In all important stages of hydrothermal precious and base metal mineralization in the central Slovak Neogene volcanic rocks the characteristic assemblages of clay minerals can be distinguished, in which illite and mixed-layer illite/smectite of various polytypes play an important role. Our study has confirmed that the illite polytypes can provide information on the thermodynamic conditions that prevailed during the formation of precious and base metal mineralization in the Central Slovak Neogene volcanic area. The results of K/Ar dating of illites from hydrothermally altered zones, such as those observed in most hydrothermal precious and base metal mineralizations in the Central Slovak Neogene volcanic area are reported for the first time. They bring new information on the time interval of the hydrothermal mineralization. The main stages took place 12.4±0.1–11.3±0.1 Ma in all the important ore districts within the Central Slovak Neogene volcanics.