Abstract

We describe the regional and micro-scale variability in composition and growth structure for hydrogenetic ferromanganese crusts based on several ROV surveys at a wide range of water depths in the Northwestern Pacific seamounts. The geochemical, mineralogical properties and micro structure which form stacked lamination strongly indicate a microstratigraphic correlation among the crusts from remotely separated seamounts. The deposits commonly cover hard rock outcrops, building maximum thickness of 12 cm. This microscopic-scale variation in chemical and mineralogical composition have been found in common and the patterns are well correlated to each other, finally resulting in variable thickness and metal contents. Here we considered the possible geological parameters controlling the grade and abundance of the deposits, are the temporal oceanographic variations and the total age of exposure in the deep waters.

Introduction

The northwestern Pacific seamounts areas are thought to yield the most abundant high-grade ferromanganese crusts in the world oceans. In fact, some institutions have carried out scientific and survey cruises for the last decade, and some nations have registered their exploration areas for Co-rich ferromanganese crusts under the UNCLOS/ISA regulations. Nevertheless, comprehensive geological interpretation or depositional model have not yet been established, but the origin and depositional processes are still yet well understood on regional, outcrop, and microscopic scales (Usui et al., 2017; Glasby et al., 2007; Hein et al., 2000). We herein summarize our geological model of formation based on our on-site observation and sampling by ROVs and submersibles together with laboratory-based analyses, and attempt to apply the proposed model of deposition and growth to exploration and economic estimation of these precious ore deposits. Our belief is that the geological model and understanding formation processes help effective, reliable, and economical methods on exploration, excavation, processing, and this meets SDGs.

Geological Background

The NW Pacific seafloor is characterized by the complex morphology of abundant and huge seamounts, where stable rock outcrops commonly occur as substrates for hydrogenetic ferromanganese crusts (Glasby et al., 2007). The province is recognized as a unique Cretaceous mantle plume region, which has produced hundreds of seamounts in the NW Pacific sea floors (Lincoln et al., 1993; Van Waasbergen and Winterer, 1993; DeLaughter et al., 2005). The seamounts originated in the Late Cretaceous (120-70 Ma) from abundant hotspots (referred to as HS) in the Middle Jurassic oceanic crust in Fig. 1 (Koppers et al., 2003). As documented by plate tectonic theory, this seamount province is located at the western edge of the Pacific Plate, subducting towards WNW, and forming major complicated plate boundaries.

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