Platinum-group minerals in gold placers in northwestern Salair

Platinum-group minerals were distinguished and studied in mining of alluvia l-deluvial gold placers In Salair. These minerals are Os-Ir-Ru alloys (all modifications), isoferroplatinum, native platinum with elevated contents of Ir (up to 8.19 wt.%), and multicomponent solid solutions mainly of Pt-Ru-I r composition. Solid metal solutions occur in association with PGE sulfides and sulfoarsenides of younger paragenesis. Laurite RuS2 forms rims and rep lacement structures in cleavage planes in the grains of Os-Ir-Ru alloys. Co operite PtS is present in multiphase of variable microparageneses (MPG) fil ling voids in isoferroplatinum. Optically homogeneous phase (Ir,Ru)(As,S)(2 ) composition intermediate between laurite RuS2 and irarsite IrAsS was foun d as inclusions and rims in Os-Ir-Ru alloys. Hollingworthite RhAsS contains 15% RuS2 end-member. Sperrylite PtAs2 has elevated concentrations of Fe an d S. Based on the composition of platinum-group minerals and their ratios, it is suggested that the paragenesis under study resulted from ophiolitic u ltramafic rocks which formed under high-temperature and high-pressure condi tions. At the early stage, mineral formation proceeded at low sulfur fugaci ty. At the late stage, sulfur fugacity increased, and PGE sulfides and sulf oarsenides formed.