Platinum ores of the Noril'sk layered intrusions: Magmatic and fluid concentration of noble metals

The disseminated sulfide ores of layered intrusions of the Noril'sk ore dis trict are ores of platinum metals. A comprehensive and detailed description of platinum potential of the disseminated ores is presented on an example of the Noril'sk-1 intrusion for the first time. The proton microprobe analy sis, scanning electron microscope with energy dispersing spectrometer, as w ell as a complex of conventional methods were used for this study. On the b asis of these instruments and methods, more than 20 minerals of platinum me tals (PGM) were determined, and quantitative characteristics and regulariti es of the platinum group elements (PGE) distribution as solid solutions in ore-forming sulfides were established. The majority of the PGM occur as coe xisting minerals in the disseminated ores. However, in other ore types (mas sive and stringer-disseminated) associated with the same intrusions, these PGM were not found. In the intrusion Noril'sk-1 PGM assemblages considerabl y differ from each other for zones with varying petrographic compositions. A zone with sulfide-bearing picrites contains minerals of the Pt-Fe system with a very limited occurrence of other minerals of Pt and especially of Pd . Rustenbergite, mineral of the Pt-Sn system, is the major mineral of the h orizon of the sulfide-bearing taxitic gabbroids, where, Pd minerals are als o common. The formation of PGM depends on the degree and conditions of the PGE solubility in the ore-forming sulfides. It was established that Ph, Ru, Ir, and, probably, Os are completely soluble in pyrrhotite and pentlandite . The following PGE contents were established in pyrrhotite and pentlandite (g/t): Rh, <3-36; Ru, <3-14; and Ir, reaching 26 g/t with an uneven distri bution. The solubility of platinum and palladium was possibly attained in t he picrite horizon. Pentlandite contains (g/t): Rh, 4-47; Ru, <3.5-14. Pent landite is characterized by a high Pd solubility. Pd contents in pentlandit es from various sulfide-bearing horizons vary from 200 to 1600 g/t. The sol ubility of Pd in chalcopyrite is a specific feature of the disseminated ore s, whereas chalcopyrite of the massive and stringer-disseminated ores does not contain soluble Pd. Data on the distribution of Mo, Se, Te, Pb, Cd, Ag, Zn, and Sn in some sulfide minerals are presented. Details of the PGE dist ribution in ore-forming sulfides and on the occurrence of the PGM and their assemblages are related to the behavior of the PGE in hyperbasite-basite m agmatic melts. The main components of the hyperbasite-basite magmatic melts are silicate, sulfide liquids, and magmatic fluid. It was shown that the e nrichment of the magmatic melt with fluid components is an indispensable fe ature of the Noril'sk intrusions. Two groups of fluid component that influe nced the behavior of the PGE were distinguished. The predominantly hydrogen -hydrocarbon component serves as a factor of the stabilization of magmatic sulfide liquids and correspondingly as a factor of the solubility of the PG E in sulfide liquids. Acid gases, including halide-hydrogen components, pla y the main role in the redistribution of the PGE between the sulfide liquid and fluid phase.