Please use this identifier to cite or link to this item: http://repositorio.unicamp.br/jspui/handle/REPOSIP/287722
Type: TESE
Degree Level: Doutorado
Title: Genese dos depositos auriferos em metaconglomerados da formação moeda, Quadrilatero Ferrifero, MG : o papel do metamorfismo e associação com a materia carbonosa
Title Alternative: Genesis of the metaconglomerate-hosted gold deposits of the moeda formation, Quadrilatero Ferrifero (State of Minas Gerais) : role of the metamorphism and the association with carbonaceous matter
Author: Pires, Paulo Fernando Ravacci
Advisor: Xavier, Roberto Perez, 1958-
Abstract: Resumo: Os metaconglomerados auríferos da Formação Moeda, Supergrupo Minas, encontram-se em porções basais dessa formação, em contato com litotipos do Supergrupo Rio das Velhas. Os metaconglomerados mineralizados formam estreitos horizontes, todavia com teores de ouro elevados, normalmente sotopostos por planos de cisalhamento tangenciais ao acamamento. Os horizontes que hospedam a mineralização aurífera consistem de metaconglomerados suportados por clastos ou matriz arenosa de granulação média; os clastos incluem seixos de quartzo de veio, com dimensões centimétricas, clastos angulosos de metachert e BIF, além de metavulcânicas ácidas. Características consideradas comuns nestes metaconglomerados incluem a presença de pirita arredondada, com ou sem inclusões de material carbonoso, comumente mostrando sobrecrescimento de pirita de segunda geração, pirofilita e, localmente, clorita. Adicionalmente também se observa que os horizontes mineralizados apresentam maior concentração da matéria carbonosa. Estas características se assemelham aos horizontes mineralizados de Witwatersrand, na África do Sul. A formação dos filossilicatos hidratados e da pirita de sobrescimento é atribuída à percolação de fluidos hidrotermais, em momento de pós-pico metamórfico, durante o desenvolvimento de zonas de cisalhamento subparalelas ao acamamento. O geotermômetro da clorita indica temperaturas em torno de 3000 C para a formação desses minerais. A matéria carbonosa relacionada à mineralização aurífera mostra-se com baixo grau de cristalinidade, de acordo com os estudos de microespectroscopia Raman. Sua distribuição e concentração encontram-se diretamente vinculadas a porções onde os metaconglomerados da base da Formação Moeda (unidade I) apresentam menor espessura, ou seja, em paleo-canais relacionados a altos estruturais. Diferentes tipos de pirita foram observados nos horizontes mineralizados, dentre os quais se destacam a pirita de sobrecrescimento, pirita arredondada compacta, pirita arredondada porosa e pirita euhedral

Abstract: The Quadrilátero Ferrífero has historically been one of the most important gold province in Brazil, with a significant number of productive greenstone-hosted lode gold deposits within the Rio das Velhas Supergroup together with those associated with the metasedimentary sequence of the Minas Supergroup. At the base of the Moeda formation, Minas Supergroup, particularly at the contact with rocks of the Rio das Velhas Supergroup, there are several medium to small size, high grade gold deposits hosted by metaconglomerates, which bear similar features with the world-class Witwatersrand gold fields in South Africa. The gold mineralization forms narrow horizons within the metaconglomerates, generally above bedding-parallel shear planes. The host metaconglomerate_ consist of different proportions of clasts of centimeter size vein quartz, metachert, BIF and felsic metavolcanics, immersed in an arenaceous matrix of medium grain size. A higher concentration of carbonaceous matter, the presence of rounded pyrite, with or without inclusions of carbonaceous matter and overgrowths of later pyrite, pyrophyllite and, locally, chlorite are the main characteristics of the gold-bearing horizons in the metaconglomerates. Pyrophyllite, chlorite and the overgrowth pyrite are interpreted as the result of hydrothermal fluids which circulated along the bedding parallel shear zones, after the peak of metamorphism. The chlorite geothermometry yielded 300°C as an estimate for the precipitation of these hydrothermal minerais and gold mineralization. The ore-related carbonaceous matter displays a low degree of cristallinity, as demonstrated by Raman micro-espectroscopy analysis, and shows more significant concentrations where the Moeda Formation metaconglomerates are thinner (e.g. Unit I), that is, in paleo channel related to structural highs. Pyrite overgrowth, rounded compact pyrite, rounded porous pyrite and euhedrical pyrite are the most frequent types of pyrite observed in the mineralized reefs, in which trace element analyses and sulphur isotopic studies were undertaken. The average concentrations of As (O, 13at%) and Ni (0,08at%) in the different types of pyrite are generally higher than the values of these elements obtained in pyrite from gold deposits (As=0,07at%; Ni=0,007at%) of the Rio das Velhas Supergrupo, considered as a source area from the metaconglomeratic pyrites. Additionally , d34S values ranging from -13%0 to -20%0 indicate a diagenetic origin for the rounded pyrite of the Batatal Formation, implying that being rounded is not a necessary pre-requisite to support a detritic origin. The porous pyrite rich with inclusions of carbonaceous matter shows d34S values between 2%0 and 4 %O, close to those d34S values, yilded by the pyrite yielded by the pyrite from the Rio das Velhas Supergroup gold deposits. Nevertheless, the former contains higher concentrations of As and Ni than the latter, whose values rarely exceed 0,23at% for As and tclose to O at% for Ni. The compact pyrite in the mineralized reefs displays d34S values ranging from -6%0 to 9%0. Reasons to explain this large d34S interval include (1) a detritic origin of the pyrite, with the large 034S variation attributed to the existence of different source areas (Hallbauer,1986; England et aI., 2002); (2) bacterial reduction of the seawater S04 to H2S (Ohmoto et aI., 1993; Strauss e Beukes, 1996); (3) mixing of oxidizíng (S04-bearing) and reducing (H2S-bearing) fluids during diagenesis or hydrothermal event. Alternative (1) implies that the atmosphere was reducing in the Archean, whereas an oxidizing nature for the Archean atmosphere has to be considered for (2) and (3). Gold occurs mainly confined to fractures, together with pyrophyllite, in rounded pyrite, in paragenesis with pyrophyllite, chlorite and skeletal pyrite (possible replacement of the carbonaceous matter), and as inclusions or fractures in overgrowth pyrite. These modes of occurrence of the gold, together with its absence in rounded compact pyrite, suggest that part of the mineralization was epigenetic and linked to the influence of hydrothermal fluids. Thus, shear zones, carbonaceous matter and the presence of conglomerate layers rich in Fe-Mg minerais seem to have played an important role in the precipitation of the gold by hydrothermal fluids. In this context, the gold occurrences and deposits hosted by the Moeda Formation metaconglomerates are interpreted as paleoplacer deposits, highly modified by hydrothermal processes along shear zones, or typical shear zone-hosted gold deposits. Gold may have been remobilized both from the host rock and from rocks of the Rio das Velhas Supergroup. Considering the depositional aspects related to the metaconglomerates, the conspicuous occurrence of carbonaceous matter in the ore zones, the presence of bedding-parallel shear zones and the similarities with Witwatersrand, possible potential areas to contain conglomerate-hosted gold mineralization are the southern sector of the Gandarela Syncline and western sector of the Ouro Fino Syncline
Subject: Ouro - Quadrilátero Ferrifero (MG)
Metamorfismo (Geologia) - Quadrilátero Ferrifero (MG)
Quadrilatero Ferrifero (MG) - Ouro
Language: Português
Editor: [s.n.]
Citation: PIRES, Paulo Fernando Ravacci. Genese dos depositos auriferos em metaconglomerados da formação moeda, Quadrilatero Ferrifero, MG: o papel do metamorfismo e associação com a materia carbonosa. 2005. 209p. Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Geociencias, Campinas, SP. Disponível em: <http://www.repositorio.unicamp.br/handle/REPOSIP/287722>. Acesso em: 4 ago. 2018.
Date Issue: 2005
Appears in Collections:IG - Tese e Dissertação

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