Characteristic Solutes in Geothermal Water from the Rehai Hydrothermal System, Southwestern China

Rehai, a high-temperature hydrothermal system located in the southern part of the Tengchong volcanic geothermal area of Yunnan Province, is characterized by intensive hydrothermal activities. The hot springs at Rehai that have been sampled so far are Na-HCO₃-Cl or Na-HCO₃ springs except for the one at Diretiyanqu (experience geothermal area) which is an acid sulfate spring. As typical characteristic solution constituents in high-temperature hydrothermal systems with magma as heat source, Cl, B and As in the Rehai geothermal waters originate mainly from the addition of magmatic fluid. However, both the mixing of magmatic fluid and the dissolution of reservoir hostrocks contribute to the enrichment of fluoride in the Rehai geothermal waters, although their fluoride concentrations are primarily controlled by the solubility of fluorite as indicated by a clear negative relation between solution fluoride and calcium concentrations. The much higher concentration of SO₄ ²⁻ in the Diretiyanqu Spring as compared to the other springs outcropping at Rehai implies a quite different geochemical genesis for this spring. The H₂S-rich vapor, separated from the deep geothermal fluid during boiling process (i.e., adiabatic cooling), can ascend to shallow aquifers where it is mixed with cold groundwaters and oxidized. Acid sulfate-rich hot springs are generally formed in this manner although only one spring of this type has been sampled during the field investigation of this study.