The dissolution of sodium-containing minerals in high-temperature geothermal systems can cause Na + to exceed 400 mg/L with high salinity. But the Na + of low-salinity geothermal water is mostly less than 100 mg/L in medium-low temperature geothermal systems. However, geothermal water with Na + up to 325.4 mg/L and total dissolved solids less than 650 mg/L was found in the Huangshadong geothermal field, which is a typical medium-low temperature hydrothermal system in South China. The water chemistry results indicate that thermal groundwater is uniformly HCO 3 -Na type with high sodium content (average 240.06 mg/L). All the thermal groundwater and shallow groundwater have the same meteoric origin based on δD and δ 18 O. According to water chemical geothermometers and multicomponent mineral equilibrium (MME) method, the reservoir temperature is estimated to be 100 to 130 • C at a maximum depth of 2.43 km. The estimation of the Cl − mixed indicator suggests that geothermal water has mixed with 51% to 72% of shallow groundwater, resulting in the reduction of Na + content in real geothermal water (Na + up to 685.2 mg/L). The simulated results of water-rock interactions indicate that mineral dissolution and ion exchange have minor contributions to Na + enrichment in geothermal water. Hydrochemical simulations and Gibbs diagrams suggest an additional source of high sodium: granite fluid inclusions are fractured into geothermal water at high temperatures. Granite fluid inclusions may only account for 3% to 5% of geothermal water, but they provide the main source of Na + in geothermal water.
The formation mechanism of high salinity geothermal water is significant for utilizing geothermal resources and mineral resources. The high salinity in geothermal water may be derived from the geothermal mother fluid or from the evaporite dissolution. It is difficult to distinguish between these two sources because they may have similar hydrochemistry. In this paper, water chemistry and stable isotopes were used to explore the high salinity geothermal water in Yanchanghe geothermal field, central China. It is a lowtemperature hydrothermal system in the inland karst area. The thermal water is Cl-Na type with high salinity (TDS > 8,400 mg/L). The modified silicon thermometer is more suitable and the reasonable result is about 58.8 ℃. The maximum circulation depth is 1.9 km. Using the temperature of hot and cold water to estimate the mixing ratio is 58%-81%. Saturation index (SI), Na/1000-K/100-Mg 1/2 and Gibbs diagram suggest that the main source of salt in geothermal water is derived from the evaporite dissolution, which provides Clof 11,264-31,279 mg/L and Na + of 9,272-21,236 mg/L. We found the combination of temperature and hydrogeochemistry can be used to investigate the formation mechanism and mixing process of high-salinity geothermal water formed in a karst low-temperature hydrothermal system.
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