The safe disposal of radioactive waste is a critical and frequently discussed topic in society. The Bundesgesellschaft für Endlagerung (BGE) was assigned with the selection of a suitable site for the disposal of high-level radioactive waste in Germany and, in the first phase of the selection procedure, has defined various selection criteria and sub-areas, seven of which cover the crystalline basement in Germany. For most of these sub-areas, little or no information is available on the crystalline host rocks and their properties, which would allow a narrowing of the site selection and identification of a suitable repository. Therefore, the AMPEDEK project establishes a reference database for the characterization of crystalline rocks in Germany. For this purpose, the database will contain information on the mineralogical, geochemical, petrophysical, and rock mechanical properties of predominantly igneous and metamorphic rocks of the Variscan basement. In the first phase, existing data from the literature or state geological offices were compiled and the structure of the database was established. In the second phase, ~500 drill cores and rock samples from quarries were analyzed in the laboratory to close existing data gaps. The goal was to sample as many different lithologies as possible to create a database that covers the majority of the relevant areas. The database currently contains ~8600 data points from 8 states and will be used to select smaller areas for Phase 2 of the site selection process and thus for detailed surface geological exploration campaigns.

Granites represent suitable crystalline host rocks for nuclear waste repositories because of their mechanical strength and apparent isotropy. However, all granites have a primary structural and petrophysical anisotropy that developed during the emplacement and crystallization of the melt. The primary anisotropy likely controls the orientation of post-magmatic structural features such as extensional fractures. This secondary anisotropy controls potential fluid pathways. Thus, a causal relationship between primary and secondary anisotropies could be an important constraint in terms of the site selection process. We present the first results of a systematic study of German felsic plutonites. We focus on samples of syn-variscan peraluminous granite plutons from two localities, namely the Fichtelgebirge and the Erzgebirge. These areas represent different tectonic settings during intrusion, i.e., compression and transtension, respectively. To estimate their primary anisotropy, we analyzed the crystallographic preferred orientation (CPO) of the rock-forming minerals. CPOs were measured using the neutron time-of-flight (ToF) texture diffractometer “SKAT” and electron backscatter diffraction (EBSD). Based on this data intrinsic bulk rock elastic properties are modeled. All granites show weak but distinct preferred orientations of the rock-forming minerals, which are coherent on a local scale. The quartz textures, for example, show similar CPOs, with point maxima of the positive rhombs combined with small-circle to crossed-girdle c-axis distributions. However, the orientation with respect to the geographic reference system strongly varies on a regional scale. We will discuss the CPOs regarding their tectonic setting and correlate the primary anisotropy with the post-magmatic fracture patterns of the particular granites.

Most high-grade metamorphic gneiss units in Germany exhibit a strong late-Variscan thermal imprint recorded by the presence of migmatites and granitic intrusions.

A nuclear waste repository in this setting is challenging because the amalgamation of various gneisses and magmatic rocks must be predicted and assessed to a search depth of 1500 m. Understanding the 3D-architecture of such basement units relies on better exposed regions than found in Germany. In this contribution, we use selected field areas to show the difficulty to predict the variability of high-grade metamorphic rock types at depth.

The Argentera Massif in the western Alps represents an ideal field analogue as it displays a variety of para- and orthogneisses and their equivalent migmatites along continuous surface outcrops. Detailed field observations from this area help to constrain the structural and lithological anisotropy of a typical Moldanubian crustal segment.

Except for the occurrence of large amphibolite lenses, gneisses and migmatites exhibit textural gradients at various scales rather than distinct lithological boundaries. Sharp boundaries on the other hand result from cross-cutting shear-zones that developed under retrograde greenshist facies conditions and discordant intrusions.

Whereas the overall 3D-geometry and anisotropy of the gneiss terrane may be described with the help of 3D models it is generally not possible to predict the spatial distribution of lithotypes and dominant textures for a given rock volume.

This contribution indicates why foliated gneisses and migmatites are less suitable as host rocks for a high-level radioactive waste repository compared with late granitoid intrusions that are more homogeneous.

The recent exploration for nuclear waste repositories includes the evaluation of safety mechanisms in case of a leakage of radiotoxic material. Possible retention mechanisms like the uptake of these elements from the conducting aquifer into ubiquitous fracture minerals like calcite have drawn much attention. In laboratory experiments, it has been shown that especially actinides are efficiently scavenged by (re)crystallization of calcite¹. To assess the long-term immobilization potential of calcite and the effect of low temperature alteration, element mobility in natural analogues needs to be studied.

A suitable study site is the Wenzel ore mine in the Black Forest (S-Germany), which was actively mined until 1823. Here, hydrothermal calcite veins are exposed at the tunnel wall. After it´s shut down the mine was naturally flooded and calcite remained in contact to groundwater for approx. 175 years until the mine was drained and reopened to the public in 1999.

Elemental distributions of La and Sr, which are considered as analogue elements for actinides and Ra are revealed by µXRF and LA-ICP-MS maps at high spatial resolution of 2 µm. First results show a complex pattern of Sr-enriched and depleted layers at the calcite-groundwater interface, which alternate at the 10 – 200 µm scale. In addition, localized enrichments of La appear, exceeding the pristine calcite concentrations by a factor of 4. Both features are interpreted as a result of secondary element mobility and discussed with respect of the retention potential of calcite during low temperature alteration.

¹Curti E. (1999) Applied Geochemistry 14: 433-445