Vol 11, No 1 (2025)

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 fine- 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 ultramafic bodies moved toward the surface. The formation of secondary native PGMs was caused by low temperatures and reducing fluid regime. Arsenides, sulfoarsenides, stibnides and sulfides 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.

The Bi mineralization is described in quartz-sulfide veins of the Nazarovskoe gold polymetallic deposit (Western Transbaikalia), which is localized within the Eravnino volcano-tectonic structure and is confined to the Nazarovskoe fault zone and the contact with a granodiorite pluton intruding volcanosedimentary rocks. The Bi minerals include Ag-Bi, Cu-Bi, Pb-Bi, and Ag-Pb-Bi sulfosalts, Bi sulfotellurides, aikinite, and native bismuth in assemblage with chalcopyrite and galena. Most Bi minerals are described for the first time both for the Nazarovskoe deposit and the entire Ozerninsky ore cluster.

The geology and petrogeochemical features of the Verkhneuralsk volcanic-plutonic association in the South Urals are characterized. The relevance of the work is determined by necessary analysis of magmatic evolution of the region to clarify ideas on geodynamic evolution of the South Urals. It is shown that this association includes comagmatic volcanic rocks of the Verkhneuralsk Sequence (trachybasalts, trachybasaltic andesites, trachyandesites, trachydacites and trachyriolites) and intrusive rocks of the Verkhneuralsk complex (subalkaline gabbroids, mozzonitoids, dioritoids and syenitoids). The structure of the Verkhneuralsk pluton is described and it is shown that it is composed of rocks of three intrusive phases. The leading mechanism of petrogenesis of the association is related to crystallization differentiation. New data on the age of the Verkhneuralsk pluton are presented and its ore (including economic Mo) potential is characterized. The igneous rocks of the Verkhneuralsk volcanic-plutonic association are typical representatives of the island-arc shoshonite series. Their localization in the western part of the East Magnitogorsk paleoisland arc is an additional argument in favor of the western fall (in modern coordinates) of the subduction paleozone at the time of their formation.