Minerals Report - International Seabed Authority

Manganese crusts that are rich in cobalt are widely distributed on
the slopes of seamounts and islands in the equatorial Pacific, on the Blake
Plateau in the north-west Atlantic, and with denser concentrations of
nodules in many parts of the ocean basins. The main factors controlling
distribution of cobalt rich manganese crusts in the Central Pacific are
described by Cronan (1983, 1985), Morgan (2000), Halbach (1983); Halbach
and Manheim (1984); and Manheim (1986) as:
(1) elevated biological activity and extraction of cobalt from
seawater by organisms;
(2) water depths less than 2000 metres in the vicinity of seamounts;
(3) manganese enrichment at certain ocean depths related to zones
of minimum oxygen;
(4) low rates or absence of turbidite sedimentation.
Clark and others (1984; 1985) state that “the most favourable areas
for cobalt bearing crusts lie on seamounts older than 25 Ma, within water
depths of 800 to 2400 m, from 5° to 15° from the equatorial zone and in
areas where two generations of crusts with ages from 16 to 9 Ma and 8 Ma
or younger occur”.
For example, cobalt concentrations in manganese-iron crusts and
nodules in the Mid-Pacific Mountain and Line Islands area increase from
less than 0.4 % at water depths of 4000 m to 1.2 % on seamount slopes and
summits less than 2500 m in depth (Gross and McLeaod, 1987). The
thickness of these crusts ranges from 2 cm in the upper slope areas to 9 cm
in the lower areas and may contain more than 16 kg/m (dry weight) of
encrusted surfaces (Halbach and Manheim, 1984).
4.5.5 How manganese nodules and crusts are formed
Manganese nodules and crusts are formed through sedimentary,
concretionary and biogenic processes. The metals contained in them are
derived from hydrothermal, diagenetic, halmyrolytic, and sedimentary
sources (Cronan, 1980; Eldesfield, 1977). The development and
distribution of nodules are influenced by a variety of regional and local
factors (Burns and Burns, 1977 – also see Cronan 2000 for references
therein). These include:
(1) their size, morphology, mineralogy, age and rate of growth;
(2) the availability, size and composition of nuclei;
INTERNATIONAL SEABED AUTHORITY 698