Typical geothermal waters in the Ganzi–Litang fault, western Sichuan, China: hydrochemical processes and the geochemical characteristics of rare-earth elements

Hydrogeochemical processes and water–rock interactions of a geothermal system are important for understanding of the hydrochemical evolutionary process and genetic mechanisms of the geothermal system. The hydrochemistry, hydrogen and oxygen isotopes, and rare-earth elements (REEs) are discussed to reveal the hydrochemical characteristics and processes of these geothermal systems in Kahui and Gezha geothermal waters in the Ganzi–Litang fault in western Sichuan, China. The results show that the geothermal waters are Na-HCO 3 type and influenced by combination of evaporite dissolution and silicate weathering. The isotopic compositions of δ 2 H and δ 18 O indicate that the geothermal water is recharged by atmospheric precipitation, and the oxygen isotope drifts in the Gezha geothermal water may be caused by water–rock interactions. The total REE content of the geothermal water is 0.059‒0.547 ug/L, and the REE contents are significantly influenced by pH and HCO 3 − , Na + , and Mn minerals. The chondrite-normalized REE pattern show geothermal water, cold springs, and surrounding rocks are rightward. The fractionation of rare-earth elements is affected by HCO 3 − complexation reactions, the dissolution of Mn minerals, and alternating cation adsorption. Moreover, the geothermal water exhibited positive Eu anomalies, which may be caused by the dissolution of feldspar minerals in surrounding rocks. In addition, the complexation patterns of REEs in geothermal water and cold springs in the Ganzi–Litang fault mainly include Ln(CO 3 ) 2 − and LnCO 3 + , and the changes in the complexation patterns are influenced by the stability constants of complexation reactions and pH.