Geochemistry of rare earth elements in the hot springs in the Simao Basin in southwestern China

Little is known about the REEs characteristics and distribution in groundwater in the Simao Basin in southwest China. In this study, hydrochemical analyses and models are used to examine the REE inorganic speciation, characteristics and controls of REE concentrations, and fractionation of spring waters in different types of rocks in the basin. REE contents in the spring waters are relatively low, which are positively correlated with TDS. Relatively high concentrations of REEs are observed in the spring waters in the red beds rather than in the carbonate rocks. Speciation calculations indicate that dissolved REEs mainly composed of LnCO 3 + and Ln(CO 3 ) 2 − complexes (the sum ranging from 30.8 to 98.2%). The carbonate complexes account for a larger proportion in the spring waters in the carbonate rocks than in the red beds. LnSO 4 + also accounts for a certain proportion up to 67.5% in the spring waters in the red beds. Relatively low pH and Eh values and high concentration of HCO 3 contribute to the REEs enrichment in the groundwater. Carbonate complexation and preferential adsorption may result in the enrichment of MREEs and HREEs. The positive Eu anomalies in the spring waters are related to the interaction of groundwater with the carbonate rocks, because Eu can exist in the Ca site of certain calcium-rich minerals (especially carbonate rocks). The negative Ce anomalies are related to redox conditions. Under the oxidation conditions, Ce 3+ is oxidized to Ce 4+ and coprecipitates with oxides of iron and manganese in the form of CeO 2 or Ce(OH) 4 .