Genesis of an exotic platinum-group-mineral-rich and Mg-poor chromitite in the Kevitsa Ni-Cu-platinum-group-elements deposit

This paper offers the first-ever study of a local chromitite stringer (~ 2 cm wide) enriched in platinum-group elements (PGE) from the Kevitsa Ni-Cu-PGE deposit. Chromite forming this chromitite is rich in Cr2O3 (29.06 37.17 wt%) and FeO (FeOtotal = 45.88 56.06 wt%) and low in MgO (0.04 0.91 wt%). The chromitite host an unusual set of mineral inclusions including: (1) platinum-group minerals of the laurite-erlichmanite (RuS2-OsS2) solid solution series, irarsite (IrAsS), sperrylite (PtAs2), hollingworthite (RhAsS), anduoite [(Ru, Os)As2], kotulskite (PdTe) and As-, Te-, Os-, and Ru- bearing unidentified phases; (2) exsolution-lamellae of ilmenite and ulvospinel along with chromite (111) and (100) planes, respectively; (3) silicates including crystallographically-oriented biotite, euhedral pyroxene and irregular-shaped amphiboles which tend to follow chromite growth planes, and (4) Fe-oxides, pyrrhotite, pentlandite, galena, barite, apatite, and native gold. Intercumulus minerals are microcrystalline Mg-rich biotite and minor amphibole. Pyroxenes (diopside) located immediately above the chromitite stringer exhibit higher CaO (23.02 25.42 wt%) and Na2O (0.16 0.55 wt%) than pyroxenes (augite) located below. We suggest that chromite saturation in the mafic melt was caused by local assimilation of a small pelitic xenolith. The xenolith also provided K and H2O explaining the occurrence of K-and volatile-rich intercumulus phases. Crystallization of chromite in this environment promoted preferential sequestering of Ir and Ru, and to a lesser extent Os, from the parental magma giving rise to an anomalous iridium-group-PGE (IPGE: Ir, Ru, and Os) enrichment.