Late Hercynian fluid percolation in the Saint-Sylvestre granite (NW French Massif 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 relatively well described in the mining area, but their extension is unknown at the scale of the whole massif. Microthermometric and microstructural studies, 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 temperature aqueous fluids least-west). The first event is characterized by high temperatures at rather low pressures, indicating a high geothermal gradient (80-90 degrees C.km(-1)) similar to those observed in the proximity of intrusions in the present-day geothermal field. It confirms the close space-time relationships between the migration of these aqueous-carbonic fluids and the Late Hercynian granites intruding the Saint-Sylvestre massif some kilometers southwards. Other fluid events, attesting a cooling of more than 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 immiscibility and cooling. These processes are common features of geothermal areas. ((C) Acadimie des sciences / Elsevier, Paris.).