Hydrothermal Activity Related to Arc-Backarc Magmatism in the Western Pacific

Abstract

A compilation of 27 sites of hydrothermal mineralization in the western Pacific was made in terms of their tectonic settings, mineral commodity, and fluid chemistry. These sites constitute about 20% of the known seafloor hydrothermal sites, which are dominated by those occurring at mid-ocean ridges (MORs). High-temperature hydrothermal activities in the western Pacific exclusively occur in association with submarine volcanism found either on backarc spreading centers, backarc rifts, or volcanic fronts in arc-backarc systems. Magmatic contribution to the hydrothermal systems in arc-backarc settings is more obvious than those in MORs, and several lines of evidence suggest direct interactions between the magma chamber and hydrothermal fluids. Hydrothermal activity at backarc spreading centers shares several common chemical, mineralogical, and morphological features with those in MORs. This is understandable because of small differences in parameters, such as magma signature and composition of oceanic crust, between those two systems. Frontal-arc volcanoes are proved to be the sites of vigorous high-temperature hydrothermal activity. Several investigated submarine calderas in the Izu-Bonin (Ogasawara) and Mariana arcs have signs of present or recent activity, indicating Ubiquitous hydrothermal mineralization regardless of their magma composition. Backarc rifting centers are particularly interesting because of their analogous tectonic settings with the Japan arc of Miocene age where many Kuroko-type volcanic massive sulfide deposits (VMSDs) formed in association with bimodal volcanisms. Some hydrothermal sites in the Okinawa Trough backarc basin represent modem examples of such mineralization. The wide variety in modes of occurrence of hydrothermal systems in the western Pacific region, in contrast to those in the MOR setting, reflects diversity in magma composition, contribution from heterogeneous island-arc crust rather than homogeneous oceanic crust, and the unique tectonic setting of the arc-backarc system. These systems provide the best field to investigate analogs to VMSDs, because most ancient VMSDs are hosted by the rock types in a subduction-related volcanic suite or an extension-related bimodal suite representative of island-arc- or continental-rift-related tectonic settings.