Newly Recognized Vein-Type Cu–Pb–Zn Mineralization in the Xiben Fluorite Deposit, South China: Constraints from S–Pb Isotopes and Sphalerite Trace Elements

A key genetic and exploration question is whether the variable spatial association between fluorite and polymetallic sulfide mineralization in the Nanling metallogenic belt (South China) reflects a common deep-seated magmatic–hydrothermal system or concealed intrusions. The recent discovery of significant, previously unrecognized vein-type Cu–Pb–Zn mineralization within the giant Xiben fluorite deposit (4.91 Mt @ 33.97% CaF2) in the southern Hunan district provides a unique opportunity to address this issue. This study presents an integrated investigation combining detailed petrography, sphalerite trace element geochemistry, and in situ sulfur and lead isotope analyses of galena. The Cu–Pb–Zn mineralization occurs as sulfide-bearing quartz veins exclusively associated with the final paragenetic stage (Stage VI), characterized by a pyrite-chalcopyrite-galena-sphalerite assemblage. Sphalerite is texturally homogeneous and compositionally distinct. It shows pronounced enrichment in Mn, Fe, and In (median values of 258, 15,287, and 349 ppm) and significant depletion in Ga and Ge (median values of 7.04 and 7.16 ppm). This signature is typical of magmatic–hydrothermal systems. In situ sulfur isotope analyses of galena yield δ34S values ranging from −7.76‰ to −0.10‰ (avg. −4.33‰), consistent with a magmatic sulfur source. Galena displays homogeneous lead isotope compositions that plot within the field of regional magmatic–hydrothermal deposits in all three Pb isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb), but are clearly distinct from local sedimentary strata and the adjacent Caledonian Penggongmiao batholith. The Cu–Pb–Zn mineralization is characterized by pervasive silicification without dolomitization, vein-type orebodies, and a simple sulfide assemblage. The geological, geochemical, and S–Pb isotopic fingerprints collectively indicate that the metals and sulfur were predominantly derived from a magmatic–hydrothermal source, with the likely involvement of concealed intrusions as suggested by regional gravity and aeromagnetic anomaly data. We propose that the Xiben Cu–Pb–Zn mineralization likely represents a distal expression of a buried magmatic–hydrothermal system, with sulfide precipitation driven by progressive cooling and decreasing sulfur fugacity (fS2) under moderate-temperature (215–311 °C, mean 281 °C), moderate-fS2 (log fS2 = −14.3 to −12.5). This study documents a new Cu–Pb–Zn mineralization type in the Xiben fluorite deposit, links it to a concealed magmatic–hydrothermal system, and suggests that fluorite orebodies in the region may be underlain by coeval sulfide mineralization, providing a new exploration paradigm for concealed systems in the Nanling belt and similar terrains.