Technical Report NTB 93-12

Geology of the Crystalline Basement of Northern Switzerland and Derivation of Geological Input Data for Safety Assessment Models

The crystalline basement of Northern Switzerland was studied in 6 deep boreholes drilled by Nagra and 16 older boreholes, geophysical surveys and regional geological and hydrogeological investigations both in the overlying sediments and in basement outcrops of the Black Forest. The sediment-covered basement in the area of interest is composed of high-grade metamorphic gneisses (mostly metapelites and meta-graywackes) that were intruded by Variscan granites and dykes (aplites, pegmatites, granite/rhyolite porphyries, lamprophyres). The late and post-Variscan evolution is characterized by a series of tectono-hydrothermal events, i.e. brittle deformation phases (faulting, fracturing) followed by hydrothermal alteration due to interaction with fluids circulating in the brittle structures. The Late Carboniferous high­temperature phases resulted in greenschist-facies alteration of the wallrocks (mainly albitization/sericitization of plagioclase and chloritization of biotite), whereas the Early Permian low-temperature phases were dominated by argillic alteration (plagioclase → illite, illite/smectite). Younger events include a kaolinitic alteration, which is accompanied only by subordinate brittle deformation, formation of vugs in pre-existing discontinuities, calcite precipitation in fractures and the formation of ore veins.

Temperature-salinity logging in the boreholes was the primary method used to detect water-inflow points, and a total of 138 inflow points were identified in the crystalline basement in 6 Nagra boreholes. The small-scale structure, mineralogy and porosity of the core materials from inflow points were used to define 3 regional types of water-conducting features, namely

  • cataclastic zones,
  • fractured zones, and
  • fractured aplite/pegmatite dykes and aplitic gneiss layers.

No major differences in the nature of water-conducting features and their transmissivities were identified between granites and gneisses. The regional classification scheme of water-conducting features obtained from the borehole data compares well with findings from the Black Forest (surface and tunnel observations). One major difference is the clustered occurrence of permeable ore-vein systems in the Black Forest, which were not found to the same extent in Northern Switzerland, either because such systems do not exist in this region or because they were not identified due to the limitations of the investigation programme.

Whereas the relative frequencies of the types of water-conducting features are depth-invariant, hydraulic conductivity varies as a function of depth. The uppermost 350 – 650 m of the crystalline basement have a conductivity in excess of 10-9 m/s in all boreholes, whereas the deeper parts have either a substantially lower conductivity (e.g. Böttstein) or a conductivity similar to that of the shallower levels (e.g. Kaisten).

The small-scale geometry of water-conducting features, including the spatial arrangement of flow porosity (e.g. open fractures, vugs/channels), of fracture infills (e.g. cataclastic matrices, vein mineralizations) and of wall rock lithologies (e.g. altered-wall rock rims around fractures, fresh rocks) is simplified in conceptual geological models. Together with data on mineralogy and interconnected wall rock porosity, these models constitute direct input to the quantification of radionuclide transport through the geosphere, accounting for advection, matrix diffusion, sorption and radioactive decay and ingrowth.