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

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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 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 The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/film, de reproduction sur papier ou sur format électronique. 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 Brazil(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 Archean 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 pyrite, 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 mineral assemblage white mica + ankerite f sidente + chlorite, and cornrnonly show flattened and partially transposed "stockwork-like" phyllosilicate-rich microveinlets and carbonate-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 + calcite + 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, nickel 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
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