—The chemical and isotope compositions of nitric and carbon dioxide thermal waters in Jiangxi Province (China) are considered. The nitric thermal waters are ultrafresh (TDS = 0.26–0.42 g/L) and highly alkaline (pH = 8.73–8.87), with excess of SiO2, F, Na+, etc. but ultralow concentrations of Ca2+, Mg2+, and Cl. The carbon dioxide thermal waters are more saline (TDS = 0.3–3.9 g/L) but have lower pH values (6.7–7.8). Major anions in both types of waters are HCO3 and Na+, but SO42−, F, CO2, and H2S also play a crucial role. The equilibria of the thermal waters with a complex of secondary minerals (carbonate, fluoride, clay, zeolite, etc.) have been calculated. The thermal-water–rock system is shown to be in the equilibrium–nonequilibrium state. During the transfer into deep horizons and back to the surface, the hydrotherms continuously dissolve all minerals that are in nonequilibrium with them (K-feldspar, anorthite, etc.) and form new minerals, which are in equilibrium with these waters (calcite, albite, etc.). The composition of the solution and the type of secondary minerals change with time because of the change in the proportion of chemical elements: Some elements are removed from the solution, while others continue to accumulate. A dynamic equilibrium between the elements entering and leaving the nitric thermal waters is established very early, when the waters are still ultrafresh, which is due to the high pH and low pCO2. This equilibrium inhibits an increase in the salinity of the nitric hydrotherms, and they remain lowly mineralized. Owing to the higher pCO2 and, correspondingly, lower pH values, the carbon dioxide thermal waters reach a dynamic equilibrium at a later stage, when their salinity is higher than 3 g/L; therefore, they are more mineralized.

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