The association of platinum-group minerals in placers of the Pustaya River, Kamchatka, Russia

We have studied the platinum-group minerals (PGM) recovered from the Pustaya River placer deposit, situated within the Koryak-Kamchatka platinum-bearing belt of Alaskan-type intrusions in eastern Russia. The specific features of the geochemistry of the PGM of this placer deposit characterize the ore-forming system in the Alaskan-type source, which was partly eroded. The PGM concentrate (160 grains) consists of platinum nuggets, except for two grains, an iridium nugget and Pd-bearing gold (15.18% Pd). The platinum nuggets are represented by native platinum, isoferroplatinum and Pt-Fe alloy with an elevated concentration of Pd (up to 9.52 wt.%). Some nuggets are surrounded by rim's of PtCu, Pt3Cu, or PtS composition. Osmium, cooperite, vysotskite, platarsite, hollingworthite, irarsite, sperrylite, keithconnite, stibiopalladinite, rhodarsenide, vasilite, rhodian pyrrhotite and unnamed phases Pd2Te, (Pt,Pd)(3)S-2, (Pt,Pd,Fe,Cu)(3)S, and (Cu,Fe,Pd,Pt,Rh)(9)S-8 occur as inclusions in the platinum nuggets. The ore forming system is characterized by a depletion in Os, Ir, and Ru, and enrichment in Pd and Cu at the late-magmatic stage and Cu at the hydrothermal-metasomatic stage. Enrichment of the melt in Pd produced Pd-rich Pt-Fe alloys. Residual portions of the melt enriched in S, Pd and Cu filled gas vacuoles in platinum nuggets, in which the further crystallization of multi-phase parageneses [vysotskite, vasilite, (Pt,Pd)(3)S-2, (Pt,Pd,Fe,Cu)(3)S, (Cu,Fe,Pd,Pt,Rh,Ru)(9)S-8, bornite and chalcopyrite] occurred. Further hydrothermal-metasomatic transformations led to the appearance of hongshiite and cooperite rims on platinum.