Platinum group minerals from chromites of Kempirsai ultramafic massif (the South Urals): new data
Yurichev A.S., Chernyshev A.I., Korbovyak E.V.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2019. V. 148. N 2. P. 76-86
Kempirsai is the largest massif of ultramafites in the South Urals, it is well known for its unique chromite ore deposits. Grains of chromespinelides in massive chromitites of the «Diamond-Pearl» deposit contain small isolated inclusions of PGE minerals represented by solid solutions of Os—Ir—Ru and Rh—Zn—Pt (later system has been revealed in the massif for the first time), disulfides of the isomorphic laurite-erlichmanite series, and sulfides of the Ir—Ni—Cu system. Some grains of the metal Os—Ir—Ru solid solutions contain up to 45 wt % of ruthenium, extending previously known boundaries of the mixing range in this triad of elements. The main trend in the Os, Ru and Ir fractionation in sulfides is enrichment of disulfides in osmium and ruthenium; iridium is mostly concentrated together with Ni and Cu in sulfides with different metal-sulfur ratios. The process of mineral-forming of these PGE associations was developing in several stages. The earliest among them are metallic solid solutions of Os—Ir—Ru and disulfides of laurite-erlichmanite series; minerals of Rh—Zn—Pt and Ir—Ni—Cu systems are related to later events connected with repeated partial melting and injection of the newly formed melt into already formed restites, or processes of the mantle metasomatism.
Key words: Kempirsai massif, chromitites, PGE minerals, chemical composition, genesis