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Processo mineralizador e modelo genético da Mina de Ouro Schramm (Sc-Brasil)

2002; Sociedade Brasileira de Geologia; Volume: 32; Issue: 4 Linguagem: Inglês

10.5327/rbg.v32i4.999

2317-4889

João Carlos Biondi, Nilton D. Franke, Paulo R. S. de Carvalho, S. Villanova,

Clay minerals and soil interactions

The primary ore fluid of Schramm Mine was aqua-carbonic, SiO 2 , CO 2 , Fe, Mg and Ca rich, with Cl, Na, K, very few sulfur, without nitrogen and methane, with Au, Ag, Ni, As and minor proportions of Zn and Co. Probably it collected Au, Ni and As from ultramafic rocks, while rising to the surface. Comparison of rare earth element models suggests that, during the formation of Schramm’s deposit, hydrothermal fluids reacted with ultramafic granulitic rocks. Primary fluid salinity varied between 0,2% and 14 wt % NaCl equiv. The use of the chlorite geothermometer and the relations of these temperatures with isochors deduced from microthermometric studies of fluid inclusions, allowed to the conclusion that siderite-ankerite and quartz veins crystallized at temperatures between 227o and 316oC and pressures between 1.35 and 0.1 kb. Into the shear zone, among granulites, the fluid precipitates carbonates, Au, Ni-As sulfides and altered rocks at temperatures from 227o to 316oC and pressures from 0.80 to 1.20 Kb (1.5 to 4.0 km). Precipitation formed carbonates veins, mineralized with gold and sulfides. It occurred especially in places with high pH, where fluid reacted with serpentinized rocks. Shear zone fracturing occurred at about 316oC, reducing pressure to about 0,1 Kb and changing pressure regime from lithostatic to hidrostatic. Meteoric water invaded the mineralizing system very fast. The consequence was the decreasing of fluid temperature to less than 227oC, the reduction on silica solubility and a bulk quartz precipitation, filling new voids opened in host rocks and also in carbonate veins. Therefore, the genetic model of Schramm gold deposit is a variant of classical “gold in shear zone deposit model” or “orogenic type deposit”, formed under low pressure and temperature conditions, equivalent to weak-incipient metamorphic grade. Differences are highlighted by changes in composition of mineralizing fluid, occurred during its rise to surface, and by the ore metals precipitation, caused not only by reactions between fluid and Fe rich host rocks, but also due the increase of the environment pH.