dimension of integration has been possible by combining a knowledge of the essential features of many Victorian goldfields with that derived from extensive work on Archaean greenstone, Witwatersrand, slate belt and other gold-only provinces,

497

G N PHILLIPS and M J HUGHES

worldwide (Phillips and Hughes, 1995). It has necessarily hinged on the belief that the Victorian gold province conforms in many ways to other major gold provinces, rather than being unique in its development. The following discussions address some of the advances relating to the geology of Victorian gold deposits.

FLUID FLOW AND STRUCTURAL CONTROL IN THE CASTLEMAINE GOLDFIELD The Castlemaine Goldfield produced 173 t of gold (Willman, 1995), mostly from very rich placer workings. Cox et al (1995) combined surface and underground mapping with microstructural, mineralogical, geochemical, fluid inclusion and stable isotope studies to elucidate many of the important controls on gold location in the quartz reefs of the Wattle Gully deposit. The deposits are in the hanging wall of a major, west-dipping reverse fault, as in several other Victorian goldfields. Auriferous quartz veins are tightly confined to an interval several hundred metres wide and several kilometres long, marking the domain of primary gold distribution, and presumably the domain of extensive fluid flow through the upper crust. The main structural control on primary mineralisation at Wattle Gully is a dilational jog on a high angle reverse fault, and continued fault movement and fluid pressure variations (as indicated by a multitude of crack-seal events) enhanced fluid interaction with what we infer were chemically favourable carbonaceous slates that led to reduction and gold precipitation. The auriferous fluid at Wattle Gully was of low salinity and had relatively non-diagnostic isotopic characteristics, consistent with a metamorphic fluid origin. Mineralising temperatures and pressures of around 300oC and 1.3 kb have been inferred.

STRUCTURAL SETTING OF THE STAWELL GOLDFIELD The Stawell field (Fredericksen and Gane, this publication) has produced 82 t of gold with indications that known resources might extend the endowment above 100 t (Fletcher, 1995). The structural setting of auriferous quartz veins in the hanging wall of a west-dipping reverse fault is similar to Castlemaine, but the host rocks include Cambro-Ordovician pelite, volcaniclastic rocks, Cambrian metabasalt, and the hornfelsed equivalents of these rocks. A combination of mapping, regional and detailed structural geology, mineralogy and metamorphic studies has provided an insight into the complexities of the Stawell mineralisation (Wilson et al, 1992; Mapani and Wilson, 1994). Detailed metamorphic petrology has suggested that depths of burial were very shallow (2 kb) at the time of D4 deformation and granite intrusion (Xu et al, 1994). Regional ductile deformation in the Stawell area pre-dated the Stawell Granite (dated around 395 Myr) in what has been inferred as D1–3 structural events (Wilson et al, 1992). Further deformation (D4) is broadly synchronous with this granite (although in detail gold introduction may pre-date the granite), and is inferred to be a Tabberabberan deformation on the basis of its NE–SW compression direction (Wilson et al, 1992). Such an interpretation might require Tabberabberan deformation to be diachronous across Victoria, as suggested by Collins (1994), and spanning a period of more than 20 Myr.

498

RELATIONSHIP OF THE MALDON GOLDFIELD TO ADJACENT GRANITE The Maldon goldfield has produced 65 t of gold, and its 56 t of primary production makes it one of the largest Victorian primary gold producers. Maldon is easily the most significant source of gold from deposits adjacent to granite, and it is an ideal place to study the changes in gold mineralisation proximal to a batholith. A combination of mapping, mineralogy, geochemistry, metamorphic petrology and quartz texture work elucidates some of the geological issues of the gold field (Hughes, Phillips and Gregory, 1997). Primary gold production has come from high grade, steeplydipping quartz veins that strike north towards the contact of a major 360 Myr granite batholith (Ebsworth, de Vickerod Krokowski and Fothergill, this publication). Mineralisation is abruptly terminated at the granite contact, and the granite is unaffected by either the gold-hosting shear zones, or the goldrelated alteration. Auriferous quartz veins are recrystallised approaching the granite, and distal arsenopyrite gives way to proximal loellingite plus pyrrhotite in the potassium feldspar zone within 1.5 km of the granite. The geochemistry of gold mineralisation at Maldon is comparable to most other Victorian gold deposits in metasedimentary rocks away from granite contacts (ie elevated values for sulphur, arsenic and gold), but with an additional contribution of molybdenum, bismuth and tungsten in close proximity to the granite only. The predictable nature of the geochemistry, with the structural, textural and mineralogical relationships observed, makes it unnecessary to propose that the Maldon deposit, or the deposits near the granite, result from a fluid dramatically different to many other Victorian gold deposits, although it does suggest a contribution from the granite. In contrast to the geochemistry, the mineralogy of the Maldon deposits is quite unusual [eg maldonite (Au2Bi), loellingite, biotite selvages on quartz veins, abundant pyrrhotite] and reflects higher temperatures than at many other Victorian gold deposits, and is consistent with contact metamorphism and the effects of significant retrograde resetting. The Maldon field is interpreted as a mineralised zone more than 6 km long intruded at its northern end by a Late Devonian granite. A thermal peak after gold mineralisation caused low pressure metamorphism in a 2.5 km wide aureole resulting in modified quartz and sulphide textures, shear zone foliation, alteration and host rock mineralogy. The nature of the mineralisation, alteration and structure provide little evidence that the granite was either the cause or source of the gold.

RELATIONSHIP OF PRIMARY GOLD DEPOSITS, PLACERS AND PALAEOPLACERS The relationship of primary to alluvial gold in Victoria has considerable impact on endowment figures and economic projections, and is one of the important inputs into genetic models. In the past, placer and palaeoplacer production have commonly been treated separately from primary production (eg Woodall, 1979). However, for the purpose of understanding the distribution and scale of primary gold mineralising systems in the province, there are benefits in viewing the primary and secondary deposits together. To gain a proper appreciation of the Palaeozoic processes that led to the Victorian gold province, it is necessary to combine primary and alluvial production. The revised endowment figures highlight the fact

Geology of Australian and Papua New Guinean Mineral Deposits

VICTORIAN GOLD PROVINCE

that there are 12 goldfields of 1 Moz or more in the Victorian gold province. The area containing these 1 Moz fields indicates the region in which there will be the highest probability of finding deposits likely to be viable today; and the styles of primary mineralisation within these 1 Moz fields indicate some (but probably not all) of the styles likely to be economic today. At many fields, the early prospectors used placer and palaeoplacer gold as a direct, and very successful, pathfinder for primary deposits. At Ballarat, highly auriferous reefs along three north-trending mineralised lines, two of 100–200 m width and the third consisting of three subzones of similar width, are drained by stream systems which have very rich placers that can be traced directly into palaeoplacers where younger basalt flows have filled old valleys (Fig 3; Taylor, this publication). The primary source is less clear at other goldfields. For example, the upper reaches of the Creswick palaeoplacer systems only contain auriferous quartz veins which have produced minor gold compared to the palaeoplacer production (but extensive basalt cover in the area may conceal larger deposits). More importantly, on a broad scale, the source area of the major palaeoplacer systems, including Creswick, is in the very heartland of Victorian primary gold deposits, namely the Ballarat–Bendigo area with its many primary deposits (Fig 2). On a wider scale, the same close relationship of rich placers to primary fields is borne out by recent comparisons with some Pacific Rim placer fields (Goldfarb, Nokleberg and Phillips, 1996). On this scale, the largest alluvial gold province in the southern Pacific (Victoria) not only coincides with the largest primary field, but the largest alluvial deposits in Victoria are remarkably close to the focus of the largest primary deposits.

GOLD DISTRIBUTION RELATED TO GEOLOGICAL ZONES The gold endowment figures show a strong focus of gold mineralisation in the Ballarat zone with marked decreases in other geological zones to the east and west, as has been known for a long while. This pattern, however, holds not only for total gold production, but also for the number of 1 Moz goldfields, number of reef mines producing over 1 t of gold, and the maximum size of goldfields in any geological zone. It probably also reflects the regional quartz vein density, total shortening across geological zones and location of high strain zones, and the distribution of anomalous but subeconomic gold values. The distribution of significant gold mineralisation in areas outside the Ballarat zone can be spatially related to high strain thrust zones. Examples are the Beaufort, Avoca and St Arnaud goldfields in the hanging wall of the Avoca and Percydale faults; the Stawell and Ararat fields associated with the Stawell–Copes Hill fault system and the Woods Point and Walhalla fields associated with the more strongly deformed eastern margin of the Melbourne zone, in the vicinity of the Mount Easton and Mount Wellington fault zones. The significance of the Harrietville and Chiltern fields immediately west of the Kiewa–Kancoona fault zone in the Tabberabbera zone is less clear. This pattern has numerous implications for exploration and gold genesis. It gives credence to the adage of ‘the best place to find a million-ounce deposit is near existing deposits’ rather than any suggestion that some areas develop many small deposits at the expense of a major one. It suggests that on the scale of geological zones, certain factors ‘went right’ for gold mineralisation in the Ballarat and neighbouring zones, but did not occur in the far east and west of Victoria. The pattern

Geology of Australian and Papua New Guinean Mineral Deposits

FIG 3 - Map of the Ballarat Goldfield showing the three parallel lines of north-trending primary mineralisation at Ballarat West, Ballarat East–Buninyong West, and Little Bendigo-Buninyong East. The alluvial deposits were rich and extensive immediately downslope from the major primary deposits, and these were traced a farther 20–30 km downstream as rich palaeoplacer deposits (‘deep leads’) beneath Quaternary basalt flows.

probably means that the volume of fluid and its gold-carrying capacity is critical on a zone scale, and on this scale (as distinct from the scale of an individual orebody) it is not the occurrence of special local structures that is of prime importance. Instead, the distribution of goldfields relative to regional scale faults suggests that the distribution of regional-scale structural ‘plumbing’ systems is important in focussing the flow of this fluid.

RELATIONSHIP OF GOLD DEPOSITS IN METASEDIMENTS, VOLCANIC ROCKS AND DYKES The three major gold mines in dykes of the Woods Point dyke swarm (the Morning Star and A1 mines near Woods Point, and the Long Tunnel mine, Walhalla) have never been integrated into the evolution of the slate-hosted deposits around Ballarat–Bendigo and the volcanic-hosted Stawell mineralisation.

499

G N PHILLIPS and M J HUGHES

Comparison with other gold provinces indicates that there are very good reasons to expect major gold deposits in both dykes and volcanic rocks, and black slates. In the absence of conflicting genetic evidence (or of any evidence other than differing host lithologies), these deposits are most reasonably part of one province and one overall event rather than being independent in time or origin. In the slate belt province of southeast Alaska, the major Juneau goldfield is dominantly hosted by a mafic dyke (Goldfarb, Snee and Pickthorn, 1993). In the Archaean greenstone gold provinces, many major gold deposits are related to iron-rich rocks (eg basalt, dolerite, banded iron formation) or less commonly carbon-rich host rocks such as black shale. In the Carlin province of Nevada, which shows similarities to some Victorian gold deposits (Hughes, Phillips and Gregory, 1997), there is a close association between gold mineralisation and pyrobitumen. These relationships of major gold deposits to iron-rich and/or carbon-rich host rocks and possibly to pyrobitumens, are attributed to the effectiveness of interaction with iron and carbon in precipitating thio-complexed gold from the low salinity fluid. There is an urgent need for detailed orebody studies which investigate these proposed controls on ore deposition. Applying these observations to the Victorian province, the predicted favourable host rocks in the Palaeozoic succession, which consists of Cambrian metabasalt, a flysch sequence of clastic metasediment, and a range of intrusions from ultramafic to felsic in composition, would be those rich in carbon (black slate) and iron (mafic dykes, metabasalt, volcaniclastic rocks). It is particularly notable that the Woods Point dyke swarm consists of hundreds of intrusions that range from ultramafic to intermediate and felsic in composition, and yet the major gold deposits are in the more iron-rich dykes of mafic composition (‘diorite’ and ‘lamprophyre’ in some earlier literature), in which iron-titanium minerals have been replaced by pyrite and rutile (or leucoxene). The nature of the wall rock alteration around deposits in the mafic dykes (ie chlorite, carbonate, mica and pyrite) is predictable given the composition of the goldbearing fluid and the host rocks. The close association of gold with dykes might be interpreted as a genetic association between the magmatic event and the source of gold. However, the dykes vary from peridotitediorite suites in the Woods Point dyke swarm and at minor deposits in dykes which occur in three other swarms which strike northwards, east of Maryborough and west of the Landsborough and Percydale faults. At the other extreme are quartz porphyry dykes at minor deposits elsewhere (eg west of Maryborough and Diamond Creek). Gold mineralisation in these dykes occurs in fractures which are systematically oriented with respect to the dyke walls and regional deformation fields in the surrounding rocks, and gold spatially unrelated to dykes occurs in sedimentary rocks of the same areas. The role of the dykes is better seen in terms of their physical response to later deformation, and their chemistry. This recognition of the probable role of carbon and iron in major gold deposits, and the observation that mineralisation can occur in most rock types, leads to the need for caution in using the term ‘slate belt’ for these deposits, or for the province. In essence, the Palaeozoic succession of Victoria is a gold province with many of the major deposits in slates.

500

GEOCHEMISTRY OF VICTORIAN GOLD DEPOSITS With some notable exceptions, most of the Victorian goldfields have a systematic and relatively simple geochemical signature comprising anomalous gold, arsenic, silver, sulphur, potassium, rubidium and carbon dioxide (Goldfarb and Phillips, 1995; Hughes, Phillips and Gregory, 1997). Minor fields in and near the Melbourne zone, of which Fosterville is the only one large enough to show on Fig 1, have anomalous to economic antimony with gold mineralisation; whereas deposits in or near granites (such as Maldon) have distinctly higher levels of bismuth, molybdenum and/or tungsten compared to all other gold deposits. This enrichment in potentially graniterelated elements is a signature additional to the main gold-only signature and generally extends less than 1 km from plutons; it does not appear to indicate a magmatic fluid origin for such gold deposits (Hughes, Phillips and Gregory, 1997). There are goldfields east of Stawell, and in the Omeo zone in eastern Victoria, that have slightly to distinctly different chemical signatures, with high silver, arsenic, copper, zinc and/or lead (Hughes, Phillips and Gregory, 1997); and from this signature a different and possibly more saline fluid is inferred, particularly for the Omeo zone, compared to most Victorian gold deposits. Sulphide ores of the Omeo zone were very small producers, and the less sulphidic ores of the eastern Stawell zone at Avoca and St Arnaud are more significant economically.

TIMING OF VICTORIAN GOLD EMPLACEMENT The timing of gold emplacement is relatively well constrained in some locations by a combination of host rock age, post-gold granites, deformation, and the essentially non-auriferous Carboniferous and younger sequences. For much of the Stawell and Ballarat zones, there is a large time range between host rock sedimentation and post-gold intrusions, with some uncertainty remaining as to the age of regional deformation. At Maldon, Early Ordovician metasedimentary rocks were mineralised before emplacement of the Late Devonian Harcourt Granite (Hughes, Phillips and Gregory, 1997), and similarly at Stawell and in the NW of the Ballarat zone (Mt Hooghly granite near Dunolly; Tarnagulla), gold mineralisation appears to predate intrusion of the 395 Myr (Early Devonian) granite. Gold emplacement appears syn- to late-deformational in the Stawell zone, and in parts of the Ballarat zone (Cox et al, 1991a) of central and western Victoria. However, the deformation is likely to be diachronous, and like the Devonian thermal event, be slightly earlier in the west. In the Stawell and Ballarat zones, economic gold mineralisation therefore pre-dates 395 and 365 Myr granites, whereas other, economically unimportant auriferous quartz veins post-date 395 Myr granites in the Stawell zone. However, in the Melbourne zone, economic gold mineralisation appears to be younger. In the Woods Point and Walhalla fields, for instance, auriferous quartz veins occur in dykes of probable Middle Devonian age which intrude the strongly deformed Early Devonian flysch sequence, whereas thick Carboniferous molasse sequences have no primary gold mineralisation, making major gold mineralisation Middle to Late Devonian in age. The widespread Late Devonian granites and acid volcanic complexes (365 Myr) appear to post-date the introduction of the major gold mineralisation mined prior to the 1980s, although some gold mineralisation may post-date this magmatic event (Hughes, Phillips and Gregory, 1997).

Geology of Australian and Papua New Guinean Mineral Deposits

VICTORIAN GOLD PROVINCE

On a broader scale, the evidence is compatible with a single period of gold introduction in any one field, with some diachroneity of that period across the province. Existing evidence permits the total range of time during which gold mineralisation occurred across the province to be at least 35 Myr. However, this corresponds to, and outlasts by at least 15 Myr, a minimum of 125 Myr of continuous turbidite sedimentation, and therefore corresponds to the final tectonic and thermal event at the close of this sedimentation. Remobilisation of existing gold mineralisation, including addition of some granite-related components, has occurred in the vicinity of some major plutons (eg Maldon) where postmineralisation dykes are closely associated with earlier quartz reefs which have probably acted as ideal plumbing systems for later fluids. There is little critical evidence to support the introduction of gold synchronous with sedimentation and/or Cambrian volcanism, nor to relate it to fluids directly sourced from individual plutons. There are several unresolved issues relating to timing of the mineralisation, and experience in other gold provinces suggests that their resolution might come from indirect studies involving field relationships and the dating of critical rock types, rather than direct attempts to date mineralisation and alteration.

STRUCTURAL GEOLOGY A number of structural studies (including a seismic transect in central Victoria) have led to significant advances in the understanding of the structural geology and especially how it relates to the distribution and controls of goldfields (Cox et al, 1991a, b, 1995; Gray, Wilson and Barton, 1991; Gray and Willman, 1991; Wilson et al, 1992). A major decollement surface near the boundary between the Cambrian metabasalt sequence and the Cambrian to Devonian flysch sequence is the sole to a series of west-dipping listric faults in the Stawell to Melbourne zones, and there may be a further decollement surface at the base of the Cambrian metabasalt, as well as duplexing within it. The dominance of this listric geometry rather than staircase faulting has been attributed to the preponderance of monotonous mudstone (slate, shale, siltstone) rather than sandstone, especially in the Ballarat zone. The major change from these west-dipping faults to SW vergence of structures occurs in the Tabberabbera zone and approximates to the eastern boundary of most major goldfields and thin-skinned tectonics (Scheibner, 1992). Major goldfields do occur in the Tabberabbera zone (eg Chiltern), but within rocks which could otherwise have been derived structurally from the Ballarat zone. This structural work has also indicated more complexity in the Melbourne zone than previously recognised, that included south-vergent thrusting, two periods of folding and an overprinting cleavage, all in the Devonian (Gray and Mortimer, 1996; Morand, Hughes and Jones, 1997). The understanding gained for the Archaean deposits of the relationship between far-field stress, orientation of rock units, and gold deposits (Ridley, 1993) can be translated to the Victorian province. In a flysch sequence where the chemically favourable host slates are incompetent compared to the surrounding rocks, dilational sites will occur where bedding in these units is oriented parallel to the far-field stress prior to deformation, resulting in saddle reefs. However, where the favourable host is more competent than the surrounding rocks, such as in greenstone belts and in thick mafic dykes, dilation will occur where units are oriented perpendicular to the far-

Geology of Australian and Papua New Guinean Mineral Deposits

field stress, resulting in stockworks and ladder veins. In part, this link accounts for the remarkable similarities between the Mount Charlotte deposit at Kalgoorlie and the A1 deposit near Woods Point, for example. The similarity of host rocks and auriferous fluid also contributes to the similarity of structural control, mineralisation and alteration between these two deposits. Closer investigation of the structures in major Victorian fields indicates a wide range of structures with relatively constant relationships to the regional stress field (eg Phillips and Hughes, 1995, see Fig 3), and also defines a common theme of repetition of structures in all major fields (eg stacked saddle reefs, multiple ladder veins and multiple reverse faults).

METAMORPHISM AND MAGMATISM An important advance in the knowledge of metamorphic petrology arose from the study of the Palaeozoic rocks in the vicinity of Stawell gold mine (Wilson et al, 1992). The thermodynamic modelling of reactions in mafic rocks applicable to the Stawell sequence has led to a much wider application to gold deposits of many ages (Powell, Will and Phillips, 1991). This modelling suggested that low grade metamorphism of carbonate-bearing mafic rocks should evolve a distinctive fluid containing water and carbon dioxide in proportions buffered by the mineral assemblage and modestly dependent upon pressure and temperature, and that the bulk of fluid evolution should accompany the greenschist to amphibolite facies transition. Comparison with reported natural fluids suggests that the theoretically predicted fluid is uncommon in nature and in most ore systems, but is virtually identical to that identified in a number of gold-only systems including slate belts, Archaean greenstone, Carlin and Witwatersrand gold deposits. To date this metamorphic fluid model is the only genetic model that predicts the composition of gold-only fluids. The model also underpins many of the links between slate belt gold provinces such as Victoria, and other gold-only types (Phillips and Powell, 1993). In a detailed study of the granitic rocks of the Lachlan Fold Belt (Chappell et al, 1991), the Central Victorian magmatic province stands out for its abundance of Late Devonian, posttectonic plutons. Some of these plutons are very high level (ie hot and water-undersaturated), and several are spatially and genetically associated with felsic volcanic centres, all confirming a major thermal event during the Devonian. This period was immediately preceded by the intrusion of ultramafic to intermediate dyke swarms, suggesting that the thermal event is deeper than just crustal in extent. Using this granite compilation for all of the Victorian gold province, it becomes clear that there is no positive correlation between gold production and granites (in fact, a negative correlation exists), and that gold mineralisation in or near granites is associated with all granite types, including I- and Stypes, fractionated and unfractionated, and mafic and felsic types (Hughes, Phillips and Gregory, 1997). Granites make up around 20% of the Victorian gold province yet deposits within granites have yielded less than 10 t of gold, mostly from outside the main goldfields region, suggesting that the non-granite parts of the Victorian gold province are two orders of magnitude more prospective for gold than the granites (Phillips, 1996). On the basis of what is known of Victorian gold mineralisation and of the Maldon deposit, and what is known in comparable gold-only provinces globally, this underrepresentation of granites as host rocks for gold-only deposits appears widespread and predictable.

501

G N PHILLIPS and M J HUGHES

LACHLAN FOLD BELT IN VICTORIA DEFINING THE TECTONIC SETTING AND TECTONIC BOUNDARIES The nature of the unexposed basement and the tectonic setting of Victoria have been contentious issues for many years (Gray, 1988), and it would appear that some of this uncertainty has arisen because the Lachlan Fold Belt is most unlike many other orogenic belts (eg Coney et al, 1990). This dissimilarity with other Phanerozoic orogenic belts is based on a number of unusual features (Coney et al, 1990; Coney, 1992), including its 800 km easterly width, its 50% shortening, lack of Precambrian basement rocks, lack of evidence for any great elevation in the past, lack of deep metamorphic rocks, and high proportion of Siluro-Devonian acid magmatic rocks. Similarities have been drawn between the Lachlan Fold Belt and Archaean greenstone belts, especially regarding their tectonics (Coney et al, 1990). There is some consensus that there was an ocean floor, island arc and subduction setting during the Cambrian, with extensive continental margin, back arc or marginal sea sedimentation on thin cratonic blocks in the Ordovician, perhaps akin to the present Bengal fan of Fergusson and Coney (1992). Episodic deformation started in the Cambrian but essentially ceased in the Lachlan Fold Belt by the Early Carboniferous with a further change to more continental and rift-related sedimentation and volcanism. Three boundaries are critical to fully understanding the Palaeozoic of Victoria, these being the Mount Wellington fault zone on the east of the Melbourne zone, the Wonnangatta line within the Tabberabbera zone, and the Kiewa fault on the east of the Tabberabbera zone (Fig 1; Collins and Vernon, 1992; Glen, 1992; Glen, Scheibner and VandenBerg, 1992; Scheibner, 1992; Collins, 1994). These boundaries are also important in understanding the controls of the distribution of gold mineralisation (Fig 2). The Mount Wellington fault zone (used here in the sense of the bounding fault zone which passes through Cambrian greenstone of the Dolodrook inlier and eastern Howqua River), has been interpreted as the boundary between the allochthonous Stawell, Ballarat and Melbourne zones of the Melbourne terrane, and the Tabberabbera zone (part of the Benambra terrane). Transport of the Melbourne terrane from further south prior to 390 Myr (Glen, 1992; Glen, Scheibner and VandenBerg, 1992), or of the Tabberabbera zone from further north, have been proposed. The Mount Wellington fault zone is an important boundary with dominantly calc-alkaline and MORB Cambrian rocks associated with the Barkly River fault zone, with a Devonian age for D1 and east vergence all to the west of the Mount Wellington fault. To the east of the same fault are MORB basalts and ultramafic rocks (the latter directly associated with the Mount Wellington fault zone) which have a pre-Devonian age for D1 and SW vergence. There is a change of granite type across this boundary, with Early and Middle Devonian granites confined to the east, whereas the granites to the west are virtually all part of the Central Victorian magmatic province. The Wonnangatta line within the Tabberabbera zone is a distinctive fault zone marking the boundary between a domain of single, SW-verging deformation to the west, and more complexly deformed rocks to the NE. This has been interpreted as the boundary between thin-skinned tectonics to the west, and thick-skinned tectonics further east (Scheibner, 1992). It has

502

recently been suggested that the age of some rocks in this SWverging zone may be Silurian, rather than Ordovician (Fergusson, 1996). The Kiewa fault separates low grade metamorphic rocks to the west from medium to high grade metamorphic rocks of the Omeo zone over much of its length (Morand, 1990). It is also a boundary based on the strontium isotopic composition of granite (Gray, 1990), and the eastern boundary of all significant gold-only deposits (Phillips and Hughes, 1995). Some of these boundaries have been explained in terms of tectonic processes. Collins (1994) inferred two periods of eastward migrating deformation, one east of the Kiewa fault during the Silurian, and one from the Stawell zone to the Kiewa fault in the Devonian. This pattern of deformation was explained by eastward delamination of the mantle lithosphere, but does not yet explain the pre-Devonian deformation in the Tabberabbera zone. The model, however, does help to explain the wide distribution of low pressure metamorphic and granitic rocks. Recent mapping of critical areas in Victoria has produced substantial revisions of the geology near the western margin of the Lachlan Fold Belt (Simpson and Woodfull, 1994; Cayley, 1995), and has indicated a complex relationship between gold, granites and deformation in central to western Victoria. The western margin is now considered by some workers to be the Moyston fault, east of the Grampians. Mapping of Silurian rocks of the Grampians area has indicated repetition through thrusting (Cayley, 1995; Cayley and Taylor, 1996) not recognised in earlier mapping, and significant deformation of the Grampians Group prior to intrusion of Early Devonian granites. Some of the relationships between folding, gold and granites can be rationalised if the Tabberabberan deformation, which is classically Middle Devonian in the Melbourne and Tabberabbera zones (but a second subordinate phase of deformation in the Tabberabbera zone), is diachronous across the province and slightly older in the west. This would be compatible with the slightly older magmatism in the west.

VICTORIA AND OTHER ‘GOLD-ONLY’ GOLD PROVINCES Comparison between the Victorian and the two other major synorogenic (‘mesothermal’) Phanerozoic gold provinces of the Pacific Rim (ie including Alaska and the Mother Lode of the Cordilleran of western North America, and the Russian Far East and Eastern Eurasia provinces) puts some focus on those aspects which might be essential in the generation of a major gold province, and those that may be less important. Eastern Eurasia and the North American Cordilleran provinces formed during the Phanerozoic when widespread Aand B-type thrusting and transcurrent faulting exposed a range of crustal levels and various ‘suspect terranes’ were accreted. In contrast, deformation and magmatism in the Tasman Orogenic Zone involved little amalgamation of terranes (especially for the Lachlan Fold Belt west of the Tabberabbera zone) and only a limited crustal section was exposed during ocean-directed thrusting in the Victorian province. A very different tectonic setting for Victoria, relative to the other provinces, is therefore indicated, although transcurrent faulting and reactivation of thrusts are common. It has been concluded that the thermal regime and its influence on fluid generation were more important in generating these major gold provinces

Geology of Australian and Papua New Guinean Mineral Deposits

VICTORIAN GOLD PROVINCE

than any particular tectonic setting (Goldfarb, Nokleberg and Phillips, 1996), although the tectonic regime may influence the plumbing system and the distribution of goldfields within a province. These more global studies also allow comparison of slate belt gold provinces with other gold types. Slate belt gold provinces have many obvious differences from greenstone, Witwatersrand and Carlin deposits, and not the least of these differences is the nature of the host sequence. However, closer inspection of these styles suggests that low salinity fluids play a role in many if not most examples, and a major thermal event is usually another key part of mineralisation. Furthermore, despite the great variation in host rocks, the role of host chemical composition can be relatively easily rationalised with the chemical behaviour of gold in low salinity fluids. It would thus appear that there are substantial differences between the major gold provinces with regard to depositional-type features, but there may be many fewer differences when considering the composition of the fluids, and in some cases the source region for the fluids and the processes which operate there.

COMMON FACTORS IN VICTORIAN GOLD GENESIS There are numerous features common to many primary Victorian gold deposits, and there are others that show great diversity. Common features include the universal structural control, the carbonate–white mica–sulphide alteration, the low salinity water–carbon dioxide fluid, the chemical composition of host rocks to major deposits, inferred relationship to a major thermal event, and a metal association dominated by gold, arsenic, antimony, potassium and sulphur with low silver, copper, zinc and lead. Great diversity exists in deposit geometry and type of structural control, the alteration minerals and their proportions, the abundance of particular granite-related components such as bismuth, molybdenum and tungsten, the type of host rock, the age of host rocks, distance to nearest granites and type of adjacent granite (if any). Importantly, the similarities can be summarised as a commonality of some deep crustal features such as fluid composition, presence of major fluid channelways and thermal event. The differences imply diversity of the upper crustal features at or near the depositional site such as host rock, host structure, alteration assemblage and spatial relationship to granites.

UNANSWERED QUESTIONS RELEVANT TO VICTORIAN GOLD GEOLOGY As understanding of the Victorian geological framework and gold improves, some aspects emerge that need resolution prior to advancing gold genetic models further. As the youngest host to major goldfields, the Woods Point dyke swarm is critical to the chronology. From stratigraphic evidence, the dykes post-date Early Devonian metasedimentary rocks, and are assumed to pre-date the weakly folded Late Devonian to Carboniferous molasse sequence which they do not intrude, and this is all compatible with their age of 380 Myr. However, this is an old K-Ar age (Marsden, 1988) and its accuracy might be dramatically improved with modern methods. Dating of the Woods Point dyke swarm together with better age constraints on the unfossiliferous, presumed Middle Devonian strata, would assist the unravelling of key gold-related processes during the Devonian.

Geology of Australian and Papua New Guinean Mineral Deposits

The age of deformation(s) in the Ballarat and especially the Stawell zones is not well reconciled with tighter constraints for the Melbourne and Tabberabbera zones. A single age for Tabberabberan deformation of Middle to Late Devonian across the province is difficult to rationalise with existing observations, so either there are distinct and separate deformation events or the Tabberabberan deformation should be viewed as markedly diachronous. The ages of dykes in these zones, some of which appear to pre-date 395 Myr granites, might further constrain deformational events. Of more importance to gold metallogeny is the major thermal event in the Lachlan Fold Belt of Victoria and this seems well constrained by the Early–Late Devonian age of granite magmatism in the Melbourne, Ballarat and Stawell zones.

GENESIS OF VICTORIAN GOLD DEPOSITS Until recently, the genesis of Victorian gold deposits has mostly relied on information from within the Victorian gold province and has not fully exploited the additional constraints placed by gold geochemistry and global gold metallogeny. Many ideas have been proposed based upon studies at isolated goldfields but until recently virtually none have covered the whole province. Almost all gold genetic models have been represented in the literature on Victorian deposits, including models which have invoked exhalative gold, Ordovician gold mineralisation, gold introduction from Cambrian to Devonian times, granite-derived, lamprophyrerelated gold, and a metamorphic fluid model for gold (Wall and Ceplecha, 1976; Cox et al, 1991b; Phillips, 1991). However, there has been little critical evaluation or testing of some of the proposals. Our preferred model for the genesis of the major Victorian gold deposits involves a metamorphic fluid derived from beneath the Early Palaeozoic flysch sequence during the Devonian thermal event. This low salinity fluid was water–carbon dioxide dominated, with a source possibly related to the pre-Ordovician mafic succession, but was emplaced along pathways that potentially incorporated minor components from higher crustal levels. Emplacement of the granites, mafic to ultramafic rocks (ie ‘diorite’ and ‘lamprophyre’ of the Woods Point dyke swarm) and felsic volcanic rocks was part of the same thermal event that generated the gold-bearing fluid, but these magmas all represent melting at considerably greater depths than the greenschist–amphibolite facies transition that sourced the auriferous fluid. Upward migration and focussing of the auriferous fluid was guided by decollement features, listric faults and, at shallow depth, smaller structures. Gold mineralisation occurred in all rock types that pre-dated the thermal event, but rocks rich in iron or carbon were most favourable for gold precipitation and hence hosted the largest fields, especially where they were fractured favourably. The most important gold-bearing region, the Ballarat zone, corresponds to the region with the greatest density of listric faults and the greatest concentration of chemically favourable host rocks. With the recognition of the close geological similarities between the Victorian gold province, other slate belt gold provinces, and many Archaean greenstone gold provinces, exploration ideas from these areas can be transferred to Victoria. For example, suggested genetic models for Victorian

503

G N PHILLIPS and M J HUGHES

gold deposits can be evaluated and ranked by looking at the effectiveness of such models in other terrains. Such an approach will not answer all the questions about the evolution of the Victorian gold province, but will provide a means of optimising the cost effectiveness of exploration and give a better chance for major discovery in Victoria, and elsewhere.

ACKNOWLEDGEMENTS The authors wish to acknowledge support and encouragement from members of the Geological Survey of Victoria and the Victorian Chamber of Mines, especially J O Reynolds and T W Dickson. Major funding to the authors from the Australian Research Council facilitated a number of specific research projects on Victorian goldfields, and especially the synthesis of the whole province and the global comparisons. Discussions with P Arden, D Arne, G Corbett, S Cox, R Goldfarb, D Groves, J Law, C Mawer, S McKnight, V Morand, R Powell, J Vearncombe, C Wilson and other colleagues have significantly influenced our ideas on Victorian gold. We thank Alliance Gold Mines NL for support and access at Maldon, and participants of the Vicgold 95 conference in Melbourne at which many of these ideas were either first aired or discussed in some detail.

REFERENCES Bowen, K G and Whiting, R G, 1975. Gold in the Tasman Geosyncline, Victoria, in Economic Geology of Australia and Papua New Guinea, Volume 1 Metals (Ed: C L Knight), pp 647–659 (The Australasian Institute of Mining and Metallurgy: Melbourne). Cayley, R A, 1995. Recent advances in understanding the structural evolution of Western Victoria, Geological Survey of Victoria Symposium, Abstracts, pp 5–7. Cayley, R A and Taylor, D H, 1996. Geological evolution and economic potential of the Grampians area, Victoria, Australian Institute of Geoscientists Bulletin, 20:11–18. Chappell, B W, English, P M, King, P L, White, A J R and Wyborn, D, 1991. Granites and related rocks of the Lachlan Fold Belt, 1: 1 250 000 scale map, Bureau of Mineral Resources, Geology and Geophysics, Canberra. Collins, W J, 1994. Upper- and middle-crustal response to delamination: An example from the Lachlan fold belt, eastern Australia, Geology, 22:143–146. Collins, W J and Vernon, R H, 1992. Palaeozoic arc growth, deformation and migration across the Lachlan Fold Belt, southeastern Australia, Tectonophysics, 214:381–400.

Douglas, J G and Ferguson, J A, (Eds) 1988. Geology of Victoria (Geological Society of Australia, Victorian Division: Melbourne). Fergusson, C L, 1996. Tectonic significance of Ordovician rocks of the Lachlan fold belt, southeastern Australia, Geological Society of Australia Abstracts, 41:137. Fergusson, C L and Coney, P J, 1992. Implications of a Bengal Fantype deposit in the Paleozoic Lachlan fold belt of southeastern Australia, Geology, 20:1047–1049. Fletcher, K, 1995. Exploration success at Stawell gold mine, in Proceedings 5th Victorian Resources Conference, pp 92–98 (J B Were & Son and Victorian Chamber of Mines: Melbourne). Glen, R A, 1992. Thrust, extensional and strike-slip tectonics in an evolving Palaeozoic orogen - a structural synthesis of the Lachlan Orogen of southeastern Australia, Tectonophysics, 214:341–380. Glen, R A, Scheibner, E and VandenBerg, A H M, 1992. Paleozoic intraplate escape tectonics in Gondwanaland and major strike-slip duplication in the Lachlan orogen of southeastern Australia, Geology, 20:795–798. Goldfarb R J, Nokleberg, W J and Phillips, G N, 1996. Tectonic setting of synorogenic gold deposits of the Pacific Rim, in Mesothermal Gold Deposits: A Global Overview, Publication 27, pp 22–28 (The Geology Department and University Extension, The University of Western Australia: Perth). Goldfarb, R J and Phillips, G N, 1995. Primary geochemistry of slatebelt gold deposits, in Proceedings 17th International Geochemical Exploration Symposium, Contribution 54, pp 236–239 (Economic Geology Research Unit, James Cook University of North Queensland: Townsville). Goldfarb, R J, Snee, L W and Pickthorn, W J, 1993. Orogenesis, high-T thermal events, and gold vein formation within metamorphic rocks of the Alaskan Cordillera, Mineralogical Magazine, 57:375–394. Gray, C M, 1990. A strontium isotopic traverse across the granitic rocks of southeastern Australia: Petrogenetic and tectonic implications, Australian Journal of Earth Sciences, 37:331–349. Gray, D R, 1988. Structure and tectonics, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 1–36 (Geological Society of Australia, Victorian Division: Melbourne). Gray, D R and Mortimer, L, 1996. Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt, Australian Journal of Earth Sciences, 43:103–114. Gray, D R and Willman, C E, 1991. Deformation in the Ballarat Slate Belt, central Victoria, and implications for the crustal structure across southeast Australia, Australian Journal of Earth Sciences, 38:171–201. Gray, D R, Wilson, C J L and Barton, T J, 1991. Intracrustal detachments and implications for crustal evolution within the Lachlan fold belt, southeastern Australia, Geology, 19:574–577.

Coney, P J, 1992. The Lachlan belt of eastern Australia and CircumPacific tectonic evolution, Tectonophysics, 214:1–25.

Hughes, M J, Ho, S E and Hughes, C E, 1996. Recent developments in Victorian geology and mineralisation, Australian Institute of Geoscientists Bulletin, 20.

Coney, P J, Edwards, A, Hine, R, Morrison, F and Windrim, D, 1990. The regional tectonics of the Tasman orogenic system, eastern Australia, Journal of Structural Geology, 12:519–543.

Hughes, M J, Phillips, G N and Gregory, L M, 1996. Victorian gold: II: Large gold deposits and granites, Geological Society of Australia Abstracts, 41:204.

Cox, S F, Etheridge, M A, Cas, R A F and Clifford, B A, 1991a. Deformational style of the Castlemaine area, Bendigo–Ballarat Zone: Implications for evolution of crustal structure in central Victoria, Australian Journal of Earth Sciences, 38:151–170.

Hughes, M J, Phillips, G N and Gregory, L M, 1997. Mineralogical domains in the Victorian Gold Province, Maldon, and Carlin-style potential, in Proceedings 1997 AusIMM Annual Conference, pp 215–227 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Cox, S F, Sun, S-S, Etheridge, M A, Wall, V J and Potter, T F, 1995. Structural and geochemical controls on the development of turbidite-hosted gold quartz vein deposits, Wattle Gully mine, central Victoria, Australia, Economic Geology, 90:1722–1746. Cox, S F, Wall, V J, Etheridge, M A and Potter, T F, 1991b. Deformational and metamorphic processes in the formation of mesothermal vein-hosted gold deposits - examples from the Lachlan Fold Belt in central Victoria, Australia, Ore Geology Reviews, 6:391–423.

504

Mapani, B S E and Wilson, C J L, 1994. Structural evolution and gold mineralization in the Scotchmans fault zone, Magdala gold mine, Stawell, Western Victoria, Australia, Economic Geology, 89:566–583. Marsden, M A H, 1988. Upper Devonian-Carboniferous, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 147–194 (Geological Society of Australia, Victorian Division: Melbourne).

Geology of Australian and Papua New Guinean Mineral Deposits

VICTORIAN GOLD PROVINCE

Morand, V J, 1990. Low-pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia, Journal of Metamorphic Geology, 8:1–12.

Ridley, J R, 1993. The relations between mean rock stress and fluid flow in the crust: With reference to vein- and lode-style gold deposits, Ore Geology Reviews, 8:23–37.

Morand, V J, Hughes, M J and Jones G N, 1997. Discussion: Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt, Australian Journal of Earth Sciences, 44:145–148.

Scheibner, E, 1992. Influence of detachment-related passive margin geometry on subsequent active margin dynamics: applied to the Tasman Fold Belt System, Tectonophysics, 214:401–416.

Phillips, G N, 1991. Gold deposits of Victoria: a major province within a Palaeozoic sedimentary succession, in World Gold ‘91, pp 237–245 (The Australasian Institute of Mining and Metallurgy: Melbourne). Phillips, G N, 1996. Maldon goldfield, Victorian gold, and granites, Geological Society of Australia (Tasmanian Division), Abstracts, July 1996, 1. Phillips, G N, Eshuys, E and Hellsten, K, 1996. Integrated exploration techniques for Archaean gold, Western Australia, in Proceedings 1996 AusIMM Annual Conference, pp 289–293 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Simpson, C J and Woodfull, C J, 1994. Geological note: New field evidence resolving the relationship between the Grampians Group and the Rocklands Rhyolite, western Victoria, Australian Journal of Earth Sciences, 41:621–624. Taylor, D, 1995. Gold mineralisation controls in the Ballarat area, Geological Survey of Victoria Symposium Abstracts, pp 3–4. Thomas, D E, 1988. The Ballarat Goldfield, in Bicentennial Gold 88, Excursion No 2 Guide, Central Victorian Gold Deposits, Publication 13 (Ed: D G Jones), p 69 (The Geology Department and University Extension, The University of Western Australia: Perth).

Phillips, G N and Groves, D I, 1983. The nature of Archaean goldbearing fluids as deduced from gold deposits of Western Australia, Journal of the Geological Society of Australia, 30:25–39.

Wall, V J and Ceplecha, J C, 1976. Deformation and metamorphism in the development of gold-quartz mineralisation in slate belts, in 25th International Geological Congress, Sydney, Abstracts, 1, pp 142–143.

Phillips, G N and Hughes, M J, 1995. Victorian gold: a sleeping giant, Society of Economic Geologists Newsletter, 21:1, 9–13.

Willman, C E, 1995. Castlemaine goldfield. Castlemaine-Chewton, Fryers Creek, Geological Survey of Victoria Report 106.

Phillips, G N and Hughes, M J, 1996. The geology and gold deposits of the Victorian gold province, Ore Geology Reviews, 11:255–302.

Wilson, C J L, Will, T M, Cayley, R A and Chen, S, 1992. Geologic framework and tectonic evolution in Western Victoria, Australia, Tectonophysics, 214:93–127.

Phillips, G N and Powell, R, 1993. Link between gold provinces, Economic Geology, 88:1084–1098. Plowman, S J, 1994. Message from the minister, Australian Mining, 86:19. Powell, R, Will, T M and Phillips, G N, 1991. Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization, Journal of Metamorphic Geology, 9:141–150.

Woodall, R, 1979. Gold - Australia and the world, in Gold Mineralization, Publication 3 (Eds: J E Glover and D I Groves), pp 1–34 (The Geology Department and University Extension, The University of Western Australia: Perth). Xu, G, Powell, R, Wilson, C J L, Will, T M, 1994. Contact metamorphism around the Stawell granite, Victoria, Australia, Journal of Metamorphic Geology, 12:609–624.

Ramsay, W R H and Willman, C E, 1988. Gold, in Geology of Victoria, (Eds: J G Douglas and J A Ferguson), pp 454–481 (Geological Society of Australia, Victorian Division: Melbourne).

Geology of Australian and Papua New Guinean Mineral Deposits

505

506

Geology of Australian and Papua New Guinean Mineral Deposits

Zurkic, N, 1998. Fosterville gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 507–510 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Fosterville gold deposits 1

by N Zurkic

INTRODUCTION The deposits are 25 km east of Bendigo in central Victoria, at lat 36o42′S, long 144o30′E, on the Bendigo (SJ 55–1) 1:250 000 scale map sheet (Fig 1). They were developed in a historic goldfield, although the Fosterville mineralisation differs from the typical Victorian discrete quartz vein deposits. Here there is only a minor association between gold and quartz veins, and gold is predominantly impregnated in porous sandstone.

TABLE 1 Fosterville resources and reserves summary at 30 December 1996. Resources1

Measured

Indicated

Inferred

Gold ‘000 t Gold ‘000 t Gold grade grade grade g/t g/t g/t

Oxide

‘000 t

(0.5 g/t Au cutoff)

2970

1.2

1581

1.0

660

0.9

(1.2 g/t Au cutoff)

2500

3.2

2162

2.5

1770

2.2

(0.5–1.19 g/t Au cutoff)

1495

0.8

1885

0.8

2282

0.8

Sulphide

Reserves

Proved

Oxide

‘000 t

(0.5 g/t Au cutoff)

1473

1

Probable

Gold ‘000 t Gold grade grade g/t g/t 1.4

506

1.3

Resources are inclusive of Reserves.

oz per year and is expected to be lifted by an additional 50 000 oz per year with the commencement of sulphide ore production in 1997.

EXPLORATION AND MINING HISTORY The field was a low grade producer, discovered in 1894. Historic production to 1982 was 1588 kg of gold, at a recovered grade of 3 g/t, mostly from open pits and underground stopes in the oxide-sulphide transition zone, between 1894 and 1909. This included production by Bendigo Gold Limited (BGL), from 1982 to 1984, of 108 kg of gold by the CIP retreatment of battery tailings. After virtual abandonment of the field by 1910, no significant work took place until 1973. Between 1973 and 1983, Lone Star Exploration NL, Noranda Australia Ltd and Apollo International Minerals NL carried out limited exploration including some drilling. These companies concluded that the target potential did not meet their tonnage criteria.

FIG 1 - Locality map.

Production at Fosterville by Perseverance Exploration Pty Ltd (Perseverance) to 30 June 1996 was 2.594 Mt of ore at 1.5 g/t gold, at a waste:ore ratio of 3.58:1, to yield 96 000 oz of gold. All production was from oxide ore, from a series of shallow pits on the Fosterville and O’Dwyers fault systems. Proved and Probable Ore Reserves in the oxide zone are 1.98 Mt at 1.4 g/t gold at a 0.5 g/t cutoff (Table 1). Current production is 40 000

1.

Senior Geologist, Perseverance Exploration McCormicks Road, Fosterville Vic 3557.

Geology of Australian and Papua New Guinean Mineral Deposits

Pty

Ltd,

BGL drilled more than 450 reverse circulation (RC) holes by mid 1989, following mapping, sampling and orientation bedrock geochemistry by BGL geologists in the period 1985 to 1987. Bulk samples of sulphide ore for metallurgical testing were obtained from a 30 m exploration shaft. BGL reported ‘an unclosed resource of approximately 3.8 Mt at a grade of 2.5 g/t gold’. This included an oxide reserve of 1 Mt at 2.6 g/t gold (McConachy and Swensson, 1990). In 1991 Brunswick NL began production from the Central Ellesmere, Fosterville and Robbins Hill open pits, but the operations closed in mid 1991 due in part to a high level of

507

N ZURKIC

project debt. In 1992 Perseverance acquired the deposit and began small scale mining of the Fosterville open pit and completed treatment of the heap leach ore produced by Brunswick NL. When Perseverance acquired the deposit the combined Measured, Indicated and Inferred Resources of oxide ore were 2.65 Mt at 1.9 g/t gold. The Inferred Resource of sulphide ore was 1 Mt at 2.9 g/t gold. Proved and Probable Reserves of oxide ore were 974 000 t at 1.7 g/t gold. Exploration for oxide ore deposits led to the discovery of a substantial sulphide resource below the Central North oxide mineralisation (Fig 3). This resource, with the known sulphide resource at Central Ellesmere, drew attention to the potential for production from sulphide ore. In 1994 it was decided to fully explore the central 4 km length of the Fosterville Fault, from the Harrington’s Hill area in the south to the Fosterville pit area in the north (Fig 2), to evaluate the potential for sulphide ore with the results shown in Tables 1, 2 and 3.

TABLE 2 Fosterville oxide and sulphide resources, 30 December 1996.

Measured

Indicated

Inferred

‘000 t Gold ‘000 t Gold ‘000 t Gold grade grade grade g/t g/t g/t Oxide (0.5 g/t cutoff) Robbin’s Hill

109

1.2

121

1.1

325

1.0

Central North

249

1.2

226

1.0

50

1.0

Central Ellesmere

54

1.1

66

0.9

79

1.0

Sharkey’s

193

0.8

176

0.8

34

0.7

Daley’s Hill

512

1.4

314

1.3

19

1.1

Farley’s

146

1.4

41

1.3

4

0.8

Rehe’s

53

0.9

14

0.8

0

0.0

Harrington’s Hill

156

1.3

190

1.0

106

0.9

O’Dwyers

987

1.0

243

0.9

12

0.7

Fosterville

276

1.1

147

1.0

30

0.7

Read’s

235

1.5

43

1.3

1

0.9

Sulphide (1.2 g/t cutoff) Robbin’s Hill

3

1.9

2

1.8

94

2.0

Central North

932

4.2

538

3.5

398

2.8

Central Ellesmere

825

3.0

818

2.5

535

2.1

Sharkey’s

51

1.6

53

1.5

51

1.5

Daley’s Hill

278

2.4

282

1.8

253

1.7

Farley’s

40

3.0

10

2.7

4

2.5

Harrington’s Hill

228

2.3

270

2.1

268

2.2

O’Dwyers

67

2.6

109

2.2

125

2.0

Fosterville

35

1.8

25

1.6

8

1.4

Read’s

41

1.5

55

1.6

34

1.7

Sulphide (0.5–1.19 g/t) Robbin’s Hill

3

0.8

3

0.8

227

0.8

Central North

184

0.8

137

0.9

165

0.9

Central Ellesmere

279

0.8

361

0.9

352

0.8

Sharkey’s

136

0.8

226

0.8

205

0.8

Daley’s Hill

207

0.8

239

0.8

205

0.8

Rehe’s Harrington’s Hill FIG 2 - Geological plan of the Fosterville area.

A feasibility study on bacterial oxidation of sulphide ore is near completion which will allow for some 300 000 oz of gold to be added to the sulphide reserve base.

REGIONAL GEOLOGY The Fosterville goldfield is in the Ballarat Trough of the Lachlan Fold Belt. The country rock is an Ordovician (Lancefieldian) turbidite sequence of alternating mudstone, siltstone and sandstone, common throughout Victoria. The Fosterville Fault, which defines the eastern limit of the northtrending Strathfieldsaye Synclinorium, transects the sequence (Cas and VandenBerg, 1988). The generally tightly folded

508

1

0.6

0

0

1

0.5

192

0.8

307

0.8

331

0.9

O’Dwyers

279

0.7

349

0.7

594

0.7

Fosterville

186

0.7

230

0.7

190

0.7

Read’s

28

0.9

33

0.9

12

0.9

sequence is poorly exposed except for the sandstone of the Sugarloaf Range to the west. The Fosterville Fault is a major, steep, west dipping fault. The fault zone has mostly undergone west side up sinistral and reverse movement, with some late stage sinistral strike slip (S King, unpublished data, 1996). It appears that shearing was transferred in the NE direction via an en echelon set of structures (Fig 2) up to and beyond the O’Dwyers line. On the O’Dwyers line a rhyolitic porphyry dyke has intruded the main O’Dwyer’s shear structure.

Geology of Australian and Papua New Guinean Mineral Deposits

FOSTERVILLE GOLD DEPOSITS

TABLE 3 Fosterville oxide ore reserves, 30 December 1996. Proved

Probable

‘000 t

Gold grade g/t

‘000 t

Gold grade g/t

Daley’s Hill

437

1.5

256

1.4

Central North–Vanessa’s

88

1.4

26

1.5

Oxide (0.5 g/t cutoff)

Sharkey’s

16

1.0

10

1.1

Farley’s

119

1.5

25

1.5

Rehe’s

57

1.3

0

0

Fosterville

29

1.8

19

2.3

Read’s

170

1.7

26

1.5

O’Dwyers

495

1.2

102

1.0

Harrington’s Hill–Sandhurst

62

1.1

42

1.0

Mineralisation is closely related to fault and shear structures, which are generally steeply dipping (Fig 3) and strike subparallel to the fold axes but cut across these at an oblique angle. The most favourable zones for mineralisation are :1.

major faults, which provide a conduit for fluids from depth,

2.

fold axes, where a zone of weakness has been created,

3.

favourable rock types which fracture, and are porous but also provide open spaces through brecciation, and

4.

an interplay of folding, faulting and favourable rock type which produces broad areas of brecciation conducive to gold deposition.

MINERALISATION Natural oxidation to depths of 30 to 60 m has released the gold in finely particulate form, with diameter of 1 to 10 µm. The gold particles occur within iron oxide, both being formed by the breakdown of the sulphide minerals. The iron oxide forms veins and fracture fill. The mineralisation occurs as zones of dispersed iron oxides with some remobilised silica. The most prospective zones were previously believed to be concentrated around the most extensive areas of primary quartz-sulphide veining, stockworking and silicification prior to oxidation. However, growing evidence suggests that this is not crucial, as high grade gold mineralisation is also homogenously disseminated through porous sandstone and to a minor extent in siltstone, with little or no stockworking or quartz veining. The primary mineralisation is a quartz-pyrite-arsenopyrite assemblage with gold occurring in the free state in minor amounts but principally within pyrite and arsenopyrite as free grains with a diameter of 1 to 10 µm. The importance of this ‘quartz-poor’ mineralisation style is that, historically, production in Victoria has been from quartz reef and alluvial systems and past exploration efforts have almost solely targeted ‘reef’ style mineralisation.

FIG 3 - Cross section on line 9033 m N, Central North pit, looking north, showing the Fosterville Fault mineralisation displaced by secondary faulting.

ORE DEPOSIT FEATURES HOST LITHOLOGY AND STRUCTURAL CONTROLS There is some lithological control of mineralisation. Sandstone with brittle fracturing has provided a favourable host to open space filling by vein quartz. In addition some mineralisation shows a homogenous ‘impregnation’ of sulphides of the porous sandstone adjacent to shear structures, with negligible quartz veining. To the NE of the Fosterville Fault, porphyritic rhyolite dykes also provide a favourable ore host, as they have a similar competency to the sandstones.

Geology of Australian and Papua New Guinean Mineral Deposits

Ore zones generally conform in shape and trend to the broader fault zones within which they exist. Ore zones may be between 2 and 30 m wide depending on the interaction of structure and rock type. On average, ore zones are 5 m wide with strike lengths to 400 m, dip to the west and are open at depth. Secondary structures (Fig 3) produce areas of higher gold grade mineralisation which may trend up to 20o off the main north (mine grid) mineralisation trend. These high grade bands are generally narrow and can only be mined in conjunction with the low grade surrounding material.

EXPLORATION METHODS The position of the Fosterville Fault and associated O’Dwyers porphyry are well known. The Fosterville Fault can be traced from the south, where it is concealed by Tertiary basalt, northward for approximately 13 km, to the north end of the mining lease (Fig 1) where it is hidden by alluvial clay cover. The fault can be traced clearly, along topographic highs and through mine workings and in unmined areas, using bedrock geochemistry. Values in excess of 100 ppb gold and 100 ppm arsenic outline currently known deposits and can be used as target thresholds. Antimony and arsenic show a close relationship with gold.

509

N ZURKIC

The intensity of mineralisation is highly variable in the fault structures (Fig 4). This leads to the conclusion that both contacts of the fault zones need to be drilled on an initial 50 m spacing to satisfactorily evaluate mineralisation. Geophysical techniques are of limited use as the position of the structure is not the issue, but rather the intensity of mineralisation. Induced Polarisation (IP) surveys have been successful in defining areas of sulphide concentrations or shallower sulphides, although generally the sulphide content is not high enough to give a wholly reliable IP response.

collected from blast holes drilled on a 5 by 2 m pattern. Grade control is by a pit technician who directs the excavator, with the ore margins defined by a combination of visible fault boundaries, assays from blast holes and, on the deeper benches, metallurgical criteria. The technician uses mining plans prepared by the mine geologist who models the ore boundaries on the bench from kriged blocks derived from the blast hole assays. Monthly reconciliations are made with the grade control model and the ore reserve model. Reserves are periodically updated to accommodate accumulating grade control and exploration data. Ore treatment is by conventional cyanide heap leaching. Characterisation of mineralisation by its level of oxidation is important, as gold recovery changes from 85% for surface ore to 1 or 2% at the base of oxidation. The primary mineralisation is refractory and gold can not be extracted from the sulphides by leaching alone. Recent metallurgical testwork reviewed a range of possible oxidation techniques, with bacterial oxidation the preferred outcome.

CONCLUSIONS As oxide ore is depleted over the next few years, the future of the Fosterville project will depend on the mining and treatment of sulphide ore. The feasibility study to be completed by early 1997 proposes that some 2.5 Mt of sulphide ore will be mined from three open pits extending to 130 m depth on the central portion of the Fosterville Fault. The 500 000 tpa bacterial oxidation treatment plant is planned for production in late 1997. Sulphide mineralisation in the Daley’s Hill and O’Dwyers areas will probably be mined from the fourth and fifth sulphide open pits.

FIG 4 - Cross section of the Central Ellesmere deposit, on line 7635 m N, looking north, showing the Fosterville Fault mineralisation.

The high grade mineralisation in the hanging wall of the Fosterville Fault in the Central North area may be won by underground mining in the future, when production of a combination of open pit and underground sulphide ore should maintain Fosterville’s output at about 100 000 oz per year.

ACKNOWLEDGEMENTS Large diameter conventional face sampling RC holes are used for resource evaluation. RC is preferred to diamond core due to the large sample provided. This is considered more useful than structural information from diamond drilling which is less relevant for open pit mining on a relatively simple structure. Drilling patterns on the Fosterville Fault are generally 25 by 25 m, and 20 by 20 m on the O’Dwyers porphyry deposits. The RC drill hole chips are logged and 2 m composites are subsampled and assayed for gold and a variety of elements useful as geochemical indicators or for metallugical quality control. Sulphur, carbon and iron levels are determined in material from potential ore zones only, as the proposed bacterial oxidation process requires a homogenous feed with respect to these elements.

MINE GEOLOGICAL METHODS

Appreciation is extended to the management of Perseverance Corporation Ltd for permission to publish this paper. The work of the geological team associated with the project is acknowledged. Thanks are also extended to S King of ERAMAPTECH for specialist structural interpretation of the Fosterville field.

REFERENCES Cas, R A F and VandenBerg, A H M, 1988. Ordovician, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63–102 (Geological Society of Australia, Victorian Division: Melbourne). McConachy, G W and Swensson, C G, 1990. Fosterville gold field, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1297–1298 (The Australasian Institute of Mining and Metallurgy: Melbourne).

A nominal mining cutoff grade of 0.5 g/t gold is used for oxide ore. The orebody is mined on 2.5 m flitches, and samples are

510

Geology of Australian and Papua New Guinean Mineral Deposits

McDermott, G J and Quigley, P W, 1998. Williams United gold deposit, Bendigo, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 511–516 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Williams United gold deposit, Bendigo 1

2

by G J McDermott and P W Quigley INTRODUCTION

The deposit is in Mining Lease 1345 at the northern end of the Bendigo Goldfield on the New Chum Anticline (Fig 1). It is 150 km NNW of Melbourne, Vic, at lat 36 o43′S, long 144o15′E on the Bendigo (SJ 55–1) 1:250 000 and Bendigo (7724) 1:100 000 scale map sheets.

Bendigo Mining NL (BMNL) has held exploration and mining licences over the Bendigo Goldfield since 1993, and has delineated a near surface open pittable deposit within the Williams United area. A Probable Ore Reserve of 1.3 Mt at 1.0 g/t for 42 000 oz of contained gold has been estimated for the deposit. This paper examines the methodology used to evaluate the complex and spatially erratic grade distribution of the deposit.

EXPLORATION AND MINING HISTORY The Bendigo Goldfield is Australia’s second largest field in terms of past production. Gold mining in Bendigo began in 1851 with the discovery of alluvial gold in Bendigo Creek. Hard rock mining commenced in 1853 and average annual production exceeded 200 000 oz until 1915. Several hundred quartz reefs were mined between 1853 and 1954, to a maximum depth of 1400 m, from 12 main parallel anticlines within a zone 4 km wide by 15 km long. Over 1300 companies held small mining leases over the goldfield and its surroundings. The closure of the Central Deborah and North Deborah mines in 1954 marked the end of over a hundred years of reef mining. The discovery of gold on the New Chum Anticline was relatively early in the mining history of Bendigo. Quartz reef mining on this anticline commenced in the mid 1850s and by 1921 all major companies had ceased operation. The most productive period was between 1860 and the late 1880s when mining occurred within 200 m of the surface. Gold was mainly mined from reefs other than traditional saddle-type reefs. More than 20 companies mined in the area of the Williams United deposit, for a total recorded gold production of 504 600 oz. Exploration and mining on the New Chum Anticline were dormant from 1921 until the 1930s when Bendigo Mines Limited carried out limited but unsuccessful exploration programs. FIG 1 - Location map for Williams United gold deposit.

Historic gold production from the Bendigo Goldfield is estimated to be 22 Moz or 684 t from discovery in 1851 until cessation of mining in 1954, ranking Bendigo as one of the most prodigious gold producing areas in Australia. Production from the New Chum Anticline is estimated to have been 2.6 Moz, ranking it as the most important line of reef in Bendigo after the Garden Gully line.

1.

Formerly Contract geologist, now Senior Project Geologist, Bendigo Mining NL, 32 Belvoir Park Road, RSD Harcourt Vic 3453.

2.

Project Geologist, Bendigo Mining NL, PO Box 2113, Bendigo Mail Centre Vic 3554.

Geology of Australian and Papua New Guinean Mineral Deposits

In 1978 WMC commenced the first modern exploration of the goldfield. They embarked on a $28 million exploration program including historic research, 64 km of drilling and 1.5 km of underground exploratory development accessed from the Williams United shaft. In 1992 BMNL acquired the Bendigo assets of WMC which, with leases already held over the Deborah line of reef (Fig 1), resulted in BMNL gaining effective control of the goldfield. Mapping, shallow reverse circulation drilling and bulk sampling programs from May 1995 to May 1997 resulted in the delineation of the Williams United deposit. An Environmental Effects Statement for a short term open pit and heap leach operation has been prepared and is currently being assessed by an independent panel appointed by the Minister for Planning and Local Government.

511

G J McDERMOTT and P W QUIGLEY

REGIONAL GEOLOGY The deposit is within the Lower Ordovician Bendigo–Ballarat zone of the Lachlan Fold Belt of eastern Australia as described by Gray (1988). The rocks of this zone are lower greenschist metamorphic grade interbedded sandstone, siltstone and shale of the Castlemaine Supergroup (Cas and VandenBerg, 1988). The sediments have been deformed into tight upright chevron folds which plunge subhorizontally either north or south. The goldfield is intruded to the south by the Devonian Harcourt Granodiorite of age 361 Myr (Richards and Singleton, 1981) and by monchiquite dykes of unknown affinity. The dykes are generally restricted to fold axial planes and some major reverse faults. Significant gold mineralisation in Bendigo is located within 12 north trending parallel anticlines. Beneath the axial trace of each anticline, sub-horizontal auriferous quartz reefs, typically 40 to 80 m high, 0.1 to 15 m wide and between 300 and 3000 m long, are developed in zones of high strain. The reefs are stacked in a vertical manner below each anticline repeating at regular depth intervals of around 200 m. Each band of mineralisation consists of at least one major controlling structure such as a fault and up to five other accessory reefs including saddles, legs and associated spur zones (Turnbull and McDermott, this publication). Quartz reef development within the goldfield is structurally controlled and is localised around fold culminations (‘domes’), reverse faults and strong rock competency contrasts.

wavelength and amplitude of 300 and 150 m respectively. The fold geometry is modified by three 150–250 m vertically spaced west-dipping transgressive reverse faults (Turnbull and McDermott, this publication) which displace the axial plane of the fold by up to 60 m in a west over east sense of transport. Four main vertically stacked auriferous quartz reef systems (St Mungo fault, Ellenborough run, Big Slate reef and Catherine reef) are associated with the three faults. The reefs were mined within the project area to a depth of 700 m prior to 1921. The uppermost of the four reefs, the St Mungo fault (SMF), has been the focus of recent exploration. The SMF is a massive quartz reef 1 to 25 m wide, which outcrops over a strike length of 850 m within the project area. The fault is inclined at 60o towards the west and is known to extend over 100 m down dip. Initiated as a bedded fault on the western limb of the New Chum Anticline, the SMF broke through the anticlinal hinge zone to pass discordantly through the opposing limb as a series of mineralised fault splays (Fig 3). On passing discordantly through the eastern limb of the fold, the SMF refracted between rock layers in a manner similar to cleavage refraction.

LOCAL GEOLOGY LITHOLOGY AND STRUCTURE The deposit outcrops along the hinge zone of the New Chum Anticline and is hosted by interbedded quartz sandstone, siltstone and shale of turbiditic origin (Fig 2). At this location o o the anticline strikes 345 , plunges north at 10 and has a

FIG 3 - Schematic cross section through the Williams United deposit, looking north.

Associated with the fault is an array of auriferous tension veins or ‘spurs’, best developed within sandstone units, that are oriented near perpendicular to the plane of the fault. Quartz spurs may be present on the hanging wall and footwall side of the fault.

MINERALISATION

FIG 2 - Geological plan of the Williams United area and proposed mine layout.

512

The gross geological controls of the location and continuity of quartz stockwork mineralisation at Williams United are relatively well understood. The principal controls of mineralisation are host rock type, location of rock competency

Geology of Australian and Papua New Guinean Mineral Deposits

WILLIAMS UNITED GOLD DEPOSIT, BENDIGO

contrasts, position and spacing of major transgressive fault sets and proximity to fold culminations (domes). All these factors are inter-related and result in the formation of dilational sites for vein development during the ductile (folding)–brittle (faulting) deformation process.

TABLE 2 Typical RC drill hole assays from Williams United. Hole No

Assays (g/t) for successive metre samples

FSR109

0.1, 0.0, 173.0, 50.1, 0.8, 0.8, 0.1, 0.7, 3.5, 0.1

The six main vein geometries identified in the Bendigo Goldfield (Turnbull and McDermott, this publication) are:

WUR001

0.1, 10.2, 1.1, 0.5, 0.0, 0.1, 0.4, 0.0, 0.2, 0.3, 1.2, 0.1

WUR002

0.3, 0.6, 0.3, 0.4, 0.2, 1.4, 0.1, 0.9, 18.7, 0.1, 1.2

1.

bedding concordant;

WUR023

0.1, 0.3, 0.2, 0.2, 0.1, 0.1, 0.1, 42.6, 0.3, 0.1

2.

transgressive;

WUR034

3.

tensional;

1.5, 0.2, 6.4, 0.5, 0.6, 0.8, 52.9, 1.6, 2.2, 0.4, 1.2, 0.5, 0.2, 1.4

4.

perpendicular;

WUR050

0.1, 319.0, 14.0, 1.3, 1.8, 0.2, 0.7

WUR065

0.1, 1.4, 0.9, 0.3, 0.4, 0.1, 0.1, 0.1, 0.1, 0.1, 7.1, 0.1, 0.5

WUR140

0.5, 0.6, 0.2, 49.4, 0.8, 0.2, 0.1, 0.2

WUR145

0.1, 0.1, 0.0, 1.2, 0.0, 0.2, 0.0, 23.1, 0.5, 0.1

WUR157

0.2, 0.9, 0.4, 0.4, 0.4, 1.6, 0.6, 0.4, 2.0, 0.1, 0.2, 1.3, 0.9, 13.8, 0.1

WUR160

0.2, 16.3, 6.8, 1.1, 0.4, 0.2, 0.2, 0.3

WUR170

0.8, 0.0, 0.5, 0.1, 0.1, 0.7, 0.1, 10.8, 0.3, 0.4, 0.2

5.

axial; and

6.

saddle.

At Williams United most of the gold mineralisation is associated with vein types 2, 3, 1, 5 and 6, in decreasing order of importance. The average vein orientations are summarised in Table 1. The type, density, dimension and continuity of veining all impact on the grade of the mineral deposit. TABLE 1 Typical Williams United vein orientations. VEIN TYPE 1 2 3

DIP

STRIKE

50–90o E or W

245o

o

50–75 W

245ο

o

296ο

o

20 NW

3

20 SE

140o

4

75–90o N or S

165o

o

COMMENTS

Conjugate set of spur veins Unmineralised AC veins

o

5

70–90 E or W

245

6

15° NW (plunge)

340o (Plunge azimuth)

Very rare vein type

The drill hole samples were analysed by a bulk cyanide leach technique, using a 1.2 kg or 3.0 kg aliquot rather than a 50 g fire assay. Orientation tests of assay methods demonstrated a significant decrease in assay variance with increasing sample size. In other words, the larger sample size tended to be more representative of grade than smaller samples for mineralisation with an erratic high nugget gold distribution such as the Williams United deposit. The difficulty in correlating high grades between cross sections warranted closer spaced RC drilling. Line spacing was decreased to a nominal 25 m, and in some instances to 6 m. Across strike drill hole spacing was maintained at a nominal interval of 25 m. Drill hole chips were logged for lithology, per cent quartz, alteration, oxidation and sample quality. All data were collated and interpreted with PC based Gemcom software.

Typical ore in the oxidised zone consists of vein quartz and variably silicified limonite-goethite (after sulphides) and psilomelane-stained sandstone and siltstone. Gold is free, occurring with colloform textured goethite and psilomelane. In the primary zone, Williams United ore consists of laminated, breccia or buck vein quartz containing visible free gold particles to 10 µm diameter, fine submicron size gold as fracture fillings in sulphide, and 2% sulphides, commonly arsenopyrite, pyrite, galena and sphalerite. The gold is generally free milling and occurs as irregular clots, filaments or stringers. The distribution of grade within the quartz stockworks is extremely erratic. Examples of typical assays, of consecutive metre samples of drill hole cuttings through the stockwork system, are shown in Table 2.

RESOURCE ESTIMATION METHODS DRILLING AND ASSAYING Following a literature search and detailed 1:1000 scale mapping and sampling, potentially economic quartz stockwork mineralisation of the SMF was tested by 50 m spaced lines of reverse circulation (RC) drill holes. This first pass drilling was directed towards defining major structures, lithology and grade distribution.

Geology of Australian and Papua New Guinean Mineral Deposits

BULK SAMPLING AND METALLURGICAL TESTING A program of bulk sampling was initiated across the SMF to provide information on metallurgical recovery, grade reconciliation against RC drill hole data and to expose quartz stockworks for mapping and sampling. Five representative 100 t bulk samples were extracted at systematic intervals across the SMF and processed through the company’s bulk sample testing plant. Five lines of 2.5 m east–west spaced grade control RC holes were drilled across the bulk sample sites prior to extraction for grade reconciliation purposes. A comparison of grade between the widely spaced exploration RC holes averaging 0.6 g/t gold, the 2.5 m spaced grade control RC holes averaging 0.7 g/t and bulk sample results averaging 1.1 g/t, suggests that the complex grade distribution in the quartz stockworks is under-sampled by drilling. The program also indicated that the SMF mineralisation is amenable to heap leaching.

GEOSTATISTICS A geostatistical approach was used to investigate the complex grade distribution pattern in the SMF and, in particular, the understatement of grade by RC drilling.

513

G J McDERMOTT and P W QUIGLEY

Initial variography on RC drill hole data at the nominal 25 by 25 m drill hole spacing was unstructured. Therefore a comprehensive grade control drilling and trial mining program over a typical 40 m strike length of the SMF was proposed. The program comprised 170 RC holes on a 2.5 by 2.5 m grid, each 15 m long, inclined at 60° and drilled towards east. Samples were collected for each metre drilled. The top 2–3 m or 2386 t were extracted for bulk sampling and reconciliation against grade control drill hole data. During the program two distinct domains of mineralisation were identified; a higher grade hanging wall zone of massive buck quartz, and a lower grade footwall zone consisting of several 1–30 cm wide tension veins or spurs. Geostatistical (variogram) analysis of both domains indicates a very high nugget effect (75% assay variance) and short scale ranges. Grade reconciliation between the 25 m spaced exploration RC holes averaging 0.3 g/t gold, the 170 grade control RC holes averaging 0.7 g/t and bulk sample results averaging 1.0 g/t confirm the trend shown by the previous bulk sample program. This significant under-reporting of grade suggests wider spaced sampling techniques (ie drill holes) do not obtain representative samples of the five main mineralised vein geometries observed at Williams United. Bulk sampling, on the other hand, tests multiple vein orientations and therefore provides a more representative grade in erratic spatially distributed gold-bearing quartz stockworks.

Grade control drilling, although still under-reporting the true grade as determined from bulk sampling, has a higher probability of intersecting more mineralised vein combinations and effectively provides a larger sample per unit area than wider spaced exploration drilling. Comparison of assay data from twinned exploration and grade control RC holes within the test area reinforces this point, as in these instances the assays are similar. This suggests that the increase in average grade on a mining block scale from exploration to grade control drilling is purely a function of sample density and sample size.

RESOURCE MODELLING The erratic gold mineralisation at Williams United suggests that the discrimination of ore and waste zones on a mining block scale, based purely on assays of drill hole samples, is virtually impossible. For this reason the resource was interpreted using a number of key features. These were quartz abundance, alteration (ie sericite, limonite, psilomelane, arsenic value and ankerite), shearing, presence of sulphides, historic stoping and grade, including all values greater than 0.1 g/t gold. The interpreted envelope of mineralisation was Laplace modelled to create a 3D entity. Drill hole assays within this 3D shape were flagged in the database as ‘SMF’. Individual high values, including a 319 g/t, a 173 g/t and a 100 g/t assay, were not top cut in the database as they were surrounded by a dense pattern of drill holes. Resource estimation was then undertaken on the mineralised entity using ordinary kriging to interpolate

FIG 4 - Graph of gold assays in g/t, for bulk samples and the RC grade control drill hole samples within each bulk sample.

514

Geology of Australian and Papua New Guinean Mineral Deposits

WILLIAMS UNITED GOLD DEPOSIT, BENDIGO

grades into 25 m along strike by 8 m across strike by 10 m vertical blocks. The ordinary kriging technique was selected because it honours the nugget values from variography and declusters the data. The end result is a global resource estimate for the deposit, which can be subdivided, on a gross scale, by geological rationale. This commitment to a global resource estimation practice means that the discrimination of internal ore and waste blocks with respect to a cutoff grade is not feasible. The consequences of applying a cutoff grade in a high nugget deposit may result in misallocation of ore to the waste dump.

RESERVE ESTIMATION INPUT The establishment of open pit ore reserves is an iterative process involving the optimisation of the resource under a number of open pit design options. These options are assessed in terms of mineability, access, blasting requirements, cost, ground support, dilution, head grade and rehabilitation. Cultural influences must also be considered in mine design and project economics. This deals particularly with current State legislation, which provides residents within 100 m of the project with the power to veto establishment of a mine. For each open pit design, a design reserve tonnage and grade estimate was produced by evaluating the pit design against the global resource model. A wall rock dilution factor of 10% has been applied in the conversion of resource to reserve. The final mine design is illustrated in Fig 2.

UPGRADING OF KRIGED GRADE ESTIMATES A plot of grade reconciliation results comparing the head grade for each bulk sample with the average grade for each close spaced RC drill hole intersecting the site from which the bulk sample was taken is shown in Fig 4. With reference to a 1:1 correlation (the y=x line) this plot demonstrates the consistent under-reporting of gold grade from drill holes below a grade of approximately 1.1 g/t. Similar grade reconciliation trends have also been observed at the Birds and Carshalton prospects in Bendigo. This trend is simply a function of sample size, drill hole orientation and the erratic distribution of gold in the deposit. Based on these results kriged resource estimates, interpolated from assays obtained from widely spaced RC drill holes, will substantially misrepresent the true grade of the deposit. The grade reconciliation study has given BMNL confidence to increase the average kriged resource and reserve grade estimates by factors determined from the graphical plot of Fig 4. This modification of grade is similar to applying a mine call factor or to cutting high assays, both acceptable database modification practices within the mining industry applied for

Geology of Australian and Papua New Guinean Mineral Deposits

the purpose of arriving at the best estimation of the true average grade (P R Stephenson, unpublished data, 1997). If grade factoring had not been applied then the resource and reserve estimates would have substantially misrepresented the true situation. Graphical grade adjustment has resulted in the overall ore reserve grade increasing from 0.8 to 1.0 g/t gold.

CONCLUSIONS Careful consideration of features such as lithology, structure and grade distribution patterns at Williams United has resulted in the development of procedures appropriate for this style of mineralisation: 1.

Assay aliquot size has been increased by between 240 and 600% to achieve the representivity required for reliable resource and reserve estimation.

2.

Grade interpolation by ordinary kriging was chosen because it honours the grade distribution pattern and effectively declusters the data.

3.

Quoting of global resource estimates is preferred to local estimation at artificial cutoff grades.

4.

Adjusting overall kriged resource and reserve grades by factors determined from grade reconciliation tests accounts for the consistent under-reporting of true grade from drill hole samples.

ACKNOWLEDGEMENTS The authors would like to thank the management of Bendigo Mining NL for permission to publish this paper. The assistance of J Vann (Geoval) for guidance on geostatistical matters is gratefully acknowledged. Discussions with P R Stephenson (P R Stephenson Pty Ltd) on factoring of the final resource and reserve estimates under the JORC Code were very enlightening. P Walklate greatly enhanced the paper through tireless drafting and K Clark is thanked for her persistent typing of the text.

REFERENCES Cas, R A F and VandenBerg, A H M, 1988. Ordovician, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63-102 (Geological Society of Australia, Victorian Division: Melbourne). Gray, D R, 1988. Structure and tectonics, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 1-36 (Geological Society of Australia, Victoria Division: Melbourne). Richards, J R and Singleton, O P, 1981. Palaeozoic Victoria, Australia: igneous rocks, ages and their interpretations, Journal of the Geological Society of Australia, 28: 395-421.

515

516

Geology of Australian and Papua New Guinean Mineral Deposits

Sebek, R S, 1998. Bailieston gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 517–520 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Bailieston gold deposit by R S Sebek

1

INTRODUCTION The Bailieston gold deposit of Perseverance Mining Pty Limited (Perseverance) is within Historic Reserve One (HR1) on Mining Licence 4784, 12 km west of Nagambie and 120 km north of Melbourne, Vic, at lat 36o14′S, long 144o03′E on the Bendigo (SJ 55–1) 1:250 000 scale and the Nagambie (7924) 1:100 000 scale map sheets ( Fig 1).

The leases containing the historic Bailieston workings were acquired from Brunswick NL in September 1992. Bailieston has a Measured and Indicated Resource of 2.1 Mt at 0.72 g/t gold over a length of 350 m and depth of 70 m. Within that resource, the pit design is based on Proved and Probable Reserves of 823 000 t at a grade of 0.76 g/t gold at a cutoff of 0.3 g/t gold for 20 100 oz contained gold. It is one of the lowest grade deposits to be mined in Australia. The operation at Bailieston comprises a 1.5 Mtpa open pit with adjacent heap leach, with a targeted annual gold production in 1996 of 7000 oz, with gold produced at the Nagambie plant. The operation poured its first gold in March 1996.

EXPLORATION HISTORY Gold was discovered at Bailieston in 1864, after which mining and processing of gold and antimony ores continued periodically until 1905. It is estimated that 22 000 oz of gold was recovered during this period. The majority of historical production is attributable to hard rock mining although isolated shallow alluvial areas were successfully worked along the entire 5 km length of the field. The Bailieston area was originally taken up as Exploration Licence (EL) 1616 by Metals Exploration Ltd (Metals Ex) in 1986. Of the four Historic Reserves located on the licence, HR1 showed the most potential. A drilling program comprising 138 holes on approximately 50 m centres for a total of 9190 m was followed by excavation of 16 trenches of varying size for a total length of 2468 m. In addition, 12 holes for 458 m were drilled by Golder Associates as part of a hydrological study. The EL was transferred to Gold Mines of Kalgoorlie Ltd (GMK) in late 1988. Like Metals Ex, this company was a member of the Bond Gold Australia Pty Ltd group of companies whose aim was to find a large tonnage deposit amenable to bulk mining in Central Victoria. As well as detailed mapping and resource calculations, GMK drilled 17 reverse circulation (RC) holes for 119 m and 14 jetstream holes for 933 m for a total of 1052 m (D Larsen, unpublished data, 1989).

FIG 1 - Location map and structural trend map of the Melbourne Trough showing outcrop traces of major faults, fold axial surface traces, granite intrusives and trend lines, modified from VandenBerg and Gray (1988).

1.

Mine Geologist, Perseverance Mining Pty Ltd, PO Box 122, Nagambie Vic 3068.

Geology of Australian and Papua New Guinean Mineral Deposits

In 1990, Brunswick NL acquired an interest in EL 1616 in a joint venture through its subsidiary company Bendigo Gold Associates, but only carried out reviews of past exploration and resource estimates (G W McConachy, unpublished data, 1990). The transfer of EL 1616 to Perseverance as EL 3223 followed the acquisition of the Fosterville gold project from Brunswick NL. Perseverance then conducted three phases of RC drilling (3222 m) aimed at closing the drilling grid down to 20 m and further testing the continuity of mineralisation and grade distribution. A sample of 1500 t was extracted to define the metallurgical properties of near surface material and to confirm the validity of standard grade control practice. Finally, three

517

R S SEBEK

costeans were dug, sampled and mapped (C L Roberts, unpublished data, 1994), to refine the interpretation of mineralisation prior to the estimation of a preliminary resource within HR1. This exploration licence was later renumbered EL 3339 following the successful amalgamation of adjacent tenements, from within which the current mining licence (ML 4784) was granted (Fig 2). Mining commenced in January 1996.

facies, probably under baric type II conditions in conjunction with low temperatures, and contrasts with the impervious nature of the Ordovician rocks. The mineralising event in the Melbourne Trough is regarded as having occurred during the Tabberabberan Orogeny. Primary gold production from within the Melbourne Trough is dominated by the Woods Point–Walhalla belt (98 t). Other significant historic production came from Rushworth (3027 kg), Costerfield (2300 kg), Diamond Creek (1870 kg) and Whroo (1240 kg). More recently, the Nagambie gold mine, 12 km to the east, ceased operations in 1996 after production of 4170 kg.

LOCAL GEOLOGY LITHOLOGY The Bailieston open pit area has little or no outcrop, and is covered by a veneer of topsoil and red-brown clay ranging in thickness from 0.5 to 2.5 m. The clay grades down into a poorly sorted alluvial gravel containing abundant well rounded quartz clasts to 10 cm in diameter. The alluvium is moderately ferruginous and rests unconformably on strongly weathered sandstone of the Dargile Formation. The alluvium varies in thickness from 1 to 4 m primarily due to scouring in local palaeochannels. The sandstone has a uniformly cream-grey coloured, highly weathered profile which overlies the oxide zone of the deposit. The weathered sandstone is generally bleached with little or no ferruginous staining or quartz veining and little mineralisation. The weathered zone is usually 2 to 3 m thick under the first bedrock exposure but is as much as 5 m thick in some places. The soft nature of the weathered sandstone allows removal without blasting.

FIG 2 - Plan of EL 3339 showing the position of ML 4784 and the Bailieston deposit with respect to the Bailieston anticline.

The underlying oxidised sediments are dominantly well laminated, fine to medium grained sandstone with minor siltstone and mudstone. The sandstone is extremely porous, and in many instances individual beds have been silicified to such an extent that they are classified as quartzite. Voids after pyrite along bedding planes are a common occurrence and occur in sandstone and quartzite.

REGIONAL GEOLOGY STRUCTURE The Bailieston goldfield occurs within a folded and faulted Ordovician to Devonian sequence of sandstone and mudstone known as the Melbourne Trough (Gray, 1988; Gray and Willman, 1991). Bounded to the west by the Heathcote fault zone and to the east by the Mount Wellington Fault zone, the Melbourne Trough (Fig 1) is roughly triangular in shape. It is intruded by Late Devonian granite and overlain by a Late Devonian–Early Carboniferous sequence of rhyolite, red bed siltstone and sandstone (Gray and Mortimer, 1996). The sediments of the Bailieston area consist of dominantly marine turbidites of the Late Silurian Dargile Formation and Early Devonian Broadford Formation. The sediments have been deformed into relatively open folds (3 to 6 km apart) with poorly developed reticulate cleavage confined to fold hinges (VandenBerg and Gray, 1988). The Dargile Formation includes laminated and current bedded sandstone, interbedded with siltstone and mudstone, while the Broadford Formation includes sandstone and siltstone with minor greywacke conglomerate intervals (M I Miller, unpublished data, 1986). The porous and permeable nature of the Silurian–Devonian sediment has resulted from metamorphism to lower greenschist

518

The field occurs along and adjacent to the hinge of the regional Bailieston anticline (Fig 2). This fold is open and upright with simple, straight, steeply dipping limbs and a complex hinge zone (G W McConachy, unpublished data, 1990). The southern half of EL 3339 contains NNE-striking fold axes which change to a NW orientation in the northern half of the licence. The Mount Black granite outcrops to the west of the area within a structural zone around which the fold axes in the extreme north swing sharply to an easterly orientation. The distribution of north-striking quartz reefs within a line of WNW trending workings fits well with a model of dextral shearing in a WNW trending zone (S King, unpublished data, 1996). Detailed mapping of trial pits by Metals Ex also showed that the shear and vein pattern is consistent with a dextral strikeslip zone. The quartz reefs align obliquely to the zone as a whole, and can be interpreted as associated riedel shear and extensional orientations to this zone (Fig 3). Antithetic shears generally develop after synthetic shears, and this can be seen at Bailieston as the sinistral shears invariably offset the dextral shears. The fact that the zone does not continue significantly to

Geology of Australian and Papua New Guinean Mineral Deposits

BAILIESTON GOLD DEPOSIT

A thick sandstone unit lies immediately to the south of the mineralised zone. This unit may have provided the competency contrast between rock types which localised shearing. Refolding of the Bailieston anticline about an east trending axis is evident from regional geology maps and magnetic images of the area.

MINERALISATION FIG 3 - Model orientation and position of extensional, synthetic riedel and antithetic riedel shears within a dextral strike-slip zone, oriented subparallel to Bailieston structures.

the WNW suggests that the mineralisation in HR1 has developed as a termination vein array to the dextral shear zone in combination with some transfer zones between dextral shear strands. Pit exposure has revealed that the overall structure is very simple with low levels of deformation. Bedding dips consistently to the SSW at moderate angles except at the closure of the Bailieston anticline in the NE side of the pit. This fold has a near chevron style core. Cleavage is sporadically developed throughout the pit but increases in intensity in the fold core. The hinge zone of the Bailieston anticline is marked by a 50 m wide zone of minor folding which is common in the closure of folds involving well bedded sediments. The fold core is subparallel to the overall dextral shear zone and may have had some control on the shear history.

Three main styles of mineralisation have been identified in the Bailieston goldfield. Present mining operations exploit low grade disseminated gold mineralisation which includes isolated high grade areas of stockwork veining and narrow quartz veins, entirely within the oxidised zone. The mineralised area is 300 m long by 150 m wide (Fig 4) and extends to 50 m deep.

DISSEMINATED ORE Bailieston is a disseminated gold deposit with a moderate near surface supergene grade dispersion. The main sources of ore are deformation zones which strike WNW and comprise ferruginous fracture zones which have little or no effect on the orientation of the moderately dipping beds (S King, unpublished data, 1996). Throughgoing features are often difficult to define on the ground and the deformation zones are discontinuous along strike, but they are easily identified by strong iron staining of the entire sediment package. As a result of brecciation, these zones are reduced to an almost powdery consistency after blasting. Widths vary from zone to zone and also along strike but are typically 10 to 15 m with blowouts of up to 20 m and bottlenecks of less than 5 m (Fig 4).

FIG 4 - Ore outlines on RL 85 (15 m below surface), based on kriging and the position of the main WNW-trending mineralised deformation zone (SW dextral boundary) with well developed extensional structures and antithetic riedel shearing. The SW dextral boundary could also be interpreted as a major synthetic reidel shear within a larger scale strike-slip zone. Note low grade halo.

Geology of Australian and Papua New Guinean Mineral Deposits

519

R S SEBEK

The mineralised deformation zones are accompanied by irregular mineralised haloes which reflect stockwork veining and selective mineralisation of favourable beds. Some sandstone beds are clearly preferentially and passively mineralised. It is not uncommon for silicification and iron staining to die out along bedding away from the WNW shear zone. Sampling of individual beds has shown that only those beds that have been silicified carry economic grades, in the range of 0.2 to 1 g/t gold. Unstained sandstone beds immediately adjacent to silicified beds are barren of mineralisation. The presence of voids after pyrite is also a good indication of economic mineralisation within such haloes, regardless of the level of iron oxide, veining or fracturing in the host. Porphyry dykes only occur within the WNW trending shear zones. The porphyry is pale brown in colour and often contains strongly limonitic and brecciated quartz stockwork veining. The porphyry intrusions are generally 1 to 3 m wide with either sharp or gradational contacts with the surrounding sediment. Porphyry has assayed up to 0.4 g/t gold, suggesting the likelihood of multiple stage mineralisation. Limited testing has established a strong correlation between gold and antimony values. The historical Black Cloud mine (1865–1882) in the north of HR1 was primarily an antimony producer, and a similar gold-antimony relationship also exists at Fosterville, 55 km to the west of Bailieston.

STOCKWORK VEINS Stockwork mineralisation is intimately associated with disseminated mineralisation (M I Miller, unpublished data, 1986) and they are mined together. The structural environment of dextral shearing provided the regime for multi-directional quartz stockwork veining in the vicinity of riedel and extensional shears. The stockwork veins are typically 1 to 5 cm thick and strongly brecciated. They contain strong limonitic staining between fractures within the quartz and as a coating separating the quartz from the host rock. The grade of the stockwork veins varies enormously, and selective sampling has shown grades of thin veinlets from 0.1 to 160 g/t gold. The grade and intensity of stockwork veining is influenced by proximity to the related shear feature.

QUARTZ REEFS Historically, most of the gold won from the Bailieston area was derived from quartz vein or reef structures on which mining was only terminated due to the influx of water below 40 m. In the few operations that persevered below the water table, the grade of mineralisation was comparable with the grade at shallower depths. At least nine significant lines of reef were worked historically at Bailieston, with grades from 15 to 300 g/t gold. The dominant north strike of the reefs is oblique to the 5 km long WNW line of workings. The width of the reefs varied between 0.1 to 1 m but was commonly in the range 0.3 to 0.5 m. Although not a priority for the current mining operation, the existence of thin high grade veins is recognised. Occasional exploration and grade control drill hole intersections in the range 1 to 5 m at 35 to 50 g/t gold serve as reminders. Selective extraction is not possible due to narrow widths and the high amount of previous stoping. However, as mining progresses,

520

potential exists for abandoned lines of reef to be exposed and selectively extracted, as a supplement to the disseminated ore being mined.

GEOLOGICAL PRACTICE Resource estimation is by kriging using 5 by 10 by 5 m blocks. Grade control drilling is on a 4 by 4 m pattern to a depth of 5 m. The cuttings from 102 mm diameter blast holes are sampled at the collar, and samples weighing between 3 and 5 kg are assayed in Bendigo for gold. Assays are received less than 24 hours after sample submission, and are added to the database where kriging is initiated using 4 by 4 by 5 m blocks. Blast hole grades are then studied in conjunction with kriged ore blocks to define extractable zones of ore which are then surveyed on to the pit floor in readiness for mining. Blasting is undertaken in 5 m benches which are then removed as two 2.5 m flitches at a waste to ore stripping ratio of 1.3 : 1.

CONCLUSIONS Mineralisation in most of the major goldfields in central Victoria is associated with quartz in well defined reefs, shoots and saddles located in partings or openings in permeable slate and siltstone. Discoveries over the past ten years have shown that should these host rocks be sufficiently permeable and located within structurally favourable environments, the potential for discovering large tonnage, low grade disseminated gold deposits is very real. Operations such as Nagambie and Fosterville have proved that low grade disseminated gold mineralisation exists and can be successfully extracted at relatively low cost. The pit will be mined until late 1997 and a further two years will be required to extract the gold remaining in the heap. An intensive exploration drilling program planned for late 1996 will focus on the southern and northern strike extensions of the deposit where sporadic exploration in the past has isolated numerous targets, which may only require limited drilling to be brought to resource and reserve status.

ACKNOWLEDGEMENTS Appreciation is expressed to the management of Perseverance Mining Pty Ltd for permission to publish this paper. The work of the small team associated with the Bailieston project is acknowledged, and thanks are also extended to S King for his valuable structural insight.

REFERENCES Gray, D R, 1988. Structure and tectonics, in Geology of Victoria, 2nd ed (Eds: J G Douglas and J A Ferguson), pp 1–36 (Geological Society of Australia, Victorian Division: Melbourne). Gray, D R and Mortimer, L, 1996. Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt, Australian Journal of Earth Sciences, 43:103–114. Gray, D R and Willman, C E, 1991. Deformation in the Ballarat Slate Belt, Central Victoria, and implications for crustal structure across southeast Australia, Australian Journal of Earth Sciences, 38:173–210. VandenBerg A H M and Gray, D R, 1988. Melbourne zone, in Geology of Victoria, 2nd ed (Eds: J G Douglas and J A Ferguson), pp 11–14 (Geological Society of Australia, Victorian Division: Melbourne).

Geology of Australian and Papua New Guinean Mineral Deposits

Turnbull, D G and McDermott, G J, 1998. Deborah line of reef gold deposits, Bendigo, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 521–526 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Deborah line of reef gold deposits, Bendigo 1

by D G Turnbull and G J McDermott

2

INTRODUCTION The Deborah line of reef (DLR) is 150 km NNW of Melbourne, Vic, in the Bendigo Goldfield (Fig 1), at lat 36o45′S, long 144o16′E on the Bendigo (SJ 55–1) 1:250 000 scale and Bendigo (7724) 1:100 000 scale map sheets. Gold production from the Bendigo Goldfield is estimated to be 22 Moz (684 t) from 1851 to 1954 (Fig 2) ranking Bendigo as the second largest goldfield in Australia behind Kalgoorlie’s Golden Mile. Production from the DLR is reported to be 218 000 oz from 470 000 t treated, ranking it as the sixth most important structure in the Bendigo Goldfield. FIG 2 - Historic gold production figures for the Bendigo Goldfield between 1851 and 1996. Note late production from the DLR in comparison with the rest of the field.

Bendigo Mining N L (BMNL) holds exploration and mining licences over the Goldfield. Exploration by BMNL has included the rehabilitation of historic workings of the DLR accessed by the refurbished 1930s Central Deborah shaft. An Indicated Resource of 113 000 t at 9.3 g/t for 34 000 oz and an Inferred Resource of 800 000 t at 6.6 g/t for 170 000 oz of contained gold has been estimated for remnant ore along the DLR.

EXPLORATION AND MINING HISTORY The Goldfield is one of the most famous and prodigious gold producing areas in Australia. Its history extends from the discovery of alluvial gold in 1851 to the cessation of reef mining in 1954, and more recently tailings retreatment during the 1980s and 1990s. Early miners recognised the repetitive nature of quartz reef structures developed along 12 parallel anticlines within a zone 4 km wide by 15 km long. Exploration within the axes of these folds (‘centre country’) became an essential feature of gold mining in Bendigo. The DLR, although discovered in the 1860s, did not have the near-surface reef potential of the adjacent lines and remained relatively unworked for 70 years. It was only through deep shaft sinking, to 630 m, and underground development of the 1930s that the DLR was brought into prominence (Fig 2). FIG 1 - Location of the DLR within the Bendigo Goldfield. Inset shows location of the four main mines along the DLR.

1.

Project Geologist, Bendigo Mining NL, PO Box 2113, Bendigo Mail Centre Vic 3554.

During the period 1931–1954 four main mines worked the DLR to recover 214 200 oz of gold at an average ore grade of 14.9 g/t (Table 1). Rapidly escalating production costs, diminished ore reserves, lack of sufficient exploration funds and a fixed gold price led to the closure of the North Deborah and Central Deborah reef mines in November 1954.

2.

Senior Project Geologist, Bendigo Mining NL, PO Box 2113, Bendigo Mail Centre Vic 3554.

WMC commenced the first modern exploration of the Goldfield in 1977. They embarked on a major program

Geology of Australian and Papua New Guinean Mineral Deposits

521

D G TURNBULL and G J McDERMOTT

TABLE 1 Historic gold production figures for the main mines of the DLR (modified after W Shywolup and J V McCarthy, unpublished data, 1988). Mine

Period

Ore (t)

Recovered grade (g/t gold)

Gold (oz)

Deborah

1932–50 144 500

11.2

51 900

North Deborah

1939–54 202 700

19.7

128 100

Central Deborah

1943–54

65 000

14.1

29 500

Monument Hill

1934–42

35 900

4.0

4700

Total

1932–54 448 100

14.9

214 200

MacGeehan (1990). On a mine scale, the DLR stratigraphy is dominated by upward-fining turbidite cycles between 10 and 50 m thick. These cyclical sedimentation packages are usually quite distinctive and can be traced along strike between the four major mines of the DLR. Individual beds, on the other hand, are less continuous and either lens out, interfinger with other beds or change facies. The North Deborah shaft section (Fig 3) provides exposure of 230 m of the locally defined stratigraphic column, from the United sandstone (bottom) to the Royal Albert shale (top).

including historical research, 64 km of drilling, and underground exploration with access via two refurbished shafts. In 1983 BMNL secured title to leases encompassing the majority of the DLR, and commenced dewatering and refurbishment of the Central Deborah shaft from which limited mapping and sampling were conducted. In 1992 BMNL acquired the remaining Bendigo assets of WMC thereby gaining control of the Bendigo Goldfield. Dewatering of the DLR workings recommenced in January 1994 and was completed to the Central Deborah 17 level in December 1995. In June 1995 a program of detailed structural mapping, underground diamond drilling, sampling and surveying of over 1200 m of accessible historic workings of the Central Deborah, North Deborah and Deborah mines was undertaken leading to resource estimation. This phase of work was completed in July 1996 and the resulting understanding of structural and stratigraphic controls of ore emplacement in the DLR has provided the geological model on which future exploration of the Bendigo Goldfield will be based.

REGIONAL GEOLOGY The DLR is within the western margin of the Bendigo-Ballarat Province of the Lachlan Fold Belt described by Gray (1988). The rocks of this province are Lower Ordovician interbedded greywacke and slate (the Castlemaine Supergroup of Cas and VandenBerg, 1988) of lower greenschist metamorphic grade. These sediments have been deformed into tight chevron folds with steeply east-dipping axial planes and subhorizontal shallow north and south plunging fold axes. The Goldfield is intruded by the Devonian Harcourt Granodiorite of 361 Myr age (Richards and Singleton, 1981) and by lamprophyre dykes of Jurassic age, dated at 150 Myr (McDougall and Wellman, 1976). The dykes were generally intruded along the fold axial planes and are associated with some major reverse faults. Major quartz reef development within the Bendigo-Ballarat Province is structurally controlled, and is intimately associated with fold axes, fold culminations (‘domes’), reverse faulting and lithological competency contrasts.

ORE DEPOSIT FEATURES STRATIGRAPHY An overview of the stratigraphy, metamorphism and deformation of the Goldfield has been presented by Sharpe and

522

FIG 3 - North Deborah shaft section showing the mine stratigraphy and major mineralised structures, looking north.

FOLDING The Deborah anticline is typical of other anticlines of the Goldfield. They strike at 340o, plunge subhorizontally to the

Geology of Australian and Papua New Guinean Mineral Deposits

DEBORAH LINE OF REEF GOLD DEPOSITS, BENDIGO

north and south and have wavelength and amplitude of 300 m and 150 m respectively. Individual folds are generally upright to locally overturned and chevron in style with interlimb angles between 40o and 50o. Fold geometry does not remain constant along the strike of the Deborah anticline and can vary from simple rollover to faulted (‘ruptured’) east over west, west over east, or a more vertical (axial) shear geometry. Many of these changes persist over a short strike length. Regional fold culminations and depressions are a feature of the Goldfield fold geometry, and the wavelength of the main fold culmination along the Deborah anticline is 1.6 km. Associated with folding is an axial plane cleavage best developed within the hinge zones in the centre country. This cleavage is manifest as a divergent slaty cleavage in shale and a convergent spaced (10 to 20 mm) fracture cleavage in sandstone. Rare kink and buckle folds of centimetre scale overprint these earlier structures. Structural evidence supports a continuous phase of deformation from ductile (folding) through to brittle (faulting) strain conditions rather than two discrete episodes. The age of folding is thought to lie within the range Benambran (Late Ordovician to Early Silurian; see Spencer-Jones and VandenBerg, 1975; VandenBerg, 1978) to Tabberabbern (Mid Devonian; Thomas, 1939; Sandiford and Keays, 1986).

(pug), slickenslides and occasionally a laminated quartz vein. Veins of this type (‘backs’) are of variable width, usually less than 0.3 m, and can thicken rapidly toward the hinge to form ‘legs’ which are part of a ‘saddle reef’ structure .

Transgressive (C) veins These are reverse faults of up to 60 m displacement and contain several structural components. They initiate as bedded faults on one limb, rupturing the anticlinal hinge zone to pass discordantly through the opposing limb. On passing through the hinge the fault is manifest as a series of fault splays with widths to 20 m. These faults may be either east or west dipping. Faults of this classification have been historically called ‘leatherjackets’, ‘fissure reefs’, ‘slides’ and ‘neck reefs’.

Tensional (T) veins These are tension vein arrays which are the product of extension along the elongation axis. Tension veins are composed of buck quartz and are best developed within sandstone horizons associated with bedded and transgressive faults. Historically veins of this type have been referred to as ‘spurs’.

FAULTING AND QUARTZ VEINING

Perpendicular (P) veins

Faults and quartz veining at Bendigo are intimately associated. A study of the fault and vein geometry has defined several structural elements. These are illustrated in Fig 4 and are briefly described below.

These are the product of extension along the fold axis and strike at right angles to bedding, associated with a-c joints. This type rarely shows significant large tonnage gold mineralisation. Historically these structures have been included in the cross course classification.

Saddle reefs (S) These result from dilation about the hinge zone often at the interface between massive sandstone and shale. The geometry of these reefs is usually modified by other structures such as transgressive faults. Two types of quartz are generally present, a laminated vein within the legs of the saddle and a massive buck quartz within the hinge or cap. The cap quartz can contain brecciated inclusions of laminated leg quartz. Saddle reef mineralisation is generally continuous along strike for several kilometres, often at high grades.

Axial plane (A) veins These structures are developed subparallel to cleavage. Veins of this type are generally composed of buck quartz. Axial veins are intimately associated with saddle reefs, transgressive faults and neck reef structures. FIG 4 - Main structural elements of an idealised Bendigo ore system: axial (A); bedded (B); transgressive (C); tensional (T); perpendicular (P) veins and saddle reefs (S).

Bedded (B) veins Bedded faults developed early in the deformation history and represent layer-parallel slip during folding. Faulting is generally confined to contacts with competency contrasts, particularly between massive sandstone and less competent shale horizons. The interface often shows 1 to 2 cm of gouge

Geology of Australian and Papua New Guinean Mineral Deposits

MINERALISATION The host of gold mineralisation along the DLR is quartz veining associated with one or all of the structures defined above. Gold is typically free and within the size range 10 µm to 4 mm. It is closely associated with late stage sulphides such as sphalerite and galena, although free gold may be present within massive buck quartz. Typical ore consists of free gold and up to 2% sulphides, mainly arsenopyrite, pyrite, sphalerite, galena, minor pyrrhotite, chalcopyrite and bournonite (PbCuSbS3). Common gangue minerals include quartz, ankerite, sericite and minor chlorite. Associated with mineralisation is a pervasive phyllic alteration halo.

523

D G TURNBULL and G J McDERMOTT

Inner and outer reefs These are structurally complex saddle reefs (type S in Fig 4) hosted by the Lady Fiona sandstone (Fig 3). The Inner reef has been worked intermittently over a strike length of 1400 m (Fig 5) for an estimated production of 73 500 t at 29.8 g/t gold for recovery of 70 500 oz (W Shywolup and J V McCarthy, unpublished data, 1988). The bulk of the production came from the east leg of the saddle. The Outer reef has been worked over a strike length of 500 m, between the Monument Hill and Central Deborah mines, for a production of 40 800 t at 6.6 g/t gold for recovery of 8700 oz (W Shywolup and J V McCarthy, unpublished data, 1988). The morphology of the two reefs changes along strike from classic small saddles to much larger structurally complex zones formed by the interaction of saddle reefs and transgressive faults. The Deborah, North Deborah and Monument Hill shaft sections (Fig 6) portray separate classic saddle geometries for the Inner and Outer reefs. The Upper Deborah fault does not appear to influence the geometry of these saddles in this area. However, at sections 124 610 m N and 124 690 m N (Figs 6 a, b) near the Central Deborah shaft there is a strong interaction between the Inner and Outer reefs. An east-dipping transgressive fault initiating in the east limb of the Inner reef is truncated by the west-dipping transgressive Upper Deborah fault coincident with the west leg of the Outer reef. This results in a chaotic zone of hinge dilation. A visible aspect of the alteration associated with the two reefs is the development of carbonate spotting, silicification and coarse arsenopyrite crystals within the host sandstone. An Inferred Resource of 45 000 t at 10.4 g/t gold for 15 000 contained oz has been estimated for remnant ore along the Inner and Outer reefs.

Deborah back This is a bedded fault (B) hosted by the Deborah Back sandstone on the eastern limb of the Deborah anticline. The

structure has been worked over a strike length of 1400 m between the Central Deborah and Deborah mines, for an estimated production of 274 600 t at 12.8 g/t gold for recovery of 112 750 oz (W Shywolup and J V McCarthy, unpublished data, 1988). The reef is composed of laminated quartz and massive to brecciated buck quartz. This structure differs from a normal bedded vein in that it terminates below the hinge. The quartz reef is 0.5 to 1.5 m wide and extends to 80 m down dip below the hinge. This contrasts with other bedded faults that rapidly pinch, typically within 20 m below the fold hinge, to uneconomic dimensions. An Inferred Resource of 31 000 t at 10 g/t gold for 10 000 contained oz has been estimated.

Deborah fault This structure is one of the dominant transgressive faults on the DLR hosted by the Deborah Back sandstone and sediment of Kingsley’s formation (Fig 3). The fault is interpreted to have a strike length of more than 1000 m and is still open to the south (Fig 5). An alteration halo consisting of pervasive sericite, coarse grained arsenopyrite and minor pyrite is developed in the host sediment. This halo usually extends 5 m out into the footwall of the mineralisation but is confined to within 5 cm of the hanging wall. Total production is estimated at 66 650 t at 13.6 g/t gold for recovery of 29 145 oz (W Shywolup and J V McCarthy, unpublished data, 1988). Typical Deborah fault ore comprises buck quartz, often vuggy, with irregular masses or laminations of sulphides, with gold as inclusions and fracture fillings within the sulphides. Limited petrographic investigations and current and historical field observations have highlighted a close relationship between gold, galena and sphalerite. A generalised paragenetic sequence for sulphides in the Deborah fault ore has been determined to be pyrite, arsenopyrite, pyrrhotite, sphalerite, galena, gold, bournonite and chalcopyrite. Gold appears to coprecipitate with the late stage sulphides and is present as composite gold-galena grains and intergrowths, which contain a trace of acicular bournonite (P M Ashley, unpublished data, 1996).

FIG 5 - Simplified longitudinal projection through the DLR, looking west, with the most significant mineralised structures defined, and location of cross sections shown on Fig 6.

524

Geology of Australian and Papua New Guinean Mineral Deposits

DEBORAH LINE OF REEF GOLD DEPOSITS, BENDIGO

FIG 6 - Isometric diagram of the DLR workings showing the main mineralised structures at shaft cross sections. Insets show significant structural features referenced in the text. All insets (a,b,c,d and e) looking north.

Figure 6 shows the position of the Deborah fault at the various mines. In the North Deborah shaft section, the Deborah fault refracts through the hinge showing repeated splaying, and hanging wall faults become progressively weaker into the east limb and terminate (Fig 3). A strong footwall fault persists into the east limb beyond the lateral extent of the quartz mineralisation. In the North Deborah shaft area the interplay between major west- and east-dipping faults is highlighted. Here the east-dipping reverse fault splays as it passes through the hinge into the west limb, displacing the west-dipping Deborah fault in a series of jogs. Resultant dilatant sites within this zone are filled with several generations of quartz veining, consisting of early laminated hanging wall quartz, abundant spurs (T) and buck quartz. A similar mineralised transgressive fault has been described at Wattle Gully (Chewton) by Potter (1990). The apex of the Deborah anticline culmination or dome, centred on the Deborah shaft (Fig 5), appears to influence the magnitude of displacement of both the major west- and eastdipping transgressive faults. The displacement on both fault sets diminishes concentrically to the north, and presumably to the south, with increasing distance away from the apex of the dome. Similar fault systems have been described from the Caledonides of southern Norway where the faults are thought to originate as break thrusts (Morley, 1994). An Inferred Resource of 708 000 t at 6.3 g/t gold for 140 000 contained oz and an Indicated Resource of 113 000 t at 9.3 g/t gold for 34 000 contained oz has been estimated for the Deborah fault from recent work.

Geology of Australian and Papua New Guinean Mineral Deposits

Kingsley’s lode A modified saddle-style reef (S) is developed within Kingsley’s formation, a large package of medium grained sandstone containing thin (zinc and arsenic>>antimony, and rare safflorite (CoAs2). Loellingite, tennantite and safflorite are more prevalent towards an ill-defined transition zone connecting the primary and supergene ores. Arsenic in the primary ore chiefly occurs as a variable solid solution of arsenic and cobalt in pyrite lattice sites, as primary cobaltite and as arsenopyrite. There is no significant correlation between copper and arsenic grades, however there is a correlation between arsenic and cobalt grades.

anomalous copper, gold, carbonate, barium as barite, fluorine as fluorite and within apatite, uranium and REE (possibly associated with apatite). The focus for mineralisation at Ernest Henry was a preexisting fracture or fault zone. Anomalous copper and gold values are associated with several NNW-trending interpreted brittle features. The nature of these features is unknown as they have not been directly intersected but there could be a link with the cooling of the Naraku Batholith which may underlie the deposit at depth. The host breccia developed primarily by fluids permeating along fractures or faults. Reaction with the primary volcanic rock resulted in replacement by magnetite, carbonate and copper-gold mineralisation. These fluids could have derived from the cooling of the Naraku Batholith which is known to be enriched in uranium and has extensive associated iron-rich alteration. The timing of the mineralisation at 1480 Myr (C Perkins, personal communication, 1994) and the batholiths at 1500 Myr (Blake et al, 1990) also supports the link. Localisation of this reaction would have been influenced by variations in temperature and pressure, host rock chemical composition and possibly by mixing and reaction with a second fluid. S Coates (personal communication, 1997) notes the local existence of similar breccias barren of magnetite, copper and gold mineralisation with primarily a carbonate±actinolite matrix. He also notes extensive local carbonate alteration and suggests that two separate alteration events gave rise to the localisation of the orebody. An earlier brecciation is proposed with carbonate±actinolite replacement of clasts, and a later alteration with the introduction of magnetite and associated copper-gold mineralisation. The focus was pre-existing zones of structural weakness with possible localisation by variations in the chemical composition of the earlier breccia matrix. The author supports the concept of a separate and widespread carbonate±actinolite alteration event but favours a single, localised mineralising event giving rise to the Ernest Henry deposit.

ACKNOWLEDGEMENTS

Barium from a limited data set consistently exceeds 200 ppm and is often present between 0.1 and 0.8% in supergene and primary ore. Barite is commonly noted as an accessory mineral.

The author gratefully acknowledges the permission of Ernest Henry Mining Pty Ltd to publish this geological description and thanks the geological staff for their input and assistance. Particular thanks are offered to S Coates and A Barber for their critical review, ideas and assistance in the compilation of this manuscript, and J Knights for his assistance with the mineralogical description.

ORE GENESIS

REFERENCES

A limited data set shows fluorine values between 100 and 3000 ppm. Fluorite and apatite are noted as accessory minerals.

The Ernest Henry deposit has many characteristics of the group of orebodies described as Proterozoic iron oxide (Cu-U-AuREE) deposits by Hitzman, Oreskes and Einaudi (1992). Related deposits include the Olympic Dam copper-uraniumgold-silver deposit in South Australia, the Wernecke Mountain breccias of the Yukon, the Kiruna iron ore district of Sweden and the SE Missouri iron ore district. Hitzman, Oreskes and Einaudi (1992) suggested that they be referred to as ‘Kirunatype’ and that they formed primarily by shallow hydrothermal processes, probably related to deep seated magmatism. As with other examples of this type, the Ernest Henry deposit has the characteristic association of dominant iron oxide with

766

Anderson, C G and Logan, K J 1992. The history and current status of geophysical exploration at the Osborne Cu & Au deposit, Mt Isa, Exploration Geophysics, 23:1–7. Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A I, 1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Blake, D H and Stewart, A J, 1992. Stratigraphic and tectonic framework, Mount Isa Inlier, in Detailed Studies of the Mount Isa Inlier (Eds: A J Stewart and D H Blake), pp 1–11, Australian Geological Survey Organisation Bulletin 243. Collins, S, 1987. The geophysics of the Starra gold/copper deposits, Exploration Geophysics, 18:20–22.

Geology of Australian and Papua New Guinean Mineral Deposits

ERNEST HENRY COPPER-GOLD DEPOSIT

Craske, T E, 1995. Geological aspects of the discovery of the Ernest Henry Cu-Au deposit, Northwest Queensland, in Recent Developments in Base Metal Geology and Exploration, pp 95–109, Australian Institute of Geoscientists Bulletin 16.

Page, R, 1993. Geochronological results from the Eastern Fold Belt, Mount Isa Inlier, AGSO Research Newsletter, 19:4–5. Webb, M and Rowston, P, 1995. The geophysics of the Ernest Henry Cu-Au deposit (NW) Qld, Exploration Geophysics, 26:51–59.

Gidley, P R, 1988. The geophysics of the Trough Tank gold-copper prospect, Exploration Geophysics, 19:76–78. Hitzman, M W, Oreskes, N and Einaudi, M T, 1992. Geological characteristics and tectonic setting of Proterozoic iron oxide (CuU-Au-REE) deposits, in Precambrian Metallogeny Related to Plate Tectonics (Eds: G Gaál and K Schulz), Precambrian Resources, 58: 241–287

Geology of Australian and Papua New Guinean Mineral Deposits

767

768

Geology of Australian and Papua New Guinean Mineral Deposits

Hodgson, G D, 1998. Greenmount copper-cobalt-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 769–774 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Greenmount copper-cobalt-gold deposit by G D Hodgson1 INTRODUCTION

EXPLORATION HISTORY

The deposit is about 35 km south of Cloncurry in NW Queensland, at AMG coordinates 451 200 m E, 7 674 500 m N, and lat 21°02′S, long 140°32′E, on the Duchess (SF 54–6) 1:250 000 scale and the Mount Angelay (7055) 1:100 000 scale map sheets (Fig 1). In 1995 Majestic Resources NL (Majestic) acquired a 75% share of the Greenmount deposit, and became manager and operator of the project. William Resources Inc owns the remaining 25%.

At Greenmount green copper stain occurs on shale and sandstone outcrops along a strike length of 2.4 km. Historical workings amount to a few shallow scrapes and one small partly filled shaft, and vague tracks from limited exploration in the early 1950s and 1960s (Ivanac and Branagan, 1960). In the mid 1980s the area was acquired by Valdora Minerals Ltd, who identified anomalous gold values in heavy mineral and bulk cyanide leach stream sediment samples from creeks draining the Greenmount area (G D Hodgson, A J B Thompson and I M Hart, unpublished data, 1988). The results encouraged Homestake Gold of Australia Limited to enter into a joint venture with Valdora.

1 40 °20 ’ E

1 4 0°40 ’E

2 0° 40’ S L Pk

C ONCURR L Pk

L Pk

Pb r L 140°30’E

L Pk br

50

0 L Pk

L Pk

Pm L

km Dugald River Zn Pb Ag

L Pk

N

L Pp r

L Pk br

L Pp r

2 0°40 ’S

L Pp r L Pk

Ernest Henry Cu Au Cloncurry EASTERN FOLD BELT Greenmount Cu Co Au

Pk L m

Selwyn Au Cu Cannington Pb Ag Zn Osborne Cu Au

L Pk Pp r L L Pk

an

P Ls

Pk s L

n

L Pp r

L Pp r Pk L m

L Pk

a t

Pk s L

L Pp r Pk s L

Pk m L

L Pp r 2 1° 00’ S

L Ps Pk s L

L Pk br

N

N

Pk s L Pk s L

Pk s L

Pk L d

As manager and operator of the joint venture, Homestake initially located anomalous gold values in soils at Greenmount. Follow-up work between 1988 and 1995 included 50 reverse circulation percussion (RC) and a dozen percussion precollared diamond drill holes (M J Cussen, R L Krcmarov, K McKenna, C C Medina, P L Paull, S Omotosho, G Rabone and J I Stewart, unpublished data, 1988–1995). An Inferred Resource of 3.6 Mt grading 0.78 g/t gold, 1.5% copper and 420 g/t cobalt was calculated (R L Krcmarov, unpublished data, 1995) before Homestake’s share of the deposit was sold to Majestic. In 1996 Majestic completed a 65 hole, 7000 m RC drill program targeting copper and cobalt mineralisation. Using block modelling (inverse power distance method) and a 0.5% copper metal equivalent cutoff, Majestic calculated an Inferred and Indicated Resource of 23.8 Mt grading 0.47% copper and 493 g/t cobalt (Majestic Resources, 1996). The gold grade has not been determined for the whole Resource but is expected to average less than 1.0 g/t.

ul a n

t

a Pp r L

0

10 kil om etre s

L Pbr

E EN D Gilded Rose breccia 1500 Myr

Mary Kathleen Group >1740 - 1790 Myr Pk br L

Limestone breccia

Pkm L

Marimo Slate

Pks L

Staveley Formation

L Pkd

Doherty Formation

Nara u Granite and e uivalents 1500 Myr Maronan Supergroup L Ps

1670 Myr

Undifferentiated

Mary Kathleen Group >1740 - 1790 Myr L Ppr

Ro mere uart ite

Malbon Group Pk L

Cloncurry beds

Pm L

1780 Myr

Undifferentiated

FIG 1 - Location and regional geological map of the Greenmount region, after BMR Cloncurry, Marraba and Kuridala Special 1:100 000 geological map sheets.

1.

Consultant, PO Box 137, South Johnstone Qld 4859.

Geology of Australian and Papua New Guinean Mineral Deposits

Because mineralisation comes close to surface and is covered by only 1 to 3 m of soil and alluvium, the top 50 to 70 m of ore could be extracted from an open pit. Trial leach tests on bulk samples of oxide material have begun. The first results suggest that the oxide mineralisation is amenable to heap leaching, solvent extraction and electrowinning. No detailed feasibility studies have been undertaken to date and no work has been done on the sulphide mineralisation at depth.

REGIONAL GEOLOGY STRATIGRAPHY The deposit lies within the Quamby–Malbon zone of the Eastern Fold Belt of the Mount Isa Inlier. The rocks of the Greenmount area, Staveley Formation arenite and Marimo Slate black shale, are part of the Middle Proterozoic Mary Kathleen Group of cover sequence 2 which ranges in age from 1790 to 1760 Myr (Blake et al, 1990).

769

G D HODGSON

450000 m E

E END 00

Dolerite gabbro

E

Six hundred metres SE of the old Greenmount shaft a sandy brecciated rock is exposed in a low NW-trending ridge. The ridge area has not been drilled in detail, but mineralisation is exposed at surface and has been intersected by drilling in two zones, one either side of the ridge (Fig 2). The breccia clasts comprise potassium feldspar–altered and hematite-dusted arenite and are very similar to some Staveley Formation rock types. However, the unit has Marimo black shale above and below, and occupies a tight, gently north-plunging and slightly inclined anticline–syncline couple. The surrounding soft shales have cleaved and the competent sandstone band has brecciated due to the tight folding. The Marimo Basin lies between two 1500 Myr granites, the Williams Batholith to the south and the Naraku Granite to the north. Irregular amphibolite bodies occur from place to place along the Staveley–Marimo contact, and a narrow fine-grained diorite dyke striking subparallel to the Staveley–Marimo contact is exposed along the southern extension of the Greenmount mineralisation. Medium grained granite is exposed in a small isolated outcrop 1 km south of Greenmount. The granite–country rock relationships are concealed.

REGIONAL METAMORPHISM AND METASOMATISM Peak regional metamorphism locally achieved potassium feldspar-sillimanite grade, but the rocks of the Eastern Fold Belt were altered subsequently by metasomatism on a regional scale. Profoundly reconstituted rocks are exposed over hundreds of square kilometres (Williams and Blake, 1993). During several tectonic events, hot hypersaline fluids were derived from the widespread evaporitic units within the various sedimentary sequences. Scapolite and/or remnant evaporite

770

Blac slate siltstone

Marimo Slate

Arenite siltstone phyllite breccia Staveley Formation

00

Calcarenite phyllite BIF Limestone limestone breccia

N

Trend of mineralisation near surface

7675000 m N

N

The Staveley Formation is a variable unit. Locally the rocks comprise calcareous, ferruginous, feldspathic and siliceous arenite, siltstone and phyllite, and also limestone and banded iron formation. On the southern flank of the Marimo Basin the Formation is an arenite or calcarenite, and though carbonate units are present it is more sandy locally than further south where, in the Selwyn area for example, it is predominantly a calcareous unit (M J Cussen, personal communication, 1996). Individual carbonate beds are separated by thin layers of silvery phyllite. In strained zones the phyllite displays a distinctive, open, pull-apart cleavage (for which the author has coined the field term ‘mackerel texture’). In the Greenmount area, facing evidence is inconclusive but it is generally considered that the Staveley Formation is overlain by the Marimo Slate. Black, variably carbonaceous and pyritic slate, siltstone and phyllite with subordinate arenite and rare limestone comprise the Marimo Slate. It is a generally recessive unit which weathers relatively easily and only forms extensive ridges around the margins of the Marimo Basin. In the Greenmount area, where the Mesozoic weathering cap has been removed, upstanding outcrops of Marimo Slate are generally bleached and silicified. Where exposed in creek beds the unit has generally weathered to a soft powdery shale.

452000 m E

50

In the area south of Cloncurry (Fig 1) the Mary Kathleen Group occupies a tectonic feature known as the Marimo Basin, between rocks of the Malbon Group to the west and rocks of the informally defined ‘Maronan Supergroup’ (Beardsmore, Newbery and Laing, 1988) to the east. Several units within the Mary Kathleen Group are not well defined and relationships between the various units and rock types are not everywhere clear.

N

unt

ua

t

unt lati n t

lati n al n i g

M

ar

tin

Cr

ee

ul an

ulati n unt a tin

N

a t

7670000 m N

all 0

500

1,000

metres

FIG 2 - Geological plan of the Greenmount deposit.

textures are common in outcrops of carbonate rock throughout the Cloncurry area.

REGIONAL STRUCTURE Williams and Blake (1993) summarised the regional tectonic, metamorphic and metasomatic history: 1.

D1 large scale nappes, thrusts, penetrative fabrics, and prograde regional metamorphism, at about 1625 Myr;

2.

Early D2 upright to overturned NW- to ENE-trending folds, S2 crenulation, and peak metamorphism, at about 1545 Myr;

3.

Late D2 shear zones subparallel to S2, start of retrograde metamorphism, and sodium metasomatism, of uncertain age;

4.

D3 ductile deformation comprising NW- to NE-trending folds and shear zones, second crenulation, retrograde metamorphism except in cordierite-garnet bearing aureoles of batholithic granitoids, at 1510–1480 Myr; and

5.

D3 brittle deformation involving faults and fractures, possible continued granite emplacement, calcium-iron skarns, alkali metasomatism (albite, potassium feldspar and phyllosilicate alteration), silicification and mineralisation, of uncertain age.

ORE DEPOSIT FEATURES LOCAL STRUCTURE On a local scale, the Greenmount deposit is controlled by structures associated with the NW-striking Staveley

Geology of Australian and Papua New Guinean Mineral Deposits

GREENMOUNT COPPER-COBALT-GOLD DEPOSIT

Formation–Marimo Slate contact (Fig 2). The author has measured numerous small scale upright tight to isoclinal folds most of which plunge gently northwards. This folding has affected both the Marimo Slate and the Staveley Formation. Prominent joints strike easterly. The evidence suggests that the Greenmount deposit occupies a position within a NW-trending sinistral shear zone several kilometres wide, and possibly related to late D2 structures identified elsewhere.

5000 m E

The Staveley–Marimo contact is not exposed, but it is probably occupied by a fault zone localised by the ductility contrast between the more massive sandy rocks of the Staveley Formation and the fissile shales of the Marimo Slate. S King (unpublished data, 1994) interpreted a steeply NE-dipping reverse fault along the contact and suggested that the dip of the fault surface is locally flattened (Fig 3). S

NE

GREENMOUNT S

NE

FIG 4 - Schematic cross section showing the Greenmount deposit located on the SW flank of a positive flower structure, modified from Woodcock and Schubert (1994).

Cover

GRCM1

GRCM47

structure where structures dip northeastwards; to the east of Greenmount structures dip steeply southwestwards.

GRCM17

CM 4 GR

ed ch lea ne yb o ye tion Cla ltera a

4

MINERALISATION L Pkm Marimo Slate blac shale

d he ac ble one ilty r s ion we rat Lo alte

water table

GD

H0

1

FAULT

GRCM2 GRCM27

d he ac ble one ilty r s ion pe rat Up alte

200 m RL

L Pkm

GRCM26

Pkm L

E END Pkm L

Marimo Slate

Pks L

Staveley Formation

d he ac ble ne o ilty r s ion LT w e rat AU Lo alte EF RS VE RE

100 m RL

FAULT

Pkm L Pks L Staveley Formation sandstone

Marimo Slate blac shale

>0 Copper metal e uivalent in drill hole Bleached one

L Pkm

No cobalt data in drill hole Inferred geological boundary 0

25

L Pks Staveley Formation sandstone

FIG 3 - Cross section on local grid line 9900 m N, looking NW.

The contact zone is complicated by late D3 north- and easttrending joints, and north- and NE-trending faults. The faults offset the contact and vertically displace segments of the mineralised zone. The north-trending structures are parallel to the strike of the axial surfaces of the small scale folds noted above. The Greenmount deposit may be situated on the western flank of a NW-trending ‘positive flower structure’ (Fig 4). Seismic work from the oil industry shows that vertical cross sections through strike-slip zones commonly show a fan-like pattern of upwardly diverging faults (eg Woodcock and Schubert, 1994). Positive flower structures are dominated by reverse oblique faults. In the Greenmount area sinistral transpression has caused a series of high angle reverse faults to develop along a NW-trending shear zone. The Greenmount mineralisation occurs on the western flank of the flower

Geology of Australian and Papua New Guinean Mineral Deposits

At Greenmount the vast bulk of known copper-cobalt-gold mineralisation is hosted by altered black shale within the basal units of the Marimo Slate, with some mineralisation along the faulted Staveley–Marimo contact. The mineralised envelope strikes NW, subparallel to the Staveley–Marimo contact, and dips moderately to steeply northeastwards. The deposit occurs beneath cover, mostly 1 to 3 m thick, along a strike length of about 500 m and drilling has defined cobalt-mineralised intercepts of altered shale, commonly 50–60 m wide but locally to 120 m. Geochemical data show that there is a general spatial relationship between copper and gold mineralisation but that their distribution is very irregular. The cobalt mineralisation forms a much more widespread and coherent envelope and appears to be related to the distribution of iron oxides on fault and joint surfaces. Local high grade patches with >1% cobalt are associated with manganese wad. Work has yet to be done on the nature of the gold and its relationship to the base metal mineralisation. Krcmarov (1995) identified the sulphide species as three varieties of pyrite, two modes of chalcopyrite, and also chalcocite and covellite, with lesser marcasite, cobaltite and rare sphalerite and pyrrhotite. Euhedral disseminated pyrite cubes occur in feldspathised rock, ragged disseminated pyrite occurs in brecciated argillically altered rock, and the vein pyrite occurs as anhedral to euhedral cubes and aggregates. In the alteration zones chalcopyrite occurs as sparsely disseminated grains, whereas in the veins it forms ragged grains or aggregates which often enclose subhedral pyrite (Krcmarov, 1995). Majestic has undertaken electron micoprobe and X-ray diffraction work on samples of mineralised drill chips and has identified the acid-soluble copper and cobalt minerals as mainly malachite and sphaerocobaltite [(Cu,Co)CO3]. From a metallurgical standpoint cobalt mineralisation manifests itself as three distinct types, namely (i) a manganese wad which leaches rapidly with SO2; (ii) copper and cobalt carbonate mineralisation; and (iii) cobalt in arsenical sulphides and pyrite,

771

G D HODGSON

which will be extracted by longer term bacterial leaching. Majestic expects to use an acid/bacterial leach regime from the start because the oxides and secondary sulphides of both copper and cobalt are mixed from surface to depth. To enhance the rate of bacterial activity in the heap pads the company is considering the use of forced air (Majestic Resources, 1997a, b). Drilling has shown that oxidation has affected the altered rocks in the Staveley–Marimo contact zone to depths of at least 150 m. All sulphide species have been modified, and malachite and chrysocolla are common near surface. Pseudomalachite [Cu5(PO4)2(OH)4], secondary pyrite and chalcopyrite are exposed in one of the shallow pits near the Greenmount shaft. Yellow-brown iron carbonates are common in drill core and percussion chips from holes intersecting the fault zone at the Staveley–Marimo contact. Within the altered black shale, but away from the immediate contact zone, red-brown iron oxides have stained fractures and joint surfaces. Geochemical analyses indicate that at least one of the pyrite species is relatively cobalt rich. Fine grained steel-grey cobaltite has been observed in drill chips but it is rare. Erythrite has not been positively identified. In the southeastern part of the deposit mineralisation occurs on both flanks of the breccia ridge (Fig 2), and the mineralised zones appear to dip more steeply here. Sulphide mineralisation is more common and fluid inclusion work indicates higher temperatures (Krcmarov, 1995).

Within the Marimo Basin at any contact where calcareous and sandy units occur adjacent to carbonaceous black shale, and wherever black shale margins are exposed, there is a strong spatial association between the black shale contacts, copper stain and elevated gold values. The contact zone is an obvious chemical trap (‘protore’?) and is usually also a zone of structural dislocation due to ductility differences between the incompetent black shale and the adjacent arenite or limestone. Stewart (1991) and Williams and Blake (1993) noted the importance of evaporites within the Staveley Formation throughout the Cloncurry–Selwyn area, and suggested an association of evaporites with gold and base metal mineralisation. This association is described in detail by Stewart (1991, 1994) and Krcmarov and Stewart (in press). At Greenmount, D2 transpression produced a positive flower structure which proved suitable for emplacement of the diorite dyke and the granite 1 km to the south. Pervasive alteration and metasomatism preceded veining. The quartz-microcline veins and albite alteration indicate that there was local hydrothermal activity focussed on structures associated with the Staveley–Marimo contact. The copper-gold mineralisation was introduced with microcline-quartz veins during late D2 or ductile D3 shearing. Joints and cross faults, formed during brittle D3 deformation, provided the locus for an apparent separate mineralising event involving cobaltiferous pyrite and/or sphaerocobaltite.

ACKNOWLEDGEMENTS ALTERATION In the Greenmount area the Staveley–Marimo contact zone and the diorite dyke have been altered by alkali metasomatism. Krcmarov (1995) recognised that near the contact the zones of bleaching, commonly tens of metres wide, are due to albite alteration. He also noted the presence of subordinate sericite after microcline and lesser amounts of hematite, rutile, tourmaline and dolomite. Patches of black manganese wad also occur from place to place in the bleached zone and manganese values are always elevated along the Staveley–Marimo contact. Black wad is less obvious within unbleached black shale. Krcmarov (1995) described the veins in detail. Quartzmicrocline veins are common and locally contain pyrite and rare chalcopyrite. The vein arrays are sporadically distributed. The structural control is unclear, though S King (unpublished data, 1994) suggested that vein arrays occur in dilation zones above flat ramps on high angle reverse faults. The veins occur at the Staveley–Marimo contact, irregularly within the bleached zones, and in apparently unaltered black shale. Oxidation occurs to depths exceeding 150 m below surface in the Staveley–Marimo contact area, notably along the steeply dipping reverse fault. Mustard coloured clay and glassy chert dominate in the fault zone.

ORE GENESIS Throughout the Cloncurry area mineralisation appears to have accompanied regional defluidisation of the crust during an early post-kinematic episode (D2). The association of economic grades of copper, cobalt and gold with black shale is somewhat unusual and the ultimate source of the metals is debatable.

772

The author thanks the directors of Majestic Resources NL for permission to publish this paper, especially G Button for his encouragement to do so, and acknowledges the contributions of the various Valdora, Homestake and Majestic geologists with whom he has worked from time to time at Greenmount since 1986. All have contributed in some way to the discovery, recognition and evaluation of the Greenmount deposit. Also thanked are R E Gould, G J Dickie and J I Stewart who reviewed drafts of the paper, and A Nieuwenburg and R McShea who drafted the figures.

REFERENCES Beardsmore, T J, Newbery, S P and Laing, W P, 1988. The Maronan Supergroup: an inferred early volcanosedimentary rift sequence in the Mount Isa Inlier and its implications for ensialic rifting in the Middle Proterozoic of northwest Queensland, Precambrian Research, 40/41:487–507. Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A I, 1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Ivanac, J F and Branagan, D F, 1960. A case history of geochemistry and prospecting in North-West Queensland, Proceedings Australasian Institute of Mining and Metallurgy, 195:25–35. Krcmarov, R L, 1995. Proterozoic geology and mineralisation of the Greenmount Cu-Au-Co deposit, Cloncurry district, MSc thesis (unpublished), University of Tasmania, Hobart. Krcmarov, R L and Stewart, J I, in press. The geology and mineralisation of the south-eastern Marimo Basin, Australian Journal of Earth Science. Majestic Resources, 1996. Annual report to shareholders (Majestic Resources NL: Perth). Majestic Resources, 1997a. March quarterly report to shareholders (Majestic Resources NL: Perth).

Geology of Australian and Papua New Guinean Mineral Deposits

GREENMOUNT COPPER-COBALT-GOLD DEPOSIT

Majestic Resources, 1997b. June quarterly report to shareholders (Majestic Resources NL: Perth). Stewart, J I, 1991. Proterozoic geology and gold geochemistry of the Marimo Basin area, Cloncurry, NW Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Stewart, J I, 1994. The role of evaporitic-shale sediment packages in the localisation of copper-gold deposits: Copper Canyon area, Cloncurry, in Proceedings 1994 AusIMM Annual Conference, pp 207–214 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Geology of Australian and Papua New Guinean Mineral Deposits

Williams, P J and Blake, K L, 1993. Alteration in the Cloncurry District: roles of recognition and interpretation in exploration for Cu-Au and Pb-Zn-Ag deposits, James Cook University of North Queensland, EGRU Contribution No 49. Woodcock, N H and Schubert, C, 1994. Continental strike-slip tectonics, in Continental Deformation (Ed: P L Hancock), pp 251–263 (Pergamon Press: Oxford).

773

774

Geology of Australian and Papua New Guinean Mineral Deposits

Fortowski, D B and McCracken, S J A, 1998. Mount Elliott copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 775–782 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Mount Elliott copper-gold deposit 1

by D B Fortowski and S J A McCracken INTRODUCTION The deposit is owned by Arimco Mining Pty Limited, a wholly owned subsidiary of Australian Resources Limited. It is at lat 21o32′S, long 140o30′E on the Duchess (SF 54–6) 1:250 000 and the Mount Merlin (6954) and Selwyn (7054) 1:100 000 scale map sheets, 140 km SE of Mount Isa and 16 km north of Arimco's Starra gold-copper deposits and the Selwyn treatment plant (Fig 1).

2

Mining is by open stoping with subsequent pillar extraction by mass blast, using development sublevels at approximately 30 m intervals. Extraction is currently at a rate of 60 000 t per month. Ore is stockpiled on surface, then trucked to the Selwyn mill where it is blended with Starra ore and treated to produce a copper-gold concentrate.

MINING AND EXPLORATION HISTORY The deposit was discovered by John Elliott in 1899 and Mount Elliott Limited was floated on the Melbourne Stock Exchange in 1906 (J Knight, unpublished data, 1992). Mine production from underground workings commenced in 1906 and the ore was initially transported by camels 400 km to the railhead at Richmond (Cherry, 1906). Open cut mining commenced in 1910 (Linedale, 1910). Ore was smelted on site between 1909 (Linedale, 1909) and 1919, and following completion of the rail link in 1911 blister copper was railed to Townsville via Cloncurry (Hishon, 1911). Recorded production from the Mount Elliott smelter totals 24 862 t of copper and 34 000 oz of gold (Blake et al, 1984) from 268 000 t of ore for an average recovered grade of 9.3% copper and 3.9 g/t gold. As some ore was derived from other mines in the area such as the Hampden Consols to the north at Kuridala, exact production figures for Mount Elliott mine are unavailable. Modern exploration commenced in 1952 with drilling and other testing by various companies including Broken Hill South Ltd (seven holes), Mount Isa Mines Ltd (three holes), Rio Tinto Southern Ltd (two holes), Anaconda Australia Inc–Union Miniere Development and Mining Corporation Limited Joint Venture (two holes) and CRA Exploration Pty Ltd (airborne surveys). Most of the holes were drilled under existing workings, and the best intercepts include 18.8 m at 4.0% copper, 2.2 g/t gold in hole BHS-2, 7 m at 3.71% copper, gold unknown in MIM-1 and 17.7 m at 2.9% copper, gold unknown in RTS-2.

FIG 1 - Geological map of the Eastern Succession of the Mount Isa Inlier, showing Mount Elliott and other major mines and deposits in the area, modified from Davidson (1989).

Production recommenced in 1994 after 75 years of dormancy and by the end of September 1996 more than 1 Mt of ore had been produced at a grade of 3.45% copper and 1.64 g/t gold.

1. 2.

Senior Exploration Geologist, Australian Resources Limited, PO Box 1929, Mount Isa Qld 4825. Formerly Senior Development Geologist, Australian Resources Limited, now Senior Computer Geologist, BHP World Minerals, Cannington Project, PO Box 5874 TMC Townsville Qld 4810.

Geology of Australian and Papua New Guinean Mineral Deposits

In 1988 the Selwyn Mining Project partners (Cyprus Gold Australia Corporation, Elders Resources NL and Arimco NL) acquired the area and commenced detailed resource definition drilling. During the period 1988 to 1993, 13 500 m of reverse circulation percussion and 8000 m of diamond drilling were completed. In May 1993 Australian Resources Ltd acquired 100% of Mount Elliott by purchasing Cyprus Gold Australia Corporation's 66.67% equity. Decline development commenced in July 1993 and the first ore was produced in July 1994. In September 1995 exploration drilling to the west of the Mount Elliott orebody discovered a separate mineralised body known as the Corbould zone. This zone has not been closed off and exploration drilling here and elsewhere at Mount Elliott continues.

775

D B FORTOWSKI and S J A McCRACKEN

RESOURCES AND RESERVES At commencement of the feasibility study for the Mount Elliott development in 1993, the combined Upper and Lower zone resource based on a block model was 2.9 Mt at 3.33% copper and 1.47 g/t gold in the Indicated and Measured categories. An additional 20 000 t at 2.42% copper and 1.37 g/t gold remained as an Inferred Resource. The Measured and Indicated Resources were subsequently converted into Probable Reserves and at December 1993 were 1.83 Mt at 3.00% copper and 1.32 g/t gold, with additional Indicated Resources of 1.003 Mt at 3.60% copper and 1.60 g/t gold and 82 000 t Inferred Resource at 2.6% copper and 1.1 g/t gold. At December 1996 the Corbould zone was estimated to contain 555 000 t at 3.35% copper and 1.50 g/t gold in the Measured and Indicated categories and a further 205 000 t at 3.70 % copper and 1.47 g/t gold in the Inferred category.

REGIONAL GEOLOGY The deposit occurs within the Eastern Succession of the Mount Isa Inlier (Fig 1) and is hosted by the Kuridala Formation. This is a Mid Proterozoic metasedimentary sequence of the Mary Kathleen Group, which is a part of Cover Sequence 2 and of 1790–1760 Myr or younger age, as described by Blake (1987). It contains several thick sill-like bodies of amphibolite (metadolerite and metabasalt) in the Mount Elliott area. The Kuridala Formation also hosts the historic Hampden group of copper-gold deposits at Kuridala, 30 km north of Mount Elliott. The area was affected by the second major period of regional deformation of the Mount Isa Inlier. The deformation consists of three phases with D1 and D2 associated with major regional metamorphism. D1 is dated at about 1610 Myr, D2 at about 1550 Myr, before emplacement of post-tectonic granites, and D3 at about 1480 Myr (Blake, 1987). The Kuridala Formation with the Staveley Formation to the west, occurs in a corridor flanked by the Gin Creek Granite to the SW, the Mount Dore Granite to the south and the Squirrel Hills Granite to the east. The Mount Dore Granite, most of the Squirrel Hills Granite and part of the Gin Creek Granite are described as post- tectonic, non-foliated, uranium-rich, A-type granites emplaced between 1500 and 1550 Myr (Blake et al, 1984). The foliated part of the Gin Creek Granite is pretectonic.

ORE DEPOSIT FEATURES LITHOLOGY AND STRATIGRAPHY

FIG 2 - Geological plan of the Mount Elliott mine area. Orebody shapes are generalised and have been projected to surface. Alphanumeric codes for rock types are explained in Table 1.

Metadolerite occurs as a sill up to 100 m thick adjacent to the hanging wall contact. Metabasalt is common in the western part of the mine area (Fig 2) and plunges east above the footwall contact. Although stratabound by the Elliott beds, observed contact relationships in outcrop and drill holes indicate that the metabasalt is intrusive and replaces host rock. The metabasalt forms the southern end of an extensive zone of magnetic metadolerite that can be traced northwards for 20 km. The Town beds, or footwall schist, consist of quartz-mica schists which may be significantly altered and in places replaced by subore grade mineralised skarn. The entire sequence is overturned (Dredge,1992) so that it is underlain by younger psammitic and calc-silicate metasediment of the Staveley Formation to the south and west of the deposit. Mineralisation is also present within this unit in the SWAN (SouthWest Anomaly) and SWELL (SouthWest Elliott) zones (Fig 2).

The Selwyn beds, at the NE end of the mine area, consist of quartzite, sandstone, schist and metasiltstone and are unmineralised.

Microdiorite dykes to several metres thick have been intersected in drill holes. These crosscut the Elliott beds but their relationship with the Town beds and Staveley Formation has not been observed. The dykes appear to follow zones of weakness including faults, and are very late stage intrusive events. They crop out in the NW of the mine area (Fig 2). Petrographic work (A Joyce, unpublished data, 1995) indicates that they are albite- and calcite-rich.

The Elliott beds host the orebody and consist mainly of carbonaceous phyllite, metasiltstone and minor schist where unaltered. Alteration progressively intensifies towards the footwall and at its peak shows an assemblage of coarse grained minerals typical of skarn type alteration. Conformable sills and dykes of metabasalt and metadolerite are common at the contacts of the Elliott beds.

The mine sequence, which includes the Elliott beds, Town beds and part of the Staveley Formation, has been subdivided by mine geologists and given alphanumeric codes. The sequence from NE (hanging wall) to SW (footwall) is shown in Table 1 and in Fig 3. All units have been metamorphosed to amphibolite facies. Units 1 to 5 in Fig 3 comprise the Kuridala Formation and unit 6 is part of the Staveley Formation.

Within the mine area the Kuridala Formation, which dips steeply NE, is informally subdivided into the Selwyn beds, Elliott beds and Town beds (Dimo, 1975) as shown on Fig 2.

776

Geology of Australian and Papua New Guinean Mineral Deposits

MOUNT ELLIOT COPPER-GOLD DEPOSIT

TABLE 1 Mount Elliott mine sequence lithology and mineralogy. Unit

Rock type

Mineralogy

Mineralisation

Comment

1A, 1B

Amphibolite-metadolerite

hbl, pl, bi, mt

py

1A sill, to 100 m thick; 1B is thin dykes

2A

Carbonaceous phyllite, metasiltstone

bi, pl, qz, gr, and

py,po

Unaltered. Occasional schist zones.

2B

Altered phyllite

pl, bi, qz, ksp, ca, fl

py

Outermost alteration halo.

3

Altered phyllite

hab, qz, sca, ca, cpx

py, po, cp, chr, ml, at, cup, tn, cu, cc

Outer skarn carapace. Hosts Upper zone orebody and part Lower zone.

4B

Undifferentiated skarn

hab, cpx, ca

py

4BX

Massive skarn pseudobreccia

cpx, hab, ca, ksp, trm, mt

py, po, cp

Host rock completely altered. Part host of Lower zone orebody.

4BM

Massive skarn pseudobreccia, magnetite rich

mt, hab, sca, cpx, ca

py, cp

Common in deeper levels below the Lower zone.

4AS

Coarse massive skarn + sulphide

cpx, ca, mt, ap, sca

cp, po, py,bn, cc

High grade crosscutting veins.

4AI

Coarse zoned skarn

cpx, ca, ap, mt, qz, amt, gyp

py

Vugs often lined with cpx, ca, qz, amt or gyp.

1C

Metabasalt

hlb, pl mt, bi, sca

Often altered to scapolitebiotite rock.

5A

Altered schist

hab, bi, qz, ksp

Intense silicification, feldspathisation.

5

Schist

bi, qz, mu and se, pl (stl, tml, gn)

Quatz boudins common. Stl, tml and gn reported.

1D

Microdiorite

hab, ca, bi, qz

Crosscuts all the above units as dikes.

6

Calc-silicate

cpx, hab, ca mt

py, cp, cu, cc, chr, ml

SWAN prospect.

6M

Calc-silicate, magnetite rich

mt, cpx, ca, hab

py, cp

Often zones with >30% mt. SWELL prospect.

amt -amethyst, and-andalusite, ap-apatite, at-atacamite, bi-biotite, bn-bornite, ca-calcite, cc-chalcocite, chr-chrysocolla, cp-chalcopyrite, cpxclinopyroxene, cu-native copper, fl-fluorite, gn-garnet, gr-graphite, gyp-gypsum, hab-hemitite-dusted albite, hbl- hornblende, ksp-potassiumfeldspar, ml-malachite, mt-magnetite, mu-muscovite, pl-plagioclase, po-pyrrhotite, qz-quartz, sca-scapolite, se-sericite, stl-staurolite, tmltourmaline, tn-tenorite, trm-tremolite.

The term pseudobreccia of Garrett (1992) has been retained for units 4BX and 4BM. It describes a rock containing ‘clasts’ that are thought to be of replacement rather than tectonic origin, ie they have less altered cores and are not of fragmental shape. Altered clasts of the original milled schist-phyllite breccia are occasionally preserved, often near the unit 3–unit 4BX contact. Rock types previously referred to as fine grained skarn (Garrett, 1992) are now referred to as metabasalt (unit 1C).

STRUCTURE The following observations are synthesised from Dredge (1992), Garrett (1992) and McLean and Benjamin (1993). The deposit occurs within the NW-trending composite Mount Elliott fault zone. The mineralisation is bounded by steeply dipping reverse faults on the hanging wall (upper crush zone) and footwall (faulted footwall schist contact). At least three phases of deformation have affected the deposit. Axial plane cleavage overprints bedding (S0) of the Elliott beds, strikes NW to NNW and dips at 50ο to 80o NE. Bedding is generally parallel to cleavage. The S1 cleavage is often tightly folded at all scales by a D2 event with amplitudes up to 100 m. The dominant SE-trending folds are doubly plunging, at 30 to

Geology of Australian and Papua New Guinean Mineral Deposits

50o to the NW and SE, and are accompanied by a very steeply dipping axial plane cleavage and associated crenulation lineation near the hinges. Folds have strong ‘z’ asymmetry, implying either asymmetric shearing during folding or that they lie on the eastern limb of a larger SE-plunging antiform. F2 folds within the Elliott beds are truncated at their faulted contact with the Town beds. There is a strong textural contrast despite a similar metamorphic grade on both sides of the fault. At depth the fault may be overprinted by alteration or intruded by microdiorite dykes. The Town beds and adjacent Elliott beds, to the west of the open cut, appear to have been folded into open flexures arranged anticlinally about a NNE axis during D3. The Upper zone of mineralisation occurs adjacent to a 15 m wide crush zone, subparallel to bedding strike and dipping at 80o to the east. Evidence of low angle, easterly dipping, dipslip reverse faulting is also apparent. Numerous easterly dipping faults at 45–60o have been observed in underground mapping in the western part of the Lower zone and Corbould zone. Faults dipping at approximately 80o to the west or east are present in the eastern part of the Lower zone. A shear zone known as ‘Jock’s fault’ which dips at 30–40o north bisects the Lower zone.

777

D B FORTOWSKI and S J A McCRACKEN

introduced with the metasomatising fluid. Massive skarn (units 4B, 4BX, 4BM) develops with potassium feldspar, calcite and clinopyroxene occupying interstices within hematite-dusted albite and eventually replacing it. This results in a final prograde assemblage dominated by clinopyroxene with minor calcite, scapolite and sphene and rare apatite. Clinopyroxenes plot on the diopside-hedenbergite line on a magnesium-iron-manganese ternary plot with slightly more magnesium than iron and virtually no manganese (Garrett, 1992). These plots are typical of prograde anhydrous alteration in calcic copper-iron-gold skarn deposits with scapolite alteration being a coeval metasomatic event. Rare garnets occur in 4AI skarn in the L1D stope on 1130 m RL. Recent analyses (D Mylrea, unpublished data, 1996) indicate that they plot on the grossularite-andradite line on an aluminium-ironchromium ternary plot of calcic garnets. The andradite component is 78%. Massive skarn is cut by coarse grained clinopyroxenescapolite-calcite-magnetite (unit 4AI) often containing chalcopyrite (unit 4AS) which replaces previously formed calc-silicates and calcite. Chlorite, epidote, calcite, sulphides and magnetite are part of the late stage retrograde alteration. The interstitial chalcopyrite and pyrite in the massive skarn (units 4BX, 4BM) were also deposited at a retrograde stage.

MINERALISATION

FIG 3 - Cross section on line 5050 E through the Mount Elliott deposit, looking grid west (299o magnetic). Shaded areas denote copper-gold mineralisation at 3% copper equivalent cutoff. The section location is shown on Fig 2.

ALTERATION Brittle fracturing and brecciation of phyllite facilitated the introduction of hydrothermal fluids and is considered to be the primary control of alteration intensity (Garrett, 1992). Evidence suggests that at least three phases of alteration have occurred. The following description of alteration is largely taken from Garrett (1992) and McLean and Benjamin (1993). Alteration of unit 2A to unit 2B represents the outermost alteration envelope and appears as a pale bleaching due to the loss of biotite and graphite with the growth of quartz-albitesericite-calcite±pyrite, pyrrhotite and rare fluorite around veinlets or along foliation. Bleaching of phyllite progressively increases with overprinting of hematite-dusted albite and minor potassium feldspar along fractures and foliation planes. With further increase in the intensity of alteration fine grained greenish clinopyroxene or amphibole occurs as veins and pervasive replacement, typical of unit 3. Other silicates such as sphene and scapolite also occur but fluorite is notably lacking. The phyllitic texture of unit 3 is progressively destroyed with depth as fracturing and brecciation increase which can lead to massive crystalline hematite-dusted albite- clinopyroxene veins to 2 m wide, particularly near the skarn contact. The deposit is not a typical skarn, as the host rocks were not carbonates or calcareous. Calcium and carbonate were

778

Several mineralised zones have been recognised over a strike length of 200 m and to a depth of 300 m below the surface (Figs 3 and 4). Mine RLs are referenced to mean sea level plus 1000 m, and the surface at Mount Elliott is at approximate RL1380 m. The Upper zone, generally above 1235 m RL, was partly mined prior to 1919. The zone contains oxidised supergene sulphides and secondary oxides (chalcocite, covellite, cuprite, malachite, chrysocolla) above 1300 m RL and primary sulphides (chalcopyrite, pyrite, pyrrhotite) below, with pyrrhotite predominant. Sulphides occur as breccia matrix with a clinopyroxene-scapolite-calcite gangue. All mineralisation occurs in the outer skarn carapace (unit 3) in irregular steeply dipping shoots. This zone extends to the surface, becoming narrower and having a conical shape with a malachite-cuprite-chrysocollalimonite-jasper gossan at the apex which was mined in the open cut. The zone is located within a NNW-trending, steeply dipping crush zone. Approximately 50 m below the Upper zone is the top of the Lower zone which is a more tabular, 35o NNE dipping zone containing ore veins to 4 m wide. The veins consist of chalcopyrite-pyrrhotite±magnetite, pyrite, clinopyroxene and calcite. Distribution of the veins is completely random with width and grade varying considerably. In contrast with the Upper zone, this mineralisation crosscuts both the skarn (unit 4BX) and hanging wall outer skarn carapace (unit 3) altered phyllite in a generally coherent dilationary zone. Peripheral zones of mineralisation form minor ‘appendages’ to the Upper and Lower zones. In a number of instances, where individual peripheral zones have been delineated in detail for stope development, they have been named separately, such as the Footwall and Wart zones. The former occurs on the footwall contact of the Lower zone, and the latter occurs to the west of the Lower zone hanging wall mineralisation and appears to be connected to it.

Geology of Australian and Papua New Guinean Mineral Deposits

MOUNT ELLIOT COPPER-GOLD DEPOSIT

The Corbould zone (Fig 2) consists of two or possibly three subparallel composite lenses. It is the most recently discovered zone at Mount Elliott and requires further definition. The zone is immediately west of, but separated from, the Lower zone. It strikes east, dips moderately at 30–60o north, and thus has a significantly different orientation from the other mineralised zones at Mount Elliott. Individual high grade lenses within the zone dip 20–30o north. The Corbould Main zone occurs as several pods and overall appears to plunge east at about 40o (Fig 4). The Corbould Footwall zone, localised along the metabasalt–footwall schist contact, is still open in a number of directions.

pyrrhotite. A zoning also exists from the outer edge of the mineralising system through to the deeper inner core as follows: (outer) po→ po-cp → cp-po→ cp-mt→ (inner), with pyrite occurring throughout.

mt-cp→ mt

Exceptions to this zoning occur. Recent mining in the lower levels (1090 m RL) of the Lower zone has shown that areas rich in pyrrhotite predominate. Sulphides have also been observed to replace clinopyroxene and calcite. Low grade chalcopyrite occurs as veins and disseminations within the outer carapace and as coarse blebby disseminations within the massive skarn.

GEOCHEMICAL TRENDS Major and minor element geochemical trends within the deposit are directly related to alteration overprinting of the phyllite-schist host sequence (Garrett, 1992). Initial bleaching of the carbonaceous phyllite of unit 2A to 2B is associated with an increase in SiO2, CaO and F, the last to 9400 ppm, and a decrease in K2O and C, as quartz and minor calcite and fluorite replace biotite and graphite. There is a strong negative correlation with F and Fe2O3, with a fluorine halo enveloping the deposit (Garrett, 1992). The introduction of albite as pervasive replacement occurs early in the alteration of unit 2B to form unit 3 with an increase in Na2O and Al2O3 and a decrease in SiO2 content. Later alteration of unit 3 occurs deeper within the system, as Na2O, SiO2 and Al2O3 content becomes lower and Fe2O3, MgO, CaO and TiO2 increase, as albite is replaced by calc-silicates, mainly clinopyroxene (Garrett, 1992). Some Na2O is retained in scapolite. The alteration of unit 3 to produce units 4B, 4BX and 4BM is similar to that which changed unit 2B to 3 but the degree of alteration is much more intense.

FIG 4 - Longitudinal projection looking 359o magnetic, through the Corbould Main zone at Mount Elliott showing resources at 3% copper equivalent cutoff, as at December 1996. The section location is shown on Fig 2.

Late stage retrograde alteration is responsible for all the economic sulphide mineralisation and its associated calcite and magnetite. At this stage high levels of the trace elements copper, cobalt, nickel and gold were introduced with sulphides. Assays of a single sample of relatively pure chalcopyrite indicate that gold, at 1.97 ppm and possibly hosted in the chalcopyrite lattice, has a strong positive correlation with copper. Chalcopyrite has elevated values for zinc at 460 ppm, silver at 7 ppm, arsenic at 8 ppm, tin at 25 ppm and mercury at 150 ppb (Garrett, 1992). Selenium values to 33 500 ppb and tellurium to 4800 ppb in massive skarn are associated with higher copper values.

In addition to its different orientation, the Corbould zone has a number of other differences from the Mount Elliott Lower zone. These include metabasalt as the dominant rock type, particularly towards the west, slightly higher average copper and gold grades, pyrite rather than pyrrhotite as the main sulphide gangue mineral, and a low magnetite content. Mineralisation is hosted almost entirely in massive skarn as apparently conformable high grade veins. The Footwall zone at Corbould also contains appreciable chalcocite and bornite which may be attributed to faulting and associated oxidation.

Assays of a single sample of pyrrhotite, with 1350 ppm cobalt and 2000 ppm nickel, show that these elements correlate strongly with Fe2O3. Selenium and tellurium levels are also high in pyrrhotite, with 55 ppm selenium and 5 ppm tellurium but gold is low at 0.04 ppm.

SULPHIDE PARAGENESIS

1.

Elements with a strong positive correlation with gold and copper are iron, cobalt, mercury, tin, selenium, silver, tellurium and bismuth.

2.

Elements with a strong negative correlation with gold and copper are sodium, potassium, rubidium, barium, chromium, yttrium, europium, samarium, ytterbium and lutetium.

Studies of sulphide paragenesis of the Mount Elliott Lower zone (Garrett, 1992) indicate that pyrite is initially replaced by chalcopyrite which is subsequently replaced by pyrrhotite. Rarely chalcopyrite replaces pyrrhotite, and pyrrhotite replaces both pyrite and chalcopyrite. Magnetite forms at the expense of

Geology of Australian and Papua New Guinean Mineral Deposits

Electron microprobe analyses of eight gold grains averaged 87–90 wt % gold and 6 –10 wt % silver (Garrett, 1992). Samples of drill core from the Corbould zone were analysed by neutron activation for a suite of 34 elements, and results (M J Cussen, unpublished data, 1996) show:

779

D B FORTOWSKI and S J A McCRACKEN

3.

Elements with a strong positive correlation with copper only are cerium, caesium, lanthanum and thorium.

ORE GENESIS AND CONTROLS Copper-gold mineralisation has clearly formed late in the deformational history and is postulated to post-date the D3 open flexure folding (McLean and Benjamin, 1993) and skarn formation. The massive skarn is a product of brecciation and metasomatic alteration of the hanging wall phyllite, footwall schist and minor amphibolite (metabasalt). The low grade interstitial chalcopyrite and pyrite in both the massive skarn (units 4BM, 4BX) and in veins in the outer carapace (unit 3) may represent late syn-skarn mineralisation. The breccia and later dilationary structures have controlled the gross geometry of the deposit, acting as ideal repositories for mineralised fluid. The source of this fluid is unknown. The timing and mechanisms for structural events have always been problematical. Brecciation occurred after the bleaching and induration stage of alteration (unit 2A to 2B). S2 cleavage displays variable orientations from clast to clast indicating that brecciation was at least post D2 (Garrett, 1992). Brittle fracturing and brecciation were probably initiated by reverse thrusting on the crush zone and footwall faults and later, or at the same time, associated with hydrothermal brecciation by skarn type fluids. These fluids were introduced along fractures, the interstices of breccia clasts and along bedding and cleavage planes. This provides an explanation for the large (2–3 m) diameter rotated blocks well away from the faults. In contrast, the very late stage coarse grained clinopyroxenecalcite-sulphide veins occupy apparent dilationary structures produced by ductile deformation. Possibly three thermochemically different fluids used the same conduits. An albite producing fluid and a later clinopyroxene-producing fluid both deposited prograde skarn mineral assemblages. A final retrograde fluid was the source of the calcite, magnetite and sulphides which were deposited as vugh fill and as replacements of calc-silicates. Studies by Garrett (1992) indicate that an initial low temperature, halogen-rich oxidising fluid produced bleaching and calcium-fluorine metasomatism. Then a hot, oxidising, sodium-rich fluid produced albitisation as well as some quartzcalcite-potassium-feldspar. Breccia zones were invaded by a very hot (450–650oC), high salinity, relatively oxidised, weakly acidic, magmatic (granitic) fluid. This fluid was CO2- and calcium-rich with a low total sulphur content (σ34S = 0 to 20/00), with the calcium possibly derived from the underlying Staveley Formation through which the fluid passed. This fluid provided the components for the final prograde calc-silicate dominated skarn mineral assemblage. Sulphur isotope determinations (Garrett, 1992) indicate that late stage sulphide values (σ34S = -5.7 to -3.6 0/00) are possibly the result of a mixing of magmatic sulphur with that leached from diagenetic (biogenic) sulphides in carbonaceous phyllites (σ34S = -12 to -10 0/00). Therefore the hot magmatic fluid may have homogeneously mixed with a low temperature, reducing, sulphur-rich, metal bearing fluid producing a hybrid hot fluid (250–350o). This fluid interacted with the prograde skarn and precipitated sulphides in response to decreasing fO2 and increasing pH. In this reducing environment magnetite was

780

precipitated in preference to hematite, and copper and gold were transported as chloride complexes. Fluid pressures were greater than lithostatic pressures in order for dilation zones to be mineralised. The presence of fluorine, as fluorite and apatite, and boron in rare tourmaline, add credence to a granitic origin for mineralising fluids (Garrett, 1992).

MINE GEOLOGICAL METHODS In the last seven years, resources at Mount Elliott and Corbould have been estimated several times, with the initial estimates based on a sectional polygonal method. The highly skewed and mixed sample distributions for gold and copper have led to geostatistical methods being used to estimate resources, and indicator kriging was selected as a distribution-free method for estimating block grades. For the Upper and Lower zones four block model estimates have now been completed, and two estimates have been completed for the Corbould zone. Detailed stope definition drilling has been used to define the search ellipsoid and variographic parameters. Indicator variography has shown that a high nugget effect (variance = 0.48–0.68 for gold and 0.4–0.6 for copper) exists for both Mount Elliott and Corbould. Most of the non-nugget variation for both gold and copper is related to short range structures with ranges from 6 to 12 m. The remaining variation can be attributed to relatively long range structures of about 50 m. An equivalence formula has been established for each of the major ore zones to reflect their metallurgical recoveries and likely economic constraints. For the Upper and Lower zones the equivalence formula is copper equivalent % = Cu% + (Au g/t x 0.82) whereas for the Corbould zone the formula is copper equivalent % = Cu% + (Au g/t x 0.65). A 3% copper equivalent lower cutoff has been used to define the perimeter of the resource in Figs 3 and 4.

ACKNOWLEDGEMENTS The authors acknowledge Australian Resources Limited for permission to publish this paper and for the support they were given by the company. In particular they wish to thank the many geologists who worked on the Mount Elliott project, past and present, including those from Cyprus Gold Australia Corporation, particularly G McLean and S Garrett. The efforts of P Goldner and R Singer in reviewing this paper are also gratefully acknowledged.

REFERENCES Blake, D H, 1987. Geology of the Mount Isa Inlier and environs, Queensland and Northern Territory, Bureau of Mineral Resources Geology and Geophysics Bulletin 225. Blake, D H, Bultitude, R J, Donchak, P J T, Wyborn, L A I and Hone, I G, 1984. Geology of the Duchess-Urandangi region, Mount Isa Inlier, Queensland, Bureau of Mineral Resources Geology and Geophysics Bulletin 219. Cherry, F J, 1906. Cloncurry. Warden's report for December, 6 January 1906, Queensland Government Mining Journal, 7(69):88. Davidson, G J, 1989. Starra and Trough Tank: iron-formation-hosted gold-copper deposits of north-west Queensland, Australia, PhD thesis (unpublished), University of Tasmania, Hobart. Dimo, G, 1975. Precambrian geology and copper mineralisation of the Mount Elliott area, MSc thesis (unpublished), James Cook University of North Queensland, Townsville.

Geology of Australian and Papua New Guinean Mineral Deposits

MOUNT ELLIOT COPPER-GOLD DEPOSIT

Dredge, C P, 1992. EPM 3370-Selwyn. Report for the twelve months ended 24th November, 1991, Cyprus Gold Australia Corporation Report No 770, Queensland Department of Minerals and Energy, C R No 23585 (unpublished). Garrett, S J M, 1992. The geology and geochemistry of the Mount Elliott copper-gold deposit, Northwest Queensland, MSc thesis (unpublished), University of Tasmania, Hobart. Hishon, P M, 1911. Cloncurry. Warden's monthly report, 4 February 1911, Queensland Government Mining Journal, 12(131):188.

Geology of Australian and Papua New Guinean Mineral Deposits

Linedale, J C, 1909. Cloncurry. Warden's monthly report, 30 June 1909, Queensland Government Mining Journal, 10(110):357. Linedale, J C, 1910. Cloncurry. Warden's monthly report, 30 September 1910, Queensland Government Mining Journal, 11(125):515. McLean, G and Benjamin, P, 1993. The geology and development of the Mount Elliott copper-gold deposit, in Symposium on Recent Advances in the Mount Isa Block, Bulletin 13 (Ed: K Williams), pp 47–54 (Australian Institute of Geoscientists: Sydney).

781

782

Geology of Australian and Papua New Guinean Mineral Deposits

Bailey, A, 1998. Cannington silver-lead-zinc deposit, in Geology of Australian and Papua New Guinean Mineral Deposits, (Eds: D A Berkman and D H Mackenzie) pp 783–792 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Cannington silver-lead-zinc deposit 1

by A Bailey

INTRODUCTION The deposit is 135 km SE of Cloncurry in NW Queensland, within the Proterozoic Mount Isa Inlier. It is at lat 21o52′S and long 140ο55′E on the Duchess (SF 54–6) 1:250 000 scale and the Selwyn (7054) 1:100 000 scale map sheets (Fig 1). Since the discovery of Cannington in 1990 by BHP Minerals Pty Ltd an intensive feasibility study program has been completed incorporating surface delineation drilling, underground mapping and drilling from a 1 in 8 exploration access decline, bench scale and pilot plant metallurgical test work and a series of mining and infrastructure studies. The total Identified Mineral Resource is 43.8 Mt at 11.6% lead, 4.4% zinc and538g/t silver (Table 1). When full production of 1.5 Mtpa commences from the Southern zone in 1998, Cannington is expected to be the world’s largest silver producer. Annual production will be approximately 220 000 t of lead concentrate and 100 000 t of zinc concentrate. TABLE 1 Cannington resource, May 1997. Zone

Category

Pb %

Zn %

Ag g/t

Southern

Measured

11.9

Mt

13.5

5.4

626

Indicated

18.4

11.5

4.2

544

Inferred

4.4

13.4

6.2

620

Total Southern Zone

34.7

12.4

4.9

582

Northern

Indicated

6.4

10.2

2.9

422

Inferred

2.7

5.9

3.4

251

Total Northern Zone

9.1

8.9

3.0

371

Total Southern and Northern Zones

43.8

11.6

4.4

538

EXPLORATION AND DEVELOPMENT HISTORY The discovery of the Cannington deposit was the culmination of several years of exploration for Broken Hill style deposits in Australia (Skrzeczynski, 1993). Part of this effort had been focussed on the Soldiers Cap Group of Mesoproterozoic age within the Eastern succession of the Mount Isa Inlier, which was considered to have similar characteristics to the Willyama Supergroup of the Broken Hill Block (S G Walters, unpublished data, 1994). Following initial exploration of tenements some 60 km to the SE of Cloncurry, which included the discovery and delineation of the Eloise copper-gold deposit, a further group of tenements was pegged to the south in areas 1.

Chief Geologist, Project Development and Technical Services Australia, BHP World Minerals, The Broken Hill Pty Co Ltd, PO Box 6062, East Perth WA 6892.

Geology of Australian and Papua New Guinean Mineral Deposits

covered by Phanerozoic sedimentary rocks, adjacent to outcrop of the Eastern succession. A regional aerial magnetic survey over the tenements in 1989 followed by interpretation of the results defined a series of anomalies. The Cannington deposit was discovered in 1990 by drill testing of one of these anomalies. Hole ANP03 intersected 20 m of mineralisation averaging 12.1% lead, 0.6% zinc and 870 g/t silver, and diamond drilling during 1990 and 1991 indicated a significant silver-lead-zinc resource. The feasibility study included development of a 1 in 8 exploration decline into the Southern zone to provide sites for closely spaced core drilling, to gain further information on the geotechnical and hydrological conditions and to provide sites in the lode horizons from which bulk samples could be taken for pilot plant metallurgical testwork (Bailey and Thomas, 1993; Roche, 1994). The study was completed in 1995 and underground mine development and surface construction commenced in early 1996. Total drilling to July 1996 comprised 281 cored surface drill holes (total 78 419 m) plus a further 221 cored underground holes for 25 896 m. The majority of the exploration and evaluation drilling has been carried out on the higher grade Southern zone of the deposit. For the feasibility study surface drill hole spacing in the Southern zone was on an approximate 50 by 50 m grid with some holes at closer spacing. Underground mining methods proposed include primarysecondary stoping in the thicker ore sections and longitudinal benching in the thinner areas. Stope voids will be paste filled. The ore process circuit will consist of sequential silver, lead and zinc flotation with the silver flotation product being combined with the lead flotation product to yield a final lead concentrate with high silver values. The expected metallurgical recoveries are 90% lead to lead concentrate with an average concentrate grade of 75% lead; 80% zinc to zinc concentrate with an average concentrate grade of 52% zinc, and silver recoveries of 85% and 5% to the lead and zinc concentrates respectively. An innovative approach to reducing the fluorine content of concentrates from Cannington has been the addition to the metallurgical circuit of a low temperature (50oC) fluorine leach process for both zinc and lead concentrates.

REGIONAL GEOLOGY The Cannington deposit occurs beneath 10 to 60 m of Cretaceous and Recent sediment, in the SE corner of the Eastern succession of the Proterozoic Mount Isa Inlier (Fig 1). The dominant lithological packages recognised within the Eastern succession are the Mary Kathleen and Malbon groups to the west of the Cloncurry Overthrust (Blake and Stewart, 1992) and the Maronan Supergroup, proposed by Beardsmore, Newbery and Laing (1988), which contains the Soldiers Cap and Fullarton River groups to the east of the Overthrust. Age relationships between these groups are uncertain due to the complex faulted and intruded contacts. The Maronan

783

A BAILEY

FIG 1 - Geological map of the southeastern part of the Mount Isa Inlier (after Walters and Bailey, in press).

Supergroup is dominated by migmatitic gneiss, schist, psammite, feldspathic psammite, calc-silicate breccia, amphibolite, pegmatite and thin banded iron formation with associated small base metal deposits. This package was derived (Blake and Stewart, 1992) from a premetamorphic sequence of predominantly immature clastic rocks with intervals of interbedded basic volcanic and minor carbonate rock horizons. Age determinations of a garnetiferous felsic gneiss from the Fullarton River Group (Page, 1993) gave a minimum age of 1677±9 Myr. It is not clear whether the zircon dated represents the precursor of the volcaniclastic component or the detrital component age. Laing (1990) postulated that the Soldiers Cap Group may have been rafted from the east on the Cloncurry Overthrust on to the Mount Isa Block during the Diamantina Orogeny which also affected the Georgetown and Broken Hill terranes.

784

Sedimentation was terminated by the Isan Orogeny, from 1520 to 1620 Myr. Deformation consisted of an early phase of NNW-verging thrust and nappe structures (Looseveld, 1989), followed by a dominant phase (D2) of east-west compression resulting in north-aligned tight, upright folds. Regional metamorphism accompanied the deformation and peaked preor syn-D2, and has been dated at approximately 1550 Myr (Page and Bell, 1986). In the Eastern succession a general trend in peak metamorphic grade exists, from upper greenschist facies in the areas around Cloncurry to upper amphibolite facies to the south, where widespread sillimanite–potassium feldspar schist and migmatitic gneiss are present. The Eastern succession has been intruded by the extensive Williams and Naraku granites (1560 to 1480 Myr), which are distinctive, fractionated I-type granite suites (Wyborn, 1992).

Geology of Australian and Papua New Guinean Mineral Deposits

CANNINGTON SILVER-LEAD-ZINC DEPOSIT

The regional magnetic data indicate that the higher metamorphic grade sequences within the Soldiers Cap and Fullerton River groups extend for considerable distances under cover to the east and SE. However the lack of outcrop in the Cannington area, high metamorphic grade and complex deformation history make detailed lithostratigraphic correlation difficult.

DEPOSIT GEOLOGY The deposit is hosted by a sequence of garnetiferous psammite within a migmatitic quartzo-feldspathic gneiss terrain. The sequence strikes north and is cut by two major NW-trending structures, the Trepell fault which separates the Northern and Southern zones of the deposit and the Hamilton fault which forms the southern limit of the deposit (Fig 2).

STRUCTURE Four deformation phases (D R Gray, unpublished data, 1992) have been recognised at Cannington: 1.

D1 - related to the early regional thrust event, produced a local schistosity and rare, minor rootless fold hinges in foliated rocks; 2. D2 - the major structural event, represented by tight, upright north-aligned folds with a well developed axial surface schistosity (S2) and a poorly represented southerly plunging lineation (L2); 3. D3 - open folds with minor crenulation of D2 fabrics; and 4. D4 - late stage brittle structures. Peak metamorphism occurred during or shortly prior to D2 deformation and reached almandine amphibolite grade. The geometry of the Southern zone sequence is controlled by a complex tight to isoclinal recumbent D2 synform which strikes north, dips 40 to 70ο to the east and plunges to the south (Fig 3). An amphibolite body (‘core amphibolite’) within the core of the fold structure separates the footwall and hanging wall mineralised sequences. The sequence is thickest in the hanging wall and has been thinned along the footwall where more intense S2 fabrics and local truncation of the sequence suggest the presence of a high-strain shear zone. Minor D3 structures have been mapped in the exploration decline and show generally ESE plunging (60o towards 120o) open folds. Open folds and occasional crenulation of S2 foliation by D3 structures are also observed in drill core. The isoclinal fold structure is displaced by a sequence of late stage faults. The interpreted Brolga fault is a zone of northstriking minor faults which cut and displace the easterly hinge zone of the isoclinal fold. More prominent are a set of NEstriking brittle-style faults which dip steeply to the NW, show predominantly dextral strike-slip displacement and have associated silica, carbonate and pyrite alteration. This set is present throughout the Southern zone and displaces the succession and lode horizons and the Brolga fault structures. These NE-trending faults are interpreted to have developed as a conjugate set to the major bounding Trepell and Hamilton faults, which in turn are characterised by intense development of breccia, clay-chlorite gouge and associated wide fracture zones. Displacement on the Trepell and Hamilton faults is interpreted by D R Gray (unpublished data, 1993) from a conjugate fracture system related to the Trepell fault as subhorizontal sinistral.

Geology of Australian and Papua New Guinean Mineral Deposits

FIG 2 - Interpreted geology and economic lodes on 900 m mine RL (350 m below surface).

The Northern zone rocks and in particular the schistose horizons are typically less deformed than their Southern zone counterparts. D R Gray (unpublished data, 1993) has interpreted the Northern zone to lie on the eastern limb of an F1 antiform structure. There are at least two stages of local D3 open folding that develop interference patterns with folds which gently plunge both to the south and to the west to NW. Near surface the Northern zone rocks generally dip to the east but at depth they dip steeply back towards the west, forming an open fold structure (Fig 4).

785

A BAILEY

FIG 3 - Diagrammatic cross section of the Southern zone on line 4700 m N, looking north.

(+200 m) sequence of quartz-garnet-sillimanite schist and foliated garnet psammite, characterised by disseminated fine grained pink almandine garnet, is developed in the hanging wall and can be traced through the synform hinge into the structural footwall sequence where it is thinned and truncated by the footwall shear and Hamilton fault system. End members of this suite are foliated sillimanite schist with garnet and massive or banded and locally foliated garnet psammite. The hanging wall schist also contains thin (0.5 to 3.0 m), finely banded pyroxmangite-hedenbergite-fayalite±quartz-garnet horizons with accessory apatite±graphite (Bodon, 1995) and locally, low grade galena-sphalerite mineralisation.

FIG 4 - Diagrammatic cross section of the Northern zone on line 5300 m N, looking north.

HOST LITHOLOGY The host migmatitic gneiss contains intercalated (0.1 to 0.5 m thick) fine grained, schistose biotite-sillimanite-quartz bands and pegmatitic quartz-feldspar bands. S G Walters (unpublished data, 1994) considers that a feldspathic sandstone with shaly bands is the likely precursor. A relatively thick

786

Microprobe analysis of a suite of garnets (Richmond, Chapman and Williams, 1996) has shown two distinct compositional groups; type A garnet with calcium garnet 22 mol %. Type B garnets are later and restricted to alteration associated with the mineralised lode horizons, the alteration selvage to pegmatite and the thin hedenbergite-pyroxmangite-fayalitequartz bands within the hanging wall sequence. The overall sequence becomes more psammitic and silicified and less schistose towards the lode horizons. In the Southern zone, within this envelope of psammitic and schistose rocks, the lodes form footwall and hanging wall sequences about a tear-shaped, south-plunging core amphibolite horizon. The amphibolite has a maximum thickness of 100 m and is a fine to medium grained, equigranular rock of hornblende and plagioclase with scattered minor quartz and accessory apatite,

Geology of Australian and Papua New Guinean Mineral Deposits

CANNINGTON SILVER-LEAD-ZINC DEPOSIT

and a weakly developed single foliation (Sheehan, 1994). Semiconformable pegmatite horizons occur throughout the deposit sequence, and were formed predominantly as a result of ‘local’ partial melts (Mark, 1993). The pegmatites were ‘emplaced’ during or prior to D2 deformation, as they are folded and show boudinage within the D2 foliation. Compositionally, the pegmatites are predominantly coarse grained potassium feldspar, quartz and plagioclase with accessory muscovite, biotite and garnet spotting, with a coarse garnet-amphibole alteration selvage. Individual horizons can be traced over considerable distances down dip and along strike. Pegmatites show partial assimilation of lode material with inclusion of sulphides and occurrences of the lead-bearing green feldspar amazonite.

SOUTHERN ZONE MINERALISATION The silver-lead-zinc mineralisation at Cannington is associated with a diverse package of siliceous and mafic rocks with extensive retrogression and alteration. A zoning of base metals is evident within the Southern zone which is consistent with the interpreted isoclinal fold structure. The lode horizons are defined by the base metal distribution. Within this semi-tabular geometry a sequence of gangue–ore geochemical and textural associations is recognised, and the lode and mineralisation types (Table 2) describe the geometry, economic, geochemical and textural relationships within the deposit. The mafic host rocks are generally defined by an overall iron content greater than 15%, but end members can contain above 30%. The package consists of moderate to coarse grains of equigranular pyroxene, pyroxenoid and olivine with local codominant or accessory magnetite and fluorspar; minor quartz and amphibole may also be present. Magnetite grains can be intensely fractured, with the fractures filled by silicates and sulphides (French, Ramsden and Walters, 1994). A clear zoning exists in the mafic assemblage with the pyroxenoid, pyroxmangite [(Fe,Mn,Ca)SiO3] associated with the outer envelope of lead-silver mineralisation, and the pyroxene,

hedenbergite [Ca(Fe,Mn)Si2O6] associated with the inner zinc mineralisation (Figs 2 and 3). Manganese-rich fayalite is associated with both phases, as are magnetite and fluorspar, although both can vary on a local scale. Widespread retrograde alteration phases occur throughout the mafic suite. These can be characterised (Bodon, 1995) as high temperature anhydrous (hedenbergite, associated with the zinc horizons and garnet), hydrous (quartz, ilvaite and amphibole), and lower temperature (pyrosmalite, chlorite, greenalite, talc and carbonate) phases. The siliceous ore hosts are mineralogically more simple than the mafic, with anhedral interlocking quartz the dominant gangue, with accessory feldspar, biotite and muscovite. The quartz is amorphous, chert-like, blue and exhibits some conchoidal fracture. Minor garnet is also present. Apatite with fluorine to 5% (French, Ramsden and Walters, 1994) is another accessory mineral, and is responsible for average phosphorus values within the siliceous package of 2000 to 3000 ppm (Fig 5). Late stage silicification is present throughout the sequence as an alteration phase and also in association with the late stage brittle-style faulting. There are local gahnite horizons within the siliceous mineralised rocks and in the hanging wall schist horizons. Magnetite and fluorspar are absent from the siliceous package. The Cannington sulphide assemblage is dominated by galena and sphalerite, with minor pyrrhotite, marcasite, arsenopyrite and chalcopyrite. The silver is contained predominantly in freibergite [Cu6(Ag,Fe)6Sb4S13], the silver-rich member of the tetrahedrite-tennantite sulphosalt series or fahlores, and in solid solution, to 1300 ppm, within galena. Other silver phases include pyrargyrite (Ag3SbS3), acanthite (Ag2S), allargentum (Ag6Sb), dyscrasite (Ag 3Sb), native and antimonial silver and a previously undescribed sulphosalt (French, Ramsden and Walters, 1994) informally called ‘canningtonite’ (4PbS.3Ag2S.3Sb2S3). Traces of proustite (Ag3AsS3) and jamesonite (4PbS.FeS.Ag.3Sb2S3) have also been recorded. In the Footwall lead lode (Nithsdale mineralisation) the silver minerals stephanite (Ag5SbS4), sternbergite (AgFeS3) and

TABLE 2 Southern zone lode and mineralisation types with gangue associations. Lode horizon (% of Southern zone resource)

Mineralisation type

Gangue–ore association

Gangue association

Footwall lead (12.3%)

Nithsdale (NS)

Mafic Pb, Ag

Pyroxmangite, magnetite, manganese-fayalite and fluorite

Warenda (WA)

Siliceous mafic Pb, Ag (low grade)

Quartz, pyroxmangite, manganese-fayalite and fluorapatite

Footwall zinc (18.9%)

Cukadoo (CK)

Siliceous Zn

Quartz, feldspar (minor) and fluorapatite

Colwell (CW)

Mafic Zn

Hedenbergite, manganese-fayalite, magnetite, pyrrhotite and fluorite

Glenholme (GH)

Siliceous Pb, Zn, Ag

Quartz, feldspar (minor) and fluorapatite

Glenholme breccia (10.9%)

Glenholme breccia (GHB)

Siliceous Pb, Zn, Ag

Quartz, feldspar (minor), carbonate and fluorapatite

Hanging wall zinc (1.4%)

Kheri (KH)

Mafic Zn (low grade)

Hedenbergite, manganese-fayalite, magnetite, pyrrhotite, fluorite and arsenopyrite

Kheri-Colwell

Mafic Zn (low grade)

Hedenbergite, manganese-fayalite, magnetite, pyrrhotite and fluorite

Burnham (BM)

Mafic Pb, Ag

Pyroxmangite, magnetite, manganese-fayalite and fluorite

Broadlands (BL)

Siliceous mafic Pb, Ag

Pyroxmangite, quartz, manganese-fayalite and fluorapatite

Hanging wall lead (56.5%)

Geology of Australian and Papua New Guinean Mineral Deposits

787

A BAILEY

FIG 5 - Median values for major and minor elements in the Southern zone lode and mineralisation types.

stetafeldite (of uncertain composition, perhaps Ag,Cu,Fe,Sb,S,H2O) have also been identified within late stage alteration (M Dugmore, personal communication, 1996). Other minor sulphides associated with the mineralisation are loellingite (FeAsS2), gudmundite (FeSbS), veenite (Pb2Sb2S5), launyaite (Pb 22Sb26S61) and bismuthinite (Bi2S3).

Footwall lead lode Two mineralisation styles are associated with this lode horizon, which is at the structural base of the deposit. These are a mafichosted high grade silver-lead mineralisation (Nithsdale style) and a down dip, lower grade more siliceous mineralisation, the Warenda type. In vertical longitudinal projection (Fig 6a), the grade.thickness product for lead (%lead x thickness) is greatest in the Nithsdale mineralisation and the grade.thickness contours define a southerly plunging shoot that is slightly to the south of, and subparallel with, the southern limit of the core amphibolite. The Nithsdale gangue assemblage is typically pyroxmangite, magnetite, fayalite and fluorite with minor hedenbergite. Pyroxmangite and fayalite have been replaced along fractures and grain boundaries by pyrosmalite and greenalite. This assemblage has a series of textures including massive equigranular pyroxmangite-fayalite and galena; coarse-banded magnetite and fluorite-galena-pyroxmangite-fayalite; and intense ductile-style milled breccia with rounded clasts of pyroxmangite, fayalite and magnetite in a matrix of sulphide and fluorite. The dominant sulphide is galena with associated silver phases, and minor sphalerite, pyrrhotite, freibergite, arsenopyrite, chalcopyrite and loellingite. French, Ramsden and Walters (1994) recorded an across-dip zoning in the Nithsdale mineralisation with pyrrhotite more abundant and coarser grained towards the hanging wall and a corresponding increase in pyroxmangite and galena towards the base. The iron content of the mineralisation has a median value of 19% (Fig 5) but can locally reach 40%, and fluorine values vary between 2 and 10% with a median of 4.4%. The transition of Nithsdale to Warenda style mineralisation down dip and along strike to the north is marked by an increasing silica content and an associated decrease in mafic

788

minerals, magnetite and fluorite. The Warenda mineralisation typically comprises irregular bands of more mafic composition, with pyroxmangite, fayalite and hedenbergite intercalated with more siliceous bands. There is only a moderate galena content with associated silver phases in this mineralisation, and it is predominantly associated with the more siliceous horizons. Minor sphalerite, pyrrhotite, pyrite and arsenopyrite are also present, and trace chalcopyrite. Magnetite and fluorite are only present in the transition areas.

Footwall zinc lode This horizon is host to approximately 20% of the Southern zone resource. It has a complex zoning of ore and gangue minerals from mafic rocks with zinc mineralisation (Colwell type) to siliceous rocks with zinc mineralisation (Cukadoo type), and siliceous rocks hosting zinc-lead mineralisation (Glenholme type). However, throughout this tabular horizon the grade of the zinc mineralisation cuts across these mineralisation styles, and when plotted in vertical longitudinal projection (Fig 6b) the zinc grade-thickness product (% zinc x thickness) defines a southerly plunging shoot which is subparallel to the southern limit of the core amphibolite. The lode is structurally above the Footwall lead lode (Fig 2) and is separated from it by a zone, approximately 15 m thick, of predominantly low grade zinc mineralisation with magnetite and minor fluorite hosted by mafic rock. Down dip and to the north where the Warenda mineralisation forms the Footwall lead lode the interburden between the lead and zinc horizons is more siliceous, and magnetite and fluorite are absent. The Cukadoo mineralisation is characterised by massive, milky white to bluish strained quartz with minor potassium feldspar, muscovite and fluorapatite, and stringers of sphalerite. Galena, pyrrhotite, arsenopyrite and chalcopyrite occur as minor veinlets and disseminations. There are small zones of intensely deformed breccia containing rounded siliceous clasts in a matrix of sphalerite-pyrrhotite. The Cukadoo mineralisation forms the up dip, southplunging segment of the Footwall zinc lode, wheras down dip it is transitional into the Colwell mafic-hosted zinc

Geology of Australian and Papua New Guinean Mineral Deposits

CANNINGTON SILVER-LEAD-ZINC DEPOSIT

FIG 6 - Vertical north-south longitudinal projections of the lodes with grade.thickness contours, distribution of mineralisation types, the extent of the core amphibolite and the intersection trace of the NE-trending faults: (a) Footwall lead lode - % lead x thickness; (b) Footwall zinc lode - % zinc x thickness; (c) Hanging wall lead lode - % lead x thickness.

mineralisation. The Colwell mineral assemblage is hedenbergite±magnetite and fluorite with sphalerite, pyrrhotite and minor galena, arsenopyrite and chalcopyrite. Milled breccia textures and wispy sphalerite-pyrrhotite-fluorite flame textures are present in some areas. Olivine, partly replaced by pyrosmalite or ilvaite, is also present. Pyroxmangite is absent or at trace levels in the Colwell mineralisation as reflected in the elemental abundance (Fig 5), where manganese is associated with the lead-silver dominant mineralisation but not with the zinc mineralisation. Further down dip and towards the hinge zone of the

Geology of Australian and Papua New Guinean Mineral Deposits

synformal structure the mineralisation changes again to the siliceous Glenholme style, and contains zinc and lead in approximately equal proportions. The Glenholme and Glenholme breccia are the only mineralisation types at Cannington with elevated levels of both zinc and lead. Both mineralisation styles are characterised by a breccia texture with silica clasts and a matrix of sphalerite, galena and minor carbonate. Minor muscovite, potassium feldspar and fluorapatite are present, with some retrograde sericite, chlorite and illite. Pyrrhotite, magnetite and fluorite are absent, except in zones transitional to the Colwell mineralisation.

789

A BAILEY

Glenholme breccia lode This mineralisation is in the hinge area of the D2 synform and is the down dip extension of the Glenholme mineralisation in the Footwall zinc lode. There are no significant mineralogical differences between the Glenholme and Glenholme breccia mineralisation types. The distinction is made on the basis of texture, the Glenholme breccia having a greater development of breccia mineralisation with small areas of highly deformed and disaggregated quartz veins and segregations, and a minor increase in antimony-bearing sulphosalts thus giving a higher antimony:silver ratio (Fig 5).

Hanging wall zinc lode Unlike its footwall equivalent this lode contains only maficrock hosted zinc (Kheri style) mineralisation. The mineralogy and elemental abundance of this mineralisation are, with only minor exceptions, identical to the footwall Colwell mineralisation. These minor exceptions are a lower zinc grade, elevated iron values, and an increase in arsenopyrite, chalcopyrite and coarse grained pyrrhotite. As in the Colwell mineralisation the textural styles vary from a common granular texture with hedenbergite, magnetite, pyrrhotite, arsenopyrite, sphalerite and minor chalcopyrite, to a more intensely milled breccia, and flame textures. In the south where the core amphibolite is absent the distinction between hanging wall and footwall lodes in the synform hinge is more problematic, and in these areas no distinction is made between the Kheri and Colwell mineralisation types and the term Kheri–Colwell is used.

Hanging wall lead lode This lode constitutes more than 50% of the Southern zone resource and comprises the mafic hosted lead-silver Burnham mineralisation and the overlying siliceous-mafic hosted lower grade lead-silver Broadlands mineralisation. Together these mineralisation types form sequences up to 100 m thick in the Southern zone and extend from the south where they are truncated by the Hamilton fault, to more localised intercepts in the north. In vertical longitudinal projection (Fig 6c) the maximum grade-thickness product for lead for the Hanging wall lead lode is positioned to the south of the core amphibolite and with the outline of the Glenholme breccia mineralisation defines an envelope enclosing the southern and down dip limits of the core amphibolite. The Burnham mineralisation is the hanging wall equivalent of the Nithsdale mineralisation, and like the Nithsdale mineralisation is characterised by a gangue of pyroxmangite, magnetite, fluorite and fayalite and a sulphide mineralogy of galena, pyrrhotite, sphalerite, arsenopyrite, freibergite and associated silver-sulphosalt phases. As with the other mafic hosted units widespread hydrous retrogression is common. Similar textural associations to those in the Nithsdale mineralisation are also present, although there is a greater degree of coarsely banded magnetite and fluorite-galenapyroxmangite mineralisation. Small scale, tight folding of banded magnetite-pyroxmangite with pyrrhotite formed in the axial plane cleavage is observed in drill core and it is possible that the folding is a contributing factor to the increased thickness of the horizon.

790

The Broadlands style mineralisation is principally a banded pyroxmangite-hedenbergite quartzite, equivalent to the Warenda mineralisation of the footwall sequence. This moderate to low grade galena with minor sphalerite mineralisation is usually associated with the more siliceous units. The form is typically lenses of predominantly pyroxmangite-hedenbergite-fayalite interbanded with lenses dominated by quartz, on a scale of 1 m or less. Other minor to trace sulphide phases include pyrrhotite, chalcopyrite, arsenopyrite and freibergite. Fluorapatite and graphite are also present in the siliceous units.

NORTHERN ZONE MINERALISATION The majority of the mineralisation in this zone lies above and is partially draped about an amphibolite horizon (Fig 4) in an equivalent position to the Hanging wall lead lode in the Southern zone, and it may therefore represent a simple faultedoff segment of this lode. Minor Glenholme lead-zinc mineralisation is also present, again in a similar position to its Southern zone equivalent but of lower grade and extent. These mineralised horizons are similar in mineralogy and texture to their Southern zone counterparts. The Northern zone also contains a consistent, steeply dipping sequence of mineralised horizons on its eastern flank. This mineralisation, termed Inveravon, is finely banded pyroxmangite-olivine±hedenbergite, apatite and quartzgalena±graphite, and is very similar in style to the horizons previously described in the Hanging wall schist in the Southern zone. Minor sphalerite is present, although grades of lead or zinc are usually low and only a very few samples reach economic grade. The Inveravon style mineralisation is also present above the Broadland mineralisation in the Northern zone (Fig 4).

GENETIC MODELS The Cannington deposit has many similarities with the Broken Hill deposit in NSW including its setting in a Middle Proterozoic sequence with both lithological and temporal affinities to the Broken Hill Block, high grade metamorphism, complex deformation and a distinct geochemical, mineralogical and economic mineral zoning. Research on genetic models for the deposit has suggested two alternatives: 1.

a late stage metasomatic mineralising event (skarn model), post-dating the major metamorphic and deformation events (Williams et al, 1996); and 2. a synsedimentary or early diagenetic mineralising event with subsequent modification and remobilisation during metamorphic and deformation episodes, followed by further modification by a late metasomatic event (S G Walters, unpublished data, 1994; Bodon, 1996; Walters and Bailey, 1996). The skarn model (Williams et al, 1996) can be summarised in the following paragenetic sequence: 1.

An original metasedimentary package which consisted of an iron-manganese-(calcium)–rich fraction representing the present lodes; and an outer iron- and manganese-rich peraluminous metasediment derived from quartz–pelite mixtures with local feldspathic fractions.

Geology of Australian and Papua New Guinean Mineral Deposits

CANNINGTON SILVER-LEAD-ZINC DEPOSIT

2.

Regional deformation (D1 and D2) with peak metamorphism reaching upper amphibolite facies pre- or syn-D2. Peak metamorphic minerals were quartz, sillimanite, potassium feldspar, biotite, type A garnet and graphite. 3. Peraluminous anhydrous iron-rich alteration with a similar mineral suite to that associated with peak metamorphism. 4. Anhydrous calcium-rich alteration with quartz, apatite, pyroxmangite, hedenbergite, fayalite, hornblende and type B garnet. 5. Hydrous iron-calcium-potassium alteration, with hornblende, biotite, pyrosmalite and dannemorite. 6. A mineralising phase with sphalerite, galena, pyrrhotite, chalcopyrite, arsenopyrite, pyrite, tetrahedrite (freibergite), magnetite and fluorite. The alternative ‘synsedimentary’ model (S G Walters, unpublished data, 1994; Walters and Bailey, in press; Bodon, 1996) can be summarised by the following paragenetic sequence: 1(i). Initial introduction and zoning of base metal and silver mineralisation with zinc dominant and lead-silver dominant horizons. By analogy with other base metal deposits the siliceous lead-silver style (Broadland and Warenda types) represents a capping to the mineralised sequence (S G Walters, unpublished data, 1994). This premetamorphic zoning could have been developed by processes associated with a volcanogenic sulphide system or a basin dewatering diagenetic system with the mineralisation controlled by primary porosity or matrix replacement. No evidence is currently available that would indicate which of the above should be the preferred model. (ii). Emplacement into the sequence of a series of tholeiitic basic sills (Mark, 1993). 2. Initial regional deformation (D1) with the resulting development of an S 1 regional schistosity. 3(i). Development of the major isoclinal south-plunging synform (D2) in the Southern zone accompanied by axial plane sillimanite schistosity (S2). Peak metamorphism is considered to be pre- or syn-D2. The prograde iron-rich metamorphic assemblage includes fayalite, type A garnet, pyroxmangite and magnetite. (ii). On a more local scale the intense D2 event caused extensive boudinage and the development of a preferred lineation (L2) on the S2 schistosity. Within the mineralised horizons remobilisation and boudinage caused thickening and the development of higher grade ‘shoots’ subparallel to the plunge of the L2 lineation and within the pressure shadow developed about the pinchout of the core amphibolite. A similar geometry is also recognised in the Northern zone with mineralisation wrapping around the closure of an amphibolite body. 4. A sequence of widespread alteration events followed (Bodon, 1996): (i) anhydrous high temperature alteration with hedenbergite–type B garnet–quartz dominant; (ii) hydrous high temperature quartz-ilvaite-hornblende alteration of olivine, hedenbergite and pyroxmangite. This includes a silicification event associated with and overprinting the Cukadoo and Glenholme mineralisation types which produced a siliceous breccia texture and destroyed the iron-manganese precursors. The

Geology of Australian and Papua New Guinean Mineral Deposits

introduction, upgrading and redistribution of silver and base metals may also be associated with both the retrograde alteration and silicification events. Fluorite may have been introduced in the waning stages of the high temperature retrogression (S G Walters, unpublished data, 1994); and (iii) hydrous low temperature alteration with pyrosmalitegreenalite dominant. 5. A sequence of brittle-style faulting episodes including the Trepell–Hamilton system, Brolga fault zone and NEtrending structures, accompanied by low temperature chlorite alteration and the introduction of silicacarbonate-pyrite alteration zones.

ACKNOWLEDGEMENTS The author would like to thank BHP Co Ltd for permission to publish this paper and in particular the management and staff of the Cannington project for their assistance. Special thanks go to P C Muhling and T J Roberts for reviewing the paper and to G A Yeates, P A Fell and A R Veale for assistance in preparing the illustrations. Many geologists have worked on the project and are acknowledged for their contribution, in particular: S G Walters, B P Grant, G A Yeates, M A Dugmore, S Konecny, M T Roche, T A Paterson, P A Fell, M J Pascoe and K C McGuckin. S B Bodon and G Davidson of CODES Key Centre, University of Tasmania, P J Williams, R G Taylor and P Pollard of James Cook University of North Queensland, and D R Gray of Monash University are also thanked for their contributions to the current understanding of the deposit.

REFERENCES Bailey, A and Thomas, M, 1993. The Cannington deposit - its discovery, geology and evaluation, in Proceedings Carpentaria and Mount Isa Regional Development Forum, pp 59–67 (The Australasian Institute of Mining and Metallurgy: Melbourne). Beardsmore, T J, Newbery, S P and Laing, W P, 1988. The Maronan Supergroup: An inferred early volcanosedimentary rift sequence in the Mount Isa Inlier, and its implications for ensalic rifting in the Middle Proterozoic of Northwest Queensland, Precambrian Research, 40/41:487–507. Blake, D H and Stewart, A J, 1992. Stratigraphic and tectonic framework, Mount Isa Inlier, AGSO Bulletin, 243:1-11. Bodon, S B, 1995. 1994 annual Ph D research report on the Cannington Ag-Pb-Zn deposit, Mt Isa Inlier, Northwest Queensland, CODES Key Centre, University of Tasmania, Hobart (unpublished). Bodon, S B, 1996. Genetic implications of the paragenesis and rareearth element geochemistry at the Cannington Ag-Pb-Zn deposit, Mt Isa Inlier, northwest Queensland, in New Developments in Broken Hill Type Deposits, Special Publication 1 (Eds: J Pongratz and G Davidson), pp 133–144, CODES Key Centre, University of Tasmania, Hobart. French, D, Ramsden, A R and Walters, S G, 1994. Mineralogical characterisation of the southern portion (line 4800N) of the Cannington lead-zinc-silver deposit, Queensland, CSIRO Division of Exploration and Mining, restricted investigation report 253R (unpublished). Laing, W P, 1990. The Cloncurry Terrane: an allochthon of the Diamantina Orogen rafted on to the Mt Isa Orogen, with its own distinctive metallogenic signature, Abstracts of Mount Isa Inlier Geology Conference, pp 19–22, Monash University, Melbourne. Looseveld, R J H, 1989. The synchronism of crustal thickening and high T/low P metamorphism in the Mount Isa Inlier, Australia. An example, the central Soldiers Cap belt, Tectonophysics, 158:173–190.

791

A BAILEY

Mark, G, 1993. Pegmatites and partial melting at the Cannington Ag-PbZn deposit, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Page, R, 1993. Geochronological results from the Eastern Fold Belt, Mount Isa Inlier, AGSO Research Newsletter, 19:4–5. Page, R and Bell, T H, 1986. Isotopic and structural responses of granite to successive deformation and metamorphism, Journal of Geology, 94:365–379. Richmond, J M, Chapman, L H and Williams P J, 1996. Two phases of garnet alteration at the Cannington Ag-Pb-Zn deposit, NW Queensland, in New Developments in Metallogenic Research: The McArthur, Mt Isa, Cloncurry Minerals Province (Eds: T Baker, J F Rotherham, J M Richmond, G Mark and P J Williams), pp 113–117; Extended Abstracts, EGRU Contribution 55, James Cook University of North Queensland, Townsville. Roche, M T, 1994, The Cannington silver-lead-zinc deposit - at feasibility, in Proceedings 1994 AusIMM Annual Conference (Ed C P Hallenstein), pp 193-197 (The Australasian Institute of Mining and Metallurgy: Melbourne).

792

Sheehan, P, 1994. The structural geology of the host rocks to Ag-Pb-Zn mineralisation at BHP’s Cannington Deposit, Eastern fold belt, NW Queensland, BSc Honours thesis (unpublished), Monash University, Melbourne. Skrzeczynski, R H, 1993. From concept to Cannington: a decade of exploration in the Eastern Succession, in Symposium on Recent Advances in the Mount Isa Block, AIG Bulletin, 13:35–38. Walters, S G and Bailey, A, in press. Geology and mineralisation at the Cannington Ag-Pb-Zn deposit - an example of Broken Hill type mineralisation in the Eastern Succession of the Mount Isa Inlier, NW Queensland, Australia, Economic Geology. Williams, P J, Chapman, L H, Richmond, J, Baker, T, Heinemann, M and Pendergast, W J, 1996. Significance of late orogenic metasomatism in the Broken Hill-type deposits of the Cloncurry district, NW Queensland, in New Developments in Broken Hill Type Deposits, Special Publication 1 (Eds: J Pongratz and G Davidson), pp 119–132, CODES Key Centre, University of Tasmania, Hobart. Wyborn, L A I, 1992. The Williams and Naraku batholiths, Mt Isa Inlier: an analogue of the Olympic Dam Granites? BMR Research Newsletter, 16:13–14.

Geology of Australian and Papua New Guinean Mineral Deposits

Adshead, N D, Voulgaris, P and Muscio, V N, 1998. Osborne copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 793–800 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Osborne copper-gold deposit 1

2

2

by N D Adshead , P Voulgaris and V N Muscio INTRODUCTION The deposit, formerly known as Trough Tank, is 195 km SE of Mount Isa, Qld, at lat 22o04′S, long 140o34′E on the Boulia (SF 54–10) 1:250 000 and Toolebuc (7053) 1:100 000 scale map sheets (Fig 1). Placer Pacific Limited owns the deposit and the surrounding Exploration Permit Minerals 9624.

mineralisation has a close spatial relationship with quartzmagnetite ironstone and, like the Starra and Ernest Henry copper-gold deposits of the Eastern Fold Belt, is an important example of this increasingly significant style of mineralisation in the Mount Isa Inlier.

EXPLORATION AND DEVELOPMENT HISTORY The long established Cloncurry mining district experienced a renewed phase of exploration following the discovery of the Pegmont lead-zinc-silver deposit in 1971. A consortium of Newmont Proprietary Limited, ICI Australia Limited and Dampier Mining Company Limited explored the southern reaches of the Eastern Fold Belt of the Mount Isa Inlier seeking comparable base metal mineralisation. An Authority to Prospect was granted in 1975 and aeromagnetic and electromagnetic surveys were flown as part of this exploration effort. Several geophysical anomalies were identified, including a strong magnetic high at Trough Tank. These anomalies were investigated by geological mapping, geophysical and geochemical surveys, plus shallow percussion drilling in areas with no basement outcrop. During 1976, seven percussion holes were drilled into two large magnetic anomalies in the Trough Tank area and the best assay from intervals of base metal–poor banded ironstone was 2 m at 0.13 g/t gold and 0.023% copper. Consequently the consortium regarded Trough Tank as unfavourable for Pegmont style mineralisation and the tenement was relinquished in late 1976.

FIG 1 - Simplified geological map of the southern part of the Eastern Fold Belt, Mount Isa Inlier. Modified from Blake (1987) and Beardsmore, Newbery and Laing (1988).

The deposit is hosted by a multiply deformed and complex sequence of metamorphic, igneous and metasomatic rocks of mid Proterozoic age that is concealed beneath 20 to 40 m of Mesozoic sediment. The epigenetic and largely hypogene 1.

Geologist, Misima Mines, PO Box 5418, Cairns Qld 4870.

2.

Mine Geologist, Osborne Mines, PO Box 5170, Townsville Qld 4810.

Geology of Australian and Papua New Guinean Mineral Deposits

In 1985, Billiton Limited and CSR Limited formed a joint venture to search the region for ironstone hosted copper-gold deposits similar to the recently identified Starra orebodies, 50 km to the NNW of Trough Tank (Fig 1). In the first year of exploration at Trough Tank, 11 reverse circulation (RC) drill holes intersected magnetite-quartz ironstone units with anomalous but subeconomic concentrations of copper and gold. Airborne and ground magnetic surveys combined with induced polarisation surveys between 1985 and 1987 outlined four distinct anomalies within the Trough Tank area. The NE anomaly, which is the largest of the four, received the most attention and was subsequently shown to identify the deposit. In June 1988, Placer Pacific Exploration Limited acquired the CSR Limited Mineral Exploration Development Group and became manager of the joint venture. Between 1985 and mid 1989, a total of 36 diamond drill holes (for 3310 m) and 80 RC holes (for 9811 m) outlined an 800 m strike length of ironstone with low grade copper-gold mineralisation. Placer persevered with the drilling program and was rewarded in late 1989 when four holes drilled into the northern portion of the NE anomaly returned high grade copper-gold mineralisation, including the ‘discovery hole’ (TTHQ029) that intersected 32 m averaging 5.8% copper and 3.2 g/t gold from 98 m. Around the same time

793

N D ADSHEAD, P VOULGARIS and V N MUSCIO

that it was becoming evident that the project was economically significant, Robert Osborne, the project geologist, suddenly passed away, and the NE anomaly was renamed ‘Osborne’ in recognition of his efforts. By June 1993 Placer was the sole owner of the deposit and surrounding tenements. Step out and delineation drilling were carried out between 1990 and 1993 as part of prefeasibility and feasibility studies. A total of 475 holes for 59 272 m of RC and 30 335 m of diamond core were drilled during this period and defined a total Measured and Indicated Mineral Resource of 11.2 Mt at 3.51% copper and 1.49 g/t gold. Project approval was granted in June 1994 and site construction commenced in August. Approximately 260 000 t of oxide and minor supergene copper ore were mined from the open pit between December 1994 and October 1995. The open pit was completed in February 1996 and all of the 968 000 t of hypogene, high grade ore from the pit had been treated by August 1996. The first ore was encountered in underground development in December 1995, with production from stopes beginning in April 1996. The current mining rate from the underground operation is 1.2 Mtpa and planned annual production rates are 29 000 t of copper and 37 000 oz of gold. At the beginning of 1996, Osborne had a total Measured and Indicated Mineral Resource of 11.3 Mt at 2.9% copper and 1.18 g/t gold, which included a Proved and Probable Reserve of 10.8 Mt at 2.96% copper and 1.21 g/t gold.

PREVIOUS DESCRIPTIONS Davidson (1989) and Davidson et al (1989) provided the first published accounts of the host rock and mineralisation characteristics at Trough Tank. Davidson focussed on documenting and understanding the geochemistry, setting and genesis of the ironstone hosted gold-copper mineralisation at Starra but a limited field season at Trough Tank convinced him that the two deposits are genetically related. In addition, sulphur isotope data obtained from Osborne samples collected by Davidson were included in a comparative study of mineral deposits from across the Mount Isa Inlier (Davidson and Dixon, 1992). Subsequent geological research on the deposit focussed on the stratigraphy and structure of the banded ironstone units (Williams, 1995) and on the host rock geology, alteration paragenesis, mineralisation characteristics and hydrothermal fluid geochemistry (Adshead, 1995).

REGIONAL GEOLOGY The predominantly metasedimentary and meta-igneous host rock sequence at Osborne is concealed beneath 20 to 40 m of sediment of the Mesozoic Eromanga Basin (Figs 1 and 3). The undeformed cover rocks onlap on to the mid Proterozoic Eastern Fold Belt of the Mount Isa Inlier and, although direct correlation has not been possible, the similarity of rock types and metamorphic grade indicate that the Osborne sequence is part of the Eastern Fold Belt. Regional mapping by the Bureau of Mineral Resources and Geological Survey of Queensland between 1950 and 1983 was compiled by Blake (1987) but field coverage of the Proterozoic outcrop to the north of Osborne, south of approximately 21o45′S and to the SW of the Squirrel Hills pluton, was limited (Fig 1). Parts of the area have been remapped by Beardsmore, Newbery and Laing (1988), the JCU Cloncurry Mapping Project 1991 (P J Williams and G N Phillips, unpublished data,

794

1991) and Pocock (1992) but the stratigraphy of the area remains problematical. Beardsmore, Newbery and Laing (1988) suggested that basement rocks to the north of Osborne can be correlated with the Soldiers Cap Group to the NNE based on lithostratigraphy (Fig 1). Furthermore, W P Laing (unpublished data, 1990) suggested that the psammitic and ironstone-bearing host rock sequence at Osborne is similar to the Mount Norna Quartzite of the Soldiers Cap Group, but the lithostratigraphic basis for this link is tenuous and the precise stratigraphic position remains uncertain. The current understanding of the stratigraphy of the Eastern Fold Belt of the Mount Isa Inlier is succinctly summarised in Blake et al (1990). The thick sequences of mid Proterozoic sedimentary and volcanic rocks experienced regional metamorphism and polyphase deformation before 1520 Myr and were subsequently intruded by numerous plutons of the largely anorogenic Williams Batholith (Fig 1). The predominantly metasedimentary package that crops out to the north of Osborne has the strong northerly-trending structural fabric that is widely developed across the Mount Isa Inlier, and is intruded by several large plutons of the Williams Batholith (Fig 1). The maximum P-T conditions attained during regional metamorphism vary across the Eastern Fold Belt but the rocks in the south and east preserve mineralogical evidence of uppermost amphibolite facies. Copper was first located in the Eastern Fold Belt in 1867 and there have been several subsequent periods with a high level of exploration and mining activity in the region. Small copper oxide±gold deposits are common to the south and east of Cloncurry but the future production of these metals in the region will be dominated by the ironstone and/or shear zone hosted deposits of Ernest Henry, Osborne, Starra and Eloise (Fig 1).

DEPOSIT GEOLOGY HOST ROCK CHARACTERISTICS The Osborne deposit can be subdivided into western and eastern domains based on significant differences in the host rock sequence and mineralisation characteristics. The nature of the Awesome fault separating the domains has been difficult to determine, but geometrical relationships noted during underground development suggest that the youngest significant movement on the fault was at a high angle and compressional (Figs 2, 3 and 4). Feldspathic psammite±thin layers of pelite is the dominant host rock type enveloping the lodes in each domain. The occurrence of pre-metamorphic banded ironstone units and associated schist define the extent of the western domain whereas a fault-bounded body of meta-ultramafic rock only occurs above the mineralisation in the northern part of the eastern domain (Fig 2). Sheet intrusions of amphibolite and post-metamorphic pegmatite and lamprophyre occur in both domains but they are more common in the eastern domain (Figs 2 and 3).

Feldspathic psammite and pelite The host sequence is dominated by poorly differentiated, pale pink to grey, sodic plagioclase-rich feldspathic psammite with locally developed pelitic bands and stromatitic migmatite. The majority of the feldspathic psammite comprises >95% Σ(albite and/or sodic oligoclase+quartz) with a complete spectrum

Geology of Australian and Papua New Guinean Mineral Deposits

OSBORNE COPPER-GOLD DEPOSIT

FIG 3 - Cross section on local grid line 21 360 N, looking north, across the middle of the Osborne deposit illustrating the distribution of the dominant host rock types. The location of the section is on Fig 2.

FIG 2 - Simplified level plan of the Osborne deposit (1200 RL) illustrating the distribution of the dominant host rock types. Modified from A-K Appleby, M Heinemann and N D Adshead (unpublished data, 1993).

between plagioclase- and quartz-rich examples, though plagioclase-rich rocks are much more abundant. Foliated biotite is invariably present in the granular, plagioclase-quartz mosaics but rarely comprises >5% of the total rock. Granular calcite is also locally common and other minor to accessory peak metamorphic minerals include magnetite, actinolitic hornblende, sillimanite, cobaltian pyrite, microcline, apatite, rutile, titanite, zircon, monazite and tourmaline. Accessory retrograde chlorite (particularly replacing biotite) and carbonate are common, locally accompanied by accessory muscovite, biotite, magnetite, quartz, hematite and epidote. Pelitic bands are rare in the feldspathic psammite sequence and have only been observed more than 200 m above the banded ironstone units and spatially associated metasomatic assemblages. Diagnostic pelitic minerals include almandine garnet, cordierite, sillimanite and microcline, with the typical feldspathic psammite assemblage of sodic plagioclase, quartz and biotite. Stromatitic migmatite is common and geothermometric and geobarometric studies incorporating the stability of the pelite mineral assemblages and evidence of melting suggest that the peak of metamorphism at Osborne occurred at 650–700oC and ~3–7 kb (Adshead, 1995).

Geology of Australian and Papua New Guinean Mineral Deposits

FIG 4 - Cross section on local grid line 21 360 N, looking north, across the middle of the Osborne deposit illustrating the distribution of the high grade copper-gold mineralisation. The location of the section is on Fig 2.

The source of the sodium enrichment in the plagioclase-rich host rocks at Osborne is contentious but the rare preservation of flame structures, ripple marks and graded bedding and absence of volcanic textures suggests that the majority of the sequence was metasedimentary rather than metavolcanic. The intensity of the sodium enrichment indicates that it is probably not a sedimentary feature but, at the current level of understanding, it cannot be determined if the metamorphosed sodic alteration is

795

N D ADSHEAD, P VOULGARIS and V N MUSCIO

due to diagenetic and/or synsedimentary hydrothermal fluids, or to metasomatism following burial and lithification (Adshead, 1995).

Banded ironstone and associated schist Banded magnetite-quartz-apatite ironstone is a distinctive host rock type at Osborne and has a strike length of at least 1.3 km (Fig 2). It occurs as two major stratiform units that strike NW and dip at 25 to 55ο to the NE in the northern part of the deposit, whereas towards the south the dip steepens to about 60o. The upper ironstone unit is 10 to 45 m thick and is separated from the much more continuous and 8 to 15 m thick lower ironstone unit by 6 to 40 m of feldspathic psammite and sporadic peraluminous schist (Fig 3). Subparallel but much thinner and discontinuous bands of mineralogically and texturally similar ironstone locally occur above and below the upper and lower units. The stratiform ironstone units have well developed and commonly folded internal banding but there are also intervals with more massive or breccia textures. The 0.2 to 10 mm wide bands are planar to lensoid and are defined by differences in the relative proportions of magnetite, quartz and apatite. Dark grey, relatively magnetite-rich layers have 25 to 60% quartz and a greater concentration of apatite, whereas the paler grey, quartz-rich layers may contain up to 30% magnetite. Magnetite, quartz and apatite invariably occur as equant crystals sharing simple grain boundaries and 120o triple point junctions. Magnetite and quartz crystals are generally inclusion free but the former locally contain very fine grained, ovoid inclusions of chalcopyrite and pyrrhotite. Magnetite does not contain exsolved lamellae of ilmenite, spinel or hematite and electron microprobe analyses indicate that it is pure iron oxide (Adshead, 1995). Apatite is a ubiquitous primary component (