The Izu-Ogasawara (Bonin) arc, Japan is an active oceanic island arc on the northeastern margin of the Philippine-sea plate, which is subducted by the Pacific plate. There are five Kuroko-type deposits on the arc, three of which are situated within an area 20 km in diameter along the volcanic front. The three Kuroko-type deposits were found based on the results of heavy mineral analysis such as sulfides together with chemical analysis of the seafloor sediments. This paper presents the results of the heavy mineral analysis and indicates potential sites for the hydrothermal sulfide mineralization in the metallogenic region of the arc.
The Izu-Ogasawara (Bonic) arc could be a place of Miocene Kurokotype deposits from the Hokuroku basin by analogy of their geographic and tectonic settings (Fujioka, 1983). Contemporary submarine volcanism may take place in submarine calderas and volcano-tectonic grabens above subduction zones representative of island-arcs or rift zones along continental margins (Sillitoe, 1982). In the Izu-Ogasawara arc, Kuroko-type deposits were identified in three calderas (Myojin knoll, Myojinsho and Bayonnaise knoll) based on data on the occurrence of sulfides and their distribution using heavy mineral analysis (Iizasa, 1993a; Iizasa, 1993b; Iizasa et al., in preparation). Heavy mineral analysis is often used for establishing their provenance (e. g. Rittenhouse, 1943; Imbrie and Van Andel, 1964). This is also a powerful method for the exploration and assessment on both active and passive hydrothermal deposits in submarine calderas and rift systems of island-arc setting. Heavy minerals in sandy sediments tend to concentrate finer-grained sand fractions (#60 mesh (0.25 mm) to #230 mesh (0.063 mm)). In this study, the 60–230 mesh subsamples were tentatively taken for heavy mineral analysis because the sample is mainly composed of sand-sized grains which are convenient for the identification of heavy mineral species.