The Soultz-sous-Forêts granitic basement represents the reservoir of an experimental Hot Fractured Rock (HFR) geothermal exchanger presently tested in the northern Rhine Graben (France), referring now to the concept of Enhanced Geothermal System (EGS). The injected fluids circulate in the natural fracture network of the granite and through its hydrothermally altered matrix. One of these fractured and altered zones, located about 800 m below the granite-sediment boundary, contains tosudite, which is a rather rare mixed-layer chlorite/smectite that crystallized here ahead of a fibrous illite/quartz/calcite paragenesis. Tosudite occurs mainly in the relics of plagioclase grains that were progressively altered by interacting with Li-bearing hydrothermal fluids percolating in the granite fractures.

The age of the hydrothermal alteration activity is inferred from K-Ar dating of varied particle sizes of the associated illite: two distinct hydrothermal episodes of illite crystallization could be set at about 63 and 18-Ma or less, without further detectable precipitation, especially during the rifting of the Rhine Graben. Precipitation of fibrous illite in the pore space of the altered granite is expected to have reduced its permeability, as frequently observed in sandstone reservoirs. Clay crystallization may, therefore, represent a significant drawback for engineering the geothermal programme, as the chemical composition of the injected fluids shall be designed to reduce and even prevent illite precipitation and promote tosudite precipitation, when mixing with the natural fluids still present in the granite.

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