Mineralogy and geochemistry of the Berong Ni-Co laterite deposit, Palawan, Philippines

Abstract

Intense weathering of the serpentinized peridotite resulted in the formation of the Berong Ni-Co laterite deposit in Palawan (Philippines). The studied laterite profiles are divided from base to top into: (i) serpentinized bedrock, (ii) saprolite horizon, and (iii) limonite horizon. Olivine in the peridotite contains a significant amount of Ni (average 0.38 wt% NiO), thus representing the main source of the ore in the laterite horizons. Six types of Ni-bearing serpentines (up to 6.65 wt% NiO) in the saprock are discriminated based on their textures and Raman signatures. Garnierite (a mixture of serpentine-, kerolite- and sepiolite-like clays) typically occurs as coatings or fracture fillings in the saprock and is characterized by strikingly high NiO content up to 34.86 wt%. Goethite and Mn-oxy-hydroxides (i.e. lithiophorite-asbolane intermediates) are the principal hosts of Ni in the limonite (up to 2.15 wt% NiO and 23.89 wt% NiO, respectively) with the latter also being the main Co carriers (up to 12.59 wt% CoO). The whole-rock XRF data show typical trends observed in other Ni laterite deposits, where Mg and Si are depleted toward the surface, and the transition metals (i.e. Fe, Al, Ni, Co, and Mn) are retained in the weathered horizons. The saprolite horizon is particularly enriched in Ni, while the lower section of the limonite horizon is enriched in Co. The mildly acidic environmental condition (pH ≤ 6.57) in the limonite horizon was found to enhance the mobilization and re-distribution of Ni from the limonite towards the lower section of the profiles. Geological observations, coupled with mineralogical and geochemical data, show that the Berong Ni-Co laterite deposit formed in two weathering processes: (i) development of the saprolite and the limonite horizons, and (ii) late precipitation of secondary phyllosilicates (i.e. garnierite), Mn-oxy-hydroxides and silica.