西南北海道積丹―洞爺地域の黒鉱型鉱床のK-Ar年代

抄録

Kuroko and kuroko-type deposits are distributed in the Shakotan-Toya district, southwest Hokkaido. The mode of occurrence of kuroko-type deposits is somewhat different from the typical kuroko deposits in the Hokuroku district. For example, the kuroko-type deposits lack any bedded ore-body. K-Ar age determination for hydrothermally altered rocks from the Kunitomi, Yoichi, Meiji, Horobetsu, Akaiwa and Otaru-Matsukura deposits has been carried out in order to compare mineralization ages of the typical kuroko deposits with the kuroko-type deposits from the view point of mineralization age.
Sericite and potash feldspar from the No.3 bedded ore-body in the Kunitomi showed same K-Ar age, 13.02±0.34 Ma and 13.24±0.31 Ma. Sericites from ore-bearing vein in the Yoichi and from a disseminated ore-body in the Meiji showed K-Ar ages of 12.39±0.32 Ma and 12.56±0.31 Ma, respectively. Ore veins within the Horobetsu deposit have ages from 15.41±0.38 Ma to 14.51±0.33 Ma. K-Ar ages obtained from hydrothermal minerals from the Kunitomi, Yoichi, Meiji and Horobetsu deposits are of middle Miocene age. These ages coincide with the typical kuroko deposits of the Hokuroku district (15.6 to 10.7 Ma).
K-Ar ages of 9.78±0.24 Ma and 9.51±0.46 Ma were obtained from a stockwork ore-body in the Akaiwa and a massive barite ore-body of the Otaru-Matsukura deposit. These results would indicate that different mineralization from main stage kuroko mineralization occurred during late Miocene time.