Pyroclastic flow deposits from a kimberlite eruption: The Orapa South Crater, Botswana

11 The Orapa Diamond Mine (Republic of Botswana) exposes a bi-lobate kimberlite 12 pipe that erupted during the Late-Cretaceous epoch (∼93 Ma) through Archaean 13 basement and volcano-sedimentary rocks of the Karoo Supergroup. Geological map14 ping of the crater zone of the South Pipe has revealed a 15–25 m thick in-situ 15 kimberlite pyroclastic flow deposit. The pyroclastic deposit fills in the crater and 16 completely drapes lower units, indicating that the parent flow originated from an 17 adjacent kimberlite pipe. The unit comprises a basal coarse lithic concentration layer 18 exhibiting imbricated clasts, which grades upwards into massive poorly sorted lapilli 19 tuff. The tuff contains abundant sub-vertical degassing structures defined by lithic 20 enrichment and depletion in fine-grained material. Degassing structures commonly 21 emanate from blocks in the basal layer. The presence of degassing structures and a 22 coarse basal layer distinguishes this deposit from pipe-filling massive volcaniclastic 23 kimberlite, which is typically homogeneous in terms of texture and clast size over 24 distances on the order of 100s metres. Studies of the thermal remnant magnetism 25 in basalt clasts from the deposit, together with serpentine–diopside assemblages, 26 indicate that it was emplaced at elevated temperatures on the order of 200–440◦C, 27 consistent with deposition from a pyroclastic flow. The lithofacies characteristics can 28 be explained by the interaction of the pyroclastic flow with the complex topography 29 of a pre-existing crater. 30