Petrographic investigations of seafloor sediments from the Kita-Bayonnaise submarine caldera, Shichito-Iwojima Ridge, Izu-Ogasawara Arc, northwestern Pacific

Abstract Seafloor sediment samples were recovered by box and gravity corers from the Kita-Bayonnaise submarine caldera, Shichito-Iwojima Ridge, Izu-Ogasawara Arc, northwestern Pacific. The samples were separated into heavy, light, and claysized fractions for assessment of the hydrothermal contribution to the sediments of the submarine caldera, and have been determined by microscopy, SEM-EDX, and XRD. Heavy fractions are composed of predominant magnetic minerals, dominant silicates and common sulfides and sulfate, minor sulfosalt, rutile, phosphates, carbonates and Mn-oxyhydroxides. Primary hydrothermal phases are composed of barite, pyrite, sphalerite, chalcopyrite, galena, fahlore, marcasite and rutile. Secondary mineral associations are usually present around galena and chalcopyrite grains. The assemblages of barite-pellet type pyrite-chalcopyrite-sphalerite and sphalerite-chalcopyrite-pyrite with galena or fahlore suggest that hydrothermal mineralization took place in the caldera. The assemblages of epidote-carbonate or chlorite in heavy transparent fractions and of sericite-quartz-feldspar in light fractions are indicative of hydrothermal alteration origin because of only dacite sampled from the caldera. Some of the smectite, chlorite, sericite and kutnahorite in clay-sized fractions could also support the existence of hydrothermal activity. Other minerals consisting of hypersthene, augite, hornblende, quartz, feldspar and biotite are observed in heavy and light fractions as detritus of dacite in the caldera. Significant amounts of sulfides and barite mixtures (ranging from about 1.0 to 3.5 wt.% in treated fractions, 0.063–0.25 mm) in gravity core samples suggest that hydrothermal mineralization took place several times in the caldera. Many lines of evidence indicate the presence of hydrothermal mineralizations associated with Ag and Th in the Kita-Bayonnaise submarine caldera. The mineralogical analyses are powerful for exploring signs of past or present hydrothermal mineralization in submarine calderas.