Secondary platinum-group minerals from a placer of the Bolshoy Sap river (Central Urals)

The object of the present study includes grains of platinum-group minerals (PGMs) from a concentrate, which was obtained during mining of a technogenic gold placer near the Pervomaisky dunite-harzburgite massif (Central Urals). The inner structure and chemical composition of grains were studied using scanning electron microscopy and electron microprobe analysis. Grains of primary Ru-Os-Ir minerals are overgrown by concentric-zonal rims of secondary PGMs of two types: 1) “corrosive” PGMs, which replace the primary PGMs, and 2) overprinted PGMs, which formed after “corrosive” PGMs. The “corrosive” PGMs comprise areas of fne- or course-grained aggregates of subgraphic (symplectite) structure and include solid solutions of the ruarsite-osarsite-irarsite series (RuAsS-OsAsS-IrAsS) and As-bearing laurite (Ru(S,As)2) after native ruthenium, As-bearing erlichmanite (Os(S,As)2) and irarsite (IrAsS) after rutheniridosmine and Fe-bearing iridium (Ir,Os,Fe), tolovkite (IrSbS) and irarsite (IrAsS) after native iridium. Aggregates of overprinted PGMs have monolithic structure and form an outer rim on the grains of primary PGMs. The overprinted PGMs include Fe-bearing native iridium (Ir,Os,Fe) and osmium (Os,Ir,Fe), Ir-Ni-Fe alloys, including garutiite (Ni,Fe,Ir), Rh-bearing tolovkite and irarsite, and Ru-bearing cobaltpentlandite ((Ni,Co,Ru)9S8). It is suggested that secondary “corrosive” and overprinted PGMs formed under conditions of decompression, pressure drop and change from reducing to oxidative regime, when ultramafc bodies moved toward the surface. The formation of secondary native PGMs was caused by low temperatures and reducing fuid regime. Arsenides, sulfoarsenides, stibnides and sulfdes of platinum group elements formed under conditions of increased fugacity of S, As and Sb and due to the change from reducing to oxidizing conditions in the upper parts of the crust.