Mineralogical Indicators of the Formation Conditions of Gabbro and Associated Copper Sulfide and Precious Metal Mineralization of the Volkovskiy Massif, Ural Platinum Belt

Abstract The paper presents data on the composition of olivine gabbro from the Volkovskiy and Kumbinskiy massifs and the composition of the rock-forming and ore minerals. Both massifs are located near the eastern contact of the Ural Platinum Belt with the Silurian−Devonian volcanic complexes of the Tagil megazone and both host copper sulfide mineralization with platinum-group metals and gold, which are associated with amphibole-bearing rock types. The earlier amphibole in olivine gabbros of both massifs is magnesiohastingsite (XMg = 0.68–0.75; 2–2.5 wt % TiO2; F/Cl = 2–6), which crystallized at T ≈ 850°C, P ≈ 2–3 kbar under oxidizing conditions (∆NNO > 1), simultaneously with titanomagnetite (3–8 wt % TiO2), bornite, and chalcopyrite. As the temperature decreased (T ≈ 750°C; ∆NNO > 1), the clinopyroxene and earlier magnesiohastingsite were replaced by magnesiohornblendite (1.32 wt % TiO2; F/Cl = 5). Later amphibole (magnesiohastingsite, pargasite, hastingsite, ferropargasite, magnesiohornblendite, and edenite) in the ore-bearing gabbro of the Volkovskiy massif has a variable composition and differs from the earlier amphibole in having a low Mg# (XMg = 0.71–0.29) and a low TiO2 content (0.10–0.96 wt %) and in being enriched in halogens, primarily Cl (F/Cl = 0.4–0.6). The later amphibole is found in a low-temperature (