ORIGIN OF NON-SULFIDE ZN AND SULFIDE ZN-PB DEPOSITS IN THE VAZANTE-PARACATU-UNAÍ TREND, BRAZIL: EVIDENCE FROM FLUID COMPOSITION FOR CONTRASTING STYLES OF MINERALIZATION

The Vazante-Paracatu-Unaí Trend contains the largest Zn and Pb deposits in Brazil. The trend is characterized by strongly contrasting styles of mineralization consisting of a Zn-Pb sulfide deposit type in the north dominated by sphalerite and galena, and a hypogene non-sulfide Zn deposit type in the south dominated by willemite. Both styles of mineralization are hosted in Proterozoic shaley dolostones. Fluid inclusions hosted by sphalerite from two Zn-Pb sulfide deposits (Morro Agudo and Fagundes) and one hypogene non-sulfide Zn deposit (Vazante) were analyzed using microthermometry and LA-ICP-MS to seek evidence about the origin of the two deposit types. Fluid inclusions in both deposit types were found to have similarly highly variable salinities, ranging from about 1 to 23 equivalent weight percent (eq. wt. %) NaCl for the sulfide mineralization and from about 5 to 22 eq. wt. % NaCl for the non-sulfide mineralization. Homogenization temperatures for both deposit types also vary widely from about 100 to 240° C in the sulfide mineralization and about 50 to 350° C in the non-sulfide mineralization. Both deposit types also contain some liquid-only fluid inclusions that likely were trapped at less than about 50° C. Fluid inclusions in both deposit types have similar proportions of Na, Mg, K, Ca, Sr, and Ba, the elements that were consistently measurable by LA-ICP-MS. Further, these elemental concentrations are consistent with those in typical modern sedimentary brines, except for K, which is higher in the fluid inclusions. Pb and Cu were not detected in fluid inclusions in the sulfide deposits, and Zn was not detected in fluid inclusions in either deposit type because of interferences from the host sphalerite matrix. Pb was detected in fluid inclusions in the Vazante non-sulfide zinc deposit, but not consistently within fluid inclusion assemblages, suggesting that the Pb was heterogeneously entrapped as accidental mineral inclusions. Ore fluid Zn concentrations may be quantifiable in future studies from Zn concentrations in dolomite based on partitioning theory. Both deposit types appear to have formed from mixing of at least three fluids of variable salinity and temperature, where the resultant fluid mixture for each deposit type had similar elemental concentrations of Na, Mg, K, Ca, Sr, and Ba.