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Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions

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Abstract

K2 is a steep-sided kimberlite pipe with a complex internal geology. Geological mapping, logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes, lithic contents and internal structures. The pipe comprises two geologically distinct parts: K2 West is a layered sequence of juvenile- and lithic-rich breccias, which dip 20–45° inwards, and K2 East consists of a steep-sided pipe-like body filled with massive volcaniclastic kimberlite nested within the K2 pipe. The layered sequence in K2 West is present to > 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass-wasting breccias generated by rock fall and sector collapse of the pipe walls: both processes occurred in tandem during the infill of the pipe. Several breccia lobes extend across the pipe and are truncated by the steep contact with K2 East. Dense pyroclastic rocks within the layered sequence are interpreted as welded deposits. K2 East represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption. This phase may have involved fluidisation of trapped pyroclasts, with loss of fine particles and comminution of coarse clasts. We conclude that the K2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement, followed by alternating phases of accumulation and ejection.

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Acknowledgements

We acknowledge funding by De Beers Group Services, UK Ltd. and De Beers Consolidated Mines. Geological staff at Venetia Mine, South Africa are thanked for on-site support. The paper benefited greatly from many useful discussions with W. Barnett, T. Gernon, S. Kurszlaukis, J. Robey, J. Schumacher, S. Trickett and A. Walters. RSJS acknowledges a Royal Society-Wolfson Merit award. Thorough reviews by J.D.L. White, T. Thordarson and J. McPhie (editor) greatly improved the science and style of the manuscript.

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Authors and Affiliations

  1. Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queen’s Road, Bristol, BS8 1RJ, UK

    R. J. Brown, M. Field & R. S. J. Sparks

  2. De Beers Group Services, Cornerstone Office, Southdale 2135, Johannesburg, South Africa

    M. Tait

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  1. R. J. Brown
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  4. R. S. J. Sparks
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Brown, R.J., Tait, M., Field, M. et al. Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions. Bull Volcanol 71, 95–112 (2009). https://doi.org/10.1007/s00445-008-0211-4

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