Ore deposit formed on a paleo-seafloor in the Japanese accretionary complex

Abstract

Basement rocks of the Japanese islands consist mainly of accre-tionary complexes younger than ca. Ma. Various types of strati-form 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. Besshi-type deposits within the Sanbagawa Belt formed by vigorous hydro-thermal activity at a pelagic mid-ocean ridge during the Late Juras-sic, 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 ferromanga-nese deposits, so-called umber deposits, are derived from hydrother-mal 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 silica-poor 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 multi-disciplinary research should clarify many of the unsolved questions related to metallogenesis. Keywords: accretionary complex, seafloor mineral resources, Besshi-type sulfide deposit, volcanogenic massive sulfide (VMS) deposit, bedded ferromanganese deposit (umber), bedded manganese deposit 1,