Minerals Report - International Seabed Authority

phosphorite deposits on the Chatham Rise off New Zealand, (~5 wt. %
phosphate) and offshore from Cochin and Bombay, India. Other major
localities include: south and south-western Africa, north-western Africa,
the western and eastern margins of South America and western Australia
(Cruikshank, 1992; Manheim, 1979).
None of these locations overlap with the extended continental shelf
regions, except for a small portion on the south-eastern margin of
Argentina.
4.2.3 Resource potential of marine phosphorite deposits
World sub-sea resources of phosphorite are probably at least of the
order of hundreds of billions of tonnes (Kent, 1980; Pearson, 1975;
Manheim, 1979). As a consequence of the prevailing economic conditions
and the alternative availability of phosphates from non-marine sources no
offshore deposits are being mined at present. Although land deposits are
large enough to meet world demands, sub-sea production may become
economically viable in local areas far removed from on-shore deposits.
Although possibly a few billion tons of offshore deposits may be classed as
para-marginal now (in those areas with already identified major deposits),
the bulk of sub-sea phosphorite resources in ELCS regions must be classed
as sub-marginal and hence of little economic value.
4.3 Marine evaporite deposits
4.3.1 What marine evaporite deposits comprise and how they are
formed
Anhydrite and gypsum (calcium sulphates), common salt (sodium
chloride), and potash-bearing minerals are termed evaporite deposits
(Holser, 1979; Peryt, 1987; Holser et al., 1988). They are formed by
evaporation of sea water and other natural brines in geologic basins of
restricted circulation. Important deposits of magnesium bearing salts are
also deposited in such basins. Elemental sulphur forms in some of them by
biogenic processes involving the alteration of anhydrite. Because rock salt
tends to flow at relatively low temperature and pressure, salt in thick beds
squeezed by the weight of a few thousand metres of overlying sediment
often protrudes upwards forming salt domes, plugs, and other structures.
Such masses, which can be a few kilometres in diameter, may bring salt to
or near the surface. They can also form structures in the intruded
INTERNATIONAL SEABED AUTHORITY 689