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ABSTRACT: Deposition of Hydrothermal and Hydrogenetic Manganese Minerals in the Bonin Arc Area, West Pacific

Akira Usui

Wide-spread distribution of hydrothermal and hydrogenetic manganese deposits have been described in the results obtained during the Hakurei-Maru cruises in the Bonin Arc areas of the West Pacific from 1984 to 1989. Manganese deposits occur in the active volcano chains, back-arc basins, remnant back-arc ridges, and oceanic seamounts.

The hydrogenetic iron-manganese deposits commonly form normal earthy black crusts and nodules, sometimes as thick as several centimeters. They are always composed of hydrogenetic iron-manganese mineral, vernadite (^dgr-MnO2) with considerable amounts of metal elements. Co and Ni contents are relatively high in the crusts from the seamounts on the Pacific Plate (up to 1.1% Co and 1.0% Ni). In contrast, the hydrothermal deposits, characterized by dense, submetallic and gray appearances, are dominant in volcanically active areas; e.g., recent submarine volcanoes and back-arc basins. Fossil hydrothermal deposits were also observed inside nodules and crusts. Their mineralogy is stable and unstable todorokites with almost negligible amount of Fe, Si, Al, Ni, Co, Zn, etc.

Bottom sampling, heat flow measurement, side-looking sonar survey, deep-sea photos, and in-site observation with the submersible Shinkai 2000 have been concentrated on a small model area, the Kaikata-Smt hydrothermal area, where a recent low-temperature hydrothermal activity is strongly suggested by a high heat flow, young radiometric ages and fissure-like microtopography of the seafloor. The probably recent hydrothermal activity has yielded thin slabs of pure manganates underlain by volcanic sand layers.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990