ORE FLUID COMPOSITION AND GENESIS OF THE MORRO AGUDO ZN-PB DEPOSIT, CENTRAL BRAZIL

The Morro Agudo deposit is an originally ~18 million tonne Neoproterozoic carbonate-hosted epigenetic ore body averaging 6% Zn and 2% Pb. Morro Agudo is the principal example of the sulfide end member of mineralization in the Vazante-Unaí Trend, the largest accumulation of Zn-Pb mineralization in Brazil. The Vazante-Unaí Trend also contains a non-sulfide end member of mineralization consisting mainly of willemite, the principal example of which is the Vazante deposit (originally ~29 million tonnes averaging 18% Zn). The present study sought to characterize the composition of sphalerite-hosted fluid inclusions in the Morro Agudo deposit in order gain insights about the deposit’s formation. Fluid inclusions were analyzed by microthermometry and LA-ICP-MS. The fluid inclusions were found to have highly variable salinities ranging from 6 to 24 equivalent weight percent NaCl that do no correlate with the wide range of homogenization temperatures of about 100 to >275° C. The results suggest mixing between a brine and more dilute fluid, each with uncertain temperatures, perhaps because of post-entrapment heating and partial thermal re-equilibration of fluid inclusions caused by the 630-610 Ma Brasiliano orogeny. The brine appears to have had higher Ca/Na and Mg/Na ratios whereas the more dilute fluid appears to have had higher Ba/Na ratios. The Morro Agudo fluid inclusions analyzed did not have detectable concentrations of ore metals. Detection limits for Pb ranged from 1’s to 10’s of ppm, and for Cu from 10’s to 100’s of ppm, though both metals are abundant in the sphalerite mineral matrix. This suggests that unless the ore fluid had very high concentrations of Zn, it could have transported high concentrations of sulfide with the ore metals allowing sulfide mineral precipitation to have occurred by cooling, pH increase, or dilution. Morro Agudo fluid inclusions have low Ca/Mg ratios, consistent with the extensive dolomite alteration and cementation in the deposit and the likely flow path of the ore fluids through dolomite rocks. Morro Agudo fluid inclusions also have high K/Na ratios compared to fluid inclusions in carbonate-hosted Zn-Pb deposits in the U.S. Ozark Plateau and in Ireland, which may be derived from abundant pelitic facies in the Morro Agudo host rocks.