Relations eaux-roches dans des aquifères gréseux profonds (sondage GPF-Sancerre)

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Abstract
A deep drill-hole has been cored in the Sancerre area under the sponsoring of the programme "Géologie Profonde de la France". Two aquifers, located in Hettangian and Triassic sandstones, were sampled. We have measured the concentrations of dissolved elements and analyzed the suspensions. In both cases, the compositions of the waters are acquired by dissolution of aluminosilicates, feldspar and evaporite minerals (sulfates and chlorides). Dissolved CO2 is the main alteration agent ; CO2 may be partly derived from deep degassing. Some neoformed minerals are common to both aquifers ; i.e. carbonates and silica. Kaolinite is also forming in the Hettangian aquifer, while gibbsite is forming in the Triassic aquifer. The relations trip between the neoformed minerals and the fluid composition corresponds to equilibrium saturation. However, the formation of kaolinite and gibbsite is only possible owing to the inhibition of fluorite precipitation. Several dissolved trace elements (Sr, Mn, Mg) have concentrations controlled by the formation of carbonates, while others (Zn, Fe, Cu, Pb, Ag) are controlled by sulfides.

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