Vasilinovskoe platinum-palladium occurrence – a new type of mineralization in the ophiolites of the Polar Urals. Communication 2. Metamorphism, <i>PTX</i> parameters and sources of matter

In this paper, ore-containing amphibolized gabbroids from the new low–sulfide ore occurrence of noble metals (Pt–Au-Pd) Vasilinovskoe, located near the Kharp town in the Yamalo-Nenets Autonomous okrug, are studied in detail. These rocks presumably belong to the Kershorsky complex, which is dated mainly to the Late Ordovician. Due to metamorphism, up to the appearance of amphibolites, a number of generations of amphiboles and plagioclases occurred; then epidote, chlorite, and some other minerals crystallized. Peak PT parameters probably reached ~6 kbar and ~700 °C; for later paragenesis, ~4 kbar and ~650 °C, decreasing to 1 kbar and ~550 °C, thus decompression occurred. For the late low-temperature transformations, chlorite geothermometer estimations reveal t = 275–100 °C. Mineralization zones are developed in these rocks (from the first cm to 50 m, sulfides 1–3 vol.%). The platinum group elements in them are represented by micron–sized palladium minerals – tellurides (merenskiite, temagamite, kotulskite, sopcheite), antimonides (stibiopalladinite, sadberite) and arsenoantimonides (arsenic stibiopalladinite, isomerthite), as well as others – moncheite, native osmium and others. The formation of noble-metal parageneses was associated with late-magmatic processes, as well as with subsequent redistribution by magmatogenic hydrothermal fluids, at temperature decreasing down to ~250 °C; pressure decreased from ~0.9–1.3 to ~0.4–0.5 kbar. The sulfur isotopic composition δ34S (‰) in pyrite varies from -4.2 to +6.3, in chalcopyrite from –1.6 to +4.2. In pyrite of southern flank of the occurrence, δ34S = –2.02 ... +2.72 %; in chalcopyrite, δ34S = –1.74 ... +0.29. According to the Pb-Th-U isotopic parameters and the sulfur isotopic composition of sulfides, the sources of the studied low-sulfide mineralization are similar to mantle-type sources.