Mineralogy and geochemistry of platinum-group elements in the zoned mafic-ultramafic intrusions of the Uralian Platinum belt, Russia

Abstract

Three styles of platinum-group element mineralization occur within the Ural-Alaskan type complexes of the Uralian Platinum belt: chromite-platinum, noble metal, and noble metal-copper ones. They are associated with dunite and chromitite (chromite-platinum), pyroxenite (noble metal) and gabbro (noble metal-copper). Three common platinum group element distribution patterns are observed for dunite and chromitite: (1) M-shaped (defined by positive Ir and Pt anomalies); (2) with positive Ir-Rh(Pt) anomalies; and (3) W-shaped with positive Os and Pt, or Os and Rh anomalies. In contrast, pyroxenite and gabbro are enriched in Pd and Au. The PGM assemblages correlate to the PGE + Au distribution patterns as exemplified by the predominance of Ir-rich Pt-Fe alloys within dunite and chromitite. The noble metal assemblages of pyroxenite and gabbro comprise tellurides, arsenides, and various Pd minerals, as well as native gold and Au-Ag alloy. An indicative Pd/Ir ratio in a succession dunite-chromitite-gabbro increases from 0.04 to 148 with a maximum value of ~5000 in massive sulfide ore of the Volkovsky that is attributable to the contrasting magma compositions defining chromite vs sulfide controls over PGE concentration.