448
Hjalmar THIEL
In the hydrothermal process the water is recirculated
to and expelled through vents into the ocean water
mass, where, close to the seafloor, the metals precipitate
with other components as mud. Their content of metals
depends on the leached rocks and the temperature. If
precipitation occurs in a trough, as in the central graben
of the Red Sea (Degens and Ross, 1969; Amann et al.,
1973; Bignell, 1978; B¨ acker, 1980; Rona, 1985), the
ore is a watery mud, somewhat drier and stiffer in the
deeper layers. When hydrothermal activity ceases, the
mud may be covered with other sediment layers; if then
it comes under pressure through tectonic shifts, rockforming processes convert the metalliferous mud into
consolidated massive sulphides. Direct precipitation of
hard material also occurs at vents, in the formation of
black and white smoker chimneys (Rona, 1982, 1985;
Scott, 1992).
Thus, polymetallic sulphides are related to the
processes at hydrothermal vents. Polymetallic sulphide
deposits have been discovered in active formation at
various localities around spreading centres (Hekinian
et al., 1983; Halbach et al., 1989b), but very little is
known about their resource possibilities that is, their
ore mass and their metal content. Scott (1992) listed
a few sites on the mid-Atlantic Ridge estimated to
amount to 5 million tonnes of sulphides. The Ocean
Drilling Programme discovered a sulphide source 94 m
thick (Duckworth et al., 1994), but its horizontal extent
was not investigated. The only ore of this type which
has known resource potential is the metalliferous mud
in the Atlantis II Deep in the central Red Sea. This
deep is one of about a dozen located in the narrow
central graben system. Below a hot brine pool [some
20 years ago, the water layer was about 200 m thick,
with a temperature of 64ºC, and salinity about 320
(Karbe, 1987)], the “sediment” starts at a depth of
about 2200 m. However, because the brine prevents the
particles sinking and consolidating, there is no clearly
demarcated seafloor (a multiple corer never collected
any sort of boundary layer, but always penetrated totally
into the fluid mud). The Atlantis II Deep covers an area
of 60 km
2 and the ore has an average depth of 11 m
(Karbe, 1987). The deeper sediments have strongly
coloured layers reflecting varying ore composition.
Mining at a rate of 100 000 metric tonnes of mud per
day would yield 2×10
6 tonnes of Zn, 0.4×10
6 t Cu,
5.7×10
3 t Co, 3.6×10
3 t Ag and 47 t Au (Guney et al.,
1984). The Atlantis II Deep resource could support
at least a 16-years’ mining operation (Mustafa et al.,
1984).
Phosphorites
For completeness one must list phosphoritic deposits, although many land resources exist; most
marine phosphorites occur on continental shelves, but
other deposits have been found on the upper reaches of
the deep sea, on continental slopes and seamounts [see
Burnett et al. (1987) for a review].
Phosphorites occur as nodules, grains, conglomerates and crusts. They penetrate several decimetres into
the sediment, so that the deposit must be classified
as three-dimensional. They develop by authigenic precipitation or by the replacement of carbonates, mainly
carbonate-fluoroapatite or francolite. The phosphoritic
material is typically associated with terrigenous, calcareous or siliceous sediments.
Most phosphoric deposits are relicts of former ages;
active deposition is occurring only off Peru and Chile,
off Namibia, off Baja California, and off eastern
Australia. Many of the deposits seem to be related
to intensive coastal upwelling with high production
levels, as is the case for the first three modern deposits;
however, the region off eastern Australia does not fall
into this category [see Thiel et al. (1998) for more
details and references].
Mining techniques
Deep-sea mining has been considered only for
polymetallic nodules and metalliferous muds, and
therefore techniques have been developed and tested
only for these two resources. However, the basic
designs with some modifications should be applicable
also for the other resource types.
The mining systems include the functions of ore
collection and transport to the ocean surface. The
simplest procedure would be the adaptation of bottom
trawling as used in fisheries by the application of rather
sturdy trawls to lift 120 tonnes of nodules per haul. This
is the latest proposal for application in the Cook Island
region (Bechtel Corporation, 1996); however, it is
difficult to imagine how such a mass of nodules should
enter and fill the trawl. The continuous-line bucket
system proposed by Masuda et al. (1971) applies the
principle of a bucket dredger. The single buckets are
attached to an endless wire, leaving the ship at the bow
and returning to the stern, scraping up nodules where
the wire loop touches the seafloor. This system has been
tested with some success, but appeared insufficiently
controllable under sea conditions. The down-going and
the up-coming legs of the wire entangled and the
exploitation was inefficient. However, the system is
Hjalmar THIEL
In the hydrothermal process the water is recirculated
to and expelled through vents into the ocean water
mass, where, close to the seafloor, the metals precipitate
with other components as mud. Their content of metals
depends on the leached rocks and the temperature. If
precipitation occurs in a trough, as in the central graben
of the Red Sea (Degens and Ross, 1969; Amann et al.,
1973; Bignell, 1978; B¨ acker, 1980; Rona, 1985), the
ore is a watery mud, somewhat drier and stiffer in the
deeper layers. When hydrothermal activity ceases, the
mud may be covered with other sediment layers; if then
it comes under pressure through tectonic shifts, rockforming processes convert the metalliferous mud into
consolidated massive sulphides. Direct precipitation of
hard material also occurs at vents, in the formation of
black and white smoker chimneys (Rona, 1982, 1985;
Scott, 1992).
Thus, polymetallic sulphides are related to the
processes at hydrothermal vents. Polymetallic sulphide
deposits have been discovered in active formation at
various localities around spreading centres (Hekinian
et al., 1983; Halbach et al., 1989b), but very little is
known about their resource possibilities that is, their
ore mass and their metal content. Scott (1992) listed
a few sites on the mid-Atlantic Ridge estimated to
amount to 5 million tonnes of sulphides. The Ocean
Drilling Programme discovered a sulphide source 94 m
thick (Duckworth et al., 1994), but its horizontal extent
was not investigated. The only ore of this type which
has known resource potential is the metalliferous mud
in the Atlantis II Deep in the central Red Sea. This
deep is one of about a dozen located in the narrow
central graben system. Below a hot brine pool [some
20 years ago, the water layer was about 200 m thick,
with a temperature of 64ºC, and salinity about 320
(Karbe, 1987)], the “sediment” starts at a depth of
about 2200 m. However, because the brine prevents the
particles sinking and consolidating, there is no clearly
demarcated seafloor (a multiple corer never collected
any sort of boundary layer, but always penetrated totally
into the fluid mud). The Atlantis II Deep covers an area
of 60 km
2 and the ore has an average depth of 11 m
(Karbe, 1987). The deeper sediments have strongly
coloured layers reflecting varying ore composition.
Mining at a rate of 100 000 metric tonnes of mud per
day would yield 2×10
6 tonnes of Zn, 0.4×10
6 t Cu,
5.7×10
3 t Co, 3.6×10
3 t Ag and 47 t Au (Guney et al.,
1984). The Atlantis II Deep resource could support
at least a 16-years’ mining operation (Mustafa et al.,
1984).
Phosphorites
For completeness one must list phosphoritic deposits, although many land resources exist; most
marine phosphorites occur on continental shelves, but
other deposits have been found on the upper reaches of
the deep sea, on continental slopes and seamounts [see
Burnett et al. (1987) for a review].
Phosphorites occur as nodules, grains, conglomerates and crusts. They penetrate several decimetres into
the sediment, so that the deposit must be classified
as three-dimensional. They develop by authigenic precipitation or by the replacement of carbonates, mainly
carbonate-fluoroapatite or francolite. The phosphoritic
material is typically associated with terrigenous, calcareous or siliceous sediments.
Most phosphoric deposits are relicts of former ages;
active deposition is occurring only off Peru and Chile,
off Namibia, off Baja California, and off eastern
Australia. Many of the deposits seem to be related
to intensive coastal upwelling with high production
levels, as is the case for the first three modern deposits;
however, the region off eastern Australia does not fall
into this category [see Thiel et al. (1998) for more
details and references].
Mining techniques
Deep-sea mining has been considered only for
polymetallic nodules and metalliferous muds, and
therefore techniques have been developed and tested
only for these two resources. However, the basic
designs with some modifications should be applicable
also for the other resource types.
The mining systems include the functions of ore
collection and transport to the ocean surface. The
simplest procedure would be the adaptation of bottom
trawling as used in fisheries by the application of rather
sturdy trawls to lift 120 tonnes of nodules per haul. This
is the latest proposal for application in the Cook Island
region (Bechtel Corporation, 1996); however, it is
difficult to imagine how such a mass of nodules should
enter and fill the trawl. The continuous-line bucket
system proposed by Masuda et al. (1971) applies the
principle of a bucket dredger. The single buckets are
attached to an endless wire, leaving the ship at the bow
and returning to the stern, scraping up nodules where
the wire loop touches the seafloor. This system has been
tested with some success, but appeared insufficiently
controllable under sea conditions. The down-going and
the up-coming legs of the wire entangled and the
exploitation was inefficient. However, the system is