Characteristics of rare earth elements in boron-rich thermal waters of the Xianshuihe Fault Belt Zone and its indication to the sources of boron

Geothermal water of high-temperature geothermal system is generally rich in high concentration of boron (B), and the research on B sources has been a hot topic for geothermal geologists. Although many researchers have conducted extensive research on the formation mechanism of high B geothermal water, it is unclear whether characteristics and migration laws of rare earth elements (REEs) in B-rich geothermal water can illustrate B sources in geothermal water. Our study focused on the B-rich geothermal water in the Xianshuihe Fault Belt Zone (XSHFZ) to explore the distribution and migration rules of B and REEs by combining with field survey and sampling, laboratory testing, hydrogeochemical modeling, and comprehensive analysis. The results showed that the maximum value of B in geothermal water was 10.50 mg/L, and the content of B in 90% of geothermal water samples was higher than 0.5 mg/L (China standard values for drinking water). The ∑REE value was 0.08-3.49 μg/L, mainly existed in the complex form of LnCO₃⁺ and Ln(CO₃)₂⁻. PAAS-normalized model and (Nd/Yb)_SN value of geothermal water in the XSHFZ showed that HREEs is enriched relative to LREEs, with significant positive Eu (an average value of δEu was 0.34) and Ce (an average value of δCe was 0.07) anomaly. Dissolving felsic mineral and carbonate rock controlled the migration of B and REEs in geothermal water. The geochemical characteristics of REEs in geothermal water can expound B's enrichment process in geothermal water to a certain extent. Our research results can expand the application of REEs in the study of B-rich geothermal water and provide a basis for illustrating the genesis study of B-rich geothermal water in similar areas.