74
Y. Fouquet
on indurated substrata (submarine volcanoes, former underwater atolls) at depths
ranging from 400 to 4,000 m. Estimations indicate that the total surface area covered by crusts is around 6.35 million square kilometres, i.e. 1.7 % of the ocean
surface area.
A very small number of underwater volcanoes (of an estimated 50,000) have
been studied in the Pacific Ocean. Cobalt- and platinum-rich deposits have the
greatest economic potential and are all located in the Pacific Ocean. On the scale of
this ocean, the deposits with the highest cobalt contents are in Polynesia (Fig. 4.6).
They appear in the form of, sometimes continuous, crusts, on the outer edges of
submarine plateaus (e.g. Tuamotu) and on volcanoes at depths of between 800 and
2,500 m.
Formation Mechanism Manganese concretions are formed from the precipitation
of Fe++ and Mn++ from seawater in the form of iron oxide-hydroxide (Fe 2 O 3 ) and
manganese oxide-hydroxide (MnO 2 ) (Fig. 4.5). Growth rates are extremely slow,
around 1 to 6 mm per million years. This is why the thickest crusts can be up to
60 million years old. This very slow growth rate means that this phenomenon is
effective and leads to high concentrations of several metals, such as cobalt and
platinum. The phenomenon is reinforced when the oxygen content in seawater is
minimal. Thus, fluctuations in platinum content could reflect the passage through
the oxygen minimum zone in the seawater column. High platinum concentrations
have been interpreted, on other sites, in terms of the oxidation of divalent platinum
present in seawater in the form of a chlorine complex, to form tetravalent platinum
trapped in iron-manganese oxides. Precipitation processes are probably also influenced and reinforced by bacterial activity. These hypotheses need to be discussed
based on more accurate field data.
Microspheroids of cosmic origin are frequent in some areas. These metal
micrometeorites contain nickel, cobalt and platinum group metals in substantial
quantities. Rough calculations nevertheless show that these micrometeorites do not
answer for the high cobalt and platinum concentrations of these crusts.
Composition Like nodules, crusts are mainly composed of iron and manganese
oxides. They are however on average three times higher in cobalt and often have
high platinum concentrations. Crusts are among the richest sources of cobalt known
on earth. These crusts could constitute the first cobalt ore, as this metal is to date
a by-product of other mining processes. Crusts in Polynesia’s EEZ have far higher
cobalt (1.8 %) and platinum (3.5 grams per tonne) concentrations than those of other
ocean areas (Co = 0.25 %). Cobalt concentrations are far higher than in lateritic ores
mined on land where the content rarely exceeds over 0.1 to 0.2 % (Fig. 4.6). On
certain sites that are very rich in platinum, this could be a non-negligible by-product. Cobalt is mainly used to make special steels. The importance of this metal is
renewed due to high demand for cobalt (production doubled between 1999 and
2006) for new technologies, in particular alloys for aviation and batteries. Around
a third of production is used in the aerospace industry. The importance of platinum
is also renewed by industry’s demand for catalysts (exhaust pipes and fuel cells).
Y. Fouquet
on indurated substrata (submarine volcanoes, former underwater atolls) at depths
ranging from 400 to 4,000 m. Estimations indicate that the total surface area covered by crusts is around 6.35 million square kilometres, i.e. 1.7 % of the ocean
surface area.
A very small number of underwater volcanoes (of an estimated 50,000) have
been studied in the Pacific Ocean. Cobalt- and platinum-rich deposits have the
greatest economic potential and are all located in the Pacific Ocean. On the scale of
this ocean, the deposits with the highest cobalt contents are in Polynesia (Fig. 4.6).
They appear in the form of, sometimes continuous, crusts, on the outer edges of
submarine plateaus (e.g. Tuamotu) and on volcanoes at depths of between 800 and
2,500 m.
Formation Mechanism Manganese concretions are formed from the precipitation
of Fe++ and Mn++ from seawater in the form of iron oxide-hydroxide (Fe 2 O 3 ) and
manganese oxide-hydroxide (MnO 2 ) (Fig. 4.5). Growth rates are extremely slow,
around 1 to 6 mm per million years. This is why the thickest crusts can be up to
60 million years old. This very slow growth rate means that this phenomenon is
effective and leads to high concentrations of several metals, such as cobalt and
platinum. The phenomenon is reinforced when the oxygen content in seawater is
minimal. Thus, fluctuations in platinum content could reflect the passage through
the oxygen minimum zone in the seawater column. High platinum concentrations
have been interpreted, on other sites, in terms of the oxidation of divalent platinum
present in seawater in the form of a chlorine complex, to form tetravalent platinum
trapped in iron-manganese oxides. Precipitation processes are probably also influenced and reinforced by bacterial activity. These hypotheses need to be discussed
based on more accurate field data.
Microspheroids of cosmic origin are frequent in some areas. These metal
micrometeorites contain nickel, cobalt and platinum group metals in substantial
quantities. Rough calculations nevertheless show that these micrometeorites do not
answer for the high cobalt and platinum concentrations of these crusts.
Composition Like nodules, crusts are mainly composed of iron and manganese
oxides. They are however on average three times higher in cobalt and often have
high platinum concentrations. Crusts are among the richest sources of cobalt known
on earth. These crusts could constitute the first cobalt ore, as this metal is to date
a by-product of other mining processes. Crusts in Polynesia’s EEZ have far higher
cobalt (1.8 %) and platinum (3.5 grams per tonne) concentrations than those of other
ocean areas (Co = 0.25 %). Cobalt concentrations are far higher than in lateritic ores
mined on land where the content rarely exceeds over 0.1 to 0.2 % (Fig. 4.6). On
certain sites that are very rich in platinum, this could be a non-negligible by-product. Cobalt is mainly used to make special steels. The importance of this metal is
renewed due to high demand for cobalt (production doubled between 1999 and
2006) for new technologies, in particular alloys for aviation and batteries. Around
a third of production is used in the aerospace industry. The importance of platinum
is also renewed by industry’s demand for catalysts (exhaust pipes and fuel cells).