Hydrothermal apatite records overprinting tungsten and gold mineralization events in the Jiangnan orogen, South China

The Huangjindong polymetallic deposit in the Jiangnan orogen provides an opportunity to resolve the contentious genetic relationship between Au and W in widespread deposits. Thick, Au-poor quartz-scheelite veins (Quartz-W Stage) are overprinted by thin, Au-rich veins (Quartz-Au Stage) and calcite-quartz veins with minor Au (Quartz-Carbonate Stage). Hydrothermal apatite of all stages provides critical evidence of fluid evolution. Overgrowth rims and dissolution-reprecipitation textures of apatite of the Quartz-W Stage, both with auriferous arsenopyrite inclusions, are related to the Quartz-Au Stage. These have higher light rare earth elements (LREEs) than apatites of the Quartz-Au Stage, with rare greater LREE enrichment indicating mobilization by Cl-rich fluids. Additionally, nucleation of Quartz-Au Stage apatites adjacent to scheelite undergoing dissolution indicates local Ca supersaturation and precipitation during fluid interaction. Fractionation factors between coexisting quartz and apatite indicate Rayleigh-dominated processes, implying that without external inputs, δ18Ofluid ratios should decrease during fluid evolution. The transition from the Quartz-Au Stage to the Quartz-Carbonate Stage reflects stronger interaction with the host slate, as indicated by higher δ18Ofluid and lower fluid/rock ratios in the latter. In contrast, the lower δ18Ofluid of the Quartz-Au Stage than the Quartz-W Stage implies a change in fluid source, as supported by distinct rare earth element (REE) contents and Sr/Y ratios in Quartz-W Stage apatite. The high δ18Ofluid and low REE, Y, and U contents of Quartz-W Stage apatite are consistent with crustal metamorphic fluids, whereas the δ18Ofluid ratios of the Quartz-Au and Quartz-Carbonate stages fall within the range predicted by fluid-rock interaction modeling for metasomatized mantle-derived fluids. Thus, isotopic data on hydrothermal apatite indicate that fractures in early, thick metamorphic quartz-scheelite veins acted as hosts for thin, auriferous veinlets deposited from mantle-derived auriferous fluid.