Fission track ages of some Neogene Tertiary acidic tuffs around the Toyoha mine, southwestern Hokkaido, Japan

Abstract

The Toyoha mine is a zinc-lead-silver vein-type deposit which is located in the Green Tuff Region of southwestern Hokkaido. Neogene Tertiary strata which consist mainly of volcanic and pyroclastic rocks are widely distributed around the Toyoha mine. They are composed of Koyanagizawa, Motoyama, Nagato, Oshidorizawa, Sanbonmata and Oheyama Formations and Younger andesite in ascending order.
Fission track age determination by zircon grains from some acidic tuffs of Motoyama, Oshidorizawa and Sanbonmata Formations was carried out using external detector method. Following fission track ages were obtained;
Motoyama Formation: 14.2±0.4 Ma and 13.3±0.9 Ma
Oshidorizawa Formation: 8.8±0.3 Ma
Sanbonmata Formation: 3.3±0.2 Ma
It is concluded that Motoyama Formation, and Nagato and Oshidorizawa Formations are correlated to Miocene Kunnui and Yakumo Formations of the standard stratigraphy of southwestern Hokkaido of Neogene Tertiary System. Sanbonmata and Oheyama Formations are correlated to Pliocene Kuromatsunai Formation respectively, and the strata of latest Miocene to early Pliocene are considered to be absent in the Toyoha mining area.