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The Geological Setting of the Lamego Banded Iron-Formation-Hosted Gold Deposit, Quadrilatero Ferr~Ferodistrict, Minas Gerais - Brgzil

The Geological Setting of the Lamego Banded Iron-Formation-Hosted Gold Deposit, Quadrilatero Ferr~Ferodistrict, Minas Gerais - Brgzil

THE GEOLOGICAL SETTING OF THE LAMEGO BANDED -FORMATION-HOSTED DEPOSIT, QUADRILATERO FERR~FERODISTRICT, - BRGZIL

Miircio André dos Santos Sales

A thesis submitted to the Department of Geological Sciences in conformity with the requirements for the degree of Master of Science

Queen's University Kingston, Ontario, Canada June, 1998 copyright O Maircio André dos Santos Sales, 1998 National tibrary Bibliothèque nationale 1*1 of Canada du Canada Acquisitions and Acquisitions et Bibliographic Services services bibliographiques 395 Wellington Street 395. nie Wellington OttawaON K1A ON4 ûttawaON KIAON4 Canada Canada

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The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts from it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. Sales, M. A. S., 1998, The Geological Setting of the Lamego Banded Iron-Formation-Hosted Gold Deposit, Quadrilhtero Ferrifero District, Minas Gerais - BraziI: M.Sc. Thesis, Department of Geological Sciences, Queen's University, Kingston (ON) - Canada.

Lamego and Cuiaba are banded iron-formation (BE)-hosted gold deposits owned by Mineraçiio Morro Velho Ltda. in the northern part of the Quadnlatero Fem'fero district (SabadCaeté region), Southeast (19" 55' S - 43" 46' W). These deposits are hosted by the Nova Lima Group, a volcano-sedimentary unit of the Rio das Velhas Supergroup, which is a typical greenstone belt sequence. The Nova Lima Group is one of the best studied supracrustal units in Brazil because of its numerous BE-hosted gold deposits and gold occurrences. Among these, the most important are the Mono Velho, Cuiabii, Siio Bento, and Raposos mines. The mineralized bodies in the Lamego deposit occur as scattered, stratiform lenses of sulphide-facies BIFs within a BIF unit that usually ranges in thickness from 1 to 8 meters ("Lamego Banded Iron-Formation"), and as stratabound lenses of sulphide-bearing cherts hosted in a chert unit that overlies the BIFs ("Pele de Onça Chert Unit"). Mineralized lenses and bodies Vary from 15-20 meters to 80-120 meters in strïke length and from less than a meter to 4 or 5 meters in thichess, and their average gold grades range fiom 3.0 g/t to 17.0 g/t. Gold occurs essentially as electmm grains (gold fineness ranges fiom 910 to 957) up to 60 pm in size included in , which is the most important sulphide phase present in mineralized sarnples (usually 80-95% of the total sulphide content). The Larnego Banded Iron-Formation and the Pele de Onça Chert Unit are positioned at a major stratigraphic transition from a lower basic volcanic sequence ("FootwalI Volcanic Unit") to an upper mixed unit consisting of epiclastic and volcaniclastic sedimentary rocks ("Upper Epic lasticNolcaniclastic Unit"). The rocks of the FootwaH Volcanic Unit, based mainly on their geochemical composition (TH2 tholeiites with calc-alkaline affinity), appear to represent the uppermost stratigraphic part of the "Middle Unit" of the Nova Lima Group. The Upper EpiclasticNolcanicIastic Unit consists of lithotogic associations that typi@ the "Upper Unit" of the Nova Lima Group. Hydrothermal alteration zones directly associated with the gold mineralization in the Lamego deposit occur in the footwall sequences of the BIFkhert units as semi-conformable (stratabound) "sheets" with a minimum vertical extension of 200 meters. Rock sequences of the Footwall Volcanic Unit situated beneath bodies of mineralized suiphide-facies BIF are characterized by a proximal alteration represented by the indicator assemblage white mica + ankerite f sidente + chlorite, and cornrnonly show flattened and partially transposed "stockwork-like" phyllosilicate-rich microveinlets and -rich veindveinlets. Proximal alteration zories grade vertically and laterally into extensive "propylitic" dista1 alteration zones typically represented by the indicator mineral assemblage clinozoisite/epidote + chlorite + + actinolite. Important systematic changes in chemistry in rocks of the Footwall Volcanic Unit during the progressive hydrothennal alteration processes include depletions in Na20, A120,, Sioz, "base metal" elements (Cu, Zn, Ni, Co), and rare-earth elements, as well as additions in CO2, K20and Cao. The Pele de Onça Chert Unit occurs not only as a strataboundtabular horizon that ranges in thickness fiom several decimeters to 4 or 5 meters, but aIso as discordant, "feeder-Iike" veidveinlet systems that are perpendicular or sub-perpendicular to the bedding surfaces of the BIFs and "merge" with the stratabuund chert unit in its lowermost stratigraphic portion. This unit is interpreted in ths thesis as a self-sealing silica cap, similar to the sinter cappings and terraces associated with modem epithmal gold deposits and the silicified tufflchert Iayers which commonly cap massive sulphide orebodies. The bulk of the gold mineralization in the Lamego deposit appean to be a product of syn-volcanic (sub-volcanic?) replacements of Fe-carbonate-rich layerdbands in carbonate-facies BlFs and carbonate-bearing cherts by gold-bearing, iron-rich sulphides (mainly pyrite + arsenopyrite) during the waning stages of a volcanic/volcano-chernicd depositional cycle. The replacement of these previously deposited volcano-chemical sedirnents is inferred to have followed the creation of pervasive lateral pwmeability by carbonate dissolution processes that occurred during the early stages of the hydrothermal activity. A clear relationship between Pele de ûnça "chert" veindveinlets and the sulphidation/repIacement processes in BFs was observed during this study. Carbonate-rich bands of carbonate-facies BIFs commonly show sulphide- enriched alteration/replacernent halos that are syrnmetrically distributed around these "chert" veins and veinlets. The mineralized environment of the Larnego deposit has been dehed and metamorphosed during the 2 major tectono-thermal events that affected the Archean supracrustal sequences of the Quadrilhtero Fenifero district: the Rio das Velhas orogeny (2780-2703 Ma) and the Transamazonian orogeny (2 125-2040 Ma). However, introduction and/or remobilization of sulphides and gold into structures and penetrative structural fabrics that developed during these orogenies were not observed. The BE-hosted gold mineralization and mineralized bodies in the Lamego deposit have been overprinted by structures and penetrative stmctural fabrics developed during the Rio das Velhas and Transamazonian tectono-thermal events. Based on lithostratigraphic correlations, the similar tectonic/structural setting, and the similar style of mineralization, it is inferred in this thesis that Lamego (3.5 million tonnes at 5.4 g/t) and Cuiaba (22.0 million tonnes at 7.8 g/t) are stratabound BE-hosted goId deposits formed contemporaneously in an identical mineralization environment. The stratigraphic sequences of the Lamego and Cuiaba deposits in the SabadCaeté region occur as "tectonic inliers" in which the uppermost stratigraphic portion of the Middle Unit of the Nova Lima Group is surrounded by younger rock sequences of the Upper Unit. These features are here interpreted to be surface expressions of kilometer-scale, highly non-cylindrical, cone-shaped, overtumed anti forms (sheath and tubular folds) that were generated during the intense WW-verging regional tectonic transportkimple shear event associated with the Transamazonian orogeny in the Quadriliitero Fem'fero district. The elongation and plunge of the Lamego and Cuiaba sheathed antiforrns and the plunge of their stratabound/stratiform BE-hosted mineralized bodies are parallel to the plunge of the regional stretching linear fabric developed during the Transamazonian orogeny. The Larnego and Cuiaba BE-hosted gold deposits represent an interesting type of volcanogenic hydrothermal gold deposit with sorne characteristics typical of gold-rich VMS deposits. The Archean greenstone belt of the Quadrilatero Femfero district (Rio das Velhas Supergroup) is metallogenetically unique in being dominated by this "unusual" type of gold deposits, to the exclusion of major occurrences of almost any other type of Archean rnineralization (polymetallic VMS deposits, deposits in komatiitic sequences, or the more typical non-BE-hosted Archean Iode-gold deposits). iii

I am indebted to Lucio Molinari (Director of Exploration for Minorco Brasil - Divis50 de Exploraçâo), Juvenil T. Félix (Vice President of Minorco Brasil Participaçoes Ltda.), GeraIdo Ibrahim Oliveira (Manager of Mineraça0 Morro Velho), and the corporations they represent for generously providing the financial support and logistical assistance that were essential for the successful completion of this MSc. project. 1 would like to express my gratitude to Dr. Robert Mason and Dr. Hmart Helmstaedt for their expert advisory support throughout al1 stages of this project. Studying under the supenision of these two devoted and patient teachers was a great pleasure and an unforgettable leaming experience. The two-week tour to the gold mines and current prospects of Mineraçào Mono Velho in the Quadrilatero Fem'fero district carried out together with Professor Robert Mason at the very beginning of this project was a colossal lesson of and "geological thinking" which I will never forget. Many thanks "Bob"! Many thanks "Herb"! The wrlter, a former Minorco's exploration geologist from 1988 to 1995, would Iike to thank his "geology-mates" fiom Mineraçiio Morro Velho and Minorco for the productive discussions and guidance during the activities and technical visits in the Nova Lima. Raposos, Santa Barbara, and SabadCaeté regions. I express my gratitude to Frederico W. R. Vieira, Edilberto Biasi, Fernando D. Gonçalves, Wilson Scarpelli, Luis H. Lisboa, Luis Carnilo Pinto, William F. Campos, Mario Corbani Filho, Eduardo Sirnoes, Carlos Alberto Si 1va. and Sebastiiio Alvarez da Costa. My former boss Jatme Duchini Jiuiior, the most optimistic and persistent exploration geologist I have ever met, is gratefully acknowledged for introducing me to the geology of the Lamego area, for our fmitful discussions, for providing al1 the necessary logistical support, and for his continuous encouragement during the course of my two field seasons in Brazil. The superb field assistants José Maria da Silva, Emani Ferreira, Adnam Vieira, and Antonio Casemiro de Moura are acknowledged for helping me during several "tough" field activities. These included geological mapping in partially flooded underground galleries, geological reconnaissance in densely vegetated areas, and the handling of rnanv tonnes of drill core boxes. 1 would also like to thank Geraldo Guilherme Silva and Rodison Lemos for helping me at the office of the Cuiaba mine with printing services and the managing of many computer data-files. The high quality of the Iithogeochemical analyses carried out in the Iaboratory of Mineraçào Moro Velho at Nova Lima is greatly appreciated. I would like to thank Irany Braga, ~lidaPereira, Theobaldo Millard, and Carmen Dias for their persistence and competency. Carlos Taciano Mendes, lerzy Adwent, and Roger innes are thanked for making the thin sections and polished sections used in this study. Dave Kempson and Dr. Peter Roeder are thanked for guidance in using the electron microprobe. Alan Grant is thanked for providing electron microprobe analyses of numerous electrum grains. Thomas Ullrich is acknowledged for assisting me with several computer software packages at Queen's University. David Regéczi is greatly acknowledged for lending, from time to time, his English literacy to this native Portuguese speaker. 1 would also Iike to thank my fhends in Nova Lima who gave me psychoIogica1 support and incentive during the completion of the field seasons in Brazil. Many thanks Orlando Casarin. Roberta Casarin, Edna Xavier, Gliucia Alves Cruz. Roberto Félix, Marcelo Fonseca. Gleyson Leone Reis, and Hélder Campos. Finally, 1 would like to thank my parents, Mircio and Nara Salles for their love. for being eternally supportive, and for the life values which they inculcated in me. TABLE OF CONTENTS

ABSTRACT ...... i ACKNOWLEDGEMENTS ...... iii TABLE OF CONTENTS ...... iv LIST OF FIGURES ...... ,...... w...... vii LIST OF TABLES .....w...... xii

CHAPTER 1 .INTRODUCTION ...... w...... 1 1.1 .Location and Access ...... 1 1 -2 .Purpose and Scope of Study ...... 1 1.3 .Methods of Investigation ...... 4 1 -4 .Previous Work ...... 5 1.5 .Thesis Organization ...... 12 1.6 .Surnrnary of the Present Contribution ...... 13

CHAPTER 2 .THE QUADRILATERO FERF~~CRO DISTRICT: REGIONAL GEOLOGY AND INTRODUCTION TO THE BANDED IRON-FORMATION -HOSTED GOLD DEPOSITS OF THE NOVA LIMA GROUP ...... 14 2.1 .Introduction and Physiography ...... 14 2.2 .Stratigraphy ...... 16 2.2.1 .Granitic-Gneissic Complex ...... 18 2.2.2 .Rio das Velhas Supergroup ...... 18 2.2.2a .Nova Lima Group ...... 20 2.2.2b .Maquiné Group ...... 73 2.2.3 .Minas Supergroup and Itacolomi Series ...... 23 2.3 .Structural and Metamorphic History ...... 25 2.4 .The BE-Hosted Gold Deposits of the Nova Lima Group ...... 29 2.4.1 . Introduction and Economic Importance ...... 29 2.4.2 .Geological Setting ...... 32 CHAPTER 3 .GEOLOGY OF THE LAMEGO PROSPECT ...... 36 3.1 .Introduction ...... 36 3.2 - Local Geological Setting of the Lamego and Cuiabii Deposits ...... 36 3.2.1 - The Nova Lima Group in the SabadCaeté Region ...... 36 3.2.2 - Local Tectonic Setting and Structures ...... 39 3.2.3 - The Larnego and Cuiaba "Eye-Shaped Structures" ...... 40 3 -3 - Stratigraphy and Rock-Types ...... 11 3.3.1 - Footwall Volcanic Unit ...... 33 3.3. la - The "Alteration Problem" and Petrography ...... 13 3.3. lb - Lithogeochemistry of the Alteration Facies ...... 57 3.3. lc - Characteristics of the Protolithic Volcanic Rocks ...... 65 3.3.2 - Lamego Banded Iron-Formation (BIF) ...... 71 3.3.2a - Introduction: "Barren" versus "Mineralized" BIF ...... 71 3.3.2b - Petrography and Lithogeochemistry ...... 72 3.3.3 - "Pele de Onça" Chert Unit ...... 79 3.3.4 - Black Carbonaceous Phyllite Unit ...... 82 3.3.5 - Upper EpiclasticNolcaniclastic Unit ...... 85 3.4 - Tectonic Structures ...... 85 3.4.1 - Planar and Linear Fabrics, Folds, and Faults ...... 85 3.4.2 .The Larnego and Cuiaba Eye-Shaped Structures: Sheath Folds? ...... 91

CHAPTER 4 = LAMEGO PROSPECT: THE GOLD MINERALIZATION ...... 101 4.1 .Lamego Prospect: Some Exploration Facts ...... 101 4.2 .Lithologic/L.ithostratigraphic Controls and Gold Grades ...... 105 4.3 ."Mineralized" and "Barren" Domains: Stratigraphie Implications ...... 112 4.4 .Mineralized Bodies: Mode of Occurrence, Size, Grade. and Petrography ...... 118 4.4.1 .Example 1: The Cabeça de Pedra Mineralized Zone ...... 119 4.4.1a .The BE-Hosted Gold Mineralization ...... 119 4.4.1 b .The Chert-Hosted Gold Mineralization ...... 125 4.4.2 .Example II: The Queimada Mineralized Zone ...... 129 4.5 .StructuraYSpatial Control of the Mineralized Bodies ...... 133 CHAPTER 5 .THE GEOLOGY OF THE LAMEGO AND CUIABA DEPOSITS: SOME CORRELATIONS. METALLOGENIC ASPECTS. AND IMPLICATIONS FOR EXPLORATION ...... 137 5.1 .Introduction ...... 137 5.2 .Stratigraphic Correlation ...... 137 5.3 .Regional Stratigraphie Position of the Larnego/Cuiaba BIF Unit ...... 142 5.4 .BE-Hosted Gold Mineralization: Characteristics and Genetic Aspects ...... 144 5.4.1 .Deposits ofthe Nova Lima Group: Genetic Aspects ...... 144 5.4.2 .Lamego and Cuiaba Deposits ...... 147 5.5 .Controls of the Gold Mineralization: Implications for Regional Exploration ...... 157 5.6 .The "Sheath Fold Inference": Local Exploration Implications ...... *...... 161

CHAPTER 6 .SUMMARY OF CONCLUSIONS ...... 162

REFERENCES ...... 168 APPENDIX A .MINERAL CHEMISTRY DATA FROM ELECTRON MICROPROBE ANALYSES (SILICATES. . ELECTRUM) ..... 179 VITA ...... oo...... ~...... 183 vii

LIST OF FIGURES

Figure 1.1 - Location of the Quadrilatero Fem'fero district (Brazil) ...... -3 Figure 2.1 - Location, limits, and sirnplified geological map of the Quadrilatero Fem' fero district ...... 1 5 Figure 2.2 - Panorarnic view of the SabadCaeté region in the northern part of the Quadrilatero Fem'fero district ...... 1 7 Figure 2.3 - Quadrilatero Fem'fero district: simplified stratigraphic column and synthesis of the available geochronological data ...... 19 Figure 2.4 - Tonnage and grade characteristics of some BE-hosted gold deposits worIdwide ...... 3 1 Figure 2.5 - Schematic cross-section showing the "flattening" of the plunge/dip of the orebodies with increasing depth - Mono Velho mine ...... 35 Figure 2.6 - Schematic cross-section showing the "flattening" of the plunge/dip of the orebodies with increasing depth - Cuiaba mine ...... 35 Figure 3.1 - Simpli fied geology of the SabarKaeté region ...... 37 Figure 3.2 - Basic geology of the Lamego prospect ...... 42 Figure 3.3 - Idealized stratigraphic sequence of the Lamego prospect - "mineralized" versus "barren" domains ...... 4.4 Figure 3.4 - Drill core samples of the altered basic volcanic rocks observed in the Footwall Volcanic Unit of the Lamego eye-shaped structure ...... 46 Figure 3.5 - Footwall Volcanic Unit - Larnego prospect: and mode of the alteration facies ...... 48 Figure 3.6 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 1 Alteration Facies ... 5 1 Figure 3.7 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 2 Alteration Facies ... 5 1 Figure 3.8 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 3/Type 4 alteration facies .. 53 Figure 3.9 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 5 Alteration Facies ... 53 Figure 3.10 - Footwall Volcanic Unit - Larnego prospect: composition of the carbonate fiom the different alteration facies ...... 54 Figure 3.1 1 - Randomly oriented, thin phyilosilicate-rich veinlets overprinted on bleached (carbonatization + sericitization) rock of the Type 4 Alteration Facies ...... 56 viii

Figure 3.12 - Footwall Volcanic Unit - Larnego prospect: chernical composition (major elements) of schists f?om the different alteration facies ...... 6 1 Figure 3.13 - Footwall Volcanic Unit - Larnego prospect: concentrations of some "base metal" trace elements ...... 66 Figure 3.14 - Chondrite-nomalized REE distributions for alterec! basic volcanic rocks ("Type 1" Alteration Facies) - Lamego prospect ...... 68 Figure 3.15 - Ti-Zr plot (Pearce and Cam, 1973) of average 'Type 1" and "TypeZ" schists compositions - Lamego and Cuiaba deposits ...... 69 Figure 3.16 - Zr/TiO,-NbN and ZrTîi02-Ce plot (Winchester and Floyd, 1977) of average 'Type 1" schist compositions - Larnego prospect ...... 70 Figure 3.17 - Typical carbonate-facies banded iron-formation of the Lamego Banded Iron-Formation ...... 73 Figure 3.18 - Typical carbonate-facies banded iron-formation of the Larnego Banded Iron-Formation ...... 73 Figure 3.19 - Thinly laminated carbonate-facies banded iron-formation of the Lamego Banded Iron-Formation ...... 74 Figure 3.20 - Thinly laminated carbonate-facies banded iron-formation of the Larnego Banded Iron-Formation ...... 74 Figure 3.21 - Photomicrograph (plane-polarized light) showing some mineralogical and textural aspects of a typically thinly laminated carbonate-facies BIF ...... 75 Figure 3.22 - Photomicrograph (cross-polarized light) showing some mineralogical and textural aspects of a typically thinly laminated carbonate-facies BIF ...... 75 Figure 3.23 - Photomicrograph (cross-polarized light) showing some mineralogical and textural aspects of a light-coloured band of a carbonate-facies BIF ...... 76 Figure 3.24 - Photomicrograph (cross-polarîzed light) showing some mineralogical and textural aspects of a dark brown band of a carbonate-facies BIF ...... 76 Figure 3.25 - Composition of carbonate minerals: "barren" and "mineralized" BIF ...... 77 Figure 3.26 - Underground exposure of the "Pele de Onça" chert ...... 80 Figure 3.27 - Underground exposure of the "Pele de Onça" chert ...... 80 Figure 3.28 - Photomicrograph (cross-polarized light) showing some mineralogical and textural aspects of typical "Pele de Onça" cherts ...... 8 1 Figure 3.29 - Photomicrograph (cross-polarized light) showing some mineralogical and textural aspects of a typical "Pele de Onça" chert ...... 8 1 Figure 3.30 - Underground exposure showing the most usual type of contact between the lower Lamego BIF and the upper Pele de Onça Unit ...... 83 Figure 3.31 .Drill core samples of schists £iom the Black Carbonaceous Phyllite Unit and Upper EpiclasticNolcaniclastic Unit ...... 84 Figure 3 -32 - Equal-area stereonet showing the distribution of the attitudes of the SI foliation planes (poles to Sz planes) ...... 87 Figure 3.33 - Equal-area stereonet showing the distribution of the plunges of the Lz linear fabric elements ...... 87 Figure 3.34 .Photograph showing the L2 Transamazonian linear fabric as an intersection of the Archean SI schistosity with the Transamazonian S, schistosity ..... 88 Figure 3.35 .Underground exposure showing the S2 foliation in carbonate-facies BIF from the SW hinge zone of the Lamego eye-shaped structure ...... 89 Figure 3.36 .Underground exposure showing the S2 foliation in carbonate-facies BIF from the SW hinge zone of the Lamego eye-shaped structure ...... 89 Figure 3.37 .Tencil structures" in highly weathered carbonaceous schists from the Upper EpiclasticNolcaniclastic Unit ...... 90 Figure 3.38 .LI linear fabric represented by the preferred orientation of individually elongated grains of in a carbonaceous schist ...... 90 Figure 3.39 .Underground exposure of a folded carbonate-facies %IFshowing an Sz axial planar foliation .SW hinge zone ...... 92 Figure 3.40 .Underground exposure of a folded sulphide-facies BIF in the SW hinge zone of the Lamego eye-shaped structure ...... 92 Figure 3.41 .Classification of highly non-cylindrical cone-shaped folds ...... 94 Figure 3.42 .Block diagrarns showing shape of sheath folds created by kinematic amplification of layenng deflections during progressive simple shear ..... 95 Figure 3.43 .Location. extent and "shape" of the Cuiaba eye-shaped structure at 4 levels of the Cuiabii mine ...... 97 Figure 3.44 .Outcrop showing steeply dipping bedded cherts of the Pele de Onça Chen Unit in the SW hinge zone of the Larnego eye-shaped structure ...... 99 Figure 3.45 .Outcrop showing steeply dipping bedded cherts of the Pele de Onça Chert Unit in the SW hinge zone of the Lamego eye-shaped structure ...... 99 Figure 3.46 .Equal-area stereonet showing the distribution of the attitudes of poles to bedding planes (So) along the entire Larnego eye-shaped structure ...... 100 Figure 4.1 .Lamego prospect: gold concentration in "active" stream sediment ...... 102 Figure 4.2 .Old workings for gold at the "Cabeça de Pedra rnineralized zone ...... 103 Figure 4.3 .Old workings for gold at the "Cabeça de Pedra" mineralized zone ...... 103 Figure 4.4 .Ancient, small-scale open-pit operation at the Lamego prospect ("Arco da Velha" rnineralized zone) ...... 104 Figure 4.5 - Panoramic view of the "LamegoPapa-Farinha" area ...... 106 Figure 4.6 - Main adit to the underground operations in the Lamego prospect ...... 106 Figure 4.7 - Underground exposure of typical sulphide-facies BIFs of the Lamego Banded Iron-Formation in the "Cabeça de Pedra" mineralized zone ...... 107 Figure 4.8 - Underground exposure of typical sulphide-facies BIFs of the Lamego Banded kon-Formation in the "Cabeça de Pedra" mineralized zone ...... 1 07 Figure 4.9 - Gold concentration versus sulphur content in mineralized BE samples from the "Arco da Velha*' and "Cabeça de Pedra" mineralized zones ...... 1 09 Figure 4.10 - Gold concentration versus sulphur content in mineralized BEsarnples from the "Balanciio/Canta Galo" orebody - Cuiaba mine ...... 1 10 Figure 4.1 1 - Goid concentration versus sulphur content in mineralized cherts from the "Cabeça de Pedra" and "Arco da Velha" mineralized zones ...... 1 1 1 Figure 4.12 - Underground exposure showing the nature of the boundaries between the Lamego Banded Iron-formation and the Pele de Onça Chert Unit .... 1 14 Figure 4.13 - Typical Pele de Onça "chert " cutting the Larnego Banded Iron- Formation close to the contact with the Pele de Onça Chert Unit ...... 1 16 Figure 4.14 - Sulphide-enriched replacement "halos" symmetrically distnbuted around a Pele de Onça "chert" vein in carbonate-rich dark bands of a BIF ...... 1 1 7 Figure 4.15 - Underground geological map - Cabeça de Pedra mineralized zone ...... 1 20 Figure 4.16 - Typical mineralized sulphide-facies BEof the Larnego Banded Iron- Formation at the Cabeça de Pedra mineralized zone ...... 12 1 Figure 4.1 7 - Outcrop of deeply weathered, mineralized sulphide-facies BIF at the SW hinge zone of the Larnego eye-shaped structure ...... 121 Figure 4.18 - Photomicrograph showing typical sulphide-nch bands of sulphide-facies BEsarnples from the Cabeça de Pedra mineralized zone ...... 123 Figure 4.19 - Photomicrograph showing typical sulphide-nch bands of sulphide- facies BIF sarnples from the Cabeça de Pedra mineralized zone ...... 123 Figure 4.20 - Photomicrograph showing an electrum grain occhng as an inclusion within a pyrite in a sulphide-facies BIF ...... 124 Figure 4.2 1 - Photomicrograph showing 2 electrtim grains occumng as inclusions within a pyrite crystal in a Pele de Onça chert ...... 124 Figure 4.22 - Grain-size distribution of electm particles hosted in sulphide-facies BIFs from the Cabeça de Pedra and Arco da Velha mineralized zones ... 126 Figure 4.23 - Variation of the gold fineness of electnim grains from the Lamego deposit and other BIF-hosted gold deposits of the Nova Lima Group .... 127

Figure 4.24 - Undermound- -- - - eeolo~icalmaD - Oueimada mineralized zone ...... 130 Figure 4.25 - Typical sulphide-facies BIF fiom the Queimada mineralized zone ...... 13 1 Figure 4.26 - Typical sulphide-facies BiF from the Queirnada mineralized zone ...... 1 3 1 Figure 4.27 - Photomicrograph showing sorne textural aspects of sulphide-rich layers of "Queimada-type" sulphide-facies BIFs ...... 1 34 Figure 4.28 - Photornicrograph showing some textural aspects of sulphide-rich layen of "Queimada-type" sulphide-facies B IFS ...... 134 Figure 4.29 - Grain-size distribution of electnim particles hosted in sulphide-facies BIFs from the Queimada mineralized zone ...... 135 Figure 5.1 - Simplified geology of the Cuiaba mine ...... 139 Figure 5.2 - Lamego prospect and Cuiaba mine: representative stratigraphic sections and stratigraphic correlations ...... 140 Figure 5.3 - Schematic cross section/block diagram showing a possible interpretation for the Cuiaba and Lamego eye-shaped structures ...... 143 Figure 5.4 - Gold concentration versus content in mineralized BEsarnples from the Balancào/Canta Ga10 orebody - Cuiaba mine ...... 149 Figure 5.5 - Gold concentration versus arsenic content in mineralized BIF samples from the Arco da Velha and Cabeça de Pedra mineralized zones ...... 150 Figure 5.6 - Gold concentration versus arsenic content in mineralized BIF samples from the Queimada rnineralized zone ...... 15 1 xii

LIST OF TABLES

Table 3.1 - Footwall Volcanic Unit: modal composition of altered volcanic rocks ...... 49 Table 3.2 - Whole-rock chemical analyses of representative, unweathered rocks of the five alteration facies of the Footwall Volcanic Unit ...... 58 Table 3.3 - Concentrations of some trace elements in representative, unweathered rocks of the five alteration facies of the Footwall Volcanic Unit ...... 59 Table 3.4 - Whole-rock chemical analyses of representative, unweathered samples of carbonate-facies BIFs f?om the Lamego Banded Iron-Formation ...... 78 Table A. 1 - Chemical composition of some rock-forming minerals fkom schists of the Footwall Volcanic Unit (Type 1 to Type 5 alteration facies) ...... 18 1 Table A.2 - Chemical composition of carbonate minerals fiom BIFs of the Larnego Banded Iron-Formation ("barren" and "mineralized" BIFs) ...... 182 CHAPTER 1 - INTRODUCTION

1.1 - Location and Access

The Lamego banded iron-formation-hosted gold deposit is located in the northem part of the Quadnlatero Ferrifero district, Southeast Brazil (Figure 1.1). nie Quadnlatero Femfero (in English, ) is an area of approximately 7,000 square kilometers centered about latitude 20" 15' S and longitude 43O 30' W in the Brazilian state of Minas Gerais @or, 1969). The district is well-known geologically not only as a consequence of its gold deposits hosted in an Archean greenstone belt sequence (Rio das Velhas Supergroup), but also for its iron deposits within a Lower clastic- chernical sedimentary unit displaying Lake Superior-type banded iron-formation sequences (Minas Supergroup). The Larnego deposit is about 20 kilometers east of , the capital of Minas Gerais and the third major Brazilian city (2,100,000 inhabitants). The study area is easily accessible by road, as it is located only 4 kilometers southeast of the city of Sabara and about 12 kilometers west of the town of Caeté (Figure 1.1).

1.2 - Purpose and Scope of Study

The Larnego gold deposit is a prospect owned by Mineraçào Morro Velho Ltda. (MMV), the second largest Brazilian gold company. MMV is a subsidiary of Minorco Brasil Ltda., a company owned by the international holding company Minorco. The banded iron-formation-hosted gold deposits of Morro Velho, Cuiabi, and Raposos, and some other smaller deposits of the same type (Faria, Bicalho, BeIa Fama, and Mono da Gloria) are owned and operated by MMV in the Quadnlitero Femfero district. For more than twenty years, this mining company has also been conducting a continuous and ppppp Brazil and the "Quadrilatero Ferrifero' district Adapted from Don (1969). Drawing by Marclo Sales (Queen's University - December, 1996). Figure 1.1 - Location of the Quadrilatero Fenifero district. systematic exploration prograrn in the district in order to replace mine reserves and discover new mineralized bodies and deposits. Mine production at the Lamego prospect should be initiated by the end of the century as part of a corporate production plan that also includes an increase in the daily production fiom the Cuiaba mine. The Cuiaba mine is only 7 kilometers from the Lamego prospect and has been operated by Mineraça0 Mono Velho since 1986. The ore of the Larnego deposit will be processed in the same metallurgical plant that now receives and processes the ore mined fiom the Cuiaba deposit. Although these 2 gold deposits have many geological similarities, they also show several fundamental differences that could influence different exploration, mine development, and metallurgical strategies and approaches. Recognizing this fact. Minorco Brasil Ltda. decided to encourage and support the present research. The main objectives of this project were the following:

A) To obtain a detailed description of the basic geology of the Lamego prospect. Fundamental and basic geological aspects of the deposit (stratigraphy, petrography, lithogeochemistry, regional tectonic setting, structures, hydrothermal alteration, and gold mineralization) were snidied in this project. B) To charactenze and describe the gold mineralization in ore-grade samples. Petrographic studies were conducted on mineralized sarnples of the Lamego prospect in order to facilitate the fuhire mine grade control and minera1 processing at the deposit. Special attention was paid to the mode of occurrence and characteristics of the electrum particles hosted in these samples. C) To identify the geological features and processes associated with, or responsible for, the generation and "control" (stratigraphic, geometric, and spatial) of the mineralized bodies in the Lamego deposit. D) To produce a bnef comparative geological study between the Lamego prospect and the Cuiaba mine in order to help the future mine development and the ongoing underground exploration at the Lamego prospect. E) To present a final surnmary containing the observed and interpreted controls of the gold mineralization in the Lamego deposit and to provide a concise list containing some relevant and useful information that should be considered for the exploration of similar deposits in the Quadrilatero Femfero region.

1.3 - Methods of Investigation

The present study involved 6 months of field work in the Quaàrilatero Fem'fero region during 1995 and 1996 (2 seasons of 3 months), and continuous research at Queen's University initiated irnmediately after the end of the first academic year of the Master of Science program. The main activities in Brazil were the following:

A) An extensive literature review centered on general geology and gold deposits of the Quadrilitero Fem'fero region. The relevant published and unpublished information for this project was compiled, analyzed, and filed for subsequent use at Queen's University. The "unpublished information" includes intemal reports, texts, and maps produced by geologists of Mineraçao Morro Veiho. B) The producing mines and current prospects of Mineraçao Morro Velho in the Quadrilatero Fem'fero district were visited together with Professor Robert Mason from Queen's University during a two-week tour in 1995. At that time, the most important geological aspects of each of these deposits were discussed with the local mine and exploration geologists, and the directions of the present research program were established. C) A field reconnaissance of the local geology and regional structural-tectonic setting of the Larnego deposit was undertaken. Despite the deep tropical , some geological profiles crossing al1 the stratigraphic units of the prospect were mapped at surface. Special attention was paid to the banded iron-formation/metachert unit that hosts the gold mineralization and to the structural parameters of the mineralized rocks. Stereonet analyses were made £?omstructural data collected in the field. D) A study of drill cores fiom the Larnego prospect and Cuiaba mine was made in order to define lithologies and stratigraphy and to study the hydrotherrnal alteration processes associated with the gold mineralization in the two deposits.

E) Detailed underground geological mapping (1 :100 scale) was completed for 2 of the 3 rnineralized zones already exposed by underground drifts and tunnels at the Lamego prospect. This activity provided essential data used to interpret the nature, ongin, and "structural control" of the gold mineralization and mineralized bodies. F) Whole-rock chemical analyses (major and trace elements) of 13 selected rock samples were cornpleted in the laboratones of Mineraçào Morro Velho.

The research was continued at Queen's University after the end of the second field season in October, 1996. The following activities were undertaken in Canada:

A) A literature review of examples of banded iron-formation-hosted gold deposits and geology of Archean greenstone belt sequences worldwide. B) Completion of peîrographic studies (transmitted-light microscopy, reflected- iight microscopy, and hand sample description) of the lithotypes that occur in the Lamego prospect. Special attention was paid to mineralized samples and also to rocks which charactenze the hydrothermal alteration patterns related to the gold mineralization. Numerous hand samples, 62 thin sections, and 40 polished sections were studied for this project. C) Completion of more detailed mineralogical studies on the main types of hydrothermally altered rocks related to the gold mineralization with the use of an ARL- SEMQ electron probe microanalyser.

1.4 - Previous Work

The publication of Dorr (1969), which is a synthesis of a systematic geological mapping program undertaken by the USGS ( Geological Survey) and DNPM (Departamento Nacional de Produçào Mineral - Brasil) between 1946 and 1962, is the base reference for the district. This geological mapping program introduced a consistent stratigraphic subdivision of the supracrustal sequences of the Quadrilatero Ferrifero and produced geological maps covering the entire district at a scale of 1 :25,000.

The USGSDNPM joint rnapping program was responsible for the recognition, narning and fint mapping of the older sequence of supracrustal rocks (the "Rio das Velhas Supergroup") that is unconformably overlain by the sedimentary rocks of the Minas Supergroup in the Quadrilatero Femfero district. This program also introduced the stratigraphic subdivision of the Rio das Velhas Supergroup into the older Nova Lima Group and the younger Maquiné Group as well as the forma1 subdivision of the Minas Supergroup into the Caraça, Itabira, and Piracicaba groups and their respective formations. The initial concepts and ideas about the structural framework, regional tectonic setting, and tectonic evolution of the rock successions of the Quadrilatero Fem'fero district were also important contributions made by Dom (1969) and the USGS- DNPM joint mapping program (see also Chapter 2). Schorscher (1978) descnbed, for the first time, ultrabasic komatiitic rocks at the base of the Nova Lima Group. Schorscher was also the first researcher to suggest that the Rio das Velhas Supergroup should be classified as a typical Archean greenstone belt sequence. Based on studies conducted in the northwest part of the Quadrilatero Ferrifer0 district, Oliveira et al. (1983) proposed a useful, informa1 stratigraphic subdivision of the undivided Nova Lima Group of Dorr (1969) into 3 units ("Basal Unit", "Intermediate Unit", and "Upper Unit"). Their Basal Unit consists of mafic to intermediate rnetavolcanic rocks interlayered with metapelites, Algoma-type banded iron-formation, and rare acidic metavolcaniclastic rocks. The Intermediate Unit is represented by intermediate/felsic volcanic rocks and volcaniclastic rocks interlayered with graphitic phyllites and horizons of banded iron-formation. The Upper Unit is composed essentially of metapelites interlayered with felsic metavolcanic rocks, metavolcaniclastic rocks, quartzites, and conglomerates. The stratigraphic subdivision of the Nova Lima Group adopted in the present dissertation is the one that was formally proposed in CPRMlDNPM (1994). This publication presented sorne results of an extensive geologicai-geophysical program that was partially fünded by local mining companies and carried out by 2 Brazilian governrnent agencies (CPRM-Companhia de Pesquisa de Recursos Minerais, and DNPM) in the Quadrilatero Femfero district during the 1992- 1995 period. CPRM/DNPM (1994) has confimed and extended to the entire Quadrilatero Ferrifero district, with a few srnaIl modifications, the stratigraphic subdivision of the Nova Lima Group previously proposed by Oliveira et al. (1983). Fontes (1993) mapped and described in detail the lithologies and lithostratigraphic successions of the Nova Lima Group at the SabadCaeté region. Drake and Morgan (1980) suggested that the Rio das Velhas Supergroup is an ailochthonous supracnistal rock sequence that was thmst ont0 a granitic-gneissic basement. In order to support their hypothesis, Drake and Morgan showed that the contacts of the granitic-gneissic terrains and the overlying Rio das Velhas Supergroup in the Quadrilatero Femfero district are highly sheared thnist surfaces associated with a large-scale tectonic transport from southeast to northwest. They also presented clear evidence of the existence of contrasting tectonic fabrics between Archean supracmstal rocks and granitidgneissic rocks.

Ladeira and Viveiros (1984) described the nature, style, and orientation of the structures and penetrative tectonic petrofabnc elements observed in supracrustal rock sequences of the district. They also made a clear distinction between a group of older structures and petrofabnc elements that are observed exclusively on lithological sequences of the Archean Rio das Velhas Supergroup, and a group of younger structures and petrofabric elements that are observed both in Archean and Proterozoic supracrustal sequences of the Quadnlitero Femfero. However (and unfortunately), Ladeira and Viveiros (1984) did not correlate the identified structures and petrofabric elements with the established regional tectono-thermal events that affected the Quadrilitero Fem'fero district (Rio das Velhas, Transamazonian, and Brasiliano orogenies - see Figure 2.3 and Section 2.3). Marshak and Alkmirn (1989) distinguished and characterized the post-Archean deformation events that affected both Archean and Proterozoic rock successions of the Quadrilatero Fem'fero district during the Transamazonian and Brasiliano orogenies. They described the Transamazonian orogeny in the district as a cornpressional tectonic event associated with intense tectonic transport and simple shear, ductile deformation, upper /lower arnphibolite facies metarnorphism, and extensive isotopic remobilization. Marshak and Alkmim also suggested that the conspicuous regional northwest-verging fold-thnist belt observed in the Quadrilitero Fem'fero and the most obvious penetrative planar and linear fabrics observed in Archean and Proterozoic supracrustal rocks were developed during the Transamazonian orogeny. These authors suggested that the effects of the would be clearly observed only in the eastem margin of the Quadrilatero Fem'fero, where a west-verging Brasiliano fold-thnist belt associated with minor folds, foliations and lineations was developed and supenmposed on earlier structures. Belo de Oliveira (1986) and Belo de Oliveira and Vieira (1987) described the major tectonic event that affected the SabarKaeté region and that produced extensive thmsts and nappes, large overturned folds with associated sheath folds, and penetrative LS tectonite fabrics. Belo de Oliveira and Vieira (1987) were the first authors to suggest that the overturned antifoms hosting the Cuiaba and Lamego deposits could represent highly stretched sheath folds produced by a regional ductile deformation event associated with intense tectonic transport and simple shear. Belo de Oliveira and Teixeira (1990) re-interpreted the deformation event defined in the SabargCaeté region by Belo de Oliveira (1986) and Belo de Oliveira and Vieira (1987) as a localized effect of the Transamazonian orogeny on the supracrustal sequences of the Quadrilitero Femfero district. Dating by U-Pb, Pb-Pb, Rb-Sr and K-Ar radiometric methods of the Archean units and tectono-thermal events that affected the Quadrilatero Fem'fero district was cornpleted by Corciani et al. (1980), Teixeira et al. (1987), Machado et al. (1989), Belo de Oliveira and Teixeira (1990), Machado and Carneiro (1992), and Machado and Noce (1993). Cordani et al. (1980) dated the metarnorphic and isotopic rernobilization events associated with the Transamazonian and Brasiliano orogenies in the southeast part of the Quadrilatero Ferrifero district. The approximate ages obtained by Cordani et al. were 2.0 Ga for the Transamazonian orogeny (Rb-Srlwhole rock) and 500-600 Ma for the Brasiliano orogeny (K-Adbiotites and amphiboles). The isotopic remobilization event related to the Transamazonian orogeny was also dated by Teixeira et al. (1987) and Belo de Oliveira and Teixeira (1990) as 2.1 to 2.2 Ga in mylonitic from the Caeté region (Rb-Srlwhole rock and U-Pbhtile). Machado et al. (1989) dated small and coreless prismatic zircons (U-Pb) From two samples of felsic volcanic rocks from the Nova Lima Group, which yielded dates of 2,776 Ma and 2,772 Ma. They interpreted these results as an indication of the time of the volcanic deposition and magmatic crystallization of some of the feelsic volcanic rocks of the Nova Lima Group.

Based essentially on U-Pb dates, Machado and Carneiro (1992) constrained the metamorphic, magmatic and isotopic remobilization events related to the Rio das Velhas orogeny to the 2,780-2,703 Ma interval. U-Pb dates were also used by Machado and Noce (1993) to define the 2,125-2,040 Ma time interval as the peak of the Transamazoniam isotopic remobilization event in the Quadrilatero Fem'fero district. Numerous publications characterize and describe the basic geological aspects of individual banded iron-formation-hosted gold deposits of the Rio das Velhas Supergroup. Graton and Bjorge (1929), Gair (1962), and Ladeira (1980, 1985) have described the geology of the Morro Velho deposit in detail. The most important studies related to the geology of the Cuiabh mine were carried out by Via1 (1980a, 1988), Vieira et al. (1991), and Vieira (1992). The Sgo Bento deposit was described by Moseley (1986), Abreu et al. (1988), Unamgem (1991), and Pereira (1992). The Raposos mine was studied by Tolbert (1962, 1964), Vial (1980b), and Vieira (1987a). The Fundamental aspects of the geology of the Bicaiho and Faria deposits were descnbed by Vial (1980~)and Moreschi (1972), respectively. Ladeira (1988, 1991) described and synthesized the more important geological aspects of the largest gold deposits of the Rio das Vehas Supergroup (Morro Velho, Cuiabii, S3o Bento, and Raposos). Basic geological information about the Larnego deposit was available only as non-published intemal reports written by exploration geologists kom Mineraça0 Morro Velho prior to this thesis. Ladeira (1980, 1985) characterized the stratigraphie, geometric and structural "controls" of the orebodies of the Moro Velho deposit. Ladeira (1 980 - p. 101) described them as stratabound and stratiform "long spindles. jIattened roak and blade-shaped' massive sulphide bodies that are congruently folded and deformed together with the host rocks associated with the deposit. This author also emphasized the fact that the more important orebodies of the Mono Velho mine are continuous down-plunge, mimicking the attitude of the most obvious penetrative linear fabric observed in that area for at least 4.800 meters. Vieira and Oliveira (1988) extended and applied the observations and descriptions about the "controls" cf the orebodies of the Morro Velho deposit made by Ladeira (1980, 1985) to some other gold deposits owned by Mineraçào Morro Velho in the Quadrilitero Fem'fero district such as Cuiabi, Raposos. Faria, Bicalho, and Bela Farna. Scarpelli (1991) suggested that the most prominent penetrative linear fabric observed in Archean and Proterozoic supracrustal rocks of the Quadrilatero Fem fero district, and the "geometric and stmctural controls" of the banded iron-formation-hosted orebodies of the Nova Lima Group (as previously descnbed by Ladeira and Vieira and

Oliveira) originated contemporaneously. According to Scarpelli (1 991 - p. 15 1 ), both of these types of geological features formed as a consequence of "cumula~ivedisplacements produced by compressive forces corning frorn the east-southeart", which transported supracrustal rock sequences to the West and northwest during the development of a northwest-verging fold-thrust beit of Proterozoic age. The hydrothermal alteration zones associated with some of rnost important banded iron-formation-hosted gold deposits of the Nova Lima Group were descnbed by Vieira (1988, 1991a). Based on geological mapping, he characterized a typical pattern of alteration zoning associated with the orebodies of the Nova Lima Group. According to Vieira (199 1a), the banded iron-formation-hosted orebodies are symmetrically enveloped by three "concentric" stratabound alteration zones ("sen~itization"~"carbonatization", and "chlontization") where the original volcanic and sedimentary rocks overlying and underlying the banded iron-formation units were hydrothermally altered. Sericitic zones are proximal in relation to the mineralized bodies, and are characterized by the mineral assemblage ankerite + sericite + quartz. Carbonatized zones tend to "envelop" sericitic zones and are identified by the assemblage ankerite + quartz + chlorite. Chloritic zones are distal from the orebodies and are characterized by assemblages composed basically of chlorite + quartz + calcite + plagioclase. The sulphides directly associated with the banded iron-formation-hosted gold mineralizations at the Cuiaba, Faria, Bicalho, and Bela Fama deposits were dated by Thorpe et al. (1984). They found that the least radiogenic dated yielded Pb-Pb mode1 ages of 2.7 Ga. Similar studies are currently being undertaken at the Morro Velho and Sào Bento deposits by E. DeWitt, C. Thorman, G. Landis, and R. Zartman fiom USGS. Nurnerous hypotheses have been proposed to explain the genesis of the banded iron-formation-hosted gold deposits in the Nova Lima Group. Earlier studies developed in the district by Graton and Bjorge (1929), Matheson (1956), and Gair (1962) interpreted the banded iron-formation-hosted gold deposits of the district as examples of "hypothermal" (terni adopted from Lindgren's genetic classifications of ore deposits, 1922 and 1933) metasomatic mineralizations related to hydrothemal fluids generated from magrnatic intrusions. Hypotheses suggesting that the gold mineralizations have a "syngenetic" ongin related to subseafloor hydrothermal activity or volcano-sedimentary exhalative processes have been supponed by Ladeira (1980, 1985, 1988, 1991), Via1 (1980a, 1980b, 1980c), Moseley (1986), Abreu et al. (1988), and DeWitt et ai. (1994, 1996). "Epigenetic" metamorphogenic/syntectonic models in which gold mineralization is related to ductile shear zones have been advocated by Belo de Oliveira (1986), Vieira (1 987b, 1988, 1991a), Vieira and Oliveira (1 988), Scarpelli (1Wl), and Pereira (1 992). There ha been general confusion in the Quadriliitero Fem'fero district about the timing relationships of the deposition of the host rocks, hydrothermal alteration, gold mineralization, , and penetrative deformation in the banded iron- formation-hosted gold deposits of the Nova Lima Group. One of the main objectives of this study is to elucidate these relationships at the Lamego deposit in order to establish rel iable guidelines for the exploration and evaluation of Archean banded iron- formation- hosted gold deposits and prospects in the Quadrilitero Fem'fero district.

1.5 - Thesis Organization

The present dissertation is subdivided into 6 chapters. Following the introductory chapter, Chapter 2 presents an overview of the regional geology of the Quadrilatero Femfero district. A brief discussion of the most relevant aspects of the geology of the banded iron-formation-hosted gold deposits of the Nova Lima Group is also included in this chapter. The local geological setting, the basic geology, and the cntical features of the gold mineralization at the Lamego deposit are described and discussed in chapters 3 and 4. Chapter 5 provides a brief comparative study between the Larnego prospect and the Cuiaba mine in terms of geological and metallogenic aspects. Chapter 6 is a synthesis of the most significant results and findings obtained by this research. 1.6 - Summary of the Present Contribution

The most important original contributions of the present research to the general understanding of geology of the banded iron-formation-hosted goid deposits of the Nova Lima Group are the following:

A) A detailed description of the lithologies, lithostratigraphic units, and stratigraphic sequence of the Lamego deposit. B) Correlation and positioning of the stratigraphic successions of the Lamego and Cuiaba deposits in the regional stratigraphy of the Nova Lima Group. C) Interpretation of the Lamego and Cuiaba ovemimed antiforms as highly stretched sheath folds (tubular folds) elongated parallei to the Transamazonian stretching lineation trend, and to the direction of simple shear and regional tectonic transport associated with the Transamazonian orogeny at the Quadrilatero Femfero district. This inference is supported by results and information obtained frorn the underground and surface mapping and structural analysis of the Lamego prospect, and from compilation of existing data, together with my own observations £kom the Cuiaba mine. D) Characterization of the pattern of the hydrothermai aiteration zones associated with the banded iron-formation-hosted goid mineralization at the Lamego deposit. E) Description of the mode of occurrence, distribution, size. and fineness of the electrum particles in mineralized sarnples fiom the Larnego deposit. F) Demonstration that the buUc of the banded iron-formation-hosted gold mineralization at the Larnego deposit is ciearly related to hydrothemal processes operating in a volcanic/sub-volcanic environment, pnor to Archean and Proterozoic penetrative deformation and metamorphism. CHAPTER 2 - THE QUADRILATERO FE~F'ERODISTRICT: REGIONAL GEOLOGY AND INTRODUCTION TO THE BANDED IRON-FORMATION-HOSTED GOLD DEPOSITS OF THE NOVA LIMA GROUP

2.1 - Introduction and Physiography

The Quadniatero Ferrifero distnct is an area of approximately 7,000 square kilometers in the Brazilian state of Minas Gerais, centered about latitude 30" 15' S and longitude 43" 30' W (Dorr, 1969). The approximate geographical limits and also some of the more important cities and toms contained in the distnct are shown in Figure 2.1. The north and west limits of the Quadrilitero Fem'fero are, respectively, the northeast- southwest trending Serra do Curral (in English, Curral Mountain Range) and the north- south trending Serra da Moeda. The south limits are defined by two east-west trending ridge-lines known as Serra Ouro Branco and Serra do Salto. According CO Ladeira (1980 - p. SZ), the east boundary of the district "is irnprecisely made" by the Serra do Caraça. The climate of the Quadrilatero Fem' fer0 region is subtropical. Temperatures nonnally range from about 6°C on the coldest nights to about 30-35°C on the warmest days. The average temperatures are about 18°C dunng the winter months and about 22°C during the summer months (Gair, 1962). The mean annual rainfall varies nonnally from 1,500 to 2,000 millimeters, and the distribution of the precipitation is seasonal. The rainy season starts in November and ends in April. A significant part of the annual average precipitation occurs between late December and early March. The topography of the region varies from moderately to very rugged. The maximum observed elevation is 2,100 meters at Serra do Caraça, and the lowest elevations (600-700 meten) occur at the bottom of valleys excavated by the major rivers. The physiography of the Quadrilatero Femfero was eloquently described by Dorr in his

1969 publication (p. A7) in the following way: "Frorn o physiographic viewpoint the 15 (caption)

Figure 2.1 - Location, limits, and sirnplified geological map of the Quadrilatero Femfero distnct. The Lamego prospect, al1 the active gold mines in the Rio das Velhas greenstone belt during the 1994/1995 period, and some of the more important cities and towns contained in the district are also shown in this map. Quadrii6tero Femfero District: Location, iirriits, and Simplifii Gedogical Map

- - Figure 2.1

RSo das Velhas Supergroup (Achean Greenstone kit Seque

Qtiadrilarero FerriJero is one of the more scenic parts of Brazil. It is marked by high and rugged peakî. long chains of hogback mountains. high . sonte deep canyons. and pleasant open valfeys.Ir is an area of great vistas and. on a cfear day /rom the top of lhe ridges and mountains one can see at least 100 km in many directions." The ridge-lines and highest elevations are supported by stratigraphic units composed of sedimentary rocks that are more resistant to weathering and erosion (quartzites and -facies banded iron-formations). The Quadrilatero Ferrifero region is characterized by savanna scmb or grassland vegetation, typical of tropicaVsubtropical regions with a well-defined dry season. Woods and forested areas are comrnon only along major drainages, and/or on lithologicaVlithostratigraphic units that typically give rise to more fertile soils (mainly basic and intermediate volcanic rocks). Some of the typical physiographic features observed in the Sabar3Caeté region (northem part of Quadrilitero Femfero) are shown in Figure 2.2.

2.2 - Stratigraphy

The Quadrilatero Fem'fero is a metallogenic district located in the southeasrem part of the SZo Francisco in eastem Brazil (Almeida, 1977) - see Figure 2.1. The region is formed by an Archean -greenstone basement overlain by an Early Proterozoic clastic-chemical sedimentary unit (Minas Supergroup), which in tum is locally overlain by a Proterozoic clastic sedimentary succession (Itacolorni Series). The Archean supracnistal rock sequences occumng in the granite-greenstone terrains of the Quadrilatero Ferrifero district have been grouped together in the Rio das Velhas

Supergroup, which is a typical Archean greenstone belt sequence (Schorscher, 1978). A simpli fied stratigraphic column showing al1 the major regional units, their lithological associations, and the available geochronological data for the Quadrilatero Fem'fero district is presented in Figure 2.3. 17 (caption)

Figure 2.2 - Panoramic view of the SabarKaeté region in the northern part of the Quadrilatero Fem' fero district. The "Serra da Piedade" (Piedade Mountain Range), which is the denomination for the NE Iimit of the "Serra do Currai" (see also Figure 2.1), and some of the mine facilities at the Cuiaba deposit are shown in this photograph.

2.2.1 - Granitic-Gneissic Complex

The most commonly observed rocks forming the basement of the supracrustal successions of the Quaddatero Fem'fero district are variably migmatized tonalitic to granodioritic gneisses (Machado et al.. 1992). The chernical composition of the tonalitic types is essentially trondhjemitic (> 70 % SiO,). and the most migmatized types usually show granitic ("sensu stricto") composition (Noce et al.. 1992). Layered arnphibolite bodies and rafts of banded iron-formation can be locally observed in association with the granitic-gneissic complexes (Machado et al.. 1992). According to Drake and Morgan (1980), Ladeira (1980), and Machado et al. (1992), most of the contacts between the granitic-gneissic complexes and the supracrustal sequences in the Quadrilatero Fem'fero are severely tectonized and marked by thmst faults. The Baçào Complex, an elliptic domain of Archean gneisses outcropping in the south-central part of the Quadnlatero Fem'fero (see Figure 2. l), was interpreted as a mantled dome (Loczy and Ladeira, 1976 in Machado et al., 1992) and as a tectonic window in the Rio das Velhas Supergroup (Gomes, 1986 in Machado et al., 1992). Machado and Carneiro (1992) obtained the oldest minimum '07~b-'06~bage (laser ablationflCPMS method) of 2.920 Ma for zircon cores in gneissic rocks collected in the Bonfim Complex (Figure 2.1). According to Machado et al. (1992), the existence of zircon cores older than 2.7 Ga (the approximate age of the isotopic remobiIization event associated with the Rio das Velhas orogeny, at the end of the Archean - see Figure 2.3 and Section 2.3) indicates the presence of a 2.8-3.1 Ma old continental sialic cmst as the basement of the supracrustal successions of the Quadnlatero Fem' fer0 district.

2.2.2 - Rio das Velhas Supergroup

The Rio das Velhas Supergroup (or Rio das Velhas Series) was formally subdivided by Dorr et al. (1957) and Dorr (1969) into the older Nova Lima Group and the younger Maquiné Group.

Cauê Fomtion: lron depobits - Take Superior-type" sequences (0.g. - ltabira district, uas Claras, Pico, Mutuco, Alegiia,3 opunema, Tirnbopeba)

2,776+23/-10Ma Pb-zircon).Pgeof cbpositim md crysY diizotlon of feisic -volcanic rock, Mxhodo et d. (i 989)

Nova Uma Group: BIF-hosted gdd deposits (0.9. - Mono Velho, Cuiaba, Si30 Bento, Rapm)

- 2,880Ma. tnfened oge for the IMd of the Rio dos Whas geenstorie -rIt bpx1lion of bmdites axl thoieiltes mmdeJr. et d., 1994). grcons) in lhe 2.92-2.70Ga htzRecords of generotlon md -remoôilizatim mts. Machado and Ccmeiro (i992). Figure 2.3 - Buaddider0 F&em dlstrlct: simplilled siraiigmphic cdumn and synihesis d the awlbbie geochiwidogical data.

2.2.2a - Nova Lima Group

Based on geological mapping around the region of Nova Lima town and on re- interpretation of the existing literature of the Quadrilatero Fem'fero district, Ladeira (1980) proposed a subdivision of the Nova Lima Group into three major units: A basal "metavolcanic unit" composed essentially of ultramafic, mafic, and felsic volcanic rocks; an intermediate thin unit (up to 150 meters in thickness) hosting metarnorphosed chernical sedirnentary rocks; and an upper sequence composed of metamorphosed impure clastic sedimentary rocks. Schorcher et al. (1982) defined the Quebra Osso Group in the eastem part of the Quadrilatero Fem'fero. The Quebra Osso Group corresponds to the lowermost part of the "metavoicanic unit" of Ladeira (1980) and is composed almost exclusively of ultrabasic volcanic rocks, including spinifex-textured peridotitic . Oliveira et al. (1983) in Via1 (199 1) proposed a usefil subdivision of the Nova Lima Group into three informal units based mainly on underground mapping at 6 gold mines located in the northwest part of the Quadrilatero Fem'fero. These units and their lithological associations are the following:

A) A basai unit composed of mafic (basic) to intermediate metavolcanic rocks interlayered with metapelites, Algoma-type banded iron-formations, and rare acidic metavolcaniclastic rocks. The authors did not recognize any ultrarnafic/uItrabasic volcanic rocks in the lowest stratigraphie levels of the Nova Lima Group in that particular region of the Quadrilatero Fem'fero district. B) An intermediate unit represented by rnafic to felsic volcanic rocks and volcaniclastic rocks interlayered with graphitic phyllite and horizons of Algoma-type banded iron-formation. C) An upper unit composed of metapelites interlayered with felsic metavolcanic rocks and metavolcaniclastic rocks, quartzites, and rnetaconglomerates. More recently, a geological and geophysical mapping program in the Rio das Velhas greenstone belt conducted by CPRM and DNPM has confirmed and expanded the stratigraphic subdivision of the Nova Lima Group proposed by Oliveira et al. (1983) across the entire Quadrilatero Fem'fero district. Some of the more informative publications produced from this program ("Rio das Velhas Project") include CPRMIDNPM (1994, 1995) and Baltazar et al. (1995). The forma1 stratigraphic units of the Nova Lima Group according to CPRMIDNPM (1994), which will be adopted in this thesis (see Figure 2.3), are the following:

A) Basal Unit - A lowermost succession composed essentially of basic and ultrabasic volcanic flows of komatiitic and tholeiitic affiliation. Lavas of intermediate to acidic composition, vo lcano-chemical sedimentary rocks suc h as banded iron- formation and chert, and clastic sedimentary rocks are minor components of the Basal Unit. B) Middle Unit - An intemediate sequence fonned mainly by intercalations of basic volcanic flows, tuffs and flows of intermediate composition, and epiclastic rocks. Banded iron-formation and chert lenses and layers are cornmonly observed within the Middle Unit. C)Upper Unit - The uppermost unit of the Nova Lima Group can be subdivided into two stratigraphic successions. The lower part of the Upper Unit is composed of acidic-intermediate volcanic and volcaniclastic rocks of calc-alkaline affinity, volcanic breccias, agglomerates, tuffs, and epiclastic rocks. The upper part of the Upper Unit is composed essentially of epiclastic rocks and graywackes.

The depositional age of the Nova Lima Group is presently not well constrained, as only very few geochronological data are available for the Quadrilitero Fem'fero district. Machado et al. (1989) and Machado et al. (1992) dated small and coreless prismatic zircons (U-Pb radiometric method) fiom two sarnples of felsic volcanic rocks fiom the

Middle Unit, which yielded dates of 2,776 (+7/-6)Ma and 2,772 (+6) Ma, interpreted as an indication of the time of the volcanic deposition and magmatic crystallization of these rocks. The contacts of the Nova Lima Group with the granitic and gneissic rocks of the Granitic-Gneissic Complex are generally obscured by shearing associated with regional thmst surfaces (Drake and Morgan, 1980 and Ladeira, 1980).

2.2.2b - Maquiné Group

The Nova Lima Group is overlain by a sequence of detntal sedimentary rocks known as the Maquiné Group. The contact between these groups is gradational, but disconformities have been observed locally (Gair, 1962; Moore, 1969; Don, 1969; and Maxwell, 1972 in Ladeira, 1980). The Maquiné Group is subdivided into the lower Palrnital Formation and the upper Casa Forte Formation (O'Rourke, 1956 in Gair, 1962; Dorr, 1969; and Ladeira, 1980). The Palrnital Formation consists of sequences of phyllites and quartz schists containing lenses of graywackes and quartzites (Ladeira, 1980). Its thickness is estimated to range from 600 to 1,400 meters (Gair, 1962). The contact of the Palmital Formation with the overlying Casa Forte Formation is gradational and rnarked by the presence of conglomeratic quartzites and conglomerate beds (Dorr, 1969).

The Casa Forte Formation has a thickness of 250 to 600 meters (Gair, 1962) and consists of sericitic and chloritic quartzites, conglomerates, and lesser amounts of sericitic and chloritic schists and phyllites (Dorr, 1969). The clastic sedimentary rocks of this formation are generally more mature (more quartzose and less sericitic and phyllitic) than the rocks of the Palmital Formation (Gair, 1962 and Dorr, 1969). 2.2.3 - Minas Supergroup and Itacolomi Series

The Minas Supergroup or Minas Series (Dorr, 1369) is an Early Proterozoic clastic-chemical sedimentary sequence that rests with angular unconfomity on the Granitic-Gneissic Comp lex and the Rio das Velhas S upergroup. According to Ladeira (1980 - p. 47), the Minas Supergroup is a typical "miogeosynclinal" succession characterized as a "paraplatforrnal assemblage of extensive

rabztlar, mature clastic sedirnenfaryroch, lirnestone. and oxide facies iron-formalion". The supergroup is formally subdivided into the lower Caraça Group, the intermediate Itabira Group, and the upper Piracicaba Group (Figure 2.3).

The Caraça Group is subdivided into two formations. The lower Moeda Formation consists of a basal conglomerate sequence overlain by mature quartzites and phyllites that is normally less than 500 meters thick (Don; 1969). The upper Batatal Formation is composed of a sequence of sericitic phyllites ranging in thickness fiom a

Few meten to more than 200 meters. It overlies the Moeda Formation bbgeneraIZyivith a slrarp contact" (Dorr, 1969 - p. A37), but the two formations may also interfinger locally on a srna11 scale (Wallace, 1965 in Dom, 1969). The Itabira Group is composed chiefly of chemical sedirnentary rocks (Dorr, 1969 and Ladeira, 1980) and is divided into two formations. The lower Cauê Formation is a sequence of banded iron-formations containing subordinate arnounts of dolomite and phyllite ranging in thickness from 300 to 500 meten (Ladeira, 1980). The contact with the underlying Batatal formation is gradational over a few centimeters or, in rare cases, I or 2 rneters (Dom, 1969). The Cauê formation is the major ndge-forming unit in the Quadrilitero Ferrifero district. Dorr (1969) pointed out that the ridges and ridge-lines are a consequence of the weathering of a thick succession of oxide-facies banded iron- formations under tropicaVsubtropical conditions. The weathenng product, a thick Iatentic carapace, is inert to chemical weathering and extremely resistant to erosional processes. The famous iron deposits of the Quadrilatero Fem'fero district are hosted in the Cauê Formation. The overiying Gandarela Formation consists of dolomites, , dolomitic banded iron-formations, and phyllites (Ladeira, 1980). Its thickness is variable, reaching a maximum of 600 meten (Ladeira, 1980). The contact between the Gandarela and Cauê formations is gradational (Dorr, 1969). The Itabira Group is overlain by the Piracicaba Group, a unit consisting of clastic sedimentary rocks ranging from conglomerate through quartzite to phyllite and graphitic phyllite as well as sporadic beds and lenses of dolomite and banded iron-formation (Dom, 1969). The Piracicaba Group is subdivided into five formations: fiom the base to the top, they are the Cercadinho Formation, Fecho do Funil Formation, Tabooes Formation, Barreiro Formation, and the Sabara Formation. According to Dorr (1969 - p. A49), most

of these formations contain "relaliveiy simple and well-drfferentiated mature sediments " (see Figure 2.3). The uppermost formation of the Piracicaba Group (Sabara Formation) is the only exception. It is composed of graywackes, chloritic schists, phyllites, tuffaceous rocks, tilloids, and lenses of banded iron-formation (Ladeira, 1980). The maximum known thickness of the Piracicaba Group is in the order of 4,800 meten (Don 1969). The Sabara Formation alone presents a total thickness of more than 3,000 meters (Ladeira, 1980). The contact between the Piracicaba Group and the Itabira Group is gradational, or locally, an erosional surface (Dorr, 1969). No angular unconformity is observed between these two units. The maximum age for the deposition of the Minas Supergroup is 2,650 Ma, based on the age of the youngest zircons (207~b-206~bages - laser ablatiodICPMS rnethod) observed in clastic rocks from the base of the Moeda Formation, the lowermost formation of the Minas Supergroup (Machado et al., 1993). The minimum age is based on the age of the Transamazonian orogeny (Figure 2.3), a regional tectono-thermal event that affected the rock successions of the Quadnlatero Fem'fero district between 2,125 and 2,040 Ma (Machado and Noce, 1993 in Renger et al., 1995). Dolomites at the top of the Gandarela

Formation were dated by Babinski et al. (1991, 1995) as 2,420 k 19 Ma (Pb-Pb/whole rock). The Itacolomi Series overlies the Minas Supergroup unconformably (Ladeira, 1980). The unit is composed dominantly of quartzites, conglomerates, and thin phyllitic lemes and layers. The thickness of the Itacolomi Senes is not known, because its uppermost observable stratigraphie horizon is limited by the present level of erosion. Octavio Barbosa (written communication, 1965 in Dorr, 1969) suggested that the series rnay have had an original thickness of 2,000 meters. According to Machado et al. (1993) and Noce (1995) in Carneiro et al. (1995), the available '*'~b-~~~~bages for detrita1 zircons suggest a maximum limit of 2,178 Ma for the begiming of the sedirnentary deposition of the Itacolomi Series.

2.3 - Structural and Metamorphic History

The supracmstal successions of the Quadnlatero Fem'fero have undergone a complex and polyphase defonnation/rnetarnorphic history resulting in multiple planar and linear fabnc elements, superposed folds, and complex overpnnted metarnorphic assemblages. Dorr (1969) identified three major periods of deformation affecting supracmstal successions of the district. The first major deformation event occurred in the Archean, afier the deposition of the Rio das Velhas Supergroup and prior to the deposition of the Minas Supergroup. The second, a poorly defined deformational event, occurred after the deposition of the Minas Supergroup and before the beginning of the deposition of the Itacolomi Senes. The third period of deformation identified by Don- was considered the strongest one, and affected al1 the supracmstal major units of the Quadrilatero Fem'fero (Rio das Velhas Supergroup, Minas Supergroup, and Itacolomi Senes). Cameiro (1992) proposed the informal designation of "Rio das Velhas tectono- thermal event" or Rio das Velhas orogeny for the Archean deformation event described by Dorr in 1969. Carneiro suggested that the Rio das Velhas orogeny affected the granite- greenstone terrains of the Quadrilatero Ferrifero district between 2.8 and 2.6 Ga. Based on the available geochronological data for the district (Rb-Sr, U-Pb, and 207~b-206~b ages), the first and main phase of the orogenic event occurred between 2.78 and 2.77 Ga, and was charactenzed, according to Carneiro et al. (1995), by crustal "reworking" (deformation and metarnorphism) and bimodal calc-aikalineltholeiitic magmatism. These authors suggested that this bimodal magmatism compnsed not only the generation and emplacement of intrusive bodies (granitic, granodioritic, tonalitic, and gabbroic composition) in the Quadrilhtero Femfero, but also the felsic volcanism observed in the upper parts of the stratigraphic colmof the Nova Lima Group. The final stages of the orogeny were marked by the post-tectonic emplacement of srna11 granitic and granodiontic bodies at 2.72-2.70 Ga (Carneiro et al., 1995). According to Ladeira and Viveiros (1984), the deformation associated with the Rio das Velhas orogeny produced northeast-verging asymmetric folding with hinges oriented along the direction S65E, and a planar fabnc ("S,"), which is invariably parallel to the sedimentary and volcanic layering ("S,,"). Dorr (1969) showed that the Archean deformation event was more intense in the western part than in the eastem part of the Quadnlatero Fem'fero district. According to Ladeira et al. (1983), the prograde metarnorphism associated with the Rio das Velhas orogeny occurred under greenschist or low-amphibolite facies conditions. The second deformation period suggested by Dorr (1969) has not been confinned or convincingly identified by subsequent geochronological and mapping studies in the district. Dorr (1969 - p. A84) believed however, that the Minas Supergroup was "warped and irpli/red' prior to the sedimentation of the Itacolomi Senes. This interpretation was based on the fact that the contact between these two major stratigraphic units is a subtle angular unconformity. The youngest (and also strongest) deformation period identified and descnbed by Dorr (1969) is now interpreted, fiom several recent structural and radiometric dating studies, as part of the "Transamazonian orogeny", an important Early Proterozoic (- 2.0 Ga) tectono-thermal event initially defined in terrains of the Guiana (northem Brazil, , , Surinam, and ). The Transarnazonian orogeny in the Quadrilatero Fem'fero was a compressional tectonic event (Manhak and Alkmim, 1989) associated with intense tectonic transport and simple shear, ductile deformation, metamorphism, and extensive isotopic remobilization. On a regional scale, the orogeny was responsible for the development of a conspicuous northwest-verging fold-thst belt (Marshak and Alkrnim, 1989) and for the large overtumed synclines and anticlines observed in the district (Belo de Oliveira and Teixeira, 1990). Regional and local stratigraphie inversions are also associated with the Transamazonian fold-thmst belt (see Figure 2.1 and Figure 3.1). Machado et al. (1992) consider that the Transamazonian orogeny in the southem Siio Francisco craton is charactenzed by tectonism, arnphibolite-facies metarnorphism, and partial melting of Archean crust at 2,059 Ma. The peak of this orogenic event in the Quadrilatero Femffero district occurred between 2,125 and 2,040 Ma (Machado and Noce, 1993 in Renger et al., 1995). The most obvious penetrative planar fabric observed in Archean and Proterozoic supracrustal rocks of the district (''StW surfaces) is an axial-planar schistosity and/or a surface of transposition produced dunng the Transamazonic event (Cordani, 1980; Marshak aqd Alkrnim, 1989; and Belo de Oliveira and Teixeira, 1990). S2 surfaces usually stnke northeast-southwest and dip southeast (Marshak and Alkrnim, 1989). A prominent penetrative linear fabt-ic ("L2*')defined primarily by minera1 lineations, stretching lineations, intersection lineations (intersection of S2 and SdIS, surfaces), and elongation of pebbles in conglornerates is also intimately associated with this tectonic event. In a regional sense, the southeasVeast plunging L, linear fabric is parallel to the direction of the Transarnazonian tectonic transport and also to the direction of vergence of the overtumed asymmetric folds associated with the orogeny (Figure 2.1 and Figure 3.1 ). The prograde metamorphic mineral assemblages in Archean and Proterozoic supracrustal rocks of the Quadrilatero Fem'fero dimict are also believed to have been generated during the Transamazonian orogeny (Cordani et al., 1980). Herz (1 978) pointed out that the 1st prograde regional metarnorphism recorded in the Quadrilatero Femifero is a Barrovian-type low-grade greenschist facies (chlorite or biotite sub-facies) event. Marshak and Alkmim (1989), however, suggested that the metamorphism associated with the Transamazonian orogeny in the district locally attained the lower amphibolite facies (staurolite in pelitic sedimentary rocks). The rnetamorphic grade in the Quadrilatero Femfero district increases to the south and east (Herz, 1978). nie S2o Francisco craton and the Quadrilatero Fem'fero district were also affected by the "Brasiliano orogeny" between 542 and 502 Ma (Cordani et al., 1980) or 650 and 500 Ma (Chemale Jr. et al., 1994). The Brasiliano orogeny in the Quadrilatero Fem'fero did not involve the addition of new matenal to the continental crust (Belo de Oliveira and Teixeira, 1990), and no Brasiliano ages have been obtained using U-Pb and Rb-Sr (whole-rock) radiometric rnethods. Brasiliano radiometric ages have only been detected From K-Ar dates in micas and amphiboles (Cordani, 1980 and Teixeira, 1985 in Marshak and Alkmim 1989). Belo de Oliveira and Vieira (1987), Belo de Oliveira and Teixeira (1990), and Manhak and Alkmim (1989) suggested that the tectonic and structural effects of the Brasiliano orogeny were better observed in the eastem margin of the Quadrilatero Ferrifero distnct, where a second phase of fold-thrust deformation was developed. Marshak and AIkmim (1989) suggested that the Brasiliano orogeny was responsible, in the eastern part of the district, for an intense west-verging tectonic transpon and for the generation of "an array of nortWsouth-trending and west-vergitrg megafolds and fairlis and associated minor folds. foliations and lineations" (p. 563) that were supenmposed on earlier structures. Marshak and Alkmim (1989) also suggested that the Brasiliano fold-thmst deformation in the Quadrilatero Fem'fero was accompanied by a new prograde metarnorphism that reached the upper greenschist facies. Momn (1988 - p. 213/214) also pointed out that Brasiliano structures and deformation "are not unformly developed" in the rock sequences of the Quadrilatero

Femfero district. He suggested that Brasiliano structures in the district "verge west to south-west and, on the regional scale, include folds such as the Moeda syncline and Mariana anticline" (see Figure 2.1 ), "meridionaliy orienled faults" along its south-eastern border, and "several norrh-north-eut oriented faults" in its south-western sector. Monitt inferred that the Mariana anticline folds earlier (Transarnazonian ?) thnist sheets and affirmed that Brasiliano surfaces cm be observed in rocks of the Minas Supergroup at the

Serra do Curral, in the northern part of the Quadnlatero Fem'fero district (Figure 2.1 ). Noce (1995) in Carneiro et al. (1995), however, suggested that the Brasiliano orogeny was not capable of promoting an intense reworking (isotopic remobilization, prograde metarnorphism, and penetrative structural fabrics) of the rock successions of the Quadnlatero Femfero district. According to Noce, the Brasiliano event in the district was responsible rnainly for the movement of cmstal blocks along pre-existing faults and for the Iocalized generation or reactivation of thst faults. Dorr (1 969) did not recognize any deformation event affecting the Archean and Proterozoic supracrustal sequences of Quadrilatero Femfero district that could be associated with, or correlated to, the Brasiliano orogeny.

2.4 - The BIF-Hosted Gold Deposits of the Nova Lima Group

2.4.1 - Introduction and Economic Importance

The Quadrilatero Femfero is an important past and present gold-producing region. Gold mining in the district started in the seventeenth century, with the metal being extracted initially only from alluvial placers. This region alone was responsible for more than half of the world's gold production during the eighteenth century (Ledru and Bouchot, 1993). Suficial and underground mining activities in "hard-rock" for primary gold were initiated in the nineteenth century, rnainly because the nch placer deposits were exhausted. The four largest known gold deposits hosted in the Rio das Velhas greenstone belt show the historical importance of the Quadrilatero Fem'fero district. The deposits of Mono Velho, Cuiaba, Silo Bento, and Raposos have been intermittently operated since 1834, 1877, 1860, and 1910, respectively (Ladeira, 1980; Ladeira, 1988; ViaI, 1988; and Abreu et al., 1988). The largest gold deposits, and also the great majority of the gold occurrences already discovered and identified in the Quadrilitero Femfero district, are hosted in the Nova Lima Group of the Rio das Velhas Supergroup. The gold orebodies and the nurnerous non-economic mineralized bodies observed in rock successions of the Nova Lima Group can be classified in two main "types". The first type would include the stratabound and stratiform orebodies hosted in banded iron-formation units. The second type wouid include various forms of gold mineralization observed in association with discordant (or stratabound but not stratiform), quartz-lode hydrothermal systems. Mineralized bodies and orebodies of the "second type" are associated with diverse lithologies and lithostratigraphic units of the Nova Lima Group. The present research is concemed mainly with the geology of one of the banded iron-formation (BIF)-hosted gold deposits of the Nova Lima Group: the Lamego deposit. The Nova Lima Group is one of the best studied Archean supracrustai units in Brazil because of its BIF-hosted gold deposits. At least 8 deposits of this category hosted in sequences of the group were being developed and operated in the Quadrilatero Fem'fero district during 199411995 (Morro Velho, Cuiaba, Silo Bento, Raposos, Faria, Bicalho, Bela Fama, and Morro da Gloria - see Figure 2.1). The Nova Lima Group also hosts numerous old workings and small gold mines operated mainly in the eighteenth and nineteenth centuries and contains many gold occurrences related to lenses and units of banded iron-formation. The tonnage and grade characteristics of sorne of the known largest BIF-hosted gold deposits in the world cm be seen in Figure 2.4. The Mono Velho (- 500 metnc tonnes of gold), Cuiaba (- 180 metric tomes), and Silo Bento (- 80 metric tomes) deposits are considered type examples for BE-hosted gold mineralization (Hutchinson and Burlington, 1984; Kerswill, 1993; and MMV-intemal reports). Tonnage and grade characteristics of some MF-hosted gold deposits worldwide. Tonnage and grade figures refer to combined past production and present reserves. (Modlfled from Kerswltl, 1993) - Marclo Soles (Queen's Unlverslty - August, 1997)

0 Deposlts In the Nova Llma Group (Rio das Velhas greenstone belt - Brazll)

Other deposits: Homestake (U,S,A,) lu In (Canada) Hi1P SO (Australla) Musselwhlte (Canada) Agnlco-Eagle (Canada) Vubachikwe (Zimbabwe)

Lamego and Culabd data: reserves + 'Infened resources' (source: MMV - lnternol reports)

Figure 2.4

1 10 Tonnage (millions of tonnes) 2.4.2 - Geological Setting

The most important known BIF-hosted gold deposits and prospects of the Nova Lima Group are associated with two distinct lithostratigraphic settings. Cuiabk Siio Bento, Raposos, Faria, and Morro da Gloria are confined to layers of regularly banded carbonate and sulphide-facies iron-formation. The Morro Velho, Bicalho, and Bela Fama deposits are hosted in a singular lithostratigraphic unit, formally termed "Lapa Seca" Formation (Ladeira, 1980). The Lapa Seca Formation has a thickness of 3 to 100 meters (average of 30 meters) and consists of massive carbonate-rich cherts associated with thin layers of tuffaceous and volcano-exhalative rocks (Ladeira, 1988). Descriptions and studies from deposits of both of the lithostratigraphic settings above (Morro Velho (Ladeira, 1980 and 1985), Cuiaba (Viai, 1988 and Vieira, 1992),

Raposos (Vieira and Oliveira, 1988 and Ladeira, 1991) and Faria (Moreschi, 1972)) have show that the BEand carbonate-rich chert units that host the mosi important orebodies of the Nova Lima Group are also found in a specific stratigraphic position. These auriferous units mark a stratigraphic transition from a basal basic (or ultrabasic) volcanic deposition regime to an upper mixed unit composed essentially of epiclastic and volcaniclastic sedimentary rocks. According to Vieira (199 1a), portions and domains of the BIF and chert units that display gold mineralization are cornrnonly enveloped by stratabound "concentric" hydrothermal alteration zones that have been charactenzed (after the regional rnetamorphism) as domains of "sencitization", "carbonatization", or "chloritization" of the original rock sequences occumng in the footwall and hanging wall of the %IF units. Ideally, sericitic zones are proximal in relation to the mineralized bodies and are charactenzed by the mineral assemblage ankerite + sericite + quartz. Carbonatized zones tend to envelop sericitic zones. They are identified by the assemblage ankente + quartz + chlorite. Chloritic zones are distal fiom the orebodies and are characterized by assemblages composed basically of chlorite + quartz + calcite t plagioclase. The gold mineralization consists of stratabound, and essentially stratiform, sulphide-rich layen and bodies that are congruently folded and deformed with the host rocks and associated stratabound hydrothermal alteration zones, and also with the entire stratigraphic sequence occurring at the deposits (Ladeira, 1991). The sulphides occur in scattered grains, in seams, and in irregular-shaped granular aggregates disposed along and replacing sideritic carbonate-rich bands of the banded iron-formations (Gair, 1962). In most cases, a direct relationship can be established between the amount of sulphides (percentage per volume) and the gold grades (grams per metric tonne) in mineralized samples. The average gold grade of individual BE-hosted orebodies of the Nova Lima

Group presently mined varies kom 6 g/t to 13 g/t.

Gold occurs in the form of electrurn, and the average AdAg ratio varies between 5: 1 and 6: 1. Electnim particles are normally observed as inclusions within sulphides or at sulphide grain boundaries (Ladeira, 1988). Very rarely, they are associated with minerals (quartz and carbonates). , pyrite and arsenopyrite are the most important sulphide minerals in the BIF-hosted orebodies. , and are commonly present, but always as very minor accessory minerals. The stratabound and stratiform BIF-hosted orebodies of the Nova Lima Group resemble, in geometrical terms, long "blades" or "sticks" displaying elliptical basal sections. Their "width" (dimension along the direction of the strike of the BIF layers) varies fkom less than 50 to 300 meters, and the thickness can Vary from 1 to 20 meters. The "length" (dimension along the plunge direction) is always the largest of the three dimensions of the orebodies. Abreu et al. (1988) pointed out that the length of the orebodies of the SZo Bento deposit along the plunge direction is always at least three times longer than the width. Some of the most important orebodies of the Morro Velho and Cuiaba deposits have minimum lengths of 800 to 4,500 meters along the plunge/dip direction. The orebodies plunge with the same attitude as the most obvious penetrative linear fabric observed in Archean and Proterozoic supracrusta1 rocks of Quadrilatero Femfero district (L,). Some of the most important and deepest mines of the district demonstrate that the plunge/dip of the BE-hosted orebodies and also the plunge of the Transamazonian linear fabnc (L2)fiatten gradationally with increasing depth in the Quadrilatero Fem'fero district. Both orebodies and Transamazonian linear fabnc at the Morro Velho mine have a plunge of 45" at surface, and 13" at the deepest level (2,450 meten in depth). According to Ladeira (1980), the decrease in plunge of the orebodies is about 1" 30' on each successive operational level of the Morro Velho mine. The orebodies of the Cuiaba deposit show a sirnilar behavior. The Balancào orebody, for example, plunges 36" at the first underground operational levei of the Cuiaba mine (Level 3), 27" at the Level 7, and 2 1" at the Level 1 1, the deepest operational level (MMV-intemal report, 1996). Schematic cross-sections showing the steady and progressive flattening of the plunge of the orebodies of the Morro Velho and Cuiaba mines are presented in Figure 2.5 and Figure 2.6. These cross-sections are oriented parallel to the plunge direction of the orebodies and also to the local Transarnazonian tectonic transport direction. 35 (caption)

Figure 2.5 - Schematic cross-section showing the "flattening" of the plunge/dip of the orebodies of the Moro Velho mine with increasing depth. The decrease of theplunge of the orebodies is about 1" 30' on each successive operational level of the mine. The plunge is 45" fiom the surface to the first operational level of the mine, and 13" at the deepest level (2,450 meters in depth). The cross-section is oriented paralle1 to the plunge direction of the orebodies and also to the local Transamazonian tectonic transport direction (see also Figure 2.1).

Figure 2.6 - Schematic cross-section showing the "flattening" of the plunge/dip of the Balanciio orebody of the Cuiaba mine with increasing depth. The Balancgo orebody plunges 36' at the first underground operational level (level 3), 27" at level 7, and 21" at level 11, which presently is the deepest operational level of the mine. The cross-section is oriented paralle1 to the plunge direction of the orebodies and also to the local Transamazonian tectonic transport direction (see also Figure 2.1). Sm meters Main Orebody - Horizontal and Morro Velho Mine Vertical Scales

-Levd 22 Morro Velho Mine - Operational Levels

2,453 meten below surface

Figure 2.5

Regional Tectonic Transport Direction N6ûW S60E Open-Pif - Surface 5ûû meters Homontal- and Vertical Scales

Sea Level

Balancao Orebody - Cuiaba Mine

-Level 1 1 Cuiaba Mine - Operational Levels

Schemdc cross-rcKtion d the orebodles with CornOPed frmMMV (1996). Figure 2.6 CHAPTER 3 - GEOLOGY OF THE LAMEGO PROSPECT

3.1 - Introduction

Chapter 3 describes the basic geological aspects of the Lamego deposit such as stratigraphy, petrography and lithogeochemistry of the rock-types, pattern of the hydrothermal alteration related to the BIF-hosted gold mineralization, stnictures, metamorphism, and regional tectonic/stratigraphic setting.

3.2 - Local Geological Setting of the Lamego and Cuiaba deposits

The Lamego and Cuiaba deposits are located in the northem part of the Quadrilatero Femfero, a few kilometers from the city of Sabara and the town of Caeté (Figure 3.1). The SabarWaeté area is mainly underlain by rocks of the Nova Lima Group (Rio das Velhas Supergroup). Sequences of the Minas Supergroup and Archean granitic- gneissic terrains occur in the northwest and east extremities of the area, respectively.

3.2.1 - The Nova Lima Group in the SabarslCaeté Regioo

The three units of the Nova Lima Group are well represented in the SabarKaeté region. According to CPRM/DNPM (1994), the Basal Unit in this region is composed essentially of intercalations of basic and ultrabasic volcanic rocks. Intercalations of intermediate to acidic volcanic rocks are not common. Lenses and layers of banded iron- formation have been observed, very locally, in the Basal Unit. The basic volcanic rocks are represented by showing variable arnounts of actinolite, chlorite, plagioclase, epidote, and quartz. The ultrabasic volcanic rocks were converted into serpentinites, talc schists, and chlorite- schists (Fontes, 1993). 37 (caption)

Figure 3.1 - Simplified geological map of the SabadCaeté region. The Lamego and Cuiaba "eye-shaped" structures (see Section 3.2.3) and the more important past gold mines and "old workings" are also shown.Note that the more important BE-hosted gold deposits and gold occurrences that have already been identified in the SabaralCaeté region are confined to the "Middle Unit" of the Nova Lima Group.

Minas Supergroup Qady Roterasoic) - phyilite. "WeSuperior-type" BIF, blomite. quamite, and conglomerate.

Rio da$ Whas Supergroup Wcheun) - Nova Uma moup - üpper bit: pelite, acidic volcaniclostic roch. epiclastic rocks.

Rio das Whas Super roup (Archecan) - Nova Umu G~OUD- Id ddle Unit: rnainlv basic and internediate volcanic rocks. kidk volcanic rocks, pelite. and Blf horizons are common.

Rio das Whas Supergroup Wchean) - Nova Uma Gioup - Basal lhk basic and ultrabasic volcanic rocks. Pcidic volcanic rocks and BIF horizons are tess common.

The Middle Unit in the Sabara/Caeté region consists of basic and intermediate volcanic rocks intercalated with acidic volcanic rocks, acidic tuffs, pelites, banded iron- formations, cherts, and carbonaceous schists (CPWNPM, 1994). The basic and intermediate volcanic rocks are represented by quartz-plagioclase-epidote-chlorite schists and quartz-plagioclase-chlorite schists (Fontes, 1993). Prirnary mineralogy and textures in acidic volcanic rocks and tuffs have been modified or destroyed by deformation and metarnorphism. As a consequence, these lithotypes are now represented by feldspathic quartz-sericite schists and feldspathic chlorite-quartz-sericite schists (Fontes, 1993). The pelitic rocks of the Middle Unit are phyllites (basically quartz + chlorite + sericite), quartz-chlorite-sericite schists, and carbonaceous or carbonate-nch quartz-chlorite-schists (Fontes, 1993). Neither biotite nor almandine gamet is observed in pelitic rocks of the region. Based on the prograde mineral assemblages shown by these pelites, it is inferred that the rocks of the Nova Lima Group in the SabadCaete region were subjected to a Iow-greenschist facies prograde metarnorphism during the Transamazonian orogeny. The banded iron-formations consist of altemating bands of quartz, and sidentidankentic carbonate, or of quartz, , and sideritic/ankentic carbonate. Nurnerous deactivated srna11 BE-hosted go Id mines and BE-hosted gold occurrences are associated with the Middle Unit of the Nova Lima Group at the SabadCaeté region (Figure 3.1). The Upper Unit in the SabariKaeté region consists of a sequence of acidic to intemediate volcaniclastic rocks (pyroclastic rocks and tuffs) and epiclastic rocks (pelites and graywackes). The original mineralogy and textures of the rocks of the Upper Unit were also extensively modified by the intense and polyphase deformation/metarnorphic events that affected the Quadrilitero Femfero district. Volcaniclastic rocks were converted into plagioclase-chlorite-carbonate-sericite schists, and epiclastic rocks into carbonaceous chlorite-sericite-quartz schists (pelites) and feldspathic chlorite-sericite- quartz schists (graywackes). 3.2.2 - Local Tectonic Setting and Structures

The more conspicuous large-scale structures observed in the SabadCaeté region are mainly related to the northwest-verging fold-thst belt developed in the Quadrilatero Fem'fero district during the Transamazonian orogeny (Belo de Oliveira and Teixeira, 1990). Thrust faults characteristically emplace older rock sequences on top of younger rock sequences and may result in major inversions of stratigraphic sections, and repetitions (or omissions) of parts of sections (Twiss and Moores, 1992). The SabadCaeté area is a good example of a regional stratigraphic inversion caused by a thnist system (Figure 3.1). In this area, the Archean Granitic-Gneissic Complex is emplaced on top of the Archean Rio das Velhas Supergroup which is in tuni emplaced on top of the Early Proterozoic Minas Supergroup. According to CPRM/DNPM (1994), the geological boundaries between these major stratigraphic units in the SabarKaeté region are marked by hstsurfaces. The Basal Unit of the Nova Lima Group lies on top of the Intermediate Unit, which, in tum, overlies the Upper Unit (Figure 3.1). The boundaries between these units in the region have been observed, in most cases, as thst or tear fault surfaces (CPRM/DNPM, 1994). The Larnego and Cuiaba deposits are positioned in a northeast- southwest "fiontal (or slightly oblique) ramp" structural domain that is bounded to the south by a set of tear faults oriented along the N70W670E direction (Figure 3.1). The most significant tectonic structures observed in rocks of the Nova Lima Group in the SabarKaeté region are the prominent regional planar (S2) and linear (L,) fabrics generated by the Transamazonian deformation event. On average, the attitude of the S, foliation in the SabarXaeté region is N40E/40°SE (Fontes, 1993). The southeast- trending L, linear fabric is parallel to the local direction of the Transamazonian tectonic transport in the SabarUCaeté region (Figure 3.1). On average, it trends S70E and plunges 30" (Fontes, 1993). Intuitively, fold axes are assurned to be subperpendicular to the direction of tectonic transport (Lacassin and Mattauer, 1985). However, some localities in the SabadCaeté region that were subjected to more intense ductile simple shear deformation

during the Transamazonian orogeny present folds with axes onented close to the plunge direction of the L, stretching lineation and to the regional tectonic transport direction. Belo de Oliveira (1986) and Belo de Oliveira and Vieira (1987) affirmed that complex noncylindrical folds and sheath folds elongated parallel to the L, stretching lineation and to the transport direction have been observed, in varied scales, in highly defoxmed domains of the SabarKaeté region.

3.2.3 - The Lamego and Cuiaba "Eye-Shaped Structuresn

Two of the most intriguing geological features of the SabarVCaete region are the so-called Lamego and Cuiaba antiforrns. These "eye foldr" or "eyed structures" (these terms were first utilized by Nicholson, 1963) are southeast-plunging overturned non- cylindrical antiforms that are outlined by the auriferous Lamego and Cuiaba banded iron- formation (BE) units (Figure 3.1). At the current erosion level, the "Lamego eye-shaped structure" (or "Lamego structure") has a length of approximately 1,800 meters, a maximum width of 450 meters, and limbs oriented along the direction N3 5E. The "core" of the structure (the stratigraphie sequence that underlies the Lamego BIF unit) consists of basidintermediate volcanic rocks. A succession of carbonaceous pelitic rocks associated with acidic volcaniclastic rocks and other epiclastic rocks overlies the Larnego BIF unit. At the current erosion level, the "Cuiaba eye-shaped structure" (or "Cuiabi structure") has a length of approximately 700 meters and a maximum width of 350 meters. As in the Lamego structure, the core of the Cuiaba structure consists of a sequence of basiclintermediate volcanic rocks. The rock sequence that overlies the Cuiaba BIF unit, on the other hand, is different fkom the sequence that overlies the Lamego BIF unit. The immediate hanging wall of the Cuiaba BIF unit consists of a sequence of basiclintermediate volcanic rocks ranging from 60 to 200 meters in thichess that is in tum overlain by a succession of carbonaceous pelitic rocks associated with acidic volcaniclastic rocks and graywackes. Some of the ideas and hypotheses developed in this study about the Lamego and Cuiaba eye-shaped structures are the following (see Figure 3.1):

A) The two distinct rock sequences that host the Lamego and Cuiaba deposits and form the Lamego and Cuiaba structures are interpreted, based on petrographic, lithogeochemical, and stratigraphic data, as sections of the same regional stratigraphic level of the Nova Lima Group. As a consequence, it has also been suggested that the Larnego and Cuiaba BIF units are "lateral equivalents" of the sarne lithostratigraphic horizon. B) The hinge zones of both the Lamego and Cuiabi structures as well as the penetrative intersection lineation (intersection of the So and Sz surfaces) observed on rock exposures of the Lamego and Cuiaba deposits are parallel to the regional Transamazonian stretching lineation (L,). C) The Larnego and Cuiaba structures are interpreted as highly stretched kilometer-scale sheath folds (tubular folds) elongated parallel to the L2 regional stretching lineation trend and also to the regional tectonic transport direction associated with the Transamazonian orogeny in the Quadrilitero Ferri fero district. Belo de Oliveira and Vieira (1987) previously suggested that the Larnego and Cuiaba structures were sheath folds. However, the interpretation made by these authors was not substantiated by field or laboratory evidence. D) The sequences of basic/intermediate volcanic rocks outcropping in the core of the Larnego and Cuiabi structures are interpreted as part of the uppermost stratigraphic portion of the Middle Unit of the Nova Lima Group.

3.3 - Stratigraphy and Rock-Types

The Lamego structure is outlined by the elliptical outcrop pattern of the Lamego Banded Iron-Formation (BIF) unit (Figure 3.1 and Figure 3.2). The core of the "closed" ovmmedcntifocm Carbonoceous phyîiiis and carbonatebearing plmge of fdd a&) feldspaftvc schists mer EprclaslicEI#can,m Ilnif).

1 Basic gedogy d the Lamego prospect &fi de@- by MMV lion: 880 meters)

antifonn (the exposed stratigraphic sequence underlying the Larnego BE unit) consists eçsentialiy of a sequence of hydrothermally altered basic and basic-intermediate volcanic rocks (basaltslbasaltic andesites). A succession of carbonaceous pelitic rocks associated with acidic volcaniclastic rocks and other epiclastic rocks, overlies the Lamego BIF unit. Both limbs of the ovemirned antifonn, the "SE limb" and "NW limb", dip to the SE (Figure 3.2). The NW limb of the structure is overtumed and, consequently, shows an inversion of the typical stratigraphic column of the deposit. Idealized stratigraphic columns of mineralized and non-mineraiized ('%amen") domains at the Lamego prospect are presented in Figure 3 -3.

3.3.1 - FootwaIl Volcanic Unit

The lowerrnost lithostratigraphic unit of the Lamego deposit (the "Footwall Volcanic Unit") outcrops in the core of the Lamego overturned antiform (Figure 3.2 and Figure 3.3). This unit has a maximum exposed thickness of about 150-200 meters and is represented by hydrothermally altered and metamorphosed basic volcanic rocks, with metarnorphic mineral assemblages characteristic of the lower greenschist facies. No unaltered basic volcanic rocks have been found in the Footwall Volcanic Unit during the course of this investigation. The unit is commonly overlain by the "Lamego Banded Iron-Formation" (Section 3.3.2) but may be locally overlain by the "Pele de Onça Chert Unit" (Section 3.3.3) or the "Black Carbonaceous Phyllite Unit" (Section 3.3.4).

3.3.la - The ''Alteration Problem" and Prtrography

The mineral assemblages observed in rocks of the Footwall Volcanic Unit at the Lamego deposit are "exotic" if compared to typical greenschist mineral assemblages in basic volcanic rocks of the Nova Lima Group fiom the northern part of the Quadrilatero Ferrifero district.

Five different types of mineral assemblages have been identified in rocks of the Footwall Volcanic Unit during this research. mese mineral assemblages, which have been narned in this thesis as hydrothermal "alteration facies", were originally established from visual estimation of the amount (percentage in volume) of key indicator minerals present in the different "lithotypes" of the unit. Subsequent laboratory studies including transmitted-light microscopy, lithogeochemical analysis, and mineral chernistry analysis using the AEU-SEMQ electron probe microanalyser in the Department of Geological Sciences/QueenTs University (see operating conditions and detailed minera1 chemistry data in Appendix A), has confimed the adopted field classification. These studies have also provided basic mineralogical and chemical information on the 5 alteration facies, which is important in understanding the hydrothermal alteration processes and in interpreting the original composition of the unaltered, protolithic volcanic rocks of the Footwall Volcanic Unit (see Section 3.3.1~). The rocks of the Footwall Volcanic Unit in the Lamego structure are interpreted in this research as originally basic (SiO2 contents between 45% and 52%) or basic/intermediate volcanic rocks (basalts or basaltic andesites), that were hydrothermally altered and metasomatized during the emplacement of mineralization in an Archean volcanic/sub-volcanic environment. These rocks were subsequently metamorphosed and had their mineralogy re-equilibrated under low-grade, greenschist facies regional metamorphism conditions.

Type 1 Alteration Facies The least altered lithotypes observed in the Footwall Volcanic Unit of the Lamego prospect have been included in the "Type 1 Alteration Facies". They are very fine- grained (largest usually smaller than 80-90 microns), weakly foliated, grayish- green coloured schists essentially composed of monoclinic epidote, chlorite, albite, quartz, actinolite, and calcite (Figure 3.4 and Figure 3.6). These rocks are generally massive and commonly "spotted" with small white-cream coloured calcite crystals. 46 (caption)

Figure 3.4 - Drill core samples of the typical hydrothermally altered basic (basichtennediate) volcanic rocks observed in the Footwall Volcanic Unit of the Lamego eye-shaped structure. These lithotypes are fine-grained (the largest crystals are usually smaller than 80-100 microns), low-grade, Barrovian metarnorphic schists. As a reference, an unaltered (?) basic volcanic rock (presently a metarnorphic greenschist) from the Cuiaba eye-shaped structure is also shown.

A) Unaltered (?) basic volcanic rock - Cuiaba mine. Average modal composition: actinolite (25% in volume), clinozoisite-epidote (25%), chlorite (20%), albite (20%), quartz (5%), and accessory minerals (5%).

B) Grayish-green schist of the Type 1 Alteration Facies - Lamego prospect. Average modal composition: clinozoisite-epidote (37% in volume), chlorite (20%), albite (15%), quartz (l4%), actinolite (7%), calcite (5%), and accessory minerals (2%). Note an incipient carbonatization characterized by a "spotted texture" (large calcite crystals within a fine-grained "greenschist" matrix) and the sparse presence of thin calcite-quartz veinlets.

C) Green coloured schist of the Type 2 Alteration Facies - Larnego prospect. Average modal composition: chlorite (40% in volume), quartz (2 1 %), calcite (1 4%), albite (l3%), sericite (7%), clinozoisite-epidote (3%), and accessory minerals (2%). Note the conspicuous foliation and the widespread presence of thin calcite-quartz veinlets.

D) Greenish-gray or greenish-beige schists of the Type 3 + Type 4 Alteration Facies - Larnego prospect. Average modal composition: ankente (3 1% in volume), quartz (27%), sericite-paragonite (18%), chlorite (IS%), albite (7%), and accessory minerals (2%). Note the conspicuous "bleaching" (carbonatization + sencitization) and the presence of "flattened" and transposed ankerite-quartz veinlets.

E) Grayish-beige schist of the Type 5 Alteration Facies - Lamego prospect. Average modal composition: ankerite (33% in volume), quartz (30%), sericite-paragonite (24%), (5%), chlorite (3%), and accessory rninerals (2%). Note the intense "bleaching" (carbonatization + sericitization) and the complex alteration pattern resulting from superimposed episodes of phyllosilicate-nch veinlet associated alteration and ankente-siderite "flooding".

Typical pnrnary (volcanic) minerals, textures, and structures have not been observed in altered rocks fiom the Footwall Volcanic Unit at the Larnego structure. Via1 (1988) and Vieira (1992) have described, however, some relict pillow lava structures and variolitic textures in similar plagioclase-quartz-chlorite-actinolite-epidote schists fiom a few underground exposures at the core of the Cuiabi structure (Cuiaba mine). These rocks from the Cuiaba structure are interpreted in this thesis on the basis of tectonic, stratigraphic, petrographic, and lithogeochemical evidences as lateral equivalents of the Footwall Volcanic Unit of the Larnego structure (see Figure 3.1 and Section 5.2). Monoclinic epidote (clinozoisite-epidote, 37% in volume), chlonte (20%), albite

(15%), quartz (14%), actinolite (7%), and caIcite (5%) are the "indicator minerals" of the Type 1 alteration facies (Figure 3.5 and Table 3.1). The accessory minerals contained in Type 1 rocks (a total of 2% in volume) are mainly unidentified opaque phases, pyrite, sphene, and . The epidotes are characterized by an average ratio of 0.21 for Fe203/(Fe203+ AI@& The chlontes are the most rnagnesium-rich types observed in rocks of the Footwall Volcanic Unit, with an average MgO/(MgO + FeO) ratio of 0.54. Electron microprobe analyses have also demonstrated that the untwinned. fine-grained plagioclase and the carbonate minera1 observed in Type 1 schists are albite and pure calcite, respectively (Figure 3.10 and Appendix A). The alteration mineral assemblage that characterizes the Type 1 Alteration Facies of the Footwall Volcanic Unit is similar to the minera1 assemblages described from propylitic alteration zones of volcanic-related epithermal gold deposits and is typical of distal alteration zones around most Archean lode gold deposits.

Type 2 Alteration Facies The Type 2 Alteration Facies includes very fine-grained (largest crystals usually smaller than 90 microns), foliated, green coloured schists composed essentially of chlonte, quartz, calcite, and albite (Figure 3.4 and Figure 3.7). Some important

"Footwall Volcanic Unit": modal composition of the altered volcanic rocks

1 l~nalteredbasic volcanic Alteration Facies (Lamego prospect) % (in volume) rocks (Cuiaba mine) "type 1" 1 "type 2" "type 3" "type 4" 7 1 not present not present not present not present not present not present not present 1 chlorite 1 20 quartz 5

white mica not present not present 17 albite 20 I calcite I 1 not present not present not ~resent

ankerite 1 not ~resent not present1- not ppresent -- 1 siderite 1 not present not present Inot present not present not present?

Petrography by Marcio Sales Table 3.1 (Queen'sUniversity - 1997) differences between Type 1 and Type 2 "greenschists" cm be recognized. Typical Type 2 Alteration Facies rocks do not contain clinozoisite-epidote or actinolite, and contain more chlonte and calcite than the lithotypes of the Type 1 Alteration Facies. White mica, which is nowhere present in Type 1 schists, may be present in small amounts in Type 2 schists.

The indicator minerals of this facies are chlorite (40% in volume), quartz (2 1%), calcite (14%), and albite (13%). Small concentrations of white mica (7%) and clinozoisite-epidote (3%) are cornmon in Type 2 schists, but they have not been observed together in any of the samples investigated. The accessory minerals detected (a total of 2% in volume) are rnainly unidentified opaque phases, pyrite, sphene, and apatite. A few crystals have been observed in Type 2 schists, usually associated with white mica. The average MgO/MgO + FeO) ratio of chlorites from Type 2 schists is 0.44 (Figure 3.5 and Appendix A). Chlorites from Type 2 schists are ncher in iron than chlorites fiom Type 1 schists. The untwinned, fine-grained plagioclase and the carbonate mineral that have been observed in Type 2 schists are albite and pure calcite, respectively (see also Figure 3.10). The accessory ''white mica" that has been observed in some sarnples of this alteration facies is a muscovite, with an average Na,O/Nu,O + &O) ratio of 0.05 (Figure 3.5 and Appendix A).

Type 3 and Type 4 Alteration Facies "Type 3" and ''Type 4" rocks are noticeably more bleached and altered than Type 2 rocks (Figure 3.4). Both of these two lithotypes are conspicuously foliated, very fine- grained (larger grains usually smaller than 100- 120 microns), greenish-gray or greenish- beige schists composed essentially of ankerite, quartz, white mica, chlonte and albite (Figure 3.5 and Figure 3.8). The distinction between types 3 and 4 schists has been based exclusively on the relative proportion of their phyllosilicate minerals. Lithotypes richer in chlorite than in white mica have been grouped in the Type 3 Alteration Facies. 51 (caption)

Figure 3.6 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 1 Alteration Facies (diarnond drill hole FPL-27). Clinozoisite-epidote (37% in volume), chlorite (20%), albite (15%), quartz (14%), actinolite (7%), and calcite (5%) are the essential minerals of 'Type 1 schists". The plane of the thin section is perpendicular to the plunge direction of the Transarnazonian regional stretching lineation.

Figure 3.7 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 2 Alteration Facies ( drill hole FPL-27). Chloite (40% in volume), quartz (21%),calcite (l4%), and albite (13%) are the essential minerals of 'Type 2 schists". Small concentrations of either muscovite (7%) clinozoisite-epidote (3%) may also be observed. The plane of the thin section is perpendicular to the plunge direction of the Transamazonian regional stretching lineation.

Conversely, schists richer in white mica than in chlonte have been included in the Type 4 Alteration Facies. The following major mineralogical and petrographic differences between Type 3 and Type 4 b'chlorite-white mica schists" and the typical "chlorite schists" of the Type 2 Alteration Facies cm be recognized (see also Figure 3.5, Table 3.1, Figure 3.10, and Appendix A): A) Type 3 and Type 4 schists are richer in carbonate minerals than Type 2 schists but contain no calcite. The only carbonate mineral observed both in Type 3 (30% in volume) and Type 4 (33%) schists is an ankente (Cao: 27.0 wt.% ; FeO: 15.0 W.%;

MgO: 12.0 W.%; MnO: 0.5 W.%; and CO2: - 45 W.%). B) Chlorite is less abundant in Type 3 (18% in volume) and Type 4 (12%) rocks than in Type 2 rocks, and chlorites from Type 3 and Type 4 rocks are ncher in iron than the chlontes from Type 2 rocks. The average MgO/(MgO + FeO) ratio of chlorites from Type 3 and Type 4 schists is 0.28. C) The white mica present in Type 3 (15% in volume) and Type 4 (22%) schists is a muscovite-paragonite displaying an average Na,O/(Na,O+ &O) ratio of 0.36. D) Untwimed albite represents approximately 9% and 4% of the volume of the Type 3 and Type 4 lithotypes respectively, compared to 13% average for Type 2 rocks. E) Tourmaline is a widespread accessory mineral (although always «1% in volume) found together with unidentified opaque phases and pyrite (a total of 2% in volume) in Type 3 and Type 4 schists, whereas it is not cornmon in Type 2 rocks.

Type 5 Alteration Facies Type 5 Alteration Facies includes the most highly altered lithotypes of the Footwall Volcanic Unit. They are very fine-grained (largest crystals usually smaller than 80- 100 microns), moderately foliated, grayish-beige (strongly bleached) schists containing carbonate minerals (up to 40% in volume), quartz (30%), and white mica (24%), with little or no chlorite (Figure 3.4 and Figure 3 -9). 53 (caption)

Figure 3.8 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 3 and Type 4 Alteration Facies (diamond drill hole FPL-27). Ankente (31% in volume), quartz (27%), white mica (l8%), chlorite (15%), and albite (7%) are the essential minerais in rocks of these two alteration facies. Lithotypes richer in white mica than in chlorite have been grouped in the Type 3 Alteration facies. Convenely, lithotypes richer in chlorite than in white mica have been grouped in the Type 4 Alteration Facies. The plane of the thin section is perpendicular to the plunge direction of the Transamazonian regional stretching lineation.

Figure 3.9 - Photomicrograph (cross-polarized light) showing the mineralogy and some textural aspects of a typical schist of the Type 5 Alteration Facies (diamond drill hole FPL-27). Ankente (33% in volume), quartz (30%), white mica (24%), and siderite (5%) are the essential minerals of Type 5 schists. Small concentrations of chlorite (3%) and albite (3%) are also present. The plane of the thin section is perpendicular to the plunge direction of the Transamazonian regional stretching Iineation.

Ca0 calcite

more altered rocks (proximal) Mg0 ankerites + Fe0 + Mn0 siderite

+ Calcite: 'Type 1 ' and 'Type 2' atteratlon facies (20 microprobe anufyses) + Ankerite: 'Type 3".'Type 4'. and 'Type 5' atteration facies (3û microprobe anaiyses) + Siderite: 'Type 5' alterotion facies - rocks usually associuted with the BIF-hosted gold mineralizution ( 10 microprobe analyses)

:ootwall Volcanic Unit - Lamego prospect: composition of the carbonate ninerals from the different alteration facies (Ca0 + Mg0 + Fe0 = 100%) Microprobe analyses by Marcio Sales at Queen's Universtty (May. 1997). Figure 3.10 The following major mineralogical and petrographic differences between Type 5 schists and the typical schists of the Type 3 and Type 4 alteration facies were observed (see Figure 3.5, Table 3.1, Figure 3.10, and Appendix A): A) Type 5 schists have similar amounts (33% in volume) of ankeritic carbonate to those observed in Type 3 and Type 4 schists. However, in addition to the ankerite, srnaII amounts (- 5% in volume) of a magnesian sideritic carbonate (FeO: 38.0 W.%; MgO: 17.0 W.%; MnO: 0.7 wt.%; Cao: 0.3 wt.%; and CO2:- 44 wt.%) occur in the Type 5 Alteration Facies. B) The white mica from schists of the Type 5 alteration facies (average of 24% in volume) is a muscovite-paragonite containing more Na,O (average Na20/(7Va20+ K,O) ratio of 0.55)than the white micas fiom Type 3 schists. C) Chlonte (average MgO/(MgO + FeO) ratio of 0.32), and albite are accessory minerals in lithotypes of the Type 5 Alteration Facies (average of 3% in volume each). D) Tourmaline crystals are more cornmon arnong the usual accessory minerai phases in Type 5 schists (unidentified opaque phases and pyrite totalizing 2% in volume).

66Stockwork-like"sericite-chlorite 66microveinlets"and thin ankerite-(quartz) veinlets/veins in Type 3, Type 4, and Type 5 rocks

Some features resembling primary (volcanogenic) reiict structures were found during this study in highly altered and metasomatized rocks of the Footwall Volcanic Unit (Type 3 to Type 5) that commonly underlie ore-grade zones of the Lamego Banded Iron-Formation. Sarnples of these lithotypes commonly show, if observed along sections perpendicular to the trend direction of the regional stretching lineation (i.e., in a "down- plunge view"), a notable pattern of randomly oriented phyllosilicate-rich (essentially chlorite + white mica) "'microveinlets" and thin ankerite-quartz veinlets which overpnnt a previously carbonatized rock (Figure 3.11 and Figure 3.4D). These features, which have never been formally described in the literature of BIF-hosted gold deposits of the Figure 3.1 1 - Drill cores of schists of the Type 4 Alteration Facies. Note randomly oriented, thin phyllosilicate-rich (chlorite + sericite-paragonite) veinlets overprinted on bleached (carbonatization + sericitization) rock. 'These relict "stockwork-like" structures were observed in samples of Type 3, Type 4 and Type 5 schists, usually on planar sections perpendicular to the orientation of the regional stretching lineation (i.e., in a "down-plunge view"). Quadriiatero Ferrifero district, are interpreted in this thesis as flattened and partially transposed Archean "stockwork-iike" structures formed during the volcanogenic hydrothermal event that was also responsible for the BIF-hosted gold rnineralization of the Lamego deposit. Flattened and transposed "stockwork-like" sericite-chlorite "microveinlets" and carbonate-rich veinlets, and stockwork-controlled magmatic and/or epithemal-style hydrothermal alteration direct1y associated with Archean lode-go Id mineralizations have been described by Mason and Melnik (1986), Mason and Brisbin (1987), and Melnik- Proud (1 992) at the Timrnins-Kirkland Lake region (Abitibi greenstone belt), by Penczak (1996) and Penczak and Mason (1997) at the Campbell mine (Red Lake greenstone belt), and by Johnston et al. (1995) and Johnston (1996) at the Hem10 deposit (Schreiber- Hem10 greenstone belt). The observed and interpreted connections between these relict "stockwork-like" stnictwes/textures, the pattern and distribution of the hydrothermal alteration zones, and the BE-hosted gold mineralization in the Lmego deposit will be clarified in the following sections of this dissertation.

3.3.lb- Lithogeochemistry of the Alteration Facies

Whole-rock chemical analyses of 2 unweathered and representative rock samples fkom each of the 5 alteration facies of the Footwall Volcanic Unit were completed in the laboratones of Mineraçào Mono Velho in Brazil. The average concentrations obtained for major, minor, and trace elements analyzed in sarnples of each of the specific alteration facies, as well as the anaiytical methods utilized (and their levels of accuracy), are presented in Table 3.2 and Table 3.3. The concentrations of the major elernents in the 5 progressive aiteration facies are presented in Table 3.2 and Figure 3.12. It has been assurned that the average chemical composition of the unaltered protolithic rock of the Footwall Volcanic Unit was similar to 58 (caption)

Table 3.2 - Whole-rock chernical analyses (major and rninor elements) of representative, unweathered rock samples from the five different alteration facies of the Footwall Volcanic Unit (Lamego prospect - diarnond drill holes FPL-22 and FPL-27), and average whole-rock chemical compositions (major elements) of unaltered TH1 and TH2 Archean thoieiites worldwide (Condie, 1981) and fiom the Midlands greenstone belt in Zimbabwe (Condie and Harrison, 1976).

The samples were analyzed in the chemical laboratories of Mineraça0 Morro Velho (Minas Gerais - Brazil). The analytical methods utilized and their corresponding ranges of accuracy are the following: SiO, (f 10%) - gravimetric analysis TiO, (t 5%), AI,O, (t 3%), Fe20, (f IO%), Mn0 (t 3%), Mg0 (k 2%),and Ca0 (+ 5%) - inductively coupled plasma/atomic emission spectrometry Fe0 (f 10%) and P205(+ 8%) - titcation Na20(f 5%) and K20(2 10%) - atomic absorption spectrometry

59 (caption)

Table 3.3 - Average concentrations of some trace elements in representative, unweathered rock samples from the five different alteration facies of the Footwall Volcanic Unit (Lamego prospect - diamond drill holes FPL-22and FPL-27).

The rock samples were analyzed in the chemical laboratones of MineraçZio Morro Velho (Minas Gerais - Brazil). The analytical methods utilized and their correspondhg ranges of accuracy are the following: B (f 4%), Cu (+ 4%), Pb (f 1O%), Zn (+ 7%), Ni (f 6%), Co (f 5%), Cr (k 3%), Ba (4 5%), Sr (+, 3%), Nb (+ I%), Zr (+ 2%), Y (+ 1%), Hf (f 7%), La (+ IO%), Ce (f. 2%), Nd (f 5%), Sm (f 6%), Eu (k 8%), Gd (f 3%), Tb (f 4%), Dy (t 5%), Er (f 4%), and Yb (f 5%): inductively coupled plasma/atornic emission spectrornetry S (+ 10%): induction/leco CS-244 Au (f 10%): fire-assay As (+ 10%): gutzeit Sb (f 2%): atomic absorption spectrornetry ------Lameao ~ros~ect:"Footwall Volcanic Unittt - 1 alteration fa& - lithogeochemirtryltrace elernents (p.p.m.) 1 "type 1" ( "type 2" 1 "type 3" 1 "type 4" 1 "type 5"

Chernical analyses by MMV (1996197). Table 3.3 Marcio Sales (Queen'sUniversity - May/June, 1997). the average composition of the typical TH2 Archean tholeiite of Condie (1981), based mainly on interpretation of "immobile" trace element data (see Section 3.3. lc for a detailed explmation). As a result of the qualitative approach of this lithogeochemical study, the very srnaIl differences in density that could be observed among rocks of the different alteration facies have been neglected, and mass balance calculations were not made. The most important "chemical trends" observed along the 5 progressive alteration facies, and some the more obvious consequences in terrns of the mineral assemblages and indicator minerals observed in each of these facies, are the following:

Loss on Ignition: "Loss on Ignition" in the lithogeochemical analyses included in this thesis corresponds to the amount of volatile components (wt.%)that can be released from the rock samples with their calcination at 1,000 OC. The average Loss on Ignition values presented in Table 3.2 and Figure 3.12 (which consist almost exclusively of CO2 + ~~0') progressively increase from 5.55 % in the least altered rocks of the Footsvall Volcanic Unit (Type 1 schists) to 16.17 % in the most altered rocks (Type 5 schists). The most important chemical component added to the system during the alteration processes was CO2. One of the most obvious consequences of this addition is the increase in carbonate content with increasing alteration (Figure 3.5). The introduction of relatively small amounts of CO2(the case of the Type 1 and Type 2 alteration facies) corresponds to the presence of calcitic carbonate (Figure 3.10). The introduction of massive arnounts of CO, (the case of Type 3, Type 4, and Type 5 alteration facies) corresponds to the presençc of ankente and, in the Type 5 Alteration Facies, to the presence of - rich siderite in addition to ankerite (Figure 3.5 and Figure 3.10). There is a corresponding decrease in the chlonte content observed in schists of the Type 3, Type 4, and Type 5 alteration facies. The Fe and Mg atoms of the rocks of the Footwall Volcanic Unit tend to be hosted in silicate minerals (actinolite, clinozoisite-epidote, and chlorite) in the less IDEAL UNALTERED I INCREASING INTENSITY OF TH2ARCHEAN 1 HYDROTHERMAL ALTERATION THOLEIITE I Distal 1Proximal I

'THZ* Archean 1 'bp9 1 ' 'bpe 2' 'Vpe 3' 'Vpe 4' 'tvw 5' tholelltea Alteraiion Faclea Alterdon Facies Alteration Faclor Alterdon Facies Alteratlon Facies (Condle, 1981) 1

Footwall Volcanic Unit - Lamego prospect: chemical composition (major elements) of schists from the ditferent alteration facies (see also Table 3.2) Chemlcal analyses by MMV - Mdrclo Sales (Queen's Unlverslty - 1997). Figure 3.12 altered rocks (Type 1 and Type 2), and in carbonate minerals (ankente and magnesian- siderite) in more altered rocks (Type 4 and Type 5). The amount of H,O+ present in schists fkom each of the alteration facies has been estimated based on the modal composition of the H20-bearing mineral phases present (chlorite, actinolite, clinozoisite-epidote, and sericite-paragonite) and their respective average H~O'contents according to chemical analyses presented in Deer et al. (1992).

The most "hydrated" lithotypes of the Footwall Volcanic Unit are the Type 2 schists (-

4.6 wt.% of ~~0')and Type 1 schists (- 3.0 W.% of ~~0').The contents of ~~0'in schists of the Type 3 and Type 4 alteration facies have been estimated as approximately 2.6 wt.% and 2.2 W.%, respectively. Schists of the Type 5 Alteration Facies (the richest rocks in the component COz) are the least hydrated rocks (- 1.4 wt.% of H,o').

SiO2: A relatively small but steady depletion in the arnounts of SiO, can be observed fiom the least to the most altered lithotypes of the Footwall Volcanic Unit (Table 3.2 and Figure 3.12). The average arnount of SiO2 in unaltered TH2 tholeiites is approximately

49.50 W.% (Condie, 1981). The average amounts of SiO, decrease from approximately 47.41 wt.% in the lest altered lithotypes (Type 1 schists) to approximately 44.48 wt.% in the most altered lithotypes (Type 5 schists). The small, but persistent, increase of modal quartz that has been observed with the increase of the alteration intensity (Figure 3.4 and Table 3.1) is probably related to the progressive depletion in A1203also observed with the increase of the alteration intensity. Part of the SiO, that would stay in the structure of Al- bearing silicates such as albite, epidote-clinozoisite, and chlorite in the least altered lithotypes, probably crystallized as quartz in the more altered lithotypes, as the more altered rocks of the Footwall Volcanic Unit are progressively depleted in Al,O, and Al- bearing silicates, and richer in CO, and carbonate minerals. A1203: There is also a decrease in the amounts of Al,O, from the least to the most altered lithotypes of the Footwall Volcanic Unit (Table 3.2 and Figure 3.12). The average arnount of Al,03 in unaltered TH2 tholeiites is approximately 15.20 W.% (Condie, 198 1). The average arnounts of Ai,O, decrease from approximately 14.55 wt.% in the least altered lithotypes (Type 1 schists) to approximately 10.07 W.% in the most altered lithotypes (Type 5 schists). As a consequence, the total amount of Al-bearing silicate phases successively decreases fiom Type 1 schists to Type 5 schists.

Total Fe (Fe,03 + FeO): Depletion of Total Fe only occurs in the most altered rocks of the Footwall Volcanic Unit (Table 3.2 and Figure 3.12). The average amount of Total Fe in unaltered TH2 tholeiites is approximately 1 1.97 M.% (Condie, 1981), very similar to the average amounts that have been observed in Type 1 (12.08 W.%), Type 2 (12.46 wt.%), and Type 3 (1 1.57 W.%) schists. The "iron-depleted" Type 4 and Type 5 schists contain arnounts of Total Fe averaging 10.21 W.% and 9.40 wt.%, respectively.

MgO:

A small and gradua1 depletion in the arnounts of Mg0 occurs from the Ieast to the most altered lithotypes of the Footwall Volcanic Unit (Table 3.2 and Figure 3.12). The average amounts of Mg0 decrease fkom approximately 4.95 M.% in the least altered lithotypes (Type 1 schists) to approximately 4.23 W.% in the most altered lithotypes (Type 5 schists). However, the average amount of Mg0 in a typical TH2 tholeiite is 6.82

W.% (Condie, 1991), which is significantly higher than the average Mg0 content for the least aItered rocks of the Footwall Volcanic Unit.

Na20: The Na,O content of schists from the different alteration facies of the Footwall

Volcanic Unit varies between 1.43 and 1.81 W.% (Table 3.2 and Figure 3.12). These concentrations are considerably lower than the average arnount of Na,O in unaltered TH2 tholeiites (which is approximately 2.70 wt.% according to Condie, 1981). in the least altered rocks, the Na is contained ahnost exclusively in albite (Type 1 and Type 2 alteration facies), whereas Na in the more altered lithotypes of the Footwall Volcanic Unit occurs in paragonite or Na-rich white mica, with minor albite.

K20: Potassium has been added during the alteration processes, as shown by the increasing amounts of K20 in more altered schists. The average amounts of K20 vas, fiom 0.90 wt.% in Type 1 schists, to 1.64 wt.% in Type 5 schists (Table 3.2 and Figure 3.12). The average arnount of K20 in unaltered TH2 tholeiites is approximately 0.69 wt.% (Condie, 198 1 ). The increase in K,O in altered rocks of the Footwall Volcanic Unit is easily detected by the presence of modal white mica.

Cao:

Calcium has also been added to the altered rocks of the Footwall Volcanic Unit, in which Ca0 varies fiom 9.79 wt.% to 12.18 wt.%, compared to the average arnount of

8.79 W.% Ca0 in unaltered TH2 tholeiites (Condie, 198 1). However, there is no clear trend of addition or depletion in the distribution of Ca0 among schists of the five different alteration facies. Ca occurs in clinozoisite-epidote, calcite, and actinolite in Type 1 schists. In the more altered schists (Type 2 to Type 5 alteration facies) Ca occurs almost entirely in carbonate minerals.

To surnrnarize, in altered rocks of the FootwalI Volcanic Unit at the Larnego deposit, there is a tendency for a progressive depletion of Sioz, Total Fe, MgO, and Na,O as well as progressive increases in CO2, K20, and Ca0 with respect to average unaltered TH2 Archean tholeiites. Trace element Concentrations:

Average trace element concentrations of rocks from a11 5 alteration facies are presented in Table 3.3 and Figure 3.13. Some of the most important observations and considerations about these results are the following:

A) A consistent and gradua1 depletion of the concentrations of some 'base metal" elements such as Cu, Zn, Ni, and Co has been observed kom the least altered Type 1 schists to the most altered Type 5 schists and also with respect to average unaltered TH2 tholeiites. The element Pb was an exception, with no substantial variation of concentration among the different alteration facies. B) Even relatively "immobile" eiements such as Zr, Nb, Y, Ti and the rare-earth elements show some mobility under the more extreme conditions typical of the Type 3, Type 4, and Type 5 alteration facies, with depletion to some extent From Type 3 rocks to Type 5 rocks. By contrast, the concentrations of Nb, Zr, Ti, and REE in rocks of the Type 1 and Type 2 alteration facies show little variation, suggesting that these elements were relatively immobile under less intense alteration regimes. Furthemore, Y appears to have been mobile even in the less intense alteration regime typical of Type 2 rocks.

3.3.1~- Characteristics of the Protolithic Volcanic Rocks

The charactenstics of unaltered protolithic rocks of the Footwall Volcanic Unit such as magma series, chemical composition, and specific type of Archean volcanism have been tentatively inferred in this thesis based on the concentration and distribution of some selected minor and trace elements (Ti, Zr, Nb, Y, and the rare-earth elements) in the least altered Type 1 rocks. As these elements, among others, are considered relatively immobile during post-consolidation alteration processes such as spilitization, subrnhne hydrothermal alteration and greenschist facies rnetamorphism (Pearce and Cam, 1973; Pearce, 1975; Condie, 1976; Winchester and Floyd, 1977; Condie, 1981; and Pearce, 1996), they could presumably be reliable indicators of ancient volcanic suites and their IDEAL UNALTERED 1 INCREASING INTENSITY OF THZARCHEAN , HYDROTHERMAL ALTERATION ,-* THOLEIITE I Distal Proxlmal t P. P.^.) 1 --

'TH2' Archean 'vp1' 'type 2' 'bv3' 'ofp4' 'type 5' IhdeIItas 1 Alteraflon Facies Alteration Facles Alteration Facios Altardon faclos Alîoration Faclos (Condle, 1981) I

Footwall Volcanic Unit - Lamego prospect: concentrations of some "basemetal" trace elements I Chemlcal analyses by MMV - Mdrclo Sales (Queen's University - 1997). Figure 3.13 differentiation products in altered metavolcanic rock sequences (Floyd and Winchester, 1978).

According to Condie (1976, 198 1), trace element distributions in tholeiitic volcanic rocks of Archean greenstone belts indicate the existence of 2 major subgroups, the TH1 tholeiites and the TH2 thoieiites. TH1 are the most common tholeiitic rocks in most greenstone belts. They are characterized by flat chondrite-normalized REE distributions as well as an overall similarity in chemistry with modem rise (MORB) and arc tholeiitic basalts. TH2 tholeiites become progressively more abundant at higher stratigraphic levels of greenstone belt sequences. They are typically charactenzed by c hondri te-normalized REE patterns "somewhat enriched" in light REE (Figure 3.14) and an overall similarity in chemistry with modem calc-alkaline and oceanic island tholeiites (Condie, 1981). The average chemical composition (major and minor elements) of TH1 and TH2 Archean tholeiites worldwide is presented in Table 3.2 (Condie, 1981). The average chemical composition of typical TH1 and TH2 tholeiites from the Midlands greenstone belt in Zimbabwe (Condie and Harrison, 1976) is also presented in the same table in order to better illustrate some of the chemical aspects of these two major types of Archean tholeiitic volcanic rocks. The chondrite-normalized REE distributions (norrnalization method: Wakita et al., 1971) obtained ftom altered basic volcanic rocks From the Type 1 Alteration Facies (the lest altered lithotypes of the Footwall Volcanic Unit) show that the unaltered protolithic volcanic rocks of the Footwall Volcanic Unit fa11 within the envelope of TH2 tholeiites (Figure 3.14). As expected for TH2 tholeiites, rocks of the Type 1 Alteration Facies also plot within the "calc-alkaline" field for modem basalts in the TVZr diagram of Pearce and Cann (1973) - (Figure 3.15). The rock-type of the hypothetical protolithic TH2 tholeiites of the Footwall Volcanic Unit has been inferred to be, based on the Zr/Ti02-Ce diagram (Winchester and Floyd, 1977), transitional between a basalt and an andesite (Figure 3.16). The results obtained with the Zr/Ti02 -Nb/Y diagram (Winchester and Floyd, 1977) have been considered somewhat ambiguous, as Y behaved as a fairly

/ MORB /

CALC - ALKALINE +* + BASALE

+ *+

Maliyomi Formation, Midlands belt (Zimbabwe) TH2 greenstone belt tholeiite (Condie and Harrison, 198 1 )

Altered basic volcanic rocks ('Type1 ' and 'Type 2' Alterotion Facies) Lamego prospect

Aitered basic volcanic rocks ('Type 1 ' Alteration Facies) Cuiab6 mine (internul reports - MMV)

------Ti-Zr plot (Pearce and Cann, 1973) of avera e "Type 1 " and 'Type 2' schirts compositions - Footwall Volcanic Una , Lamego and Cuiaba deposits. M6rcio Sales (Queen's University - April, 1997). Figure 3.15 + Samples from the Lamego prapect

sutialkoline ixtsatt l

cornendite pantellerite -

phonoiite hchyt8 + Sarnples from the Larnego prospect

Zr/TiO,-Nb/Y and Zr/TiO,-Ce plots (Winchester and Floyd, 1977) of average 'Type 1 ' schist compositions - Footwall Volcanic Unit, Lamego prospect. Figure 3.16 mobile element during the hydrothermal alteration processes, becoming progressively depleted in each of the successive 5 alteration facies of the Footwall Volcanic Unit (see Y concentrations in the different alteration facies - Table 3.3).

3.3.2 - Lamego Banded Iron-Formation (BIF)

3.3.2a - Introduction: "Barren" versus uMineralized" BIF

The altered basic (or basichtemediate) volcanic rocks of the Footwall Volcanic Unit are overlain by the Lamego Banded Iron-Formation (Figure 3.2 and Figure 3.3). This unit is composed of carbonate and sulphide-facies banded iron-formations (BIFs), and usually ranges in thickness kom 1 to 8 meters (locally up to 15 meter thick). The unit is not developed everywhere in the Lamego eye-shaped structure. Locally, the rock sequences of the Footwall Volcanic Unit occur in direct contact with carbonaceous phyllites and -rich schists of the "Black Carbonaceous Phyllite Unit" or with cherts of the "Pele de Onça Unit" (Figure 3.2 and Figure 3.3). The boundaries between the Lamego Banded Iron-Formation and the schists of the Footwall Volcanic Unit are always sharp and have been easily mapped both at surface and underground. Carbonate-facies BIF is the most cornmon lithologic type of the Larnego Banded Iron-Formation. It is generally characterized by lack of sulphides and gold mineralization. Sulphide-facies BIFs occur only in restricted portions of the Lamego structure and usually contain anomalous gold. In detail, there is a complete gradation between "barren" carbonate-facies BFs and "mineralized" sulphide-facies BIFs in the Larnego Banded Iron-Formation. The present section describes only the volumetrically much more important barren carbonate-facies BE. The sulphide-facies BIF is described separately in Chapter 4. 3.3.2b - Petrography and Lithogeochemistry

The carbonate-facies BIF is composed of altemating light and dark bands varying in thickness fiom a few millimeters to several decimeters (figures 3.1 7 to 3.20). ïhese bands are interpreted as the original sedimentary or volcanogenic-sedimentary beds. The light-coloured, usually white, bands are composed essentially of very fine-grained granoblastic-polygonal quartz (the larger crystals commonly are 35 to 40 microns in diameter) associated with subordinate amounts of equally fine-grained ankerite, siderite, pyrite, and chlorite (figures 3.21 to 3.23). The dark bands can be brown or dark brown in colour, and are cornposed essentially of equally fine-grained (less than 50 microns), granoblastic-polygonal aggregates of "Mg-poor ankerite", siderite, and quartz associated with variable arnounts of an "amorphous carbonaceous material" that resembles graphite (figures 3.21, 3.22, and 3.24). Occasional crystals of chlorite, sericite, albite, and apatite

(?) were also observed in these carbon and carbonate-rich bands. The chemical compositions of the 2 types of carbonate minerals observed in the dark bands of typical barren carbonate-facies BE were analyzed with the use of an electron probe microanalyzer, and the results are presented in Figure 3.25 and Appendix A. The more abundant carbonate mineral is a Mg-poor ankente showing the following average chemical composition (wt.%): Cao- 25.5%; FeO- 24.5%; MgO- 4.0%; and MnO- 1.5%. The other carbonate minera1 phase present is a sidente presenting the following composition (wt.%):FeO- 52.5%; MgO- 3.0%; MnO- 2.0%; and Cao- 0.5%. Whole-rock chemical analyses of 3 representative unweathered samples of carbonate-facies banded iron-formation were completed in the laboratones of Mineraçào Morro Velho in Brazil, and the results are tabulated in Table 3.4. The sarnples are nch in

SiO, (70-80 W.%), contain 6- 13 wt.% Total Fe and 3.5-7.0 wt.% Cao, and are deficient in Mg0 (average of 0.23 W.%), &O3 (average of 0.07 W.%), Na,O (< 0.01 W.%), and

K,O (< 0.01 W.%). The total Fe contents are well below the 15% lower limit suggested by James (1954) in defining a banded iron-formation, but the Lamego BIF is nevertheless 73 (caption)

Figure 3.1 7

Figure 3.17 and Figure 3.18 - Typical carbonate-facies banded iron-formation of the Lamego Banded Iron-Formation.

Figure 3.18

74 (caption)

Figure 3.19

Figure 3.19 and Figure 3.20 - Thinly laminatecl carbonate-facies banded iron-formation of the Lamego Banded iron-Formation.

Figure 3.20

75 (caption)

Figure 3.21

Figure 3.21 (plane-polarized light) and Figure 3.22 (cross-polarized light) - Photomicrographs showing some mineralogical and textural aspects of a typically thinly larninated carbonate-facies BIF (Lamego Banded Iron-Formation). The light-coloured, usually white bands shown in figures 3.17 to 3.20 are essentially composed of very fine- grained granoblastic-polygonal quartz (the larger crystals are commonly 35 to 40 microns in diameter) associated with vev subordinate amounts of equally very fine-grained ankente, siderite, pyrite, and chlorite. The dark bands are essentially composed of equally very fine-gained (less than 50 microns) granoblastic-polygonal aggregates of "Mg-poor ankente", siderite, and quartz associated with variable amounts of an "amorphous carbonaceous material" that resembles graphite.

Figure 3.22

76 (caption)

Figure 3.23 - Photomicrograph (cross-polarized light) showing some mineralogical and textural aspects of a typical light-coloured or white band of a carbonate-facies BE &om the Larnego Banded Iron-Formation. White bands are composed essentially of fine- grained, granoblastic-polygonal quartz (larger crystds 35 to 40 microns in diameter) associated with very subordinate amounts of equally fine-grained ankente, siderite, pyrite, and chlorite.

Figure 3.24 - Photornicrograph (cross-polarized light) showing some rnineralogical and textural aspects of a typical dark brown band of a carbonate-facies BIF from the Larnego Banded Iron-Formation. Dark brown bands are composed essentially of fine-grained (less than 50 microns), granoblastic-polygonal aggregates of "Mg-poor ankente", siderite, and quartz associated with variable amounts of an "amorphous carbonaceous matenal" that resemb les graphite.

Ca0 calcite

Mg0 Fe0 + Mn0 magnesite siderite

+ Carbonate minerals from 'bartenucarbonate-facies BIF (4û microprobe analyses) + Carbonate mineral from 'mineraliredusulphide-facies BIF - 'Queimado' mineralized body (20 microprobe analyses) + Carbonate mineral from 'mineralized' sulphide-facies BIF - 'Arco da Velha' and 'Cabeça de Pedra' mineralized bodies (20 microprobe analyses)

- Composition of carbonate minerals: 'barren" and 'mineralized" BIF :Ca0 + Mg0 + Fe0 = 10û?Ie) Microprobe analyses by Marcio Sales ut Queen's University (May. 1997). Figure 3.25 78 (caption)

Table 3.4 - Who le-rock chemical analyses of representative, unweathered rock samp les of carbonate-facies BIF from the Lamego Banded Iron-Formation.

The rock sarnples were analyzed in the chemical laboratories of Mineraça0 Morro Velho (Minas Gerais - Brazil). The analytical rnethods utilized and their corresponding ranges of accuracy are as follows: SiO, (f 10%) - gravirnetrk anatysis TiO, (t 5%), AI,O, (+ 3%), Fe203(f IO%), Mo0 (f 3%), Mg0 (f Z%),and Ca0 (k 5%) - inductively coupled plasmdatomic emission spectrometry Fe0 (k 10%) and P,O, (+ 8%) - titration Na20 (f 5%) and K20(f 10%) - atomic absorption spectrometry "Barren" carbonate facies BIF - lithogeochemistry (major elements)

Si& Ti4

A1203 Total Fe Mn0 Ma0 Ca0 NazO K20 pz06 L.O.I.* Total

Chernical analyses by MMV (96197). Table 3.4 * - loss on ignition Marcio Sales (Queen's University - August, i997). clearly part of a spechum of chernical sedimentary rocks which are comrnonly mapped as "banded iron-formations".

3.3.3 - "Pele de Onça" Chert Unit

The term "Pele de Onça" (in English, "Black-Jaguar Skin") chert has been utilized by exploration geologists from Mineraça0 Morro Velho, since the beginning of the 1980s, as a field name for the siliceous, massive (sometimes slightly bedded in an outcrop scale), dark-coloured rocks which occur in a fairly continuous stratigraphic horizon overlying the Lamego Banded Iron-Formation (Figure 3.26 and Figure 3.27). This stratigraphic unit ranges in thickness from many decimeters to 4 or 5 meters (rarely up to 10 meters) and is referred to in this thesis as the "Pele de Onça Chert Unit" (Figure 3.3). Similar to the lower Larnego Banded Iron-Formation, the Pele de Onça Chert Unit is not developed everywhere in Lamego structure and is often represented by a thin layer of less than a rneter in thickness. The Peie de Onça chert is composed essentially of quartz (usually more than 85% in volume) with subordinate arnounts of carbonate and with py-rite as an important accessory mineral. Quartz occurs as large crystals (the largest ones are usually 2 millimeters in length) showing deformation larnellae, undulatory extinction, and "serrated boundaries that have been progressively recrystallized as very fine-grained, granoblastic-polygonal quartz aggregates (crystals smaller than 40 microns) and also as completely recrystallized domains containing only very fine-grained, granoblastic- polygonal grains (Figure 3.28 and Figure 3.29). Although the Pele de Onça chert has been described as essentially a "massive siliceous rock", poorly defined bedding surfaces andlor a relict layering (sedimentary or volcanogenic deposition) have occasionally been observed in outcrops and underground exposures of the Pele de Onça Chert Unit (Figure 3.26). These surfaces and layers are always concordant with the bedding (S,) shown by the underlying BIFs of the Lamego Banded Iron-Formation and with layers and lenses (fkom several decimeters to 1 or 2 80 (caption)

Figure 3.26

Figure 3.26 and Figure 3.27 - Underground exposures of the "Pele de Onça" chert in the SW hinge zone of the Lamego eye-shaped structure. Traces of the poorly defined bedding surfaces observed in rock sequences of the Pele de ûnça Chert Unit can be seen in Figure 3.26 (parallel to the length of the pocketknife).

Figure 3.27

81 (caption)

Figure 3.28

Figure 3.28 and Figure 3.29 - Photomicrographs (cross-polarized light) showing some rnineralogical and textural aspects of typical "Pele de Onça" cherts fiom the SW hinge zone of the Lamego eye-shaped structure. These rocks are composed essentially of quartz (usually more than 85% in volume), with very subordinate amounts of carbonate. Pyrite may be an important accessory mineral. Quartz occurs as large crystals (the largest ones are usually 2 millimeters in length) showing deformation lamellae, undulatory extinction and "serrated" boundaries, and as very fine-grained (usually 10 to 40 microns in size), granoblastic-polygonal aggregates.

Figure 3.29 meters in thickness) of carbonaceous phyllites and graphite-rich schists that are comrnonly observed as intercalations in the upper stratigraphic portions of the Pele de Onça Chert Unit. The boundaries between the Pele de Onça Chert Unit and the underlying BiFs of the Lamego Banded Iron-Formation were previously considered as essentially sharp and tabular (Figure 3.30). However, the present investigation has shown that these boundaries are much more complex, and that they are also related to hydrothermal mineralizing processes. Considering the connections observed between the siliceous rocks of the Pele de Onça Chert Unit and the E3IF-hosted gold mineralization during this study, some other aspects related to this unit (including the nature of the boundaries with the Lamego Banded Iron-Formation) will be discussed in Chapter 4.

3.3.4 - Black Carbonaceous Phyllite Unit

The Pele de Onça Chert Unit is overlain by the Black Carbonaceous Phyllite Unit (Figure 3.2 and Figure 3.3). The Black Carbonaceous Phyllite Unit is continuous throughout the Lamego structure, ranges in thickness From 2 to 18 meters (average of 10 to 12 meters), and is composed of highly sheared, fine-grained (crystals smaller than 20 microns) black phyllites and graphite-rich schists (Figure 3.3 1-A). The contacts between the Black Carbonaceous Phyllite Unit and the Pele de Onça Chert Unit are sharp and were easily defined in outcrops and underground exposures. Layers and lenses of black carbonaceous phyllites and graphite-rich schists (fkorn several decimeters to 1 or 2 meters in thickness) cornmonly occur as intercalations in the upper stratigraphic portions of the Pele de Onça Chert Unit. The black phyllites anaor graphite schists are composed essentially of "dusty" particles of carbonaceous matenal (graphite?) and crystalline aggregates of sericite, carbonate, quartz, chlorite, and plagioclase. The most common accessory rninerals observed in thin sections are pyrite, rutile, and tourmaline. Schists/phyllites of this unit typically contain thin 1ensesAayers (fiom less than a millimeter to several millimeters in Figure 3.30 - Underground exposure in the SW hinge zone showing the more usual semi-conformable type of contact that is observed between the lower Lamego Banded Iron-Formation and the upper Pele de Onça Chert Unit (contact marked with red paint). Figure 3.3 1

A) Drill core samples of typical carbonaceous phyllites/graphite-rich schists fiom the Black Carbonaceous Phyllite Unit.

B) Drill core samples of typical schists fiom the Upper EpiclasticNolcaniclastic Unit. thickness) composed essentially of fine-grained carbonate and quartz that are concordant with the S, bedding surfaces (Figure 3.3 1-A).

3.3.5 - Upper EpiclasticNolcaniclastic Unit

The Black Carbonaceous Phyllite Unit is overlain by the Upper EpiclasticNolcaniclastic Unit, a succession of epiclastic and volcaniclastic sedimentary rocks which has a minimum thickness of about several hundred meters (Figure 3.2 and Figure 3.3). The rocks of this unit occur in altemating light and dark-coloured beds that range in thickness fkom a few millimeters to several decimeters (Figure 3.3 1-B). The dark beds are very fine-grained (crystals smaller than 20 microns), and consist mainly of sericite, chlonte, quartz, and "dusty" carbonaceous material (graphite?) with subordinate plagioclase and carbonate. The light beds, which are fine-grained (crystals smaller than 50-100 microns), are composed essentially of quartz, carbonate, plagioclase, sencite, and chlonte and are devoid of carbonaceous material. The contacts of the rock sequences of the Upper EpiclasticNolcaniclastic Unit with the underlying carbonaceous phyllites and graphite-rich schists of the Black Carbonaceous Phyllite Unit are gradational over several decimeters. nie rocks of these 2 units are interpreted as "mixed" sediments possessing contributions of both epiclastic and volcaniclastic/tuffaceous origins.

3.4 - Tectonic Structures

3.4.1 - Planar and Linear Fabrics, Folds, and Faults

The Archean planar fabnc associated with the Rio das Velhas orogeny (SI surfaces) is less obvious than the Transamazonian S, planar fabric in rocks of the Lamego structure. SI surfaces were observed and described in this research only in incompetent (usually hydrothermally altered) phyllosilicate-rich rocks, where they occur as a penetrative schistosity which is invariably concordant with the primary S, bedding (Figure 3.34) and overprinted by Transamazonian fabrics. Archean planar fabncs parallel to S, surfaces and folded by Transamazonian structures have been reported by Vieira (1992) in phyliosilicate-rich rocks from the Cuiaba mine and by Fontes (1 993) in pelitic rocks from the SabarKaeté region. The orientations of the S, foliation planes measured in rocks fiom many parts of the Lamego stnicture cm be seen in the equal-area stereonet presented in Figure 3.32. N54E/44"SE is an average for the attitude of the S2 planar elements. The strike direction and dip angle of the S, foliation planes in the Lamego structure Vary from N40E to N75E and fkom 35OSE to 60°SE, respectively. In the more incompetent, phyllosilicate-rich lithologies (schists and phyllites fkom the Footwall Volcanic Unit, Black Carbonaceous Phyllite Unit, and Upper EpiclasticNolcaniclastic Unit), the S2 foliation is represented by a well-developed schistosity (Figure 3.34). The S, foliation in more competent rocks such as banded iron-formations and cherts, on the other hand, is represented by a planar fabnc consisting of persistent, closely-spaced fkactures which may be filled with remobilized quartz (Figure 3.35 and 3.36). The S2 schistosity/cleavage planes make small angles with the S, bedding surfaces in the NW and SE limbs of the Larnego structure (Figure 3.2 and Figure 3.34), but they intersect each other at high angles in the SW and NE hinge zones of the structure (figures 3.2,3.35,3.36). The L2 linear fabric is represented by a small number of structural elements in rocks of the Lamego stnicture. Most of the L, lineations observed in outcrops and underground exposures are intersection lineations of the St foliation planes with the planes of bedding (So) andlor with the Archean SI cleavage planes (Figure 3.34). The plunge of the L2 lineation is also easily determined in weathered outcrops by measuring the orientation of the longest dimension of the "pencil structures" formed by the intersection of bedding planes with the S2 foliation (Figure 3.37). L, mineral Iineations may also be defined by the preferred orientation of individually elongated grains of quartz in thin sections (see Figure 3.38). The hinge lines of isoclinal to open, 87 (caption)

Figure 3.32 - Equal-area stereonet showing the distribution of the attitudes of the S, foliation planes (poles to S, planes) measured in outcrops and underground exposures of the Lamego eye-shaped structure.

Figure 3.33 - Equal-area stereonet showing the distribution of the plunges of the L2 linear fabric elements rneasured in outcrops and underground exposures of the Lamego eye-shaped structure. S2 FOLIATION- LAMEGO NE-SHAPED STRUCTURE N

CONTOUR LEGEND:

Pole Concentration(%O total per 1.O % of area): Min. Contour - 4.5 % Contour lnterval - 4.5 % Max. Conc. - 29.5 % - maximum - omhide: ~54~/U4*SE

- Equal-Areo Stereonet - - Fsher Distribution - - Lower Hemisphere - - 164 Data Entries - - 164 Poles Plotted - s Figure 3.32 Data Collecteci by Mdrcio Sales and uNIGEO

CONTOUR LEGEND:

Pole ConcentfationC%of total per 1 .O % of oreal: Min. Contour - 6.5 % Contour lnterval - 4.5 % Max. Conc. - 45.0 % W= - maximum -

- EquaCArea Stereonet - - Fisher Distribution - - Lower Hemisphere - - 202 Data Entries - t - 202 Poles Piotted - S Figure 3.33 ~otaCollected by Mdrcio %les and UNIGEO - Equal Area Stereocret - -Sc!!mnet - - Lower Hemaphere -

Figure 3.34 - Underground exposure in the SE limb of the Larnego eye-shaped smicture showing a Type 5 schist of the Footwall Volcanic Unit close to the contact with the Lamego Banded Iron-Formation. The photograph shows the Lz Transarnazonian linear fabric represented both as an intersection of the Archean SI schistosity with the Transarnazonian 5& schistosity and as a stretching lineation. The Si schistosity surfaces show in this exposure are parallel to the bedding surfaces (So) in BIFs of the Larnego Banded Iron-Formation. The S2 schistosity usually makes small angles with the SIand Sa foliation planes in the limbs of the Lamego eye-shaped structure. 89 (caption)

Figure 3.35

Figure 3.35 and Figure 3.36 - Underground exposures of carbonate-facies BIFs in the SW hinge zone of the Lamego eye-shaped structure. The S2 foliation is represented, in more competent rocks such as banded iron-formations and cherts, by a planar fabric consisting of persistent, closely-spaced fractures that are very often filled with remobilized quartz. The S2 planes invariably make a high angle with the S, and S, planes in the two hinge zones of the Larnego structure. Small displacements (apparent slips in the order of centimeterdseveral decimeters) were commonly observed along Sz surfaces in BFs.

Figure 3.36

90 (caption)

Figure 3.37 - "Pencil structures" formed by the intersection of the bedding fissility with the S, foliation planes in highly weathered carbonaceous schists from the Upper EpiclasticNolcaniclaçtic Unit (SW hinge zone - Lamego eye-shaped structure).

Figure 3.38 - Photomicrograph (plane-polarized light) of a carbonaceous schist fiom the Upper EpiclasticNolcaniclastic Unit (SW hinge zone - Lamego eye-shaped structure). Note that this section was cut parallel to the plunge direction of the L, linear fabric, which is defined by the preferred orientation of individually elongated grains of quartz.

Transarnazonian folds are also parallel to the L, linear fabric elements. These folds are characterked by S2 axial planar foliations (Figure 3.39 and Figure 3.40). An equal-area stereonet showing the distribution of the plunges of the L, linear fabric elements measured in outcrops and underground exposures of the Lamego prospect is show in Figure 3.33. S81E/32' (or 099/32O) is an average for the plunge of L, linear elements measured in the different geographicaVstmcniral domains of the Lamego structure. The Sz schistosity surfaces observed in phyllosilicate-rich rocks at the Lamego prospect, and also at the Cuiaba mine (Vieira, 1992), were weakly crenulated (microfolded) during a subsequent deformation event of unknown age. Locally, these crenulations have developed as typical crenulation cleavages. The average attitude of the axial surfaces and axes of these late crenulations or microfoldings (S, planar fabric and L3 linear fabnc) at the Lamego prospect is N05-20E/70a-80°SE and 170-185/1S0-30°, respectively. The Lamego Banded Iron-Formation and the Pele de Onça Chert Unit are commonly displaced by high-angle, oblique-slip to strike-slip faults that are invariably oriented along a WISE direction (Figure 3.2). The apparent relative movement along these faults is either dextral or sinisixal, and the observed displacements between the faulted blocks are always smaller than 80-90 meters (Figure 3.2).

3.4.2 - The Lamego and Cuiabh Eye-Shaped Structures: Sheath Folds?

Sheath folds and tubular folds are highly non-cylindrical, cone-shaped folds. According to Skjemaa (L989), highly non-cylindrical, cone-shaped folds are classified based on the hinge line angle and on "the ratio of the length of rhe cone ais to that of the long axis of the cross section of the structure at the position where the hinge angle is rneasured' (Figure 3.41). According to this author, sheath and tubular folds are defined in the following way: "vin a non-cyiindrical fold a cross-section can be selected so that the hinge angle o measured where the cross-section culs the cone is less than 909 and the length of the 92 (caption)

Figure 3.39 - Underground exposure of a folded carbonate-facies BIF showing an S, axial planar foliation (SW hinge zone - Lamego eye-shaped structure). Note cuspate structures associated with prominent brown carbonate layer. These structures are a consequence of a competency contrat between the carbonate layer and the adjacent siliceous layers.

Figure 3.40 - Underground exposure of a folded sulphide-facies BIF in the SW hinge zone of the Larnego eye-shaped structure. The sulphide-rich layers are clearly folded (note S, cleavage surfaces as an axial planar foliation) and thus pre-date the penetrative Transamazonian deformation. The plunge of the L, linear fabnc is also defined by the orientation of the hinge lines of isoclinal to open Transamazonian folds observed in varied scales at the Larnego deposit. Note cuspate structures formed as a result of competency contrasts between di fferent layers in the BE. cone mis is more than a quarter of the length of the long mis y of the cross-section, the fold is a sheath foid. A zighl sheath fold, with w < 20" and x:y > 1, can be tenned a tubular fold." Tubular folds are mostly found in rock sequences buiit up of layen with altemating rheological properties and/or cornpetencies (Skjemaa, 1989) that are intensely deformed in shear zones, basal nappe complexes, and diapir margins, where the bulk deformation process is progressive simple shear (Cobbold and Quinquis, 1980). in ductile systems undergoing bulk simple shearing and intense tectonic transport, tubular folds are assurned to be developed by "kinernatic amplifications" of previously formed structural elements or "deflections" such as large fold axes that were initially subperpendicular to the tectonic transport direction (Cobbold and Quinquis, 1980 and Malavieille, 1987) - (see Figure 3.42). These kinernatic amplifications should produce cwilinear folds that becarne successively more elongated during the evolution of the processes of progressive simple shear deformation and tectonic rransport (Lacassin and Mattauer, 1985 and Malavieille, 1987). Lacassin and Mattauer (1985) have show that tubular folds formed during intense progressive simple shear and tectonic transport under ductile conditions in the Monte Rosa nappe in the Swiss Alps exhibit the following structural features and characteristics:

A) Tubular folds are elongated parallel to the direction of regional tectonic transport. B) Tubular folds are associated with rocks displaying a prominent stretching linear fabric that would lie close to the direction of the tectonic transport and simple shear. C) The hinge zones of "eye-shaped structures", defined as the outcrop pattern of a tubular fold at a given horizontal erosional level, and the axes of associated coeval rnesoscopic and microscopic folds are also parallel to the previously mentioned prominent stretching linear fabric. Sheath Fold: (Skjernaa, 1 989) w > 20' but c 90' x/y > 0.25 but < 1

Tubular Fold: (Skjernaa, 1989)

Classification of highly non-cylindrical cone-shaped folds Adopted from Skjernaa (1 989) Figure 3.41 y Shear Strain Progressive slmple shear Stretc hing Lineation

Block diagrams showing shape of sheath folds created by kinematic amplification of layering deflections during progressive simple shear From Molavlellle ( 1987) Figure 3.42 The geological data and information collected and compiled 60m the Lamego and Cuiaba deposits during this research strongly suggest that the Larnego and Cuiaba eye- shaped structures (Figure 3.1) are "sheathed" ovemimed antiforms. Figure 3.43 shows the location, extent, and shape of the Cuiaba stmcture at different operational levels of the Cuiaba mine. This illustration shows that the perimeter and area of the structure progressively narrow upwards, and enlarge downwards, suggesting that the Cuiaba sheaWtubu1ar antiform would have "closed~'upwards prior to being eroded. The "upwards narrowing", or "downwards enlargement" of the Cuiaba sheath/tubular antiform can also be inferred based on the small, but systematic, variations in plunge that have been observed at the different "geographic/structural domains" of the Cuiaba structure (SW hinge zone, NE hinge zone, N limb, and S limb) at various levels of the Cuiaba mine (Figure 3.43 and Figure 5.1). The average plunges of the Transarnazonian linear fabric elements, which are everywhere parallel to the direction of the elongation and plunge of the Cuiaba ovemirned antiform, indicate that the o angle of the Cuiaba sheathhbular antiform is 12" (the SW hinge zone systernaticaily plunges in a S56E direction and the NE hinge zone systematically plunges in a S68E direction, indicating as a consequence, a "downwards divergence", or "upward convergence" for the ~wohinge zones). Furthemore, the N limb has steeper average plunges than the average plunges shown by the S limb. According to the structural and morphologie parameters proposed by Skjemaa

(1989), the Cuiabh antifonn should be classified as a tubular fold. The o angle of the antifonn is inferred to be close to 12". For obvious reasons, the total length of the x axis is unknown, but any hypothetical cross-section cutting the Cuiaba antifonn orthogonally to the plunge/elongation direction between Level 11 and Level 18 would show that its x axis is longer than its y axis (Figure 3.43). The geological mapping completed during this research has indicated that the Larnego structure is also a sheathed ovemimed antifonn (Figure 3.2). The SW hinge zone plunges S66E (Figure 3.44 and Figure 3.45) and the NE hinge zone of the structure

plunges S88E, parallel to the average plunge of linear fabric elements measured at surface and underground. Therefore, the two hinge zones of the Larnego structure diverge

downwards with increasing depths and converge upwards, as in the case of the Cuiaba antiform. The o angle of the Lamego sheathed antifom is 22', compared to the 12' angle of the Cuiaba tubular antifom. This interpretation is also supported by the NW limb of the Lamego overtumed antiform (the limb that presents an inverted stratigraphic column)

having steeper average plunges (from 34' to 40') than the average plunges shown by the SE limb (from 28" to 34'). The average plunge (or elongation) of the Lamego sheathed overtumed antiform

has also been inferred based on the distribution of the attitude of the bedding surfaces

(poles to So planes) collected along the entire Larnego structure and plotted in a rr- diagrarn equal-area plot (Figure 3.46). The plunge of the girdle axis obtained in this stereographic projection, S81E/36° (or 099/36"), has the sarne attitude as the average plunge that has been obtained for the L, Transamazonian linear fabric elements for the entire Larnego structure, which is S81E132" (see Figure 3.33). On a more regional scale, the elongation of the Lamego and Cuiaba sheathed antiforms and the average plunges of the L, Transamazonian linear fabric elements appear to lie very close to the local transport directions of the conspicuous northwest- verging Transamazonian fold-thnist belt observed in the SabarKaeté region (Figure 3.1 and Figure 2.1). 99 (caption)

Figure 3.44

Figure 3.44 and Figure 3.45 - Outcrops showing steeply dipping bedded cherts of the Pele de Onça Chert Unit in the SW hinge zone of the Lamego eye-shaped structure.

Figure 3.45

BEDDING PLANES (Sd - LAMEGO ME-SHAPED STRUCTURE N

CONTOUR LEGEND:

Pole Concentration(%01 total per 2.0 % of areo): Min. Contour - 3.0 % Contour Interval - 3.0 % Max. Conc. - 18.5 % E

- glrdle mis - plunge: 099136~ or SB I €/36'

- Equal-Areo Stereonet - - Wher Distribution - - Lower Hemisphere - - 35 1 Data Entries - - 351 Poles Plotted - S

Figure 3.46 MO Collected by Mdrcio sates and UNIGEO lnterpretotion and drawing by Marcio Sales (Queen's Universrty - November. 1996).

Figure 3.46 - Equal-area stereonet showing the distribution of the attitudes of poles to bedding planes (So) measured in outcrops and underground exposures along the entire Lamego eye-shaped structure. The average plunge of the Lamego sheathed overtumed antifom was inferred based on the distribution of the attitude of the bedding surfaces (poles to So planes) in a n-diagram equal-area plot. Note that the trendplunge of the girdle axis obtained in this stereographic projection (S81 W36" or 099/3ti0) is sirnilar to the average trendplunge obtained for the Lz Transamazonian linear fabric elements in Figure 3.33 (S81E132" or 099132"). 101

CHAP'ER 4 - LAMEGO PROSPECT: THE GOLD MINERALIZATION

4.1 - Lamego Prospect: Some Exploration Facts

The Larnego auriferous BIF unit was detected as a result of a regional stream sediment survey conducted by MMV in many parts of the Quadrilatero Fem'fero district during the late 1970s and early 1980s. Nurnerous active strearn sediment samples showed concentrations of gold (kom 0.5 to 3.2 parts per million in the < 2 mm grain-size fiaction) in creeks of the "Lamegopapa-Farinha" area (Figure 4.1). This justified subsequent follow-up activities that consisted mainly of geological reconnaissance prospecting, geological mapping (1 :2,000scale), and rock sampling. Very early in the follow-up phase, the exploration geologists from MMV realized that the stream sediment anomalies were caused by a folded BIF unit, and that this displayed some striking geological sirnilarities to the well-known BIF unit of the Cuiaba deposit. The geological reconnaissance activities discovered numerous old workings (eighteenth century ?) in the mineralized BIF unit of the Lamego-Papa Farinha area (figures 4.2, 4.3, 4.4, and 4.5). The gold occuffences and old workings were mapped and carefuliy sampled. An early understanding of sorne of the "key" exploration aspects of the Lamego deposit including the nature of the BIF-hosted gold mineralization; the size and grade of the typical mineralized bodies; and the structural, geomeûic, and spatial "controls" of the mineraiized bodies was obtained during these preliminary investigations. After the geological reconnaissance phase, the Lamego BE unit was mapped in detail (1 500 scale) and extensively sampled inside the exploration properties owned by MMV, prior to diamond drilling. During the 1980s, after several carnpaigns, 19,900 meters of dnlling were completed in order to test mineralized domains of the Lamego Larnego BIF + Pele de Onca Chert Unit + Black ~arbonaceousPhyllite Unit

Footwall Volcanic Unit

Gold concentration (p.p.m.) in the < Smm grain-size fraction of the active drainage sediments

1 Lamego Prospect: gold concentration in "active' stream sediments Geoctiemical suwey by Mineraçdo Morro Velho. Data compiled from MMV's interna1 reports. Drawing by Marcio Sales (Queen's University - June, 1997). Figure 4.1 1O3 (caption)

Figure 4.2

Figure 4.2 and Figure 4.3 - Old workings for gold at the Lamego prospect ("Cabeça de Pedra" mineralized zone - see also Figure 3.2).

Figure 4.3

Figure 4.4 - Ancient, small-scale open-pit operation at the Lamego prospect ("Arco da Velha" rnineralized zone - see also Figure 3.2). eye-shaped structure. By 1990, the drill indicated resources of these properties were estimated as approximately 10,000 tonnes of "'ore" per vertical meter at 6.5 g/t or 16,000 tonnes at 5.0 g/t (information based on intemal reports by Mineraçiio Morro Velho). From 1993 to 1996, the three richest mineralized bodies previously detected on the surface drilling carnpaigns were explored and evaluated using underground drifts (approximately 2,000 meters of gallenes) and underground drilling (Figure 4.6). During 1995-1996, a program of deep drilling confirmed the continuity of the gold mineralization to a depth of 460 rn below surface. According to Mineraça0 Morro Velho, the cornbined geological resources fiom these three mineralized bodies are approxirnately 3.5 million tonnes of ore at 5.4 g/t.

4.2 - Lithologic/Lithostratigraphic Controls and Gold Grades

Al1 "ore-grade" rnineralized bodies (average gold grade 2 3.0 g/t) detected and evaluated to-date in the Larnego prospect are confined to the Lamego Banded Iron-

Formation and to the Pele de Onça Chert Unit. In the Larnego Banded Iron-Formation, they are associated with a specific lithologic type, the "sulphide-facies BIF". Sulphide- facies BIFs in the Larnego deposit, similar to the "barren" carbonate-facies BIFs described in Section 3.3.2, are composed of altemating siliceous light bands and iron-rich dark bands that ordinarily Vary in thickness fkom a few millimeters to several decimeters (Figure 4.7 and Figure 4.8). However, the iron-bearing minera1 phases present in the dark bands of the sulphide-facies BIFs are not only iron-bearing carbonates, but also iron-rich sulphides (essentially pyrite + arsenopyrite). The most important petrographic aspects of the sulphide-facies BIFs from the Lamego deposit are descnbed in Section 4.4.1 and Section 4.4.2. Economic gold grades in typical sulphide-facies BIF samples frorn the Lamego and Cuiaba deposits are usually related to the presence of sulphide minerais. In most cases, a direct relationship cm be established between the arnount of sulphides 106 (caption)

Figure 4.5 - Panoramic view of the "Larnego/Papa-Farinha"area (SabarSCaeté region).

Figure 4.6 - Main adit to the underground operations. Approximately 2,000 meters of exploration tunnels were completed at the Larnego prospect by Mineraçào Mono Veho between 1993 and 1996.

107 (caption)

Figure 4.7

Figure 4.7 and Figure 4.8 - Underground exposure of typical sulphide-facies BIFs of the Lamego Banded Iron-Formation in the "Cabeça de Pedra" mineralized zone. Sulphide- facies BIFs in the Lamego deposit consist of altemating siliceous light bands and iron- rich darWLgolden" bands that Vary fiorn a few rnillimeters to several decimeters in thickness. Petrographic studies on sulphide-rich bands showed that they are composed essentially of iron-rich sulphides (usually from 35 to 80% in volume, essentially pyrite + arsenopyrite), Mg-poor ankerite, and quartz.

Figure 4.8

(percentage per volume) and gold grades (grams per tonne). Clear illustrations of this fact are provided in Figure 4.9 and Figure 4.10, where the sulphur contents (W.%) and the gold grades (gt) of mineralized sulphide-facies BIF samples from the Lamego prospect ("Cabeça de Pedra" and "Arco da Velha" mineralized zones, see Figure 3.2) and Cuiaba mine ("'Balanciio/Canta Galo" orebody, see Figure S. 1) deposits are plotted in a cartesian diagram. The important conclusions obtained fiom these diagrams are the following:

A) Gold grades in typical sulphide-facies BIF samples from these 2 mineralized zones of the Larnego eye-shaped structure are usually lower than 10 glt. Grades between 10 and 20 g/t have been less often detected. B) Gold grades in sulphide-facies BIF samples fkom the Balanc2o/Canta Ga10 orebody of the Cuiaba mine Vary f?om less than 5 g/t to more than 40 g/t. However, grades between 10 and 20 g/t have been commonly observed. C) The inferred regression trendlines (linear equations) presented in Figure 4.9 and Figure 4.10 indicate that, ideally, for a given sulphur content (wt.%) or sulphide content (mode), sulphide-facies BEsamples from the Larnego deposit display lower gold grades than sulphide-facies BIF samples from the Cuiaba mine. D) The obtained R-squared values suggest that the "reliability" of the linear equation inferred for samples of the Cuiaba mine is considerably higher than the "reliability" of the linear equation inferred for samples of the Lamego prospect. As a consequence, visual estimations of gold grades based on sulphide contents (mode) have been more difficult to achieve in BE samples fkom the Lamego prospect than in those from the Cuiaba mine.

Economic goid grades are also associated with sulphides in mineralized cherts of the Pele de Onça Chert Unit. However, visual estimations of gold grades based on sulphide content (mode) in this lithotype are considerably more problematic (Figure 4.11). In some situations, chert samples lacking appreciable amounts of sulphides have shown "attractive" gold values (> 10 glt) or, convenely, sulphide-rich specimens in 109 (caption)

Figure 4.9 - Gold concentration versus sulphur content (wt.%) in mineralized sulphide- facies BIF samples (gold grades 5 1 g/t) of the Lamego Banded Iron-Formation fkom the "Arco da Velha" and "Cabeça de Pedra" mineralized zones. A regression trendline (linear equation) attempting to establish the relationship between the amount of sulphur and the gold grades in mineralized samples is shown. Gold Concentration versus Sulphur Content (BIF Samples) - "Arco da Velha" and "Cabeca de Pedra" Mineralized Zones -

Sulphur Content (% of S - W.%) Compilation and diagram by Marcio Sales. Data: MMV - Queen's University (1W6). Figure 4.9 1 1O (caption)

Figure 4.10 - Gold concentration versur sulphur content (W.%) in mineralized sulphide- facies BE sarnples (gold grades 2 1 g/t) fiom the "Balanc3o/Canta Galo" orebody - Cuiaba mine. A regression trendline (linear equation) attempting to establish the relationship between the amount of sulphur and the gold grades in mineralized sarnples is show. Gold Concentration versus Sulphur Content (BIF Samples) - "BalancaolCanta Galo" Orebody - Cuiaba Mine

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 CI Sulphur Content (% of S - wt.%) CIO Compilation and diagrarn: Marcio Sales. Figure 4.10 Data: MMV - Queen's University (1996). I 1l (caption)

Figure 4.1 1 - Gold concentration versus sulphur content (wt.%) in mineralized cherts (gold grades 1 1 g/t) of the Pele de Onça Chert Unit fiom the "Cabeça de Pedra" and "Arco da Velha" mineralized zones. Gold mineralization usually has been associated with the presence of sulphides in cherts of the Pele de Onça Chert Unit. However, abnormally high or very high gold grades (from 20 to 641 g/t) have been locally obtained in sorne sulphide-f?ee or sulphide-poor (sulphur < 2.0 wt%) chert sarnples. mineralized bodies have show b'uneconomic" grades (c3 g/t). The relevant petrographic aspects of mineralized cherts nom the Pele de Onça Chert Unit are described in Section 4.4.1b.

4.3 - "Mineralized" and "Barren" Domains: Stratigraphic Implications

From an exploration and mining point of view, the Lamego eye-shaped structure has been informaily subdivided in this study into two types of "geologic domains" based on a group of features systematically observed during underground geologic mapping and drill core logging. "Barren domains" are devoid of economic BIF-hosted gold mineralization. Their most relevant characteristics are as follows (see Figure 3.3):

A) The Lamego Banded Iron-Formation is composed alrnost exclusively of carbonate-facies BIFs in typically barren domains of the Lamego eye-shaped structure. Sulphide-facies BIFs and sulphide-bearing carbonate-facies BIFs are absent or very minor components of this unit in these domains. B) The average thickness of the Pele de Onça Chert Unit is comrnonly, but not everywhere, more modest in barren domains than in typically mineralized domains. Pele de Onça cherts in barren domains are invariably devoid of sulphides and goid mineralization. C) The rock sequences of the Footwall Volcanic Unit that lie imrnediately beneath the carbonate-facies BIFs of the Larnego Banded Iron-Formation in barren domains are composed of the lest altered and rnetasornatized schists previously described in Section 3.3.1. The most cornmon altered volcanic rocks observed in barren domains are 'Type 2" and "Type 1" schists. However, a 1 to 3 meter-thick layer composed of 'Type 3" schists regularly occurs along the contact beh~eenthe Footwall Volcanic Unit and the Lamego Banded Iron-Formation. D) in typical barren domains, the contacts between the Larnego Banded Iron- Formation and the Pele de Onça Chert Unit are invariably sharp and essentially tabular (Figure 3.30).

"Mineralized domains" of the Lamego eye-shaped structure are less cornmon and have the following characteristics (see Figure 3.3):

A) In mineralized domains, the Lamego Banded Iron-Formation is composed exclusively of sulphide-facies BIFs (less cornmon case) or of "associations" of sulphide- facies B IFS, sulphide-bearing carbonate-facies BR, and sulphide- fiee carbonate- facies BIFs. The distribution pattern of these "associations" is extremely cornplex, comprising sharp or gradational, vertical and lateral lithologic/facies variations. B) The average thickness of the Pele de Onça Chert Unit is generally greater in mineralized than in barren domains. Pele de Onça cherts in mineralized domains are commonly sulphide-bearing and mineralized in gold. C) The Footwall Volcanic Unit imrnediately beneath the sulphide-facies BIFs of the Lamego Banded Iron-Formation in typical mineralized domains is composed of the most altered and rnetasomatized schists described in Section 3.3.1. A IO to 20 meter-thick "layer" composed almost exclusively of "Type 5" and "Type 4" schists has been mapped al1 along the lower contact of the Larnego Banded Iron-Formation in mineralized domains.

The nature of the boundaries between the Lamego Banded Iron-Formation and the Pele de Onça Chert Unit: evidence for epigenetic volcanic/sub-volcanic gold mineralization

Detailed underground mapping in mineralized domains of the Lamego structure showed that the contact between the lower Lamego Banded Iron-Formation and the upper Pele de Onça Chert Unit is not always sharp and entirely planar as initially interpreted. This is illustrated in Figure 4.12. The Pele de Onça "cherts" occur not only in an overlying stratabound tabular horizon that ranges ir. thickness Eom several decimeters to t 14 (caption)

Figure 4.12 - Underground exposure ("SE lirnb"/Lamego eye-shaped sîructure) showing the nature of the boundaries between the Larnego Banded bon-formation and the Pele de Onça Chert Unit. The Pele de Onça "cherts" occur not only in a stratabound tabular horizon that ranges in thickness fiom several decimeters to 4 or 5 meters, but also as interconnected, discordant "feeder-like" vein/veinlet systems that are oriented perpendicular or sub-perpendicular to the bedding surfaces and "merge" with the Pele de Onça Unit in its lowennost stratigraphie portion. The width of the individual 'khert" veins in these systems varies from less than a centimeter to 1.5 meters (see also Figure 4.13). Note that the carbonate-rich bands of the BIFs show sulphide-emiched alteratiodreplacement halos that are syrnmeûically distributed around Pele de Onça 'chert" veinsheinlets (see also Figure 4.14). The intercalated carbonate-fkee siliceous bands of the sarne rocks, however, have not been replacedaltered close to the contacts of the sarne veinsheinlets.

4 or 5 meten, but also as intercomected, discordant "feeder-like" veidveinlet systems that are oriented perpendicular or sub-perpendicular to the bedding surfaces of the BPs and "rnerge" with the Pele de Onça Chert Unit in its lowermost stratigraphic portion. The width of the individual "chert" veins in these systems varies &om less than a centimeter to 1.5 meters in the underground exposures mapped (Figure 4.13). Based on its stratigraphic position, it is inferred that the Pele de Onça Chert Unit was deposited at the waning stages of a volcanic/volcano-exhalative depositional cycle. The chert is interpreted in this research as a possible subaqueous Archean analogue of modem hydrothermal cherts and/or siliceous sinter horizons deposited in epithermal deposits by neutral-pH hot-spnng waters. According to Sillitoe (1993 - p. 412/413), sinter horizons are essentially subaenal products that are generated "where upflowing hydrothemal fluidr discharge at the earth 's suface and precipitate silica". Sillitoe, however, has also descnbed subaqueous silica precipitation from hydrothermal fiuids as "dense. delicatelv Zaminated, srratiform chert" in epithermal gold deposits in volcanic settings. A fundamental relationship was observed between the Pele de Onça "chert veins/veinlets" and the stratifom BIF-hosted gold mineralization in "poorly" mineralized domains of the Larnego Banded Iron-Formation, and also at the margins of some of the BIF-hosted mineralized bodies during detailed underground mapping. At these sites, the carbonate-rich bands of carbonate-facies BIFs commonly show sulphide-enriched alteratiodreplacement halos that are symrnetrically distributed around the Pele de Onça "chert" veinsheinlets (Figure 4.12 and Figure 4.14). The intercalated carbonate-free, light coloured siliceous bands of the same rocks, however, have not been replaced/altered close to the contacts of the sarne veinsheinlets. The replacement of iron-nch carbonates by iron-bearing sulphides in carbonate- rich bands has also been inferred based on petrographic studies on carbonate-facies and sulphide-facies BEs. Although the carbonate-nch bands of typical sulphide- free carbonate-facies BIFs usually contain 2 types of carbonate minerals (ankente + siderite), siderite was never observed in the carbonate-nch bands of typical sulphide-facies BIFs Figure 4.13 - Underground exposure C'SE limb'/lamego eye-shaped strucm showing a typical Pele de Onça "chert" vein cutting the Larnego Banded Iron- For rnatio close tc 1 the contact with the Pele de Onça Chert Unit. These veins are usuall riente perpenc iicular or sub-perpendicular to the bedding surfaces (So) of the BIFs an merge with th e Pele de Onça Chert Unit in its lowennost stratigraphie portion (see a Figui 4.12). Figure 4.14 - Close-up view of sulphidesnriched alteration.replacement "halos" symrnetrically distributed around a Pele de Onça "chert" vein/veinlet (right corner of the photograph). These replacement features were observed in dark carbonate-rich bands of BIFs occhgin b'poorly" mineralized domains of the Lamego structure and also at the margins of some of the BIF-hosted mineralized bodies. Note that the sulphide content of the carbonate-rich bands tends to decrease merfiom the contacts with the Pele de Onça "chert" veinheinlet. The intercalated carbonate-fkee, light coloured siliceous bands were not replaced/altered close to the contacts with the veidveinlet (see also Figure 4.12). (see Figure 3.25). The only carbonate mineral observed in sulphide-facies BIFs of the Larnego Banded Iron-Formation is the "relatively iron-poor" ankente. It is inferred, consequently, that an important part of the iron from the siderite in non-replaced, sulphide-fiee, carbonate-rich bands has been used to stabilize/crystallize iron-rich sulphides in bands that were replaced by sulphur-rich fluids during the hydrothermal alteration processes. The relationships observed between the Pele de Onça veins/veinlets, the stratabound Pele de Onça Chert Unit, and the sulphidation/replacement processes in carbonate-facies BKFs at the margins of mineralized bodies strongly suggest that the gold mineralization in the Lamego deposit is epigenetic in relation to its main host unit (Lamego BIF) and essentially related to hydrothermal alteration processes operating in an Archean volcanic/sub-volcanic environment. Further evidence to support this syn- volcanic/epigenetic hypothesis is presented in Section 5.4.

4.4 - Mineralized Bodies: Mode of Occurrence, Size, Grade, and Petrography

The ore-grade bodies identified and evaluated in the Lamego Banded Iron- Formation are scattered, stratiform lenses of sulphide-facies BIF that Vary fiom 15-20 meters to 80-120 meters in length along the strike direction and nom less than a meter to

4 or 5 meten in thickness. Their average gold grade ranges From 3.0 g/t to 17.0 g/t. Ore- grade bodies in the Pele de Onça Chert Unit have been observed mainly as thin stratiform lenses (fkom less than a meter to 2.5 meters in thickness) having a maximum strike length of 50-60 meters that usually occur at the lowermost stratigraphie portion of the unit, along the contact with the BIFs of the Lamego Banded Iron-Formation. Their average gold grade ranges from 5.0 to 10.0 gh. The ore-grade mineralized bodies of 2 of the 3 most important mineralized domains of the Larnego structure (the "Cabeça de Pedra" and "Queimada" mineralized zones) were mapped underground in detail (1:100 scale) and are described petrographically in this study. The ore-grade mineralized bodies of the third mineralized domain (the "Arco da Velha" mineralized zone) were also studied petrographically in this thesis. Some of the more important results obtained from these activities are presented in Section 4.4.1 and Section 4.4.2.

4.4.1 - Example 1: The Cabeça de Pedra Mineralized Zone

The Cabeça de Pedra mineralized zone is located in the SW binge zone of the Lamego eye-shaped structure (Figure 3.2). Six stratiform "lenticular" ore-grade mineralized bodies were mapped underground in this zone at Level 1 (elevation: 880 meters). Five of them are confined to the intermediate and uppemost stratigraphic portions of the Larnego Banded Iron-Formation, and one is hosted in the lowermost stratigraphic layer of the Pele de Onça Chert Unit (Figure 4.15). It is important to notice, however, that the typical stratigraphic section of the Larnego deposit is inverted in the Cabeça de Pedra mineralized zone as a consequence of the localized presence of Transamazonian SE-plunging overturned folds.

4.4.la - The BIF-Hosted Gold Mineralization

The 5 ore-grade mineralized bodies hosted in the Lamego Banded Iron-Formation at the Cabeça de Pedra mineralized zone Vary from 15-20 meters to 40 meters in strike length and from 1.5 to 4.0 meters in thickness. They are composed of sulphide-facies BIFs that are characterized by alternating siliceous light bands and sulphide-rich "golden" bands that Vary in thickness kom several millimeters to several decimeters (Figure 4.16 and Figure 4.17). Petrographic studies of sulphide-rich bands of typical sulphide-facies B IF samp les from the Cabeça de Pedra and Arco da Velha mineralized zones show that they are composed essentially of iron-rich sulphides (usually from 35 to 80% by volume), Mg- poor ankerite (see Figure 3.25), and quartz. Siderite was not observed in sulphide-rich bands of BIFs from the Lamego deposit. The siliceous, light coloured bands of these Detailed underground geological map d the "Cabeça de Pedmnminenilii zone &eVd1) lhdergomd gedogicd mapping by Maa'o Sdes 1 996). Scmpli md iiogadesdpti~of~ampfchanetsm&~~m=byNlLa. Wngby MkWo Sdes -3- - ml, 1997)

121 (caption)

Figure 4.16 - Underground exposure of a typical mineralized sulphide-facies BIF of the Larnego Banded Iron-Formation at the "Cabeça de Pedra" mineralized zone.

Figure 4.17 - Deeply weathered. mineralized sulphide-facies BIF of the Lamego Banded Iron-Formation in an outcrop located at the SW hinge zone of the Lamego eye-shaped structure. Under tropicaVsubtropica1 humid climatic conditions, carbonate minerals are extensively ieached and sulphides are oxidized and converted into iron hydroxides.

samples are petrographically sirnilar to those described in typical carbonate-facies BE samples (Section 3.3.2). Among the sulphides, pyrite (usually 90-95% of the total sulphide content) and arsenopyrite (usually 2-8%, less commonly 10-20% of the total sulphide content) are aiways the most important phases present in the sulphide-rich bands. Pyrrhotite, chalcopyrite, sphalerite, and galena occur only as very minor accessory minerals and represent less than 2% by volume of the total sulphide content of the sulphide-rich bands. A fine-grained "arnorphous carbonaceous matenal" that resembles graphite and a few crystals of chlorite, sericite, and were also observed in sulphide-rich bands of the sulphide-facies BIF samples investigated. Pyrite occurs as euhedral to subhedral cxystals that commonly range from 20 microns to 5 or 6 millimeters in diameter and may include essentially euhedral crystals of arsenopyrite (Figure 4.18 and Figure 4.19). Tiny anhedral inclusions of pyrrhotite, chalcopyrite, and galena (usually less than 50 microns) are commonly observed in the larger pyrite crystals. Arsenopyrite (S- 19 wt.%; As- 48 M.%; and Fe- 33 W.%; according to Salas et ai., 1996) is invariably seen as euhedral, or euhedral to subhedral, pnsmatic or diamond-shaped crystals that Vary from 40 microns to 1 millimeter in length and are devoid of inclusions of the other sulphide phases. Based on similar textural observations made on sulphide-facies BIF sarnp les f?om the Cuiaba deposit, Bosho ff (mineralogical report M/79/l76, hg10 Arnerican Corporation) in Via1 (1 980a) inferred that arsenopyrite was the first sulphide phase to crystallize in the gold-bearing sulphide- rich bands of the Cuiaba Banded Iron-Formation. However, arsenopyrite crystals that had partially overgrown previously formed pyrite crystals were also observed in samples of the Lamego Banded Iron-Formation during this study. More than 95% of the electrum grains and particles detected in ore-grade sulphide-facies BIF samples ffom the Cabeça de Pedra and Arco da Velha rnineralized zones are included within pyrite crystals (Figure 4.20). Electrum grains were very rareiy observed as inclusions within arsenopyrite, at sulphide grain boundaries, or associated 123 (caption)

Figure 4.1 8

Figure 4.18 and Figure 4.19 - Photomicrographs (plane-polarized reflected light) showing typical sulphide-rich bands of sulphide-facies BEsamples from the "Cabeça de Pedra" mineralized zone. Pyrite (usually 90-95% of the total sulphide content) and arsenopyrite (usually 2-8%, less commonly 10-20% of the total sulphide content) are always the most important sulphide phases present in these bands. Pyrrhotite, chalcopyrite, sphalerite, and galena occur ody as very minor accessory minerals and represent less than 2% by volume of the sulphide content of the sulphide-rich bands. Pyrite (Py)occurs as euhedral to subhedral crystals that cornmonly range fiom 0.02 to 5 or 6 millimeters in diarneter and may include essentially euhedral crystals of arsenopyrite. Arsenopyrite (Asp) is invariably seen as euhedral, or euhedral to subhedral, prismatic or diamond-shaped crystals that Vary fiom 0.04 to 1 millimeter in length. Arsenopyrite crystals are devoid of inclusions of the other sulphide phases. However, arsenopyrite crystals that had partially overgrown previously formed pyrite were also observed during this study.

Figure 4.19

124 (caption)

Figure 4.20 - Photomicrograph (plane-polarized reflected light) showing an electrum grain (Au) occurring as an inclusion within a pyrite crystal in a sulphide-facies BIF £kom the Larnego Banded Iron-Formation. Petrographic studies completed in this study revealed that more than 95% of the electnim grains and particles in "ore-grade" sulphide- facies BIF sarnples (> 3 or 4 g/t) fiom the "Cabeça de Pedra" and "Arco da Veiha" minerdized zones are included within pyrite. The "size" of the graindparticles (the length of the longest dimension of the grains in a polished section) varies f7om less than 1 micron to 60 microns, with the highest frequency observed between 2 and 15 microns (see also Figure 4.22).

Figure 4.21 - Photomicrograph (plane-polarized reflected light) showing 2 electrum grains (Au) occurring as inclusions within a pyrite crystal in a Pele de Onça chert. Petrographic studies completed on representative sulphide-rich (sulphides > 10% b y volume), high-grade (> 8.0 g/t) sarnples of Pele de Onça chert fkom the "Cabeça de Pedra" and "Arco da Velha" mineralized zones indicated that the mode of occurrence and size of the electnim grains hosted in cherts and sulphide-facies BIFs are remarkably similar. More than 92% of the electnim grains detected in sul~hide-richPele de Onça chert samples are inclusions smaller than 80 microns within pyrite crystals. with gangue minerals (quartz and carbonates). The "size" of the graindparticles (the length of their longest dimension in a polished section) cornmonly varies fkom less than 1 micron to 60 microns, with the highest fiequency observed between 2 microns and 15 microns (Figure 4.22).

The fineness of the electnim grains (1000 x Au I (Au + Ag)) included within pyrite crystals from sulphide-facies BIFs and Pele de Onça cherts Erom the Cabeça de Pedra, Arco da Vziha, and Queimada mineralized zones was detennined by electron microprobe analyses, and the results nom samples of these 2 distinct mineralized lithotypes of al1 3 mineralized zones are rernarkably homogeneous. The values of gold fineness in more than 85% of the grains analyzed (total of 30 grains in 8 different samples) Vary fiom 91 0 to 957 (average of 924). The lowest and highest values of fineness in electrum grains from sarnples of the Lamego deposit are 81 1 and 957, respectively. These values are relatively high when compared to similar data available from other BE-hosted gold deposits of the Nova Lima Group (Figure 4.23). The range of fineness of the electm grains from the Cuiabi, Morro Velho, and Silo Bento deposits is from 751 to 907 (Rodngues et al., 1995), 802 to 866 (Ladeira, 1988), and from 821 to 938 (Rodrigues et al., 1999, respectively.

4.4.lb - The Chert-Hosted Gold Mineralization

The only ore-grade mineralized body hosted in the Pele de Onça Chert Unit at the Cabeça de Pedra mineralized zone is a stratiform lens that has a length (along strike) of 50 meters and varies in thickness fkorn less than a meter to 2.5 meters. Similar to other mineralized lenses hosted in this stratigraphic unit of the Larnego structure, this lens is positioned very close to the contact with the lower Larnego Banded Iron-Formation. Its average gold grade is 9.5 g/t. The mineralized lenses and bodies hosted in the Pele de Onça Chert Unit at mineralized zones of the Larnego structure are composed of poorly bedded or massive, 126 (caption)

Figure 4.22 - Grain-size distribution of elecbum particles hosted in sulphide-facies BIF samples from the "Cabeça de Pedra" and "Arco da Velha" mineralized zones. The "size" of the grains (the length of the longest dimension of a grain in a polished section) varies from less than 1 micron to 60 microns, with the highest fiequency between 2 microns and 15 microns. Number of electrum grains detected BIF-hosted deposits range and average -Nova Lima Group- fineness of elect~m source grains (AU and ~g - wt.%) l I I I8l 1 910 957 This nierls Lamego prospect 1- 1 Cabeça de Pedra + Quamada I 1 I + Afco da Veiha I I I minefolued zones I 1907 I 75 1 Cuiaba mine I 1 I I I I 1 I I I Morro Velho mine Oo21 I 1- 1- I I I l I I I I 1 821 I 938 I Sao Bento mine Rodrigues et al. ~rI ( 1995) I I I 800 900 1000

Figure 4.23 - Variation of the gold fineness ( 1O00 x Au / (Au + Ag)} of electrum grains fkorn Lamego and other BIF-hosted gold deposits of the Nova Lima Group. Note that the average values obtained in sarnples from the Lamego deposit are relatively high when cornpared to sirnilar data from other BIF-hosted gold deposits of the district. Microprobe analyses of elecmgrains by Alan Grant (Queen's University). Diagram adapted from Rodrigues et al. (1995). sulphide-bearing cherts. The maximum total sulphide content in individual "heavily" mineralized chert sarnples (> 8.0 ut) is approximately 20-294 by volume. Sulphides in mineralized cherts occur as massive concentrations along thin layers (up to several centimeters in thickness) parallel to bedding surfaces (S,,), or as disseminations. Similar to the mineralized sulphide-facies BIF sarnples descnbed in the previous section (4.4. la), the iron-nch sulphide phases in mineralized Pele de Onça chert sarnples are essentially pyrite (usually more than 90% of the total sulphide content by volume) and arsenopyrite (usually 2- 1 O%, less comrnonly 10-20%). Pyrrhotite, chalcopyrite, sphalerite, and galena occur only as very rninor accessory minerals representing less than 2% by volume of the total sulphide content of these samples. The mode of occurrence and the main texh.mil aspects of the sulphide phases hosted in mineralized Pele de Onça chert and sulphide-facies BIF sarnples are remarkably similar. Pyrite in mineralized cherts occurs as euhedral to subhedral crystals that commonly range from 50 microns to 7 or 8 millirneters in diameter and sometimes include essentiall y euhedral crystals of arsenop yrite. Tiny anhedral inclusions of pyrrhotite, chalcopyrite, and galena (usually less than 50 microns) are comrnonly observed within larger pyrite crystals. Arsenopyrite is invariably seen as euhedral, or euhedral to subhedral, prismatic or diarnond-shaped crystals that Vary from 50 microns to 1 millimeter in length. Inclusions of other sulphide phases are comrnonly not observed in menopyrite crystals of mineralized Pele de Onça cherts. Microscopie studies on polished sections of representative, sulphide-rich

(sulphides > 10% in volume), high-grade (> 8.0 g/t) samples of Pele de Onça chert in mineralized bodies fiom the Cabeça de Pedra and Arco da Velha mineralized zones have shown that the mode of occurrence, grain-size, and fineness of the elecûum grains hosted in these sarnples and in representative sarnples of hi&-grade sulphide-facies BIF (see Section 4.4. la) are remarkably similar. More than 92% of the electrum grains detected in sulphide-rich chert sarnples are inclusions smaller than 80 microns within the p*te crystals (Figure 4.21). However, abnormally high or very hi& gold grades (from 20 to 641 g/t) have been locally obtained in some sulphide-free or sulphide-poor (sulphides < 2-3% in volume) chert samples fkom several mineralized lenses and bodies hosted in the Pele de Onça Chert Unit (Figure 4.11). In these samples, large "free" eleccnim grains (up to 4 rnillimeters in length) that locally are visible in the field have been observed in association with quartz grains. Large electm grains not included in sulphide minerals, "visible gold", and high gold grades that are not directly related to the presence of massive quantities of sulphide minerals have not been observed during this study in sarnples of mineralized sulphide- facies BIFs fiom the Lamego Banded Iron-Formation.

4.4.2 - Example II: The Queimada Mineralized Zone

The Queimada mineralized zone is located in the ovemimed NW Iimb of the Lamego eye-shaped structure (Figure 3.2). The Lamego Banded Iron-formation in this mineralized zone consists of a folded and segrnented ("boudinaged"), thin stratiform lens (fiom less than a meter to 2.5 meters in thickness) that has a length of at least 1 10 meters along strike (Figure 4.24) and is cornposed essentially of high-grade sulphide-facies BFs. Individual channel samples collected in underground exposures of this lens indicated

variable high gold grades from 10 to 50 g/t. The average grade has been estimated as 17 dt. The sulphide-facies BIFs mapped and sampled in the Queimada mineralized zone are somewhat different fkom the sulphide-facies BIFs in the Lamego Banded Iron- Formation in other parts of the Lamego deposit such as the Cabeça de Pedra and Arco da Velha mineralized zones. The "Queimada-type" sulphide-facies BEis a "layered massive sulphide" essentially composed of carbonate, pyrite, and arsenopyrite (Figure 4.25 and Figure 4.26). Quartz was observed in heavily mineralized samples both as an unimportant accessory mineral (more cornrnon case) or as a major minera1 constituent. The layering, which is everywhere parallel to the bedding surfaces (S,), is defined by an alternation of

L Deîuiled underground geokgical map d the 'laceirnada" mineralkd oone @vei 1 )

13l (caption)

Figure 4.25

Figure 4.25 and Figure 4.26 - Typical sulphide-facies BIF kom the "Queimada" mineralized zone. The "Queimada-type" sulphide-facies BIF is a "layered massive sulphide" composed essentially of ankerite, pyrite, and arsenopyrite. Quartz may occur both as an unimportant accessory mineral (more cornmon case) or as a major minera1 constituent. The layering, which is always parallel to bedding surfaces (S,), is defined by an altemation of distinct thin layers (usually fiom a few millimeters to many centimeten in thickness) showing variable relative arnounts of ankerite, pyrite, arsenopyrite, and quartz.

Figure 4.26 distinct thin layers (usually nom a few millimeters to many centimeters in thickness) showing variable relative amounts of carbonate, pyrite, arsenopyrite, and quartz. Carbonate is usually the rnost abundant minera1 in the Queimada-type sulphide- facies BiFs, and samples with 60-70% of modal carbonate are not uncornmon. Optical microscopy and electron microprobe analyses have shown that the only carbonate mineral present in this lithotype is ankente (see Figure 3.25), which occurs mainly as subhedral to anhedral crystals varying in size fiom less than 50 microns to 1 millimeter. The amounts of sulphides present in typical Queimada-type sulphide facies BIFs (mode) Vary nom 25 to 55%. Among the sulphides, pyrite (of'ten 75-90% of the total sulphide content) and arsenopyrite (often 8-25%, less commonly 25-55% of the total sulphide content) are always the most important phases. Pyrrhotite, chalcopyrite, sphalerite, and galena occur only as very minor accessory minerals. Together they usually represent less than 2% by volume of the total sulphide content. Pyrite occurs as euhedral to subhedral crystals that commonly range from 20 microns to 2 or 3 millimeters in diameter and may include euhedral crystals of arsenopyrite. Tiny anhedral inclusions of pyrrhotite, chalcopyrite, and galena (usually less than 50 microns) are commonly observed in the larger pyrite crystals. Anenopyrite is invariably seen as euhedral, or euhedral to subhedral, prismatic or diamond-shaped fine-grained crystals that Vary From 10 microns to 0.5 millimeters in length and are devoid of inclusions of other sulphide phases. The arsenopyrite crystals may also occur in aggregates of very fine-grained crystals (< 50 microns). Quartz usually represents only 5 to 15% of the volume of the Queimada-type sulphide-facies BE. Other cornrnon accessory minerals of this lithotype are chlorite, sericite, and an extrernely fine-grained carbonaceous matenal (graphite ?). A few crystals of scheelite were observed in some of the samples investigated. Detailed petrographic studies of representative ore-grade sarnples of sulphide- facies BIFs from the Queimada mineralized zone revealed that more than 95% of the electmm grains are included within pyrite crystals (Figure 4.27 and Figure 4.28). Rare electnim grains have also been observed as inclusions within arsenopyrite, at sulphide grain boundaries, or associated with gangue minerals (quartz and carbonates). The average size of the electrum grains (the length of the longest dimension in a polished section) hosted in ore-grade BIF samples from the Queimada mineralized zone is slightly smaller than the average size of the electnun particles hosted in ore-grade BIF samples from the Cabeça de Pedra and Arco da Veiha mineralized zones. Electrum grains from the Queimada mineralized zone Vary kom less than 1 micron to 35 microns, with the highest fkequency observed between 0.3 microns and 5 microns (Figure 4.29).

4.5 - StructuraVSpatiaI Control of the Mineralized Bodies

Geological observations made in "old workings", available diarnond drilling data, and information recently gathered in this study have confirmed that the stratabound and stratifom mineralized bodies of the Lamego deposit plunge with the same attitude (trend and plunge) as the orientation of the local Transamazonian linear fabric elements (L3. Since these mineralized bodies are congmently folded and deformed with their host units and associated stratabound "footwall" alteration zones, as well as with the entire stratigraphie section of the deposit, they also plunge with the same attitude of the plungelelongation of the kilometer-scale Larnego sheathed oveminied antiforni. The small and systematic variations in the average plunge of the L2 linear fabnc elements observed along the different stnicturaVgeographic sectors of the Larnego eye- shaped structure (Section 3.4.2) are associated with identical srnaIl and systematic variations in the average plunge of the mineralized bodies at the Larnego deposit. The mineralized bodies located in the Cabeça de Pedra mineralized zone (SW Hinge Zone, see Figure 3.2) and "P.A. mineralized zone" (NE Hinge Zone) plunge approximately S66E/31° and S88E/3 1°, respectively. Mineralized bodies situated in the SE Limb of the Lamego structure plunge with "intemediate" trends between S66E and S88E. Mineralized bodies located in the "Arco da Velha" and "Arco NE" mineralized zones (Figure 3.2) plunge approximately S79E/32' and S81/32', respectively. Mineralized 134 (caption)

Figure 4.27

Figure 4.27 and Figure 4.28 - Photomicrographs (plane-polarized reflected light) showing some textural aspects of sulphide-rich layers of "Queimada-type" sulphide- facies BiFs. Pyrite (Py) occurs as euhedral to subhedral crystals that commonly range from 20 microns to 2 or 3 millimeters in diarneter and may include euhedral crystals of arsenopyrite. Arsenopyrite (Asp) is invariably seen as euhedral, or euhedral to subhedral, prismatic or diamond-shaped, fine-grained crystals that Vary fiom 10 microns to 0.5 millimeters in length and are devoid of inclusions of the other sulphide. The arsenopyrite crystals may also occur in aggregates of very fine-grained crystals (c 50 microns). Petrographic studies of representative ore-grade samples of sulphide-facies BIFs nom the "Queimada" mineralized zone also revealed that more than 95% of the electnun grains (Au) are included within pyrite crystals.

Figure 4.28

135 (caption)

Figure 4.29 - Grain-size distribution of electrum particles hosted in sulphide-facies BIF samples from the "Queimada" rnineralized zone. The average "size" of the electrum grains (the length of the longest dimension of a grain in a polished section) hosted in ore- grade sarnples from the Queimada mineralized zone is slightly smaller than the average size of the grains hosted in ore-grade samples fiom the "Cabeça de Pedra" and "Arco da Velha" mineralized zones. Electrum grains from the Queimada mineralized zone usually Vary in size from less than 1 micron to 35 microns, with the highest frequency between 0.3 microns and 5 microns. Electrum grains hosted in sulphide-facies BIF sarnples from the "Queimada" mineralized zone

mm0s- ZDIZEE Ci (Y O YOYOYOY O Y OYOA 2 W$W- NOgg Sire of the electrum grains (10" mm)

Petrography by Marcio Sales Figure 4.29 (Queen's University - 1996). bodies located in the ovemuried NW Limb have steeper average plunges than mineralized bodies located in the SE Limb. For example, the inferred average plunge for the mineralized bodies in the Queimada mineralized zone is S79E/3S0-36". Similar to other important gold deposits of the Quadrilatero Fem'fero district, the plunges/dips of the mineralized bodies of the Lamego deposit are expected to flatten gradationally with increasing depth. The "plunge lengths" (dimension along the plunge direction) of the more important mineralized bodies of the Larnego deposit are inferred to be at Ieast 4 or 5 times longer than their respective "strike lengths". The deep dnlling program conducted by MMV during 199511996 confirmed the continuity of the more important mineralized bodies of the deposit (the ones that have a strike length of approximately 100-120 meters) to a minimum depth of 460 meters below surface. CHAPTER 5 - TAE GEOLOGY OF THE LAMEGO AND cUIABA DEPOSITS: SOME CORRELATIONS, METALLOGENIC ASPECTS, AND IMPLICATIONS FOR EXPLORATION

5.1 - Introduction

Chapter 5 compares and correlates some fundamental geological aspects of the Lamego and Cuiaba deposits. Information f?om the Cuiaba deposit has been compiled kom a literature review, studies of drill cores, and visits to underground mining operations. The most important goals of this chapter are the following:

A) To correlate and position the stratigraphic sections of the Lamego and Cuiaba deposits in the regional stratigraphy of the Nova Lima Group (SabadCaeté region). B) To show that the nature and the critical features of the BIF-hosted mineralization in these two deposits are remarkably similar. C) To infer, based mainly on data and information from the Lamego deposit, the origin and relative timing of the BIF-hosted gold mineralization in these two deposits. D) To suggest a possible explanation of the fact that Cuiaba is a considerably richer deposit than Larnego. E) To briefly discuss the practical implications of some findings and interpretations made in this study for the exploration of BIF-hosted gold rnineralizations both on the regional (Quadrilkero Fem'fero district) and local scales (SabarUCaeté region).

5.2 - stratigraphic Correlation

Some of the Fundamental geological aspects of the Cuiaba eye-shaped structure and a re-interpretation of the nature of its main lithostratigraphic units (after Vial, 1980a, 1988 and Vieira, 1992) are presented in Figure 5.1. A regional correlation of the stratigraphic sequences of the Lamego and Cuiaba deposits is inferred in Figure 5.2. The lowermost stratigraphic sequence exposed in the Cuiaba structure (the "core" of the antifonn) at Level 3 has a minimum thichess of 50-60 meters and consists of weakly foliated carbonate-quartz-piagioclase-actinolite-chlote-idote schists with local pillow structures, and foliated epidote-quartz-carbonate-plagioclase-chlt schists (Vial, 1980a, 1988 and Vieira, 1992). Petrographic studies on drill cores completed as part of this research, combined with previous petrographic descriptions of the rock-types occurring in the Cuiaba mine (Vial, 1980a and Vieira, 1992), have indicated that the schists of this "lowermost" unit are indistinguishable, mineralogically and texturally, bom the typical 'Type 1" and "Type 2" schists observed in the Foohlrall Volcanic Unit of the Lamego eye-shaped structure (Figure 5.2).

The lowermost unit in the Cuiaba structure is overlain by a 15-30 meter-thick sequence of albite-chlonte-white mica-quartz-carbonate schists, albite-white mica- chlorite-quartz-carbonate schists, and white mica-quartz-carbonate schists. This sequence underlies the aurïferous Cuiaba BIF Unit (Figure 5.1 and Figure 5.2) and its rocks are, rnineralogically and texturally, similar to the highly altered/metasomatized rocks of the Footwall Volcanic Unit (''Type 3" to 'Type 5" schists) that underlie the Lamego BIF Unit in mineralized domains of the Larnego structure (Figure 3.3 and Figure 5.2). The carbonate-facies and sulphide-facies BIFs of the Cuiaba BE Unit have been described in detail in Vial (1980a, 1988), Vieira (1992), and Rodrigues et al. (1994, 1995). There are no major compositional, texhual, chernical or structural differences between the banded iron-formations of the Cuiaba and Lamego deposits. Occurrences of sulphide-facies BIFs are, however, much more comrnon and widespread in the Cuiaba structure than in the Larnego structure. The thickness of the Cuiaba BE Unit nomally varies from 6 to 15 meters (Vial, 1988), whereas that of the Lamego BI.Unit varies from 1 to 8 meters.

Iered basic volcanic rock (Type 3" to "Type 5" schists) vh'h minor epidostic~olcmiciasticrocks.

PEtered basic volcanic rocks (Type 1" to "Type 2" schisfs).

-4%'omdmed antiforni (and plmge of fdd cms)

Explanution:

Banôeâ im-formafiuntarlphid. and carbonate-facies)and 'Pek de Onça* choit.

*Ihted vokank rock. *Type 5 AîteroHan Facies' (ankerite + qua* + mricit.-potagonite + siciorth).

Altored basic vokanic rock. 'Type 1 Altermon Facks' (cîinozobne-e~mh + cnbrtte 4 ai- + qwm + adino~ite + calcite).

Reprerentative shatigraphk sections of the Lam O prospect and Cuiabd nmine. ~~a%rndill cotes. The "Pele de Onça Chert Unit" is also present in the Cuiabi structure. Continuous or discontinuous layers of Pele de Onça cherts (nom less than a meter to 5 meters in thickness) "capping" the sulphide-facies BEsequences in underground exposures of the "Balancâo/Canta Galo", "Fonte Grande Sul", and "GaIinheiro Extensao" orebodies were observed during this research. The Pele de Onça cherts have been informally designated "smoky quartz" by MMV during the mining operations at Cuiaba. The BE/chert units in the Cuiaba deposit are also invariably overlain by a layer of carbonaceous phyllites and graphite schists that has approximately 10 meters in average thichess. These carbonaceous phyllites and graphite schists are correlated with the 'Black Carbonaceous Phyllite UnitT'of the Lamego structure. The "Black Carbonaceous Phyllite Unit" in the Cuiaba structure is overlain by a relatively thick (up to 100 meters in thickness) "hanging wall volcanic sequence" composed essentially of more or less altered basic volcanic rocks. This hanging-wall sequence is not present in the Lamego deposit, where the Black Carbonaceous Phyllite Unit is always in direct contact with the "Upper EpiclasticNolcaniclastic Unit" of the Lamego structure (figures 3.2, 3.3, 5.1, and 5.2). The lower part of the hanging wall volcanic sequence at Cuiaba consists of highly altered and metasomatized basic volcanic rocks that are, mineralogically and texturally, similar to the more altered schists of the Footwall Volcanic Unit in the Lamego structure ('Type 3" to "Type 5" schists - see Figure 5.1 and Figure 5.2). The intemediate part is composed, according to Vieira (1992), of a sequence of unaltered (or only slightly altered) pillowed basic volcanic rocks represented by plagioclase-actinolite-epidote-chlorite schists. The uppermost part of the hanging wall volcanic sequence consists of altered basic volcanic rocks and carbonaceous phyllites/schists (Figure 5.1 and Figure 5.2) and is overlain by the Upper EpiclasticNolcaniclastic Unit. The stratigraphie sections of the Lamego and Cuiabi deposits thus can be correlated, with a reasonable level of confidence, from their lowermost sequences (Footwall Volcanic Unit, the core of the two eye-shaped structures) to the unit of carbonaceous phyllites and graphite schists that overlies the BIFs and/or cherts (Black Carbonaceous Phyllite Unit). However, 2 significant differences between these sections are the unique presence of the hanging wall volcanic sequence and the much thicker development of the mine stratigraphic sequence (BE + cherts + graphite schists) at the Cuiaba deposit (see Figure 5.2).

5.3 - Regional Stratigraphie Position of the LamegoKuiaba BIF Unit

The Larnego and Cuiaba BEunits are interpreted in this study to be located at (or very close to) the boundary between the Middle Unit and the Upper Unit of the Nova Lima Group (Figure 3.1). The basic volcanic rocks of the Footwall Volcanic Unit, mainly on the basis of their geochemical composition (TH2 tholeiites with calc-alkaiine affinity), are tentatively correlated with the uppermost basic and basichtermediate volcanic rocks of the Middle Unit of the Nova Lima Group. The Upper EpiclasticNolcaniclastic Unit essentially consists of lithologic associations that typifi the Upper Unit of the Nova Lima Group. Figure 3.1 also shows that the more important BIF-hosted gold deposits and gold occurrences already identified in the SabariXaeté region are confined to the Middle Unit of the Nova Lima Group. The Lamego and Cuiaba eye-shaped structures are interpreted in this shidy as kilometer-scale "tectonic inliers" in which the uppemost stratigraphic portion of the Middle Unit is surrounded by younger rock sequences of the Upper Unit as a consequence of a highly non-cylindrical, cone-shaped folding (sheath or tubular folds) in the SabardCaeté region. Cobbold and Quinquis (1980) and Malavieille (1 987) have suggested that such folds are a consequence of intense simple shearing and regional tectonic transport. They probably developed as "kinematic amplifications" of regional northwest-verging overtumed folds with axes that were initially subperpendicular to the transport direction. Such kinematic amplifications would produce curvilinear folds which becarne successively more elongated during simple shear deformation and tectonic transport (Figure 5.3).

It is suggested in this study that the Cuiaba and Lamego sheathed antiforms "nucleated" as a consequence of the competency contrast between the highly altered, incompetent, phyllosilicate-rich volcanic rocks which envelop the mineralized BIFs, and the competent mineralized BIFs and cherts. As tubular and sheath folds are mostly found in layered rock sequences with altemating rheological properties and/or cornpetencies (Skjemaa, 1989), sheathed or tubular antiforms would be preferentially nucleated in specific portions of the regional LamegoKuiaba BIF unit that were more highly altered and mineralized prior to the onset of the penetrative Transarnazonian deformation.

5.4 - BIF-Hosted Gold Mineralization: Characteristics and Genetic Aspects

"Iron-fonnatior~~fonn a major part of the large group of chernically precipitated siliceour and carbonaceous metalliferous sediments considered collecrivety as the stratafer group. ThlS group of hydrolithic sedirnents includes many d~xerenttypes of interrelated facies. Tile most common and besr known of these are the iron-formation, associated sedimentay rnanganese depositx ... stran~onnpoljmetallic (iron, rnanganese. , . . nickel, gold, siIver, . ) sulphide depositr.... sedintenta~halarive(SEDEQ deposirs, and other hydrolithic sediments that bear economically important amounrs of gold, . mngsten. nickel, banie. fluonie. and rare-earth elemenfi." (Gross, 1993 - p. 167).

5.4.1 - Deposits of the Nova Lima Group: Genetic Aspects

Initially, the Morro Velho mine and some other BIF-hosted gold deposits of the Nova Lima Group were described in the literature as classic exarnples of "hypothermal'' (term adopted fiom Lindgren's genetic classifications of ore deposits, 1922 and 1933) metasomatic deposits formed by hydrothermal solutions originated from deep-seated granitic intrusions. Graton and Bjorge (1929, 193l), Matheson (1 956), Gair (1 962), and Park and MacDiaxmid (1975) were the most prominent supporters of this general genetic model. According to hem, the paragenetic association of arsenopyrite, pyrrhotite, pyrite, gold, quartz, scheelite, and wolfiamite in orebodies, and also the observation of textures suggesting replacement of the host rocks (Lapa Seca Formation and BIF units), would be diagnostic features of deep-seated, high-temperature metasornatic processes caused by "hypothennal" granitic fluids. However, occurrences of granitic stocks or felsic intrusives were never reported at the vicinity of the BF-hosted gold deposits of the Nova Lima Group.

Since the early 19803, "syngenetic" hypotheses related to subseafloor hydrothermal activity or volcano-sedimentary exhalative processes have been advocated to explain the genesis and some critical features of the most important deposits of the Nova Lima Group. Syngenetic volcanogenic-exhalative models to explain the genesis of individual or groups of BIF-hosted gold deposits of the Nova Lima Group have been proposed by Ladeira (1980, 1985: Morro Velho), Vial (1980a: Cuiaba), Vial (1980b: Raposos), Vial (1980~:Bicaho), Moseley (1986: Sao Bento), Ladeira (1988, 1991: Morro Velho, Cuiabi, Raposos, and Siio Bento) Abreu et al. (1988: Sâo Bento), and DeWitt et al. (1994, 1996: Morro Velho and Sao Bento). According to Kerswill (1993 - p. 187/189), in a typical syngenetic stratiform BE-hosted gold deposit al1 the chemical components (iron, , , sulphur, arsenic, gold, , copper, COz, and tungs ten) are "depositedfrom hydrothermal fluids during chemical sedimentation or eady diagenesis". Kerswill proposed that a syngenetic deposition should occur "directly onto the sea floor from plumes or brine pools or just below the sediment-water inteflace from fluids reacring with penneable sediments". A syngenetic-volcanogenic hypothesis for the BE-hosted gold deposits of the Nova Lima Group is strengthened by the following geological features:

A) The BE-hosted auriferous sulphide bodies and lenses of the Nova Lima Group are not only stratabound (confined to horizons of BIF or chert), but are also stratiform. The mineralized bodies, lenses, and layers are invariably concordant with the bedding (S,) and congmently folded together with the host lithostratigraphic units. B) The hydrothermal alteration zones associated with the gold mineralization are essentially stratabound and concordant with the BWchert horizons. C) The majority of the more important auriferous BIF units and the Lapa Seca Formation are in a special stratigraphic position, at a major stratigraphic transition from a basal mafic (or ultramafic) volcanic sequence to an upper mixed unit composed essentially of epiclastic and volcaniclastic sedirnentary rocks. The importance of this specific stratigraphie position could be explained by the fact that waning stages of volcanic depositional cycles are cornmonly charactenzed by hydrothermal activities including exhalations of stem and other gases and vapours from fumaroles and metal- rich hot spnngs. D) Despite the fact that several of the most important BIF-hosted gold deposits of the Nova Lima Group display alteration zones both at the footwall and hanging wall of the mineralized bodies (Vieira, 1991 a), some examples in which alteration features and alteration mineral assemblages are confined to footwall sequences have also been reported. These relationships are compatible with gold mineralkation being pnmarily related to processes associated with the volcanic/volcanogenic environment. E) Galena crystals directly associated with BIF-hosted gold mineralizations From sorne of the more important deposits of the Nova Lima Group were dated (Pb-Pb radiometric method) by Thorpe et al. (1984: Cuiabi, Faria, Bicaiho, and Bela Fama deposits) and DeWitt et al. (1996: Sgo Bento deposit). The lest radiogenic sarnples analyzed yielded mode1 ages of 2.7 Ga. These results are similar to the U-Pb ages obtained by Machado et al. (1989) for the deposition and magrnatic crystallization of some felsic volcanic rocks of the Nova Lima Group (see Section 2.2.2a).

However, more recently, metamorphogenic "epigenetic" modeis related to ductile shear zones have been widely applied to explain the genesis and some geological features of many Archean gold deposits. The BE-hosted gold deposits of the Nova Lima Group have been interpreted as metamorphogenic and -related by Belo de Oliveira (1 986: mineralizations in the Caeté region), Vieira (1 98%: Raposos), Vieira (1988: al1 of the more important deposits of the Nova Lima Group), Vieira and Oliveira (1988: Cuiabi, Raposos, and Faria), Vieira (1991a: Cuiabh, Raposos, and Faria),

Scarpelli (199 1: al1 of the more important deposits of the Nova Lima Group), and Pereira (1992: S3o Bento). A hypothesis considering a shear zone-related metarnorphogenic origin for the BIF-hosted gold deposits of the Nova Lima Group is supported, according to these authors, by the following features: A) The orebodies are "structurally and spatially controlled". The attitude (trend and plunge) of the auriferous orebodies mimics the orientation of the linear fabric developed in the Quadrilatem Femfero district during the Transamazonian orogeny. B) Some of the thicker and more massive BIF-hosted auriferous orebodies of the Nova Lima Group are Located in hhge zones of folds generated during the Transamazonian orogeny. C) Replacement textures (sulphidation) directly associated with gold mineralization are observed on auriferous BWchert units. Vieira (199 1a) affimed that replacements of iron-rich rninerals (sidente, ankente, and magnetite) by pyrite and arsenopyrite are commonly observed in iron-rich bands of mineralized BIFkhert units in some deposits of the Nova Lima Group. According to Vieira, these replacements can occur symmetrically or asymmetrically distributed from fractures, sheared surfaces, and discordant quartz veins.

5.4.2 - Lamego and Cuiaba deposits

The nature and critical features of the BE-hosted gold rnineralization in the Cuiaba and Lamego deposits are remarkably similar. The mineralized bodies hosted in the Cuiaba BIF unit are stratiform lenses/Iayers of sulphide-facies BEthat Vary fiom Iess than a meter to 8-10 meters in thickness and have up to several hundred meters in lateral length (along the strike of the BIF unit). Their average gold grades Vary from 5.5 g/t to 13 glt, and there is a direct relationship between gold grades and arnounts of sulphides/sulphur contained in individual mineralized sulphide-facies BIF sarnples (Figure 4.10). Gold occurs as electrum grains (the average gold fineness is 840 according to Rodngues et al., 1995) that usually have less than 60 microns in diameter and are enclosed in pyrite crystals (Vial, 1988). The mineralized sulphide-facies BIFs of the Cuiaba and Lamego deposits are very similar petrographically and chemically, but at least one important di fference was observed between representative samples fiom the 2 deposits. Mineralized sulphide- facies BIFs kom the Cuiaba deposit normally contain arsenopyrite only as a very minor accessory mineral and, as a direct consequence, their As203concentrations are low, very often below 0.60 wt.% and cornmonly between 0.05 and 0.40 wt.% (Figure 5.4). Mineralized sulphide-facies BIFs fiom the Larnego deposit, on the other hand, commonly contain arsenopyrite as an important accessory mineral and, as a result, their As,Ol concentrations are cornmonly higher and more variable than the As203concentrations in similar samples fiom the Cuiaba deposit. The As203 concentrations in mineralized samples from the Cabeça de Pedra and Arco da Velha mineralized zones Vary from 0.10 to 2.0 wt.% (Figure 5.5). Samples of typical Queimada-type sulphide-facies BIFs contain highly variable As,03 concentrations between 0.50 and 5.0 wt.% (Figure 5.6). Individual sulphide-facies BIF samples from Cuiaba and Larnego do not show a direct relationship between As,03 concentrations (and modal arsenopyrite contents) and gold grades (see figures 5.4, 5.5, and 5.6). However, a precise knowledge of the average As20, concentrations in the mineralized bodies of the Lamego deposit is critical, as even relatively small increases in arsenic could negatively affect recovenes of gold from Larnego processed at Cuiaba (Queiroz rnetallurgical plant). The differences in As,03 concentration between ore-grade samples fiorn the hvo deposits have to be carefilly managed by MMV in the future. Vieira (1991a - p. 324) stated that the bulk of the gold mineralization in the Cuiaba deposit "consists of pyrite and arsenopyrite which replace the iron mineral bands in banded iron-fomations". According to this author, carbonate-rich bands of the banded iron-formations are symmetrically or asymmetncaIIy replaced by pyrite and menopyrite at the boundaries with fractures and "interconnected quartz veim" that CU^ the bedding surfaces of the host rocks at high angles. Vieira (1991% 1992) has interpreted these discordant "interconnected quartz veins" associated with the sulphidation of the 149 (caption)

Figure 5.4 - Gold concentration versus arsenic content in mineralized BIF samples (gold grades 2 1 g/t) from the "BalancàolCanta Galo" orebody, Cuiaba mine. Mineralized sulphide-facies BFs fkom the Cuiabi deposit usually contain arsenopyrite as a very minor accessory minera1 and, as a direct consequence, their As203concentrations are low, very ofien below 0.60 wt.% and commonly between 0.05 and 0.40 W.%. Note that there is not a direct relationship between gold grades and arsenic contents. Gold Concentration versus Arsenic Content (BIF Samples) - "BalancaolCanta Galo" Orebody - Cuiaba Mine

0.00 0.20 0.40 0.60 0.80 1 .O0 1.20 1.40 1.60 1.80 C1 Arsenic Content (Xof As203- W.%) s Compilation and diagram by Marcio Sales. Figure 5.4 Data: MMV - Queen's University (1996). 150 (caption)

Figure 5.5 - Gold concentration verstu arsenic content in mineralized BEsamples (gold grades 2 1 g/t) of the Larnego Banded Iron-Formation from the "Arco da Velha" and "Cabeça de Pedra" mineralized zones. The As203concentrations in these samples usually Vary from 0.10 to 2.0 wt.%. Note that there is not a direct relationship between gold grades and arsenic contents. Gold Concentration versus Arsenic Content (BIF Samples) - "Arco da Velha" and "Cabeca de Pedra" Mineralized Zones - Lamego Prospect

0.00 0.20 0.40 0.60 0.80 1.O0 1.20 1.40 1.60 1.80 Arsenic Content (% of Asz03- W.%)

Compilation and diagram by Marcio Sales. Figure 5.5 Data: MMV - Queen's University (1096). 15 1 (caption)

Figure 5.6 - Gold concentration versus arsenic content in rnineralized BIF samples (gold grades t 1 g/t) of the Larnego Banded Iron-Formation fiom the "Queimada" mineralized zone. The As203concentrations in these samples usually Vary from 0.5 to 5.0 wt.%. Note that there is not a direct relationship behueen gold grades and arsenic contents. Gold Concentration versus Arsenic Content (BIF Samples) - "Queimada" Mineralized Zone - Lamego Prospect

O .O0 1.O0 2.00 3.00 4.00 5.00 6.00 7.00 8.00 CI Arsenic Content (% of AS& - W.%) 2

Compilation and diagram by Marcio Sales. Figure 5.6 Data: MMV - Queen's University (1996). carbonate-nch bands in BIFs (and with the processes that altered/metasomatized the sequences of volcanic rocks at the footwall and hanging-wall of the Cuiaba BE unit) as ""metamorphogenic" and "shear zone-related. Quartz veins and hydrothermal alteration would be, according to the author, coevai and directly related to the regional penetrative

defonnation event associated with the Rio das Velhas orogeny. Vieira (1991a, 1992) affirmed that the stratabound and symmetric alteration zones developed in both the footwall and hanging-wall of the Cuiaba BIF unit; the discordant quartz veins associated with the sulphidation processes; and the gold-bearing sulphide-facies BEs are al1 direct consequences of an essentially ductile, transcurrent shear zone developed strictly parallel to the original sedimentary/volcanic bedding surfaces (So) and hosted precisely within the Cuiaba BIF unit. It is suggested in this thesis, based on the lithostratigraphic correlation, the similar structural setting of the mineralized stratigraphie sequences (the sheathed overtwned antiforms), and the sirnilar style of mineralization, that Cuiaba and Lamego are stratabound BE-hosted gold deposits which formed contemporaneously in an identical mineralization environment. The present writer also believes that the Larnego deposit may provide, as a consequence of being hosted in a less "modifieci" and less intensely alteredmineralized domain of this environment, some genetic information that may not be easily obtainable in a highly modified and altered/mineralized domain such as the Cuiaba deposit. The geological information gathered during this study indicates, however, that the BIF-hosted gold rnineralization in the Larnego depoçit appears to be related to Archean sub- seafloor (sub-volcanic?) hydrothexmal alteration processes, prior to the tectonic inversion, penetrative deformation, and metamorphic events which affected the Archean supracmstal sequences of the Quadrilatero Femfero district. The BE-hosted gold mineralization at

Lamego also was, at least in part, ""epigenetic" in nature, as shown by the replacement of carbonate-rich layers in banded iron-formations by gold-bearing, iron-nch sulphides. These findhgs contribute to, and modi@, previous discussions and interpretations of the genetic aspects of the BIF-hosted gold deposits of the Nova Lima Group that have invariably considered "syngenetic" volcano-exhalative hypotheses and "epigenetic" rnetamorphogenic/syntectonic shear-zone related hypotheses. Evidence contributed by this thesis suggests that the bulk of the BIF-hosted gold mineralization in the Larnego and Cuiaba deposits is both svn-volcanic/sub-volcanicand "e~ieenetic" in nature. Furthemore, these deposits have characteristics similar to hot spring-type epithermal gold deposits and go Id-nch volcanogenic massive sulphide (VMS) deposits. A "syn-volcanic/epigenetic" hypothesis for the BE-hosted gold mineralization in the Lamego and Cuiaba deposits is suggested based essentially on the following observations:

A) The more important BIF-hosted gold deposits and occurrences already identified in the SabarKaeté region are invariably within the uppermost stratigraphic portions of the "Middle Unit" of the Nova Lima Group, very close to, or at the contact of the "Upper Unit" (Figure 3.1 ). B) The BIFIchert units that host the mineralized bodies of the Larnego and Cuiaba deposits (Larnego/Cuiaba Banded Iron-Formation and Pele de Onça Chert Unit) are in a distinctive stratigraphic position, at a major stratigraphic transition from a lower basic volcanic sequence (Footwall Volcanic Unit) to an upper mixed unit composed essentially of epiclastic and volcaniclastic sedimentary rocks (üpper EpiclasticNolcaniclastic Unit). Waning stages of volcanic depositional cycles are cornrnonly characterized by the presence of hydrothermal activity and the development of volcanogenic massive sulphide mineralization. C)The Pele de Onça Chert Unit is interpreted in this thesis as a self-sealing silica cap, similar to the siliceous sinter cappings and terraces associated with epithermal gold deposits, and the silicified tuWchert layers which commonly cap massive sulphide orebodies. D) Hydrothermal alteration in the Larnego deposit occurs only in the footwall

sequences (Footwall Volcanic Unit) of the auriferous zones in the Larnego BIF Unit and - Pele de Onça Chert Unit. The hydrothermal alteration zones are, in relation to the BIFkhert units, semi-confomable (stratabound) "sheets" showing a minimum vertical extension of 200 meters. Highly altered domains of the Footwall Volcanic Unit situated beneath bodies of mineralized sulphide-facies BIF are characterized by a "proximal"

alteration represented by the indicator mineral assemblage white mica + ankerite t siderite + chlorite (Figure 3.3 and Figure 3.5). Proximal alteration zones grade vertically and laterally into propylitic "distal" alteration zones typically represented by the indicator minera1 assemblage clinozoisite/epidote + chlorite + calcite f actinolite (Fi,we 3.3 and Figure 3.5). Similar hydrothermal alteration pattern has been described in rnetal-rich hot spring-type epithermal deposits associated with seafloor/sub-seafloor volcanism (e.g.,

Sillitoe, 1993) and in volcanogenic gold deposits in VMS environments (e.g., Hannington et al., 1997). E) At Cuiabi, hydrothermal activity appears to have continued afier the deposition of the mineralization, and alteration extends into the lower parts of the hanging wall volcanic sequence (Figure 5.1 and Figure 5.2). This is also a typical feature of some VMS systems. F) The flattened and partially transposed "stockwork-like" phyllosilicate-nch microveinlets and carbonate-rich veinsheinlets observed in highly alteredmetasomatized zones of the Footwall Volcanic Unit underlying ore-grade zones at the Larnego deposit are interpreted in this study as evidence of the volcanogenic/epithermal-style nature of the mineralizing hydrothermal alteration processes (Figure 3.4D and Figure 3.1 1). G) Some of the important systematic changes in chernistry occuired in rocks of the Footwall Volcanic Unit of the Lamego deposit during the progressive hydrothermal alteration/metasomatisrn include depletions in Na20, A120,, SiO2, and "base metal" elements (Cu, Zn, Ni, and Co) as well as additions in CU2, K20and Ca0 (Figure 3.12 and Figure 3.13). Base metal depletion patterns in semiconformable, epidote-quartz (i carbonate, + actinolite, + chlorite) alteration zones developed in the footwall sequences of some volcanic-associated massive sulphide (VMS) deposits have been described by Hodgson and Lydon (1977), Spooner (1977), Richardson et al. (1987), Gibson (1990), Franklin (1993), and Gibson and Kerr (1993). Na20 depletion and CO2 and K,O addition patterns are also comrnonly observed in the footwall sequences of VMS deposits (Franklin, 1993). H) There is clear evidence that the replacement/mineralization of the carbonate- rich bands of BIFs was, at least in part, coeval and directly related to the emplacement of Pele de Onça veins and veinlets and also to the volcanogenic deposition of the stratabound Pele de Onça Chert Unit at the Lamego prospect (Figure 4.12 and Figure 4.14). These veins and veinlets of Pele de Onça chert that were observed in the Lamego deposit are the equivalents of Vieira's "interconnected quartz veinr" (Vieira, 1991 a - p. 324), that they cut bedding surfaces at high angles and fil1 structures which also fed the sulphidation of carbonate-rich bands in the Cuiaba deposit. 1) The carbonate-facies banded iron-formations of the Larnego and Cuiaba BE units were, very possibly, poorly lithified sedimentary rocks which had their primary lithological permeability increased by carbonate dissolution processes operating during the early stages of the volcanic/sub-volcanic hydrothemal alteration processes. This "increased" permeability would allow an efficient lateral flow for the percolating sulphur- bearing metalliferous fluids (relatively reduced, near-neutral pH fluids?) that replaced the carbonate-rich bands in originally sulphide-fkee carbonate-facies BIFs (Figure 4.14). J) The stratifonn BIWchert-hosted mineralized lenses, layers, and bodies and their associated stratabound alteration zones in the Lamego deposit clearly pre-date the penetrative Transamazonian deformation, as they are folded, inverted, overprinted, and transposed by Transamazonian structures (figures 3.2, 3.40, 4.24, and 5.1). Introduction and/or remobilization of sulphides and gold into Transamazonian structures and penetrative structural fabrics were not observed in the Larnego deposit. The structuraVspatia1 control of the mineraiized bodies of the Lamego and Cuiaba deposits (which is a direct consequence of the plunge/elongation of the Larnego and Cuiaba sheathed overturned antiforms) is, however, Transamazonian in age. K) The essentially ductile transcurrent shear zone of Rio das Veihas age developed entirely within the Cuiaba BIF unit and parallel to the bedding surfaces (S,) which, according to Vieira (1991a, 1992), would be responsible for the bulk of the BIF- hosted gold mineralization were never found during this study neither in the Cuiaba mine nor in the Larnego prospect. There is no evidence for intraformational brecciation or mylonitization within the BE units as suggested by Vieira to support his shear zone hypothesis.

The basic explanation as to why Cuiaba is a larger and ncher deposit than Larnego, may be provided by their respective stratigraphic sections (figures 3.2, 5.1, and 5.2). In the Cuiabâ eye-shaped structure, the volcano-chernical units @IFS + cherts) are underlain by a continuous unit composed of highly altered/metasomatized basic volcanic rocks ('Type 3" to 'Type 5" schists) and overlain by a sequence composed essentially of altered and unaltered basic volcanic rocks. The BIF Unit in the Cuiaba structure is extremely nch in occurrences of mineralized sulphide-facies banded iron-formation. Furthermore, the mine sequence package at Cuiaba (BIFs + cherts + graphite schists) is signi ficantly thicker than at Lamego. The volcano-chemical units in the Lamego structure, however, are only locally underlain by the more aItered/metasornatized basic volcanic rocks of the Footwall Volcanic Unit ('Type 3" to "Type 5" schists) and there are no volcanic rocks above the Larnego BIF Unit. A sequence of sedimentary rocks containing mixed epiclastic and volcaniclastic contributions overlies the BIFs and cherts in the Larnego structure. Furthermore, the BIF unit in the Lamego structure contains only locally significant occurrences of sulphide-facies mineralized banded iron-formation. These basic lithostratigraphic differences appear to indicate that the paleogeographic position of the Cuiaba deposit was more proximal to an Archean volcanic center(s) than that of the Lamego deposit. In addition, the hydrothermal system at Cuiaba appears to have been more potent and more active over a longer penod of time than the hydrothermal system at Lamego. The BE-hosted gold deposits in the SabadCaeté region of the Quadrilatero Fem'fero district appear to represent an interesting type of volcanogenic hydrotherrnal gold deposit with some characteristics typical of gold-rich VMS deposits. The Rio das Velhas Supergroup is metallogenic unique in being dominated by this unusual type of gold deposits, to the exclusion of major occurrences of almost any other type of Archean mineralization (polymetallic VMS deposits, nickel deposits in komatiitic sequences, or the more typical non-BE-hosted Archean Iode-gold deposits).

5.5 - Controls of the Gold Minerakation: Implications for Regional Exploration

A concise "list" containing some information and suggestions that are expected to be helpful for future exploration programs aiming the discovery or evaluation of BLF- hosted gold deposits in the Nova Lima Group is presented in this section. This list, which to a certain extent represents points of view and interpretations of the author, was subdivided into several main topics that are the following: stratigraphic Setting of the Host BIF Units

The majority of the more important auriferous BIFkhert units of the Nova Lima Group occur in a special stratigraphic context, at major stratigraphic transitions from basal volcanic sequences to upper mixed units cornposed essentially of epiclastic and volcaniclastic sedimentary rocks. The present study presented the case of the Larnego and Cuiabi deposits (and some other significant BIF-hosted gold occurrences in the SabargCaeté region), which are interpreted to be hosted in the uppermost part of the Middle Unit of the Nova Lima Group, very close to, or at, the contact with the Upper Unit. The Raposos deposit is hosted within a BIF unit that is underlain by a sequence of ultrabasic volcanic rocks and overlain by a sequence of epiclastic and volcaniclastic sedirnentary mcks (Vieira and Oliveira, 1988). The Lapa Seca Formation, which is the host unit of the Morro Velho deposit, occurs in a contact zone between a basal sequence of basic vo lcanic rocks and an upper sequence of epiclasticho lcanic lastic sediments (Ladeira, 1980, 1985).

Proximity to Volcanic Centers

The interpretation that the bulk of the gold mineralization in the Lamego and Cuiaba deposits is a result of syn-volcanic replacements of carbonate-facies BIFs (sulphidation) implies that the selection and pnoritization of exploration areas for BE- hosted gold rnineralization in the Nova Lima Group should be made as a function of relative proximity to the Archean volcanic centers. According to Gross (1 993), Precambrian sulphide and carbonate-facies BIFs were deposited close to high-temperature effisive centers and oxide-facies were deposited around cooler vents and in areas distal from active hydrothermal discharge. It is worth noting that the largest BIF-hosted gold deposits of the Nova Lima Group are hosted in BIF units composed of mixed sequences of carbonate and sulphide-facies BIFs. Oxide- facies BiFs are commonly not found, or are very minor components, in auriferous BIF units at the vicinity of the large deposits. Oxide-facies BIFs are, however, comrnon laterally (along the regional strike of the BIF units), invariably far away from domains hosting expressive gold mineralization.

The Pele de Onça chert, a lithology that was observed in the Lamego and Cuiaba deposits both as a stratabound layer overlying the BLF unit and as "feeder-like" veins and veinlets should also be used as a favourable indication of proximity to volcanic centers and mineralized zones. The Pele de Onça Chert Unit is interpreted in this study as a subaqueous Archean analogue of the modem hydrothermal cherts and/or siliceous sinter horizons comrnonly found in epitherrnal gold-silver deposits. Anornalous thickening of volcanic sequences and recurrent volcanic activity after the sedimentation of the volcano-chemical units (BIR and cherts), as observed in the stratigraphic section of the Cuiaba eye-shaped structure, may also be interpreted as an indication of proximity to volcanic centers.

Hydrothermal Alteration and Mineralization

The sequences of basic volcanic rocks that underlie rnineralized portions of the BE units in the Lamego and Cuiaba structures are charactenzed by "proximal" alteration zones represented by the indicator minera1 assemblage white mica + ankerite + siderite f chlorite. Proximal alteration zones grade vertically and laterally into extensive propylitic "distal" alteration zones typically represented by the indicator mineral assemblage clinozoisite/epidote + chlorite + calcite k actinolite. Sequences of altered basic volcanic rocks also overlie the BIFkhert units in the Cuiaba structure. Based on the geological features descnbed in the present study, the following alteration patterns are important in selecting areas with potentiai for hosting BIF-hosted mineralization associated with volcanic rocks in the Nova Lima Group: A) Highly altered/metasomatized volcanic rocks in mineralized stratigraphic sections should ideally contain abundant Na-rich white mica and ankeritic carbonate together with some sideritic carbonate. Prospects containing foohvall alteration zones represented essentially by the indicator minerals calcitic carbonate, chlorite, epidote, and actinolite (propylitic alteration) should receive a lower degree of pnority. B) The presence of "stockwork-like" phyllosilicate-rich microveinlets and carbonate-rich veinsheinlets in altered volcanic rocks underlying mineralized zones of the Larnego BIF unit in the Lamego structure suggests that these stnicturaVtextural features may also be used, in conjunction with the appropriate mineral alteration assemblages, in defining "'high priority" exploration areas and properties. C) "Feeders", interconnected veins, and veinlets of Pele de Onça cherts cutting the banded iron-formations are important features to be noted in outcrops and drill cores, although they are extremely difficult to observe in drill cores. Exploration areas showing suiphidation processes and/or gold anomalies associated with these types of features should, for obvious reasons, be prioritized. D) Lilhogeochemistry could be used as an accessory "tool" in defining the intensity of the alteration/metasomatism processes undergone by the footwall (or hanging-wall) volcanic sequences of the BF units. In altered rocks of the Footwall Volcanic Unit at the Lamego deposit, there is a tendency for a progressive depletion of SiO,, A120,, Total Fe, MgO, and Na20 as well as progressive increases in CO,,&O, and Ca0 with respect to unaltered rocks. Consistent and gradua1 depletions of the concentrations of some "base metal" elements (Cu, Zn, Ni, and Co), rare-earth elements, and Y were also observed for increasing levels of alteration/metasomatism. E) The presence of a sequence of altered volcanic rocks overlying an auriferous BIF unit is also interpreted in this study as a favourable exploration parameter.

Size and Structural Control of the BIF-Hosted Mineralized Bodies

The BIF-hosted auriferous bodies of the Nova Lima Group are relatively small, both laterally and in thickness. The strike length of the bodies varies fiom less than 50 meters to only several hundred rneters (generally less than 200 meters), and the thickness comrnonly varies from 1 to 8 meters. A cluster of small bodies may thus be laterally contained in only 1 kilometer. As a consequence, "follow up" exploration activities for BF-hosted gold rnineralization in areas of high potential should consider a minimum space of 100-200 meters between exploratory geochemistry/geophysics traverses. A 100 meter interval would commonly be adequate enough to define the potential of geochemical or geophysical anomalies delineated in soi1 geochemistry or ground-based geophysical surveys. The more important BIF-hosted mineralized bodies and orebodies of the Nova Lima Group plunge with the same attitude as the rnost obvious penetrative linear fabric observed in Archean and Proterozoic supracmstal rocks of Quadrilatero Femfero district (the Transamazonian L2 linear fabric). The notable "structural control" shown by the orebodies as well as their relatively small strike length rnakes the completion of careful field-based geological/stnicturaI mapping prograrns mandatory, pnor to designing diamond dnlling campaigns.

5.6 - The 'Sheath Fold Inference": Local Exploration Implications

Two interpretations made in this study have important implications for the exploration and discovery of new BIF-hosted gold deposits on a more localized scale. The inferences that the Lamego and Cuiaba BIF units represent a single stratigraphie unit located at the contact of the Middle Unit with the Upper Unit of the Nova Lima Group and that the Lamego and Cuiaba eye-shaped structures are expressions at surface of kilometer-scale, sheathed ovemimed antiforms open new exploration perspectives in the SabarWaeté region. Based on the interpretation presented in Figure 5.3, the "roofs" or hinge zones of other kilometer-scale sheathed antiforms, representing lateral extensions of the mineralized Cuiaba-Lamego BIF Unit, may occur as subsurface expressions that could be reached by diamond drilling (say, 100-400m or even more, below the present level of erosion). Initial indirect indications of these hypothetical "blind" sheathed antiforms could (or should) be obtained, pnor to drilling, by geophysical surveys andor geological mapping programs aiming the identification of kilometer-scale domal structures in the SabarKaeté region. Care should be taken in interpreting magnetic surveys, as the BIFs and the BIF-hosted gold mineralization in the Cuiaba and Lamego deposits contain extremely insignificant concentrations of magnetic minerals (pyrrhotite and magnetite) and probably are zones of low magnetic susceptibility. Electromagnetic sweys could be used effectively in the search for such blind orebodies. Electromagnetic systems with superior resolution and depth penetration would be especially appropriate for this geological situation. CHAPTER 6 - SIJMMARY OF CONCLUSIONS

The most important conclusions and interpretations made during the course of this study are the following:

Gold Mineralization - Description

A) The "ore-grade" mineralized bodies (average gold grade t 3.0 g/t) in the Lamego eye-shaped structure are confined either to the Larnego Banded Iron-Formation or the Pele de Onça Chert Unit. Economic gold grades in BIF and chert sarnples are usually related to the presence of sulphide minerals, which are essentially pyrite with subordinate amounts of arsenopyrite. A direct and positive relationship is commonly established between the amount of sulphides and the gold grades in individual mineralized BiF and chert samples. B) More than 95% of the electnim grains detected in ore-grade sulphide-facies BIF samples fiom the Lamego deposit using an optical microscope are included within pyrite crystals. They were very rarely observed as inclusions within arsenopyrite, at sulphide grain boundaries, or associated with gangue minerals. The electrum grains commonly varied £kom less than 1 micron to 60 microns in diameter and showed a gold fineness varying fiom 910 to 957. C) The ore-grade mineralized bodies in the Lamego Banded Iron-Formation are scattered, stratiform lenses of sulphide-facies BIF that Vary fiom 15-20 meters to 80-1 20 meters in strike length and fiom less than a meter to 4 or 5 meters in thickness. Their average gold grades Vary fiom 3.0 g/t to 17.0 g/t. Ore-grade mineralized bodies in the Pele de Onça Chert Unit occur as thin stratiform lenses of sulphide-bearing chert (from less than a meter to 2.5 meters in thickness) with a maximum strike length of 50-60 meten. These tend to occur in the lowermost stratigraphie portion of the unit, at (or close) to the contact with the Larnego Banded Iron-Formation. Their average gold grades vary from 5.0 to 10.0 g/t.

Hydrothermal Aiteration and Gold Mineralization

D) Hydrothemal alteration in the Lamego structure was observed only in the footwall sequences (Footwall Volcanic Unit) of the auri ferous volcano-chemical sedirnentary rocks (Lamego BIF and Pele de Onça Chert Unit). These "footwall" hydrothermal alteration zones are, in relation to the BWchert wiits, semi-conformable (stratabound) "sheets" showing a minimum vertical extension of 200 meters. Hydrothermal alteration in the Cuiaba structure, however, is observed both in the footwall and hanging wall rock sequences of the Cuiaba BE. E) In the Lamego deposit, highly altered domains of the Footwall Volcanic Unit located beneath mineralized sulphide-facies BIF sequences are characterized by "proximal" alteration zones represented by the indicator minera1 assemblage white mica + ankerite + siderite t chlonte. Proximal alteration zones grade vertically and laterally to propylitic "distal" alteration zones typically represented by the indicator minera1 assemblage clinozoisite/epidote + chlonte + calcite i actinolite. Ideally, "barren" domains of the Lamego Banded Iron-Formation are underlain by propylitic alteration zones. F) Some of the important systematic changes in chemistry in rocks of the Foohwall Volcanic Unit of the Larnego deposit during the progressive hydrothermal alteration/metasomatism include depletions in Na20, A1203,Si02, "base metal" elements (Cu, Zn, Ni, and CO), and rare-earth elements, as well as additions in CO2, KzOand Cao. G) Flattened and partially transposed "stockwork-like" phyllosilicate-rich microveinlets and carbonate-rich veinsheinlets were observed in highly altered zones of the Footwall Volcanic Unit underlying ore-grade zones at the Lamego deposit. These features/stmctures are interpreted in this study as evidence of the volcanogenic/epithemal nature of the mineralizing hydrothermal alteration processes. FI) The chens of the Pele de Onça Chert Unit in the Lamego deposit occur not only as a "capping" stratabound/tabular horizon that ranges in thickness from several decimeters to 4 or 5 meters, but also as discordant, "feeder-like" veidveinlet systems that are oriented perpendicular or sub-perpendicular to bedding surfaces and "merge" with the stratabound chert unit in its lowermost stratigraphic portion. Carbonate-rich bands of carbonate-facies BEs commonly show sulphide-e~chedalteration/replacement halos that are symrnetrically distributed around these chert veins/veinlets. The Pele de Onça Chert Unit is interpreted in this thesis as a self-sealing cap, similar to the sinter cappings and terraces associated with modem epithermal gold deposits and the silicified tufllchert layers which comrnonly cap massive sulphide orebodies.

Gold Mineralization - Genetic Aspects

r) The polarization of the discussion and interpretation of the genetic aspects of the BE-hosted gold deposits of the Nova Lima Group between "syngenetic" volcano- exhalative hypotheses and "epigenetic" metarnorphogenic/syntectonic shear zone-related models has to be questioned and possibly revised. It is inferred in this thesis, based on some cntical geological features observed in the Larnego deposit such as stratigraphic setting, pattern of the hydrothemal alteration zones associated with the mineralization, and relative timing of the replacement-type mineralization, that the bulk of the BIF- hosted gold mineralization in the Larnego deposit is both "syn-volcanic" and "epigenetic" in nature. J) The Larnego and Cuiaba BIF-hosted gold deposits appear to represent an interesting type of volcanogenic hydrothermal gold deposit bearing some characteristics typical of gold-rich VMS deposits.

Stratigraphic Setting and Regional stratigraphic Positioning

K) Similar to other BIF-hosted gold deposits in the Nova Lima Group, the Lamego Banded iron-Formation and the Pele de Onça Chert Unit are located at the stratigraphic transition between a lower sequence composed of altered basic volcanic rocks (Footwall Volcanic Unit) and an upper mixed unit composed of epiclastic and volcaniclastic sedimentary rocks (Upper EpiclasticNolcaniclastic Unit). L) The stratigraphic sections of the Lamego and Cuiaba deposits can be comelated fkom their lowermost sequences (Footwail Volcanic Unit) to the unit of carbonaceous phyllites and graphite schists that overlies the BIFs and cherts (Black Carbonaceous Phyllite Unit). However, there are 2 significant differences between these sections, narnely the unique presence of the hanging wall volcanic sequence as well as the much thicker development of the mine stratigraphic sequence (BiFs + cherts + graphite schists) at the Cuiaba deposit. M) The basic differences between the Lamego and Cuiaba lithostratigraphic sections mentioned in "Ln appear to indicate that the paleogeographic position of the Cuiaba deposit was more proximal to an Archean volcanic center(s) than that of the Lamego deposit. In addition, the hydrothemal system at Cuiaba appears to have been more potent and more active over a longer period of time than the hydrothemal system at Lamego. N) The Lamego and Cuiaba BIF nits are interpreted in this study as located at (or very close to) the stratigraphic boundary between the "Middle Unit" and the "Upper Unit" of the Nova Lima Group. The basic volcanic rocks of the Footwall Volcanic Unit. mainly on the basis of their geochemical composition (TH2 tholeiites with calc-alkaline affinity), are tentatively correlated with the uppermost stratigraphic portion of the Middle Unit of the Nova Lima Group. The Upper EpiclasticNolcaniclastic Unit is composed essentially of lithologic associations that typiw the Upper Unit of the same group.

Tectonic Structures and Structural Controls

O) The Lamego and Cuiaba eye-shaped structures are interpreted as "tectonic inliers" in which the uppermost stratigraphic portion of the Middle Unit of the Nova Lima Group is surrounded by younger rock sequences of the Upper Unit. These structures are also interpreted as the surficial expressions of highly stretched kilometer-scale sheathed overtumed antifoms elongated parallel to the L, regional stretching lineation trend and also to the regional tectonic transport direction associated with the Transamazonian orogeny in the Quadri latero Fem' fer0 district. P) "Roofs" or hinge zones of other kilometer-scale sheathed antiforms, representing lateral extensions of the mineralized Cuiaba-Lamego BIF Unit, may occur as subsurface expressions that could be reached by diarnond drilling (say, 100-400m or even more, below the present level of erosion) in the SabarKaeté region. Initial indirect indications of these hypothetical "blind sheathed antiforms could (or should) be obtained, pnor to drilling, by geophysical surveys and geological mapping programs aiming the identification of kilometer-scale dornal structures in the Sabar3Caeté region. Q) in the Lamego structure, the stratiform BIFIchert-hosted rnineralized lenses and bodies, as well as their associated stratabound alteration zones, clearly pre-date the penetrative Transamazonian deformation, as they are folded, overprinted, and sometimes transposed by Transamazonian structures. Introduction and/or remobilization of suiphides and gold into Transamazonian structures and penetrative stmctural fabrics were not observed in the Lamego deposit during this study. R) The stratiform mineralized bodies of the Lamego and Cuiaba deposits are "smicturally and spatially controlled". The attitude (trend and plunge) of the orebodies mirnics the orientation of the linear fabric developed in the Quadrilatero Fem'fero district during the Transamazonian orogeny. Since these mineralized bodies are congruently folded and defonned with their host units and associated stratabound "footwall" alteration zones, as well as with the entire stratigraphie section of these deposits, they also plunge with the same attitude as the plunge/eIongation of the kilometer-scale Lamego and Cuiabii sheathed overturned antiforms. S) Structures and textures of Archean age indicating intraformational tectonic brecciation or mylonitization of the Lamego Banded Iron-Formation were not observed in mineralized zones of the Lamego structure.

T) Archean planar fabrics associated with the Rio das Velhas orogeny (S, surfaces) are less obvious than the Transamazonian S2 planar fabric in rocks of the Lamego structure. S, surfaces were observed and described in this research only in incompetent phyllosilicate-nch rocks (usually hydrothemdly altered), where they occur as a penetrative schistosity which is invarïably concordant with the primary S, bedding and overprinted, folded and transposed by Transamazonian fabrics and stmctures. REFERENCES

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Siiicates and Carbonates

The mineral chemisw data of individual silicate and carbonate minerals reported in this study were completed by Miircio Sales on the ARL-SEMQ electron microprobe in the Department of Geological Sciences at Queen's University. Nine element analyses (Si, Ti, Al, Fe, Mg, Mn, Ca, Na, and K) were made in al1 microprobe work using the energy dispersive technique (EDS).The operating conditions utilized were the following: - an accelerating voltage of 15 KV. - a nominal beam current of 40 nA. - an idealized beam diameter of 2 Fm (white micas and plagioclases were scanned to avoid significant losses of Na and K). - an EDS spectra collection time of 200 seconds. The standard procedure for analyses of silicate and carbonate minerals at Queen's University includes corrections for matrix effects using the method of Bence and Albee (1968) utilizing the correction coefficients of Albee and Ray (1970) as modified by Dr. Peter Roeder fiom Queen's University (Penczak, 1996). Sarnples of standards were systematically analyzed during the course of the microprobe work in order to assure an adequate level of precision and accuracy of the results. The chemical composition of silicates and carbonates presented in this appendix are mathematical averages (mean) calculated after many individual analyses carried out on numerous crystals of several representative samples of each lithologic group of interest. Electrum grains

The range and average of the gold fineness of electm grains contained in mineralized sulphide-facies BIF sarnples fiom the Lamego deposit presented in this thesis (see Figure 4.23) are based on results of analyses completed by Alan Grant on the ARL- SEMQ electron microprobe in the Department of Geological Sciences at Queen's University. Gold and silver analyses were made in al1 microprobe work using the ener, dispersive technique (EDS). The operating conditions utilized were the following: - an accelerating voltage of 15 KV - a nominal beam current of 40 nA - an idealized beam diameter of 2 pm - an EDS spectra collection tirne of 60 seconds The standard procedure for analyses of electnim grains at Queen's University includes ZAF correction and the utilization of severai NBS AdAg ailoy wires as standards in order to assure an adequate level of precision and accuracy of the results. Depending on the grain size, between two and five analyses were made on the cores and nms of each electnim grain. No relevant variation or zonation in the AdAg ratio was observed in individual grains investigated. Table A.1 - Mineral Chemistry Data (Average and Standard Deviation) chernical composition of some rock-forming minersls fmm schlsts of the Footwall Volcanic Unit 1 i 1L TYMI-. Alteration Facies 1 dinozoisite (12) chlorite (10) plagiodase (8) calate (1 0) WT % (S. dev.) SiO2 38.93 (0.35) 28.1 (0.46) 73.27 (0.27) 0.34 TiOz 0.06 0.03 0.00 0.00 Al203 27.42 (1.35) 20.89 (0.66) 21.O0 (0.18) 0.34 Total Fe 7.50 (1-64) 18.1 (1.32) 0.31 0.40 (0.11) Mg0 0.07 21.36 (0.97) 0.00 022 (0.08) Mn0 0.10 0.38 0.00 0.59 (0.08) Ca0 24.55 (0.51) 0.1 1 0.53 (0.07) 57.16 (1.92) NazO 0.02 0.07 6.93 (0.44) 0.04 K20 0.01 0.02 0.00 0.00 total (without H90* or CO4 98.66 89.06 102.04 59.09 -- -

Type 2 Aiteration Facies chlorite (15) calab (10) plagiocase (10) white mica (8) \KT % (S. dev.) Si02 26.25 (0.81) 0.33 70.36 (1.26) 47.09 (1.90) Ti02 0.00 0.00 0.00 0-44 &203 22.8 (2.01) 0.35 20.13 (0.43) 32.89 (0.89) Total Fe 21.28 (0.99) 0.82 (0.20) 0.23 2.11 (0.34) &IO 16.76 (0.92) 0.43 (0.1 5) 0.00 1.09 (0.52) Mn0 0.25 0.51 (0.11) 0.00 0.03 Ca0 0.08 56.19 (1.n) 0.39 (0.23) 0.59 0.00 Na& 0.04 9.88 (0.62)-. 0.49 r K~O 0.03 0.00 0.00 8.50 (0.50) total (without H20* or CO2) 87.45 58.67 100.99 93.23

Tvm 3 Alteration Facies 1 .- - 1 ankente (IO) chlorite (10) white mica (16) plagiodase (2) WT % (S. dev.)

Ca0 27.13 (0.61) 0.18 0.13 0.20 (0.04) Na20 0.00 0.17 2.56 (1.27) 8.65 (0.73) K20 0.01 0.00 4.46 (2.561 0.00 total (without HzO' or COz) 55.47 87.53 93.59 99.51 I Type 5 Alteration Facies ankefite (10) white mica (18) siderite (10) chlorite (10) plagioclase (5) WT % (S. dev.) SiO, 0.52 48.51 (1.061-. 0.49 24.6 Il. .O1 -) 69.88 (0.26)- - TiO2 0.00 0.16 0.00 0.01 0.00 N203 0.48 36.83 (1 -61) 0.45 22.71 (0.96) i9.n (o.la) Total Fe 13.37 (0.73) 0.77 (0.31) 37.89 (0.94) 27.06 (0.63) 0.52 Mg0 13.55 (0.33) 0.42 (0.42) 16.95 (0.72) 12.47 (0.34) 0.00 Mn0 0.45 (0.10) 0.00 0.65 (0.08) 0.1 3 0.00 Ca0 27.51 (0.69) 0.16 0.31 (0.14) 0.12 0.19 Na20 0.12 4.23 (1.31) 0.03 0.00 8.65 (0.62) K2O 0.02 3.49 (2.66) 0.00 0.01 0.00 totaf (without H30' or C02) 56.02 94.63 56.n 87.1 1 99.01 siderite (18) - number of crystals analysed. Microprobe analyses by Marcio Sales at Queen's Univei Table A.2 - Mineal Chernistry Data (Average and Standard Deviation)

chemical composition of carbonates from BlFs of the Lamego Banded Iron-Formation I ankerites and siderites from "banen" carbonate-facies BlFs ankerite (20) siderite (20) WT % (S. dev.) Si02 0.55 0.62 1102 0.00 0.00 &203 0.36 0.43 Total Fe 24.05 (126) 52.87 (1.39) Mg0 3.94 (0.59) 2.95 (0.96) Mn0 1.91 l0.34) 1.80 (0.47) Ca0 25.49 (0.72) O.%-(0.27) Na20 0.00 0.00 K20 0.01 0.01 I total (without CO2) 56.20 59.34 I ankerites from "minealized* sutphide-fades BlFs Queirnada-t.vDe (20) C. de Pedra/A. da Velha-h/De (201 WT % (S. dev.) Si02 0.1 1 0.27 TiOn 0.00 0.00 &zO~ 0.34 0.30 Total Fe 16.8 (2.1 6) 18.98 (2.04)

Mn0 1.79 (0.32) 2.28 (0.36) Ca0 26.32 (1 .OS) 25.6 (0.36)

- I total (withaut CO2) 55.49 54.86 1

siderite (20) - number of crystals analysed. Microprobe analyses by Marcio Sales at Queen's University. IMAGE EVALUATION TEST TARGET (QA-3)

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