Atypical Mineralization Involving Pd-Pt, Au-Ag, REE, Y, Zr, Th, U, and Cl-F in the Oktyabrsky Deposit, Norilsk Complex, Russia

Abstract

Highly atypical mineralization involving Pd-Pt, Au-Ag, REE, Y, Zr, U, Th, and Cl-F-enriched minerals is found in zones with base metal sulfides (BMS; ~5 vol.% to 20 vol.%) in the eastern portion of the Oktyabrsky deposit in the Norilsk complex (Russia). The overall variations in Mg# index, 100 Mg/(Mg + Fe2+ + Mn), in host-rock minerals are 79.8 → 74.1 in olivine, 77.7 → 65.3 in orthopyroxene, 79.9 → 9.2 in clinopyroxene, and An79.0 → An3.7. The span of clinopyroxene and plagioclase compositions reflects their protracted crystallization from early magmatic to late interstitial associations. The magnesian chromite (Mg# 43.9) trends towards Cr-bearing magnetite with progressive buildups in oxygen fugacity; ilmenite varies from early Mg-rich to late Mn-rich variants. The main BMS are chalcopyrite, pyrrhotite, troilite, and Co-bearing pentlandite, with less abundant cubanite (or isocubanite), rare bornite, Co-bearing pyrite, Cd-bearing sphalerite (or wurtzite), altaite, members of the galena-clausthalite series and nickeline. A full series of Au-Ag alloy compositions is found with minor hessite, acanthite and argentopentlandite. The uncommon assemblage includes monazite-(Ce), thorite-coffinite, thorianite, uraninite, zirconolite, baddeleyite, zircon, bastnäsite-(La), and an unnamed metamict Y-dominant zirconolite-related mineral. About 20 species of PGM (platinum group minerals) were analyzed, including Pd-Pt tellurides, bismuthotellurides, bismuthides and stannides, Pd antimonides and plumbides, a Pd-Ag telluride, a Pt arsenide, a Pd-Ni arsenide, and unnamed Pd stannide-arsenide, Pd germanide-arsenide and Pt-Cu arseno-oxysulfide. The atypical assemblages are associated with Cl-rich annite with up to 7.54 wt.% Cl, Cl-rich hastingsite with up 4.06 wt.% Cl, ferro-hornblende (2.53 wt.% Cl), chlorapatite (>6 wt.% Cl) and extensive solid solutions of chlorapatite, fluorapatite and hydroxylapatite, Cl-bearing members of the chlorite group (chamosite; up to 0.96 wt.% Cl), and a Cl-bearing serpentine (up to 0.79 wt.% Cl). A decoupling of Cl and F in the geochemically evolved system is evident. The complex assemblages formed late from Cl-enriched fluids under subsolidus conditions of crystallization following extensive magmatic differentiation in the ore-bearing sequences.