| Abstract: | Platinum group minerals (PGM) are rather widespread as admixture in gold placer deposits in Tuva. The present paper reports new data on PGM in the Kundus gold placer confined to the Kaakhem ophiolite belt. The minerals are mainly represented by solid solutions of the Os-Ir-Ru system. They make up rims of sulfoarsenides, sulfides, and arsenides of the platinum group elements (PGE) developed after primary minerals. PGMs of this placer always contain traces of Pd (0.33–1.58 wt %), Cu (0.29–0.50 wt %), and As (0.03–2.17 wt %), as well as Ni and Sb (within the detection limit). Typomorphic features of minerals along with the set of main elements and isomorphic trace-elements in the major and secondary mineral species, suggest that sources for the studied placer was represented by the Alpine-type ultramafics and associated chromitites. We cannot also rule out that PGM mineralization was influenced later intrusions that promoted the formation of rims of sulfoarsenides, sulfides, and arsenides of PGE. The PGM rims are marked by the S and As isomorphism, which characterizes the composition of mixtures rather than independent mineral types (end members of isomorphic series). In one case, minerals are represented by the isomorphous mixture of sulfoarsenides with a limited role of sulfides; in another case, by arsenides with a limited role of sulfoarsenides. |
