The newly discovered Baiganhu W-Sn ore district in Qimantag of East Kunlun orogenic belt provides a key window to insight into the W-Sn mineralization in Northwest China. In this paper, the authors present results from the ⁴⁰Ar/³⁹Ar dating of two muscovite samples collected from the ore-bearing quartz veins in the Baiganhu W-Sn ore district, which yielded two ⁴⁰Ar/³⁹Ar plateau ages of 422.7 ±4.5 Ma and 421.8 ±2.7 Ma, respectively. These two samples also yielded consistent (within errors) isochronal and inverse isochronal ages of 424 ±15 Ma and 418 ±24 Ma, respectively, suggesting that the analytical results are reliable. The new plateau ages show that the mineralization occurred in the Late Silurian, associated with the tectonic-thermal events induced by the closure of Proto-Tethys. The post-subduction continental collision caused the formation of granitic magmas sourced from remelting of the metalliferous metamorphosed Proterozoic sediments. The W-Sn mineralization resulted by the hydrothermal fluids exsolved from the granitic magmas during their upward emplacement.