Source Analysis of Sodium of Low-Salinity High-Sodium Geothermal Water in Huangshadong Geothermal Field from East Guangdong

The Na⁺ content in the geothermal water in the high-temperature geothermal system generally exceeds 300 mg/L, and the salinity is also large (TDS > 1 000 mg/L). In the medium- and low-temperature geothermal system, the Na⁺ content of low-salinity geothermal water is generally less than 160 mg/L. However, geothermal water with Na⁺ as high as 325.4 mg/L and TDS less than 650 mg/L was found in the Huangshadong medium-low temperature geothermal system in Guangdong. It is difficult to explain their formation with typical hydrogeochemical reactions (mineral dissolution, ion exchange, etc.). The water chemistry results show that the chemical types of geothermal water are all HCO₃-Na type, with high sodium content (average 240.06 mg/L). The results of hydrogen and oxygen isotopes show that both geothermal water and shallow groundwater have the same atmospheric source which is in the Yaokeng mountainous area. According to the hydrochemical geothermometer and the multicomponent mineral equilibrium (MME), the thermal storage temperature is estimated to be 100-130 ℃ and the maximum circulation depth is 2.43 km. Cl^- is used as a mixing ratio calculation indicator to reveal that the proportion of shallow groundwater mixed into geothermal water is 51%-72%, The actual content of Na⁺ in deep geothermal water should be as high as 685.2 mg/L. The simulation results of water-rock interaction indicate that mineral dissolution and ion exchange make a minor contribution to Na⁺ enrichment in geothermal water, and also reveal the existence of additional sources of Na⁺ in geothermal water. Granite fluid inclusions are tiny but widespread among crystalline mineral grains, and its average Na⁺ content is 11 758.9 mg/L. In the case of geothermal heating, the fluid inclusions at and near the fracture and granite fissure network expand and rupture, and the fluid is mixed into the geothermal water, providing average of 83% of Na⁺ for the geothermal water. Therefore, granite fluid inclusions may be the main source of Na⁺ for the low-salinity and high-sodium geothermal water in medium-low temperature geothermal system.