Effingen Member in the Baulmes Quarry (near Orbe, Yverdon): Calcite-celestite veins as indicators of past fluid flow

Veins in Baulmes show two stages of mineralisation separated by a compressional event that produced stylolites. Both mineralizing stages are characterized by the precipitation of similar minerals; calcite and celestite in the Effingen Member, and calcite only in the Sequanian limestone. In addition to these minerals, Stage 1 veins hosted in the Effingen Member locally bear sphalerite, barite, and pyrite. Stage 1 minerals are affected by the compressional event and minerals are cracked or sheared, while Stage 2 minerals are (almost) unaffected by tectonic strain. Sealing of vein structures by vein minerals is more frequent in Stage 1 than in Stage 2,where drusy textures are the rule. The succession of extensional (veins) and compressional structures (stylolite) might suggests vein formation was related to the stress release occurring during rock failure (earthquakes) while stylolite might record the compressional peak previous to rock failure. The veining and folding appear to be genetically related but the exact chronology of veining within the folding process is not solved at this stage. The story is probably complex, likely with inversions of movement along fault (vein) structures during the folding process. Veins can be split into two families, with the first being oriented NW and showing subvertical dip, and the second being oriented NE, and showing variable dip towards NW to SE. The first family of veins is apparently related to minor dextral and sinistral shears, while the second family trends parallel to the fold axis and it interpreted to correspond to syn-folding bed-to-bed stress accommodation, locally showing sinistral movement. Stage 1 and Stage 2 vein minerals are found in both vein families, which means these were connected at the time of mineralisation,and therefore synchronous. A large part of the veins show Stage 2 mineralisation overprinting Stage 1, thus meaning that even if sealed by minerals, vein structures are longstanding weaknesses that can be reactivated at any time in the future in response to tectonic stress and act as permeable structure. Veins can be followed for a maximum of about 5 meters (maximum about 5 cm thick) and are apparently not organized along major tectonic structures but occur as disseminated clusters, consistent with local stress accommodation during Jura folding. Although no major fault-vein structures were found in the investigated area, it is possible that such structure exist SW of the mine, in the continuation of the WNW-ESE fault-thrust structure that continues to Yverdon-les-Bains. The Baulmes anticline axis shows a dextral offset along this lineament that could probably be related to major fault-vein structures. Isotopic data on vein celestite and calcite can be explained with three fluid reservoirs: 1. the Triasic, probably within the Muschelkalk, rich in sulfate, poor in Sr 2. the Effingen Member porewater, rich in Sr (poorly radiogenic), with positive δ13C values 3. the Malm limestone aquifer, poor in Sr (slightly more radiogenic), containing water with negative δ13C values Sulfur and oxygen isotope data of celestite from Stages 1 and 2 hosted in the Effingen Member can be explained with sulfate that was derived from the Triassic evaporite, most likely from the Muschelkalk aquifer. In contrast, Sr isotopes of vein celestite and calcite broadly mimic the values obtained for corresponding whole rock carbonate and thus suggests the Sr source is local,from the Effingen porewater (and Sr sorbed on clay minerals) or the Malm limestone groundwater, respectively. This scheme is similarly valid for carbon isotopes of vein calcite in the Effingen Member, but not for the Malm limestone, where vein calcite shows negative δ13C values in disequilibrium with the host rock, indicating groundwater contained biogenic carbon in isotopic disequilibrium with the host rock carbonate