Evidence for hydrothermal activity in the earliest stages of intraoceanic arc formation: implications for ophiolite-hosted hydrothermal activity

Forearcs within intraoceanic arcs preserve a record of earliest arc volcanism that took place at the initiation of subduction. In this study we have investigated the submarine section of the Bonin Ridge, which forms part of the Izu-Bonin intraoceanic arc and/or forearc system. The crustal stratigraphy of this forearc is found to have many similarities with suprasubduction zone ophiolite sequences, including the occurrence of sheeted dikes overlain by depleted tholeiites and/or boninitic magmatism. We propose that the Izu-Bonin forearc is potentially analogous to suprasubduction ophiolites. Hydrothermal alteration, together with sulfide disseminations, is exposed on some of the subaerial Bonin Islands. Based on geologic observations, the hydrothermal activity appears, in general, to have been contemporaneous with the boninite and boninitic andesite host volcanics. The sulfide mineral assemblage associated with the hydrothermal alteration is dominated by pyrite accompanied by minor chalcopyrite, sphalerite, and pyrrhotite. These sulfides are mainly found as disseminations in pillow breccias and hyaloclastite, or along the margins of dikes, but are also found as sulfide nodules in hydrothermal clay-rich zones. This forearc mineral assemblage contrasts with the Pb-rich mineralization associated with the Quaternary submarine hydrothermal deposits found along the main Izu-Bonin arc. Hydrothermal alteration in the Eocene Izu-Bonin forearc represents the style of hydrothermal activity generated by magmatism immediately after the initiation of intraoceanic subduction. Similarities in the style of hydrothermal activity and in the stratigraphy of the hosting volcanic sequence imply that the Bonin Ridge can be regarded as an in situ (i.e., not obducted or dismembered) analogue for a suprasubduction ophiolite including Cyprus-type massive sulfide deposits