Geochemistry and sources of boron and strontium of geothermal waters from the Pearl River Delta region, South China: Implications for water-rock interactions

Pearl River Delta (PRD) region of South China is abundant of geothermal water resources. However, the development and utilization level of geothermal resources in this region is relatively low, due in part to the lack of fundamental geochemical research. To access water-rock interaction processes of the PRD geothermal system, we analyzed the geochemistry of geothermal waters by combining trace elements (B, Sr, and Br) and isotopes (δ¹¹B and ⁸⁷Sr/⁸⁶Sr) with conventional tracers (major elements) that had been reported. The Cl/Br ratios (from 31 to 639) confirm the multi-source salinity of marine origin, precipitation, and a minor dissolution of halite. Major ions chemistry highlights the influence of ions exchange, the dissolution of carbonate, silicate, and sulphate minerals as well as the contribution of seawater. The Cl/B ratios (61 to 22,583) suggest interactions with carbonate rock and felsic rock, input of seawater, and groundwater mixing. Boron isotopic compositions (δ¹¹B) range between −9.22 and +39.78 ‰. Sr contents and ⁸⁷Sr/⁸⁶Sr ratios are more homogeneous, falling between 0.06 and 32.26 mg/L and between 0.71239 and 0.72121, respectively. The B and Sr isotopic signatures show that three processes contribute to geochemistry of geothermal waters: 1) water/rock interaction involving marine carbonate rock, evaporite, and granitoid or/and gneiss, 2) seawater intrusion, and 3) shallow groundwater mixing. Major-trace element chemistry and these isotope systematics on their own indicates essential information on the aspects of fluid origin or water-rock interaction processes, and however provide a more comprehensive understanding of the geothermal system in the PRD region, South China.