COMPOSITION OF THE PLATINUM-GROUP MINERALS IN THE SALMON RIVER PLACER DEPOSIT, GOODNEWS BAY, ALASKA

Abstract

The composition and the associations of platinum-group minerals from the Salmon River placers, Goodnews Bay, Alaska, are compared with those of assemblages in other Alaskan-type complexes. Pt–Fe alloy, with an average value 25 at.% Fe, corresponds to isoferroplatinum (Pt3Fe). Ir concentrations in isoferroplatinum reach 15.4 wt.%, and Rh concentrations, as much as 2.27 wt.%. Tulameenite, which replaces the isoferroplatinum grains, appears as a rim with distinct boundaries. It forms a Pt2CuFe–PtFe series and also contains up to 2.64 wt.% Ni. Iridium and osmium occur as inclusions in the isoferroplatinum matrix and are depleted in Ru. The Ir content of osmium reaches 26 at.%, and the proportion of Os in iridium varies from 12 to 20 at.%. The concentration of Pt in iridium reaches 21.79 wt.%, which exceeds the limit of solubility of Pt in Ir at the maximum temperature of equilibration (~850°C). The composition of PGE-sulfarsenides corresponds to irarsite, with accessory platarsite and osarsite components: (Ir,Pt,Os)AsS. As predominates over S where substantial Pt exists, and excess S is present in irarsite with a high Os content. Solid solution along the erlichmanite (OsS2) – sperrylite (PtAs2) series is restricted. A wide range of compositions and levels of minor elements (Ir, Pd, Rh) in Pt–Fe alloys from different Alaska-type occurrences reflects geochemical features of the ore-forming system at the source. Fractionations of the PGE in the ore-forming system, from Ir-rich nuggets to Rh-rich and to Pd-rich alloys, reflect the decrease in temperature of the ore-forming system. Compositional variation and temperature range of alloy genesis decrease in the order from Goodnews Bay and Inagli to Galmoenan, Fifield, Nishnii Tagil, then to Tulameen and Yubdo, and down to Pustaya and Itchayvayam occurrences.