The distribution and mineral hosts of silver in eastern Australian volcanogenic massive sulfide deposits

Silver, an important by-product in volcanogenic massive sulfide (VMS) deposits of eastern Australia, is enriched mainly in Zn-rich zones; in a few deposits it is enriched in Cu-rich zones. Minerals that contain significant amounts of Ag include galena, tetrahedrite and chalcopyrite. The contribution of Ag sulfosalts, native silver and Ag tellurides to the total Ag budget is generally small. The hosts of Ag in VMS deposits vary spatially as follows: (1) in Cu-rich zones, Ag occurs mainly in chalcopyrite or Pi-rich galena; (2) in overlying Zn-rich zones, Ag occurs mainly in galena and, to a lesser extent, tetrahedrite; (3) in barite-bearing zones, Ag occurs mainly in Ag-rich tetrahedrite. The geochemical factors that seem to influence the mineralogical distribution of Ag in VMS deposits include: (1) temperature, (2) the relative abundances of semi-metals in the mineralizing fluids, (3) fractional crystallization of tetrahedrite-tennantite minerals, and (4) redox conditions during ore deposition. Higher temperatures and more reduced conditions favor partitioning of Ag into chalcopyrite, and then galena. Silver partitions into tetrahedrite under lower temperature, oxidized conditions, assisted by fractional crystallization, hence enriching later-precipitated tetrahedrite in Ag and Sb.