Geological setting
Located in the northern portion of the French Massif Central, the Limagne Basin consists of a hemi-graben filled with Tertiary sedimentary rocks such as arkoses, sandstones and conglomerates 14,15 (Figure 2). Three main normal faults can be distinguished separating the Tertiary sediments and the Variscan basement, which is composed of Variscan granitoids and metamorphic rocks. Some Quaternary basalt flow can also be observed (Figure 2B). The Eastern fault and the SW-NE Aigueperse fault meet at the N-S Clermont Ferrand Fault (Figure 2B) 16. This basin is also characterized by the presence of numerous CO2-rich thermomineral waters with high temperature springs reaching 33° C at the surface, indicating fluid circulation17. Five years ago, Calcagno et al. 18 identified a good geothermal potential for the basin, based on its thermal anomaly.
Outcrops at the boundary between the basement and the cover show quartz veins indicating the initial stages of the functioning of this hydrothermal field (Figure 3A). Sites where the vein density was highest were assumed to represent singular points in terms of hydrothermal recharge/discharge of the basin. Overall, the veins observed are sub-vertical with fillings of comb-textured quartz. On the western border vein orientations of N90-120E dominate whereas on the eastern edge, the directions are from N90E to N25E; finally, in the south the orientations are around N25E.
Fluid flow within the Limagne basin:
The estimation of fluid flow velocity must be based upon the growth mode. Most of the comb quartz crystals observed in longitudinal section by C-L image display pyramidal growth (Figures 1B, 3B). We also note some crystals with prismatic growth (Figures 1A, 3B). However, it was found that most of the quartz crystals formed exhibit a pyramidal growth mode (i.e., r face) and that the m (prismatic) face grows only passively 19. Nevertheless, in our case, taken from nature, in some cases we observe growth along the m face.
In order to compare equivalent faces, we prepared thin sections perpendicular to the <c> axis of the quartz grains, in the case of comb-textured quartz this means parallel to the vein wall (Figure 1C). Here, we consider that the main growing mode is on the trigonal pyramid; however, in both cases the thickest growth band, on the opposite face from the thinnest gives the hydrothermal flow direction (Figure 1). A single quartz grains may record more than one flow direction during its crystallization, which can be in the upward, downward or lateral direction (Figure 1C).
Accordingly, in our work we have deduced the velocities of the fluid flow for both growth modes. In the eastern part of the basin (Sans-Soucis area, Figure 2B) we find a downward flow reaching 10-6 to 10-4 m.s-1 when considering the prismatic growth, and 10-7 to 10-5 m.s-1 for the pyramid (Figures 2D,E, 4). Within the Charbonière-les-Varennes area, we observe firstly a downward fluid flow, followed by a lateral flow direction with a large range of fluid flow velocities from 10-7 to 10-4 m.s-1 or less when using the pyramidal growth mode (Figure 4). To the south, in the Roure area, we record only a lateral fluid flow with velocities of ca. 10-6 to 1-5 m.s-1 or 10-6 to 10-7 m.s-1 (Figure 2E,D, ; 4). As for the Southern region, the Ris area records a mainly lateral flow with a wide range of fluid flow velocities of ca. 10-6 to 10-3 m.s-1 (Figure 2 E,D; 4). We also note the presence of quartz grains with no asymmetry in the growth bands, indicating a low velocity fluid flow. Moreover, lower velocities are recorded for the downward flow. Here, we can see that the flow is discontinuous over time and changes in velocity and direction.