During the 1960s and 1970s the great importance of massive sulfide deposits in volcanic rocks finally came to be realized. Commonly referred to as VMSDs; for volcanogenic massive sulfide deposits, they were, during two decades of increasingly intensive research, found to range in age from the Archean to the present and to be encased in volcanic rocks that almost always displayed some evidence of submarine extrusion. Exploration geologists reclassified many previously known deposits as members of the class, and then, using successful combinations of geological, geophysical, and geochemical tools, discovered large numbers of new ones. By 1983, more than a thousand volcanogenic massive sulfide deposits have been recognized. They are now major suppliers of Cu, Pb, Zn, Ag, and Au around the world and rank second only to porphyry coppers in economic importance among nonferrous metallic mineral deposits (Rose et al., 1977).
Despite the obvious importance of volcanogenic massive sulfide deposits, an unusually large number of questions concerning their origins and formative processes remain. Unresolved questions led, during 1976, to the idea of a multidisciplinary, cooperative research project on these deposits. The idea arose during discussions between Japanese and U. S. scientists and, appropriately, the specific deposits suggested for attention, the Kuroko, have been the longest studied type of volcanogenic massive sulfide deposits and the first ones recognized as being of submarine, exhalative origin (Ohashi, 1920).
The U.S.-Japan-Canada Cooperative Research Project on the Genesis of Volcanogenic Massive Sulfide Deposits commenced in 1978. Sponsored by the National Science Foundation in the
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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.