Petrology of kimberlite from the DeBruyn and Martin Mine, Bellsbank, South Africa

The micaceous kimberlite from DeBruyn and Martin Mine, Bellsbank, South Africa, is composed of olivine, phlogopite, spinels, and perovskite in a groundmass of calcite and phlogopite. Three types of phlogopite are present: macrocrysts [Mg/(Mg+Fe) ≃ 0.93, FeO (3–3.5 wt.%), TiO₂ (0.5–0.7 wt.%), Cr₂O₃ (0.4–0.45 wt.%), low Fe₂O₃, ΔT (Fe³⁺ in tetrahedral sites = 0.01–0.05]; groundmass phlogopite [Mg/(Mg+Fe) ≃ 0.88, FeO (≃ 6.0 wt.%), TiO₂ (≃ 1.5 wt.%), Cr₂O₃ (0.5–0.6 wt.%), high Fe₂O₃, ΔT = 0.429–0.666]; and phlogopite aggregates associated with olivine xenocrysts [Mg/(Mg+Fe) ≃ 0.90, FeO (4.5–5 wt.%), TiO₂ (0.4–0.7 wt.%), no ferric iron, and high Cr₂O₃ (2.7–3.4 wt.%)]. The phlogopite macrocrysts crystallized from the kimberlite magma prior to emplacement; groundmass phlogopite appears to have crystallized from late-stage kimberlitic fluids after emplacement; and the phlogopite aggregates are products of disaggregation of ultramafic nodules. Spinels show crystallization trends different from those of mica-poor “basaltic kimberlites.” The Mg–Al chromite is the earliest spinel to crystallize and is followed by titaniferous magnesian chromite and titaniferous magnetite. The crystallization trend is toward decrease in A1₂O₃ and increase in TiO₂ and Fe₂O₃. The sharp decrease in A1₂O₃ content from Mg–Al chromite to titaniferous chromite is attributed to the crystallization of groundmass phlogopite, which depleted the kimberlitic liquid in Al. Perovskite shows a remarkable enrichment in REE and Nb relative to other South African kimberlites. Perovskite also is highly enriched in LREE and appears to be the major REE-bearing phase in kimberlite from the DeBruyn and Martin Mine. The high concentrations of REE and other incompatible elements in this kimberlite and the wide range of concentrations displayed by these elements in different kimberlites from the Bellsbank region suggest that kimberlite magma was derived from a garnet lherzolite source that was affected by metasomatism in the upper mantle via equilibration with LKEE-enriched CO₂ and H₂O vapors.