Bakayskoe gold occurrence (Southern Urals): Ore mineralogy and formation conditions

Research subject . In morphological terms, the Bakayskoe gold occurrence comprises an area of fracture and crushing of sublatitudinal strike in the granitoids of the Turgoyak massif (C1-2), which contains quartz veins with sulfide mineralization. The vein zone is about 1.5 km in length. The Turgoyak massif is located on the border of the Magnitogorsk and Central Ural areas, being a satellite of the Syrostan massif. Materials and methods . Samples collected in old workings and ore stockpiles were studied by optical and SEM microscopy. Fluid inclusions in vein quartz were studied by thermoscryometry. Results. The predominant ore minerals were found to be pyrite and galena; rarely, the ores contain chalcopyrite, tetrahedrite, sphalerite, sulfosalts, and bismuth sulfotellurides. In the studied samples, gold is present as both native gold (primary and supergene), and tellurides and sulfides. Ores are partly oxidized. The average and median values of the homogenization temperature (Tg) of primary and primary-secondary inclusions from ore quartz are about 242– 247°C. The average and median Tg of quartz from shale is slightly lower (222 and 215°C, respectively), with a significant scatter of values. The concentration of salts in NaCl equivalent, determined by the melting point of the last ice crystal, varies from 1.4 to 13.0 wt % in FI from the host shales and 0.2–5.6 wt % in ore quartz. Such wide variations in salinity in all the studied cases may indicate changes in FI by secondary processes.