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 of the world. However, they seldom represent an economically exploitable resource in oxide-type deposits due to their negligible ore-grade [1]. In this study we report the mineralogy and chemistry of Ni-bearing phyllosilicates in the Wingellina deposit (Western Australia), an oxide-type Ni-Co laterite with 168 Mt of limonitic ore reserves grading 0.98% Ni and 0.08% Co (owned by Metal X Ltd.), 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 comprehends 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 [2], the highest Nigrades are commonly observed within the saprolite horizon [3]. The present study was conducted on saprolite samples originating from two cores, drilled through the Wingellina laterite profile in correspondence of two diverse parent rocks: a gabbro and a peridotite. The mineralogy and chemistry of the Ni-bearing phyllosilicates was investigated by X-Ray Powder Diffraction on clay aggregates and by Electron Microprobe Analyses (EMPA). The studied samples reveal 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 analyses on clay aggregates of the gabbro-derived saprolite samples revealed that the smectites mainly are of the dioctahedral type, likely corresponding to montmorillonite and nontronite. The EMPA analyses evidenced that these clays have the highest Ni concentrations (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 serpentines occurring in the samples likely belong to the lizardite polytype, and only show limited Ni enrichments (up to 3 wt% NiO). These minerals occur as aggregates with mesh-like structure, mimicking the original texture of the replaced magmatic minerals, and are partially overprinted by skeletal Fe-oxy-hydroxides and dioctahedral smectites (most likely vermiculite and nontronite). These serpentines could have formed in relationship with the hydrothermal alteration of the ultramafic parent bedrock, and could have been lately replaced by smectites during the lateritization. All these features would suggest that the peridotite-derived saprolite section is the result of both hydrothermal and supergene processes.