The Vasilinovskoe Platinum–Palladium Occurrence As a New Type of Mineralization in Ophiolites of the Polar Urals. Report 2. Metamorphism, PTX Parameters, and Sources of Ore Matter

Abstract 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 Kershor 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. The peak PT parameters probably reached ∼6 kbar and ∼700°C, with these parameters being ∼4 kbar and ∼650°C for later paragenesis, decreasing to 1 kbar and ∼550°C; thus, decompression occurred. For the late low-temperature transformations, chlorite geothermometer estimations reveal that t = 275–100°C. Mineralization zones are developed in these rocks (from the first centimeters 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, etc. 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 δ 34 S (‰) 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, δ 34 S = –2.02…+2.72%; in chalcopyrite, δ 34 S = –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.