<i>In-situ</i> geochemical and S-Pb-Sr isotopic constraints on fluid origin and evolution of the Heilongtan-Xiejiagou intrusion-related gold deposit, Tongbai Orogen, central China

Abstract The mechanisms governing the formation of complex mineral assemblages and fluid chemistry in structurally controlled gold deposits remain poorly understood. Textural, chemical, and isotopic characteristics of hydrothermal pyrite and apatite from gold veins provide crucial constraints on fluid compositions, which can potentially reflect episodic variations in source reservoirs and/or changes in fluid physicochemical conditions. This study presents detailed textural observations, in-situ trace element analyses, and S-Pb isotopic data for multiple texturally distinct pyrite generations from the Heilongtan-Xiejiagou structurally controlled gold deposit in the Tongbai Orogen. Integrated with hydrothermal apatite U-Pb ages and Sr isotopic compositions, these results offer new constraints on the deposit’s fluid source, evolutionary history, and genetic mechanism. Two primary types of gold mineralization are identified: (1) altered rock-type ores (predominantly disseminated and veinlet-style) dominated by invisible gold-bearing arsenian pyrite (particularly Py2c and Py3) and minor base metal sulfides, accompanied by intense wall-rock alteration (muscovitization, sulfidation, carbonatization, silicification); and (2) vein-type ores (quartz veins), characterized by visible gold, As- and Au-poor pyrite (Py4 and Py5), and abundant base metal sulfides, with limited alteration halos dominated by muscovite and quartz. Five mineralization stages (I–V) are distinguished, corresponding to seven texturally and temporally distinct pyrite generations: Py1 (Stage I), Py2a–Py2c (Stage II), Py3 (Stage III), Py4 (Stage IV), and Py5 (Stage V). LA-ICP-MS trace element analyses reveal systematic compositional differences between pyrites from the two ore types: altered rock-type pyrites (Py1–Py3) are enriched in Au (median 15 ppm in Py1, 195 ppm in Py3), As (median 2872 ppm in Py1, 26868 ppm in Py2b), Co (median 90 ppm in Py1, 177 ppm in Py3), and Ni (median 80 ppm in Py1, 75 ppm in Py3). Stage II Py2 exhibits a core-mantle-rim zoning, with an Au- and As-poor Py2a core, a porous and inclusion-rich Au-As-enriched Py2b mantle, and an oscillatory-zoned Au-As-rich Py2c rim). In contrast, vein-type pyrites (Py4–Py5) are severely depleted in Au (median 0.01 ppm in Py4, 0.03 ppm in Py5) and As (median 3.1 ppm in Py4, 1.1 ppm in Py5), with significantly lower concentrations of other trace metals (e.g., Co ≤ 32 ppm, Ni ≤ 95 ppm). Oscillatory zoning of lattice-bound As and Au in altered rock-type pyrites (Py2b and Py3) is attributed to cyclic fluid immiscibility and vapor condensation driven by pressure fluctuations. Subsequent fluid immiscibility and cooling promoted the co-precipitation of visible gold and base metal sulfides in vein-type ores. Sulfur isotope compositions (δ3⁴S) of sulfides from Stages I–III altered rock-type ores range from −1.7‰ to 9.5‰, indicating a mixed sulfur source involving magmatic and host-rock components. In Stages IV–V vein-type ores, sulfides and baryte display δ3⁴S values of −3.9‰ to 5.6‰ and 8.2‰ to 14.5‰, respectively; texturally equilibrated pyrite and baryte yield a calculated total sulfur δ3⁴S value of 4.9‰, consistent with a predominantly magmatic sulfur source. Pyrite and galena exhibit consistent in-situ Pb isotope compositions across all mineralization stages, pointing to a common lead source. Vein-hosted calcite (Stages IV–V) and scheelite (Stage V) display ⁸⁷Sr/⁸⁶Sr ratios of 0.7089–0.7102, indicative of magmatic Sr. In contrast, altered rock-type ankerite (Stages II–III) and apatite (Stage II) show more radiogenic Sr isotopic compositions (0.7115–0.7142), suggesting intense fluid-rock interaction. Hydrothermal apatite intergrown with Stage II pyrite yields a U-Pb age of 133.2 ± 5.0 Ma, consistent with the previously reported Rb-Sr isochron age (132.6 ± 2.7 Ma) for vein-type ores and the emplacement age of adjacent Qijianfeng granite (140–130 Ma). Collectively, field observations, textural analyses, geochronological data, compositional studies, and S-Pb-Sr isotopic results demonstrate that the Heilongtan-Xiejiagou gold mineralization was derived from a single Early Cretaceous magmatic-hydrothermal fluid system. Spatial and temporal variations in fluid physicochemical parameters (e.g., temperature, pressure, oxygen fugacity) and fluid-rock interactions account for the observed mineralogical diversity within this structurally controlled gold system.