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The chemical and isotopic compositions and the origin and formation conditions of the nitric and carbon dioxide thermal waters in Jiangxi Province (China) are examined. The differences between these nitric and carbon dioxide thermal waters are shown. The nitric thermal waters are ultra-fresh and high alkaline with abundant SiO2, F, Na, Li, B, Sr, Rb, etc. but low concentrations of Ca, Mg, Cl, Ag, V, Pb, Zn, Co, etc. The carbon dioxide thermal waters are distinguished by higher salinity but lower pH values. The predominant anions are HCO3? and Na+. The thermal waters' composition peculiarity is also determined by SO42?, F?, CO2 and H2S. The special focus is on the thermal waters' origin and the geological conditions of the recharge and discharge zones. The saturation degree of thermal waters with various secondary minerals (carbonates, fluorides, clays minerals, zeolites, pyrogenetic minerals, etc.) is also calculated.

The thermal water – rock system is shown to be an equilibrium-nonequilibrium system. While ascent to the surface, studied thermal waters continuously dissolve minerals that are far from equilibrium and form new minerals that are in equilibrium with water. Over time, the solution composition, type of secondary minerals, and chemical element proportions change because some elements precipitate from the solution and the rest continue to accumulate. In nitric thermal waters, the dynamic equilibrium of elements entering and precipitating from the solution is achieved during early stages when the water is ultra-fresh, which creates high pH values and low PCO2. This equilibrium state decreases the total dissolved solids (TDS) growth of nitric thermal waters, which stay low mineralized. Carbon dioxide thermal waters have higher PCO2 and, accordingly, lower pH values, thus achieving dynamic equilibrium during later stages when their TDS exceeds 3?g/l. Therefore, carbon dioxide thermal waters are more mineralized. The origin of redundant elements, particularly F, in thermal waters is considered in the paper, and we show that the source of fluorine is simple minerals of igneous origin.