秋田県馬場目—黒毛沢地区の地質層序と鉱化時期

抄録

The Babame-Kurogezawa districts in Akita are underlain by the metamorphosed Cretaceous granitic rocks at the basement. The Neogene Tertiary system, which overlies the basement, consists of the Kitanomata, Hikarizawa and Onnagawa formations in ascending order. The Kitanomata formation is of terrestrial nature as evidenced by the occurrence of welded tuff. The lower part of the Hikarizawa formation consists of sandstone-mudstone member and volcanic conglomerate member in ascending order. Plant fossils occur in the former and marine fossils such as calcareous nannofossils occur in the latter. The beginning of the marine transgression is estimated to have occurred later than 17.95 Ma. The middle to upper parts of the Hikarizawa formation are characterized by bimodal submarine volcanism with mudstone, being assigned to the Nishikurosawa stage. The Onnagawa formation consists of hard mudstone and basaltic pyroclastic rocks. The Babame and Hikarizawa kuroko deposits are hosted at the middle part of dacitic member and the host horizon is different from that of the Hokuroku district where kuroko deposits are hosted within upper part of the dacitic lavas of the Nishikurosawa stage. However, mineralization periods are thought to be not so different between them. Meanwhile, auriferous quartz veins are distributed at the Haginari deposit in the Babame district and the adjacent Kurogezawa district. K-Ar ages of their altered host rocks indicate 11.4±0.3Ma for the former and 53.5±1.3Ma for the latter. The former age is normal for the mineralization period of the surrounding area, but the latter age corresponds to the period of the basement metamorphism.