Geochemical behaviors of rare earth elements in granite-hosted geothermal systems in SE China

Abstract

Rare earth elements (REEs) are effective indicators in tracking water-rock interactions in the hydrothermal systems. Subject to multiple environmental effects, none a unified conclusion has been achieved to generalize the mechanism of diversifying the REEs in various thermal systems. Low-medium temperature hydrothermal systems occurring in fractured-granite exist widely in South China. However, geochemical behavior of REEs in geothermal water is not well studied though there are lots of REEs deposits. This study focuses on hydrogeochemical characteristics of REEs and constrains from salinity. Results show that the TDS (total dissolved solids) mutation due to transfer of sedimentary environments from coastal to relatively inland areas caused the great differentiation of REEs and their species. When TDS is greater than 1 g/L, Ln3+, LnOH2+, LnSO4+, and LnF2+ (Ln represents REEs) prevail and fractions of them except LnOH2+ are positively correlated with TDS. In contrast, when TDS is less than 1 g/L, LnCO3+ and Ln (CO3)2− constitute the most important part and mutually compete in fluids. A positive Eu anomaly is proved to originate primarily from the preferential dissolution of Eu-rich minerals and the seawater intrusion somewhat contributed to the negative Ce anomaly. This work first proposed the salinity effect on REEs by analysis of the in-situ sampling thermal waters that may have been neglected before and provided a new perspective to comprehensively understand the occurrence and migration mechanisms of REEs in different hydrothermal systems.