Trace element mobilization in the monzogranite of Soultz-sous-Forêts

In the context of combined raw material and energy production, public attention focusses increasingly on geothermal power plants. The Upper Rhine Graben (URG) in southwest Germany and eastern France is of special interest as it represents a possible geothermal reservoir with raw material potential.

From around 1300 different oil as well as geothermal wells and thermal water wells, knowledge can be acquired about the reservoir rocks of different depths. In the URG, the Triassic sandstones and Variscan crystalline rocks have the greatest geothermal potential. Furthermore, from these wells, the geochemical properties of the fluids can be studied and related to the reservoir rocks.

Up to now, only limited knowledge is available about the recent fluid-rock interactions releasing trace elements to the fluid. There are several hypotheses about the reaction kinetics and transport mechanisms of the elements and water. One of the hypotheses is that trace elements are released by the alteration of feldspars and mica of the Variscan crystalline basement. To corroborate the hypothesis, initial mineralogical and geochemical analyses were carried out on partially altered monzogranite core samples which were extracted from different depths while constructing the geothermal power plant in Soultz-sous-Forêts (France).

The first results of geochemical and mineralogic analysis revealed a need to concretize the hypothesis. Based on the direct comparison between the unaltered mineral and the alteration product, it was determined that trace elements are released but can also be fixed. Thus, the question arises whether these alteration processes lead to the present fluid composition.