Mineralogical and petrological assessment of selected granites and granodiorites from the Odenwald area, Germany: a crucial step toward the site selection for the GeoLaB underground infrastructure?

Abstract

GeoLaB (Geothermal Laboratory in the Crystalline Basement) started in the beginning of 2023 to plan and build an underground geoscientific laboratory in a fractured crystalline basement. One of the potential selected sites is the Odenwald complex (Hessen, Germany) due to its geology (fractured crystalline basement) and petrology (Tromm granite). Considerable efforts and investigations were recently implemented to evaluate, whether this site is a suitable location for the realization of GeoLaB. In the initial exploration stage, surface rock samples were collected in the Odenwald (Streitsdöll, Hammelbach and Ober-Mengelbach) area for mineral and petrological investigations. The sampling strategy aims for different structural contexts within the same lithology, e.g. a non-fractured granite, one located in the fault damage zone, and one located in the cataclastic core zone. The rock samples are macroscopically characterized by well-formed feldspar/plagioclase and mica (biotite/muscovite). Mineralogy and petrology are fundamental for investigating the composition and the occurrence of hydrothermal alteration. This influences rock properties such as porosity-permeability and also the response to applied stress. A first set of eight samples was investigated by means of X-ray powder diffraction XRD (quantitative estimation of the mineral assemblage, rock classification), electron microprobe analyzer EMP (determination of the mineral geochemistry, hydrothermal alteration and microstructures) and X-ray fluorescence XRF (whole chemistry and trace elements). The granites/granodiorites are composed of quartz, plagioclase, felspar (andesine based on the Na-Ca geochemistry), and mica (biotite and muscovite). Apatite, magnetite, rutile and monazite were detected as accessories, thus enabling geochemical dating. Three samples (Streitsdöll) show hydrothermal alterations in the form of kaolinite or clay phases with similar mineral chemistry Al2Si2O5(OH)4 at the plagioclase rims. Traces of metasomatic processes could be observed in the images acquired with the EMP. The quantitative mineral assemblage evaluation also indicated different types of plutonic rocks: granodiorite and granite based on the QAPF (Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)) diagram. A compositional variation with depth can be expected based on the mineral heterogeneity. This hypothesis will be verified by analyzing cores samples from exploration drillings planned for 2024. Besides seismic and geophysical campaigns, additional fieldwork focusing on structural geology, rock sampling, and geomechanical experiments will be conducted to develop a baseline to scientifically assess whether the Odenwald site is a suitable location to build the GeoLaB.   Keywords: underground laboratory, crystalline basement, fractured granite, mineral composition variation, hydrothermal alteration.