The geology of the Letseng Kimberlites, Lesotho
Mining operations at the Letseng Diamond Mine in Lesotho have facilitated the study of the fresh kimberlite in the two diatremes (Main and Satellite pipes), and the included lower crustal and upper mantle nodules. The present day erosion level of these diatremes is close to the transition from diatreme to crater facies in the model of a kimberlite pipe. The two pipes contain a variety of distinctive kimberlites. The zonation of large and dense xenoliths in the Main pipe garnetiferous kimberlite is believed to reflect the near-surface emplacement by a process of fluidisation. The geochemistry, xenolith and diamond contents of the two pipes indicate diverse origins despite their close proximity. REE abundances together with strontium and neodymium isotopic evidence indicates kimberlite genesis by a small degree of partial melting of slightly depleted chondritic mantle. Kimberlite dykes, both older and younger than the pipes, indicate some chemical and mineralogical evolution of the parent magma. The peridotites are chromite and/or garnet bearing lherzolites and harzburgites similar to those from other Lesotho kimberlites. Textures vary from coarse to mosaic porphyroclastic and extreme fluidal and LAD varieties. All garnet-bearing xenoliths display coronas on the garnets resulting from retrograde reaction to spinel facies. In some cases reaction has gone to completion. Granuloblastic aluminous spinel lherzolites and garnet/spinel lherzolites are interpreted to derive from normal garnet lherzolite by a process of reaction, deformation, chemical homogenisation and re-equilibration during diapiric upwelling. Several peridotites are interpreted to show chemical disequilibrium and do not plot on a smoothly curving 'fossil geotherm'. This disequilibrium is believed to result from readjustment of primary phase compositions during diapiric upwell. A synthesis is presented of the kimberlite genesis in the upper mantle, the subsequent diapiric ascent and the surface emplacement.