DEPOSITION OF HYDROGENETIC AND HYDROTHERMAL MANGANESE MINERALS IN THEOGASAWARA (BONIN) ARC AREA, NORTHWEST PACIFIC

A widespread distribution of hydrothermal and hydrogenetic manganese d eposits is described in the results of the Hakurei-Maru cruises conduc ted in the Bonin Are areas of the West Pacific from 1984 to 1989. Mang anese deposits occur in the active volcano chains, back-are basins, re mnant back-are ridges, and oceanic seamounts. The hydrogenetic iron-ma nganese deposits commonly form earthly black crusts and nodules on the topographic highs of inactive ridges and old seamounts, sometimes as thick as 10 cm. They are always composed of the iron-manganese mineral vernadite. Co and Ni contents are relatively high in the crusts from the seamounts in the open Pacific Ocean (up to 1.1% Co and 1.0% Nt). T wo generations of distinct chemistry and texture are typical of these crusts, which can be compared to reported thick crusts from the Centra l Pacific seamounts. The hydrothermal deposits, characterized by dense , submetallic, and gray appearance, are dominant in the recent and als o in past submarine volcanoes of the island-are systems. Evidence of p ast hydrothermal activity was ascertained as fossil hydrothermal manga nese deposits inside the hydrogenetic nodules or beneath the hydrogene tic crusts over the Tertiary island are. Their component minerals are considered to be todorokite and birnessite (stable and contractible up on dehydration), having almost negligible amounts of Fe, Si, Al, Ni, C o, Zn, Pb, Cu, etc. In the small model site, the Kaikata Seamount hydr othermal area, the presumably recent hydrothermal activity has yielded thin slabs of pure manganate deposits growing downward within volcani c and sand layers. Early diagenetic influence is negligible on nodules of this area.