Quantitative assessment of the relative roles of sulfide liquid collection, magmatic degassing and fluid-mediated concentration of PGE in low-sulfide ores of the Norilsk intrusions

Low-sulfide ores in the upper marginal series of the Norilsk intrusions are distinct from the main massive and disseminated sulfide orebodies in a high platinum-group element (PGE) tenor up to 1,300–1,700 ppm PGE + Au in 100 % sulfide, lower Cu/Pd of ∼100–300 and increased Pt/Pd of ∼0.2–2. These features have previously been considered either as a result of collection by sulfide liquid at extremely high R-factor (silicate to sulfide melt mass ratio) or postmagmatic sulfide removal, although the decoupling between PGE grade and PGE tenor may suggest an additional process of PGE concentration in addition to partitioning into immiscible sulfide liquid. In this study, the distribution of diverse platinum-group minerals (PGM) and their spatial associations with sulfide-silicate assemblages were investigated by automated mineralogy techniques to recognize the mechanism of the PGE concentration. Mineral assemblages were studied from 2D surfaces of polished sections and mineral separate blocks using the Mineral Liberation Analysis and TESCAN Integrated Mineral Analysis in combination with X-ray microtomography and lab- and synchrotron-based microbeam X-ray fluorescence mapping collected over slabs and thick sections, representative of the 3D sub-surficial layer. Palladium minerals (mostly stibnides, bismuthides, tellurides and arsenides) prevail over Pt minerals in all three intrusions (Norilsk 1, Talnakh and Kharaelakh) based on ∼6,600 identified PGM grain statistics from both thin sections and heavy mineral separates. Among Pt minerals, sperrylite and Pt-Fe alloys occur in an equal proportion in Norilsk 1 whereas the alloys are exceptionally rare in Talnakh-Kharaelakh low-sulfide ores. The PGM grains are predominantly locked within secondary silicates with a percentage of their mutual boundaries of ∼50 rel. % in consistency with a high abundance (35–90 area %) of secondary silicates in mineralized rocks. No correlation between the proportions of secondary silicates and PGMs as well as with the degree of their spatial affinity was found. The preferred confinement of PGM grains to the disseminated chromite schlieren is observed in the studied samples, although, among PGMs, only Pt sulfides demonstrate significant spatial affinity (15–34 rel. % mutual boundary) intergrowing with chromite. The spatial association between PGM and base-metal sulfides (∼20 rel. %) is notably higher than the proportion of sulfides in rocks (1.8–2.8 area %) that is interpreted as a quantitative proof of the cogenetic link between PGM and sulfides. However, the significant mm-scale detachment of the PGM grains from the sulfide blebs is persistent throughout the mineralized sections. Element mapping techniques reveal widespread textures of early magmatic degassing (such as amygdules, segregation vesicles, fluid caps and halos), the common tight intergrowths of PGM with volatile-rich minerals and the presence of PGM grains nucleating on the former gas bubble walls. These features are all distinct from the textures of postmagmatic hydrothermal-metasomatic alteration. We propose that an initial composition of sulfide liquid was modified due to S partitioning into a fluid phase during early magmatic degassing. Therefore, the high PGE tenor is not solely a result of sulfide-silicate exchange but was likely enhanced by early magmatic S loss during degassing and PGE fluid transport.