Lithium (Li) is currently much sought after worldwide, mainly for use in the batteries of electric vehicles and static Battery Energy Storage Systems (BESS), which are expected to expand rapidly in the context of the energy transition. To a lesser extent and for a very much smaller market, rubidium (Rb) is also of interest, particularly for applications in fiber optics, semiconductors, monolithic laser fabrication, emerging quantum-computing devices, atomic clocks, etc.
According to recent literature on the geochemical characteristics of deep geothermal fluids, the Upper Rhine Graben (URG), located along the French/German border, is one of the regions where these fluids exhibit some of the highest concentrations of Li in the world. This makes the URG one of the most promising areas for Li extraction in combination with geothermal energy production for heat and electricity. These hot brines (> 150 °C) extracted from deep boreholes (≥ 2500 m), characterized by relatively high salinity of around 100 g/L or more, are very rich in Li (> 150 mg/L), and also in elements such as Rb, caesium (Cs), strontium (Sr), and boron (B). This new type of resource significantly enhances the attractiveness of Enhanced Geothermal Systems (EGS) in the URG, while also emphasizing the need for a quantitative resource assessment, even if only preliminary.
The main aim of this study is to provide a first-order estimate of the potential Li and Rb resources contained in the deep geothermal brines of the URG. For lithium, this work builds upon a previous study conducted by BRGM in 1991, updated to incorporate recent geoscientific knowledge and newly available data. The earlier study estimated Li resources ranging from to 300 to 2200 kilotons of Li metal, with an average value of 1000 kilotons, based on a mean Li concentration of about 155 mg/L in the URG brines.
In the present study, revised estimates range from 1044 to 15,952 kilotons of Li metal, with an average value of 6243 kilotons of Li metal, using an updated mean Li concentration of 174 ± 16 mg/L. These resources are relatively significant when compared with global world production of Li in 2022 and 2023 (146 and 180 kilotons of Li metal, respectively). For rubidium, the estimated potential resources range from 150 to 2301 kilotons, with a mean value of about 900 kilotons, based on a mean Rb concentration of 25.1 ± 2.7 mg/L in the URG brines. The narrow ranges of variability in Li, Rb and B concentrations and in Li and B isotope values in the URG brines, seem to confirm that mica alteration and illite precipitation in the geothermal reservoirs of the Lower Triassic-Permian sandstones and granite basement, at temperatures close to 225 ± 25 °C, constitute the main sources and controlling processes for Li, B and Rb.
Based on these results and literature data, a revised conceptual model of circulation of deep geothermal fluids in the URG is proposed. While the URG holds considerable and promising potential resources of Li and Rb, two key challenges must be addressed: identifying zones that are sufficiently permeable and productive to ensure economic viability, and minimizing induced seismicity risks or other environmental impacts in order to secure public acceptance and local population support.