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Letšeng diamond mine, Lesotho: a variant of Kimberley-type pyroclastic kimberlite emplacement

Abstract

The Letšeng Diamond Mine comprises two ~91 Ma kimberlite pipes. An update of the geology is presented based on the 2012–2017 detailed investigation of open pit exposures and all available drillcores which included mapping, logging and petrography. Each of the steep-sided volcanic pipes comprises a number of phases of kimberlite with contrasting diamond contents which were formed by the emplacement of at least four batches of mantle-derived magma. The resulting range of textures includes resedimented volcaniclastic kimberlite (RVK), Kimberley-type pyroclastic kimberlite (KPK), coherent kimberlite (CK) and minor amounts of hypabyssal kimberlite (HK). The pipes are compared with KPK occurrences from southern Africa and worldwide. Many features of the Letšeng pipes are similar to KPK infilled pipes particularly those of the widespread Cretaceous kimberlite province of southern Africa. The differences displayed at Letšeng compared to other large KPK pipe infills described from around the world are attributed to the marginal or melnoitic nature of the magma and the upper diatreme to crater setting of the Letšeng pipes, where processes become extrusive. It is concluded that the pipes comprise a variant of Kimberley-type pyroclastic kimberlite emplacement. The classification of many of the Letšeng rocks as KPK is important for developing the internal geology of the pipes as well as for predicting the distribution of diamonds within the bodies.

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Acknowledgements

The authors would like to thank Letšeng Diamonds (Pty) Ltd. and Gem Diamonds Ltd. for permission to publish the information presented in this paper. Over the past four years multiple site visits were completed and these would not have been possible without the support of the geology department and staff at the Letšeng Mine. We appreciate the comments by Kimberley Webb, reviews by three anonymous experts, and suggestions by guest editor D. Graham Pearson, as they improved this manuscript. Expert drafting by Meilani Smith is much appreciated.

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Correspondence to Casey M. Hetman.

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Editorial handling: G. Pearson

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Hetman, C.M., Scott Smith, B., Robey, J. et al. Letšeng diamond mine, Lesotho: a variant of Kimberley-type pyroclastic kimberlite emplacement. Miner Petrol 112, 365–382 (2018). https://doi.org/10.1007/s00710-018-0642-3

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Keywords

  • Type IIa diamond
  • Tuffisitic kimberlite
  • Resedimented volcaniclastic kimberlite
  • Kimberley-type pyroclastic kimberlite
  • Hypabyssal