Clay mineralogy of the nickel laterite ore developed from serpentinised peridotites at Murrin Murrin, Western Australia

Abstract

Samples from the saprolite and extensive smectite zone (up to 25 m thick) of the Murrin Murrin lateritic nickel deposit, developed from serpentinised peridotites, were studied using optical microscopy, XRD, SEM and TEM-EDX. In the saprolite zone, the initial mesh-texture of the parent rock is still well preserved. The mineralogy is dominated by serpentine, smectite and maghemite. Two kinds of smectites occur in this zone: saponites and smectites with chemistries intermediate between two dioctahedral end-members (Fe nontronite, Fe montmorillonite) and a trioctahedral one (Mg + Ni saponite). In the smectite zone, serpentine and saponite completely disappear, however some relicts of the initial mesh-texture exist. Smectite and maghemite are the dominant minerals and goethite appears at the top of this zone. Smectites from the smectite zone have a complex chemistry which appears intermediate between four dioctahedral end-members (Fe Nontronite, Fe montmorillonite, Al beidellite, Al montmorillonite) and a trioctahedral one (Mg + Ni saponite). Smectites in fissures exhibit a greater variability in Al vs Fe + Cr content than smectites from plasma; this suggests a supply of Al by the percolating fluids. Nickel, essentially concentrated in the smectite zone (1.3 wt%), is located in the octahedral sheets of the plasma and fissure-smectites with Ni content up to 0.30 per half unit-cell. The oxygen and deuterium isotopic compositions of the smectites show that they crystallised at low temperature in contact with meteoric fluids. The δ18O values of Murrin Murrin smectites are negatively correlated with their Fe + Cr content.