| Abstract |
The origin of elemental enrichment in geothermal brines (e.g. Li, Sr, Mg, B) from deep Upper Rhine Graben (URG) reservoirs is complex, involving multiple lithologies and fault-controlled flow paths. Better understanding the processes driving chemical enrichment in geothermal water could help optimize resource development, particularly if critical raw materials (e.g., lithium) are to be extracted. However, the mechanisms of fluid–rock interaction that led to the current geochemical composition of URG brines remain poorly understood. In this study, strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were measured in geothermal fluids and in granite and sedimentary rocks from three deep geothermal wells at Soultz-sous-Forêts (GPK-1, GPK-2, EPS-1) in France. Over 50 rock samples were analyzed, with ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7087 to 0.7863. This wide isotopic variability mainly reflects mineralogical composition but can also indicate progressive water–rock interaction. For instance, in granites affected by argillic alteration, illite formation (which incorporates K and thus ⁸⁷Rb) can drive ⁸⁷Sr/⁸⁶Sr ratios towards higher values over time due to radioactive decay. The isotopic similarity between the Soultz-sous-Forêts brine (0.7112 to 0.7114) and fresh monzonite granite (0.7087 to 0.7133) suggests that granite may contribute to the Sr content in the brine. However, fractured Triassic sedimentary rocks also show Sr isotopic signatures close to those of the fluids, indicating that fluid–rock interactions extend beyond the granite basement. These findings highlight the utility of Sr isotopes as tracers of fluid provenance and hydrothermal alteration in deep geothermal systems. |