Geochemistry of thermal waters in the Changbaishan volcanic geothermal system, Northeast China—implications for vapor-liquid separation controls on geothermal fluid composition

Abstract

This work studied the geochemistry of the Changbaishan volcanic two-phase geothermal system in northeastern China. The fluid geochemistry and existence of hot fumaroles (with temperatures greater than 100 °C) and sinters imply the occurrence of vapor-liquid separation at depth, and the B/Cl and Na/Cl ratios imply a second vapor-liquid separation process. The extremely low B/Cl ratios and Hg concentrations below the detection limit in the sampled well water indicate that the process in the deep reservoir has approached the late stage. Statistical analysis shows that Na, Mg, Li, Sr, B, Fe, Zn, HCO3-, and Cl- are the characteristic elemental constituents of the geothermal system, and the type A elements (Fe, Zn, Li, B, Sr, and Mg) are mainly controlled by the vapor-liquid separation in the reservoir. As the water rises, the concentrations of the type A elements are controlled by fluid-rock interactions and the dilution process. In contrast, the type B elements (Cl-, Na, and HCO3-) are mainly controlled by the vapor-liquid separation accompanying the input of magmatic volatiles. The Na/Li geothermometer indicates that the first vapor-liquid separation process occurs at 297 °C, and the Na–K–Ca geothermometer indicates that the second vapor-liquid separation process occurs at 328 °C. With the isotopic fractionation characteristics at different temperatures, our reassessment of the δ2H and δ18O data implies that the water in the hot springs originates from different mixtures of precipitation and steam-heated water that has experienced multiple separation processes. In addition, a large flux of CO2 has lowered the δ18O values, causing a shift to the left of the local meteoric water line.