High-grade sediment-hosted Cu mineralization at Yushui, South China: In situ Cu-Pb-S-Nd isotopes unravel multiple metal sources

Abstract Yushui is a high-grade stratiform base metal deposit (291 Mt at 3.5% Cu, 4.3% Pb, 2.9% Zn, 112 g/t Ag) located at a sedimentary unconformity between Lower Carboniferous red sandstone and Upper Carboniferous dolostone and limestone in South China. The genesis of the Yushui deposit is much debated, with several models put forward, including Carboniferous (ca. 308 Ma) seafloor hydrothermal formation, Triassic (ca. 223 Ma) epigenetic sediment-hosted stratiform Cu (SSC) mineralization, or Middle–Late Jurassic magmatic-hydrothermal genesis. The Yushui deposit is mainly composed of massive Cu mineralization, which is divided into a lower chalcopyrite-rich part and an upper galena-rich part, with vein-style orebodies cutting through the massive ore. We combine in situ sulfide S-Pb-Cu isotope and trace-element analysis with apatite in situ Nd isotope and trace-element analysis to unravel the ore genesis and metal sources of the Yushui deposit. The lower chalcopyrite-rich massive orebody contains variable amounts of heavy rare earth element (HREE) minerals and uraninite. The sulfides have a radiogenic Pb isotopic signature (208Pb/204Pb: 37.368–39.902; 207Pb/204Pb: 15.324–15.991; 206Pb/204Pb: 17.926–23.642), variable δ34S values (−16.8‰ to −6.8‰), and a wide range of δ65Cu values (−1.02‰ to +1.98‰). Radiometric data on uraninite and HREE minerals coexisting with chalcopyrite indicate a Triassic (ca. 223 Ma) SSC-style basinal brine origin. In contrast, the upper galena-rich part of the massive orebody and the vein-style orebody display a linear array of Pb isotopic ratios (208Pb/204Pb: 37.217–39.910; 207Pb/204Pb: 15.239–15.965; 206Pb/204Pb: 17.535–18.736), magmatic sulfur signatures, and markedly lower δ65Cu values (down to −1.70‰). These isotopic characteristics are consistent with those of Middle–Late Jurassic Cu deposits and associated magmatic rocks in southeastern China, and coupled with the mineralization age of vein-style orebodies (ca. 163 Ma), thus indicate a magmatic-hydrothermal origin. Moreover, apatite Nd isotope and trace-element data indicate that the mineral system is genetically related to both Triassic basinal brines and Middle–Late Jurassic magmatic-hydrothermal fluids. Collectively, the data suggest that the ore-forming metals at Yushui were derived from Triassic oxidized basinal brines that leached the footwall sandstone and from Middle–Late Jurassic magmatic-hydrothermal fluids. There is no evidence for Carboniferous seafloor hydrothermal activity contributing Cu. Cu isotope mass-balance calculations suggest that magmatic-hydrothermal fluids likely contributed ~40% to 70% of the Cu in samples that exhibit mixed S-Cu isotope characteristics. Therefore, the Triassic SSC mineralization was enriched and overprinted by Middle–Late Jurassic magmatic-hydrothermal fluids. Our results indicate multiple metal sources were involved in generating the high-grade mineralization and that these findings might be applied to analogous deposits worldwide. The study highlights the advantages of coupling in situ Cu isotopes with traditional isotope and trace-element geochemistry for identifying metal sources in complex mineral systems.