Characteristics, geochronology, and formation conditions of W mineralization and its relationship with Au mineralization in the Zhaishang gold deposit, West Qinling Orogen, China

Abstract

The West Qinling Orogen is an important Au-W ore belt; however, the spatio-temporal and genetic relationships of W and Au ores remain unclear. This study takes the Zhaishang deposit, one of the largest Au-W deposits in the West Qinling Orogen, to trace their connections. The geological observation reveals that high-grade W ores are commonly characterized by extensional hydrothermal veins. In contrast, high-grade Au ores are hosted by breccia zones, which cut the W ore-controlling structures, even though the W and Au ores overlap locally. The mineral assemblages of W ores associated with carbonatization and silicification consist of scheelite and pyrite with few polymetallic sulfides, in contrast to the abundance of polymetallic sulfides and native gold in Au ores linked with silicification. Textural and in situ trace element analyses reveal that the scheelites show a decrease in Na, U, Mo, Nd, and calculated Eu anomaly values and an increase of Th concentrations and Ce anomaly values from early- to late-generation scheelites. This finding, in conjunction with mineral assemblages and previous thermodynamic modeling, suggests a decrease in salinity and oxygen fugacity (fO2) of the oxidized ore-forming fluids of W mineralization, as opposed to the reduced fluids of Au mineralization. The plots of scheelite at Zhaishang in the ternary LREE-MREE-HREE diagram are compatible with those of W deposits related to magmatic-hydrothermal fluids. In situ U-Pb dating of scheelite from W ores constrains the precise timing of W mineralization at ca. 227.9 ± 2.3 and 225.6 ± 3.0 Ma in a collisional orogenic setting. The unequivocal crosscutting relationship, contrasting mineral assemblages, and physiochemical conditions suggest that the W and Au mineralization at Zhaishang are due to two separate mineralization events. The identification of overlapped W and Au mineralization events at Zhaishang is crucial for guiding the regional exploration of W and Au deposits.