Geochemistry of geothermal fluid with implications on circulation and evolution in Fengshun-Tangkeng geothermal field, South China

Abstract

This paper aims to determine the recharge sources, geochemical evolution and secondary reactions during ascending process of geothermal fluid with wellhead temperature of 92 °C in the Dengwu power plant of Fengshun area through analysis of hydrogeochemical and isotopic compositions of both geothermal water and gases. Results show that geothermal water for power generation is slightly alkaline HCO3-Na type with TDS of 291–369 mg/L and it is N2 dominated. It is originally recharged by precipitation from surrounding mountainous areas with altitude of 475–683 m and circulates in the granite reservoir for about 7 ka to 9 ka years as evidenced by 14C and tritium dating after infiltration through faults systems of NE, NW and EW orientations. Under reservoir conditions, dissolution of silicate minerals like albite, microcline, muscovite and plagioclase leading to alkaline metal elements of K, Na, Ca and Mg and trace elements of Li, Sr and Rb releasing into the fluid. In addition, previous dissolved Ca2+and Mg2+ by eluviations exchange with Na+ and K+ absorbed on silicate reservoir and this process greatly improved concentrations of Na+ and K+, resulting in the dominant proportions of Na+ (91.3–95.8 meq%) in the geothermal water. And dissolution of CO2 coming from air-saturated precipitation and organic matter or biogenesis promotes the water-rock interactions in granite reservoir. Helium and neon isotope analysis indicate that up to 12.2% of volatile of mantle migrate into the deep geothermal fluid through the regional scale faults system, e.g., Locus Mountain fault. During the ascending process, mixing with shallow groundwater and RongJiang River water make the geothermal water with lower temperature and TDS outflow along the NW orientated fault in forms of springs and artificial boreholes. It is speculated that the 5760 m3/day of geothermal fluid for power generation from fractured media of granite reservoir are mixture of original geothermal fluid in deep, shallow groundwater and Rongjiang River water.