The nickel ores mined in New Caledonia for one century are
mainly silicate ores. These ores are nickel enriched saprolites,
formed by partial weathering of peridotites (predominantly
harzburgites and some dunites), serpentinised to varying degrees.
Saprolites constitute the lower horizon of lateritic soils,
sometimes up to 100 metres thick, developed on peneplains now
greatly modified by erosion. Saprolites are generally overlain by
laterites, a more weathered horizon, where the main components
are iron oxy-hydroxides with a low nickel grade. The structure of saprolites depends essentially on the
serpentine content of peridotites and, on the other hand,
serpentine minerals may have a high nickel content and, so,
represent important nickel bearing minerals in silicate ores. Two types of serpentines can be found in nickel ores from New
Caledonia: primary serpentines, formed by hydration of olivine at
temperatures below 400C, during overthrusting of
peridotites in late-Eocene (Paris, Andreieff and Coudray,
1979), are generally very abundant; secondary nickeliferous serpentines, associated with other
sheet silicates in garnierites, are now relatively scarce but
garnierites were selectively mined one century ago, because
of their high nickel grade.