Osmium, sulphur, and helium isotopic results from the giant Neoproterozoic epithermal Imiter silver deposit, Morocco: evidence for a mantle source

The giant epithermal Ag–Hg deposit of Imiter (Anti-Atlas, Morocco) was investigated to study the transfer of chalcophile elements from the mantle to superficial crustal levels. The origin of metals and ligands (sulphides) was deduced from S, fluid inclusions, He, and Re/Os data obtained on sulphide phases and gangue minerals. Sulphur isotope analyses show the existence of two distinct isotopic reservoirs, one found in pyrite from the surrounding black shale country rocks (δ34SCDT=−38‰) and the other in pyrites associated with a synchronous rhyolitic dome (δ34SCDT=−7 to −2‰). The δ34SCDT values of the silver mineralization event range from −28 to −2‰ and are interpreted as resulting from preferential degassing of SO2 in ascending fluids, as well as mixing between the magmatic isotopic reservoir and a country rock reservoir. Helium isotope analyses of sulphides and gangue minerals yield similar results, with 3He/4He ratios ranging from 0.76 to 2.64Ra. These data and the absence of 20Ne in the analyzed fluid inclusions suggest a mantle origin for the fluids associated with the epithermal silver event. Os isotopic ratios have been measured for the first time in Ag° and Ag–sulphosalts. These data and those obtained on other sulphide phases directly associated with the Ag mineralization show measured 187Os/188Os ratios of 0.142–0.197 indicating a dominantly mantle source for the associated Os. Assuming that Os and Ag are derived from the same source, the black shale country rock provided less than 10% of the silver metal stock. tudy highlights the conditions required to transfer 8000 metric tons of Ag (and other elements such as Hg, Co, Ni, Pb, Zn) from the mantle, i.e. (i) an efficient vector such as the sulphide ligand, (ii) a major and sudden break in the geodynamic environment similar to the one which occurred near the Precambrian–Cambrian transition to provide pathways from the mantle wedge and the lithospheric base to the surface and, (iii) a strongly extensional tectonic setting to provide traps for the concentration of metals in ore deposits.