Fluid and metal sources of the Wenquan porphyry molybdenum deposit, Western Qinling, NW China

摘要

The Wenquan porphyry moybdenium deposit, Western Qinling, NW China, with a resource of 247 million tonnes at 0.048% Mo, formed during Triassic collision between South China and North China blocks. Ore fluids at Wenquan vary widely in composition from single-phase, low-salinity aqueous to low-salinity vapor, and hypersaline inclusions. Fluid inclusion assemblages in a quartz-molybdenite vein contain >60 vol% vapor, are equant to negative-crystal shaped, and form clusters or distinct inclusion trails. They yield homogenization temperatures of 285-295 degrees C, with an estimated trapping temperature of 425 degrees C, suggesting a paleodepth of about 5 km calculated at a pressure correction of 100-150 MPa. Hydrothermal K-feldspar from early stockwork veins related to potassic alteration have calculated delta O-18(fluid) values of -1.9 parts per thousand to +1.9 parts per thousand. Hydrothermal sericite from an overprinting phyllic alteration associated with late quartz-pyrite veins has calculated delta D-fluid values between -68 and -60%, and delta O-18(fluid) values from -3.7 to +1.4 parts per thousand. These isotopic data suggest that both early- and late-stage fluids are dominated by magmatic fluids, with influx of meteoric water during the late stage. Exsolution of volatiles from magma in a late-stage open system, results in more variable delta D values than an earlier closed-system.