Brine-granite interaction and lithium leaching in a continental rift (Soultz-sous-Forets, Rhine Graben, France)

At Soultz-sous-Forets, the geothermal reservoir was identified around thirty years ago by drilling down to depths of 5000 m. It comprises the base of the Mesozoic series and the underlying fractured granitic basement. Fluid inclusion data have shown near-continuous mixing trends between two main fluid end-members: i) brines interpreted as of primary evaporitic origin with low Cl/ Br ratio (100-200), which evolved then by fluid-rock interaction in the granite, and ii) dilute recharge fluids originating from high points. These mixing terms cover much wider ranges than the pumped geothermal fluid. The brines identified at depth of up to 5000 m indicate very significant penetration from the sedimentary series, and the extent of fluid mixing is most likely favoured by the dynamics of extension and convection developed by the heat flow associated with the continental rift. LA-ICP-MS and crush-leach analyses of the fluid inclusions show that lithium concentrations are high (up to 1000 ppm), up to three orders of magnitude higher than seawater. Therefore, the enrichment of the reservoir in lithium and other metals cannot be explained by the pre-concentration of primary brines by evaporation alone. Enrichment occurs in damage envelopes along fault zones through fluid-rock interactions and illite (+ quartz) development at the expense of ferromagnesian phyllosilicates, constituting the primary Li source with a content of 300 to 700 ppm Li. Using a simplified assessment of the ratio between the lithium available in the rock around 30-50 ppm, and the lithium in solution (200 ppm) in current fluids, an extraction of 2.5% to 10% of the Li contained in the granite can contribute to the mass of Li available in the reservoir. This average estimate is compatible with the much greater losses identified in damage areas where Li concentrations in weathered granite fall to values of 10 to 16 ppm.