Mineralogical and geochemical alterations in the Opalinus Clay and surrounding formations provide information about the long-term stability of this hydrogeochemical system

The Opalinus Clay is the chosen host rock for the Swiss deep geological disposal of nuclear waste, and it is also investigated in Germany. For the long-term integrity of the disposal site, temporal and spatial stable geochemical conditions are essential. The identification of mineralogical and geochemical alterations in the Opalinus Clay and surrounding formations gives insights on their stability in the past, and thus enables an assessment for the future.

To detect the mobility of chemical elements and associated mineralogical changes, a 58 m long borehole was drilled at the Mont Terri Rock Laboratory (Switzerland). Drilling was conducted from the Opalinus Clay through the entire underlying Staffelegg Formation, which includes two water-bearing sections. The rock members and their transitions were characterised with a variety of analytical methods.

The results show that many trace elements are particularly enriched at the upper transitions of the water-bearing sections. At the top of the Rietheim Member (Posidonia Shale), this enrichment is accompanied by an increase in pyrite, which can be interpreted differently. On the one hand, the depositional conditions may have led to enhanced pyrite formation and the associated pyritization of trace metals. On the other hand, the overlying rock layers could have acted as cap rock for mobile elements in the pore water, leading to the observed accumulations. Consequently, to distinguish between depositional or diagenetic and alteration or mobilisation features is crucial for the interpretation of the analytical results in order to assess the long-term stability of Opalinus Clay and surrounding formations.