Gas geochemical characteristics and heat source mechanisms of a fault-controlled geothermal system: A case study from the western Wugongshan area, South China

Geothermal systems controlled by active fault zones represent a critical component of sustainable energy exploration, yet their fluid genesis and heat source mechanisms remain poorly constrained in complex tectonic settings. This study investigates the chemical composition and isotopic data of dissolved gases from multiple water bodies in the western Wugongshan area to elucidate the sources of hydrothermal volatiles, reservoir temperature, lithospheric thermal structure, and heat source mechanisms. Our findings reveal a distinctive gas assemblage dominated by N₂ (74.51–90.83 vol%), with subordinate O₂ (3.58–18.19 vol%) and CO₂ (1.17–5.93 vol%). Helium isotopic signatures (0.08–0.16 Ra) indicate a primary crustal radiogenic origin (97.79–99.21 %), while δ¹³C_CO2values (−11.9 ‰ to −21.9 ‰ VPDB) and δ¹³C_CH4 values (−49.8 ‰ to −74.8 ‰ VPDB) suggest that CO₂ predominantly originates from crustal organic sources and CH₄ is primarily generated through thermogenic and microbiological processes. The application of IMG and CO₂-CH₄ gas isotope geothermometers indicates that the temperature of the deep gas source varies between 100.2 °C and 197.3 °C, which is marginally above the temperature range of the geothermal water reservoir, recorded at 120.3 °C to 137.1 °C. Furthermore, integrated analysis of ³He/⁴He, borehole temperature measurements, and rock thermophysical properties indicate that the lithospheric thermal structure of the study area is classified as “hot crust-cold mantle,” with a crust-mantle heat flow ratio ranging from 1.36 to 1.56. Finally, the heat source mechanism of the fault-controlled geothermal system in the western Wugongshan area was presented: (1) thermal accumulation of blanket-shaped heat flow conduction and Moho surface thinning diffluence; (2) radioactive decay heat from crustal concentrated layers; and (3) convective heat from deep faults. This study provides a new perspective on the heat source mechanisms of fault-controlled geothermal systems and serves as an important reference for the exploration and utilization of geothermal resources with similar origins.