Assessment for co-production of critical metals (lithium, copper, strontium) and geothermal heat in various geological formations of the North German basin

Abstract

Several sedimentary formations and lithologies such as limestone from the Muschelkalk or sandstone from the Bunter or from the Rotliegend are considered for geothermal exploitation in the North German Basin. The production and analysis of geothermal fluids from deep wells in this area already showed an increased content of certain critical raw materials (CRM) in some brines indicating that a co-production of geothermal heat and CRM could be economically interesting. In this study, the overall potential of 23 geological formations for CRM co-production of the 4 km deep geothermal well from the Groβ Schönebeck site was assessed with focus on the CRM elements lithium (Li), copper (Cu), and strontium (Sr). For this, the total elemental content of 23 rocks samples from different depths between 1.4 and 4.1 km was determined. Highest Sr was identified in the limestones of Muschelkalk (2330 mg/kg) and Werra formation (2980 mg/kg). Copper was mainly enriched in the Kupferschiefer (214 mg/kg) and the Upper Rotliegend sandstones (79 mg/kg), and Li mainly in the Rotliegend conglomerates (up to 200 mg/kg).

To determine how strong and to which minerals these CRM are bound within the rocks, leaching and sequential extraction experiments were performed on five selected formation rock samples that are considered either for geothermal exploitation (Muschelkalk, Buntsandstein, Rotliegend sandstone) and/or as potential source for the CRMs Li, Cu, Sr (Permo-Carboniferous volcanic rocks, Ohre anhydrite). These experiments revealed that Sr was predominantly bound to carbonates indicating incorporation in calcite minerals. For Cu no clear source rock was identified although a volcanic origin seems most likely. Lithium was strongly bound to all investigated rocks (<5 % of total Li was extractable) indicating a binding to the silicate phases.