Economic accumulations of bentonites as well as other exploitable types of clays and zeolites on the SW margin of the Kremnicke vrchy Mts. were formed by alteration of acidic vitritic volcanoclastics of the Sarmatian-Pannonian Jastraba Formation. The alteration took place during diagenesis of the sediments in a freshwater environment in an open or semiclosed hydrological system. The vertical zonation of alteration products represented from top to bottom by zones of perlite, smectite and high-silica zeolites, is a reflection of changes in pH and chemical composition of pore solutions percolating through the volcanoclastic complex. The original diagenetic mineral zones were locally modified by contact-thermal and hydrothermal effects of younger rhyolite bodies.