Geophysical and geological methods for exploration and evaluation of deep-sea cobalt-rich ferromanganese crusts: A case study in the south pacific

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Usui, Akira
Holding Date 21 September 2008

Ferromanganese crusts on topographic highs in the Pacific Ocean are believed to be potential resources of cobalt, nickel, and platinum. Earlier shipboard geological and geophysical studies have focused on estimating the total reserves of the deposits and the grades of these metals. This paper summarizes our attempt to obtain more realistic distribution patterns and configuration of the deposits on some seamounts in the South Pacific Ocean. Morphology of the seamounts and micro-topography of the sea floors are described using shipboard acoustic remote sensing, deep-tow sonar images, and TV/camera. The narrow-beam echo sounding and sea floor photo images tell us the rough distribution of possible rock outcrop or gravels, but not necessarily the coverage of ferromanganese crusts. Sampling of the rocks and sediments is necessary to understand the distribution of surface and sub-surface materials, though it does not always represent typical distribution of the bottom materials. Based on using rock drills, dredges, deep-tow TV/camera, and side-looking sonar across some narrow-beam sounder mapped areas, we would recommend a combined interpretation together with geological consideration to decipher the evolution of seamounts, surface geology, the formation environments of rocks and ferromanganese crusts. Because a universally effective mapping system of seafloor ferromanganese crusts is not yet established, it is more realistic for evaluation of the deposits to integrate the acoustic remote sensing data and ground truth sampling data. Some combined data and interpretations are presented on much smaller scales on individual seamounts with complicated topography and distribution of deposits. For example, we find typically abundant distribution of thicker crusts on the radiating minor ridges but usually less over valleys between the ridges, which allows for a more realistic mapping of surface ferromanganese distributions based on geological consideration.