Aspectos tectono-estruturais do "Greenstone Belt" de Piumhi, MG em relação a mineralização de cromita e ouro

AUTOR(ES)
DATA DE PUBLICAÇÃO

1996

RESUMO

The Piumhi greenstone belt of Archaean age in SW Minas Gerais State, Brazil, is located within the limits of the Tocantins and São Francisco provinces of Almeida et aI. (1981). This belt is made up of a lower volcanic sequence , the Ribeirão Araras Group, consisting of komatiites, tholeiites, acid and intermediate volcanic rocks, overlain by metasediments of the Paciência and Lavapés Groups. Banded iron formation and chromite in ultrabasic rocks form a part of the latter succession. Gold mineralization, although of minor importance, occurs in the lower stratigraphic unit - the Ribeirão Araras Group. Regional structures that affected the greenstone belt are an initial oblique, tangentiallow-angle thrust with south to north movements, and later transcurrent SE-NW faults related to the Capitólio Shear Zone (ZCC). Gold mineralization is associated with the latter deformation event, which was responsible for hydrothermal alteration of host volcanic rocks in local shear zones and formation of mineralized quartz-carbonate-sericite veins. Geochemical analyses of altered rocks reveal an increase in silica and potash and a concomitant decrease in MgO, CaO, TiO2 and MnO during this process. On the other hand, the lenses and pods of chromite in intensely sheared ultrabasic rocks of the Lavapés Group may owe their emplacement to the first tangential movements and accompanying deformation. Platinum Group Elements in the chromite indicate a possible ophiolite affinity ofthe enclosing ultrabasic rocks, and the chromite is tentatively classified as podiform

ASSUNTO(S)

ouro geoquimica cromita geologia estrutural - piumhi (mg)

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