Chemical tracing and isotopic dating of authigenic illite-type minerals in centimeter-sized gouges of the ‘Main Fault’ within the Opalinus Clay (Switzerland)

Abstract Various minerals have crystallized in centimeter-sized gouges and in adjacent scaly clays of the Main Fault intersecting the Opalinus Clay within the Mont Terri underground rock laboratory (Switzerland). Authigenic illite-type crystals, for instance, formed idiomorphic overgrowths on older detrital mica-type agglomerates in the gouge matrices and in their highly deformed surrounding damage zones. This heterogeneous illite crystallization was induced by local temperature increases due to the friction during faulting and to circulation of hot fluids, more in the highly deformed gouge rims than in the gouge matrices themselves. The aggregation of authigenic illite with detrital counterparts in the size separates made it difficult to do any quantitative distinction, and therefore any direct isotopic age dating. This incomplete separation of the detrital from authigenic illite in size separates of the gouges and the scaly clays, even down to nanometer size, needed a theoretical K-Ar age extrapolation for the authigenic illite fraction at ~8.8±0.9 Ma. This estimated age is based on the K content of a theoretically pure authigenic fraction determined by in situ analysis relative to estimated K contents of the detrital counterparts. The superposition of the micro-X-ray fluorescence Na and Cl maps visualizes a few veins filled with salts across an examined gouge, showing that fluids flowed through it, as well as through its highly deformed rims, while diffusing discretely into the surrounding matrix. The combined Fe and S micro-X-ray fluorescence maps of the examined materials also showed numerous pyrite agglomerates systematically associated with P, which indicates an occurrence of organic matter related to pyrite alteration. This organic matter induced local reducing conditions that were confirmed by REE spectra in the newly crystallized clay minerals of the fault features, carried by flowing fluids into the gouges. No geochemical argument suggests that the gouges are more or less permeable, at present, than the nearby undeformed Opalinus Clay. However, they were certainly more or less permeable at one point, probably during faulting, as K had to be supplied to explain the illite authigenesis detected in them and, to some extent, in the nearby scaly clays. Such permeability is also demonstrated by the systematic occurrence of salts in the micro-veins throughout the observed gouge and in its highly deformed rims that had to crystallize from fluids flowing during the deformation episode. Clearly, both the shape of the drains in and around the gouges and their systematic infilling by salts exclude technical artifacts claimed sometimes to deny any opening of the gouge volumes.