Sulfide and selenide PGE mineralization in chromitites of the Dunzhugur ophiolite massif (East Sayan, Russia)

Abstract

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 local-ized 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 stud-ied by scanning electron microscopy. The enrichment of chromitites with a refractory platinum group-ele-ment (PGE) sulfides and alloys in the Os-Ir-Ru system (IPGE) testify that the for-mation 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 partic-ipation 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 mag-netite–hematite association are log fO2 (–30.5) at 300օC and log fO2 (–40.5) at 200օC. The minimum sulfur fugacity value was determined by the stability line of laurite at 300օC, log fS2 = –20; the maximum sulfur fugacity values—by the stability line of iron and nickel sulfides: log fS2 –4.5 at 300օC and log fS2 –10.5 at 200օC. The selenium fugacity at T = 300օC is log fSe2 (–8…–13); at T = 200օC, log fSe2 (–12…–17). Au–Ag selenides were formed at T = 200օC, 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.