Karstification of calc-silicate rocks and its role in country rock homogeneity: results of petrological, geochemical and hydrological studies of an area suitable as a deep radioactive waste depository nearby the Temelín nuclear power plant.

The locality Janoch (ETE-south) situated nearby the Temelín nuclear power plant (Czech Republic, South Bohemian Region) was chosen out of nine possible localities as a suitable deep radioactive waste depository. Based on further geological, geochemical, geophysical and hydrological studies a suitable area for a potential radioactive waste depository was defined. Furthermore, in this area two homogeneous blocks (eastern block and western block) representing the insulation part of the depository were defined. Both homogeneous blocks are located in the Varied unit of the Moldanubian domain composed of metamorphic rocks (up to Precambrian in age) intruded by Variscan igneous rocks. The only inhomogeneity are intercalations of calc-silicates, marbles and quartzites and also some brittle structures related to the presence of the intercalations. New geological and geophysical mapping helped to improve on the existing results and defined dip and dip direction of the intercalations, their thickness and orientation as well as their range in depth. Petrological, geochemical and hydrological research focused at karstification features related to the presence of isolated lenses of marbles and calc-silicates. Marbles are alternating with calc-silicates, but also with non-karst rocks. They vary in colour, grain size and often contain high proportion of insoluble minerals. In the Moldanubian domain, typical surface karst features as sinkholes, surface stream sinking or springs of karst water are rare and the role of mechanical erosion by ground water on cavity formation and growth is very limited. The origin of the karst features here is mostly related to corrosion. Corrosion is either controlled by the rain water or more commonly by chemically aggressive water from the non-karst host rock that accumulates easily in the more permeable marbles. Dissolution occurs mostly near cracks, joints, faults and fault zones. Some cavities can develop along contacts between marbles and non-karst rocks, which are often related to the presence of brittle structures. The back-reaction of calcium carbonate precipitation is not important here and results only in formation of secondary surface features as for example travertine filling of cavities in non-karst rocks. Besides that, on the locality of ETE-south (Janoch), one of the marble intercalations developed a small karst system with several small caves with speleothems, karst spring and calcareous tufa deposit. The largest cave was around 15 m long and was almost completely destroyed by marble mining activities in the early 1950s. [ABSTRACT FROM AUTHOR]