Calcite, calcite-celestite and celestite veins were identifed from the bottom of the Opalinus Clay up to the Malm limestone. Most of the sampling effort for veins was focused on the NE trending Main Fault, a thrust structure related to the Jura folding and thrusting, where sub-millimeter thick calcite (±celestite) veins are common. The few accessible (no shotcrete on the gallery wall) NW trending and sub-horizontal celestite-calcite veins located below and above the Main fault were also sampled. Isotopic analyses (Sr, S, O, C) were performed on whole rock carbonate, diagenetic sulfides, evaporitic sulfates (Triassic) and vein minerals (calcite and celestite) from samples distributed along a profile from the Muschelkalk to the Malm. Isotope data suggest that veins formed during a single geotectonic event that allowed fluids to flow across the Opalinus Clay, with the main fluid source located in the underlying Trias. Petrographic data combined with structural information indicate that vein mineral precipitation was syntectonic and most likely occurred during the Jura folding and thrusting. Sulfate-rich fluids were apparently expelled upward from the Trias aquifers (probably Keuper) through active faults and mixed with local fluids.