Mineralogy and petrogenesis of intrusive rocks of the Kutuevsky Au-Cu-porphyry ore occurrence (Southern Urals)

Research subject. Mineralogical, petrological and geochemical features and formation conditions of gabbro-diorites and diorites composing the ore-bearing intrusions of the Kutuevsky Au-Cu porphyry ore occurrence in the Main Uralian Fault zone (Southern Urals). Materials and Methods. The content of rock-forming oxides was determined by the wet chemistry analysis, trace elements – by ICP-MS (ELAN 9000 mass spectrometer) and ICP-AES (ICPE-9000 atomic emission spectrometer). The study of the chemical composition of minerals was carried out using a Tescan Vega Compact scanning electron microscope with an Oxford Instruments Xplorer 15 energy-dispersive analyzer. P-T formation parameters were assessed using mineral geothermobarometers. Results and conclusions. The rock-forming minerals of gabbro-diorites and diorites are plagioclase with primary composition close to andesine (An = 32.83–34.43%) and clinopyroxene, represented by augite (Wo43.9–44.0En45.2–45.5Fs10.6–10.8) and diopside (Wo45.2–47.9En42.1–44.8Fs9.2–10.3). Accessory minerals include zircon, magnetite, titanite, apatite, and titanomagnetite. Clinopyroxene is largely replaced by green hornblende (6.956–7.169 a.f.u. Si, 0.73–0.76 Mg/(Mg+Fe2+)) of unknown genetic nature. Clinopyroxene crystallization occurred at T = 1010–1072°C and P = 1.35–1.78 kbar. The intrusive rocks of the Kutuevsky ore occurrence and other porphyry occurrences of the MUF zone in the Southern Urals have similar petro-geochemical characteristics that correspond to igneous rocks formed at the early stages of development of ensimatic island arcs. They have a moderate potassium composition, normal alkalinity, calc-alkaline or transitional tholeiitic-calc-alkaline composition. Spinel peridotites of the lithospheric mantle, metasomatized by subduction fluids, are assumed to be the source of magmas for gabbro-diorites and diorites of the ore occurrence. Crust contamination processes also influenced the composition.