Platinum-group minerals and the genesis of the sulfide PGE-Cu-Ni deposit “Ore Horizon 330” of the Monchegorsk Pluton, Kola Region, Russia

Abstract

The sulfide PGE-Cu-Ni deposit “Ore horizon 330” (OH330) of the reef type is located within the orthopyroxenite of the Sopcha intrusion, which is part of the Paleoproterozoic layered Monchegorsk pluton (Monchepluton) in the Kola Region. OH330 is a sill-like body with a thickness of 4–6 m, traced along the entire perimeter of the Sopcha intrusion at a distance of 3.3 km and a width of 1.2 km. The OH330 deposit was studied in two well-exposed sections of its western flank. Here, OH330 is a thinly layered horison that lies among the host olivine orthopyroxenite. It consists of interlayers of dunite, harzburgite, and orthopyroxenite containing fine sulfide dissemination with increased contents of Ni (1290–5220 ppm), Cu (up to 2320 ppm), and platinum group elements (PGE) (0.43–1.40 ppm of the PGEtot). It is assumed that the OH330 parental magma was significantly contaminated with crustal material, saturated with sulfur and chalcophile elements. The separation of immiscible sulfide droplets during cooling led to the development of ore sulfide concentrations. The association of platinum group minerals (PGM) in harzburgite is represented by the predominant PtFe alloys and subordinate PGE bismuth-tellurides, arsenides, and tellurides. Compared to harzburgite, PGE bismuth-tellurides and tellurides are widely distributed in orthopyroxenite, whereas arsenides, PtFe alloys, and sulfides are minor. The PGE-Cu-Ni ore formed in a relatively wide temperature range from 1000 to 900 °C (base metal sulfides) to 900–600 °C (PGE sulfides and arsenides, and PtFe alloys) and ended at a temperature of 600–400 °C with a crystallization bismuth-tellurides and tellurides of Pt and Pd.