Elemental and isotopic tracing of mineral infillings from various microstructures of a fault system into fine-grained sediments: which interacting fluids?

Abstract

Elemental contents and Sr isotopic data of calcite and celestite infillings from various tectonic microstructures of the so-called “Main Fault” that occurs in the Opalinus Clay Formation (Switzerland) were used to elaborate on the geochemical impact of this structural system on the fine-grained host sediments. The chemical data of the leachates from vein, slickenside and gouge infillings are significantly different from those of leachates from diffuse calcite of the undeformed and of the highly deformed Opalinus Clay rock matrix called scaly clays. For instance, the fluids flowing within the microstructural veins and slickensides could not derive from pore waters of the host sediments, as the data show that the flowing waters diffused from veins into the matrix. The fluid/rock interactions in the gouges were also different from those in the veins and the slickensides, probably due to an additional pressure-solution impact in the former, complemented by a further local supply of organics from very close sediments. The infillings of the vein and slickenside micro-features and of some scaly clays that precipitated from bi-carbonated and/or sulfated flowing fluids yield an almost constant 87Sr/86Sr ratio of 0.70774 ± 0.00001 (2\(\upsigma\)). This constrained Sr isotopic value points to a unique homogeneous external source for the tectonic-related fluids that were of probable marine origin, signing those of the early, but also of the late faulting event. Notably different from those seeping presently throughout the host rocks, the initial flowing fluids were apparently related to a late-Eocene marine invasion into the Delémont Basin that occurred during the contemporaneous rifting of the Rhine-Graben tectonic system. The flowing fluids induced by the late Main-Fault system obviously used, at least, some if not most of these earlier tectonic microstructures, as they yield the Sr isotopic signature of their infillings. The regional evolution includes then an initial Rhine-Graben tectonic episode with fluids resulting from a flooding of the deposited sediments and into the faults of the regional active Rhine-Graben rifting by contemporaneous seawater. These fluids precipitated precisely calcite and celestite infillings in the microstructures induced by the rifting that remained then unaffected until the renewed tectonic activation during the late Eocene in the Mont Terri and its fault system. This new tectonic–thermal episode created a new generation of microstructures during the thin-skinned deformation of the Jura Belt within and around the Main Fault, in addition to those from earlier Rhine-Graben event that already existed in the host rocks. Connections between the two generations of veins could be observed at the wall of a new gallery in the rock laboratory. The flowing fluids involved in this late belt event moved apparently along these newly created tectonic microstructures, as well as along older ones when accessible. This duality allowed the preservation of the same initial Sr isotopic signature by partial dissolution of the initial infillings that precipitated about 30 million years earlier in the initial drains of the Rhine-Graben fault system.