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 Pustay a River placer deposit, situated within the Koryak-Kamchatka platinum-beari ng 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 PG M concentrate (160 grains) consists of platinum nuggets, except for two gra ins, an iridium nugget and Pd-bearing gold (15.18% Pd). The platinum nugget s are represented by native platinum, isoferroplatinum and Pt-Fe alloy with an elevated concentration of Pd (up to 9.52 wt.%). Some nuggets are surrou nded by rim's of PtCu, Pt3Cu, or PtS composition. Osmium, cooperite, vysots kite, platarsite, hollingworthite, irarsite, sperrylite, keithconnite, stib iopalladinite, rhodarsenide, vasilite, rhodian pyrrhotite and unnamed phase s 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 character ized by a depletion in Os, Ir, and Ru, and enrichment in Pd and Cu at the l ate-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 w hich the further crystallization of multi-phase parageneses [vysotskite, va silite, (Pt,Pd)(3)S-2, (Pt,Pd,Fe,Cu)(3)S, (Cu,Fe,Pd,Pt,Rh,Ru)(9)S-8, bornit e and chalcopyrite] occurred. Further hydrothermal-metasomatic transformati ons led to the appearance of hongshiite and cooperite rims on platinum.