Hydrogeochemistry and Formation of Low Temperature Geothermal Waters in South Hunan, China

The south of Hunan is rich in low temperature geothermal water resources, yet their formation mechanism has not been well investigated. This study focused on the occurrence conditions of geothermal water and estimation the thermal reservoir temperature and the circulation depth of geothermal water in this area. The hydrogeochemical analyses of 23 geothermal water samples are proceeded with the help of Origin, PHREEQC and Aquachem software. The results show that the main hydrochemical type of geothermal water in the study area is HCO₃·SO₄–Ca·Mg and HCO₃–Na. The δD and δ¹⁸O of the geothermal water samples show a linear right-up trend, indicating the geothermal waters were recharged by meteoric water. The mineral saturation index was calculated by PHREEQC, which indicated that the quartz and chalcedony minerals were close to the saturation state. The reservoir temperature range of geothermal water is estimated to be 38–122°C by using a silica (quartz) geothermometer, with an average of 73°C. The geothermal water circulation depth range calculated by the geothermal gradient is 1–4 km, with an average of 2.13 km.