Mineralised fractures (veins) record the composition of the fluid at the time of mineral precipitation, that is they are archives of the evolution of ground- and porewater. The combination of U/Pb and U/Th dating, clumped isotope thermometry (see also Section 4.3.5) and other isotope proxies allows reconstruction of the palaeohydrogeological evolution, notably for the time period older than the memory window of the porewater tracer profiles (Section 4.6) or from the analyses of groundwater composition (Section 4.5.5). Comparing such information to present-day values for pore- and groundwater provides an insight into the long-term behaviour of the aquitards (geological barriers), particularly with respect to fluid flow along fractures. Note that hydromechanical processes leading to self-sealing of fractures are discussed in detail in Section 5.7.
This section summarises the investigations of vein mineralisation in the TBO drill cores and draws conclusions on past transport in the aquifer-aquitard systems. It builds on previous studies on palaeo-fluid flow in the Mesozoic cover in Northern Switzerland by Pearson et al. (1991; old Nagra boreholes), Nagra (2001; Benken borehole), de Haller et al. (2011; Oftringen borehole), Mazurek & de Haller (2017; Mont Terri rock laboratory), Mazurek et al. (2018; Schlattingen borehole) and Looser (2022; clay pit of Frick).