Minerals Report - International Seabed Authority

CHAPTER 5
COBALT-RICH FERROMANGANESE CRUSTS: GLOBAL
DISTRIBUTION, COMPOSITION, ORIGIN AND RESEARCH
ACTIVITIES
1. Abstract
James Hein, Senior Geologist
United States Geological Survey, California, United States of America
Cobalt-rich ferromanganese crusts occur throughout the global ocean
on seamounts, ridges, and plateaus where currents have kept the rocks swept
clean of sediments for millions of years. Crusts precipitate out of cold
ambient seawater onto hard-rock substrates forming pavements up to 250 mm
thick. Crusts are important as a potential resource for primarily cobalt, but
also for titanium, cerium, nickel, platinum, manganese, thallium, tellurium,
and others. Crusts form at water depths of about 400-4000 m, with the
thickest and most cobalt-rich crusts occurring at depths of about 800-2500 m,
which may vary on a regional scale. Gravity processes, sediment cover,
submerged and emergent reefs, and currents control the distribution and
thickness of crusts. Crusts occur on a wide-variety of substrate rocks, which
makes it difficult to distinguish the crusts from the substrate using remotely
sensed data, such as geophysical measurements. However, crusts can be
distinguished from the substrates by their much higher gamma radiation
levels. The mean dry bulk density of crusts is 1.3 g/cm3 , the mean porosity is
60%, and the mean surface area is extremely high, 300 m2 /g. Crusts generally
grow at rates of 1-6 millimetres per million years. Crust surfaces are
botryoidal, which may be modified to a variety of forms by current erosion.
In cross-section, crusts are generally layered, with individual layers
displaying massive, botryoidal, laminated, columnar, or mottled textures;
characteristic layering is persistent regionally. Crusts are composed of
ferruginous vernadite (δ-MnO2) and X-ray amorphous iron oxyhydroxide,
with moderate amounts of carbonate fluorapatite (CFA) in thick crusts and
minor amounts of quartz and feldspar in most crusts. Elements most
commonly associated with the vernadite phase include manganese, cobalt,
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