Primary platinum-group minerals in the placer of the Bol’shoy Sap River (Middle Urals) and the problem of the indicator value of the osmium-ruthenium trend for the compositions of natural hexagonal Os-Ir-Ru alloys.

Research subject. Primary platinum-group minerals from the gold placer of the Bolshoy Sap River (Middle Urals) in the southern frame of the Pervomaisk ophiolite-type massif. Methods. The chemical composition of minerals was studied by scanning electron microscopy (JEOL-JSM6390LV) and electron microprobe analysis (Cameca SX 100). The sulfur isotopic composition of laurite and erlichmanite grains was determined using a laser femtosecond ablation system (NWR Femtosecond UC with laser Pharos 2mJ-200-PPam and harmonics module HE-4Hi-A) attached to a MAT-253 mass spectrometer (Thermo Fisher Scientific). Results. A wide species composition of primary platinum-group minerals was revealed, represented by native minerals of the Os-Ir-Ru (osmium, iridium, ruthenium, rutheniridosmine) and Pt-Fe (by stoichiometry close to the composition of isoferroplatinum) systems, as well as Ru-Os sulfides (laurite, erlichmanite). Iridium grains contain isoferroplatinum lamellae, which are a product of solid solution decomposition, as well as the inclusions of cuproiridsite, Ru-bearing pentlandite, kashinite, and tolovkite. Inclusions in isoferroplatinum are represented by braggite, rhodium and palladium sulfides (Pd-Rh-S), and Pd-bearing (5.78 wt % Pd) native gold. Variations in the composition of natural hexagonal Os-Ir-Ru alloys reflect the presence of three trends (i.e., ruthenium, osmium-iridium, and osmium-ruthenium). The sulfur isotopic values of laurite and erlichmanite grains ((1.0–2.5) ±0.2‰) are consistent with derivation of sulfur from a sub-chondritic source, reflecting a minor contribution of crustal sulfur during mantle-crustal interaction processes. The prevalence of primary platinum-group minerals in placers from various platinum-bearing zones of the Middle Urals was analyzed. In the western Serov-Nevyansk zone, Os-Ir-Ru alloys of osmium-iridium and ruthenium trends are common, as well as Pt-Fe minerals of the tetraferroplatinum series PtFe – tulameenite PtFe_0.5 Cu _0.5 – ferronickelplatinum PtFe _0.5 Ni_0.5. Os-Ir-Ru alloys of the osmium-ruthenium trend were established only in the eastern Salda-Sysert and Alapaevsk zones. Os-Ir-Ru alloys of ruthenium and osmium-iridium trends, native iridium and isoferroplatinum are widespread. Conclusions. The wide species composition of primary PGMs in the placer is due to the polygenic nature of chromitites, which is typical of ophiolite massifs in the Middle Urals. The high-temperature Os-Ir-Ru alloys of the ruthenium trend, as well as Os-Ru sulfides, are associated with laterally secreted chromites in the dunite-harzburgite complex. Metasomatic and reactive metasomatic chromitites in the dunite-verlite-clinopyroxenite complex serve as sources of natural Os-Ir alloys of the osmium-iridium trend and Pt-Fe alloys. The highest temperature Os-Ir-Ru alloys of the ruthenium trend, as well as Os-Ru sulfides, are associated with lateral secretion chromitites in the dunite-harzburgite complex. Metasomatic and reaction-metasomatic chromitites in the dunite-wehrlite-clinopyroxenite complex serve as bedrock sources of natural Os-Ir alloys of the osmium-iridium trend, and Pt-Fe alloys. The most likely reason for the appearance of the osmium-ruthenium   trend in the chemical composition of natural hexagonal Os-Ir-Ru alloys is the recrystallization of primary high-temperature solid solutions during metamorphic transformations at lower temperature conditions and the change of the oxidative regime to a reducing regime.

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