Contribution of Hydrothermal Processes to the Enrichment of Lithium in Brines: Evidence from Water–Rock Interacting Experiments

Over the world, the available lithium (Li) resources are reserved mainly in closed-basin brines, with high Li concentration (> 150 mg/L) and low Mg/Li ratio (< 10) being critical for Li extraction using precipitation-based methods. In order to investigate the enrichment of Li over Mg during the formation of Li brine deposits, batch water–rock interacting experiments between igneous rocks and aqueous solutions were carried out under low (25, 50 and 75 °C) and high (200, 300 and 400 °C) temperature conditions. Our results show that for the experiments using water and accomplished under 25 °C, the Mg and Li concentrations vary from 0.470 and 0.782 mg/L in the solution interacted with Li-rich granite, to 5.626 and < 0.002 mg/L in that interacted with basalt, with Mg/Li ratio being slightly higher than those of the igneous rocks. By contrast, while a NaCl or Na₂SO₄ solution was used, the Mg and Li concentrations can be improved by up to tens of times, and the Mg/Li ratio also increased slightly. Lastly and above all, with increase in the water–rock interacting temperature from 25 to 400 °C, the Mg and Li concentrations in all solutions vary conversely and the Mg/Li ratio decreases by orders of magnitude, leading to the formation of Li-rich brines with very low Mg/Li ratios at temperatures above 200 °C. By comparing the results from our experiment to those from Li-rich springs, rivers and closed-basin brines, we conclude that water evaporation over time is fundamental for the concentration of Li in brines, meanwhile high-temperature hydrothermal processes are key to the formation of Li brine deposits with low Mg/Li ratios.