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

Abstract

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 >60vol% vapor, are equant to negative-crystal shaped, and form clusters or distinct inclusion trails. They yield homogenization temperatures of 285–295°C, with an estimated trapping temperature of 425°C, suggesting a paleodepth of about 5km calculated at a pressure correction of 100–150MPa. Hydrothermal K-feldspar from early stockwork veins related to potassic alteration have calculated δ18Ofluid values of −1.9‰ to +1.9‰. Hydrothermal sericite from an overprinting phyllic alteration associated with late quartz-pyrite veins has calculated δDfluid values between −68 and −60‰, and δ18Ofluid values from −3.7 to +1.4‰. 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 δD values than an earlier closed-system.Molybdenite and pyrite have δ34S values ranging from 1.1‰ to 6.6‰, indicating that sulfur at Wenquan had a magmatic source. A linear relationship between δ34S values of sulfides separated from early potassic alteration and late phyllic alteration could reflect incorporation of isotopically heavy evaporate sulfate into source magmas from underlying Devonian sedimentary rocks during late alteration. The δ56Fe whole-rock values of altered porphyries range from 0.08‰ to 0.26‰, similar to δ56Fe values of 0.15‰–0.32‰ for pyrite from quartz veins related to both the potassic and phyllic alteration assemblages. The δ56Fe values of pyrite are positively correlated to, but in general slightly lighter than, those of altered porphyries, indicating similar metal sources. Moreover, both altered porphyries and the pyrite are progressively enriched in heavy Fe isotopes from biotite, through potassic to late phyllic alteration, consistent with isotopic evolution from an early lithostatic load to a late hydrostatic load.The Triassic intrusive rocks and fluids responsible for mineralization were derived from a deeply-sourced hybrid mantle-crustal magma crystallizing at a paleodepth of 5km under lithostatic load. External Late Triassic meteoric or Devonian formational fluids became a part of the hydrothermal system during the post-fracturing final stage of ore formation under a hydrostatic regime. The formation of the Wenquan magmatic-hydrothermal systems spans the ductile-brittle transition based on a normal thermal gradient as constrained by the temperature range of 550–300°C. The early, high-temperature potassic alteration developed under a ductile regime, whereas the later, low-temperature phyllic alteration correlates to a brittle environment, where hydraulic rock fracturing enhances permeability and fluid circulation. Most sulfur and metals were precipitated from vapor-rich fluids resulting from cooling and expansion of a single-phase fluid exsolved from a hybrid magma.