Minerals Report - International Seabed Authority

trace metals. These include vanadium (0.06%), molybdenum (0.05-0.1%), and
platinum (0.14 to 5 ppm) [20-21].
The richest cobalt-crusts deposits appear to be concentrated at water
depths of 800-2 000 m. Some scientists have considered a link with the oxygen
minimum zone as a possible reason for their formation [22]. However, as in
fossil stratigraphies, such crustification is an indication of a lacuna of
sedimentation, either by a hiatus (no deposition) or by intermediate erosion.
Similar encrustations, associated with nodules, were found in cores made on
top of seamounts in the Indian Ocean [23] and later in many DSDP cores.
They were proved lacunae of sedimentation. Observations of current activities
that prevent sediment deposition have been recorded during several surveys
[15].
It seems that there are two kinds of cobalt-rich crusts deposits:
• Flat deposits on top of sunken atolls, where the crust covers old
coral reef formations;
• Inclined deposit on the flanks of volcanic seamounts, where the
crust covers volcanic breccias and associated sediments.
The dimensions of flat deposits can be 50-200 km 2 with 70-90% of the
area covered by encrustations. Their topography is relatively even, with
slopes less than 5%. Cracks form an irregular pattern that cup up the crust
and the underlying material to several decimetres deep. The corresponding
slabs are one to several square meters wide.
Slope deposits are inclined up to 25%, as the flanks of old volcanoes.
Crusts cover more or less consolidated sediments as well as hard basaltic rock
and breccias. Evidence of sliding along the slope has been recorded.
The thickness of the crust can be 2 to 10 cm, sometimes up to 20 cm,
but the structure and composition varies from top to bottom. Generally, only
the few first millimetres have very high cobalt content (up to 3%). Cobalt
grade decreases with depth, as well as manganese and iron, because of mixing
with the underlying material. When this material is composed of calcareous
phosphorite, there is a corresponding increase of the phosphorus and calcium
contents. Therefore, only the first few centimetres (2 to 3) of a deposit have an
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