Geologic Structures of the Field of Metallic Mineralizations in the Central Hokuroku District, Akita Prefecture

Abstract

Lithologic characters and stratigraphic succession of the Miocene formations in this area can be summarized, from oldest to youngest, as follows: Formation III; rhyolite and basalt in the Early-Middle Miocene (Nishikurosawa stage): Formation IV; alternations of mudstone and tuff in the Middle Miocene (Nishikurosawa-Onnagawa stage): Formation V; alternations of mudstone and tuff breccia in the Middle-Upper Miocene (Onnagawa stage).
The tectonic elements are three uplift belts and two basins. They are separated by NNE-trending faults. Analyses of isopach and lithologic mud-tuff ratios of the formations support the conclusion that the pyroclastic sedimentations were controlled by differential movements of the elements, and submarine volcanisms took place along the boundaries between the uplift and basin blocks. The base metal mineralizations of veins and some black ore deposits are associated with the volcanisms and confined in the Formations III and IV.