10
Y. Fouquet and D. Lacroix
Cross-Cutting Challenges by Metal Type
The main metals liable to be extracted from the deep-sea environment are of different types.
Type A comprises base metals that are subject to probable economic tension
(zinc, copper, manganese, cobalt, nickel, lead, barium, silver and precious metals
with high heritage value: gold).
Type B is composed of a few critical metals with high technological potential
and major supply risks (indium, germanium, cadmium, antimony, mercury—linked
to zinc—and selenium, molybdenum, bismuth—linked to copper—on sites rich in
hydrothermal sulphides, rare earths in crusts and nodules). Platinum and platinum
group metals (on crust sites, with uncertainties over the risks of substitution in uses,
as 200 years of possible consumption remain based on current supplies) also belong
to this category. Another resource is natural hydrogen found in hydrothermal fluids
of sulphide chimneys on hydrothermal sites associated with mantle rock.
Hydrothermal Sulphides: Relative Certainties and Uncertainties
Submarine hydrothermal activity is a consequence of plate tectonics and volcanic
activity. These processes generate the oceanic crust at divergent boundaries that
form the 60,000 km of mid-ocean ridges. The presence of heat and faults promotes
the circulation of fluids in the oceanic crust. This hydrothermal activity is an important metal concentration mechanism, causing metals to accumulate in the form of
sulphide deposits (Fig. 1.3). Hydrothermal sulphide deposits result from the circulation of seawater through the oceanic crust under the effect of high thermal gradients.
They are found on all submarine structures of volcanic origin.
According to their location, they show great diversity in their physical and geological characteristics and the types of metals that can be mined. These differences
are controlled by physical processes (temperature and depth for instance) and, to a
greater extent, by the type of rocks through which the hydrothermal fluids circulate
(various volcanic rocks, mantle rocks, sediments). This type of ore is well known in
fossil deposits mined on land and previously formed below the sea. A small proportion of the copper, zinc, silver and gold mined on land is produced from this type of
deposit, some of which also contain lead, cobalt and barium.
The first hydrothermal mineral deposits associated with hot brine (70 °C) were
observed in 1962 in the Red Sea. The first black smokers (350 °C) were discovered
on the East Pacific Rise in 1978, at a depth of nearly 3,000 m. After 30 years of exploration in all the world’s oceans, the discovery of almost 150 hydrothermal sites
(Fig. 1.4) shows the importance of extraction, transport and concentration processes
for metals associated with submarine volcanic activity. Sulphide mineral deposits
are now known to exist at depths of between 800 and 4,100 m. Hydrothermal fields
have been identified in the main geodynamic contexts (slow- and fast-spreading
ridges, back-arc basins, island arcs) and on various substrata (basalt, andesite, dacite, sediment, ultramafic mantle rock).
Y. Fouquet and D. Lacroix
Cross-Cutting Challenges by Metal Type
The main metals liable to be extracted from the deep-sea environment are of different types.
Type A comprises base metals that are subject to probable economic tension
(zinc, copper, manganese, cobalt, nickel, lead, barium, silver and precious metals
with high heritage value: gold).
Type B is composed of a few critical metals with high technological potential
and major supply risks (indium, germanium, cadmium, antimony, mercury—linked
to zinc—and selenium, molybdenum, bismuth—linked to copper—on sites rich in
hydrothermal sulphides, rare earths in crusts and nodules). Platinum and platinum
group metals (on crust sites, with uncertainties over the risks of substitution in uses,
as 200 years of possible consumption remain based on current supplies) also belong
to this category. Another resource is natural hydrogen found in hydrothermal fluids
of sulphide chimneys on hydrothermal sites associated with mantle rock.
Hydrothermal Sulphides: Relative Certainties and Uncertainties
Submarine hydrothermal activity is a consequence of plate tectonics and volcanic
activity. These processes generate the oceanic crust at divergent boundaries that
form the 60,000 km of mid-ocean ridges. The presence of heat and faults promotes
the circulation of fluids in the oceanic crust. This hydrothermal activity is an important metal concentration mechanism, causing metals to accumulate in the form of
sulphide deposits (Fig. 1.3). Hydrothermal sulphide deposits result from the circulation of seawater through the oceanic crust under the effect of high thermal gradients.
They are found on all submarine structures of volcanic origin.
According to their location, they show great diversity in their physical and geological characteristics and the types of metals that can be mined. These differences
are controlled by physical processes (temperature and depth for instance) and, to a
greater extent, by the type of rocks through which the hydrothermal fluids circulate
(various volcanic rocks, mantle rocks, sediments). This type of ore is well known in
fossil deposits mined on land and previously formed below the sea. A small proportion of the copper, zinc, silver and gold mined on land is produced from this type of
deposit, some of which also contain lead, cobalt and barium.
The first hydrothermal mineral deposits associated with hot brine (70 °C) were
observed in 1962 in the Red Sea. The first black smokers (350 °C) were discovered
on the East Pacific Rise in 1978, at a depth of nearly 3,000 m. After 30 years of exploration in all the world’s oceans, the discovery of almost 150 hydrothermal sites
(Fig. 1.4) shows the importance of extraction, transport and concentration processes
for metals associated with submarine volcanic activity. Sulphide mineral deposits
are now known to exist at depths of between 800 and 4,100 m. Hydrothermal fields
have been identified in the main geodynamic contexts (slow- and fast-spreading
ridges, back-arc basins, island arcs) and on various substrata (basalt, andesite, dacite, sediment, ultramafic mantle rock).