The newly discovered Shuangjianzishan Ag-polymetallic deposit is in the southern part of the Great Xing'an Range of Inner Mongolia in NE China, and is China's largest Ag deposit with a 21,665 t silver reserves. Three types of hydrothermal mineralization are evident: relatively early disseminated and stringer mineralization, succeeded by a breccia-hosted and younger vein-type mineralization. Preliminary studies show that the deposit might represent a mixed ore system where the early disseminated and stringer mineralization is overprinted by the breccia-hosted and vein-type mineralization. Muscovite Ar–Ar and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) U–Pb dating of zircon, and geochemical analyses were completed at the deposit. The zircon U–Pb dating for the argentiferous quartz-veins yield a crystallization age of 148 ± 1 Ma, and muscovite coexisting with sulfides yields an Ar–Ar plateau age of 147 ± 2 Ma. Both dates, being the same within error indicate that the mineralization is ca. 148 Ma, which is a date that coincides with a wide-spread ca. 159–144 Ma magmatic event in the region. Stable isotopic data (δ 18 O H2O and δD) collected from fluid inclusions from the quartz vein-type mineralization are characterized by systematic and small variation, and this difference reflects the higher fluid fluxes and smaller amount of rock-buffering for the quartz veins. The δ 13 C values of −10.6 to −14.3‰ and δ 18 O values of 0 to 5.1‰ for calcite from the veins indicate that the carbon in the mineralizing fluid had a mixed magmatic and highly evolved meteoric fluid source. In addition, the δ 34 S values of 28 sulfides range from −4.70 to 1.40‰ which are values markedly close to that of granitic rocks and the mantle, indicative of a magmatic source. Lead-isotopes values for sulfides are indicative of a mixed mantle and crustal source. Many features of intermediate-sulfidation epithermal deposits are present at Shuangjianzishan, which is therefore interpreted as an intermediate sulfidation deposit derived from a mixed source during a phase change in temperature and pressure interpreted as being related to fluid mixing and boiling.