Mineralogical, chemical and isotopic perturbations induced in shale by fluid circulation in a fault at the Tournemire experimental site (Aveyron, France)

Abstract

Nuclear waste confinement systems in shales depend, among other factors, on their low permeability, alkaline pH and low Eh. These properties may be altered close to fractures, in the presence of fluids in chemical disequilibrium with the host formation. Since shales are well known for their self-sealing response to fracturing, the question of possible perturbations close to faults is not often treated. In order to understand how the shale may be affected in such cases, a mineralogical, chemical and isotopic study of a fractured zone was conducted at the French Institute for Nuclear Safety and Radioprotection (IRSN) experimental site. This site is located in a 250 m thick, fractured, Domerian and Toarcian shale layer, located between two aquifers, near Tournemire, on the western margin of the Causses basin (France). Oxygen isotopic analyses of calcite veins from the last tectonic phase suggest that cements crystallised from both pore water and meteoric fluids. Variable Fe content in calcite and presence of pyrite indicate that fault cements crystallized in different reduced environments. In the shale, δ 13C values are similar in the cement and in calcite veins. In the studied fault, however, δ 13C are closer to values measured in calcite from veins in the overlying limestone. This suggests that the fault allowed fluids from the upper aquifer to penetrate the shale. In the host rock, variations in Ca and Sr concentrations and δ 18O suggest that carbonate is altered and partly recrystallised up to 1 m from the main fault zone (MFZ). Ba and Zn are slightly concentrated near the MFZ as authigenic baryte and pyrite. Evidence of partial REE mobilisation was found. Variations of U and Th concentrations are quite low and suggest that if they are mobilised, they do not reach the MFZ.