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Depositional processes in a kimberlite crater: the Upper Cretaceous Orapa South Pipe (Botswana)



The Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross-cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain-size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the ∼70 m thick stratiform crater-fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater-fill are interpreted as rock-fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub-unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub-unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small-scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra-basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub-aerial settings.

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