Associations of platinum-group minerals of the Noril'sk copper-nickel sulfide ores

Abstract

Significant variations in modes of occurrence and composition of platinum-group minerals in different types of ores of the Noril9sk deposits are described. In disseminated sulfide ores the main platinum-group minerals are represented by isoferroplatinum and cooperite whereas in massive ores a great diversity of palladium intermetallic compounds and tetraferroplatinum commonly occur. Pd and Pt sulfides, tellurides, and bismuthotellurides are characteristic of veinlet-disseminated and breccia ores.A short review of the platinum-group mineralogy shows that, in addition to platinum-group minerals commonly found in other copper-nickel deposits, the Noril9sk ores are characterized by: (1) a wide spectrum of platinum-group element partners (Sn, Pb, Cu, Bi, Fe, Ni, As, Sb, Te, Se, Ag) in the platinum-group minerals, (2) a large number of ternary compounds of Pd, Sn, and Cu, and (3) a wide distribution of Pd-Pb minerals.The textural features observed in the massive ores indicate that the platinum-group minerals formed at a late stage in the crystallization history of the ores. These textural features include metacrystals and veinlets of platinum-group minerals in the main ore-forming sulfides--pyrrhotite, chalcopyrite-group minerals, and pentlandite--and the close association of platinum-group minerals with galena, sphalerite, and Cl-bearing djerfisherite. Polymineral intergrowths of platinum-group minerals are also noted. Of great interest are zonal intergrowths of these minerals, testifying to the transformation of the earlier platinum-group minerals by reaction with a residual solution.Different conditions of formation are represented by different types of platinum-group mineral assemblages. These are distinguished as those crystallizing directly from a sulfide melt and represented by cooperite and isoferroplatinum in disseminated ores and those formed from the residual liquid enriched in Pt, Pd, Sn, Pb, As, Sb, Bi, Cu, Ag, Au, and volatile components represented by various intermetallic compounds of palladium in massive ores composed of chalcopyrite-group minerals.