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

Casey Hetman, Barbara Scott Smith, Jock Robey, Teboho Nkotsi, Mohapi Mohapi, Thabang Mohapi
Monday, September 18, 2017
First presented: 
International Kimberlite Conference

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.

Feature Author

Casey Hetman

Casey has 20 years of experience focused on the exploration and evaluation of primary diamond deposits. He has dedicated much of his career to the macroscopic and petrographic investigation of drillcores and mining exposures for the purpose of identifying and characterizing geological domains defined by different grades. This information is combined with other data sets—including geochemistry, grade information, geophysics, and modern volcanological principles—to generate 3D models in Gemcom.

Casey has been involved in geological investigations for the purposes of exploration, resource development, and mine planning for kimberlites in Canada, Greenland, Botswana, Lesotho, Angola, Zimbabwe, and Sierra Leone.

Principal Geologist
SRK Vancouver
SRK Latin America