K-Ar Ages of Sericite in Hydrothermally Altered Rocks around the Toyoha Deposits, Hokkaido, Japan

The Toyoha mine located in the Green Tuff Region of southwestern Hokkaido is well known for the occurrence of polymetallic vein-type deposits. Miocene strata composed of lava and pyroclastic rocks host the deposits and are hydrothermally altered. The age of formation of the deposits has been estimated to be middle Miocene. However, K-Ar age determination for sixteen sericites in hydrothermally altered rocks from the Toyoha deposits balls in a range from 2.93 to 0.96 Ma which are considered to represent the duration of hydrothermal activity responsible for the formation of the Toyoha deposits. The gangue sericite of Izumo vein showed a very young age of 0.49•}0.04 Ma. It has been estimated, on the basis of cross-cutting relations, that the mineralization stages of the deposits are mainly divided into two. The present results supported this observation that the Izumo vein belongs to the later stage. Sericites from the Okuiburi mineralized zone and the Yunosawa geothermal alteration zone showed the K-Ar ages of 2.90•}0.07 Ma and 2.40•}0.10, 2.39•}0.09 Ma, respectively. Therefore, it is likely that hydrothermal activity at the Okuiburi mineralized zone and the Yunosawa alteration zone is contemporaneous with that of the early stage of the Toyoha deposit.