Platinum-group element mineralogy and geochemistry of chromitite of the Kluchevskoy ophiolite complex, central Urals (Russia)

We report the results of investigation of chromitites occurring in the Kluchevskoy ophiolite complex of the Russian Urals. The chromite composition suggests crystallization from a boninitic magma in a supra-subduction zone geodynamic setting. The investigated chromitites are enriched in Os–Ir–Ru over Rh–Pt–Pd, as typical of the mantle hosted ophiolite chromitites. Consistent with the geochemical data, the Platinum Group Mineral (PGM) assemblage is dominated by Ru–Os–Ir phases, whilst specific Rh–Pt–Pd minerals are absent. Two distinct paragenetic assemblages have been recognized: 1) primary magmatic PGM (laurite, erlichmanite, osmium, iridium, unnamed Ir–Ni–S, cuproiridsite, irarsite and ruthenarsenite) and 2) secondary PGM formed by desulfurization of primary sulfides at low temperature (ruthenium). Comparison of the studied chromitites with those hosted in the mantle of the Kempirsai, Ray–Iz and Voikar–Sininsky ophiolites has shown that all these chromite deposits form in the same geodynamic environment. The differences in the temperature calculated on the Fe–Mg exchange between olivine–spinel and observed in the PGM assemblage suggest that the Kluchevskoy chromitite suffered the effects of the metasomatism to a lesser extent compared with Kempirsai and Ray–Iz chromitites.