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The Leishan-Rongjiang antimony ore field (LAOF) is in a unique geotectonic location in the uplift between the Youjiang and Xiangzhong basins. This paper focuses on two representative deposits in the LAOF: the Bameng and Peize antimony (Sb) deposits. We analyzed fluid inclusions (FIs) in stibnite and coexisting quartz, as well as the sulfur isotopic composition of stibnite, to better understand the nature of the ore-forming fluid and the metallogenic process. The FIs data from samples of the stibnite and coexisting quartz indicate that the ore-forming fluids were characterized by low-temperature (150-210 °C), low-salinity (1.5 wt%-6.0 wt% NaCl equiv.), and low-density (0.872-0.961 g/cm). The δS values of stibnite (−8.21‰ to 3.76‰, average = −6.30‰) fall in between the sulfur isotopic compositions of the mantle and of biogenic sulfur in sedimentary rocks. However, the δS values (−4.41‰ to +0.04‰, average = −2.49‰) of the ore-forming fluids are generally closer to the sulfur isotopic composition of the mantle source, indicating that the sulfur in the LAOF was mainly sourced from the mantle, but with possible involvement of biogenic sulfur. In addition, FIs petrography and ore deposit geology show that fluid boiling resulted from an abrupt decrease in pressure, which may have triggered the precipitation of stibnite. We conclude that low-temperature, dilute hydrothermal fluids with mixed origins migrated along the regional fault and interacted with the wall rock, extracting the ore-forming materials. Then, the ore-forming fluids were injected into the fault fracture zones. [ABSTRACT FROM AUTHOR]