Late Hercynian fluid percolation in the Saint-Sylvestre granite (NW FrenchMassif Central): fluid inclusion data on the Razes-Saint-Pardoux area

The different fluid percolations in the granitic complex of Saint-Sylvestre (Northwest French Massif Central) at the end of the Hercynian orogeny is r elatively well described in the mining area, but their extension is unknown at the scale of the whole massif. Microthermometric and microstructural st udies, outside any mineralized zone, show that different fluids circulated among three sets of microfissures: i)aqueous-carbonic fluids CN40 degrees E and N150 degrees E), ii)aqueous vapours and liquids least-west) similar to those considered as linked to the granite episyenitisation, and iii) low t emperature aqueous fluids least-west). The first event is characterized by high temperatures at rather low pressures, indicating a high geothermal gra dient (80-90 degrees C.km(-1)) similar to those observed in the proximity o f intrusions in the present-day geothermal field. It confirms the close spa ce-time relationships between the migration of these aqueous-carbonic fluid s and the Late Hercynian granites intruding the Saint-Sylvestre massif some kilometers southwards. Other fluid events, attesting a cooling of more tha n 200 degrees C after the previous fluid stages, seem to be related to the incoming of meteoric fluids in the system, followed by local fluid immiscib ility and cooling. These processes are common features of geothermal areas. ((C) Acadimie des sciences / Elsevier, Paris.).