The Evolution of Sulfide Melts as a Key Factor in the Distribution and Concentration of Platinum-Group Elements in Norilsk Ores

Abstract

The Norilsk–Talnakh magmatic sulfide Cu–Ni–PGE (platinum-group elements) deposits were formed by the accumulation of metals in immiscible sulfide melt comagmatic with the parental mafic–ultramafic magma. In this study, the main types of magmatic sulfide ores of the Norilsk–Talnakh deposits are considered as manifestations of different stages in the evolution of the initial sulfide melts. In the context of the overall evolution of Norilsk sulfide melts, the earliest ores are Cu-poor pyrrhotite ores with high concentrations of Rh and IPGE (Os, Ir, and Ru), which were discovered at the Talnakh deposit. The second stage of sulfide melt evolution was marked by the formation of most disseminated ores and Cu- and PGE-poor massive pyrrhotite ores. The massive and disseminated ores were formed independently from each other, but generally correspond to the melts with identical compositions. The only exception is low-sulfur PGE-rich ores from the Upper Gabbroid rocks of the differentiated intrusions, which were affected by wall rock assimilation and early magmatic degassing. It has been shown that the concentrations of ore components in the disseminated sulfides, which are examples of in-situ crystallized droplets of immiscible sulfide melt, vary depending on the composition and degree of fractionation of the parental silicate magma. During the final stage, the crystallization of the residual sulfide melts led to the formation of Cu-rich ores with high Pt and Pd contents. The compositions of these main ore types are compared with the compositions (including trace elements) of their base metal sulfides (BMS). All element dependencies in the massive ores follow the fractional crystallization trend of the sulfide melt. PGE in Norilsk ores are concentrated in distinct platinum-group minerals (PGM) and occur as trace elements in BMS. Rhodium and IPGE are concentrated in pyrrhotite, pentlandite, and pyrite; Pt is occasionally found in pyrite; whereas Pd is found predominantly in pentlandite. The concentration of Pd in pentlandite increases from the Cu-poor to Cu-rich ores. Based on a detailed analysis with the application of several methods, the Pd-rich pentlandite (containing 4.84 wt % Pd) from massive primary magmatic Cu-rich MSS–ISS ores is thought to have been formed by a high-temperature mechanism involving a reaction with sulfide melt. Using X-ray absorption spectroscopy (XAS), the oxidation state of Pd in pentlandite (2⁺) and its occurrence in the form of a solid solution, in which Pd apparently replaces Ni in the pentlandite structure, were identified for the first time.