日本列島付加体中に胚胎する古海洋底で生成した鉱床

Abstract

Basement rocks of the Japanese islands consist mainly of accretionary complexes younger than ca. Ma. Various types of stratiform and/or massive ore deposits that formed on a paleo-seafloor are hosted within the Japanese accretionary complexes. In the present study, we review recent progress on metallogenetic research and outline unsolved problems related to these types of deposits, such as the Besshi-type sulfide deposits, bedded ferromanganese deposits, and bedded manganese deposits related/unrelated to greenstone. Besshitype deposits within the Sanbagawa Belt formed by vigorous hydrothermal activity at a pelagic mid-ocean ridge during the Late Jurassic, and sulfide ore was preserved by the concomitant Late Jurassic Ocean Anoxic Event. Besshi-type deposits, which are closely associated with in-situ greenstone in the Northern Shimanto Belt, formed by ridge subduction during the Late Cretaceous. Certain aspects of the genesis of other Besshi-type deposits in the mélange zone of the Northern Shimanto and Chichibu Belts remain unresolved, although the Tsuchikura deposit occurs as an olistolith. Bedded ferromanganese deposits, so-called umber deposits, are derived from hydrothermal sediments at the periphery of the mid-ocean ridge, whereas bedded manganese deposits closely associated with greenstones were originally produced as hydrothermal sediments around seamounts. Bedded manganese deposits that occur within pelagic chert sequences without greenstone are considered to be formed by drastic changes in the redox state of deep water due to the influx of oxic and silicapoor surface seawater into anoxic and high-manganese stagnant deep water. However, whether bedded manganese deposits without greenstone are hydrothermal or hydrogenous in origin remains controversial. Although the problems are complex, future multidisciplinary research should clarify many of the unsolved questions related to metallogenesis.