Genesis of the Sihe Pb-Ag-Au Deposit in the Western Qinling Metallogenic Belt, China: Constraints from Apatite U-Pb and Mica Rb-Sr Geochronology and S-Pb Isotopes

The Sihe Pb-Ag-Au deposit in the West Qinling metallogenic belt (WQMB) offers a critical opportunity to investigate the timing and sources of Indosinian polymetallic mineralization. This study integrates apatite U–Pb and mica Rb–Sr geochronology with in situ S–Pb isotopes to constrain the formation age, metal sources, and genetic model of the deposit. Four hydrothermal stages are recognized: Stage I (quartz–pyrite veins), Stage II1 (quartz–pyrite–arsenopyrite–gold veins), Stage II2 (quartz–polymetallic sulfide–gold veins), and Stage III (quartz–carbonate veins), with Stages II1 and II2 representing the main Pb–Ag–Au mineralization events. These stages are hosted in the metamorphosed volcanic-sedimentary rocks of the Sinian–Ordovician Danfeng Group and are structurally controlled by NEE-trending secondary faults. Apatite grains from Stage II1 mineralization yield a weighted mean 207Pb/235U age of 235.7 ± 9.2 Ma (MSWD = 0.022), interpreted as the main mineralization age, whereas mica Rb–Sr dating yields a well-fitted isochron age of 266.4 ± 6.7 Ma (MSWD = 0.82), which is interpreted as a Middle Permian thermal event unrelated to ore formation, although its regional significance remains to be independently verified. The δ34S values of pyrite, arsenopyrite and galena range from 4.50‰ to 8.05‰ (mean = 6.38‰), significantly higher than typical magmatic sulfur (0‰–5‰), which is consistent with a mixed sulfur source with significant sedimentary contribution and subordinate magmatic input. Lead isotopes of galena are highly homogeneous (206Pb/204Pb = 18.340–18.371; 207Pb/204Pb = 15.774–15.808; 208Pb/204Pb = 38.909–39.133) and plot within the orogenic lead field, overlapping with the range of Indosinian granitoids in the WQMB but showing slightly elevated 207Pb/204Pb and 208Pb/204Pb ratios, suggesting a genetic link to Indosinian magmatism, although a unique magmatic metal source cannot be demonstrated and the relative contributions of magmatic versus crustal reservoirs cannot be quantified without Pb isotope data from the host rocks. The ~236 Ma age aligns with early Indosinian magmatism (248–234 Ma) and predates the regional magmatic–metallogenic peak (220–210 Ma). The Sihe deposit is therefore interpreted as a hydrothermal vein-type deposit formed during the compression-to-extension transition of the Indosinian orogeny (~236 Ma), representing an early Indosinian polymetallic mineralization episode within the regional gold–polymetallic metallogenic system. This integrated geochronological and isotopic approach provides a robust framework for unraveling multi-stage mineralization and metal sources in orogenic settings.