K-Ar ages were measured on five whole-rock samples of felsic volcanics and clays from the Hiraki mine. The results are: (1) 70.0±1.5 Ma for the underlying Kamogawa rhyolite; (2) 68.9±1.6 Ma and 69.1±1.6 Ma for the least-altered and altered rhyolitic non-welded tuff (= ore horizon), respectively; (3) 67.6±1.5 Ma for the unconformably overlying rhy-olitic Hiraki welded tuff; (4) 63.8±1.5 Ma for clays from the fracture zone crosscutting the whole volcanic sequence. These ages are fairly consistent with the volcanostratigraphy in the mine area, indicating that various volcanisms and mineralization took place within a time span of a few million years in the latest Cretaceous to the earliest Tertiary Periods. The unconformity recognized, therefore, means a minor time-gap, not a long cessation of volcanism, at least in the Hiraki mine area.