Two Episodes of Au and Ag–Pb–Zn Mineralization in the Kangshan Deposit, the Southern Margin of the North China Craton: Constraints from C–O–S Isotopes and Sphalerite Rb–Sr Dating

Polymetallic deposits are significant repositories of both bulk commodities and critical metals. However, determining the ages of multiple mineralization events in these deposits remains a persistent challenge. The Kangshan Au-polymetallic deposit, located in the southern margin of the North China Craton, exhibits distinct mineralization zonation, transitioning from deep-seated gold mineralization to shallow Ag–Pb–Zn mineralization, making it a natural laboratory for investigating composite mineralization processes. Based on mineral paragenetic assemblages, the ore-forming process can be divided into four stages: quartz with minor pyrite (Stage 1); quartz–pyrite–chalcopyrite with minor native gold (Stage 2); polymetallic sulfides–quartz–ankerite with native gold (Stage 3); and quartz–calcite–fluorite (Stage 4). In situ sulfur isotope analysis of pyrite from Stage 3 yields δ34SV-CDT values of 7.30‰–8.30‰, higher than the typical magmatic range and indicative of a heavier crustal (marine-sulfate) sulfur component incorporated into the late-stage ore fluids. Sphalerite from Stage 3 has an initial 87Sr/86Sr ratio of 0.708576 ± 0.000056, suggesting a mixed crust–mantle source. Calcite from Stage 4 has δ13CPDB values of −4.63‰ to −3.45‰ and δ18OSMOW values of 15.76‰–16.41‰, indicating that the late-stage ore-forming fluids were contaminated by carbonate wall rocks. Rb–Sr isotopic dating of sphalerite yields an age of 109.2 ± 2.3 Ma for the Ag–Pb–Zn orebodies, indicating that they formed in the Early Cretaceous, approximately 20 Myr later than the regional gold mineralization event (131.7 ± 4.6 Ma). This temporal gap suggests two distinct magmatic-hydrothermal mineralization events rather than a continuous evolutionary sequence. Integrating our results with the regional magmatic geochronological framework, we propose that the gold mineralization and Ag–Pb–Zn mineralization at the Kangshan deposit represent two separate hydrothermal events. Both events formed in an extensional tectonic setting associated with the destruction of the North China Craton and were spatially superimposed within the same fault system. The deposit is classified as a medium–low-temperature, post-magmatic hydrothermal vein system with a weak epithermal affinity.