In tropical and subtropical climates intense weathering of ultramafic rocks may cause significant cobalt and nickel enrichment in surficial residual deposits known as laterites. Cobalt dispersed in silicates and sulphides within the host rock is remobilised and deposited in weathered layers as hydroxides and oxides near the surface and as silicate at deeper levels. These deposits are generally about 20 metres thick and mid-Tertiary to recent in age. They are principally worked for nickel with cobalt as a by-product. The cobalt is contained within limonite and goethite as well as erythrite and asbolite. At deeper levels, weathering of ultramafic rocks is less intense and the nickeliferous mineral garnierite is formed.
Serpentine-rich zones in saprolite at the base of laterites restrict the circulation of groundwater and thus the amount of cobalt enrichment. It also interferes with the processing of the ore as individual grains need to be crushed in order to liberate ore minerals from gangue intergrowths. Grades of cobalt in laterite deposits vary widely in the range 0.1 to 1.5% Co.
Topography plays an important role in the formation of laterite deposits. The most extensive deposits are found on gently dipping slopes where groundwater can freely circulate to encourage weathering. Therefore deposits are often associated with areas of gentle tectonic deformation causing slow uplift. Important examples are found in New Caledonia and Cuba due to large areas of serpentinised peridotites and ideal weathering conditions.
Reprinted with kind permission from the British Geological Survey. BGS © NERC. ALL Rights Reserved. 2016