Mineralogical and geochemical features of the East Aktahs gold ore deposits in the South Urals

Abstract

Introduction: Recently, the interest in performing mineralogical studies at gold deposits has increased due to the fact that crust-mantle interactions are reflected in the mineral composition of ores. The article presents the results of mineralogical, microthermometric, and isotope-geochemical studies of the East Aktash gold and quartz ore deposits located in the south of the Bashkir meganticlinorium (BMC) in the Avzyan ore region (AOR). Methods: The mineralogical study was conducted using X-ray diffraction analysis. The chemical composition of the samples was determined using X-ray fluorescence spectroscopy, inductively coupled plasma atomic emission spectrometry (ICP-AES), and photometric analysis. The Au and Ag content were determined using atomic absorption spectroscopy. The isotopic composition of sulphur was determined using the MI-1201 mass spectrometer (Russia), and the isotopic compositions of carbon and oxygen were determined using the DELTA V Advantage mass spectrometer (Germany). The microthermometric study of fluid inclusions of quartz was performed using the LinkamTHMSC-600 thermocryogenic table. Results and discussions: As for microthermometric characteristics (Teut=–34.8...–28.4°С; Тgon=228–382°С), the pyrite-ankerite-quartz and quartz veins are similar to the quartz veins from the gold deposits of the AOR and are different from the metamorphogenic quartz formed as a result of regional postdiagenetic transformations of the Riphean deposits of the region. Sulphur in the ore deposits shows the greatest correlation coefficients with Rb, Ва, К2О, V, F, Fe2O 3, Ni, and Sc, as well as with such minerals as chlorite and shale. As for gold, sulphur shows weak negative correlation. The values of δ34S pyrites are within the range from 1.3 to 14.0‰. The isotopic compositions of carbon and oxygen in ankerites are –5.0...–3.7‰ and 19.2...20.9‰ respectively, while the inclosing limestones of the Katav formation have the values of 1.4 and 20.0‰. Conclusions. The formation of the East Aktash ore deposits was caused by the deep migration of fluids. According to the isotopic data on ankerite, which is syngenetic to quartz veins, the fluids related to the magmatic source. The calculations of the pressure of fluids by the homogenising temperature of СО2 to fluid inclusions of quartz of the East Aktash ore deposits give the values of 1280–1420 bar, so the quartz veins are assumed to be formed at a depth of about 4 km. The temperatures of the fluids were 250–400°С. Pyrites in the ore deposits were formed with the participation of sulphur from the crust. The lack of correlation between the gold content in the rocks and the concentration of sulphur in them, as well as the trace presence of Au in the pyrite concentrates, indicate that presence of gold in the ore deposits is not related to sulphides.