Evidence and effects of fluid circulation on organic matter in intramontane coalfields (Massif Central, France)

Abstract

Recent evidence for a Late Carboniferous hydrothermal event responsible for Au–As mineralization within the Variscan belt of the French Massif Central adds a supplementary episode to the already rather complex thermal history of this area. To better understand this history, 45 coal samples from various sites in the Massif Central were studied petrographically (reflectance analysis) and geochemically (Rock-Eval pyrolysis). The results of this study suggest that the studied coal was buried to 1500 m and that the coalification took place within 25 Ma, probably ending at the boundary between the Early and Late Permian (marked by the Saalic orogeny). Two thermal end-members basins were identified: (i) the Carboniferous of Bosmoreau-les-Mines (Limousin) and West Graissessac (Montagne Noire) showing geothermal paleoflow values between 150 and 180 mW m −2, and (ii) the Stephanian of Argentat and Détroit de Rodez (SW Massif Central) with values estimated at between 100 and 120 mW m −2. By plotting the T max and R o values on a diagram, the samples were grouped into two populations, the first showing a positive correlation between R o and T max and the second with higher T max values than expected after R o values. Selected samples of the second group are also characterized by a high Oxygen Index (OI) that increases with T max. These divergence between R o and T max associated with a high OI may be the result of the circulation of slightly oxidizing hot fluids subsequent to coalification. The other kind of R o– T max divergence seems to be linked to local, particularly high thermal activity, especially in Graissessac and Bosmoreau basins. It is interpreted as being due to a difference in response of these two maturity indicators, respectively to the intensity and duration of the thermal events (e.g., short-lived hydrothermal circulation and thermal domes of regional extent).