Abstract: Numerous Sb deposits occur in the western segment of the East Kunlun Orogenic Belt, yet their ore-fluid evolution, sources of ore-forming materials, and mechanisms of stibnite precipitation remain poorly constrained. This study focuses on the Xiaoerkule, Huangyangling, and Hongshanding Sb deposits. Based on detailed deposit-scale geological investigations, we conducted fluid-inclusion microthermometry on quartz, in situ trace-element analyses of quartz, and in situ sulfur-isotope analyses of stibnite from successive mineralization stages, aiming to characterize the ore-forming fluids, trace the sources of metals and sulfur, elucidate the mechanisms of Sb precipitation, and constrain the deposit genesis. The ore-forming fluids in all three deposits are dominated by aqueous liquid-vapor inclusions and belong to medium- to low-temperature, low-salinity H₂O-NaCl systems. Homogenization temperatures during the main ore stage are concentrated at 180–200 °C, with estimated mineralization depths of 1.5 to 1.9 km. From early to late stages, both fluid temperature and mineralization depth decrease systematically. Quartz from the three deposits is characterized by generally low Ti concentrations, consistent with the fluid-inclusion microthermometric results. At Xiaoerkule and Huangyangling, Al, Li, and Sb concentrations in quartz of main ore stage increase notably, indicating that fluid acidification and Sb supersaturation played important roles in stibnite precipitation. In contrast, quartz from Hongshanding displays high Sb but low Al concentrations, suggesting that stibnite precipitation was mainly controlled by meteoric-water mixing and sustained cooling. Stibnite δ³⁴S values are predominantly negative to slightly positive, clustering at approximately −4‰ at Huangyangling, −2‰ at Hongshanding, and ranging from −1.0‰ to +4.9‰ at Xiaoerkule. These isotopic compositions indicate that the ore-forming sulfur was mainly derived from the Middle Permian Huangyangling Formation, with local addition of magmatic sulfur. Fluid boiling occurred during the main ore stage at Xiaoerkule and Huangyangling, where stibnite precipitation was jointly controlled by decompression boiling, rapid cooling, and fluid acidification; at Hongshanding, it was governed primarily by meteoric-water mixing and sustained cooling. Collectively, the Sb deposits in the western East Kunlun formed in a Late Triassic post-collisional to intracontinental extensional setting. A regional magmatic–tectonic thermal event drove deep circulation of meteoric water, enabling hydrothermal fluids to leach Sb and S from the Huangyangling Formation and transport them upward along NE–NNE-trending faults. Ore deposition was subsequently triggered by the combined effects of the above mechanisms. The deposits are therefore classified as medium- to low-temperature hydrothermal vein-type Sb deposits.