Apr 8, 2008 - Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions. R. J. Brown &a
Bull Volcanol (2009) 71:95–112 DOI 10.1007/s00445-008-0211-4
RESEARCH ARTICLE
Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions R. J. Brown & M. Tait & M. Field & R. S. J. Sparks
Received: 19 July 2006 / Accepted: 4 March 2008 / Published online: 8 April 2008 # Springer-Verlag 2008
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
Editorial responsibility: J McPhie R. J. Brown (*) : M. Field : R. S. J. Sparks Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queen’s Road, Bristol BS8 1RJ, UK e-mail:
[email protected] M. Tait De Beers Group Services, Cornerstone Office, Southdale 2135, Johannesburg, South Africa
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. Keywords Kimberlite . Volcanic conduit . Pyroclastic . Explosive eruption
Introduction Conduit deposits derived from the explosive eruption of kimberlite magma occur throughout the geological record, but no such eruptions have been witnessed and we know comparatively little about the near-surface eruptive behaviour of such magmas. Kimberlite magmas are characterised by low viscosities (0.1–10 Pa s), by SiO2 contents of 80 boreholes over the past 30 years, accounting for >22 km of core. Over 250 thin sections taken from surface outcrops and drillcore have been examined. The geology was modelled in 3D using computer software (Gemcom GEMS™). A lithofacies-based approach has been adopted with lithofacies distinguished on the basis of grain size, lithic clast abundance and composition.
Terminology K2 kimberlite pipe The term macrocryst is used for anhedral crystals, typically >0.6 mm in diameter inferred to be mantle-derived xenocrysts; phenocryst for euhedral–subhedral crystals, 20 m in diameter (Fig. 2a,
Bull Volcanol (2009) 71:95–112
99
or two types of basement rock (predominantly gneiss, schist, amphibolite, dolerite or quartzite). Clasts are angular with joint-bounded surfaces, although a minor proportion of them are moderately to well-rounded. Some clasts are intensely fractured and show jigsaw-fit textures. The poorly sorted fine-coarse ash matrix is composed of abundant country rock and xenocrysts (particularly biotite, amphibole, quartz and feldspar), serpentine, clay and minor olivine crystals. Calcite occurs as a patchy pore-fill in some Br units.
b). The margins of some blocks are highly fragmented with jigsaw-fit textures (Fig. 2a). Most megablocks have been rotated relative to their orientation in the adjacent wall rock and some have undergone as much as ∼90° rotation about a sub-horizontal axis. Megablocks are found throughout K2 West (Fig. 1d) and the largest are associated with the country rock breccia lithofacies (Br). Country rock breccia (Br) Country rock breccias comprise >90 vol.% lithic lapilli and blocks with an interstitial matrix (Fig. 2c). They occur in units several metres thick to >30 m thick around the western margin of K2, interbedded with other breccia lithofacies (e.g., Fig. 2d); several Br units extend across the whole of K2 West at depth (Fig. 1d). The clast population in the country rock breccias is commonly dominated by one
a
Matrix-supported kimberlite breccia This volumetrically important lithofacies comprises 20– 50 vol.% angular to well rounded lithic lapilli and blocks in an altered kimberlite-derived matrix containing abundant free olivine crystals (Fig. 3a). The clast population is
b
d c
e
Fig. 2 Country rock breccias (Br) in K2 West. a Jigsaw-fit textured country rock megablock has been comprehensively fractured but fragments have not been rotated or moved. b Country rock breccia lithofacies showing abundant basement clasts in sparse matrix (bench 6). Mine benches are 12 m high. c >20 m diameter gneiss country rock megablock in country rock breccia (Br). Block has been rotated ∼90°
about a horizontal axis relative to in-situ country rock. Metre-rule for scale. All observations from 72–96 m bps. d Inward-dipping contact between country rock breccia and clast-supported volcaniclastic kimberlite breccia in K2 West. e Jigsaw textured gneiss clasts in core (DDH221, 243.78 m)
100
dominated by gneiss, schist, amphibolite and dolerite with subordinate marble and basalt. Dolerite accounts for >40% of the clast population in some units between 370–570 m bps, below the outcrop level of a dolerite sill in the country rock. Extremely well-rounded basement lithic lapilli and blocks are common and many exhibit concentric-foliated margins of altered rock around fresh cores. Angular, platy fragments apparently derived from these altered layers occur in the matrix (see Fig. 3a). Scattered country rock megablocks occur in association with this lithofacies. The long axes of platy lapilli and blocks dip inwards at ∼ 40– 80° locally within 40% clast population) with subordinate amounts of other basement rocks (see Kurszlaukis and Barnett 2003). Jigsaw-fit clast textures are common (Fig. 2e). Approximately 5–15% clasts are moderately to extremely well-rounded and display altered concentric foliated margins as found in the matrixsupported volcaniclastic kimberlite breccia lithofacies (Fig. 2b). This lithofacies occurs conformably interbedded with the other K2 West lithofacies as metre to tens-of-metres-thick layers, lenses and pods (Fig. 1d). Volcaniclastic kimberlite This lithofacies occurs in units up to several tens of metres thick. It is lithologically and petrographically similar to the matrix-supported kimberlite breccia lithofacies but contains
