Fluid Evolution and Ore Genesis of the Juyuan Tungsten Deposit, Beishan, NW China

The newly discovered Juyuan tungsten deposit is hosted in Triassic granite in the Beishan Orogen, NW China. The tungsten mineralization occurred as quartz veins, and the main ore minerals included wolframite and scheelite. The age, origin, and tectonic setting of the Juyuan tungsten deposit, however, remain poorly understood. According to the mineralogical assemblages and crosscutting relationships, three hydrothermal stages can be identified, i.e., the early stage of quartz veins with scheelite and wolframite, the intermediate stage of quartz veinlets with sulfides, and the late stage of carbonate-quartz veinlets with tungsten being mainly introduced in the early stage. Quartz formed in the two earlier stages contained four compositional types of fluid inclusions, i.e., pure CO2, CO2-H2O, daughter mineral-bearing, and NaCl-H2O, but the late-stage quartz only contained the NaCl-H2O inclusions. The inclusions in quartz formed in the early, intermediate, and late stages had total homogenization temperatures of 230–344 °C, 241−295 °C, and 184−234 °C, respectively, with salinities no higher than 7.2 wt.% NaCl equiv (equivalent). Trapping pressures estimated from the CO2-H2O inclusions were 33−256 MPa and 36−214 MPa in the early and intermediate stages, corresponding to mineralization depths of 3–8 km. Fluid boiling and mixing caused rapid precipitation of wolframite, scheelite, and sulfides. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition and from magmatic to meteoric, as indicated by decreasing δ18Owater values from early to late stages. The sulfur and lead isotope compositions in the intermediate-stage suggest that the Triassic granite was a significant source of ore metals. The biotite 40Ar/39Ar age from the W-bearing quartz shows that the Juyuan tungsten system was formed at 240.0 ± 1.0 Ma, coeval with the emplacement of granitic rocks at the deposit. Integrating the data obtained from the studies including regional geology, ore geology, biotite Ar-Ar geochronology, fluid inclusion, and C-H-O-S-Pb isotope geochemistry, we conclude that the Juyuan tungsten deposit was a quartz-vein type system that originated from the emplacement of the granites, which was induced by collision between the Tarim and Kazakhstan–Ili plates. A comparison of the characteristics of tungsten mineralization in East Tianshan and Beishan suggests that the Triassic tungsten metallogenic belt in East Tianshan extends to the Beishan orogenic belt and that the west of the orogenic belt also has potential for the discovery of further quartz-vein-type tungsten deposits.