Kuroko-Type Massive Sulfide Deposits of Japan: Products of an Aborted Island-Arc Rift
L. M. Cathles, A. L. Guber, T. C. Lenagh, F. Ö. Dudás, 1983. "Kuroko-Type Massive Sulfide Deposits of Japan: Products of an Aborted Island-Arc Rift", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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The Green Tuff belt of Japan and its Kuroko-type exhalative volcanogenic massive sulfide deposits exhibit many unusual features which are best explained if the Green Tuff belt and the deposits formed as the result of an aborted attempt to rift the volcanic chain and open a new marginal sea. This failed rift hypothesis can account for the distribution of mining districts in the Green Tuff belt, the observed extensive and substantial premineralization subsidence and postmineralization uplift in the belt, its volcanic evolution, and other features. The premineralization subsidence results from the same mechanism proposed for the formation of rift valleys at mid-ocean ridges—a dynamic loss of fluid pressure in asthenospheric material that must well up as the lithosphere extends. The Green Tuff belt of Japan is strikingly similar in geology, tectonics, and mineral deposits to Archean greenstone belts, suggesting that Archean belts may be failed rifts. Ore deposits formed in failed rifts have a high probability of permanent incorporation into stable cratons because the deposits are underlain and surrounded by sialic crust and are near sea level. Areas of crustal extension are ideal sites for the kind of very vigorous hydrothermal circulation required to form massive sulfide deposits because they combine unusually high rates of heat input into the crust with a tensional, fracture-dilating stress environment. It is suggested that all exhalative massive sulfide deposits may form in rift settings. Differences between traditional deposit classes reflect mainly the different geologic settings of the rifts. Exploration implications of the failed rift hypothesis are detailed.
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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.