Mineralogical and isotope-geochemical indicators of the genesis of titanomagnetite-copper-sulfide ores in the Volkovsky massif, Middle Urals

Research subject. Mineral assemblages of sulfides, iron-titanium oxides and amphibole from titanomagnetite-copper-sulfide ores of the Volkovsky massif, Middle Urals. Aim. Identification of the physicochemical conditions for the formation of titanomagnetite-copper-sulfide ores based on a comprehensive study of sulfide assemblages, iron-titanium oxides and amphibole using mineralogical-geochemical and isotopic analytical methods.

Methods. The chemical composition of amphibole, sulfides and iron-titanium oxides was studied using a CAMECA SX 100 electron probe microanalyzer at the Institute of Geology and Geochemistry, UB RAS. Sulfur isotope data for sulfide samples (22 analyses) were obtained using a laser femtosecond ablation system (NWR Femtosecond UC with Pharos 2mJ-200-PP laser and harmonics module HE-4Hi-A) and a MAT-253 mass spectrometer (Thermo Fisher Scientific, Germany) at the Far East Geological Institute, Far Eastern Branch of the Russian Academy of Sciences (Vladivostok).

Results and conclusions. It has been established that the early magmatic stage occurred under closed-system conditions (Т_ox ≈ 657 ± 17°C, Т_amp ≈ 845 ± 30°C) at oxygen fugacity (logfO₂) values controlled by the FMQ buffer. Disseminated sulfide mineralization is represented by an assemblage of chalcopyrite (from –0.3 to –0.7‰), consistent with a mantle sulfur source, and subordinate bornite showing initial signs of fractionation (δ³⁴S from –1.3 to –2.0‰). The halogen profile is characterized by the dominance of fluorine (F/Cl = 2.4). The transitional stage records the onset of fluid-driven destabilization and a shift in the halogen regime (F/Cl drops to 0.5). Iron-titanium oxides are characterized by a wide range of formation parameters (Т = 516–704°C, mean 590 ± 50°C) and logfO₂instability (±3.2 orders relative to FMQ), indicating non-equilibrium conditions in an open system. A stabilization of the bornite isotopic composition (δ³⁴S = –1.1‰) is observed while mantle signatures are preserved in chalcopyrite, along with the appearance of sperrylite (PtAs₂) associated with cobaltite.  The fluid-metasomatic stage is associated with intensive high-temperature (604 ± 22°C) chloride metasomatism  (F/Cl = 0.2). Under high oxidizing potential (consistently above FMQ), the removal of iron from the sulfide composition  and a sharp negative sulfur isotope shift were recorded: from –2.1 to –2.2‰ in bornite and down to –2.9‰ in chalcocite. Myrmekitic textures of chalcocite and Cu-excess bornite (Сu_5.16Fe_0.96s₄) containing inclusions of merenskyite (PdTe₂) were formed by the exsolution of a high-temperature bornite solid solution (Bn_ss). This process occurred under the influence of oxidized chloride fluids. The established correlation between the decrease in the F/Cl ratio in amphiboles, the increase in copper content in bornite, and the negative δ³⁴S shift can serve as a prospecting criterion for identifying rich PGE-copper-sulfide ores in analogous massifs.

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