New Data on Platinum Group Element Bearing Chromitites from the Kurtushiba Ophiolite, Western Sayan

Abstract—Small grains of platinum group minerals (PGMs) have been found in chromitites of the Ergaksky and Kalninsky ultramafic massifs, which are marginal northeastern fragments of the Kurtushiba ophiolite belt in Western Sayan. Along with previously described PGMs, the Cu–Ru–Os solid solution, arsenic-bearing (As up to 3.6 wt %) laurite with minor Pt, Ni, and Co, platinum group element (PGE) sulfides of the MeS and Me3S4 types, sulfoarsenide with the high Rh content (12.7 wt %) in chromitites of the Ergaksky massif, and laurite and sulfoarsenides in chromitites of the Kalninsky massif were found for the first time. The PGM compositions indicate a high formation temperature of Ru–Os sulfide (1200–1250°C) in both massifs, as well as a high partial sulfur pressure during the formation of chromitites and syngenetic PGMs in the Ergaksky massif. The PGM assemblages were formed during several stages. The Os–Ir–Ru alloys and the laurite–erlichmanite series disulfides (predominantly partitioning Ru) are early; the Cu–Ru–Os alloys, sulfides (Ru,Fe)3S4, (Ru,Ni,Fe)S, and PGE sulfoarsenides are late and were crystallized under the influence of reductive fluids as a result of PGE remobilization and redeposition.