Geoscientific Characterisation and Interpretation (Geosynthesis) within the Preliminary Safety Assessment in the German Site-Selection Procedure for a High-Level Nuclear Waste Repository
Bundesgesellschaft für Endlagerung (BGE), Germany
After implementation of the Repository Site Selection Act (StandAG) in 2017, the Federal Company for Radioactive Waste Disposal mbH (BGE mbH), as the German waste-management organization, started the site-selection procedure for a nuclear repository for high-level radioactive waste in Germany. On the way towards the repository site with the best possible safety, the site-selection procedure is required to be a participatory, transparent, learning and self-questioning process based on scientific expertise. With the Sub-areas Interim Report published in 2020, first results were presented, outlining sub-areas with favorable geological conditions in preparation for defining the site regions for surface exploration. The identified 90 sub-areas with favorable geological conditions cover approximately 54% of the area of Germany. Currently, one of the main tasks in the site selection procedure is to conduct the representative preliminary safety assessments for each sub-area.
Apart from the technical descriptions of the repository system, the geoscientific characterization and interpretation (Geosynthesis) of the host rock, the overburden and the geological processes serve as a basis for the safety assessment. The main character of the Geosynthesis is therefore to compile all geoscientific information, relevant to the safety of a repository. Additionally, we describe how the Geosynthesis could be used to identify potentially suitable areas within large sub-areas. These areas with the most favourable geological conditions will then be evaluated in more detail during the representative preliminary safety assessments.
Element partitioning during hydrothermal alteration at ultramafic-hosted mineralized systems: insights from the fossil Marmorera-Cotschen hydrothermal system (Platta nappe, SE Switzerland)
1Laboratoire de Géologie, CNRS-UMR 8538, Ecole Nationale Supérieure de Paris, France; 2Géosciences Rennes, CNRS-UMR 6118, University of Rennes 1, France; 3Department of Geology, Trinity College Dublin, Dublin, Ireland; 4IFREMER Centre de Brest, DRO/GM, France; 5Institut des Sciences de la Terre d’Orléans, UMR 7327, University of Orléans, France
Ultramafic-hosted mineralized systems commonly form massive sulphides at the seafloor which are enriched in base (Cu, Zn, Ni), critical (Co) and precious (Au, Ag) metals. In present-day settings, the limited conditions of observation at the seafloor prevents a complete understanding of these hydrothermal systems, especially concerning deep hydrothermal processes. A way to unravel deep hydrothermal processes that occur in these systems is to focus on fossil analogues preserved on-land which noticeably well crop out in mountain belts.
We adopted this strategy here and focused on a mineralized system preserved in the Platta nappe (SE Switzerland), a remnant of the Jurassic opening of the Alpine Tethys Ocean. The geometry and petrographic assemblages of the hydrothermal system, previously established, served as a base for the present study. We performed a geochemical tracing both on whole rocks and in-situ metal-bearing phases (sulphides and oxides) sampled at three distinct structural positions of the hydrothermal system. Among the geochemical tracers, Co, Ni and Se appear as good proxies to constrain hydrothermal processes. Indeed, at given structural position, the Co/Ni ratio increases in the most mineralized and altered sample suggesting this ratio is linked to the intensity of hydrothermal alteration. Also, towards the top of the system, a general trend showing respective decrease and increase of the Co/Ni ratio and of the Se content in metal-bearing phases was observed. The evolution of these geochemical tracers together with petrographic evidences supports a genetic model for the Marmorera-Cotschen hydrothermal system involving hydrothermal fluid progressively mixing with seawater.
Source of metals in ultramafic-hosted VMS deposits: insight from the Troodos ophiolite and ODP Hole 735B
1KIT, Germany; 2Mineralogical State Collection Munich
Volcanogenic massive sulfide (VMS) deposits associated with mafic-ultramafic rocks show strong structural control and are located at or in the vicinity of low angle detachment faults such as oceanic core complexes (OCC) in mid-ocean ridge environments. These ultramafic VMS deposits are variably enriched in precious (Au-Ag) critical (Co) and the base metals Cu, Zn, and Ni but the source of the metals remains poorly known. The Troodos ophiolite, Cyprus, and the ODP Hole 735B on the Atlantis Bank are investigated to better characterize the source of metals and the deposit genesis. The ODP Hole 735B recovers gabbroic rocks down to 1508 meters below seafloor (mbsf) and shows evidence for high temperature hydrothermal alteration in the upper 250 mbsf. There the rocks are significantly depleted in Cu and S and primary magmatic sulfides are absent, implying efficient metal mobilization. Similarly, the Troodos ophiolite shows evidences for relics of OCC with seafloor-related high temperature hydrothermal alteration and associated massive sulfide mineralizations. Within the western Limassol Forest complex in the Troodos ophiolite, the Dhierna main shear zone separates serpentinized ultramafic rocks from sheeted dykes. Here, massive sulfide mineralizations enriched in As, Au, Co, Cu and Ni are observed and are characterized by pyrrhotite, pentlandite, chalcopyrite, cubanite and cobaltite. Additionally concentrations of Cu, Zn, Ni and Co in the peridotites decrease with increasing serpentinization towards the detachment fault also implying metal mobilization during hydrothermal alteration along the detachment fault.