Structural Studies of Kuroko Deposits
Among Kuroko deposits in the Hokuroku district, some differences are observed in the modes of occurrence and the characteristics of ore which are due mainly to the different environments of ore formations. The Furutobe deposit, a typical Kuroko deposit, is composed of the siliceous ore zone and the stratiform ore zone and occurs in and on dacitic lavas and pyroclastics. The Furutobe deposit may have formed in the area where active volcanic action occurred before ore formation. The sea bottom consisted mainly of unconsolidated dacitic pyroclastics and well-fissured dacitic lavas which gave a favorable condition for upwelling of hydrothermal solutions and circulation of seawater. The siliceous ore zone is composed of fissure and interstice-filling ore which formed in dacitic lavas and pyroclastics. The stratiform ore zone is divided into two types by modes of occurrence: (1) the primary ore deposits which seem to have been formed on the sea floor and in the unconsolidated tuffs; (1) ore deposits which are believed to have been transported and redeposited in a trough by slumping from the primary ore deposits.
The Matsuki ore deposit, a somewhat peculiar Kuroko deposit, contains a group of unit orebodies consisting mainly of high-grade yellow ore and occurs in mudstone and dacitic pyroclastics. The Matsuki ore deposit may have formed in the area where the preceding volcanism was minimal and the sea bottom consisted mainly of mudstone. The mudstone, overlying the permeable dacitic pyroclastics, behaved as an aquiclude layer against the rising hydrothermal solution and circulating seawater. The ore deposit is considered to have been formed in unconsolidated mudstone and dacitic pyroclastics from the modes of occurrence of the deposits and the existence of benthonic foraminifera in the mineralized zone.
Figures & Tables
This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.