The Upper Rhine Graben (URG) hosts favorable conditions for geothermal energy usage in the crystalline basement, where temperatures higher than 200 °C were reached in 5 km depth. Formation fluids in the basement rocks are highly saline and Na–Cl dominated. Natural seismicity or artificial stimulation activity during geothermal reservoir development are able to produce fractures. Subsequently, unaltered rock surfaces get in contact with the brines and a geochemical-mineralogical alteration process will be initiated. The alteration products and the type of mineral reactions on the fracture surfaces have a major influence on the fracture permeability and, thus, the life-time of a geothermal installation. Autoclave batch-type experiments were carried out to reproduce and comprehend the initial granite alteration processes under conditions found in the URG. We used granite samples with a fresh fractured surface and an intact rock fabric to represent natural fracture surfaces. A 1.5 and 2 molal Na–Cl solution has been used in the experiments, which is compositionally and in salinity strength close to natural URG basement brines. Experiment temperatures were 200 °C and 260 °C, inspired by maximum URG reservoir temperatures. Solid and fluid samples were investigated before and after the experiments. One of the main mineral reaction is the dissolution of quartz at the (sub-)surface, weakening the rock fabric and increasing the permeability. Biotite chloritization is the most reactive process in the experiments. Partly chloritized biotites of the initial samples completely transformed into chlorites at sample surfaces. Thereby, hydrobiotite and a corrensite-like mineral phase occur as intermediate phases. The chloritization accompanies with a delamination of the phyllosilicate sheets and therefore, a volume expansion of the mineral grains. Feldspar shows only minor reactions in the experiments. During the autoclave experiments the initial solution changed considerably. It reflects the mineral reactions during the experiments. Comparison with natural deep basement rocks, e.g. from the German continental deep drilling project (KTB) and the Soultz-sous-Forêts geothermal project, show very similar processes at the beginning of fracture alteration in these settings. In accordance with the observations of alteration features in deep crystalline basement we derive, (1) quartz dissolution and biotite chloritization are the main mineral reactions in the early stages of fracture alteration in deep crystalline basement rocks under geothermal conditions; (2) in these early stages rock fabric will be further weakened by the mineral–brine reactions, resulting in a propagation of the fracture into the adjacent rock and thus, an increased fluid permeability in the early period of the fracture alteration.

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