Overview of Mineral Resources in the EEZ

Abstract

There are at least six classes of EEZ mineral deposits, aggregates of sand and gravel, metallic placer minerals, phosphorites, black smoker related polymetallic sulphides and associated metalliferous sediments, manganese nodules and cobalt-rich manganese crusts. Of these, aggregates and placers are shallow water deposits and have been mined for several decades and whose extraction poses few technological difficulties other than those associated with mining in progressively deeper or more hostile environments. Phosphorites are mid-depth deposits, generally not more than a few hundred metres deep, which have yet to be mined offshore. Prospective deposits such as those on Chatham Rise off New Zealand are coarse gravels 0.8–5.0 cm in size. Proposed mining techniques involve a hydraulic lift system incorporating various separators, as yet still at the drawing board stage. Black smoker deposits are composed principally of Cu, Zn and Fe bearing sulphide minerals which precipitate from hydrothermal solutions discharging at high temperature onto the ocean floor. Most occurrences are at mid-ocean ridge crests at around 3500 m depth, but they also occur in island arcs, sometimes at shallower depth. The nature of the deposits depends on many factors including their temperature of formation, substrate rock composition and rock-water reactions taking place below the sea floor. Resource feasibility studies on these deposits have only been carried out in the Red Sea where in the 60 km2 Atlantis II Deep there are 30 million tonnes (m.t.) of Fe, 2 m.t. of Zn and 0.5 m.t. of Cu, together with significant amounts of silver. A pre-pilot test mining operation has been carried out there. Manganese nodules are mainly non-EEZ deposits being concentrated in the proposed future International Seabed Area (ISA) of UNCLOS, more than 200 miles from land. However, important nodule deposits occur within the EEZ’s of some Pacific island nations such as the Cook Islands and on the Blake Plateau in the United States EEZ, which might be mined preferentially to the ISA ones. The largest deposits are rich in Co rather than in Ni and Cu, the two most economically important metals in the ISA. Cobalt-rich manganese crusts have been known for almost as long as the nodules, but have only recently been thought of as an economic deposit. They are attached to hard rock substrates mainly on seamounts between 800–2400 m depth and pose considerable mining problems in that, unlike nodules, they would have to be broken off before recovery. Average Co in the deposits is around 1%, but recovered ore will contain less Co than this due to dilution by substrate material and may be no richer in Co than the 0.5% Co typical of the Co-rich nodules mentioned above. Recent resource assessments for Co in both nodules and crusts in the EEZ’s of the Cook Islands and parts of Kiribati have shown much greater quantities of Co in the nodules than in the crusts, suggesting that the relative resource potentials of these deposits in EEZ’s needs to be reassessed.