A mina de ouro do Faria, Minas Gerais: um deposito estratiforme associado a um complexo vulcano-sedimentar

João Batista Moreschi

Resumo


The Faria gold mine, here interpreted as a stratiform deposit associated to a volcano-sedimentary complex, is located in the south of Nova Lima, State of Minas Gerais, Brazil, within the well known Quadrilátero Ferrífero area. Lens of carbonate facies iron formation, which is part of a metassedimentary sequence. Graphitic phyllite is a rock commonly associated with the iron formation beds. Two metavolcanic sequence of basic affiliation delimite, both in the base and in the top, the metassedimentary sequence where locally occur tuffaceous rocks. The mineralized zones are biologically controlled by the carbonatic facies of the iron formation, where the sulphide minerals are preferentially distributed in the carbonatic rich horizons, resulting in a banded structure to the ore. Local tectonic deformations indicated by breccia and shearing zones are interpreted as the probable cause of the formation of the massive ore. The main opaque minerals in the ore are pyrite, pyrrhotite and arsenopyrite in that order, with less amounts of chalcopyrite, cubanite, sphalerite, galena, tetrahedrite, magnetite, ilmenite, hematite and native gold .It is suggested a primary sedimentary concentration of the ore, including the gold, which was simultaneous to the deposition of the chemical sediments of the iron formation. This hypothesis is based in local and regional aspects of the geology observed in the district and in the geochemical properties of the gold, according to the experimental results which has been published about the subject. As a result of this postulation, the iron formations within the Nova Lima Group appear to be the regional stratigraphic and paleogeographic controls of possible mineralized zones for gold. The knowledge which we have at the present do not permit to conclude that the basic volcanism was the only source of the ore, although it could have been an important process which caused the enrichment of the ore materials in the sedimentary environment. Tectonic deformation and metamorphism of the previously enriched sediment may be responsible for some of the epigenetic aspects of the ore.

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