Disposal of radioactive waste in geological formations - Huelgoat granite massif (Finistere). Confirmation of favorable characteristics - First phase, first part - Petrographic and structural studies

The geological studies (petrographic, geochemical and structural) presented in this report are part of a first phase of work dedicated to the Huelgoat granite massif (Central Finistere, France) in the context of deep radioactive waste disposal issues. This first phase has enabled us to identify the different types of granite that make up the massif and their genetic relationships, as well as the influences of the different phases of brittle tectonics that have affected it. A summary of the geodynamic history of the granite massif is proposed, highlighting facts with practical implications for storage problems, such as the overall geometry, rooting and fracturing of the massif. This geological work was carried out in parallel with the gravimetric interpretations (report 84 SGN 102 GP8), which we have taken into account in order to eliminate contradictory hypotheses. The way the granite was emplaced points to a diapiric ascent of the pluton, giving it a flattened balloon shape. Structural studies show that the massif is moderately to weakly fractured locally, with major fracturing tending to cluster into N 120 deg. E corridors. Morphokinematic data emphasize the potential of the N 120 and N 165 deg. E fracture families to open up. In conclusion, we eliminate a priori unfavorable zones based on petrographic, structural and geodynamic criteria, and delimit a restricted zone for which we propose further work, in order to define the major problem corresponding to the delimitation of major discontinuities.