Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 136

equivalents; (2) an appropriate balance of temperature and rainfall (high temperatures
favour Fe-rich laterites); and (3) a pronounced alternation of wet and dry seasons.
5.4.2 Ni Laterites
If it is accepted that an island half way across the world is part of the French nation,
then the biggest mines in France are in New Caledonia. The nickel deposits of the
island were found in 1864 by Jules Garnier who gave his name to the main ore
mineral, a mixture of pale green or apple-green phyllosilicates with the approxi-
mate composition mineral, (Ni, Mg) 3 Si 2 O 5 (OH) 4 . The deposits have been mined
almost continuously since 1875 and have seen the development of many innovative
procedures for the extraction of Ni from lateritic ores. During the early years very
rich garnierite ores containing up to 15% Ni and averaging 2.5% Ni were exploited
but these are now largely exhausted and mining has turned the less rich goethitic
ores (1.3-1.6% Ni). The long and involved history of the development of a new
deposit in the Goro mine, from its conception in 1993 to planned production in
2013, is given at http://www.valeinco.nc/pages-eng/propos/history.htm . The New
Caledonian deposits are thought to contain about a third of the world's supply of Ni;
and when other major lateritic deposits in Indonesian, Cuba and Australia are
included, the figure reaches 50-60%.
Just as with bauxite, the extraction of Ni metal from the silicates and hydroxides
that constitute lateritic nickel ore requires abundant energy. The lateritic ores contain
abundant bound water, which must be driven off, and have high magnesium contents
which means that conventional smelters must run at very high temperatures. A wide
variety of alternative extraction processes have been developed New Caledonia and
Cuba and these are being applied with variable success in the extraction of ore in other
countries. Because of the high energy consumption, the viability of a nickel laterite
project depends crucially on the price of energy, which is dictated by the oil price;
when it is low, lateritic deposits gain a competitive edge over magmatic deposits.
Table 5.6 lists the advantages and disadvantages of the two types of Ni deposit.
Nickel laterites develop when ultramafic rocks are exposed to protracted
weathering in hot humid climates. In most of areas where deposits are known -
New Caledonia, Indonesia, Cuba, etc. - the ultramafic rocks come from the lower,
mantle portions of ophiolites. A notable exception is the deposits in Western
Australia which have formed on intrusions of komatiitic lineage, the same rocks
that host magmatic ore deposits.
The laterization process is directly comparable to that of bauxite formation and
the parallel is brought out in the comparison of the two profiles in Fig. 5.9 . Nickel
laterites develop when either the primary minerals of the ultramafic rocks, olivine
or pyroxene, or secondary serpentine that replaces these minerals, are subjected to
weathering under conditions similar t those that produce bauxite. Elements such as
Si, Mg, Ca, which make up close to 90% of the parent rock, are removed and the
less mobile Ni (and Fe) are retained. Through this process the Ni content increases