Contrasting PGM assemblages in the Verkh-Neivinsk and Alapaevsk chromite deposits: Implications for the source of the HSE in the Urals ophiolite-type massifs

In order to gain further insights into the origin of platinum-group minerals (PGM) and the source of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) in the chromite deposits associated with the Urals ophiolite-type massifs, we carried out a mineralogical, HSE abundance, and Pt-Re-Os isotope study of chromitites and PGM from the Verkh-Neivinsk and Alapaevsk dunite-harzburgite massifs in the Middle Urals. The chromitites are characterized by negatively-sloped Bulk Silicate Earth (BSE)-normalized HSE patterns, consistent with the predominance of PGM of the Ir-group platinum-group elements (PGE), i.e., Os-Ru-Ir alloys and RuOs sulfides, over the PGM of the Pd-group PGE, i.e., stibiopalladinite and geversite. These two groups of PGM are interpreted to represent the primary and secondary mineral assemblages, respectively. The observed HSE patterns in the studied chromitites are typical of those formed in supra-subduction zone (SSZ) settings. The near-chondritic average initial γ¹⁸⁷Os and μ¹⁸⁶Os values in the chromitites and PGM of the Verkh-Neivinsk massif indicate that its HSE budget was derived from the convecting upper mantle source that evolved with time-integrated near-chondritic Re/Os and Pt/Os ratios. These features are also typical of the sources of most Archean and Proterozoic mafic-ultramafic rocks worldwide. In contrast to the Verkh-Neivinsk massif rocks, the Alapaevsk massif chromitites show radiogenic initial γ¹⁸⁷Os values indicating evolution of the mantle source of these rocks with a supra-chondritic time-integrated Re/Os ratio. This long-term enrichment in Re relative to Os could be the result of interaction of the source peridotites with ¹⁸⁷Os-enriched melts derived from ancient recycled oceanic crust.