Magmatic sedimentation and carbonatitic differentiation in kimberlite sills at Benfontein, South Africa

At Benfontein, near Kimberley, South Africa, three sills of kimberlite intrude Dwyka shales and overlying Karroo dolerite. Each sill results from numerous injections of kimberlite that have consolidated to give the sill a layered appearance. Many layers show magmatic sedimentation features and cumulus textures, and, although some show in situ differentiation, other layers result from pre-injection differentiation. The transporting, intercumulus liquid was carbonate-rich and some layers have differentiated to form a carbonate rock composed of the intercumulns calcite; this, on trace element and isotopic data, shows strong affinities with carbonatite. In one of the sills one calcite layer has migrated diapirically into overlying layers in the sill. These sedimentation features, combined with thermal metamorphism of country-rock shales and the presence of quench calcite and apatite, are interpreted as evidence that the kimberlite was injected as a highly mobile fluid, comprising megacrysts of olivine, garnet, pyroxene, mica and picroilmenite in a hot carbonatitic liquid from which olivine, magnetic spinel, perovskite, apatite, calcite, dolomite, ankerite and quartz crystallised. The evidence that the transportation medium was a warm carbonatitic liquid is directly opposed to earlier hypotheses proposing that kimberlite is intruded as a cold or plastic paste, and also supports proposals of a genetic link between kimberlite and carbonatite.