In situ Cu isotopes reveal the mechanism of Sn-Cu mineralization: A case study from the Jinkeng deposit, South China

• In situ Cu isotope data show large fractionation between Sn-rich and Cu-rich ores. • Cu isotopes and modeling show water–rock interaction crucial in Sn-Cu mineralization. • Cu source of the Sn-Cu coexisting deposit may be the Cu-rich country rocks. This study presents high-resolution in situ Cu isotope data for chalcopyrite from the Jinkeng Sn-Cu deposit, Guangdong Province, China, with the aim of understanding how the Sn-Cu mineralization was formed. Results show that early chalcopyrite (quartz-cassiterite stage) has the lowest δ 65 Cu values (–0.01 to +0.02 ‰), similar to δ 65 Cu values (∼0‰) in most granites. In contrast, chalcopyrite from the subsequent cassiterite-sulfide stage exhibits the highest δ 65 Cu values (+0.90 to +0.97 ‰), comparable to those of the volcanic country rocks (+0.87 to +1.00 ‰). δ 65 Cu values in late chalcopyrite gradually decrease (from +0.79 to +0.45 ‰) across the latest ore-forming stages, from quartz-sulfide to quartz-calcite-sulfide mineral assemblages. Observed variation in δ 65 Cu is explained by water–rock interaction. Quantitative geochemical modeling also demonstrates that the Cu isotopic compositions of chalcopyrite record variable degrees of water/rock ratios during water–rock interactions. Therefore, our results suggest that water–rock interaction plays a critical role in providing Cu for Sn-Cu ore systems. The study further indicates that in situ Cu isotope analysis provides an accurate and efficient tool for diagnosis of magmatic-hydrothermal processes.