Tomamaeite Cu3Pt in native osmium from river sediments in the Nizhnie Sergi Region, Central Urals

Abstract

The article is devoted to tomamaeite Cu3Pt from inclusions in a native osmium grain from alluvial river sediments in the Nizhnie Sergi region, Sverdlovsk oblast. It is the fourth fnding of this mineral in the Urals. In addition to tomamaeite, native osmium Os0.36Ir0.33Ru0.31 contains the inclusions of digenite (or, possible, roxbyite), millerite, Fe-Ni-Pt alloy, an unidentifed Pd mineral, magnesiohornblendite, and low-Ca pyroxene. In composition, tomamaeite is close to ideal formula Cu3Pt and contains minor amounts of (wt. %) Pd (up to 2.0), Ir (up to 1.7), Ru and Ni (up to 0.6), and Fe (up to 0.2). In addition to native osmium, two grains of native ruthenium Ru0.54Os0.23Ir0.23 and Ru0.40Os0.31Ir0.29 are found in sediments. Our data suggest that PGE minerals in river sediments could be sourced from mafc-ultramafc massifs located in the Nizhnie Sergi area. The formation of tomamaeite and coexisted minerals is related to the process of postmagmatic transformation of the primary PGE mineral assemblage, probably, during serpentinization of ultramafc rocks.