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.