ABSTRACT:

During submersible dives small-scale variability of Mn deposits were described on seamounts in the NW Pacific with reference 10 microtopography and geological evolution. Co-rich hydrogenetic crusts and nodyles occur as large talus deposits on Cretaceous guyots. These are probably supplied from older Mn crust deposits through mass movement. Tectonic deformation of the guyot may have caused landslides and sediment rework, which occasionally prevented development of crusts. Hydrogenetic crusts and nodules cover entirely the flank of early-Miocene submarine volcanoes. The crusts cover volcanic rocks, while nodules rest on calcareous sediments. Local faulting and mass movement may have affected development of the deposits, showing evidences of rollover of nodules and breakup of crusts. Fossil hydrothermal Mn precipitates serve as nodule nuclei and crust substrates, suggesting past low-temperature hydrothermal activity. Modem hydrothermal deposits were discovered on an active submarine volcano. Hydrothermal water is probably emanating through fractures in the substrate, and produces pure Mn precipitates near the sea floor. This may be a type of lowhydrothennal Mn mineralization in submarine volcanoes. The submersible observations give insights on growth environments of Mn deposits and" geotechnical properties of bottom materials. Further observations and in-situ experiments are required to define physicochemical and biological environment and geotechnical properties. Submersible observation is highly recommended for exploration, mining technology and development, and environmental inpact assessment.

INTRODUCTION

General patterns of distribution and mode of occurrence of manganese deposits have been described on regional scales most intensively in the northeastern, central and southern Pacific and Indian Oceans by some leading countries since 1960s. Our knowledge of geosciences and geotechnology is not sufficient yet in understanding micro-environments and growth processes of the deposits, although shipboard observations and measurements with an aid of recent technology have revealed the general picture of deep-sea floors and marine manganese deposits.

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