Experimental and geochemical evidence for magmatic origin of Li-Rb-Cs in geothermal waters of the Yangbajing-Gulu Rift, Tibet

Geothermal waters in certain regions of Tibet, particularly those characterized by intense tectonic activity, are exceptionally enriched in lithium (Li), rubidium (Rb), and cesium (Cs). However, the primary sources of these elements within Tibetan geothermal systems remain debated. Clarifying whether these elements are derived predominantly from water-rock interaction or magmatic fluid input is essential for a comprehensive understanding of the region’s geothermal systems. Previous studies lacked quantitative analysis methods using water-rock interaction simulation experiments. To address this gap, we conducted laboratory simulations using 10-million-years-old biotite granite from the Yangbajing area, performing high-temperature and high-pressure experiments at 200 °C and 20 MPa over 62 days. The results yield a Li:Rb:Cs ratio of approximately 48:64:1, which starkly contrast with the ratio observed in natural geothermal waters (around 5.7:1:2.1). This significant discrepancy suggests that there must be an additional geochemical process that strongly modifies the Li, Rb, and Cs ratios in natural geothermal waters. Moreover, in the Yangbajing-Gulu rift, the concentrations of Li, Rb, and Cs show a strong correlation with Cl, a tracer indicative of magmatic fluid input, while the correlations between δ¹⁸O and the concentrations of Li, Rb, and Cs are weaker. The geothermal waters exhibit a Cs > Rb pattern, similar to other geothermal systems with magmatic fluid input. Combined with geophysical evidence of shallow melt bodies beneath the rift, these findings indicate that magmatic fluid input is a key factor controlling the enrichment of Li, Rb, and Cs in the geothermal waters of the Yangbajing-Gulu rift. This study highlights the magmatic source mechanism for rare metal supply in Tibetan geothermal waters and provides critical insights into the metallogenic models of Tibetan geothermal systems.