Granitic/rhyolitic and sedimentary reservoir rocks of the Upper Rhine Graben – trace element mobility during fluid-rock interaction

To better constrain the leaching behaviour of potential reservoir lithologies in the Upper Rhine Graben and the associated mobilisation of specific elements into fluids, we conducted leaching experiments at 70 °C on Permian rhyolites and Rotliegend sediments as well as Triassic sandstones and limestones. Initial rock compositions were determined by petrographic analysis of thin sections, focusing on mineralogy, fabrics, and reaction textures. The results reveal that elemental release occurs at highly variable rates controlled by both rock properties (mineral abundance, fluid inclusions, mineral composition and solubility) and fluid characteristics (composition, pH, Eh, temperature). Grain size and porosity, though critical in natural systems, are less significant here due to the use of powdered samples. Saline fluid inclusions, together with the dissolution of feldspars and biotite, exert a significant control on the concentrations of Rb, Sr, Li, Na, K, and Cl in the leachates for most lithologies, whereas in the limestone samples the high solubility of calcite masks these effects. These factors interact in complex ways, leading to distinct element correlations in the leachates. Element concentrations are further constrained once solutions reach saturation with respect to specific minerals. The combined influence of these parameters governs both leaching kinetics and the extent to which elements are mobilised or retained.