Timing of formation and overprinting for the oldest VMS deposit (Tieshajie Cu deposit) in South China

The Tieshajie copper deposit is located in the northeastern part of the Qin-Hang metallogenic belt in South China. There is an ongoing debate regarding its ore-forming type, with theories proposing between ancient seafloor volcanic massive sulfides (VMS) and magmatic-hydrothermal deposits. This study conducts detailed investigations on geological features, zircon-apatite-titanite U–Pb geochronology, and in-situ sulfur isotope studies to constrain the genesis and evolutionary history of the deposit. The zircon U–Pb dating of the host rock, spilite, yielded a concordant age of 1015 ± 6 Ma. Combined with the presence of abundant massive ores and in-situ sulfur isotope compositions, this indicates that the main ore-forming event of the Tieshajie copper deposit is related to VMS mineralization, recording the oldest VMS ore-forming process in South China. Sulfur isotopic simulation of the chalcopyrite (δ 34 S = 4.90–5.73‰) suggests that thermochemical reduction of seawater sulfates is the dominant sulfur source with possible contributions from the surrounding rocks. We also identified a small quantity of vein-type ores and evident alteration features such as potassic, silicic, and fluorine alterations. U–Pb dating of hydrothermal apatite and hydrothermal titanite yielded isochron ages of 164.0 ± 25 Ma and 163.0 ± 19 Ma, respectively. These are consistent with the hydrothermal alteration age (166.0 ± 18 Ma) recorded by apatite in the host rock. Additionally, microscopic observations of the co-occurrence of ilmenite, magnetite, and apatite suggest that the Tieshajie copper deposit underwent modification by oxidized magmatic-hydrothermal fluids during the Yanshanian period. Considering the regional tectonic evolution, these two ages may correspond to the rifting oceanic island environment of the Late Mesoproterozoic and the hydrothermal overprinting processes related to the subduction of the Pacific Plate during the Yanshanian period. In summary, this study provides new insights into the genesis and evolution of the oldest VMS deposits in the Qin-Hang metallogenic belt and offers guidance for the study of ancient VMS deposits globally.