Kimberlite wall-rock fragmentation processes: Venetia K08 pipe development |
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| Authors: | W P Barnett S Kurszlaukis M Tait P Dirks |
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| Institution: | (1) Mineral Resources Management, De Beers Group Services, P/Bag X01, Southdale, 2135, South Africa;(2) Present address: SRK Consulting, Suite 2200, 1066 West Hastings Street, Vancouver, V6E3X2, BC, Canada;(3) Kimberlite Petrology Unit, De Beers Canada Inc., Toronto, Ontario, Canada;(4) School of Earth and Environmental Sciences, James Cook University, Townsville, Australia |
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| Abstract: | Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its
feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa,
which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.%
chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia
layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation.
Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared
breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented
rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation
formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation.
Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic
in nature, with a total energy output of 2.7 × 109 kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite
intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe. |
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