Petrological-geochemical features of ore-bearing effusive and intrusive rocks of the Nikolaevskoe gold-porphyry deposit (the Southern Urals)

Abstract

Object. The article considers the results of geochemical studies of ore-bearing effusive and intrusive rocks of the Nikolaevka gold deposit, located in the zone of the Main Uralian fault in the Southern Urals. The deposit belongs to the gold-porphyry type which unconventional for the Urals and poorly studied here. Methods. The rock composition was determined by chemical analysis (IG UFRC RAS), mass-spectrometry with inductively coupled plasma on the ELAH 9000 quadrupole mass-spectrometer (IGG UB RAS) and X-ray fluorescence analysis on the VRA 30 spectrometer (IG UFRC RAS). Results. It is established that gold-porphyry mineralization associated with an island-arc volcanic-intrusive complex that unites plagiophyric and pyroxene-plagiophyric basalts, their tuffs, gabbro intrusions and ore-bearing series of dikes plagiophyric dolerite-porphyrites and gabbro-diorite-porphyrites. The rocks of the complex, including ore-bearing dikes, have normal alkalinity, tholeitic and transition from tholeitic to calc-alkaline composition. Among the volcanogenic formations, faunistically dated in the zone of the Main Ural fault in the Southern Urals, by a number of geochemical parameters (for example, the ratios of Zr/Nb and Nb/Th) the effusive and intrusive rocks of the deposit are the closest to the volcanics of the pyrite-bearing Baymak-Buribay Formation (D 1 e 2 ), lying at the base of the section of the Magnitogorsk island arc, and, in all probability, are their age analog. At the same time, the gold-bearing complex differs from the Baymak-Buribay Formation by a general increased iron and titanium content, with a reduced magnesia of all rock types, as well as the absence of boninites and acid volcanics, the predominance of porphyry rock types, which can be due to the geodynamic conditions of its formation. Conclusion. Geodynamic analysis of geochemical data using diagrams (La/Sm)N–TiO 2 and V–Ti/1000 suggests that the Nikolaevka deposit was formed in the articulation zone of the island arc and the back-arc basin. Gold-porphyry mineralization associated with the Late Emsian volcanic processes was isolated in the Southern Urals for the first time.