77
4 Characteristics and Formation Process
accumulations. To date, no field study using a manned submersible or remote-operated underwater vehicle has been carried out on this type of deposit in the ocean.
Hydrothermal Polymetallic Sulphides
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. Quantitative estimations show that the
quantity of seawater that flows through ocean rocks is around 1.3 to 9 × 10
11
tonnes
a year, evacuating 36 to 44 × 10
18
calories a year of heat energy. These figures represent the equivalent of the flow of the total mass of ocean waters through hydrothermal systems every 5 to 11 million years. This is therefore a major phenomenon on
a global scale. According to these calculations, since the formation of the oceans,
around 4 billion years ago, all the water of the oceans has flowed through the oceanic crust several hundred times.
This hydrothermal activity is an important metal transport and concentration
mechanism, causing metals to accumulate in the form of sulphide deposits. This
type of ore is well known in fossil deposits mined on land and formed below the
sea over geological periods. A portion of the copper, zinc, silver and gold mined on
land is produced from this type of deposit, some of which also contain cobalt and
barium. For instance, over 30 % of silver resources mined on land come from fossil
sulphide deposits formed below the sea during geological periods.
The first deposits observed only represented a few tens of thousands of tonnes
of polymetallic sulphides. Today, several hydrothermal fields are known whose dimensions and mineral contents are similar to land-based mines, i.e. several million
to tens of millions of tonnes.
Location The first hydrothermal mineral deposits associated with hot brine (70 °C)
were observed in 1962 in the Red Sea Rift. 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 nearly150 hydrothermal sites (Fig. 1.4) demonstrates the importance of metal extraction, transport and
concentration processes related to submarine volcanic activity. Sulphide mineral
deposits are now known to exist at depths of between 800 and 5,000 m. Through
exploration, hydrothermal activity has been discovered and its diversity specified
in varying geodynamic contexts. Volcanic areas in which hydrothermal activity is
found can be divided into four types of environments: fast-spreading ridges, slowspreading ridges, back-arc basins and intraplate volcanoes. So-called slow-spreading ridges, such as the Mid-Atlantic Ridge, spread at rates of no more than two
centimetres a year, while fast-spreading ridges spread at rates of up to 18 cm a year
in the south of the East Pacific Rise, north of Easter Island.
Fast-Spreading Ridges: Many Small, Unstable Hydrothermal Fields The first black
smokers were discovered in 1978, on the East Pacific Rise, at the latitude of Mexico
4 Characteristics and Formation Process
accumulations. To date, no field study using a manned submersible or remote-operated underwater vehicle has been carried out on this type of deposit in the ocean.
Hydrothermal Polymetallic Sulphides
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. Quantitative estimations show that the
quantity of seawater that flows through ocean rocks is around 1.3 to 9 × 10
11
tonnes
a year, evacuating 36 to 44 × 10
18
calories a year of heat energy. These figures represent the equivalent of the flow of the total mass of ocean waters through hydrothermal systems every 5 to 11 million years. This is therefore a major phenomenon on
a global scale. According to these calculations, since the formation of the oceans,
around 4 billion years ago, all the water of the oceans has flowed through the oceanic crust several hundred times.
This hydrothermal activity is an important metal transport and concentration
mechanism, causing metals to accumulate in the form of sulphide deposits. This
type of ore is well known in fossil deposits mined on land and formed below the
sea over geological periods. A portion of the copper, zinc, silver and gold mined on
land is produced from this type of deposit, some of which also contain cobalt and
barium. For instance, over 30 % of silver resources mined on land come from fossil
sulphide deposits formed below the sea during geological periods.
The first deposits observed only represented a few tens of thousands of tonnes
of polymetallic sulphides. Today, several hydrothermal fields are known whose dimensions and mineral contents are similar to land-based mines, i.e. several million
to tens of millions of tonnes.
Location The first hydrothermal mineral deposits associated with hot brine (70 °C)
were observed in 1962 in the Red Sea Rift. 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 nearly150 hydrothermal sites (Fig. 1.4) demonstrates the importance of metal extraction, transport and
concentration processes related to submarine volcanic activity. Sulphide mineral
deposits are now known to exist at depths of between 800 and 5,000 m. Through
exploration, hydrothermal activity has been discovered and its diversity specified
in varying geodynamic contexts. Volcanic areas in which hydrothermal activity is
found can be divided into four types of environments: fast-spreading ridges, slowspreading ridges, back-arc basins and intraplate volcanoes. So-called slow-spreading ridges, such as the Mid-Atlantic Ridge, spread at rates of no more than two
centimetres a year, while fast-spreading ridges spread at rates of up to 18 cm a year
in the south of the East Pacific Rise, north of Easter Island.
Fast-Spreading Ridges: Many Small, Unstable Hydrothermal Fields The first black
smokers were discovered in 1978, on the East Pacific Rise, at the latitude of Mexico