Hydrogeochemical features and formation mechanisms of geothermal circulation water in Lushan, Jiangxi, China

• The hot springs exhibit fluoride enrichment and elevated metasilicic acid (H 2 SiO 3 ) concentrations. • The estimated recharge elevation is 718–924.8 m, with a reservoir temperature of 120.8°C and a circulation depth of 3389 to 3786 m, accompanied by 37–75% cold-water mixing. • Inverse water–rock interaction modeling quantitatively identified key dissolution/precipitation reactions along three flow paths. To support the scientific development and utilisation of geothermal resources in the Lushan area (Jiangxi, China), this study integrates major- and trace-element hydrochemistry, ionic-ratio diagnostics, stable hydrogen–oxygen isotopes and hydrogeochemical modelling (geothermometry and PHREEQC inverse simulations) to elucidate the hydrochemical characteristics and deep circulation mechanism of the geothermal system. The results indicate that the geothermal waters are dominated by the HCO 3 -Na and HCO 3 -Ca facies. Their chemical composition is primarily controlled by rock weathering and is further modified by water–rock interaction and cation exchange during deep circulation. Fluoride concentrations range from 0.02 to 16.49 mg·L −1 , and several hot spring sites meet the naming threshold for fluoridated (therapeutic) mineral water; fluoride enrichment is jointly governed by the presence of F-bearing lithologies and minerals, water–rock interaction, cation exchange, pH and temperature. Stable isotope evidence suggests that the geothermal waters are predominantly recharged by meteoric precipitation, with estimated recharge elevations of 718.0–924.8 m, and experience variable degrees of shallow cold-water mixing during ascent. The reservoir temperature is estimated to be approximately 120.8°C, and the maximum circulation depth ranges from 3389 to 3786 m. Inverse modelling further indicates that solute evolution is driven by coupled dissolution-precipitation reactions involving calcite, dolomite, albite and anorthite, together with ion-exchange processes. Overall, meteoric water infiltrates along fault-fracture systems, undergoes deep circulation and heating and ascends along structural conduits to discharge as hot springs. These findings provide quantitative constraints on circulation depth, reservoir temperature, mixing processes and dominant reaction pathways, thereby offering a scientific basis for geothermal resource management and hot spring utilisation in the Lushan region.