NICKEL LATERITES THROUGH GEOLOGICAL TIME

Nickel laterite deposits are formed by the prolonged and pervasive weathering of ultramafic rocks, generally, in tropical to sub-tropical climates. The nature of the resulting deposit is a function of many global and local factors including the nature of the parent rock, the tectonic setting, topography, groundwater chemistry and drainage. However, climate is a first order control on the formation of a lateritic weathering profile.

Nickel laterites can be classified on the basis of ore mineralogy as hydrous silicate deposits (e.g. SLN operations, New Caledonia), clay silicate deposits (e.g. Murrin Murrin, Australia), and oxide deposits (e.g., Moa Bay, Cuba). The hydrous silicate deposits form in humid savanna-tropical rainforest areas that have high annual temperatures and total rainfall. Many of the major deposits producing presently have formed in areas where the rainfall is often seasonal in nature. The clay silicate deposits can form in humid savanna climates with poor drainage but are often associated with drier climates; some possibly form in semi-arid environments. Oxide deposits likewise form in savanna climates but may be modified in semi-arid environments (Butt, 1979). There are, however, some deposits found in higher latitudes with more temperature climates today e.g. the Urals, Eastern Europe, Oregon. Many of these are older and are thought to have formed in tropical or savanna environments when they were near the equator and have since been preserved.

There is significant evidence for several major periods of laterite and bauxite development in the Tertiary and most of the currently producing Ni-laterite mines formed in that time period. Evidence of Ni-laterite deposits forming before the Tertiary is rare, though evidence of bauxite formation is more common through geological time and is often used as a palaeoclimate proxy. Periods of climatic conditions conducive to laterite formation are considered to have occurred in the Carboniferous, Late Permian, Eocene-Oligocene, Miocene and Pliocene (Bardossy & Aleva, 1990). The linkage of periods of favourable climatic conditions to the formation of nickel laterites in the Paleozoic and Mesozoic is poorly understood, but is a potentially fruitful exercise for predicting as yet undiscovered deposits in ancient profiles.