Letšeng Diamond Mine, Lesotho: A Variant of Kimberley-type Pyroclastic Kimberlite Emplacement

The Letšeng Diamond Mine comprises two adjacent kimberlites, the Main and Satellite pipes. Very low grade and low frequency of high value stones preclude use of standard evaluation methods, increasing the need for high confidence geology models. New results of drillcore investigations, including core logging, country rock dilution measurements, indicator mineral abundances and petrography are integrated with open pit mapping and macrodiamond production data to present updated 3D geological models of the Main and Satellite pipes .

 Letšeng was emplaced ~90 Ma and forms part of a Cretaceous kimberlite province extending across southern Africa. The Letšeng bodies are steep-sided volcanic pipes that are infilled with multiple asymmetric phases of kimberlite separated by near-vertical, sharp, cross-cutting internal contacts. There are associated marginal carbonate-cemented country-rock breccias and sub-vertical kimberlite dykes that can occur within a zone of well-developed carbonate veining adjacent to the pipes. The pipe infills are dominated by kimberlite closely resembling Kimberley-type pyroclastic kimberlite (KPK; formerly tuffisitic kimberlite; Scott Smith et al. 2013) that contains common large megaxenoliths of massive and brecciated country rock basalt. Other textural varieties include hypabyssal kimberlite (HK), transitional HK-KPK and resedimented volcaniclastic kimberlite (RVK). Each phase of kimberlite represents a separate magmatic system. In each KPK there is a continuum of crystallization from the magmaclast selvages to the intermagmaclast matrix. As documented elsewhere, the HK-KPK transition involves an increasing (i) degree of deuteric replacement of olivine, (ii) amount of incorporated country rock xenoliths and (iii) textural modification of the magma prior to solidification subsurface within the diatreme. These conclusions negate some other proposed modes of emplacement.