Contrasting compositions and textures of ilmenite in the Catoca kimberlite, Angola, and implications in exploration for diamond

The Catoca group-I kimberlite, the only currently active diamond-producing mine in Angola, was emplaced in the northeastern part of the Lucapa structure. We focus here on compositional and textural variations in ilmenite from drill-core material, in the hope of elucidating events before and during the emplacement of the kimberlitic magma. We have characterized four main variants of ilmenite, with enrichments in Fe3+, Mg, Mn and nearly stoichiometric ilmenite, and in seven textural classes, and have distinguished crystals of variable size, ranging from micro- to megacrysts. Most ilmenite is found to derive, through a complex process, from replacement of Fe3+-rich ilmenite, presumably originating by mantle metasomatism at a relatively high fO2. This Fe3+-rich ilmenite reacted with fluids under reducing conditions, producing Mg-rich ilmenite. The Mn-rich ilmenite is produced by interaction with a late CO2-rich fluid. The Mg-rich ilmenite is here clearly a minor phase and a late product of replacement. The absence of fresh Mg-rich ilmenite and the occurrence of Fe3+-rich ilmenite do not seem to be convincing arguments to exclude the presence of diamond crystals in a kimberlite. Compositional attributes must thus be considered with caution, and only in light of textural studies, in exploration programs.