Co, Sc, and Ni enrichment in the Cagdianao nickel laterite deposit, Dinagat Islands, Philippines

Abstract

Abstract Nickel laterites are soils that are derived from the weathering of mafic and ultramafic rocks. These deposits are being extracted mostly for their economic Ni and Fe content. However, in recent studies, the potential of nickel laterites to hold significant amounts of critical metals such as Sc and Co are already being investigated. In the Philippines, nickel laterite deposits form through the weathering of ophiolitic units. One of these is the Cagdianao nickel laterite deposit which was derived from the weathering of the ultramafic units of the Dinagat Ophiolite Complex in Dinagat Island, Surigao Del Norte, Philippines. The bedrock of the Dinagat Ophiolite Complex is dominantly comprised of harzburgite. This harzburgite bedrock is overlain by a nickel laterite profile consisting of horizons of Mg- and Si-rich rocky saprolite, earthy saprolite, Fe-rich limonite, and an Al-rich topsoil. Drillcores and field exposures of the nickel laterite profile were observed, sampled, and subjected to petrography, whole rock geochemistry (XRF and ICPMS), and mineral chemistry (LA-ICPMS) analyses. In terms of major elements, Fe is dominant in the limonite zone (up to 59.76 wt.%). Mg and Si are more enriched in the rocky saprolite layer (up to 28.41 and 37.41 wt.%, respectively). In terms of the critical minerals, Ni is more enriched in the saprolite zone (up to 29936 ppm) while Co and Sc are more enriched in the limonite zone (up to 1080 and 86 ppm, respectively). Elemental mapping of the bedrock samples reveals the strong affinity of Ni to olivine and serpentine. Sc was revealed to have a greater affinity with orthopyroxenes. Lastly, Co is observed to have a slight affinity with olivine. These results show a relative enrichment of Co in the profile, particularly in the limonite. Co within the laterite profile also has a very strong affinity with MnO. Similarly, the highest concentration values for Sc were also observed in the limonite. It also exhibits an affinity for Fe2O3 and Al2O3. Lastly, Ni was observed to have a strong affinity with olivine and serpentine minerals in the bedrock and the rocky saprolite horizons. Within the deposit, Ni is the main commodity while Co, Sc, and REEs can be considered as by-products.