Evolution of the ore-forming fluid of the Awanda gold deposit in southwestern Tianshan orogenic belt, Xinjiang
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The Awanda gold deposit, a recently defined medium-size gold deposit in Baicheng County, Xinjiang, is located in the southwestern Tianshan orogenic belt. The gold mineralization occurs primarily within subsidiary faults in ore-hosting schist rocks. The sulfide mineral assemblage is dominated by arsenopyrite, pyrrhotite, loellingite, with minor pyrite and gudmundite; and the sulfide mineral contents in the gold ore are less than 15%. The wall-rock alteration in the deposit includes silicification, sericitization and minor carbonatization. Combining microthermometric analyses of fluid inclusions and the arsenopyrite geothermometer, authors discuss the evolution of the ore-forming fluids of this deposit. It was shown that fluid inclusions in quartz grains from the ore consist of major two-phase aqueous inclusions and minor three-phase CO2-bearing ones. The two-phase aqueous inclusions have a bimodal-distributed homogenization temperatures of 188-380? and salinities of 6.9%-20.7%NaCl, while the CO2-bearing inclusions have the homogenization temperatures of 238-347? and salinities of 2.8%-7.0%NaCl. The arsenopyrite geothermometer shows that the sulfide assemblage of arsenopyrite+pyrrhotite+loellingite in Awanda gold deposit formed in a high-temperature condition ranging from 345? to 420?, and there are some low-temperature arsenopyrites that can not to be applied by the arsenopyrite geothermometer. It can be synthesized that the ore-forming fluid experienced from the high-temperature epoch to the low-intermediate-temperature one. In the high-temperature epoch, the ore-forming fluid are characterized by the high homogenization temperatures of 270-380?, the trapping temperatures of 345-420? from arsenopyrite geothermometer and the calculated trapping pressures of 74-142 MPa (the metallogenic depths of 2.8-5.4 km evaluated by lithostatic pressure system), and the ore-forming fluid belonged to a low-intermediate-salinity H2O-CO2-NaCl system and depositing high-temperature arsenopyrites and other sulfides. In the low-intermediate-temperature epoch, the ore-forming fluid are characterized by the low-intermediate homogenization temperatures of 188-270?, the trapping temperatures less than 304? but larger than 270?, and the calculated trapping pressures lower than 104 MPa but higher than 52 MPa, and the ore-forming fluid belonged to a low-intermediate-salinity H2O-NaCl system and forming low-temperature arsenopyrites and other sulfides. According to the geological characteristics and the evolution of the ore-forming fluid of the Awanda gold deposit, it can be conclued that the deposit belongs to the epizonal-mesozonal orogenic gold deposit.
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