Wookey Hole, United Kingdom
Wookey Hole, United Kingdom

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Fontana G.,University of Oxford | Niocaill C.M.,University of Oxford | Brown R.J.,Durham University | Sparks R.S.J.,University of Bristol | Field M.,DiaKim Consulting Ltd
Bulletin of Volcanology | Year: 2011

Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained (the Cretaceous A/K1 pipe, Orapa Mine, Botswana, and the Cambrian K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions, layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Basalt lithic clasts in the layered and massive vent-filling pyroclastic deposits in the A/K1 pipe at Orapa were emplaced at >570°C, in the pyroclastic crater-filling deposits at 200-440°C and in crater-filling talus breccias and volcaniclastic breccias at <180°C. The results from the K1 and K2 pipes at Venetia suggest emplacement temperatures for the vent-filling breccias of 260°C to >560°C, although the interpretation of these results is hampered by the presence of Mesozoic magnetic overprints. These temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. The temperatures are also comparable to those obtained for pyroclastic deposits in other, silicic, volcanic systems. Because the lithic content of the studied deposits is 10-30%, the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300-400°C hotter than the palaeomagnetic estimates. Together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites, the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits. This study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies. © 2011 Springer-Verlag.

White J.L.,University of Bristol | Sparks R.S.J.,University of Bristol | Bailey K.,University of Bristol | Barnett W.P.,Consulting Inc. | And 3 more authors.
South African Journal of Geology | Year: 2012

Field relations, petrography and bulk rock geochemistry constrain the emplacement mechanisms and pedogenesis of a kimberlite sill complex exposed in water tunnels at Wesselton Mine, Kimberley. The sill complex, together with associated dykes, is precursor to the main Wesselton pipe and contains: 1. aphanitic kimberlite with a carbonatitic groundmass of calcite, perovskite, spinel, serpentine and phlogopite, with variations from rocks rich in olivine microphenocrysts to rocks with little or no olivine; 2. macrocrystic kimberlite containing entrained harzburgite microxenoliths, olivine macrocrysts and strongly reacted orthopyroxene; and, 3. a 'green' aphanitic kimberlite. The sills were emplaced as multiple intrusions into the upper Dwyka shales immediately below a Karoo dolerite sill in a zone several metres thick. Emplacement of the sill complex was governed by the rigidity of the dolerite sill, which prevented the magma from breaching the surface. Feeder dykes to the sills were influenced by a regional crustal stress system with the principal vector of horizontal compression being between eastwest and northwest-southeast. Serpentinisation has pseudomorphed olivine, replaced calcite, apatite and phlogopite in the groundmass and thereby changed the bulk compositions. Geochemcal and mineral chemistry data on spinels and phlogopites indicate intrusion of several magma batches with different petrogenetic histories. The magmas are transitional to carbonatite with compositional variations linked to partial melting, assimilation of xenocrysts, entrainment of xenoliths, xenocrysts and macrocryst, fractional crystallization and flow differentiation during ascent and emplacement. © 2012 March Geological Society of South Africa.

Gernon T.M.,University of Southampton | Field M.,DiaKim Consulting Ltd | Sparks R.S.J.,University of Bristol
Journal of the Geological Society | Year: 2012

The Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, Canada, is a complex segmented diamond-bearing ore-body. Detailed geological investigations suggest that the kimberlite is a multi-phase intrusion with at least four magmatic lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal kimberlite have been identified. Key observations are that the olivine-rich lithofacies has a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Heterogeneities in the lithofacies are attributed to variations in intrusion thickness and structural complexities. The geometry and distribution of lithofacies points to magmatic co-intrusion, and flow segregation driven by fundamental rheological differences between the two phases. We envisage that the low-viscosity OPK magma acted as a lubricant for the highly viscous ORK magma. The presence of such low-viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (>60 vol.%) are able to reach the surface during kimberlite eruptions. We also document the absence of crystal settling and the development of an unusual subvertical fabric of elongate olivine crystals, which are explained by rapid degassing-induced quench crystallization of the magmas during and after intrusion.

Buse B.,University of Bristol | Sparks R.S.J.,University of Bristol | Field M.,DiaKim Consulting Ltd | Schumacher J.C.,University of Bristol | And 2 more authors.
Bulletin of Volcanology | Year: 2011

The BK9 kimberlite consists of three overlapping pipes. It contains two dark varieties of massive volcaniclastic kimberlite, informally termed dark volcaniclastic kimberlite (DVK). DVK(ns) is present in the north and south pipes and is interbedded with lenses of basalt breccia at the margins of the pipes. DVK(c) is present within the central pipe where it is overlain by a sequence of basalt breccias with interbedded volcanogenic sediments. The features observed within the DVK units of the BK9 kimberlite provide strong evidence for gas fluidisation of the accumulating pyroclastic material. These include the massive interior of the pipes, marginal epiclastic units, well-dispersed country-rock xenoliths and small-scale heterogeneities in lithic clast abundance. The upper portions of the central pipe provide a record of the transition from pyroclastic eruption and infill to passive epiclastic infilling of the crater, after the eruption has ceased. The wall-rock of the BK9 kimberlite dips inwards and is interpreted as post pipe-fill subsidence of the adjacent country rock. The two DVK units contain interstitial, silt-sized pyroclasts. The DVK(ns) has a higher fraction of former melt and displays evidence of incipient welding, as a result of differences in eruption dynamics. These units demonstrate that whilst DVK is comparable in many respects to MVK and forms part of a spectrum of volcaniclastic rocks formed by fluidisation, it differs in frequently containing silt-sized particles and including agglutinated and welded varieties with a high melt fraction. The DVK varieties, studied here, also have a distinctive hydrothermal assemblage, resulting from the abundance of low-silica accidental lithic clasts. Both the hydrothermal alteration and the abundance of silt-sized particles contribute to the DVKs distinctive dark colour. © 2011 Springer-Verlag.

Buse B.,University of Bristol | Schumacher J.C.,University of Bristol | Sparks R.S.J.,University of Bristol | Field M.,DiaKim Consulting Ltd
Contributions to Mineralogy and Petrology | Year: 2010

Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F-, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350-250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits. © 2010 Springer-Verlag.

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