Ore petrology of chromite-PGE mineralization in the Kempirsai ophiolite complex

Summary The platinum group minerals (PGM) in chromite ores of the Kempirsai ophiolite massif, located south of the Ural Mountains, are extremely varied in composition and represented predominantly by alloys, sulfides, arsenides, and sulfosalts of the iridium-group PGE (IPGE). The earlier Ir-Os-Ru alloys prevail over the later Cu-Os-Ru, Cu-Ir, Ni-Ir, Ni-Os-Ir-Ru, and Ni-Ru-Os-Fe alloys rich in base metals (BM). The earlier Ru-Os disulfides crystallize coevally with Ir-Os-Ru alloys, whereas the later sulfides are represented by compounds with a variable stoichiometry and a wide miscibility of Ni, Cu, Ir, Rh, Os, and Fe. Phase relations of PGE alloys with PGE-BM alloys, sulfides and sulfoarsenides confirm that deposition of these minerals was defined by a general evolution of PGE fractionation in the mineral-forming system but not by a super-imposed process. The leading mechanism of PGM crystallization is thought to be their dendritic growth during gas-transport reactions from low-density gaseous fluid enriched in PGE. The representative technological sampling of 0.5 million tons of an ore showed that the average PGE content in chromite ore is 0.71 ppm which leads to an evaluation of the PGE resources to be no less than 250 tons. Hence, the Kempirsai deposit is not only a giant chromium deposit, but also a giant deposit of IPGE: Ir, Ru, and Os. The size parameters of PGM and their aggregates suggests that the PGE may be recoverable in separate concentrates.