Mapping lithofacies within the D/K1 kimberlite pipe at Letlhakane, Botswana: An assessment of petrographic, geochemical and mineralogical indicators

Mapping and characterisation of the Letlhakane D/Kl kimberlite pipe has allowed a detailed reconstruction of rock forming events and petro- genetic processes during near-surface emplacement of this kimberlite. This study combines different disciplines within geology, petrogra phy, mineral chemistry and bulk rock geochemistry to delineate sep arate mappable lithofacies, and uses these groupings to understand the petrological processes responsible for forming each deposit and how these relate to physical emplacement conditions, chemical com positions of the parental magmas and sub-volcanic intrusion and em placement processes. The primary geological field data was acquired by extended mine visits to examine drill cores through detailed logging and quarry face mapping, and describing variations in petrographic textures between lithofacies. Groundmass spinel compositions were used to establish mineralogical variations within discrete lithofacies and show that some lithofacies were derived from separate batches of magma and that mixing had taken place between lithofacies. Analy sis of perovskite, abundant at Letlhakane, was used with spinel com positions to estimate the crystallisation sequence of the kimberlite groundmass inferred changes in oxygen fugacity in the magma may have been particularly important. Multivariate statistical analysis of the bulk rock geochemistry provided confirmation of the lithofacies groupings which appear to represent discrete units of kimberlite. This also demonstrated a grainsize relationship with the geochemistry. The major conclusions from this thesis show that the D/Kl kimberlite pipe was produced by dynamic and energetic eruptive processes which took place over sufficient time to allow crystallisation of one phase of deposits before the eruption of another. The final chapter combines all the field observations and analytical results into a hypothetical phase diagram for kimberlite magma, extrapolated from high to low pressures and temperatures, and a new emplacement model. This model is specific to the Letlhakane D/Kl pipe, but it may prove to be applicable to other kimberlite bodies