Y. Copard et al., Evidence and effects of fluid circulation on organic matter in intramontane coalfields (Massif Central, France), INT J COAL, 44(1), 2000, pp. 49-68
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 vari
ous sites in the Massif Central were studied petrographically (reflectance
analysis) and geochemically (Rock-Eval pyrolysis). The results of this stud
y suggest that the studied coal was buried to 1500 m and that the coalifica
tion took place within 25 Ma, probably ending at the boundary between the E
arly and Late Permian (marked by the Saalic orogeny). Two thermal end-membe
rs basins were identified: (i) the Carboniferous of Bosmoreau-les-Mines (Li
mousin) and West Graissessac (Montagne Noire) showing geothermal paleoflow
values between 150 and 180 mW m(-2), and (ii) the Stephanian of Argentat an
d Detroit 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 s
amples were grouped into two populations, the first showing a positive corr
elation 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 ch
aracterized by a high Oxygen Index (OI) that increases with T-max. These di
vergence 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 coalifica
tion. The other kind of R-o-T-max divergence seems to be linked to local, p
articularly 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., shore-lived hydrothermal circulation and thermal dom
es of regional extent). (C) 2000 Elsevier Science B.V. All rights reserved.