Lithium-rich geothermal brines in Europe: An up-date about geochemical characteristics and implications for potential Li resources

Lithium (Li) is a strategic metal - especially for batteries in electric vehicles - for which worldwide demand is constantly increasing. Presently, several investigations are examining if a part of lithium could be extracted from deep European geothermal fluids. Among the data from geothermal and hydrocarbon wells found in the literature review carried out by BRGM and EIFER, only six areas stand out with deep fluids containing high Li concentrations from 125 to 480 mg/l in Italy, Germany, France and the United-Kingdom. Except the UK fluid, which has a relatively low salinity (TDS = 19 g/l) and reservoir temperature (around 52 °C), these deep Li-rich fluids are Na-Cl brines (TDS ≥ 56 g/l) with high concentrations of Na (> 18 g/l) and Cl (> 25 g/l) and high temperatures (≥ 120 °C and up to 380 °C in Italy). If high TDS and temperature values seem to be key factors for triggering high Li concentrations in such fluids, these two factors alone cannot be sufficient. Indeed, our study confirms that Li concentrations not only depend on temperature and fluid salinity, but also on the type of reservoir rock and its mineralogical constituents, as demonstrated by the good results obtained using two different Na-Li thermometric relationships existing in the literature. One fits well with the brines from low to high temperature (120-250 °C) reservoirs in deep tectonic sedimentary basins over a crystalline basement (France, Germany) and the other with the fluids from ultra-high temperature (≥ 300 °C) reservoirs in volcano-sedimentary environment (Italy). If the high chloride concentrations values in these brines mainly depend on the fluid origin (evaporated seawater or freshwater, halite dissolution, primary neutralization fluids or parent-geothermal fluids in high-temperature and -pressure volcanic environments, water mixing, etc.), the other aqueous major species are mostly controlled by hydrothermal water-rock interaction processes. At these temperatures (≥ 120 °C), the fluid-rock interaction processes are generally dominated by plagioclase and K-feldspar dissolution, followed by albitization of these minerals, dissolution of white micas and biotite, precipitation of illite, and chloritization. According to the two Na-Li thermometric relationships, existing mineralogical and isotopic data, this study suggests that the main sources of Li are white micas and biotite dissolution. Among the European areas, it shows that the Upper Rhine Graben (URG) along the French/German border is probably the most promising zone. For the URG geothermal brines, the main source and control of lithium, at about 225 °C, at reservoir depth, is probably the up to 450-m-thick micaceous continental sandstone of Triassic Buntsandstein. A minor contribution from the granite basement can also not be excluded. Though the Li concentration values of these brines (≥ 150 mg/l) seem to be favourable for geothermal Li exploitation, it is essential to make an as accurate as possible estimate of the Li resource of these brines, design the Li extraction process, and examine the economic conditions of its exploitation.