Sources of Ore-Forming Materials and Physicochemical Evolution of Qingshuitang Veined Pb–Zn Deposit in the Qin-Hang Metallogenic Belt, South China: Insight from In Situ Pb Isotopes and Fluid Inclusions

The Qin-Hang Metallogenic Belt (QHMB) of South China includes abundant structurally controlled hydrothermal vein-type Pb–Zn deposits, which are predominantly hosted within low-grade metamorphosed sedimentary sequences. However, due to the lack of effective constraints on the sources of ore-forming materials and ore fluid physicochemical conditions, their genesis remains unknown. The Qingshuitang, as a representative Pb–Zn deposit in the center of the QHMB, is characterized by multi-phase alteration/mineralization. The in situ Pb isotopes of multi-phase galena (206Pb/204Pb: 18.368–18.416, 207Pb/204Pb: 15.697–15.721, 208Pb/204Pb: 38.741–38.829) at Qingshuitang suggest that the ore lead is primarily sourced from the upper crust, and regional Cambrian–Ordovician sedimentary strata represent one possible source of metals, although contributions from basement and local granitoids cannot be ruled out. Fluid inclusions (FIs) preserved within mineralization Stages II, III and IV are primarily liquid-rich, and the observed eutectic-melting temperature is approximately −21.8 °C, which suggest that the ore-forming fluid is dominated by the H2O-NaCl aqueous system. FI microthermometric data obtained for Stages II–IV reveal that the fluids have a wide ranges of temperatures (102–395 °C) and salinities (0.71–19.45 wt% NaCl equiv.). For sphalerite-hosted fluid inclusions, salinity, density, and pressure show an overall decreasing trend from Stage II through Stage III to Stage IV, while a prominent temperature drop occurs from Stage II to Stage III, and a relatively stable low-temperature hydrothermal condition is maintained in Stage IV. No valid primary fluid inclusion microthermometric data are available for Stage I, which consists primarily of barren quartz veins. Fluid mixing, coupled with decreases in temperature, may have controlled Pb–Zn precipitation, based on the large variations in temperatures and salinities of FIs. Integrating previous research data, we propose the deposit is a sedimentary-hosted deposit similar to CD Pb–Zn systems that is controlled by crosscutting faults and related to hot brines with possible magmatic–hydrothermal contributions.