Silver sulfotellurides from volcanic-hosted massive sulfide deposits in the Southern Urals

Summary This paper addresses Ag-sulfotellurides occurring in volcanic-hosted massive sulfide deposits of the Southern Urals. Cervelleite-like minerals were identified in ores from the Gayskoe, Yaman-Kasy, Severo-Uvaryazhskoe, Tash-Tau, and Babaryk deposits, where they occur in ores containing chalcopyrite, galena, sphalerite, tennantite ± bornite. Other Ag- and Te-bearing minerals (electrum, hessite, stromeyerite and Ag-bearing chalcocite) are present in the association. A benleonardite-like mineral associated with sylvanite and native tellurium was found as a metastable phase in paleohydrothermal tubes relics from the Yaman-Kasy deposit. Formation of the sulfotellurides indicates relative low fTe₂ in the hydrothermal systems, insufficient for formation of most S-free tellurides. The significant Cu enrichment in cervelleite relates to the association with bornite. Broad variations in composition and physical properties of cervelleite-like sulfotellurides allow the supposition of the presence of several, as yet unnamed mineral species, which can be distinguished by Cu contents, Te/S ratios, and presumably by crystal structure.     

Silicification of peridotites at the stalemate fracture zone (Northwestern Pacific): Reconstruction of the conditions of low-temperature weathering and tectonic interpretation

During cruise SO201-1b of the joint Russian-German expedition on the R/V Sonne in 2009, mantle peridotites affected by varying secondary alteration were dredged on the eastern slope of the northwestern segment of the Stalemate transverse ridge adjacent to the eponymous fracture zone. The collection discussed in this paper included four samples of silicified serpentinites after dunites and 11 lherzolite samples serpentinized to a varying degree. The abundance of amorphous silica and quartz, very high SiO₂ content (up to 88.7 wt %), and unusually low MgO (up to 1.4 wt %) in the serpentinized dunites strongly distinguish these rocks from the known products of hydrothermal alteration and low-temperature (seafloor) weathering of peridotites in the oceanic crust. In order to determine the conditions and processes resulting in the silicification of peridotites at the Stalemate Fracture Zone, thermodynamic modeling accounting for the kinetics of mineral dissolution implemented in the GEOCHEQ program package was used in this study. The results of modeling allowed us to suppose that the geochemical and mineralogical effects observed in the silicified serpentinized dunites of the Stalemate Fracture Zone are consequences of low-temperature deserpentinization of oceanic materials under subaerial conditions.