Influence of mineralogical heterogeneities on the mechanical behavior of a shear zone in gneiss, Schauinsland Mine, Germany

Abstract Enhanced geothermal systems including fault zones and fracture systems are characterized by geological heterogeneities leading to area dependent mechanical behavior. The interplay between fault zone porosity architecture and mechanical behavior is a key factor to understand the fault zone tectonic stability. Through the sampling across a clay-rich shear zone including host rock, transition zone and next to the main core zone, mechanical and mineralogical investigations presented a transition from a brittle to ductile failure behavior towards the main core zone. High compressive strength values were achieved for rock samples with high clay content which is an indicator of the reorganization of strong mineral phases within a weak compacted clayey matrix. Moreover, tensile strength parameters portrayed a decrease in strength towards the main core zone presenting a weakening effect due to the mineralogical composition. The increase in clay content related to the hydrothermal alteration of primary biotite and feldspar into clay minerals led to an increase in porosity. These different strength and hydraulic behaviors demonstrate the importance of the combined analysis of different disciplines to identify fault zone areas which are sustainable for geothermal exploitation.