Présentations Colloques

Oral Presentation
Session 8.08: Mineral and Thermal water: an indicator of deep processes and source of economically valuable minerals
Taillefer Audrey
Hydrothermal circulations in Eastern Pyrénées - brittle and ductile faults both act as drains for upward circulations.
Hydrothermal systems in orogenic contexts without a heat source such as volcanic or plutonic activity are poorly studied. These systems are characterized by a fast upward ascent of hydrothermal fluids from a possible hot reservoir at depth to the spring at the surface. Faults are zones of heterogeneity of permeability acting as drains or barriers for hydrothermal fluids. In Eastern Pyrénées, hot spring alignment along the major Têt fault is an ideal case to understand the relations between faults and hydrothermal circulations. Cartography and field study shows that hot springs always emerge in crystalline rocks, especially at the contact between gneiss and metapelites, and mostly in the Têt fault footwall. Hot springs are locate near two types of faults- 1) Near the Têt brittle fault, at the intersection with other subsidiary faults, in the highly fractured damage zone. The core zone is composed of fine matrix rich cataclastic faults rocks. 2) In ductile faults, where we observe intersections between dissolution caves along the mylonitic foliation, and the regional fracturing parallel to principal faults. Numerical models coupling the Darcy law with heat transfer equation in a porous media shows that fluids with temperatures corresponding to our study area emerge in the permeable damage zone in the footwall of the fault. The permeability of the different constituents of the area (fault core, fault damage zone, bed rock) is the major factor controlling temperatures. So, faults control upward circulations of hydrothermal fluids by further processes- 1) The juxtaposition between impermeable metapelites and cataclasite with fractured permeable gneiss force fluids to stay in the Têt footwall. 2) The connexion of the Têt fault with other subsidiary faults induces damage zones, and thus fractures intersections, forming very efficient drains. 3) Ductile faults are anisotropic drains, with a good permeability parallel to the mylonitic foliation. This last point has practically never been identified. A better comprehension of these hydrothermal systems has important implications for geothermal exploration and permeability of basement aquifer.