Petrology and geochemistry of scandium in New Caledonian Ni-Co laterites

The growing demand for scandium (Sc), essential for several modern industrial applications, drives the mining industry to develop alternative Sc sources. In such context, significant Sc concentrations (similar to 100 ppm) were recently reported in several Ni-Co lateritic oxide ores developed after mafic-ultramafic rocks. This contribution examines the distribution of Sc in Ni-Co laterites from New Caledonia, the sixth largest Ni producer worldwide. Representative lateritic profiles were selected based on the protolith type and include dunite, harzburgite and lherzolite protoliths, wherein the Sc content, determined by the relative proportion of olivine and pyroxene, ranges from < 5 ppm in dunite to > 10 ppm in lherzolite. In Ni-Co laterites, dissolution and leaching of primary Mg-rich silicates leads to the residual enrichment of iron as ferric oxides/oxyhydroxides in the upper horizons. Downward remobilization and trapping of Ni and Co lead to their local enrichment to economic concentrations, with maximum grades reached in the saprock/saprolite and in the transition horizons, respectively. In contrast, maximum Sc enrichment occurs in the yellow limonite horizon, where Sc-bearing goethite contains about ten times the Sc content of the parent rock. Consequently, harzburgite- and lherzolite-derived yellow limonites yield maximum Sc concentrations up to 100 ppm, together with moderate Ni and Co concentrations. There, Sc is potentially a valuable by-product that could be successfully co-extracted along with Ni and Co through hydrometallurgical processing. In addition to peridotite-hosted laterites, hornblende-rich amphibolites yield elevated Sc up to 130 ppm. The saprolitization of amphibolites leads to the formation of a goethite-gibbsite-kaolinite mixture with Sc concentrations > 200 ppm. There, goethite is the main Sc carrier with up to 800 ppm Sc. Therefore, despite their relatively limited volumes, amphibolite-derived saprolites may also represent attractive targets for Sc in New Caledonia. It is proposed that three main factors control the distribution and intensity of Sc enrichment in laterites derived from mafic and ultramafic rocks: (i) the initial Sc content of the parent rock, (ii) the development of goethite-dominated, yellow limonite after long-lived tropical weathering, and (iii) the local remobilization of Sc from the uppermost horizons through dissolution/recrystallization of goethite and partial replacement of goethite by hematite, thus leading to downward Sc concentration in the yellow limonite.