Permeability variation in crystalline rocks due to low-grade solution phenomena

Permeability of crystalline rocks depends on parameters such as density and interconnectivity of fractures and pores. While in pristine crystalline rocks porosity is usually considered to be low, low-grade solution phenomena such as the formation of episyenites occur occasionally and may cause a local dramatic increase in porosity and permeability. These solution phenomena can be effective in otherwise unaltered rocks and may result in the preferential removal of certain mineral phases, especially of quartz so that porosities correspond to the spatial distribution of the previously existing mineral phase if no subsequent mineralization occurs (e.g., Pennacchioni et al., 2016). Using light-optical and scanning electron microscopy, X-ray tomography, micro-XRD, as well as digital image analysis, the differences in connectivity and hence permeability between, for example, quartz-depleted granite, gneiss, and schist can be characterized and quantified. We demonstrate that such porosities do not necessarily result in high permeabilities in an undeformed granodiorite from the Central Gneiss unit of the Tauern Window (Lago di Neves area, Italy), since former quartz aggregates are not interconnected due to their relatively late crystallization age and the preservation of the magmatic fabric; however, in the case of moderate mylonitic deformation, quartz as rheologically weak phase forms interconnected aggregates and layers. Its dissolution results in an extremely increased permeability. Therefore, not only the content and grain size but also the distribution, shape and alignment of minerals are crucial for rock permeability and need to be carefully investigated when searching for a final repository of highly radioactive waste in crystalline rocks. Especially since local shear zones may form in otherwise undeformed intrusive bodies, a detailed structural analysis beyond the exclusion of the presence of fractures is required to mitigate the risk of a long-lasting nuclear waste disposal.