Figure 9.11

Smectite silicate nickel laterite. Smectite silicate nickel laterites are generally developed by weathering of now serpentinised clinopyroxene-bearing perodotitic ultramafics rocks, mainly in stable shield areas. The best known deposit, Murrin Murrin in Western Australia, occurs in a semi-arid environment, but was formed under humid conditions, in poorly drained terrain of low relief. Due to the poor drainage, the regolith is less strongly leached than more free-draining situations in the same region. Consequently, the rocks weather to form a thick clay-rich saprolite that retains elements such as magnesium, calcium and sodium hosted in smectite clays. The low permeability of the smectites further impedes drainage. Magnesium released during more arid phases is retained in the profile as magnesite. In the upper part of the saprolite, nontronite, a sodium-iron smectite, is dominant; this hosts nickel and forms the ore. Close to the surface, preferential weathering along joints and fractures has led to enhanced leaching and collapse, with solution structures infilled with oxidised materials derived from the surface. In places, these structures penetrate to depths of 20–30 m (source: C. Butt).