Quantification of Ni laterite mineralogy and composition: a new approach

Abstract

Quantification of the chemical and mineralogical composition of Ni-bearing laterites is challenged by their complex nature, with the distribution of Ni and other metals controlled by a number of phases. Anew approach to predict the Ni (and Fe, Mg) composition of Ni laterites more reliably was tested using samples from drill holes from the Siberia North and Highway deposits. These deposits, selected as examples of (siliceous) oxide-style lateritic deposits, form part of the Kalgoorlie Nickel Project (KNP) in the north-eastern Yilgarn, Western Australia. Diffuse reflectance spectra over the 380–2500 nm wavelength range, measured using the CSIRO HyChips™ automated scanning system, were integrated with a multi-element dataset for 1 m-composite pulp samples using partial least-squares regression (PLSR) analysis. PLSR cross-validation models were developed to predict Ni, MgO and FeO, contents in samples with bulk composition limits of >0.08 wt% Ni and <6 wt% MgO. Calibration and validation models showed a strong, linear trend for characterising FeO and, to a lesser extent, Ni content, whereas reflectance spectroscopic-chemical models for predicting MgO content were less reliable. This reflected the prevailing mineralogy and associated element distribution within the oxide laterite profiles. For example, Fe and Ni distribution is controlled by relatively few minerals—predominantly goethite and, to a lesser extent, the Fe3+-bearing smectite, nontronite. Magnesium is associated with a range of Mg-bearing minerals, some of which are compositionally complex and include serpentine, talc, chlorite, smectite and carbonates (e.g. dolomite and magnesite). The MgO abundance alone does not reflect the varied mineralogy of samples with relatively high MgO contents, which can be spectrally distinct. Hence, samples with similar Mg contents, although comprising Mg-bearing minerals with distinct reflectance spectra, were difficult to model (e.g. smectite vs Mg carbonates). The findings of this study demonstrate that characterisation of Ni laterite chemistry may be achieved as part of routine logging of either drill cuttings or diamond drill core using PLSR analysis, but a detailed understanding of the mineral–element association is essential for reliable predictive analysis.