The Upper Rhine Graben (URG) is a highly promising area for geothermal lithium (Li) extraction. Li concentration in deep geothermal brine circulating in naturally fractured Visean and Triassic reservoirs exceed 150 mg/L and are combined with significant water flows exploited by geothermal plants. However, there are still a lack of knowledge in the fluid-rock interactions leading to present high Li concentration in the geothermal brine. To address the behavior of Li in the crystalline reservoir during hydrothermal alteration, 36 granite rock samples were selected from 3 deep geothermal wells at Soultz-sous-Forêts (GPK-1, GPK-2 and EPS-1) in France. These samples were analyzed both chemically and mineralogically to assess the impact of dissolution/precipitation on major elements together with Li precipitation in mineral phases or solubilization in the brine. In total, 4 different main alteration facies from low to high alteration grades were identified in the 36 granite samples (fresh granite, propylitic alteration, argillic alteration, argillic alteration and fractured) that displayed significant chemical, mineralogical and textural changes. The Li concentration of the total rocks range from 18 to 1938 ppm and could be attributed to secondary mineral precipitation (illite, illite/smectite and chlorite/smectite mixed layers, quartz, carbonates, barite) and hydrothermal alteration by partial dissolution of the main minerals (biotite, chlorite, feldspars) present in the granite. These results highlight the importance of fluid-rock interactions due to hydrothermal circulation in the mobilization of Li in the reservoir and refine the story behind high Li concentration in the URG brine.

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