Hydrochemistry characteristics and genesis mode of hot springs in the Luolou River basin, Yunnan, China

The hydrochemistry and genesis mechanism of thermal springs hold great significance for the exploitation and utilization of low to intermediate temperature geothermal resources. Under the influence of a concealed fault zone, thermal springs in the Luolou River basin, Yunnan Province, are distributed along both sides of valleys and exposed in Quaternary loose deposits and Permian basalt strata. It belongs to low to intermediate temperature thermal spring water, with temperatures ranging between 34.5 and 50.0℃, pH is between 7.62-9.73, TDS is between 262-702 mg/L. Based on the hydrochemical and isotopic tests of thermal spring samples and nearby cold water samples from the Luolou River basin, this study explored the hydrological cycle, hydrochemical evolution, and genesis mode of the thermal springs and groundwater using the hydrogeochemical method. The results show that the thermal spring water has a hydrochemical type of HCO₃-Na due to the mixing of HCO₃-Ca type shallow cold groundwater. Furthermore, the chemical compositions of the thermal springs and cold groundwater are influenced by the leaching of silicate and carbonate minerals. Hydrogen and oxygen isotope analysis indicates that the thermal springs and cold groundwater are primarily recharged by meteoric precipitation, with recharge elevation ranging from 2007 to 2307 m. The recharge area is the mountain range of the valley, such as the northwest Maer Mountain, the north Guogai Mountain and the central Zuojia Mountain. The thermal springs exhibit reservoir temperatures ranging from 68.4 to 150.0℃, as estimated using silica geothermometers, silica-enthalpy mixing model, and multi-mineral equilibrium simulations. The mixing ratio of cold water is estimated to be 77.9%-90.5% by silica-enthalpy mixing model. Regarding the genesis mode of thermal springs in the Luolou River basin, the atmospheric precipitation infiltrates along the fissure or karst channel, and is then heated by terrestrial heat flow during deep underground circulation with a depth of 2 872-3 724 m. Afterward, driven by the high temperature and the density difference between cold and hot water, the water upwells along the concealed Bailiancun fault zone, exposed as low to intermediate temperature thermal springs in the Quaternary strata. The results of this study will provide a basis for the exploitation and utilization of hot springs in the Luolou River basin and exploring the exploitation potential of the same type of low to intermediate temperature geothermal resources.