The giant South China low-temperature metallogenic domain (LTMD) hosts many low-temperature hydrothermal systems (e.g., the Youjiang Au-As-Sb-Hg ore deposits, Xiangzhong Sb-Au ore deposits, and Xiangqian Hg ore deposits), which provide ~80% of the global Hg metal production. Here, we study the Hg isotopic composition of the Xiangqian Hg deposits and re-evaluate available data on the Youjiang Au-As-Sb-Hg deposits and Xiangzhong Sb-Au deposits. Cinnabar samples from eight Xiangqian Hg deposits display negative to near-zero Δ¹⁹⁹Hg values, suggesting that their Hg is from two sources: (1) the asthenospheric mantle with near-zero Δ¹⁹⁹Hg values, and (2) the Precambrian basement with negative Δ¹⁹⁹Hg values. Such mixing is also observed in the Youjiang Au-As-Sb-Hg deposits and Xiangzhong Sb-Au deposits. Combining the ages of these deposits, we infer that plate subduction and collision during Caledonian, Indosinian, and Yanshanian tectonic-thermal events triggered lithospheric extension, asthenospheric upwelling, and basement remobilization within the South China craton, leading to the generation of magmatic-hydrothermal fluids and metal-rich crustal fluids to form low-temperature hydrothermal systems in the LTMD. This work demonstrates that Hg-bearing low-temperature hydrothermal deposits in China have distinct δ²⁰²Hg-Δ¹⁹⁹Hg patterns from those in Europe and North and South America, implying that Hg isotopes can be used for tracing the sources of Hg in the global market.