Abstract
Barite-bearing silica chimneys and crusts were found during alvin dives in a back-arc basin called the Sumisu Rift of the Izu-Bonin arc, northwest Pacific. These silica deposits mainly occur on the flank of a rhyolite lava dome of the bimodal volcanic suite along the rift axis. Analogous hydrothermal activity occurred around 15 Ma in the northeast Japan arc; the formation of Kuroko deposits at this time was closely related to submarine rhyolitic volcanism of bimodal suite in a back-arc rift. The chimneys are composed of filamentous amorphous silica and minor amount of barite and iron oxide amorphous to X-rays. The concentrations of minor elements Fe, Mn, Ba, Pb, Zn, Cu, As, and Sb are similar to those in the hematitic chert layers which constitute the uppermost part of Kuroko orebodies. Theδ 34S andδ 18O values of barite from the Sumisu Rift have ranges of 21.7–22.3 and 8.8–10.3‰, respectively. The oxygen isotopic values, which are about 2‰ higher than those of the Kuroko barite, indicate that the temperature of formation was less than 150°C. This is supported by the dominance of one phase (liquid) inclusions in the barite crystals and occurrence of amorphous silica. Such a low formation temperature has also been deduced for the hematitic chert of the Kuroko deposits. Therefore, it is suggested that the silica deposits in the Sumisu Rift are the present-day analog to the Kuroko chert. This hydrothermal activity appears to be responsible for the observed high contents of manganese in surface sediments of the Sumisu Rift basin. Equivalent regional zoning of distal bedded manganese deposits around Kuroko deposits is observed at the Hokuroku basin.
Published Version
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