PGE mineralization in marginal sulfide ores of the Chineisky layered intrusion, Russia

Summary We have undertaken a detailed study of platinum group element (PGE) mineralogy and geochemistry of disseminated sulfides associated with the marginal zone of the Chineisky layered mafic intrusion. Towards an intrusive contact the marginal zone reveals a gradual progression from gabbro-gabbronorites towards monzodiorite. Sulfides occur in all the rocks of the marginal zone including exocontact sandstone. They occur mainly as pyrrhotite, chalcopyrite, and pentlandite and show progressive enrichment in Cu towards the intrusive contact. In the same direction, PGE mineralogy reveals the following systematic changes: (1) size of PGE mineral grains decreases from 50 µm up to 1 µm; (2) the association of Pd minerals with Ni and Co arsenide and sulfarsenides becomes stronger; and (3) the composition of PGE minerals changes for palladium: Pd-Sn → Pd-As → Pd-Sb → Pd-Te → Pd-Bi; for platinum: Pt-Fe + PtAs 2 → PtS + PtAs 2 → PtAs 2. This zoning pattern is interpreted as the result of fractional crystallization of an immiscible sulfide melt, with the residual liquid, enriched in Cu, PGE, and volatile elements, being expelled towards the periphery of the intrusive body. PGE minerals also decompose in an oxidation zone. The most stable of them are paolovite and sperrylite, which both accumulated in placers derived from the massif. Pd is removed from the decomposed minerals, and then absorbed by brown iron and goethite in the oxidation zone.