Scheelite as an archive of distinct ore-forming fluid pulses and later hydrothermal alteration in the Huxingshan W deposit, South China

The formation of magmatic-hydrothermal tungsten deposits typically involves multiple pulses of ore-forming fluids, which lead to multi-stage mineralization and alteration of primary ore minerals; however, the origin and evolution of these fluid pulses and the associated alteration processes remain poorly understood. In this study, we analyzed the textures, trace element concentrations, and Sr isotopic compositions of two types of scheelite (designated as Sch-A and Sch-B) collected from a scheelite−quartz−muscovite−calcite vein and a scheelite−quartz−muscovite−fluorite vein, respectively, in the Huxingshan tungsten deposit, South China. Cathodoluminescence (CL) imaging reveals a two-stage growth of both Sch-A and Sch-B: the primary crystallization of Sch-A-1 (homogeneous, dark-blue CL) and Sch-B-1 (oscillatory-zoned, dark CL), followed by their subsequent partial replacement by Sch-A-2 (porous, turbid, light-blue CL) and Sch-B-2 (homogeneous, bright CL), through fluid-mediated coupled dissolution-reprecipitation (CDR) processes. Geochemical variations indicate that Sch-A-1 precipitated from an oxidizing, fluorine-poor fluid characterized by lower Sr and rare earth element (REE) concentrations and less radiogenic 87Sr/86Sr ratios (0.70954−0.71013), whereas Sch-B-1 precipitated from a reducing, fluorine-rich fluid characterized by higher Sr and REE concentrations and more radiogenic 87Sr/86Sr ratios (0.71758−0.71771). The two geochemically distinct fluid pulses probably originated from two separate magmatic-hydrothermal pulses released by an undiscovered intrusion in the Huxingshan district, and were later partially altered during fluid-rock interaction. During CDR processes, the chemical compositions of altered Sch-A-2 and Sch-B-2 were partially modified or almost completely re-equilibrated compared to the primary Sch-A-1 and Sch-B-1, respectively. Results show that hydrothermal alteration via CDR processes can effectively mobilize trace elements, alter 87Sr/86Sr ratios, and cause significant REE fractionation as well as disturbance in the U-Pb isotopic system of scheelite. Our study shows that multiple fluid pulses—especially late-stage F-rich exsolved fluids—are critical for tungsten enrichment in magmatic-hydrothermal deposits. Scheelite effectively records these discrete pulses, but secondary overprinting may distort U-Pb, REE, and Sr isotopic proxies, limiting their use for fluid source and dating studies.