Vol 11, No 1 (2025)
Mineralogy of ore deposits
Secondary platinum-group minerals from a placer of the Bolshoy Sap River (Central Urals)
Abstract
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.



Co and Ni minerals in ores of the Mikheevskoe porphyry Cu deposit (South Urals)
Abstract
Co and Ni minerals are described in ores of the Mikheevskoe porphyry copper deposit (South Urals, Russia). Minerals of the cobaltite–gersdorfite series (CoAsS–NiAsS), violarite (FeNi2S4), millerite (NiS), pentlandite (Ni5.5Fe3.4)8.9S8.0 and melonite (NiTe2) are identified. They overgrow pyrite, chalcopyrite and bornite and belong to late low-temperature assemblages. Cobalt and Ni are also incorporated in pyrite. The latter, being the most widespread mineral of the deposit, is a major carrier of these metals. The variability of Co and Ni minerals is determined by the presence of serpentinites, which replaced ultramafic rocks.



Bismuth minerals of the Nazarovskoe gold polymetallic DEPOSIT (Western Transbaikalia)
Abstract
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.



Characteristics of the unique Y-HREE-F-RICH pegmatite system revealed by zircon geochemistry: a case study from Mt. Ploskaya amazonite deposit, Kola Peninsula
Abstract
An amazonite-quartz-albite pegmatite body of the Mt. Ploskaya intrudes a metavolcanic complex of the Keivy Terrane (Kola Peninsula) and is unique in its diversity and an assemblage of ore and accessory Y-Yb-Ta-Nb-Be-Pb-F mineralization. The studied zircon grains have a heterogeneous structure associated with the presence of relics of primary zircon (lighter in BSE regime) in a matrix of altered mineral (darker in BSE regime). The composition of relics and matrix is characterized by a decreased content of Zr (0.67–0.81 apfu) and increased content of Hf (0.13–0.15 apfu) and Yb (0.01–0.02 apfu). The composition of the matrix has a deficit of the analytical sum, which can vary 3 to 5 wt. %, and a higher content of non-formula elements: CaO and UO2 (>1 wt. %) and Na2O and ThO2 (>0.2 wt. %). The average total rare earth element (REE) content of the relict zones and the matrix is 19400 ppm and 27400 ppm, respectively. The average contents of Y and some heavy REEs (HREE: Yb and Lu) have a similar distribution: 19370 (15420 and 2430) ppm in relicts compared to 27390 (21740 and 3140) ppm in the matrix. An increased Ce/Ce* ratio of the zircon matrix indicates the phase crystallization under more oxidizing conditions compared to relics. The content of volatile components (H2O, F, and Cl) of the zircon matrix is 5–8 times higher relative to the relics. In this case, a H2O:F:Cl ratio of zircon varies from 35:5:1 in relics of igneous zircon to 20:5:1 in products of its hydrothermal alteration. The study of zircon of the Mt. Ploskaya pegmatite shows a selective accumulation of a number of trace elements and volatiles in residual fluids during the magmatic-hydrothermal evolution of the system and the unique HREE enrichment as a consequence of metasomatic alteration of the protolith.



Петрология и геохимия магматических комплексов
Late Devonian Verkhneuralsk volcanic-plutonic association (East Magnitogorsk Paleoisland arc, South Urals): age, petrology, geochemistry, ore potential and geodynamic setting
Abstract
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.


