Lateritization process of peridotites in Siruka, Choiseul, Solomon Islands

Abstract

The lateritic weathering crusts exposed in Siruka, Choiseul Islands, Solomon Islands, were developed on the expense of serpentinized peridotite underlain by Siruka schists and Voza lavas with a subhorizontal contact. The lateritic profiles consist of three generalized zones: bedrock, saprolitic zone (weathered and decomposed zone) and the limonitic zones. The profiles demonstrate variations in depths and continuity but illustrate mineralogy and geochemical affinity down profile and are analogous to saprolitic nickel laterite deposits. Silica and magnesia in the bed rock and the saprolitic zones have been removed and only the residual elements (iron, chromium, aluminium, manganese, cobalt and nickel) remain in the limonitic zone. These elements are relatively concentrated as a result of the removal of the soluble elements. Nickel is associated with silica and magnesia, as lizardite ormixed gels (garnierite nickel ore) at the weathering fronts. On the other hand, nickel, with generally low concentrations (<2% NiO), is also hosted by several secondary mineral phases (goethite and Mn-oxyhydroxides locally) as residuals in the surface limonitic zone. Cobalt, on the other hand, is generally of higher grade (up to 0.9% CoO) and is associated with cryptocrystalline and crystalline manganese-oxyhydroxides. Cobalt and manganese are concentrated in the limonitic part of the profiles (~3-7m depth) of which are poorly protected against erosion. The behavior of nickel is different from that of cobalt and manganese as its relative mobility is a little higher than them. Nickel is suggested to have been leached out of the limonitic concretions, where it is first concentrated with cobalt and manganese, and migrates to the bottom part of the profiles. Significant supergene nickel enrichment occurring in the saprolitic zone indicated that water had percolated downward to a very low water table at depth during weathering. The structure and mineralogy of the weathering profile is consistent with the alternating wet and dry periods of the studied area.

Keywords: Siruka ultramafic rocks, laterization, Choiseul-Solomon Islands