Key points:

• Mineralised fractures (veins) can be observed in all stratigraphic units. The Opalinus Clay in particular shows only few. They are thin (thickness mostly 0.5 – 3 mm) and are mainly found in narrow zones (1 – 2 metres thick). Between such zones, the Opalinus Clay is essentially free of veins.
• The combination of U-Pb dating, clumped isotope thermometry and isotope signatures of vein minerals allow constraining the palaeohydrogeological evolution and the behaviour of the geological barrier over timescales beyond the memory window of the porewater tracer profiles.
• In general, the data indicate evolutions of the mineral-forming fluids from modified seawater-type prior to the Eocene times towards meteoric waters in the Cenozoic.
• The investigated veins in the Opalinus Clay precipitated from fluids with less negative δ¹⁸O values compared to the present-day porewater and/or at higher temperatures. This suggests vein formation prior to Pliocene cooling.
• Based on δ¹³C and Sr-isotope data, an internal evolution of the mineral-forming fluids is conceivable for the Opalinus Clay, however, they could also attest to an efficient rock-buffering.