Genesis of Precambrian iron and manganese deposits: Proceedings of the Kiev Symposium, 20-25 August 1970

Genesis of Precambrian iron and manganese deposits

Genèse des formations précambriennes de fer et de manganèse

Proceedings of the Kiev Symposium,

Actes du colloque de Kiev,

20-25 August 1970

20-25 août 1970

i\

'

Résumés e n français

Unesco

Paris 1973

Earth sciences

Sciences de la terre

9

In this series / Dans cette collection: 1. The seismicity of the earth,1953-1965, by J. P.Rothé / La séismicité du globe, 1953-1965,par J. Rothé. 2. Gondwana stratigraphy.IUGS Symposium,Buenos Aires, 1-15 October 1967 / La estratigrafía del Gondwana. Coloquio de la UICG,Buenos Aires, 1-15 de octubre de 1967. 3. Mineral map o$ Africa. Explanatory note / Carte minérale de l’Afrique.Notice explicative. 1/10O00 000. 4. Carte tectonique internationalede l’Afrique.Notice explicative / International tectonic map of Africa. Explanatory note. 1/5O00 000. 5. Notes on geomagneticobservatoryand survey practice,

by K. A. Wienert. 5. Méthodes d‘observationet de prospectiongéomagnétiques, par K. A. Wienert. 6. Tectoniquede l’Afrique/ Tectonics of Africa. 7. Geology of saline deposits.Proceedingsof the Hanover Symposium,15-21 May 1968 / Géologie des dépôts salins. Actes du Colloque de Hanovre, 15-21 mai 1968. 8. The surveillance and prediction of volcanic activity. A review of methods and techniques. 9. Genesis of Precambrianiron and manganese deposits. Proceedingsof the Kiev Symposium,20-25 August 1970 I Genèse des formations précambriennes de fer et de manganèse.Actes du Colloque de Kiev,20-25 août 1970. 10. Carte géologique internationale de l’Europe et des régions riveraines de la Méditerranée. Notice explicative / Internationalgeological map of Europe and the Mediterranean region. Explanatory note. 1/5O00 O00

11. 11. 12. 13.

(Édition multilingue: français, anglais, allemand, espagnol,italien, russe/ Multilingual edition: French, English,German,Spanish,Italian,Russian). Geological map of Asia and the Far East. 1/5O00 000. Explanatory note. Second edition. Carte géologique de l’Asie et de l’Extrême-Orient. 1/5O00 000.Notice explicative. Deuxième édition. Geothermal energy. Review of research. Carte tectonique de l’Europeet des régions avoisinantes. 1/2500 000.Notice explicative/Tectonic map of Europe and adjacent areas. 112 500 000.Explanatory note. (A paraître/To be published.)

Published by the United Nations Educational,Scientific and Cultural Organization, 7 Place de Fontenoy, 75700 Paris Printed by Presses Universitaires de France,Vendôme Publié par l’organisationdes Nations Unies pour l’éducation,la science et la culture, 7,place de Fontenoy,75700 Paris Imprimerie des Presses Universitaires de France,Vendôme ISBN 92-3-001107-X (Paper/ Broché) ISBN 92-3-001108-8 (Cloth/Relié) L.C.NO 73-79858

0 Unesco 1973 Printed in France

The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Unesco Secretariat concerning the legal status of any country or territory, or of its authorities,or concerning the delimitations of the frontiers of any country or territory.

Les désignations employées et la présentation adoptée ici ne sauraient être interprétées comme exprimant une prise de position du Secrétariat de l’Unesco sur le statutjuridique ou le régime d’un pays ou d‘un territoire quelconque,non plus que sur le tracé de ses frontières.

Foreword

Avant-propos

The Precambrian is of very special significancein the evols ution of the Earth’s crust.It representsalmo st seven-eighth ofthegeologicalhistory of our planet. During this period of time, lasting approximately 4,000million years, the basement of continentalland masses and the deposits of iron and manganese ore were formed.These latter are of world-wide significanceboth in quantity and extent. They form part of the natural resources of the geographical environment and their study is important both for developed and developing countries. The study of Precambrian rocks and ore deposits includes various theoretical and practical aspects-economic, mineralogical, geochemical, tectonic. The research methodology applied to the Precambrian is very specific and fundamentally different from that used for other geological eras. Straightforward time-stratigraphicalmethods are not applicable here because of the lack of palaeontological criteria, destroyed by metamorphism.Successive granitizations form a complex which is very often difficult to bring into conventional order. In an attempt to throw some light on these complex geological phenomena, Unesco, in collaboration with the International Association of Geochemistry and Cosmochemistry of the InternationalUnion of Geological Sciences and the Academy of Sciences of the Ukrainian S.S.R., organized a symposium on the geology and genesis of Precambrian iron-manganeseformations and ore deposits.At the invitation of the Academy of Sciences,the meeting was held in Kiev from 20 to 25 August 1970.Some sixty specialists coming from twelve countries met at the Main Conference Hall of the Academy of Sciences of the Ukrainian S.S.R. and presented papers at this meeting. The participants were welcomed by R . V. Babiychuk, Minister of Culture of the Ukrainian S.S.R. and Chairman of the Ukrainian National Commission for Unesco,Opening addresses were also given by Academician N.P. Semenenko, Chairman of the symposium, and D r K. Lange of the Natural Resources Research Division of Unesco. In order to provide a systematic consideration of the problems, the programme was divided into four sections:

L’ère précambrienne a une importance toute particulière dans l’évolution de l’écorce terrestre. Elle couvre les sept huitièmes de l’histoire géologique de notre planète. Pendant cette période, qui a duré approximativement quatre milliards d’années,se sont formés le socle des masses continentales et les gisements de fer et de manganèse. Ces gisements précambriens présentent un intérêt mondial à la fois sur le plan de la quantité et sur celui de l’étendue.Ils font partie des ressources naturelles du milieu géographique et leur étude est utile tant aux pays développés qu’à ceux qui sont en voie de développement. L’étude des roches et gisements de minerais précambriens revêt divers aspects théoriques et pratiques :économiques, minéralogiques, géochimiques, tectoniques. Les méthodes de recherches appliquées, lorsqu’il s’agit du Précambrien, sont très spécifiques et diffèrent fondamentalement de celles qui sont utilisées pour d‘autres ères géologiques. Les méthodes stratigraphiquesde datation ne sont pas applicables ici,faute de critères d‘ordre paléontologique, dont l’absence est due au métamorphisme. Les granitisations successives ont donné naissance à un ensemble complexe qu’il est très souvent difficile de classer dans l’ordre conventionnel. Afin d‘essayer d‘éclairer quelque peu ces phénomènes géologiques complexes, l’Unesco, en collaboration avec l’Association internationale de géochimie et de cosmochimie de l’Union internationale des sciences géologiques et l’Académie des sciences de la République socialiste soviétiqued’Ukraine,a organisé un colloque sur la géologie et la genèse des formations précambriennes de fer et de manganèse. Sur l’invitation de l’Académie des sciences, la réunion s’est tenue à Kiev du 20 au 25 août 1970. U n e soixantaine de spécialistes venus de douze pays se sont réunis dans la grande salle des conférences de l’Académie des sciencesde la Républiquesocialistesoviétique d‘Ukraine et ont présenté des communications. Les participants ont été accueillis par M. R . V. Babiychuk, ministre de la culture de la RSS d‘Ukraine et par le président de la Commission nationale ukrainienne pour l’Unesco; des discours d‘ouverture ont été prononcés par M . N. Semenenko,

I. Genesis and types of iron-silicate and ferruginous cherty formations,their position in geosynclinal sedimentary or volcanic sequences and the relation between these and analogousmanganese-bearingformations. II. Absolute age dating of iron-silicate and ferruginous formations and their position in the Precambrian stratigraphic sequence. Analogous formations from the Phanerozoic. III. Differing degrees of metamorphism,the mineral facies and the petrographic nomenclature of ferruginous rocks such as ferruginous quartzites, taconites, jaspilites, itabirites. IV. Genesis of high-gradesecondary iron and manganese ores from iron-silicate and ferruginous formations and ores, metasomatic processes and processes of oxidation in them. An exhibition of Precambrian/manganese rocks was arranged: consisting of samples from the U.S.S.R., in particular from the Ukrainian S.S.R.,as well as samples brought by foreign participants. Immediately following the meeting, from 25 to 30 August, a field trip was organized to the well-known Krivoyrog deposits. The Ukraine occupies a leading position in industrial mining and exploration of Precambrian iron formations, and this visit enabled participants in the symposium to make comparisons and correlations with rocks of Precambrian iron formations from elsewhere. The symposium was the first international gathering toprovide an opportunity for a wide exchange of results obtained through studies of rather intricate problems concerning the nature and specific features of the unique ironbearing metamorphic Precambrian strata of the Earth. As a result of a broad discussion of the presented papers, it was recommended that further studies be made on basic regularities of occurrence, distribution and genesis of Precambrian iron-manganeseore formations, with special attention to modern geological,mineralogical,geochemical,and geophysicalmethodsand researchtechniques. The symposium also drew attention to the need for the determination and detailed investigation of ironmanganese deposits, including studies of interrelations between chert-iron-manganesedeposits, including studies of interrelations between chert-iron-manganeseand volcanogenic formations. It was considered that one of the first tasks to be undertaken should be the systematizationand classification of the rocks of chert-iron-manganeseformations,the correlationofnomenclatureoftheserocksindifferentcountries, the elaboration of a unified system of nomenclature for iron rocks in different regions of the world, and the study of analogues of these rocks in conditions of different degrees of metamorphism. A second important task was also recommended: intensification of investigations on the problem of formation of secondary ores, study of characteristic features of these ores in zones of oxidation and supergene alterations along with formation of iron-richores related to hypogene processes.

académicien, président du colloque,et par M.K.Lange, de l'Unesco (Division des recherches relatives aux ressources naturelles). Afin d'assurer l'examen systématique des questions, le programme a été divisé en quatre sections. I. La genèse et les types de formation de silicate de fer et de chert ferrugineux,leur position dans les séquences sédimentaires ou volcaniques géosynclinales et les relations entre ces dernières et les formations manganésifères analogues. Il. La datation absolue des formations de fer et de silicate de fer et leur position dans la série stratigraphique précambrienne. Les formations analogues phanérozoïques. III. Différents degrés de métamorphisme,faciès des minéraux et nomenclaturepétrographique des roches ferrugineuses telles que les quartzites, taconites,jaspilites et itabirites ferrugineux. IV. La genèse des minerais de fer et de manganèse secondaires à haute teneur,à partir des formations de minerais de fer et de silicate de fer, les processus métasomatiques et les processus d'oxydation qui s'y rattachent. Une exposition de roches manganésées précambriennes a été présentée. Elle était composée d'échantillons provenant de l'URSS,en particulier de la RSS d'Ukraine, ainsi que d'échantillons apportés par des participants étrangers. Immédiatement après le colloque une visite,qui a duré du 25 au 30 août, a été organisée aux célèbres gisements de Krivoyrog. L'Ukraine occupe une place prépondérante dans l'exploitation minière industrielle et l'exploitation des formations de fer précambriennes et cette visite a permis aux participants de faire des comparaisons et d'établir des corrélations entre les roches des formations ferrugineuses précambriennes. Ce colloque a été la première rencontre internationale qui ait permis un large échange de résultats d'études consacrées à des questions relativement complexes concernant la nature et les caractéristiques des remarquables couches précambriennes métamorphiques ferrugineusesde l'écorce terrestre. A la suite d'une ample discussion des communications présentées, les participants ont estimé qu'il y avait lieu de procéder à d'autres études sur les constantes fondamentales de la présence, de la répartition et de la genèse des formations précambriennes des minerais de fer et de manganèse, en se préoccupant particulièrement des méthodes et des techniques modernes de recherche géologique, minéralogique, géochimique et géophysique. Ils ont en outre souligné la nécessité de délimiter et d'étudier en détail les gisements de fer et de manganèse en recherchant notamment les relations entre les formations de chert-fer-manganèseet les formations volcanogéniques. L'une des premières tâches devrait être, a-t-onestimé, de systématiser et de classer les roches des formations de chert-fer-manganèse,d'établir la concordance des nomenclatures de ces roches en vigueur dans différents pays, d'élaborer une nomenclature unifiée des roches ferrugi-

It was agreed that publication of the Proceedings of the symposium would be a valuable contribution to geological and geochemical sciences,and while the Academy of Sciences of the Ukrainian S.S.R.has undertaken to provide such a publicationin the Russian language,Unesco was asked to ensure publication in English [with summaries in French). The undoubted success of this symposium was assured on the one hand by the preparatory work undertaken by the InternationalAssociation of Geochemistry and Cosmochemistry, and in particular its President, Professor E.Ingerson,and on the other hand by the excellent organization of the meeting in Kiev by the Academy of Sciences of the Ukrainian S.S.R. Special thanks are due to the Chairman of the Organizing Committee, Professor N.P.Semenenko. The papers presented at the symposium are reproduced in this ninth volume of the Earth Sciences series. The selection of material, the points of view, and the opinions presented are those of the authors and are not necessarily endorsed by Unesco.

neuses des différentes régions du monde et d'étudier les roches analogues à différents degrés de métamorphisme. U n e autre tâche importante a également été recommandée :l'intensification des recherches sur la formation des minerais secondaires,l'étude des traits caractéristiques de ces minerais dans les zones d'oxydation et d'altération supergene ainsi que l'étude de la formation des minerais riches en fer liée aux processus internes. Les participants ayant estimé que la publication des Actes du colloque constitueraitune aide précieuse pour les sciences géologiques et géochimiques, l'Académie des sciences de la RSS d'Ukraine s'est chargée d'assurer cette publication en langue russe,et l'Unesco a été chargée d'en assurer la publication en anglais (avec résumés en français). Le succes incontestablede ce colloque est attribuable, d'une part, au travail préparatoire accompli par 1'Association internationalede géochimie et de cosinochimie,et en particulier par son président,le professeur E.Ingerson, et, d'autre part, à la façon remarquable dont l'Académie des sciences de la RSS d'Ukraine a organisé la réunion à Kiev. L e président du comité d'organisation, le professeur N. P. Semenenko, doit être tout particulièrement remercié. Le présent ouvrage, qui fait partie de la collection ( ( Sciences de la terre », reproduit les communications présentées au colloque. Les opinions qui y sont exprimées n'engagent évidemment que leurs auteurs.

Contents

Table des matières

Genesis and types of iron-silicate and ferruginous cherty formations,their position in geosynclinal sedimentary or volcanic sequences and the relation between these and analogous manganese bearing formations / Les types de formations de silicate de fer et de chert ferrugineux; leur genèse, leur position dans les séquences sédimentaires ou volcaniques géosynclinales et les relations entre ces dernières et les formations manganésifères analogues

The depositional environment of principal types of Precambrianiron-formations Milieux dans lesquels se sont déposés les principaux types deformationsprécambriennes defer [Résumé] G.A . Gross Archaean volcanogenic iron-formationof the Canadian shield Laformation defer volcanogéniquearchéenne du bouclier canadien[Résumé] A . M . Goodwin The facial nature of the Krivoyrog iron-formation Lesfaciès desformationsferrugineuses du Krivoyrog [Résumé] A,I. Tugarinov,I.A . Bergman and L.K.Gavrilova Jacobsitesfrom the Urandi manganese district,Bahia (Brazil) Jacobsites du district de manganèse d’Urandi,Bahia (Brésil) [Résumé] E.Ribeiro Filho Time-distribution and type-distributionof Precambrian iron-formationsin Australia Répartition de l’âgeet du type desformationsprécambriennesdefer en Austr.alie[Résumé] A . F.Trendall The origins of the jaspilitic iron ores of Australia Les origines des minerais de fer jaspilitique d’Australie[Résumé] R. T. Brandt Occurrence and origin of the iron ores of India Manifestationset origine des minerais de fer de l’Inde [Résumé] M . S. Krishnan Precambrian iron ores of sedimentary origin in Sweden Minerais defer précambriens 21 caractèressédimentaires,en Suède [Résumé] R . Frietsch The ferruginous-siliceousformations of the eastern part of the Baltic shield Lesformations defer siliceux dam lapartie orientale du bouclier baltique [Résumé] V. M.Chernov Precambrian ferruginous-siliceousformations associated with the Kursk Magnetic Anomaly Lesformations defer siliceux du Précambrien dans la région de l’anomaliemagnétique de Koursk [Résumé] N.A . Plaksenko,I. K . Koval and I. N.Shchogolev Structural-tectonicenvironments of iron-oreprocess in the Baltic shield Precambrian Environnement tectoniqueet structuraldesprocessus deformation de minerai defer dans le Précambrien du bouclier baltique [Résumé] P. M . Goryainov

15 20 23 33

35 39

41 47 49 55 59 66 69 75 77 82 85 86

89 94 95 98

Geology of the Precambrian cherty-ironformations of the Belgorod iron-oreregion Géologie desformationsprécambriennes defer siliceux dans le gisement de Belgorod [Résumé] Yu. S. Zaitsev Iron-formationand associated manganese in Brazil Formation de fer et de manganèse en association,au Brésil [Résumé] J. Van N.Dorr II The Precambrian iron and manganese deposits of the Anti-Atlas Gisements de minerai de jer et de mangatièse dans le Précambrien de l’Anti-Atlas[Résumé] G.Choubert and A.Faure-Muret Tectonic control of sedimentation and trace-element distribution in iron ores of central Minas Gerais (Brazil) Le contrôle tectonique de la sédimentation et la répartition des éléments-tracesclans les minerais de fer de la partie centrale de l’&tut de Minas Gerais,au Brésil [Résumé] A. L. M.Barbosa and J. H.Grossi Sad

1 o1 1Ó3 105 112

115 123 125

131

Absolute age dating of iron-silicate and ferruginous formations and their position in the Precambrian stratigraphic sequence. Analogous formations from the Phanerozoic / L a datation absolue des formations de fer et de silicate de fer et leur position dans la série stratigraphique précambrienne. Les formations phanérozoïques analogues

The iron-chertformationsof the Ukrainian shield Géologie et genèse desformations de fer siliceux du bouclier cristallin d’Ukraine [Résumé] N.I?. Semenenko Occurrences of manganese in the Guianas (South America) and their relation with fundamental structures Les indicesde manganèse dans les Guyanes (Amériquedu Sud) et leurs relations avec les structuresfondamentales [Résumé] B. Choubert Precambrian ferruginous-siliceousformations of Kazakhstan Les formations de fer siliceux dans le Précambrien du Kazakhstan [Résumé] I. P. Novokhatsky Geology and genesis of the Devonian banded iron-formationin Altai, western Siberia and eastern Kazakhstan Géologie et genèse de laformation dévonienne defer rubaiié dans I’Altai,la Sibérie occidentale et le Kazakhstan oriental Désumé] A . S. Kalugin Genesis of high-grade iron ores of Krivoyrog type Genèse des minerais de fer à haute teneur de Krivoyrog [Résumé] Y.N.Belevtsev Effusive iron-silicaformations and iron deposits of the Maly Khingan Les formations de fer siliceux eflusif et les gisements de fer du Maly Khingan [Résumé] E.V. Egorov and M.W.Timofeieva Effusive jasper iron-formationand iron ores of the Uda area L a formation du minerai de fer à jaspe effusifet les minerais defer de la région d’Ouda[Résumé] E.L. Shkolnik

135 141 143 150 153 156

159 164 167 177

181 184 187 189

Differing degrees of metamorphism, the mineral facies and the petrographic nomenclature of ferruginous rocks such as ferruginous quartzites, taconites, jaspilites, itabirites / Différents degrés de métamorphisme, faciès des minéraux et nomenclature pétrographique des roches ferrugineuses telles que quartzites ferrugineuses, taconites, jaspilites et itabirites

Mesabi, Gunflint and Cuyuna Ranges, Minnesota (United States of America)

193

Les chaînes de Mesabi, Gunflintet Cuyuna dans le Minnesota, aux ÉLats-Unisd’Amérique[Résumé] G.B. Morey

206

Physico-chemicalconditions of the metamorphism of cherty-ironrocks Les conditionsphysico-chimiquesdu métamorphisme desformations de fer siliceux [Résumé] Y.P.Melnik and R . I. Siroshtan The Serra do Navio manganese deposit (Brazil) Le gisement de manganèse de Serra do Navio, au Brésil [Résumé] W.Scarpelli

209 215 217 227

Genetic studies on the Precambrian manganese formations of India with particular reference to the effects of metamorphism 229 Étude génétique des formations de manganèse précambrien en Inde avec références particulières aux efsets du 239 métamorphisme [Résumé] S. Roy Precambrian ferruginous formations of the Aldan shield 243 Formationsferrifères du Précambrien inférieur du bouclier d’Aldan[Résumé] 246 I. D.Vorona,V. M . Kravchenko,V. A. Pervago and I. M . Frumkin O n the issue of genesis and metamorphism of ferromanganese formations in Kazakhstan 249 Formation et métamorphisme des roches ferrugineuses de diverses époques dans les provinces du Kuzakhstan [Résumé] 253 V. M . Shtsherbak,A . S. Kryukov and Z.T. Tilepov Genesis of high-grade secondary iron and manganese ores from iron-silicate and ferruginous formations and ores, metasomatic processes and processes of oxidation in them / Genèse des minerais de fer et de manganèse secondaires à haute teneur, à partir des formations de minerais de fer et de silicate de fer; processus métasomatiques et processus d’oxydation qui s’y rattachent

Iron-formationsof the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites Formations de fer du groipe de Hamersley, en Australie occidentale :exemples typiquesd’évaporitesprécambriennes en varve [Résumé] A.F. Trendall Geology and iron ore deposits of Serra dos Carajás,Pará (Brazil) Géologie et dépôts de minerai de fer de la Serra dos Carajás,Pará,Brésil [Résumé] G . E.Tolbert,J. W.Tremaine,G.C.Melcher and C. B. Gomes Enrichment of banded iron ore, Kedia d’Idjil (Mauritania) Enrichissement des minerais zonés de fer de la Kedia d’ldjilen Mauritanie [Résumé] F.G.Percival Iron ores of the Hamersley Iron Province,Western Australia Les minerais de fer d’Hamersley,en Australie occidentale [Résumé] W.N.MacLeod Significance of carbon isotope variations in carbonates from the Biwabik Iron Formation, Minnesota Significationdes variationsdesproportions des isotopesdu carbone dans les carbonatesdes gisementsdefer de Biwabik,dans le Minnesota [Résumé] E.C. Perry Jr and F.C.Tan Genesis and supergene evolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon) Genèse et évolution supergène du gisement sédimentaireprécambrien de manganèse de Moanda, au Gabon [Résumé] F. Weber The Belinga iron ore deposit (Gabon) Les minerais de fer de Bélinga,au Gabon [Résumé] S. J. Sims Itabiriteiron ores of the Liberia and Guyana shields Les minerais de fer d’itabirite du Libéria et du bouclier guyanais [Résumé] H. Gruss Structuralcontrol of the localization of rich iron ores of Krivoyrog Déterminationstructurale de la localisation des minerais de fer à haute teneur de Krivoyrog [Résumé] G.V. Tokhtuev Iron deposits of Michigan (United States of America) Gisements de fer du Michigan,aux États-Unisd’Amérique[Résumé] J. E.Gair

257 268 271 279 281 288 291 297 299 304 307 320 323 332 335 357

361 364 365 374

Problems of nomenclature for banded ferruginous-chertysedimentary rocks and their metamorphic equivalents

377

List of participants/Liste des participants

381

Genesis and types of iron-sihcateand ferruginous cherty formations,their position in geosynclinal sedimentary os volcanic sequences and the relation between these and analogous manganese-bearing formations

Les types de formations de silicate de fer et de chert ferrugineux; leur genèse, position dans les séquences sédimentaires ou volcaniques géosynclinales et les relations entre ces dernières et les formations manganésifères analogues

The depositional environment of principal types of Precambrian iron-formations G.A. Gross Geological Survey of Canada, Ottawa 4,Ontario (Canada)

Iron-formationscomposed of thinly bedded chert and iron minerals which contain at least 15 per cent iron are probably the most abundant chemically precipitated sedimentary rocks known. They occur in a wide variety of geological environmentsand because of the diversity in chemicalproperties of their elemental constituents are highly sensitive indicators of the depositional environments in which they formed.M u c h of the geologicalliteratureon these rockshas been based on separate iron ranges or formations and interpretations from these specific studies have been applied to the whole group of cherty ferruginous sediments. Interpretations and extrapolations are frequently made without distinguishing adequately the diversity in depositional, tectonic, chronological and host rock environments iii which the many different lithological varieties of these chemical sediments occur. The purpose of this paper is to distinguish differences between some of the principal geological environments where siliceous iron sediments occur and to recognize variations in the physical and chemical characteristics of banded cherty iron-formationsas found in these different environments.Itis necessaryto distinguish and define the various types of depositionalenvironmentsof these rocks before concepts and hypotheses pertaining to their origin and genesis can be satisfactorily evaluated and the geological significance of iron-formations fully appreciated. Of the broad group of iron-richsediments, only the banded cherty iron-formation sediments are considered in this paper. The oolitic chamosite-siderite-goethite clay-rich rocks commonly referred to as ironstones are recognized as a distinctly separate type of iron sediment,They formed in different environments than the cherty iron sediments and probably have a different origin and source of iron. The separategroup ofcherty ironsedimentswhich are associated with a wide variety of sedimentary and volcanic rocks indicate pronounced diversity in conditions in their sedimentary environments. The cherty iron-formations are chemically precipitated sediments and the many different sedimentary facies demonstrate the changes in physical and chemical environment during their deposition.The distinct

variationsingeologicalenvironment and physicaland chemical characteristics of the cherty iron-formations are such that it cannot be assumed that all of these rocks havesimilar sources of iron and silica and similar genetic affinities. It is highly probable that there are other fundamental factors affecting the origin of these sediments which have still not been recognized. Because there are relatively few examples of cherty iron-formationin rocks of Mesozoic age or younger and apparently no modern examples exist where banded cherty iron sediments are forming today,w e have no complete contemporary model or guide to the geological parameters affecting the origin of these special sediments. For these reasons investigations of the depositional environment of cherty iron sediments have to be comprehensive both in scope and in definition of sedimentary featuresand environmentif the mode of origin of these rocks is to be understood. Detailed comparisons of iron ranges throughout the world may provide a composite picture of the complex factors and conditions which contribute to the deposition of iron-formations. It has proved highly instructive to classify or group the cherty iron-formations according to general features and characteristics of their depositional environments and the kinds ofsedimentaryrocks associatedwith them.In Canada the name ‘Algomatype’ has been used in recent years to designate cherty iron-formationsand their equivalentfacies variants that are intimately associated with volcanic rocks and greywacke type sediments in eugeosynclinalbelts. The iron-formations associated with quartzites,dolomites and black slates in continental-shelfenvironments are classified as ‘LakeSuperior type’.This broad classification may not be entirely satisfactory for all occurrences of cherty ironformation,but it servesto distinguish the two main environments in which cherty iron sediments most frequently occur. The Lake Superiortype ofiron-formationformsprominent iron ranges of middle to late Precambrianage in nearly all of the shield areas of the world. Most of the geological literature on cherty iron-formationsis based on this type of iron sediment and it is the host rock, or protore, for

Unesco, 1973. Genesis of Precambrian iron and niunganese deposits. Proc. Kiev Syrnp., 1970. (Earth sciences, 9.)

15

G.A.Gross

the largest and best known iron ore deposits in cherty ironformations. Lake Superior type iron-formations are characteristically thin-banded cherty rocks with iron-rich layers representing various sedimentary facies. Oxide facies are composed of magnetite,hematite or mixtures of these oxide minerals which were deposited mainly as primary iron oxides. Silicate minerals in the silicate facies commonly range from greenalite and minnesotaite to stilpnomelane, cummingtonites and grunerite to hypersthene depending on their rank of metamorphism. Carbonate facies are representedpredominantly by siderite associated with magnetite or iron silicates but ankerite and ferrous dolomites are prevalent where carbonate is associated with hematite-rich facies.Sulphide facies of this type of iron-formationusually consist of fine-grainedcarbon-rich mudstones with interlayered chert or siliceous shale. Characteristic features of the various facies of this type of iron-formationhave been described by Gross (1965), James (1954)and others. Granules and oolites composed of both chert and iron minerals are typicaltexturalfeatures of these sediments and they are practically free of clastic material except in the transitional border zones or in distinct well-defined members within the formation. The alternate or rhythmic banding of iron-rich and iron-poor cherty layers, which normally range in thickness from a few millimetres to 1 metre,is a prominent feature.Individuallayers may pinch and swell to give a wavy-bandedmember or the uniformity of the layering may be disrupted by nodular or stubby lenses of chert and jasper, by rare occurrences of crossbedding, or by cherty forms resembling in shape and structure ‘Collenia’or ‘Crystozoan’growths in limestones formed by algal colonies.Tension,syneresisand desiccation cracks are present in some chert granules and nodules,and styolites are common.Textures and sedimentaryfeatures of this type of formation are remarkably alike in detail wherever examined, although certain sedimentary features are more prominent in some formations than in others. The close associations of this type of formation with quartzite and black carbonaceous shale, and commonly also with conglomerate dolomite, massive chert, chert breccia,and argillite,are recognized throughout the world. Volcanic rocks,either tuffs or flows,are not always directly associated with Superior type iron-formation,but they are nearly always present in some part of the stratigraphic succession.The sequencedolomite,quartzite,red and black ferruginous shale,iron-formation,black shale and argillite, in order from bottom to top,is so common on all continents that some investigatorshave been led to believe in the past that it is invariable. However, stratigraphic studies have shown that, although there is a persistent association of these sedimentary rocks, the successionmay differ in local areas; it does so for example in the Labrador geosyncline. Quartzite and red to black shale lie below the ironformation and black carbonaceous shale above it, but the presence of other sedimentary rocks and their position in the stratigraphic succession may vary from place to place, even in a single range or sedimentary belt. 16

Continuous stratigraphic members of Superior type iron-formation commonly extend for hundreds of miles along the margins of ancient continental platforms or geosynclinalbasins.The formationsmay vary in thickness from a few tens ofmetres to severalhundred metres and occasionally up to 1,000metres, but their persistence is truly remarkable. The rock successions in which the ironformations occur usually lie unconformably above highly metamorphosed gneisses,granites or amphibolites,and the iron-formations are, as a rule, in the lower part of the succession. In some places they are separated from the basement rocks by only a few metres of quartzite,grit and shale or,as in certain parts of the Gunflint Range, they lie directly on the basement rocks. However, in most areas they occur at least some hundreds of metres above the basement rocks. The Lake Superior type iron-formationsare present in late Precambrian rocks in nearly all parts of the world and possibly in some early Palaeozoic rocks (O’Rourke, 1961). They apparently formed in fairly shallow water on continental shelves or along the margins of continental shelves and miogeosynclinal basins, and consist of sediments derived from the adjacent land mass and also some material from the volcanic belts within the basin. It is still considered uncertain as to whether the iron and silica in this type of iron-formationwere derived from the eroding of a land mass or a volcanic source. This type of siliceous formation is the protore or host rock for the rich hematite-goethite orebodies of the Lake Superior region in the United States, Quebec-Labradorin Canada,north-westernAustralia, Orissa and Bihar states in India, Krivoyrog and Kursk areas in the U.S.S.R.,in Brazil and for many other major iron deposits in the world. Algoma type iron-formations are present in nearly all of the early Precambrianbelts of volcanic and sedimentary rocks in the Canadian shield, in parts of the Australian shield and in belts of similar rock of Palaeozoic and Mesozoic age in many other regions. This type of ironformation is characteristically thin-banded or laminated with interlayered bands of ferruginous grey or jasper chert and hematite and magnetite. Massive siderite and carbonate beds, iron-silicatemineral facies and iron-sulphidemineral facies are frequently associated in the formationbut are less abundant than the oxide facies.In the Michipicoten area of Ontario, massive siderite and pyrite-pyrrhotitebeds form part of the formation. Single iron-formationmembers of this type range from more than a hundred metres to less than 1 metre in thickness and rarely extend more than a few kilometres along strike.A number of these lenticular beds may be linked together or distributed en échelon throughout a belt of volcanic and sedimentary rocks. The Algoma type iron-formationsare intimately associated with various volcanic rocks including pillowed andesites, tuffs, pyroclastic rocks, or rhyolitic íìows and with greywacke,greygreen slate, or black carbonaceous slate. Tuff and finegrained clastic beds or ferruginouscherts are interbedded in the iron-formationand detailed stratigraphicsuccessions

The depositionalenvironment of principal types of Precambrian iron-formations

show heterogeneous lithological assemblages. These ironformations have streaked lamination or layering,and oolitic or granular textures are apparently absent, except in rare casesin post-Precambrianrocks.The associated rocks indicate a eugeosynclinal environment for their deposition and a closerelationshipin time and spaceto volcanic activity. The direct association of Algoma type iron-formations with centres of volcanism or volcanic activity is recognized in a number of volcanic belts in the Canadian shield. Rhyolitic and daciticvolcanicrocks are usually thickest and most abundant in the succession of volcanic sedimentary rocks in and around the ancient volcanic centres.In general the Algoma iron-formationsoverlap the bulk of the acidic volcanic material and are in turn covered by andesitic volcanic rocks and associated greywacke type of sediments. Sulphideand carbonatefacies of iron-formationoccur at or near the centres of volcanism and the oxide facies are usually distributed farther away,even where they are almost entirely enclosed by clastic sediments. Carbonate and silicate facies occur near the centres of volcanism, but a general zonalrelationship from sulphide through carbonate to oxide facies of Algoma type iron-formationis commonly found. This direct relationship between the type of ironformation facies and the various kinds and distribution of volcanicrocks leaves little doubt about the geneticrelationship of these cherty iron sediments and volcanic processes. Thin beds of graphitic schist or black carbon-rich mudstones are commonly associated with Algoma type iron-formationand occur mainly in parts of the succession where volcanic rocks are more abundant than the greywacke sediments. Much of the fine clastic material in the black mudstone may be derived from tuff and volcanic ash and collected in depressions in the depositional basin. Usually they contain pyrite and pyrrhotite and parts of them have appreciable amounts of lead, zinc and copper. Black mudstones of this type are closely associated with stratiform base metal sulphide deposits and are one of the common host rocks in which the thin-banded and layered sulphide beds occur. The black mudstones may be a facies of the Algoma type iron-formationand occur in the same bed or member as oxide and carbonate facies.They also occur as separate beds or horizons which are closely associated with thicker beds composed of other facies of ironformation. Algoma type iron-formations are widely distributed in the volcanic-sedimentary belts in the older parts of the Canadian shield and some of the better known examples of this type of iron-formationoccur in the Michipicoten District, near Kirkland Lake, Moose Mountain Mine, Timagami Lake, the Kapico Iron Range,north of Nakina, at Red Lake, Bruce Lake and Lake St Joseph in Ontario. Examples of Ordovician age occur near Bathurst in northern N e w Brunswick and northern Newfoundland and some of Mesozoic age on Vancouver Island. Iron-manganese formations of Algoma type are of particular interest but are relatively rare compared with the frequency of occurrence of the iron-rich beds. Ironmanganese formationswere deposited under much the same

conditions and in a similar geologicalenvironment to those for typical Algoma iron-formation.The manganese content may range from nearly pure cherty manganese sediment to cherty sediments with a low manganese to iron ratio. Examples of this type of cherty sediment are found in the Karazdhal range in the U.S.S.R. and near Woodstock in N e w Brunswick, Canada. These appear to be formed by volcanic exhalativeprocessesand are classified with Algoma type iron-formation. Nearly all of the cherty iron-formationscan be classified satisfactorily in these two principal environmental types. Many of the iron-formations and their associated rocks are highly metamorphosed and their sedimentary environments can only be interpreted from the relict sedimentary features that are still recognizable. Many other iron-formationsare not known in detail and their immediate geological setting or depositional environment has not been studied or reported. A n interesting iron-formationin an unusual geological setting extends along the Yukon and Mackenzie District border in north-western Canada. The Snake River ironformation forms a succession of jasper and blue hematite beds more than 150 metres thick which occur near the base of the Rapitan formation;a crudely stratified,poorly sorted conglomerate at least 1,500 metres thick. The Rapitan conglomerate lies between two angular unconformíties. It overlies a thick succession of dolomite, shale, gypsum and shale, shaly carbonate,limestone, and quartzite which may be Lower Cambrian in age but is believed to be Precambrian. The Rapitan conglomerate is overlain by dark shale and silty dolomite of late Cambrian age. The exact age of the Rapitan formation and the enclosed ironformation is still not known. The Rapitan formation as a whole is composed of conglomeratic siltstone and shale, siltstone and silty shale with about 10 per cent of its volume made up of rounded to subangular coarse fragments mostly in the 1-5 centimetres size range with isolated boulders up to 5 metres in dimension. The coarse fragments consist of carbonate, basic igneous rocks, sandstone,quartzite and shale in decreasing order of abundance. M u c h of the conglomeratic siltstone in the lower part of the formation associated with the iron-formationis highly ferruginous and dark red to maroon in colour. Parts of the Rapitan formation some distance from the thicker iron-formationcontain a high proportion of coarse fragmental volcanic rocks and considerable tuffaceous material. The iron-formationhas an average iron content of 46per cent and is composed ofinterlayered bright red jasper and fine-grained deep blue hematite beds which range in thickness from thin laminae to several centimetres. The jasper and hematite layers are mostly well-segregated,but some hematite beds have conspicuousroundnodules ofred, grey or buff chert 0.5-1 centimetre in size which may make up 20 per cent of the hematite layer. Granular or oolitic textured beds were not found in the iron-formation.Other common siliceous layers and beds are deep red to maroon in colour and are made up of very fine-grainedclastic m u d

17

G.A.Gross

in a highly siliceous matrix. There are numerous thin lenticular beds of coarser clastic material distributed throughout the iron-formation. Some of the fine-grained silty material is composed of tuff fragments and coarser fragments are similar in composition to the coarse fragments in the main conglomerate.T w o thin but continuous silty sandstone beds, one near the base and one near the top of the iron-formationsequence,have been used as horizon markers for correlation of detailed stratigraphy. Thin laminae and beds of ankeritic and dolomitic carbonate are interlayered with the chert and hematite in some parts of the iron sequence. The iron-formationappears fresh and there is little evidence of metamorphism. Primary sedimentary and diagenetic features are well preserved and much can be determined about the sedimentary environment and nature of these beds. Differential compaction features, slump and glide structures, intraformational breccias composed of cherty iron-formationfragments,scour and €ill structures,tension and syneresis cracks are all conspicuous throughout the iron-formation.Many of the coarsefragmentalbeds appear to have been m u d flows which spread over beds of partly consolidatediron-formationcausing distortionand disturbance of the underlying bedding in the iron-formation.The iron-formationoverlying the m u d flow is straight,undisturbed,horizontally bedded jasper and hematite. In some places mud flows were observed which had scoured and cut channels in the soft iron-formation5 metres deep,and tens of metres wide. Large blocks of iron-formationare suspended in the m u d flow and iron-formation fragments in the flow are most abundant near the walls of the channel. The suggestion has been made that some of the large isolated boulders found in the iron-formation,which caused warping and depression of the underlying chert beds,were rafted by ice and dropped in the soft semi-consolidated cherty iron-formation.Most of these lie along thin seams of conglomerate and tuffaceous material and the writer believes that this material is the product of explosive volcanism which took place during the deposition of the ironformation. N o volcanic vents or diatremes have been identified, but the occurrence of tuffaceous layers and volcanic materials in the conglomerate and iron-formation are evidence of volcanic activity during the deposition of these rocks. The thick lenticular iron-formationdescribed here is exposed over a width of 10 miles (16kilometres)and extends laterally for more than 30 miles (48 kilometres). It thins towards the east and west, is terminated at the uiiconformity surface to the north and its extent to the south, where it dips under youngerstrata,has not been determined. The total dimensions of this iron-formation,either for the thicker lenticular zone or for its completelateral extent,are not known. Thinner beds of lithologically similar ironformation,which may be a continuation of this same stratigraphic zone, have been observed in isolated occurrences for more than 200 miles (320kilometres) to the north-west and also for some considerable distance to the south-east. 18

The Rapitan formation represents a rapid filling of a deep basin depression with poorly sorted and stratified silty and conglomeratic material. Chemical precipitation of the iron and silica of the cherty iron-formationhas taken place at the same time as the inpouring of the silty conglomerate and the two types of sedimentation, clastic and chemical, have been superimposed on one another. The ironformationisfreshand relatively unmetamorphosed.Primary sedimentary features indicate that alternate chemical precipitation of silica-and hematite-richlayers was interrupted by the influx of m u d flows and conglomerate which spread over the partly consolidated chert and hematite,in places scouring channels in the soft iron-formation.The conglomerateand iron-formationarebelievedto have been deposited in a broad depression or basin on the ocean floor, and slumping and flow of unconsolidated rocks from adjacent fault scarps or basin shelves may have been triggered by movement along bordering faults or by explosive volcanic activity. Some of the fine-grainedclastic beds impregnated with hematite appear to be tuff or volcanic ash that settled in soft hematite ooze. The hematite and silica are believed to have been transported in solution by hot fumarolic waters and precipitated when these solutionswere discharged on the sea floor along fault zones (Gross, 1965). The Snake River iron-formation may be the product ofexhalative-sedimentaryprocessesand thereforehave a very closegenetic aíñnity to the main g o u p of iron-formations classified as Algoma type. The origin of the Snake River iron beds may be closely analogous to the siliceous iron, manganese and base-metal deposits at present being precipitated in the deeps of the Red Sea (James, 1969). The Snake River iron-formationrepresents a voluminous influx of chemicalIy precipitated iron and silica into a basin that was being rapidly filled by conglomerate and coarsesilt.There is no apparent genetic relationshipbetween the source and manner of derivation of the two types of sediment. In the case of the Algoma type iron-formations, the chemically precipitated iron and chert beds deposited contemporaneously with a great variety of volcanic and sedimentary rock and the specific genetic relationship between the chemical sediment and the various kinds of clastic and volcanic material is subject to conjecture and interpretation.Important empirical relationships of different facies of iron-formationwith certain phases of volcanic activity or kinds of volcanic rock and sediments, and the zonal distribution of different iron-formation facies and exhalative deposits around volcanic centres, leave little doubt that deposition of iron-formationand the volcanic rocks are both expressions of a common igneous-volcanic phenomenon. In the case ofthe Lake Superior type ofiron-formation, very thick successions of chemically precipitated silica and iron sediment have been deposited in sequences of normal and common types of continentalshelf sediment.In many of these areas there was contemporaneousvolcanic activity and deposition taking place along the outer edge of the shelf or basin. A possible common source for the iron and silica

The depositional environment of principal types of Precambrian iron-formations

in the iron-formation aiid the quartzites, dolomites and argillaceous sediments has been proposed by postulating deep chemicalweathering of a land mass and specialerosion and sedimentation conditions to account for the whole assemblage of sedimentary rocks. Geological models based entirely on these concepts of erosion, transportation and deposition of the iron and silica have not provided a satisfactory explanation for the origin of this type of ironformation.The problem of the sourceand origin of the iron and silica has not been solved conclusively by appealing to exhalative-sedimentaryprocesses related to volcanic activity in the adjacent volcanic belts. The writer believes, however, that the source of iron and silica most probably lies in the volcanic belt rather than in an eroded land mass. This opinion is based more on comparison of common features and aspects in the environments of Algoma and Lake Superiorformationsand analogies which may be made between the two types. It is expected that continuing detailed study of the depositional environments of both Algoma and Lake Superior types of iron-formation will provide examples of iron ranges depositedunder conditions intermediate between the volcanic eugeosyncline environment of the Algoma type and the stable continental shelf environment of the Lake Superior type. If this proves to be the case, then the two prominent types of environment now recognized can be considered as two depositional models or sedimentary expressions with the iron and silica derived or supplied from a common kind of source and by a common phenomenon. Recognition of the two principal types of cherty ironformation and characteristicfeatures of their depositional environment is an important step towards determining the critical or essential geological processes and features that are involved or related to the origin and development of these chemical sediments. Only some of these processes or features are mentioned here in a qualitative way and it is not possible in this short paper to elaborate on their significance or implication with regard to the source of iron and silica and the origin of cherty iron-formations. There are also many important economic implications related to these typical environments which are being considered in mineral exploration. Distinctive characteristics of the different kinds of iron ore derived from the principal types of iron-formationhave been recognized and described in the literature on iron-oredeposits and will not be elaborated here. Recognition of the characteristic features of the types of iron-formation plays an important part in the evaluation of newly discovered or developed iron-oredeposits.Probably one of the most significant factorsrelating to the type of iron-formationis recognition of the kind of manganese, base-metal or gold deposits that may be associated with it. Important stratiform base-metal sulphide deposits in the same geological environment as Algoma type ironformation are recognized as faciesvariants ofsulphidefacies or iron-formationand, like the iron-formation,are considered to be exhalative-sedimentary volcanic deposits. There is little doubt about the genetic relationship of these

stratiform base-metalsulphide deposits with sulphide,carboiiate and oxide facies of iron-formation,and recognition of this fact has fostered new and highly rewardingconcepts in mineral exploration in the Canadian shield.The empirical association of gold deposits and Algoma type ironformation has been recognizedfor many years.In the past, some have explained this relationship on a structuralbasis, believing that the brittle cherty iron-formationswere a favourablehost rock for quartzvein development.Evidence is n o w being accumulated to show that the carbonate and some of the sulphide facies of Algoma type iron-formation are source beds for gold and probably silver which were later concentrated in veins and stockworks associated with the iron-formation. It is noted that the composition and physical characteristics of some of the stratiform base-metal sulphide deposits associated with Algoma type iron-formationare very similarand directly comparablewith the contemporary layered siliceous sulphide sediment being deposited in the deeps of the Red Sea. It is highly probable that deposits in the Algoma type iron-formationand Red Sea environments are both products of deep-seatedmagmatic processes centred along major faults or tectonic features in the crust, Fumarolic activities and circulation of water caused by near-surface thermal gradients have probably given rise to the solution and transport of large quantities of silica and metallic ions in both cases.In the Algoma type environment there has been a prominent deposition of volcanic rock contemporaneous with the discharge of these metal bearing solutions and deposition of their salts,while in the Red Sea solutions are being discharged from the deepseated fault systems without active volcanism. Referring briefly to the global distribution of cherty iron-formations,it is noted that many of the major Precambrian iron-formationsof the world lie close to or parallel to the borders of the continentalmasses. This is the case for iron-formationnear the west coast of the African continent and those in South America along its east coast, and for the distribution of iron-formations in India and Australia. These iron-formations are Precambrian sediments in ancient shield terrain which may have been closely related to, or even parts of, the sanie depositional basins and tectonic belts prior to continental drifting and segmentation of the principal Precambrian land masses. The type of cherty iron-formation,its associated rocks and depositionalenvironment for each of these iron belts, need to be defined and compared in detail to determine whether the iron ranges now on the borders of different continents may at one time have formed parts of the same depositional basins and tectonic belts. This comparison of the type and environmentof iron-formationbelts is of course dependent on better determination of the age of sedimentation of the iron beds and much inore detailed information on the chronological sequence of events in each of the ironformation ranges. The writer believes that many of the Precambrian iron ranges and their depositional basins may be closely related to, ifnot parts of,the same sedimentarysequences of rocks 19

G.A Gross

which were separated during the segmentation and drifting of the continents. The iron-formations may be closely related to major deep-seated fault and tectonic systems of global dimensions which existed in the Precambrian land mass and have not been recognized because of continental drift.The separation of large volumes of iron and silica and their transportation by fumarolic water or by circulation of water currents caused by thermal gradients along these tectonic zones may be related to deep-seated igneous aiid volcanic processes.Ifthis is the case,we can then appreciate some fundamentalreasons and basic causes for finding this large group of cherty sediments in such a diversity of depo-

sitional environments,The fundamental reasons for finding voluminous sequences of silica, iron and other metallic elements on continental shelves, in volcanic-sedimentary rock assemblages in eugeosynclines, or in thick sequences of conglomerate,as in the case of the Snake River ironformations, will not be found by exclusive studies of the sedimentation in typical iron-formationenvironments. These answers will most likely be found through study of major tectonic features and the associated deep-seated igneous processes which may have had a common genetic relationshipto all of these distinctive sedimentary environments of iron-formation.

Résumé Milieux dans lesquels se sont déposés les principaux types de formations précambriennes de fer (G. A.Gross)

Les formations de fer, veinées de silex, qui sont réparties très largement dans toutes les régions du bouclier précambrien se rencontrent dans deux types principaux de milieux ; d'où le nom qui leur a été donné en Amérique du Nord : ( (Algoma ) )et ( (Lac Supérieur ) ) . L e type ((Algoma 1) est étroitement lié à la fois par sa genèse et par sa localisation aux roches volcaniques. O n pense qu'il a été produit par des processus d'exhalation volcanique dans un milieu eugéosynclinal.I1 consiste dans une grande variété de faciès sédimentaires qui vont de l'oxyde de fer siliceux aux faciès des carbonates,silicates et sulfures. Ce type est largement distribué dans les roches volcaniques archéennes dans tout l'ensemble du bouclier canadien. L e type ((Lac Supérieur ))s'est déposé sur la plateforme protérozoïque et dans les environs du plateau continental.Il est associé avec la quartzite,la dolomite et l'argile schisteuse noire, et avec du tuf en moindre quantité et d'autres roches volcaniques. C e type de formation de fer siliceux atteint des épaisseurs de plusieurs centaines de

pieds et est distribué de façon continue sur des centaines et m ê m e des milliers de kilomètres près de la ligne de côte des anciens continents. U n exemple remarquable de ce type de formation de fer est la région du lac Supérieur et le géosynclinal du Labrador dans le bouclier canadien. U n e image précise de la position relative des zones couvertes par le bouclier précambrien avant la dérive des continents est nécessaire pour l'étudedes milieux sédimentaires où s'est forméle fer siliceux.Les zones où l'on trouve le fer dans l'hémisphèrenord à l'intérieur des masses continentales actuelles ont pu être préservées dans un milieu phanérozoïque tectonique relativement stable. Les régions où se rencontre le fer près des bordures des masses continentales actuelles dans les régions équatoriales et dans l'hémisphère sud semblent avoir été fragmentées à la suite de la dérive des continents. Des comparaisons entre des milieux où se sont formés les dépôts des formations de fer devraient permettre de reconstruire les principales zones de dépôts de fer, les vastes plateaux continentaux ainsi que les environs des bassins où les deux types se sont déposés avant la dérive des continents.

Bibliography/ Bibliographie CANADA. GEOLOGICAL SURVEY OF CANADA. 1963. Geology of northern Yukon territory andnorthwestern district of Mackenzie. Ottawa, Geological Survey of Canada.(Map 10-1963 .) GOODWIN, A. M . 1962. Structure, stratigraphy and origin of

iron formations,Michipicoten area,Algoma district,Ontario, Canada. Bull. Geol. Soc. Amer., vol. 73, p. 561-86. GROSS, G. A.1965.Iron-formation,Snake River area, Yukon and Northwest territories; Report of activities; Field, 1964. Geol. Surv. Pap. Can., 65-1, p. 143. . 1965-68. Geology of iron deposits i?z Canada. Ottawa,

-_

Geological Survey of Canada. (Econoinic Geology Report no. 22.) Vol.I: General geology and evaluation of iron deposits (1965);Vol. II: Iron deposits, Appalachian and Grenville

20

regions (1967); Vol. III: Iron ranges of the Labrador geosyncline (1968). JAMES, H.L. 1954.Sedimentary facies of iron-formation.Econ. Geol., no.49,p. 235. . 1966. Data of geochemistry,sixth edition, chapter W. Chemistry of the iron-richsedimentary rocks.Prof. Pap, US. Geol. Surv., 440-W. .1969. Comparison between Red Sea deposits and older ironstone and iron-formation; Hot brines and recent heavy metal deposits in the Red Sea. Edited by Egon. T. Degens and

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David A.Ross. New York, N.Y., Springer. O'ROURKE, J. E.1961,Paleozoic banded iron-formation.Econ. Ceol.,vol. 56, p. 331-61.

The depositional environment of principal types of Precambrian iron-formations

SAPOZHNIKOV, D. G. 1963. Karadzhal'skoe zhelezo-margantsevoe rnestorozhdenie [The Karadzhal iron-manganese deposit]. Transactions, Institute of the Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, no. 89,p. 12395. Moscow, U.S.S.R. Academy of Sciences. (In Russian.) (Unpublished translation by the Canada Department of the Secretary of State, Bureau for Translations).

ZELENOV, K.K.1958.O n the discharge of iron in solution into the Okhotsk Sea by thermal springs of the Ebeko volcano (Paramushir Island). C.R. Acad. sci. U.R.S.S.,vol. 120, p. 1089-92. (In Russian; English translation published by Consultants Bureau Inc.,1959,p. 497-500.) -. 1970. Survey of world iron ore resources. New York, N.Y., United Nations.(Sales no. E.69,II. C.4.)

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Archaean volcanogenic iron-formation of the Canadian shield A. M.Goodwin Department of Geology, University of Toronto,Canada

Introduction Iron-formationis widely distributed in Archaean (older than 2,500m.y.) rocks of the Canadian shield. Although individual iron-formations are comparatively small, their wide distributioncompensates in total quantity.Thus total estimated iron-ore reserves in Archaean iron-formation amount to 35,000million tons with an average content of 25-30 per cent Fe. This constitutes25 per cent of the total estimated iron-oreresource in Canada (Gross, 1968). This paper demonstrates the genetic relationship of Archaean iron-formationto volcanism by focusing on three increasing levels of relationships: (a)the Helen iron range

where the quantities of silica present in the iron-formation are equivalent to those leached from subjacent footwall volcanic rocks;(b) the Michipicoten area where basin analysis has revealed the genetic relationship of iron facies to basin bathymetry and volcanic centres; and (c) the Canadian shield with exclusive regional relationship ofArchaean ironformation to volcanic-richgreenstone belts.

Helen Iron Range The Michipicoten area, situated in southern Superior tectonic province (Fig. i), is underlain by Archaean

Michi picoten

FIG.1. Location of Michipicoten area and HeIen iron range. Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

23

A,M.Goodwin

FIG.2. Geology of Helen iron range. supracrustalrocks of Michipicoten Group and by younger intrusions.In the centralpart,which includes the main iron range, the Helen iron-formation forms a part of a thick, varied volcanic succession.Situated at the top of thick felsic pyroclasticsand overlainby mafic flows,it occupies a unique stratigraphic position at an abrupt felsic-mafic volcanic interface (Fig. 2). Structurally,the Helen iron-formationand enclosing volcanic rocks have been overturned to the north about an east-trendingfold axis;the rocks dip steeply southward but face to the north.They plunge eastward at 30-45 degrees.

The main banded chert member is from 400to 1,000ft (120-300 m) thick. It is typically composed of alternating bands of white to grey chert and pale brown siliceous sider-

Stratigraphic tops

H E L E N IRON F O R M A T I O N

Thisiron-formationcomprises three distinctiveand mutually transitionalfacies which are in descending stratigraphicsuccession,banded chert,pyrite and sideritemembers (Fig. 3). In addition, thin discontinuous chert zones are present within and at the base of the siderite member. 24

-, i

FIG.3. Cross-sectionof Helen iron-formation.


below the present bedrock surface; the ore distribution, although 205

G.B. Morey

modified by erosion, indicates that the bedrock surface nearly parallels the present surface and was, therefore, one of low relief. The actual time of ore iormation is not completely documented. Peterman (1966) showed that the Cuyuna rocks were metamorphosed about 1,750 m.y. ago and later affected by ‘. . .“hydrothermal” leaching . . .’about 1,460 m.y. ago, an interpretation consistent with the geologic evidence outlined by Schmidt (1963). Unfortunately, the time of second-stage ore formation cannot be as precisely established.If it is inferred that both the secondstage Cuyuna ores and the Mesabi ores developed approximately during the same time-interval,some limits can be

placed on that interval,The ore deposits must have been formed prior to Late Cretaceous time on a previously developed bedrock surface of low relief. Parham (1970) has shown that a thick regolith was developed in Mesozoic time prior to the early Late Cretaceous on a peneplain that extended from Manitoba,Canada to at least southern Minnesota. Previously Symons (1966) suggested, on the basis of palaeomagnetic data, ‘. . .that meteoric solutions weathered the primary Animikie iron-formationsduring the Mesozoic-Cenozoic to form. . . ore deposits.’ Thus, it appears likely that the weathered natural ores were the consequence of a prolonged period of weathering during late Mesozoic time.

Résumé Les chaines de Mesabi, Gunflint et Cuyuna dans le Minnesota, aux États-Unis d’Amérique(G. B. Morey)

Le Minnesota est l’un des plus grands producteurs de minerai de fer du monde. L a plus grande partie du minerai provient des formations de fer du Précambrienmoyen dans les chaînes de Mesabi, Cuyuna et Gunflint. L a chaîne de Mesabi est une bande étroite de formation de fer qui s’étend sur près de 200 kilomètresà travers la partie septentrionale du Minnesota. C‘est le plus grand producteur du monde avec une production de 2,7 milliards de tonnes brutes de minerai depuis 1892. Sur ce total, environ 809 millions de tonnes brutes (soit 30 %)ont été concentrées soit à partir de minerais naturels à faible teneur,soit à partir de taconite contenant de la magnétite. Durant les quinze dernières années, la production de minerai naturel a diminué et,en 1968, 59 %de la production totale de la chaîne était du concentré de taconite. L’extrémité ouest de la chaîne de Mesabi disparaît sous des strates du Crétacé et du Pléistocène; cependant, la formation de fer suit un tracé sinueux pour se rattacher aux formations de fer de la chaîne de Guyuna dans la partie centre-estdu Minnesota. Depuis sa découverte en 1904, la chaîne de Cuyuna a produit et expédié environ 103 millions de tonnes brutes.Dans les années récentes,la production de minerai naturel a diminué de 81 %, passant de 3,6millions de tonnes en 1955 à 698O00 tonnes en 1968. Cependant, à l’inverse de ce qui se passe dans la chaîne de Mesabi, aucun concentré de taconite n’est en production courante. L a partie est de la chaîne de Mesabi est tronquée par le complexe de Duluth du Précambrien supérieur mais un prolongement de la formation de fer émerge de nouveau du complexe de Duluth ?i environ 60 kilomètres au nord-est sur la chaîne de Gunflint dans le district de Thunder Bay dans l’Ontario et dans la partie adjacente du Minnesota. On ne trouve aucun minerai naturel sur la chaîne de Gunñint et l’on ne peut guère espérer trouver du minerai de taconite que dans la petite partie de la chaîne qui se trouve dans le Minnesota. 206

L a minéralogie des formations de fer inoxydées comprend du quartz (silex), de la magnétite, de la sidérite, de la stilpnomelane et de la minnesotaïte avec de moindres quantités d’hématite,de calcite,de dolomite,de chamosite, de greenalite et de chlorite. A l’intérieur d‘une zone de contact métamorphiqueautour du complexe de Duluth,la formation de fer contient du quartz, de la magnétite, des amphiboles, des pyroxènes, du grenat et de la fayalite.L a teneur moyenne de la formation de fer inoxydée est d’environ 29 % de fer, 46 % de silice et 0,9 % d‘alumine. Quant à leur structure,les chaînes de Mesabi et du Gunflint présentent un homocline peu accusé de direction est-nord-estet plongent de 5 à 15” vers le sud-est.Cette direction générale est modifiée par plusieurs plissements en travers dirigés vers le nord et par de nombreuses failles d‘orientation nord-est,est et nord-ouest.Par contre, la structure de la chaîne de Cuyuna est complexe.Les roches sont étroitement plissées dans une série de plis de direction générale nord-est,localement isoclinales et généralement renversées vers le nord-ouest; on trouve des plissements en travers dirigés vers le nord et quelques petites failles. Des structuresmoins importantes(petits plissements,failles et monoclines) ont joué un rôle important pour la locaíisation des gisements de minerai naturel sur les chaînes de Mesabi et de Cuyuna. Les concentrations de minerai naturel offrent une grande variété de formes et de dimensions.Les donnéesgéologiqueset chimiquesdontnous disposonsactuellementindiquent que les minerais naturels sont le résultat de solutions qui se sont déplacées le long des zones perméables OU l’oxydation et la lixiviation se sont produites. L a source et la nature de ces solutions sont inconnues.Les observations faites sur la chaîne du Cuyuna laissent peiiser qu’il y a eu deux périodes d’altération,une période ancienne hydrothermalesuivie beaucoup plus tard par une période de désagrégation et de désintégration. Les observations faites sur la chaîne de Mesabi corroborent la thèse que la lixiviation et l’oxydation se sont produitesà l’époquecénozoïque par l’action des eaux de surface et des procédés normaux

Mesabi, Gunflint and Cuyuna ranges,Minnesota (United States of America)

de désagrégation. Les gisements de minerai naturel sont généralement composésde quartz,de martite,d’hématite et degoethiteet,en quantitémoindre,demagnétite,d‘oxyde de manganèseet de kaolinite.Bien que la chimiedes gisements

de minerai naturel soit étroitement reliée à la composition des strates dont ils sont dérivés, la teneur moyenne en fer est d’environ 59 %; la quantité de silice varie entre 2 et 10 % et celle d’aluminiumvarie de moins de 1 à 6 %.

Bibliography/Bibliographie BONNICHSEN, B. 3 969. Metamorphic pyroxenes and amphiboles in theBiwabikIronFormation,Dunka River Area,Minnesota.

.

Spec. Pap. Miner. Soc. Amer..,no. 2, p. 217-39.

BRODERICK, T.M.1920.Economic geology and stratigraphy of the Gunñint iron district, Minnesota. Econ. Geol., vol. 15, p. 422-52. CANNON, W.F.;GAIR, J. E.1970. A revision of stratigraphic nomenclature for Middle Precambrian rocks in northern Michigan. BdI. geol. Soc. Amer., vol. 81, p. 2843-6. CLEMENTS, J. 1903.The Vermilion iron-bearingdistrict of Minnesota. Monogr. U.S.geol. Sirrv.,no. 45,463 p. FAURE, G.;KOVACH, J. 1969. The age of the Gunflint Iron Formation of the Aniniikie Series in Ontario,Canada. Bull. geol. Soc. Amer., vol. 80, p. 1725-36. FRENCH, B.M . 1968.Progressive contact metamorphism of the Biwabik Iron-formation,Mesabi range, Minnesota. Bull. Mim.geol. Surv.,no. 45, 103 p. GOLDICH, S. S.;NIER, A.O.;BAADXAARD, H.; HOFFMAN, J. H.; KRUEGER, H.W. 1961. The Precambrian geology and geochronologyofMinnesota.Bull. Minn.geol.Surv.,no.41,193p. GOODWIN, A. M.1956. Facies relations in the Gunflint Ironformation.Econ. Geol., vol. 51, p. 565-95. GRIFFIN, W.L.;MOREY, G. B.1969.The geology of the Isaac Lake Quadrangle,St Louis County, Minnesota.Spec. Publ. Minn. geol. Sirrv.,no. 8,57 p. GROUT, F.F.; WOLFF, J.F.SR.1955.The geology ofthe Cuyuna district, Minnesota. Bull. Minn. geol. Surv., no. 36, 144 p. GRUNER, J. W.1924.Contributionsto the geology of the Mesabi range, with special reference to the magnetites of the ironbearing formation west of Mesaba. Bull. Minn. geol. Surv., no. 19, 71 p. -.

1946. The mineralogy andgeology of the tacoriites and iron ores of the Mesabi range, Minnesota. St Paul, Minnesota,

Office of the Commissioner of the Range Resources and Rehabilitation, 127 p. HANSON, G.N.; MALHOTRA, R.1971.K-Arages of mafic dikes and evidence for low-graderegional metamorphism in northeastern Minnesota. BuII. geol. Soc. Ainer. (in press). HUNT, T.S. 1873.The geognosticalhistory of the metals. Trans. Amer. Inst. min. (metall.) Engrs., vol. 1, p. 331-95. HURLEY, P. M.; FAIRBAIRN, H.W.; PINSON, W.H.; HOWER, J. 1962.Unmetamorphosedminerals in the Gunflint Formation used to test the age oftheAnimikie.J.Geol.,vol.70,p.489-92. IRVING, R.D.1883.The copper-bearingrocks of Lake Superior. Monogr. US.geol. Surv., no. 5, 464 p. JAMES, H.L.1954.Sedimentary facies of the Lake Superiorironformations.Econ. Geol., vol.49, p. 235-93. . 1955.Zones ofregionalmetamorphism in the Precambrian of northern Michigan. Bull. geol. Soc. Amer.., vol. 66, p. 1435-88. . 1958.Stratigraphy of pre-Keweenawanrocks in parts of northern Michigan, Prof. Pap. U S . geol. Surv., 3144, p. 27-44.

__

__

.

1960. Problems of stratigraphy and correlation of Precambrian rocks with particular reference to the Lake Superior region. Amer. J. Sei., Bradley volume, vol. 258-A, p. 104-44. 1966. Data of geochemistry, 6th Edition. Chapter W. Chemistry of the iron-richsedimentaryrocks.Prof. Pap. U.S. geol. Surv., 440-W,61 p.

LABERGE,G.L. 1964. Development of magnetite in ironformations of the Lake Superiorregion.Econ. Geol.,vol. 59, p. 1313-42. . 1966. Altered pyroclastic rocks in iron-formationin the Hamersley Range, Western Australia. Econ. Geol., vol. 61, p. 147-61. .1967. Evidence on the physical environment of ironformation deposition (Abc.).13th Annual Meeting Institute on Lake Superior Geology M a y I-2,1967,East Lansing,Michigan, p. 25.

LEITH, C.K . 1903. The Mesabi iron-bearingdistrict of Minnesota. Monogr. US.geol. Sirrv.,no. 42, 316 p. MARSDEN, R. W.; EMANUELSON, J. W.; OWENS, J. S.; WALKER, N.E.;WERNER, R. F. 1969. The Mesabi Iron Range, Minnesota.Ore deposits of the United States,vol. 1, chap.25, p. 518-37. New York, American Institute of Mining (and Metallurgical) Engineers. MENGEL, J. T.JR.1965.Precambrian taconite iron-formations. A special type of sandstone, Geological Society of Americu Program for 1965 Annual Meetings, Nov. 4-6, 1965, Kansas City, Missouri,Abstracts,p. 106. MOREY, G.B. 1969. The geology of the Middle Precambrian Rove Formation in northeastern Minnesota. Spec. Publ. Mim.geol. Surv., no. 7, 62 p. -; OJAKANFAS, R.W.1970. Sedimentology of the Middle Precambrian Thomson Formation, east-central Minnesota. Rep. Invest. Minn. geol. Surv.,no. 13, 32 p. MURRAY, A. 1957. Report for the year 1856. p. 145-90. Geological Survey of Canada (Report of Progress for 1853-5455-56). OWENS, J. S.;TROST, L.C.; MATTSON, L.A. 1968.Application of geology at the Butler and National Taconite Operations on the Mesabi range. Society of Mining Engineers reprint Number 68-1-345, Minneapolis Meeting, Sept. 1968. PARHAM, W.E.1970. Clay mineralogy and geology of Minnesota’s kaolin clays. Spec. Publ. Minn. geol. Surv., no. 11 (in press). PERRY, E.C.JR.; BONNICHSEN, B. 1966.Quartz and magnetite: oxygen-18-oxygen-16 fractionation in metamorphosed Biwabik Iron-formation.Science, vol. 153, p. 528-9. PETERMAN, 2.E. 1966. Rb-Sr dating of Middle Precambrian metasedimentary rocks of Minnesota. Bull. geol. Soc. Amer., vol. 77, p. 1031-44. SCHMIDT,R. G.1963.Geology and ore deposits of the Cuyuna North range,Minnesota.Prof. Pap. U.S.geol. Surv.,no.407, 96 p.

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SIMS, P.K.; MOREY, G.B.;OJAKANGAS, R.w.; GRIFFIN, w.L. 19680. Stratigraphic and structural framework of the Vermilion district and adjacent areas,northeastern Minnesota (Abs.). 14thAnnual Institute on Lake Superior Geology,M a y 6-7, 1968, Superior, Wisconsin, p. 19-20. , . > . __ . 1968b.Preliminary geologic map of the

Vermilion district and adjacent areas, northern Minnesota. Minri. geol. Surv.Misc.Mup M-5. SLOAN,E.R. 1964;The Cretaceous system in Minnesota.Rep. Invest. M i m . geol. Surv., no. 5, 64 p.

SPURR, J. E.1894,The iron-bearingrocks of the Mesabi range in Minnesota.Bull. M i m . geol. Surv., no. 10,268 p. SYMONS,D.T.A.1966.A paleomagnetic study of the Gunflint, Mesabi, and Cuyuna iron ranges in the Lake Superiorregion. Econ. Geol., vol. 61,p. 1336-61. SANTON, T.L.1931.Fort William and Port Arthur,and Thunder Cape map areas,Thunder Bay district,Ontario.Mem. geol. Siirv. Can., no. 167,222 p. TRENDALL, A. F. 1968. Three great basins of Precambrian

208

banded iron formation deposition.A systematic comparison, Bull.geol. Soc. Amer., vol. 79,p. 1527-44. VAN Hrse, C.R.;CLEMENTS, J. M.1901. The Vermilion ironbearing district. 21st Annu. Rep. US. geol. Sirrv.,part 3, p. 401-9. ;LEITH, C. K.1911,The geology of the Lake Superior region.Monogr. U.S.geol. Surv., no. 52, 641 p. WHITE, D.A.1954.The stratigraphyand structureof the Mesabi range,Minnesota. Bull.Minn. geol. Surv., no. 38,92 p. WINCHELL, H.V. 1893. The Mesabi Iron Range. Annu. Rep. Minn.geol. Surv., vol. 20,p. 111-80. WOLFF, J. F.1915.Ore bodies ofthe Mesabi range.Engng.Min. J. (Press), vol. 100,p. 89-94, 135-9, 178-85,219-24. . 1917.Recent geologic developments on the Mesabi range, Minnesota. Trans.Amer. Inst.min. (metall.) Engrs.,vol. 56, p. 142-69. YODER, H.S. 1957.Isograd problems in metamorphosed ironrich sediments.Annu. Rep.Geophys. Lab.,p. 232-7.Washington,Carnegie Institute,(Yearbook 56).

Physico-chemicalconditions of the metamorphism of cherty-ironrocks Y.P.Melnik and R. I. Siroshtan Institute of Geochemistry and Physics of Minerals, Academy of Sciences of the Ukrainian S.S.R.

Cherty-iron rocks make up a considerable part of any iron-formation.These rocks are characterized by a variety of mineral phases (from hematite-bearing nonsilicate jaspilites to silicate-bearingcherts and slates), and by a similarity in bulk chemical composition.For example,jaspilites and slates contain different mineral phases, but are very similar to each other in their silica and total iron content, whereas other rock-formingcomponents are in subordinate quantities and do not play any important role in mineral formation. Thus, the chemical composition of such rocks is characterized by the predominance of iron and silica over other components. This characteristic enables us to separate cherty-iron rocks into a separate iron-siliceous isochemicalgroup.This peculiarity of mineral formation in the system Fe,O,-Fe-SiO, has been described by Korzhinsky (1940) and Semenenko (1966). Korzhinsky maintains that, at the early stages of metamorphism, the formation of paragenetic hematite to magnetite was accompanied by the inert behaviour of oxygen.Increased metamorphic alteration is accompanied by a certain activity of oxygen, which results in the replacement of hematite by magnetite ; the former becomes unstable in high-temperaturemineral associations. Semenenko considers that the activity of iron depends on the presence of ferrous oxide,which reacts with SiO,to form silicates; ferric oxide enters into reactions with silica only when chemical potential of Na,O is high. When Fe0 and Fe,O, are both present in the sediment, the ore mineral magnetite forms first; the remaining F e 0 reacts with SiO, to form ferrous silicates. The equilibrium of the iron ore minerals (hematite, magnetite and siderite) in metamorphic rocks was investigated both on the basis of thermodynamic calculation (Hawley and Robinson, 1948; Holland, 1959; Kornilov, 1969; Melnik, 1964a,b, 1966a, 196901,and on the basis of experimental data (French and Rosenberg,1965;Melnik, 19666;Seguin, 1968; Shunzo, 1966). Peculiarities of mineral equilibrium involving the participation of fayalite also have been studied (Melnik and Jarotschuk, 1966). The data obtained helped the more complete understanding

of the role of other components (graphite) and mineral associations (hematite +magnetite, magnetite +fayalitej in elucidating the conditions of metamorphism in chertyiron rocks, and in the creation of the controlled oxidation-reductionsystem-via buffers-that fix the fugacity of oxygen-foz (Eugster, 1961; French, 1966; dames and Howland, 1955). The physico-chemicalinvestigations cited above were, to a certain extent,approximate and not exhaustive because of the absence of thermodynamic constants for a number of rock-forming minerals (amphiboles, micas, chlorites), inaccuracy in calculations of tabular constants (siderite, ferrosilite, fayalite, etc.j, considerable discrepancies between calculated and experimental data,and the absence of the data pertaining to the characteristics of fluid phases under high pressure.Apart from this,many thermodynamic calculations were treated only approximately,without due regard to the effect of pressure. This paper reports thermodynamicanalysis of mineral equilibria carried out using a new system of thermodynamic constants,which included such hydrosilicates as grunerite and minnesotaite. All calculations have been made in accordance with a very precise technique with due regard to the effect of pressure on solid phases (correction for AV> and for fluid phases (correction for the fugacity coefficient -y>. Below w e consider the metamorphic peculiarities of cherty-ironrocks of different lithological composition.

Metamorphism of silicate iron-formation It is believed that, after the processes of sedimentation and diagenesis, in rocks of this type the stable silicate containing ferrous iron is a mineral of the minnesotaite type (ferrous taicj,silica is in excess, siderite is absent, the presence of ferriferous oxide is possible,and the fluid phase in the intergranularspace consists dominantly of water.

Unesco, 1973. Genesis of Precanzbriari iron and manganese deposits. froc. Kiev Syrnp., 1970. (Earth sciences, 9.)

209

Y ,P.Melnik and R.I.Siroshtan

Under progressive metamorphism the low-temperature transformation of minnesotaite to grunerite takes place according to the dehydration reaction:

7 Fe,Si,O,,(OH), tt 3 Fe,Si,O,,(OH), + 4 SiO,-I- 4 H20 (i) The P-T curve for this reaction (Fig. l), at low and moderate pressures, lies in the interval of 250°-280" C and is characterized by a reverse slope. It is worth mentioning that the given position of the curve cannot be thought of as reliably fixed because thermodynamic constants of minnesotaite are based on scarce experimental data. The absence of minnesotaite may testify to the beginning of metamorphism under green schist facies conditions. Grunerite is stable from the beginning stages of regional metamorphism and is a typicalmineral in iron cherty rocks that have been metamorphosed under both green schist and amphibolite facies conditions. But at the top of the amphibolite facies at temperatures of 640°-690"C grunerite is decomposed (Fig. 1) according to the reaction:

2 Fe,Si,OzZ(OH),

+. 7 Fe,SiO, + 9 SO,+ 2 H,O (2) 1°C 800

700

into minerals (fayalite and quartz) stable under granulitic facies conditions of association. Ferrous pyroxene-ferrosilite-bearing assemblages are not stable at any temperatures whenever the pressure is below 15,000 bar, as indicated by the positive value of AZ, for the solid phase reaction: 2 FeSiO,+Fe,SiO, + SiO, (3) but where more than 10-15 per cent molecular magnesium is present, the direction of the reaction is reversed, A summary mineral equilibria diagram with Ig fol-T co-ordinatesat a pressure of 5,000 bar is given in Figure 2. O

-10

-2c

I

1

-30

I P foz -4c

-50

'

600

green schist

arnphiboliie

facies

facies

-60

IO00

T'K 500

400

granulite facies I100

1200

-

FIG.2. Metamorphism of silicate iron-formation (diagram Ig fon-T).Diagram for P,= P/= C (Pa,o, P,?, Po,>= 5 kbar. Isolines for lgf;I,/fH,o are shown as dotted lines. Fe, iron; Hem,hematite; Mgt, magnetite;Fay, fayalite;Cru, grunerite; Min, minnesotaite.

As can be seen,silicate equilibria with magnetite according 300

to reactions

6 Fe,Si,0Z2(OH),

+7 O,

200

3 Fe,SiO, 100

FIG.1. P-T curves ofmetamorphic reactions in iron cherty rocks with excess of silica.To the right of the curve the predominant fluid component is shown. C,graphite; Hem,hematite; Mgt, magnetite;Fay,fayalite;Cru,grunerite;Min,minnesotaite;Sid, siderite. 21 o

3c 14 Fe,O, + 48 SiO,+ 6 H,O + O,e 2 Fe,O, + 3 SiO,

(4) (5)

are buffered and control, with stable T and P, = the fugacity of oxygen. The facies boundaries also are shown on this diagram. W e consider as very important the confirmation by thermodynamic data of the instability of hematite with ferrous silicate under P-T conditions characteristic of any metamorphic facies. Because in essential fluids water can consist of decompositionproducts only (not counting neutral gases), we

Physico-chemicalconditions of the metamorphism of cherty-ironrocks

'

Total diagrams of equilibria

i, Gru \

t

-6

-4

-2

O

c2

+4

16

-6

-4

-2

O

1.2

14

16

-6

-4

-2

1.2

O

t4

t6

Detailed diagrams of mineral equilibria

t4

I

I

+2 -3 -2 -I green schist facies

o

tl

-3

-2

*I

o

+i

amphibolite facies

-3

-2

-I

o

tl

granulite facies

(ai

(cl

FIG.3, Metamorphism of silicate iron-formation (diagrams In (a), (b) and (c), T=6OO0K (327O C),800"K

(527"C), and 1,000"K (727"C), respectively. Fe, iron; Hem, hematite; Mgt, magnetite;Fay,fayalite;Cru, grunerite.

can also construct diagrams with PHz0-PH2-T co-ordinates. However, diagrams with lg -lg fH,-T co-ordinates and, especially their isothermal section, are more useful. Such sections are shownin Figure 3 for temperaturescorrespondingto the changes in metamorphicfacies conditions.l From diagrams (b) and (c) the composition of fluid and, in particular,the hydrogen content,which is in equilibrium with the mineral association in question, can easily be defined. Pure water is an oxidizer in relation to ferrous silicate. When a considerable quantity of water enters the rocks, this can lead to replacement (partial or complete) of barren minerals by magnetite according the reactions:

quite possible in hydrothermal activity-is required for the oxidation of 1 g grunerite into magnetite. But for the oxidation of magnetite into hematite via a similar process (a variant of hypogene martitization) an enormous quantity of water is needed and, as such,the phenomenon can be only of local importance.

lgfE20-lgfHz).

3 Fe,Si,O,,(OH),+ 4H,O

= 7Fe304+ 24 SiO,+ 7 Hz (6) 3 Fe,Si04+ 2 H,O

= 2 Fe304+ 3 SiOz+ 2 H,. (7) Thus,under the above-mentionedamphibolite facies conditions, approximately 100-150 g pure water-a quantity

Metamorphism of carbonate iron-formation The diagnostic features of carbonate iron-formation are the occurrence of siderite in paragenesis with quartz and the occurrence of Fe oxides,both magnetite and hematite; hydrosilicates with ferrous iron do not occur. Thenature of carbonate iron-formationmetamorphism depends, to a certain extent, on the presence of hematite 1. Isobar numbers correspond to P,=P~=~(FH,IJ +PE,+ Po?),kbar. Thin incline lines-isobars

Ig fo,.

211

Y.P.Melnik and R.I. Siroshtan

because, in such cases,under comparatively low temperatures (300"C)and PeO1= 2,000bar (Fig. i) the following reaction is possible: FeCO, +Fe,O, = Fe,O,+ CO,.

(8)

But the equilibrium assemblage siderite + heinatite+magnetite depends greatly on pressure in as much as the slope of the P-T curve defining a decarbonatization reaction is much steeper than that for dehydration reactions. It is believed that Auid consists dominantly of carbon dioxide, which is why the above-mentionedreaction does not define precisely the lower temperaturelimit of the green schist facies and why sideriteand hematite sometimesoccur with grunerite at temperatures up to 390"-420"C and Pcoz = 5,000-7,000bar. After disappearance of hematite at the completion of this reaction, pure FeCO, remains unchanged because, at temperatures lower than 400'-500" C,the formation of magnetite from carbonate requires the presence of oxidizers that must be derived from outside the system.In Figure 4, with Pco2 = 5,000bar, the phase limit of bivariante quilibrium of the siderite +magnetite assemblage corresponds to the temperature interval 38O0-5OO0 C. Above 400°-500" C dissociation of siderite is theoretically possible according to the reaction: .3 FeCO, = Fe,O, +2 CO2+ CO

(9) but the proportion CO:CO,=l :2,as required by the O

-10

-20

1

equation,is metastable because of the dissociationofcarbon monoxide to form graphite:

2CO=C+COz. (10) It has been shown by many investigators that graphite, whether newly formed or already present in the rock, is an oxygen buffer which can regulatefo, and fc0 in the carbonate ñuid. The line of graphite stability divides the diagram (Fig. 4) into two parts. Only the minerals whose fieldsof stability are crossed by this line-siderite, magnetite and fayalite-can be found in equilibrium with graphite. Mineral associations in the shaded area of Figure 4 cannot exist,as it is physically impossibleto createsuch low values of fog in carbonate rocks. Mineral assemblages found in an unshaded field are stable only in the absence of graphite. Analogous observations should be taken into consideration when studying the isothermic sections of diagrams with lg fco2-lg feo co-ordinates for the temperatures of various metamorphic facies (Fig. 5). Five petrological conclusions drawn from the analysis of Figures 4 and 5 are as follows. First,heinatitecannotexist in equilibrium with graphite at any temperature. Under green schist faciesconditions, hematite must react to form siderite: 2 Fe,O, + C 4-3 CO,+4FeCO,

(1 1)

or magnetite:

6 Fe,O,

+C+

4Fe,O, 4-CO,

(12) depending mainly onfeo,. Under amphiboliteand granulite facies conditions,reduction is only possible according to reaction (12). Second,siderite is a stable mineral up to temperatures of the beginning of amphibolite facies. Third,the equilibrium transformation of siderite into fayalite is thermodynamically impossible as the stability fields of these minerals are separated at any temperature by the magnetite field along the graphite join.Reaction

2 FeCO,+ SiO,= Fe2Si04+2 CO,

(13) is not an equilibrium reaction.Only the phase transformation of siderite into magnetite by reaction (9) is possible, reduction of magnetite to fayalite then follows. Fourth,at temperatureshigher than 500"-600"C under amphibolite facies conditions,the association of magnetite with graphite (Fig. 5(b) and (c)) becomes unstable as a result of the reaction:

-3c

kfo,

1-41

-51

2 Fe,O, + C + 3 SiO,+3 Fe,Si04 + CO,. c5 1

500

600

700

800

900

T"K

1000

II00

1200

-

FIG. 4. Metamorphism of carbonate iron-formation(diagram lgfo,-T). Diagram for Ps= Pf= C(Pco,, Pco,Po-= 5 kbar.In a dotted line isolines Ig fe0/fco~are shown.Area of metastability under the line of graphite is shaded.C,graphite;Fe,iron;Hem, hematite; Mgt, magnetite; Fay, fayalite;Sid, siderite.

212

(14)

Finally,under granulite facies conditions,graphiteis stable only with fayalite. By using the diagrams (Figs. 4 and 5), one can find equilibrium fluid compositions. Because increased temperature causes the graphite field to become smaller, the carbon monoxide content in any fluid in equilibrium with graphite must increase. Carbon dioxide, as well as water, can be an oxidizer for silicates containing ferrous iron, but in this case still

Physico-chemicalconditions of the metamorphism of cherty-ironrocks

greater quantities are required. This is why formation of magnetite in such a way can hardly be of ore-making importance.

could appear in the course of conjugated oxidation-reduction reactions involving the participation of COz, evolved in the disassociation of carbonates, for example reaction (9) or the reaction

Metamorphism of silicate-carbonate iron-formation

3 Fe,SiO,

$2 Fe,O,

-1- 3 SiO,+2 CO (1 5)

with further dissociation of CO as per reaction (10). For this reason it is necessary in the analysis of mineral equilibria to build sectional diagrams along the line of graphite stability (Fig. 5). Using fGo, and fHs0 as independent variables, such a section at a constanttemperaturewill represent the surface on which oxygen fugacityis controlled everywhere by the presence of graphite (Fig. 6), as per the reaction: c -1- O, CO,. (16)

A great number of cherty-ironrocks are not represented by purely silicate or carbonate types, but by combined silicate-carbonateones.Because the rock-formingminerals are ferrous silicates and siderite,the presence of a certain quantity of graphite, in the role of oxygen buffer, is required. Graphite could be formed by the metamorphism of organic carbon originally present in the sediment, or it

I

+2 CO,

I

I

t6

+5

+4 +3

+2

+I

-4

-2

O

+2

+4

-4

+6

green schist facies ia)

.

-2

+2

0

+4

-4

+6

-2

O

amphibolite facies

granulite facies

íb)

(C)

FIG.5. Metamorphism of carbonate iron-formation (diagrams Ig fco,-lg feo). Numbers of isobars correspond to Ps=P~ =C(Pco2, Pco, Po,),kbar.Area of metastability is shaded.In

t2

t4

+6

(a), (b) and (c), T=600° K (327O C), 800"K (527' C), and 1,000"K (727"C), respectively. C,graphite; Fe, iron; H e m , hematite;Mgt, magnetite; Fay, fayalite; Sid, siderite.

+? C6

Ig fco2

I

+3

+2 ti

o

-1

41

+2

+3

+4

+5

o

.I

+1

green schist facies

amphibolite facies

granulite facies

(a)

íb)

(C)

FIG. 6.Metamorphism of silicate-carbonateiron-formation(diagrams Ig feo,-lg fH,o in the plane of graphitestability). Isobar numbers correspond to P,=PI=C(PC,?, PH~o, PH?,PGO,PO,), kbar. Thin incline lines denote isobars lg fH2;thin dotted lines

c2

+3

+4

+5

denoteisobars lg foz;thin hachures denote isobars Ig fco. In (a), (b) and (c), T = 600" K (327"C),800"K (527"C),and 1,000"K (727" C), respectively. Mgt, magnetite; Fay, fayalite; Gru, grunerite;Sid,siderite. 213

Y.P. Melnik and R.I. Siroshtan

This enables us to include in the diagram isobars of oxygen fugacity and isobars of carbon monoxide fugacity as per reaction (10). If foz and ffis0are known, one can easily determine for every point in the diagram the fugacity of hydrogen, according to the reaction of water decomposition: 2 H2O 2 H2-1- 0 2 (17) and draw corresponding isobars. The resulting diagrams make it possible to carry out a detailed analysis of mineral equilibria and to determine at a given T and P the content of any of five volatile components of the fluid phase. Let us consider, first of all,the relation of siderite and grunerite, as determined by the reaction: 7 FeCO,+ 8 Si02+ H,O

= Fe,Si,O,,(OH),+7 COz (18) Siderite+ grunerite (-I quartz) inequilibriumoccupy a wide field under P-T conditions of low-temperaturemetamorphism, and this is the main mineral association in green schist facies rocks (Fig. 6(a)). However, under amphibolite facies conditions (Fig. 6(b)) this mineral association theoretically can remain only under a very high fluid pressure of more than 7,000-8,000bar. Within this stability field the formation of grunerite depends not on temperature but rather on the relation of CO,and HzOin fluid.Thus,the main cause for the development of grunerite in metamorphic rocks is the presence of a sufficient quantity of water,which provides a stablehigh value for PHtoand the relation Pzz0:Pco,with the expenditure of water according reaction (18). One can assume that the development of grunerite through metamorphism is connected with processes involving the dehydration of chlorites,hydromicasand other hydro-minerals,to form the slatebeds which are interlayeredwith iron cherty formation. The equilibrium siderite+grunerite+magnetite is monovariant, and at pressures of 4,000-8,000 bar corresponds to a temperature of 420°-530" C.At higher temperatures, the bivariant equilibrium grunerite+magnetite exists, at 550°-650° C,it is replaced by the equilibrium grunerite +fayalite and above 670"-700"C by fayaliteonly (Fig. 6(c)). The above data prove that,in fluids of complex composition, temperatures of equilibrium reactions involving the participation of only one volatile component drop markedly,and the appearance of fayalite becomes possible under amphibolite facies conditions.

Metamorphism of oxide iron-formation The rocks of this type of iron-formationare represented by sediments originally having iron hydroxides that were transformed during diagenesis into goethite and, possibly, into hydromagnetite or magnetite. The transformation .of 214

goethite into hematite takes place prior to metamorphism at temperaturesup to 120°-180"C.Themetamorphic transformation of hematite into magnetite according to the reaction 6 FezO,&4 Fe,O, + O2 (19)

is possible only in the presence of reducing agents (free carbon,gaseous CO or HJ.Where no reducing agents are present, the association of hematite + magnetite is quite stablein all facies of metamorphism,includingthe granulite facies. The equilibrium of hematite with other minerals has been considered in previous sections of this report.

Certain peculiarities of low-temperature metamorphism of iron cherty formation A great number of iron cherty rocks which have undergone metamorphism to the green schist facies are characterized by the close and frequent interlayering of various beds that have different compositions and modes of formation. In some places, within a distance of some centimetres, a bed of siderite-hematiteis replaced by one of gruneritemagnetite,and silicate-richbeds having graphite are interlayered with hematite-magnetite beds that contain no free carbon.A detailed investigation of layered rocks has provided enough evidence to suppose that specific equilibrium conditions occur in a limited volume (mosaic or local equilibrium); this equilibrium volume is characterized by a fluid phase of quite different composition. Probably diffusion, not only of the solid phases, but also of the fluid phase as well, was limited at low temperatures. These separate volumes of the metamorphosed rocks can be considered as closed systems, and iron cherty formation,taken as a whole, can be treated as a number of closed systems. Under amphibolite facies conditions, there is a tendency towards the equalization between beds of the fluid composition and thus towards a reduction in the diversity of mineral associations. The analysis of separate groups of rocks that belong to different metamorphic facies reveals the presence of a certain metamorphic zoning in the iron cherty formations of the Ukrainian Shield.

Metamorphism of iron cherty formations and ore deposition Banded iron cherty rocks make poor ores,their value being determined by their magnetite contents. Magnetite crystallization probably took place in early stages of metamorphism via the reduction of hematite by carbon according to reaction (12),or by the reaction of hematite and

Physico-chemicalconditions of the metamorphism of cherty-ironrocks

siderite according to reaction (8), and under metamorphic conditionsnear the amphibolite facies grade by the thermal dissociation of siderite according to reaction (9). Thus, metamorphism at low and moderate temperatures contributes greatly to an increase in ore quality. However, the introduction of water at these metamorphic stages has a negative effect, as it results in the formation of grunerite according to reaction (18) instead of magnetite.

Under amphibolite and granulite facies conditions, a surplus of graphite leads to the development of silicate -grunerite or fayalite-at the expense of magnetite, thus reducing the ore quality to a certain extent. However, introduction of a considrablequantity of pure water sometimes may lead to a reverse process; that is oxidation of silicatesto magnetite.

Résumé Les conditions physico-chimiques du métamorphisme des jòrmatioiis de jeu siliceux (Y.P. Melnik, R.I. Siroshtan) 1. Les roches de fer siliceux,qui ont une grande extension dans les limites du bouclier ukrainien, se rencontrent non seulement en strates qui contiennent de riches dépôts de minerai de fer,mais aussi en minerais à faible teneur,dont la concentration est facile,Les formations de fer siliceux sont des sédiments chimiques métamorphosés ; le fer et la silice dans ces roches proviennent de fumerolles hydrothermales actives au cours d'un volcanisme subaquatique dans des zones géosynclinales. 2. Les roches de fer siliceux sont de différents types pétrographiques.En raison des proportions différentes des minerais,des silicates et des carbonates,ces roches peuvent être classées en trois types : les roches de minerai, dans lesquelles le composant principal est la magnétite ou l'hématite ; des roches à faible teneur, dans lesquelles le quartz joue un rôle important avec d'autres minerais ; les roches sans minerai, dans lesquelles seuls les silicates et le quartz sont les minéraux constituantla roche. 3. Les principaux minéraux des roches de fer siliceux (qui consistent en Sioz, Feo,Fe,O,, H,O et CO,)sont le quartz, la grunerite, la fayalite,la magnétite, l'hématite et la sidérite.L a silice est un composant en excès toujours présent dans les associationsminérales sous formede quartz. Il en est de m ê m e de Fe,O,,qu'on rencontre dans les conditions de basse température et qui détermine l'apparition de l'hématite. L a présence de grunerite ou de sidérite dépend du rapport H,O/CO,. 4.Suivant les conditions dans lesquelles les roches de fer siliceux se sont métamorphosées, on peut distinguer trois faciès de métamorphisme : le schiste vert, l'amphibolite et la granulite.Les roches de fer siliceux des trois faciès se rencontrent dans la région structurale KrivoyrogKremenchug. Cela reflète l'irrégularité des conditions thermodynamiquesau cours du métamorphisme.Des différences dans la composition minérale des roches de fer siliceux sont le résultat de changements dans les conditions du métamorphisme tout le long du filon. 5. Les paramètres physicochimiques de la stabilité de i'hématite, de la magnétite, de la sidérite,de la grunerite, de la fayalite et du graphitedans les minerais de fer siliceux métamorphosés sont à la base d'un nouveau système de

constantes thermodynamiques des minéraux. Les diagrammes d'équilibre minéral ont été établis pour des condi- '. tions P-T de faciès de schiste vert, d'amphibolite et de granulite pour des compositions H,O/CO,d'un fluide (en coordonnées log foi et T ; log fco, et log fco ; log fHz0 et log fH2; log fco, et 1% fH@ etc.). 6. Les limites supérieuresde température des associationsminérales typiques sont calculées à partir des données thermodynamiques ; elles s'accordent avec les observations pétrographiques : Sidérite+hématite +magnétite 200-400"C ; Sidérite+ grunerite+magnétite + graphite 370-500" C ; Sidérite +magnétite +graphite 400-550"C ; Fayalite +magnétite +graphite 500-600" C ; Grunerite +fayalite + quartz 640-690" C. 7. O n trouve que l'hématite ne peut pas coexister en équilibre avec les silicates de Fe2+ (grunerite, fayalite) ou avec le graphite dans les roches de fer siliceux quels que soient P,T et la composition du fluide.L a transformation métamorphique de la sidérite (+quartz) en fayalite est peu probable, puisque dans les conditions P-T du faciès de schiste vert la sidérite doit se transformer en grunerite ou en magnétite, suivant la valeur du rapport CO,/H,O dans le fluide. L'association magnétite -1- quartz + graphite devient instable dans les conditions du faciès amphibolitique et se transforme en grunerite ou fayalite stable, suivant la valeur de Ces processus expliquent l'abaissement de la qualité du minerai du fait du métamorphisme, résultat des transitions des minerais de fer (magnétite, sidérite) en silicates. 8. L a comparaison des diagrammes et des données pétrologiques permet de montrer que l'équilibre a le caractère d'une mosaïque aux premiers stades du niétaniorphisme. Les roches situées dans des régions séparées diffèrent beaucoup, après métamorphisme, quant à la composition du hide (variétésfo,,fEZo, fC,> et quant à la teneur en composés volatils, essentiellementl'eau. 9. Si l'on suit la composition minérale des roches de fer siliceux tout le long du filon de Krivoyrog-Kremenchug, on observe un zonage dans le faciès métamorphique. La partie centrale de cette région consiste en roches de faciès granulitique qui,en direction du nord et du sud,remplace les faciès amphibolitique et de schiste vert. Vers le sud de Krivoyrog le stade du métamorphisme est encore plus 215

Y.P.Melnik and R. I.Siroshtan

élevé, avec des roches représentées par un faciès aniphibolitique. Les variations qu'on observe dans les types de rocheç de fer siliceux dénotent des variations de P-T au

cours du métamorphisme.L'âge du métamorphisme est de 1 900 à 2 100 millions d'années d'après des datations par la méthode K-Ar.

Bibliography/ Bibliographie EUGSTER, H.1961. Physico-chemicalproblems of rocks and ores formation. Vol. I, Moscow, Academy of Sciences of the U.S.S.R. (In Russian.) FRENCH, B. M.1966. Rev. Geophys.,vol. 4,no. 2, p. 223. FRENCH, B. M.; ROSENBERG, P. E. 1965. Science, vol. 147, no. 3663, p. 1283. HAWLEY,J.E.; ROBINSON, S.C.1948.Econ.Geol.,vol.43,p. 603. HOLLAND, H.G.1959. Econ. Geol., vol. 54, no. 2. JAMES, H.L.;HOWLAND, A.L. 1955. Bull.geol. Soc. Amer., vol. 66, no. 12, p. 1580. KORNILOV, N.A.1969. C.R.Acad. Sei. URSS,vol. 184,no. 4, p. 939. (In Russian.) KORZHINSKY, D. S. 1940. Factors of mineral equilibria and mineralogical facies of depth, Bull. Inst. Geol. Acad. Sei. USSR,vol. 12, no. 5. (In Russian.) MELNIK, Y.P. 1964a. Acad. Sci. USSR,Geol, Rudn. Mestorozdenij, no. 5, p. 3. (In Russian.) __ . 19646. Acad. Sci. UIcSSR, Geol. Zum., vol. 5, no. 5, p. 16. (In Ukrainian.)

216

-.

1966a.In:Problemstheory and experimentin oreformation, p. 58. Kiev, Naukova Dumka. (In Russian.)

19666. In: Research on nature and artificialformation of mineuals, p. 120. Moscow, Nauka. (In Russian.) -. 1969a. In: Problems of genesis of precambriun iron rocks, p. 259. Kiev, Naukova Dumka. (InRussian.) -. 1969b. Acad. Sei. UIcSSR,Geol. Zurn., vol. 29, no. 4, p. 13. (In Russian.) MELNIK, Y.P.;JAROTSCHLK, M.A. 1966. In: Problems theory and experiment in ore formation, p. 98. Kiev, Naukova Dumka. (InRussian.) , 1970.Acad. Sci. USSR,ZapiskiMiner. Ob.,vol. 99,no.1, p. 3. (In Russian.) SEGUIN,M.1968. Nat.canad.,vol. 95,no. 6,p. 1195,p. 1217. SEMENENKO,N.P. 1966. Metamorphism of Mobile Zones, Kiev, -.

Naukova Dumka. (In Russian.) SHUNZO, Y.1966. Econ. Geol.,vol. 61,no. 4,p. 768.

The Serra do Navio manganese deposit (Brazil)' W.Scarpelli Industria e Comercio de Minerios S.A.(Brazil)

Introduction

The Gneisses

The Serra do Navio manganese deposit is in the Federal Territory of Amapá,in northern Brazil (Fig.1). Production started in 1957 and up to the end of 1969,10million metric tons of washed ore had been produced, most exported to North America and Europe.The average grade of the commercial,beneficiated ore varies from 48 to 50 per cent of manganese. Beneficiation consists of crushing,washing and classification by size and density. The deposit is part of the PrecambrianGuyana Shield, at the left bank of the River Amazon. In Serra do Navio and vicinity this shield is composed essentially of gneisses, amphibolites, schists and quartzites (Table 1) plus pegmatites and quartz veins.

Gneisses are the most common rocks in the neighbourhood of Serra do Navio. The other metamorphic rocks seem to occur as inclusions in them. The predominant type of gneiss is leucocratic and composed essentially of quartz, microline and/or oligoclase, and biotite, occasionally with dark hornblende-richzones parallel to the foliation. In some places a gneiss with very high quartz content forms prominent ridges. This type of gneiss probably is the product of metamorphism of a silica-richrock, possibly quartzite.

TABLE1. Stratigraphiccolumn of the Serra do Navio district Series

Group

General description

Lithologic units

Serra do Navio

Metasediments

Jornal

Amphibolites

Amphibolites Schists Quartzites

Gneisses

Gneisses

Manganese protores Graphitic facies Biotitic facies Quartzose facies

Amapá

???

-

The oxide ore bodies are the product of secondary enrichment of protores which occur in the Serra do Navio group, outcrop in topographic ridges,and are mined from open pits.

The Jornal group The amphibolites of the Jornal group are second to the gneisses in areal extent. They are not uniform in texture or mineralogical composition, varying considerably even in short distances.The predominant mineral is green hornblende, followed by andesine-oligoclase and, in variable percentages, magnetite, titanite, diopside, tremolite, carbonate, and sulphides.Quartz occurs in small veins. Differences in adjacent bands of amphibolite are conspicuous. The bands vary in grain size (fine to medium or coarse-grained), in structure (well or poorly developed foliation), in texture (presence or absence of oriented minerals) and in mineralogical composition.It is very possible that the amphibolites are derived from a heterogeneous rock sequence.This possibility is reinforced by the occurrence within the amphibolitesof belts of quartzites,gneisses and biotite schists concordant with the foliation. These belts show that the rock column from which the amphibolite originated by metamorphism was not homogeneous. From these observationsthe origin of the amphibolites cannot be inferred.It is relatively certain that at least part of the sequence is of sedimentary origin,as testified by the 1. With permission of Industria e comercio de Minerios S.A.,ICOMI, Rio de Janeiro, Brazil.

Unesco, 1973. Genesis of Precambrian iron and mungunese deposits. Proc. Kiev Synzp., 1970. (Earth sciences, 9.)

217

W.Scarpelli

* Y)

SCALE

-

1:6.000.000

CONVENTIONS

0

city Village

__t_

FIG.1. Geographic location of the Serra do Navio manganese district.

218

Roilroa,d

The Serra do Navío manganese deposit (Brazil)

as determined from thin sections. It must be emphasized that themineralpercentagesof these rocks changemarkedly from layer to layer and place to place, thus the tabulated data are given only to illustrate the variety of observed minerals and mineralogicalcomposition.Quartz and biotite are the most common minerals, followed by graphite,muscovite, sillimanite, garnet (usually almandine), plagioclase (oligoclase, rarely andesine), andalusite, sulphides, and other less frequent minerals. The predominant sulphide is pyrite, followed by chalcopyrite and arsenopyrite. The manganese protores occur as lenses in the upper part of the graphitic facies.There are two types of protore. The thicker and richer in manganese is composed essentially of rhodochrosite, followed by manganese-bearing silicates such as spessartite,tephroite and rhodonite. The thinner lenses,poorer in manganese,are composed of spessartite, amphiboles,quartz, and graphite. In the quartzose facies there are layers very rich in calcium carbonate and silicates. They occur in two types. The thicker,which can be described as marbles, are composed essentially of calcite pIus some calcium silicates,The thinner are composed of coarse-graineddiopside,tremolite, calcite, and pyrrhotite and can be called a calc-schist. The bedding of the metasediments, which survived at least three metamorphic phases and is recognizable in the field,was preserved by the developmentof the metamorphic foliation parallel to it (with some exceptions) and by the great differences in composition of individual layers.

belts of quartzites and the belts of diopside-calcite-tremolite-titanite.O n the other hand, some intercalated bands of gneiss have remnants of a typical porphyritic texture, indicative of an igneous origin. N o field or microscopic evidence was found indicativeof the origin of the amphibolite layers. At one point in the River Amapari there is a good exposure of the contact zone between the gneisses and the amphibolites,N o gradationalchange was observed in either of these two rocks toward the contact,which is parallel to their foliation.At the actual contact zone the gneiss alternateswith the amphibolite in a seriesofcontinuousunfolded bands, each one about 0.3-1 .Om thick. It is possible that there is no great difference in age between the rocks from which the gneisses and the amphibolites originated,as both had the same metamorphic history.

The Serra do Navio group Above the amphibolites there is a sequence of metasediments composed of quartzites,schists and carbonate-rich layers (Fig. 2). These units alternate in a relatively cyclic pattern and are subdivided into three distinct facies, a quartzose, a biotitic,and a graphitic. The minerals which occur in these facies are almost the same, but occur in variable percentages. Table 2 shows the mineral compositioii of these facies

TABLE 2. Composition of the metasedimeiitary facies of the Serra do Navio groupl Biotitic facies (13 samples)

Quartzose facies (16 samples) Mineral Average

( %)

Quartz Biotite Graphite Muscovite Plagioclase Silimanite Andalusite Staurolite Garnet Cordierite Tremolite Diopside Titanite Carbonate Hornblende Tourmaline Sulphide

Maximum and minimum

( %)

Number of positive samples

Average

( %)

Maximum and minimum

Graphitic facies (14 samples)

( %)

Number of positive samples

Average

(%)

60-3

16

29

50-20

13

25

10

53-20 8-traces

13

14

8-0

340

4

25-0

13 10 5

26

3

12 3

35-0

12 12 9 5

34 4

8

33-0 9-0 25-0

8

2

5 1 15

25-0 16-0 1-0

10

17-0 traces-O 35-0

6 5 11

7 6 6 O

30 9

4

4 traces 5

O

-

8-0

O 8 2 6 2 3

20-0

12

6-0 15-0 5-0

10

10

50-0

I

10-0 6-0

1 1 3 3

2

Epidote

3

Chlorite

1

7-0

5 6

4 traces 8 2 O O O O O

25-0 18-0

1 1 1 1

3-0

-

3-0 5-0 5-0

2

O O O

O O 12 9 1 4

4 3

-0

O traces traces O 2 1 5 5

Maximum and minimum

(P4)

38-8 340 45-15 13-0

Number of

positive samples

14 13

14 8

25-0

4

5-0 20-0

8 8 10 10

10-0 10-0

-

O O O

1-0

1

1-0

-

1 O

6-0

10 10

2-0 30-0 45-0

4 5

1. Excluding the manganese protorcs and the quartz-free calc-siliceous layers of the quartzose facies.

219

FIG. 2.Geologicalmap ofthe Serra do Navio District.Contour lines at 50 metre interval.Contacts are inferredfrom the known data. At the centrethe ‘y’shaped metasediments of the Serra do

Navio group (SNV)overlying the Jornal group (J), and, at West,the gneisses (G). The dotted areas represent the outcrops and float of the manganese ore bodies.

The Serra do Navio manganese deposit (Brazil)

MANGANESE PROTORES

There are two types of manganese protore,one carbonatic and one siliceous,or garnetiferous (Table 3). The carbonatic protore has an average 31 per cent of manganese and is composed principally of rhodochrosite, followed by spessartite,occasionally with veins and bands of tephroite and rhodonite. As accessories there are sulphides (sphalerite, niccolite,gersdorfite), graphite and orthoclase.The texture is mosaic,sometimes disturbed by shearing.In the groundmass of rhodochrosite spessartite crystals grew to a maxim u m diameter of 0.5-1.0mm. TABLE 3. Chemical analysis of protore samples A

Mn Fe SiO2 A1203

CO2

C Ca0 MgO

NazO K2O

s

As P Ignition loss

B

C

D

E

F

G

3.6 35.7 27.7 4.8 3.7 2.8 32.6 15.3 49.7 12.7 1.9 3.6 8.9 27.5 1.2 19.3 8.6 3.4 8.2 7.9 4.3 0.6 3.5 0.8 1.7 3.1 1.7 0.3 0.4 0.2 t0.05