Fluoride enrichment in geothermal waters based on H, O, Sr, and Li isotopes of the Taiyuan Basin, North China

Hydrochemical characteristics and fluid evolution are crucial for understanding geothermal water origins, assessing water quality, and evaluating regional seismic activity. This study investigates the hydrogeochemical processes responsible for elevated fluoride concentrations in the geothermal waters of the Taiyuan Basin. The investigation employed a combined approach, utilising water chemistry and isotopic data (δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ⁷Li) from three distinct water sample types. These analyses elucidate the sources and mechanisms controlling fluoride enrichment in geothermal systems. The fluoride concentrations in these waters, ranging from 6.19 mg/L to 21.08 mg/L, were primarily influenced by water–rock interactions, mixing processes, and cation exchange. The isotopic data (δ¹⁸O and δD) indicate a meteoric origin for the geothermal waters and substantial interaction with carbonate and silicate minerals at reservoir temperatures between 110 °C and 140 °C. Elevated ⁸⁷Sr/⁸⁶Sr ratios (up to 0.716827) and δ⁷Li values (ranging from 8.2 ‰ to 20.4 ‰) further highlight extensive water–rock interactions, particularly with carbonate, evaporite, and clastic rocks. These processes facilitate the release of fluorides and other ions, thereby contributing to the increased salinity of geothermal fluids. Cation exchange, particularly between Na⁺ and Ca²⁺, further promoted fluoride retention in the solution, while evaporite dissolution contributed to the observed high salinity. The mixing between deep geothermal waters and shallower groundwater also influences geochemical variability. However, fluoride enrichment primarily occurs within the deeper geothermal reservoirs. This study highlights the complex interactions between hydrogeological, geochemical, and isotopic characteristics in controlling the fluoride concentrations within the Taiyuan Basin.