Oxide minerals in a layered kimberlite-carbonate sill from Benfontein, South Africa

Abstract

The lower sill at Benfontein, South Africa, shows a high degree of magmatic sedimentation to kimberlite, oxide-carbonate, and carbonate layers. The iron-titanium oxide minerals are similar in the carbonate-rich and silicate-rich layers and are represented by titaniferous Mg-Al chromite, Mg-Al titanomagnetite, magnesian ilmenite, rutile, and perovskite. The spinel crystallization trend was toward enrichment in Mg and Ti and depletion in Cr; this trend is similar to that observed in many kimberlites. The ilmenite has Mg and Cr contents within the range observed in kimberlites and lacks the Mn enrichment observed in ilmenites from carbonatites. Perovskite in silicate-rich and carbonate-rich layers shows similar total REE contents and LREE enrichment and lacks the remarkable Nb enrichment observed in perovskite from carbonatites. These new data on the iron-titanium oxide minerals in the lower Benfontein sill do not support a genetic relationship between kimberlites and carbonatites.