Unique Cu-rich sulphide ores of the Southern-2 orebody in the Talnakh Intrusion, Noril’sk area (Russia): Geochemistry, mineralogy and conditions of crystallization

We present new geochemical and mineralogical data from the Southern-2 orebody, located in the South-Western branch of the Talnakh intrusion (Russia) and composed of massive and disseminated sulphides. The Southern-2 orebody consists dominantly of chalcopyrite along with lesser pentlandite and subordinate cubanite, bornite and pyrrhotite, and differs from Cu-rich ores of the Oktyabr'sky deposit due to the absence of moikhukite-talnakhite assemblages. Ores from the Southern-2 orebody are highly enriched in copper, nickel and platinum-group elements (PGEs): Cu up to 28.12 and 10.58 wt%, Ni up to 6.29 and 2.45 wt%, and Pt + Pd up to 220 and 173 ppm in the massive and disseminated ores respectively. The ratios of Cu/Ni = 3.85–7.34 in massive and 2.64–6.58 in disseminated ores. The ratios of the platinum to iridium group elements (PPGE/IPGE) reaches more than 40000. These features indicate an extremely different degree of fractionation of the sulphide melt during the formation of ores. The sequence of crystallization of the PGE minerals (PGMs) on the basis of textural information was as follows: Pt-Pd-Sn (rustenburgite, atokite) → Pd-Cu-Sn (stannopalladinite) → Pt-Fe-Cu-Ni (tetraferroplatinum) → Pd-Ni-As (majakite) → Pd-As (palladoarsenide) → Pd-Pb(Bi) (zvyagintsevite, polarite and Pd5Pb3 unnamed phase) → Au-Cu-Pd (tetra-auricupride and auricupride) → Au-Ag alloys. This is consistent with experimental data on PGE minerals. The PGM assemblage was caused by the evolution of a significantly fractionated sulphide melt that was enriched in Cu, Ni, Pt, Pd, Pb and Au, and as a result produced minerals with peculiar compositions (e.g., Cu-rich stannopalladinite, Pb-rich polarite, Ni-rich palladoarsenide), which do not occur in other ores. Furthermore, Pt-Fe-Cu-Ni alloys are abundant and sperrylite is absent, which is a very common phase in all ores of the Noril'sk area. The composition of pentlandite with Ni/(Ni + Fe) ratios ranging from 0.48 to 0.68 indicates that it formed between 560 °C (Ni-poor) to 500 °C (Ni-rich) and −10.5 to −7 lgfS2, respectively. The PGMs and Au-minerals were formed at temperatures ranging from 550 to 225 °C. The specificity of sulphide (with PGMs) association is due to the evolution of the Cu-rich (and Ni-rich) melt under elevated sulphur fugacity conditions. The mechanism of filter-pressing, which precipitated sulphide drops at the bottom of the chamber, and penetration of sulphide melts into the country rocks is supported by the same geochemical composition of disseminated and massive ores.