Behaviour of metals in the geothermal fluid system of the Upper Rhine Graben: Lithium

In order to increase the economic potential of geothermal power plants in view of environmental protection and sustainability, there are considerations to extract critical raw materials (CRM), such as lithium, from the geothermal fluids. The Upper Rhine Graben (URG) in southern Germany is of particular interest as it represents a hydrothermal fluid reservoir with large CRM potential. Apart from the sedimentary strata with different reservoir rocks and associated fluids, their Variscan basement is exhumed in the Vosges and the Schwarzwald. Unconformity related hydrothermal vein-type and Mississippi-Valley-Type deposits evolved from paleofluids in the region. Recent fluids precipitate mineralogically and chemically similar scalings in the power plants indicating their potential for CRM extraction from these fluids. These access to all parts of a geological system in the URG area accounts for its consideration as a natural laboratory. Therefore, this study deals generally with metal provenance, reservoir processes, transport and precipitation mechanisms. One of the hypotheses regarding those processes that lithium is released into the geothermal fluid by hydrothermal alteration of basement feldspars and mica. To corroborate the hypothesis, different reservoir rocks and fluids in various depths are mineralogically and geochemically investigated. The fluids analyzed to this point show different trace element distributions, which are associated with changes in Cl/Br and Rb/Cs ratios and might thus indicate an interaction of the fluids with the reservoir rocks. A comparison of modern fluid chemistry with the geochemistry of altered minerals, will help to link CRM provenance and content in modern fluids with economic potential.