Sulfide and Selenide PGE Mineralization in Chromitites of the Dunzhugur Ophiolite Massif (East Sayan, Russia).

The paper presents the first data on PGE sulfide and selenide mineralization formed in podiform chromitites at different stages of evolution of the Dunzhugur ophiolite massif. The chromite bodies are localized in listvenitized aposerpentinites, which are part of the Dunzhugur ophiolite complex. The forms of occurrence, microstructural features and compositions of platinum-group minerals in chromitites were studied by scanning electron microscopy. The enrichment of chromitites with a refractory platinum group-element (PGE) sulfides and alloys in the Os–Ir–Ru system (IPGE) testify that the formation of magmatic platinum-group minerals (PGMs) occurred under mantle conditions. Primary PGMs were replaced by iridium sulfoarsenides and sulfoantimonides due to the interaction of mantle peridotites and chromitites with As–Sb-bearing fluid generated during dehydration and melting of the subducting slab. Native osmium was formed as a result of desulfurization of magmatic PGE-bearing sulfides with the participation of reduced fluids at the serpentinization stage. The replacement of Ru–Os sulfides by selenides of these metals in chromitites could occur at the stages of subduction or obduction at high oxygen fugacity under the influence of acid hydrothermal/metamorphogenic fluids. A qualitative assessment of physicochemical parameters of the formation of ruthenium selenides was carried out. Oxygen fugacity values estimated from the magnetite–hematite association are log fO₂ (–30.5) at 300°C and log fO₂ (–40.5) at 200°C. The minimum sulfur fugacity value was determined by the stability line of laurite at 300°C, log fS₂ = –20; the maximum sulfur fugacity values—by the stability line of iron and nickel sulfides: log fS₂ < –4.5 at 300°C and log fS₂ < –10.5 at 200°C. The selenium fugacity at T = 300°C is log fSe₂ (–8...–13); at T = 200°C, log fSe₂ (–12...–17). Au–Ag selenides were formed at T = 200°C, log fS₂ (–9...–10.5); log fSe₂ is from –13.5 to –20.5, log fO₂ (–40). An important factor for the possibility of replacement of Ru–Os sulfides by selenides is Se occurrence in the ore-forming system. The source of Se may be the substance of the subducting slab—volcanogenic–sedimentary rocks containing fragments of Se-rich hydrothermal–sedimentary sulfide ores. They were involved in the processes of magma genesis and accompanying hydrothermal circulation. [ABSTRACT FROM AUTHOR]