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 (3 wt % MnO), bornite (±chalcocite or digenite), and minerals of precious metals (Pd−Te and Au−Ag). The enrichment of the later amphibole in Cl (F/Cl = 0.4–0.6), in contrast to the earlier magnesiohastingsite (F/Cl = 2–6), suggests the influx of fluid from an external source. Our data indicate that postmagmatic processes have played a significant role in forming the copper sulfide and precious-metal mineralization in the Volkovskiy massif and, likely, also in other massifs of the Ural Platinum Belt.
Anikina et al. (Mon,) studied this question.
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