Mineralogical zoning of the PGE–Cu–Ni orebodies in the central part of the Oktyabr'sky Deposit, Norilsk District, Russia

Abstract

Mineralogical features of two orebodies or lenses (C-3 and C-4) at the central part of the Oktyabr'sky deposit were identified. Multidirectional mineralogical zoning in the northern and southern orebodies is shown, confirming the hypothesis of their formation from various magmatic flows, which have individual features and their own modes of formation. The southern C-3 and northern C-4 orebodies differ in their mineralogical associations: the first one is characterized by a high-sulfur association of sulfides, and the second one contains minerals with a sulfur deficit (talnakhite, sugakiite). Variations in Fe and Ni ratios in pentlandite are controlled by the volatility of sulfur during ore crystallization. Direct crystallization zoning is observed in the disseminated ores of the C-4 orebody (borehole RT-107), where the fugacity of sulfur (fS2) increases from bottom to top. Whereas in orebody C-3 (borehole RT-30) fS2 decreases in the same direction. The identified reverse zoning coincides with the vectors of the evolution of ore systems in various blocks of the Main Ore Body (MOB) of the Oktyabr'sky deposit. The difference in typomorphic features of disseminated ores of the southern and northern orebodies is confirmed by differences in the associations of platinum group element minerals (PGMs). Disseminated ores in picritic gabbro-dolerites and massive pyrrhotite ores in the exocontact of the intrusion within the southern orebody differ in the specialization of PGMs: the first is characterized by minerals of the Pd-Bi-Sb-Te system, the latter -only Pt minerals. The similarity of these types of ores lies in the similar reverse mineralogical and geochemical zoning from top to bottom along the sections, caused by the evolution of the sulfide melt in this direction. The formation of reverse zoning of disseminated ores (zones of "marginal reversion") is probably due to the action of the mechanism of repeated influx of a melt of an increasingly primitive composition into the upper parts of the crystallizing