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Abstract:

The origin of hydrothermal karst cavities was connected with the Variscan hydrothermal process. The cavities were formed and filled by crystalline calcite. The process was accompanied by the intensive dolomitisation. Younger phase of hydrothermal karstification was not connected with vein-filling, but with the deep circulation of groundwater, probably associated with neovolcanic activity in the Bohemian Massif. This is supported by pollen grains and decomposed volcanic ash in speleothems which were formed after the major phases of speleogenesis. It is supposed that caves in the Konûprusy Devonian were formed in confined aquifer under phreatic and batyphreatic conditions. Thermal conditions appeared when paleogeothermic gradient was increased due to intensive neovolcanic activity. Hydrothermal karstification partly changed the morphology of caves. The maximum temperatures were stated to 60-700 C from large calcite crystals precipitated under phreatic and deeply phreatic conditions. The piezometric level was situated above limestones in Upper Cretaceous platform siliciclastics as indicated by numerous subvertical phreatic tubes („depressions") filled with sunkened Cretaceous and Tertiary sediments after the water buyoancy support decreased. Popcorn-like silicified Konûprusy Rosettes can be result of decrease of thermal water level and mixing with infiltrating meteoric waters. Outer zones of large calcite crystals with precipitation temperatures of about 400 C can indicate the gradual cooling of the whole system.