Mineralogy and chemistry of Ni-bearing phyllosilicates from the saprolite zone of the Wingellina Ni-Co laterite deposit (Western Australia)

Ni-phyllosilicates represent the main ore minerals in many ‘silicate-type’ Ni-laterites around the world. However, they seldom represent an economically exploitable resource in oxide-type deposits as a result of their low grades (Butt and Cluzel 2013). In this study, we report the mineralogy and chemistry of Ni-bearing phyllosilicates in the Wingellina deposit (Western Australia; owned by Metal X Ltd.), an oxide-type Ni-Co laterite with 168 Mt of limonitic ore reserves grading 0.98% Ni and 0.08% Co derived from the weathering of the olivine-rich mafic to ultramafic layered-intrusion of the Giles Complex (Mesoproterozoic). In this deposit, the lateritic profile typically consists of a well-developed limonitic unit and a less voluminous saprolite horizon. Even though the economically exploitable Ni-Co resources are located in the Mn- and Fe-hydroxides rich zones of the limonite (Putzolu et al. 2018), the highest Ni-grades are commonly observed within the saprolite horizon (Putzolu et al. in press). The present study was conducted on saprolite samples originating from two cores drilled through the Wingellina laterite profile developed over two diverse parent rocks: a gabbro and a peridotite. The mineralogy and chemistry of the Ni-bearing phyllosilicates were investigated by the X-Ray Powder Diffraction (XRD) analysis of clay aggregates and by Electron Microprobe Analysis (EMPA).

The studied samples have various mineralogical characteristics. The mineralogy of the saprolite zone lying on the gabbro bedrock is dominated by smectite clays, with less abundant goethite and hematite. The XRD analysis on clay aggregates of the gabbro-derived saprolite samples revealed that the smectites are generally dioctahedral, likely corresponding to montmorillonite and nontronite. The EMPA indicates that these clays contain the highest Ni concentrations of any samples from the study area (up to 12 wt-% NiO). The mineralogy of the peridotite-derived saprolite samples is more complex because they contain both 15 Å - and 7 Å-spaced phyllosilicates (i.e. smectites and serpentine). The serpentine within the samples is most likely lizardite polytypes that are only somewhat Ni enriched (up to 3 wt-% NiO). These minerals occur as aggregates with mesh-like structures, mimicking the original texture of the replaced magmatic minerals, and are partially overprinted by skeletal Fe-oxy-hydroxides and dioctahedral smectites (most likely saponite and nontronite). This serpentine could have formed as a result of the hydrothermal alteration of the ultramafic parent bedrock and could have been lately replaced by smectite during the lateritization. All these features suggest that the peridotite-derived saprolite section is the result of both hydrothermal and supergene processes.