FLUID INCLUSION AND ZINC ORE MINERAL GEOCHEMISTRY OF THE VAZANTE DEPOSIT, BRAZIL

Large hydrothermal non-sulfide zinc (NSZ) deposits, i.e. zinc deposits in which willemite dominates over sphalerite, have been found in several sedimentary basins around the world. Unlike other sediment-hosted base metal deposits, particularly Mississippi Valley-type (MVT) deposits, little is known about the fluids that precipitated NSZ deposits. The present study used LA-ICP-MS and microthermometry to characterize the composition of fluid inclusions hosted by quartz and sphalerite from the Vazante deposit in central Brazil, the world’s largest NSZ deposit. Primary fluid inclusions hosted by willemite were not found.

All fluid inclusions analyzed had homogenization temperatures ranging from about 100 to 170° C. In quartz-hosted fluid inclusions, total dissolved solids (TDS) contents ranged mostly from about 120,000 to 160,000 ppm, with a smaller set ranging from about 65,000 to 110,000 ppm. In sphalerite-hosted fluid inclusions, TDS contents ranged mostly from about 160,000 to 180,000 ppm. None of the fluid inclusions contained Zn and Cu above their detection limits of 10’s to 100’s of ppm. Ba was rich in sphalerite-hosted fluid inclusions, averaging about 1700 ppm, but poor in quartz-hosted fluid inclusions, averaging about 50 ppm. Conversely, Pb was detected in many quartz-hosted fluid inclusions, in which it averaged about 200 ppm, but occurred below its detection limit of 1’s to 100’s of ppm in sphalerite-hosted fluid inclusions. Atomic K/Na and Ca/Na ratios were similar in quartz- and sphalerite-hosted fluid inclusions, averaging about 0.12 and 0.04, respectively. However, Sr/Na ratios were much higher in sphalerite-hosted than quartz-hosted fluid inclusions (0.006 vs. 0.0009). Mg never occurred above its detection limit of 100’s to a few thousand ppm. Mineral matrix analyses showed sphalerite to be relatively rich in Cd and Pb but poor in other typical trace elements such as Mn, Fe, Cu, and Ge. Willemite contains a broad array of trace elements, with Fe, Pb, As, and Cd existing in the highest concentrations, respectively. Both sphalerite and willemite contain up to several ppm Ag.

Vazante ore fluids have TDS contents resembling basinal brines, but are more dilute than most MVT ore fluids. Vazante ore fluids are also more enriched in K and depleted in Ca and Mg compared to most basinal brines and MVT ore fluids.