Geology, mineralogy, and geochemistry of the Vazante Northern Extension zinc silicate deposit, Minas Gerais, Brazil.

摘要

The Vazante Northern Extension is a continuation of the structurally controlled, hypogene nonsulfide zinc deposit from the Vazante Mine, Minas Gerais, Brazil. The deposit is hosted in Neoproterozoic carbonates of the Serra do Poco Verde Formation located in the Brasilia Fold Belt along the western margin of the Sao Francisco Craton. The Northern Extension deposit is hosted within a shear zone that strikes 200 to 225 and dips 30°-70° NW and is associated with substantial folding that occurred in the same deformation (D2) event and later transcurrent faulting. The shear zone is composed of a tectonic-hydrothermal breccia that can be subdivided into four types: Type 1 Breccia-- distally located from the willemite (Zn2SiO4) ore, weakly brecciated dolostone with some Fe-carbonate alteration; Type 2 Breccia--Strong Fe-carbonate alteration, intense brecciation; Type 3 Breccia--hematite replacement breccia, usually proximal to the Type 4 Breccia; Type 4 Breccia--willemite ore breccia, containing three generations of hypogene willemite (Zn2 SiO4) as well as hematite (Fe2O3), and franklinite (ZnFe2O4). In the southern part of the deposit, late sulfide-rich veinlets are observed cutting the hypogene zinc silicate mineralization, and contain galena, sphalerite, native silver, covellite, and stromeyerite (Cu1-xAg1-xS). Geochemical studies conducted on the tectonic-hydrothermal breccia showed a strong regional signature enriched in Ag, Cd, Pb, and Zn. The presence of willemite as opposed to sphalerite and the ubiquity of hematite suggest that the zinc silicate mineralization has formed from the mixing between a low sulfur, acid, oxidizing metalliferous basinal brine similar to the fluids found in MVT deposits with a cooler, more oxidizing, meteoric fluid.