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commercialisation, information on the depth extent and therefore the size of the deposit
and the type of mineralisation and alteration are extremely important. Drilling by the
Ocean Drilling Program Leg 158 at the active TAG hydrothermal mound at the Mid-
Atlantic Ridge 4 has indicated a total tonnage of 2.7 million tonnes of sulphide above and
1.2 million tonnes below the seafloor 5. It was also found that high concentrations of base
and precious metals are confined to the upper few metres of the mound, which basically
consists of breccias with varying proportions of pyrite, silica, and anhydrite that would
not be economically recoverable. Initially it was though that the mound entirely consists
of polymetallic massive sulphides.
Except for the TAG mound, the Middle Valley site at the Juan de Fuca Ridge, and
the Atlantis II Deep in the Red Sea, depth information is not available for any of the
known seafloor sulphide deposits. Research and resource assessment of these deposits
rely on surface samples only. As drilling of hydrothermal systems by the Ocean Drilling
Program will be the exception rather than the rule, reliable portable drilling and coring
devices are required for research and industry. It has to be demonstrated that these
systems are actually capable of drilling and coring several tens of meters of massive
sulphide and rock at the seafloor. The presently available technology is an encouraging
start but needs to be further developed in order to make drilling at the seafloor to depths
of 50-100 m a routine operation by any research vessel and to reveal reliable information
on the depth extent of mound and chimney complexes.
After the resource potential has been adequately established by grid drilling
similar to land-based operations, exploitation and recovery will be the next challenges.
Selective mining using large TV-controlled grabs similar to those used for exploration are
an option, however, continuous mining appears to be the only economic alternative.
Continuous mining systems used by De Beers Marine offshore Namibia for the recovery
of diamonds from water depths of about 100-150 m appear to be convertible to massive
sulphide mining. These systems consist of large (7 m diameter) rotating cutter heads that
are attached to a flexible drill string through which the diamond-bearing sediment is
airlifted onto the ship where further processing takes place. The redesign of such a
system for massive sulphides and/or altered gold-bearing rocks seems possible. A weight
on head of several tens of tonnes controlled by the ships drill rig in combination with a
heave compensator would allow to crush sulphides and altered rock in situ and to
convert them to a slurry which can than be airlifted to the mining vessel from which the
ore slurry would be transferred to a cargo freighter for transport to a processing plant.
There is no question that the design and construction of suitable seafloor mining systems
for polymetallic massive sulphides and gold-rich rocks are major tasks for research
institutions and the off-shore industry which need to be met within the next few years.
96 International Seabed Authority