Metallogeny of the Sarsuk polymetallic Au deposit in the Ashele Basin, Altay Orogenic Belt, Xinjiang, NW China: Constraints from mineralogy, fluid inclusions, and He-Ar isotopes

The Sarsuk polymetallic Au deposit is located in the Devonian volcano-sedimentary Ashele Basin of the Altay Orogenic Belt (AOB), Xinjiang. Mineralization in the deposit is confined to an area of 400 m long and 50-120 m wide, and occurs as dense massive, banded, disseminated, stockwork, and veinlet ore structures. Exhalative sedimentary, subvolcanic hydrothermal, and supergene periods have been distinguished. Although syn-depositional ores can be found, the main orebodies from this period have been eroded. Thus, the orebodies being mined are hosted mainly in a rhyolite porphyry subvolcanic intrusion within the volcanic conduit of a hydrothermal system. Electron microprobe analysis of the main ore minerals indicates that pyrite is the main Au-beating mineral. The Au occurs mainly as either inclusions or in fissures in the form of tellurides, and is closely associated with Ag. Zn/ Fe ratios of sphalerite vary from 11.37 to 251.36, with an average of 96.65, and the formation temperatures of chlorites are 155-206 degrees C, with an average of 175 degrees C. The compositions of sphalerite and chlorite indicate that the main mineralization occurred in a low- to moderate-temperature environment. Homogenization temperatures of fluid inclusions from ores of the main mineralization stage vary generally from 130 degrees C to 390 degrees C, with peaks at 140 degrees C, 190 degrees C, and 310 T. Their corresponding salinities range from 3.0 to 10.0 wt.% NaCl equivalent, with densities from 0.56 to 1.03 g/cm(3). The main mineralizing fluid of the deposit is characterized by high-low temperatures, moderate-low salinities, and moderate-low densities. The He-3/(4) He ratios of fluid inclusions in pyrite are 0.02 to 0.44 Ra, and the percentage of mantle-derived He ranges from 0.06% to 7.45%. Ar-40/Ar-36 ratios range from 319.6 to 458.5, and the percentage of Ar-40* varies from 7.53% to 35.55%. Both fluid characteristics and He and Ar isotope compositions suggest that the ore-forming fluids were a mixture of magmatic fluid and seawater. The CO2 -rich fluids in the ores suggest that boiling occurred. The mixing of a high-temperature and high-pressure magmatic fluid with deeply circulating seawater caused a decrease in temperature, and phase separation in the ore-forming fluids. Thus, the precipitation of the most economically important ore materials was the result of this mixing process. (C) 2016 Elsevier B.V. All rights reserved.