Potential formation mechanisms of early diagenetic displacive veins in the Permian Boda Claystone Formation

The Boda Claystone Formation is the most promising host rock for the disposal of high-level radioactive waste in Hungary. The formation has several properties obstructing the migration of radionuclides; however, structural heterogeneities may nonetheless act as migration pathways. Therefore, understanding the tectonic evolution of the formation is vital. Previous research has identified four different vein generations in the BAF-2 well. This study deals with the generation of veins that contain characteristic microstructures, such as curved wall rock inclusions in bands, trails and cone-in-cone arrangements. Based on optical, cathodoluminescence and electron microscopy, together with X-ray fluorescence and Raman microspectroscopy, these veins appear to have potentially formed during early diagenetic processes. Vein development was continuous and occurred along horizontal surfaces. The abovementioned structural elements were formed by growing crystals; however, primary crystal morphologies were presumably replaced during diagenetic and hydrothermal processes. A theoretical model is described, which could be responsible for the horizontal and continuous vein opening and simultaneous crystallisation in sediments with high (but easily degraded) effective porosity. According to this model, compaction, pressure solution, elevated pore fluid pressures, seepage forces and the force of crystallisation can all play important roles in the formation of these types of veins.