Development of Submerged Sedimentary Basin and Environment of Kuroko Mineralization in the Hokuroku District

Abstract

The Hokuroku district, which belongs to the "Green Tuff region" of Northeast Japan, is well known for distribution of the typical Kuroko deposits, as Kosaka, Shakanai, Hanaoka and others. The field examination and investigation of many drilling cores indicate that the district has a basin satructure of about 25 km (E-W) by 30 km (N-S). The ore deposits mainly occur along the east and west marginal parts within the basin.
During the Nishikurosawa and Onnagawa stages of the Miocene, the basin was formed in consequence with progressing volcanic activity and subsidence of surface materials at the same time. The acidic volcanisms accompanying Kuroko mineralization and other acidic to basic volcanism were most active along both the sides of the basin, where the basement rock had been sheared.
On the basis of Lehman's theory, the tectonic history was analyzed. A tension environment at the Nishikurosawa stage or later is responsible for the acidic volcanism (including Kuroko mineralization) of the Hanaoka and Kosaka mines areas. The volcanism at the late Onnagawa and Funakawa stages followed the zone of a maximum compression. Degree of subsidence in the Hokuroku basin seems largely dependent on the amount of erupted materials from the magma chamber.