Genesis of the Caijiagou Gold Deposit in the West Qinling Orogen, China: Constraints from In Situ Trace Element Analysis, Sulfur Isotopes of Sulfide, and Apatite U–Pb Dating

The newly discovered Caijiagou gold deposit is hosted in metasedimentary rocks of the Silurian Taiyangsi Formation in the West Qinling Orogen (WQO), central China. Three mineralization stages are recognized: (1) the quartz–sericite–pyrite stage (Stage I), characterized by fine-grained, disseminated pyrite (Py1) and arsenopyrite (Apy1) with trace amounts of native gold; (2) the main ore-stage, the quartz–polymetallic sulfide–ankerite stage (Stage II), which contains coarse-grained euhedral pyrite (Py2) and arsenopyrite (Apy2) intergrown with ankerite and native gold; and (3) the post-ore calcite stage (Stage III), which is barren. Hydrothermal alteration assemblages include silicification, sericitization, pyritization, and carbonatization. In situ trace element analyses show that gold is mainly hosted in Py2 (avg. 2.86 ppm Au) and Apy2 (avg. 1.95 ppm Au), with consistently low Co/Ni ratios (<0.6) in ore-stage sulfides, supporting a metamorphic fluid origin. Sulfur isotope values (δ34S = +6.47‰ to +11.96‰) point to sulfur derived from thermochemical reduction of marine sulfate. Apatite from the main ore stage (Stage II) yields a U–Pb lower intercept age of 237 ± 4.3 Ma (MSWD = 0.89), placing the mineralization in the Middle Triassic (Indosinian orogeny). These results are consistent with a sediment-hosted orogenic gold model, and the geochemical and geochronological signatures provide insights into the ore-forming process and regional metallogeny of the WQO.