A study on the geothermal circulation system of granite: An example from the Lancang area, Yunnan

This study assesses the geothermal water circulation process of a granite geothermal system in the Lancang area, Yunnan. Cycling processes are studied using water chemistry and isotope analysis techniques. The results show that the water chemistry type of the hot springs in Lancang is Na-HCO3, with TDS values ranging from 246 to 724 mg/l. The granite pluton, as the main heat source, has a high radiogenic heat generation rate (4.17 μW/m3 on average). δD and δ18O in Lancang are close to the global atmospheric and local precipitation lines, indicating that geothermal water originates from atmospheric precipitation. Moreover, the mountainous area from 1295.3 to 1624.5 m above sea level is the main precipitation recharge area. In addition, the main ions of geothermal water mainly come from the dissolution, weathering, and cation exchange of silicate minerals in granite, and Ca2+ and Mg2+ in the water are exchanged with Na+ and K+ in the reservoir, significantly increasing the Na+ and K+ concentrations in the water. The reservoir temperatures are 126.60 °C to 151.32 °C based on the SiO2 geothermometer, and the depths range from 2283.05 to 4024.59 m. The NW- and NE-oriented fault system in Lancang serves as a channel for geothermal water to exit the ground and form hot springs; this water is also mixed with cold water during upwelling, and the mixing ratios are 63 %-84 %. Finally, the model of geothermal water circulation in the Lancang is established.