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The extension of the western limb, Bushveld Complex (South Africa), at Cullinan Diamond Mine

  • R.G. Cawthorn (a1) and N. McKenna (a1)

Abstract

Mafic rocks of the Bushveld Complex at the southeastern end of the western limb, intersected in bore core from the Cullinan Diamond Mine, are described. A 260 m thick ultramafic body of orthopyroxene and chromite cumulate rocks, with mg# – 100*Mg/(Mg+Fe) – values from 77 to 84 and 0.25 to 0.5% Cr2O3 in the pyroxene, is considered to have affinity to the Critical Zone. Such an interpretation considerably extends the eastern limit of Critical Zone rocks of the western limb of the Bushveld Complex. The whole-rock composition of the lower, chilled basal contact of this body has 10% MgO and 500 ppm Cr, and is comparable to magmas considered parental to the Bushveld Complex. Due to intrusion of a younger sill, the upper contact is not preserved in the bore core. The cumulate rocks have higher interstitial component, inferred from incompatible trace element abundances (Zr, Ti and K), than normal Critical Zone rocks, interpreted to be a result of more rapid cooling due to proximity to the basal contact. The near-constancy of mg# in the pyroxene in the entire succession suggests that large volumes of magma flowed through this conduit, with only the liquidus phases of orthopyroxene and chromite being precipitated.

Five generations of sills, intruded into the underlying metasedimentary rocks, are identified. The oldest is tholeiitic, and was metamorphosed prior to the emplacement of the Bushveld Complex. The second equates to the magma proposed as being parental to the Bushveld Complex (2060 Ma). The third represents the products of differentiation of that magma. The fourth is syenitic, and related to the Pienaars River Alkaline Complex (1430–1300 Ma). The fifth is tholeiitic (1150 Ma), and cuts the Cullinan kimberlite.

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Barnes, S.J. (1986) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contributions to Mineralogy and Petrology, 93, 524531.
Barnes, S.J. and Hoatson, D.M. (1994) The Munni Munni Complex: Western Australia: Stratigraphy, structure and petrogenesis. Journal of Petrology, 35, 715751.
Bartlett, P.J. (1994) Geology of the Premier Diamond Pipe. Proceedings of the XVth CMMI Congress, South African Institute of Mining and Metallurgy, Johannesburg, 201214.
Buchanan, D.L. (1975) The petrography of the Bushveld Complex intersected by boreholes in the Bethal area. Transactions of the Geological Society of South Africa, 78, 335348.
Cameron, E.N. (1978) The Lower Zone of the Eastern Bushveld Complex in the Olifants River trough. Journal of Petrology, 19, 437462.
Cameron, E.N. (1980) Evolution of the lower Critical Zone, central sector, eastern Bushveld Complex. Economic Geology, 75, 845871.
Cameron, E.N. (1982) The upper Critical Zone of the eastern Bushveld Complex - precursor of the Merensky Reef. Economic Geology, 11, 13071327.
Cawthorn, R.G. (1988) Discussion of metamorphic evidence for early post-Bushveld sill in the Penge Iron Formation, Transvaal Sequence, Eastern Transvaal. South African Journal of Geology, 91, 134136.
Cawthorn, R.G. (1996) Re-evaluation of magma composition and processes in the uppermost Critical Zone of the Bushveld Complex. Mineralogical Magazine, 60, 131148.
Cawthorn, R.G. and Webb. SJ. (2002) Connectivity between eastern and western limbs of the Bushveld Complex. Tectonophysics, 330, 195209.
Cawthorn, R.G., Davies, G., Clubley-Armstrong, A. and McCarthy, T.S. (1981) Sills associated with the Bushveld Complex, South Africa: an estimate of the parental magma composition. Lithos, 14, 114.
Cawthorn, R.G., Barnes, S.J., Ballhaus, C. and Malitch, K. (2005) Platinum-group element, chromium and vanadium deposits in mafic and ultramafic rocks. Economic Geology, 100th Anniversary Volume, pp. 215249.
Cousins, C.A. (1959) The structure of the mafic portion of the Bushveld Complex. Transactions of the Geological Society of South Africa, 62, 179189.
Davies, G., Cawthorn, R.G., Barton, J.M. and Morton, M. (1980) Parental magma to the Bushveld Complex. Nature, 287, 33–5.
Eales, H.V. (2000) Implications of the chromium budget of the western limb of the Bushveld Complex. South African Journal of Geology, 103, 141150.
Eales, H.V. and Cawthorn, R.G. (1996) The Bushveld Complex. Pp. 181230 in: Layered Intrusions(Cawthorn, R.G., editor), Elsevier, Amsterdam.
Eales, H.V., Teigler, B. and Maier, W.D. (1993) Cryptic variations in minor elements Al, Cr, Ti and Mn in Lower and Critical Zone orthopyroxenes of the western Bushveld Complex. Mineralogical Magazine, 57, 257264.
Hall, A.L. (1932) The Bushveld Igneous Complex of the Central Transvaal. Geological Survey of South Africa, Memoir, 28, 560 pp.
Harmer, R.E.J. (1985) Rb-Sr isotopic study of units of the Pienaars River Alkaline Complex, north of Pretoria, South Africa. South African Journal of Geology, 88, 215224.
Hatton, C. and von Gruenewaldt, G. (1987) The geological setting and petrogenesis of the Bushveld chromitite layers. Pp. 10943 in: Evolution of Chromium Ore Fields(C.W.|Stowe, editor). Van Nostrand Reinhold Co., New York.
Jansen, H. (1977) The geology of the country around Pretoria. Explanation of sheets 2527DA, DB, DC, DD and 2528CA, CB, CC, CD. Geological Survey of South Africa, Pretoria, 135 pp.
Lombaard, B.V. (1932) Felsites and their relationships to the Bushveld Complex. Transactions of the Geological Society of South Africa, 35, 125190.
Maier, W.D. and Bowen, M.P. (1996). The UG2-Merensky Reef interval of the Bushveld Complex, northwest of Pretoria. Mineralium Deposita, 31, 386393.
Maier, W.D. and Eales, H.V. (1994) Plagioclase inclusions in orthopyroxene and olivine of the UG2-Merensky Reef interval: regional trends in the western Bushveld Complex. South African Journal of Geology, 97, 408414.
Mitchell, A.A. (1990) The stratigraphy, petrography and mineralogy of the Main Zone of the Northwestern Bushveld. South African Journal of Geology, 93, 818831.
Mitchell, A.A. and Manthree, R. (2002) The Giant Mottled Anorthosite: a transitional sequence at the top of the Upper Critical Zone of the Bushveld Complex. South African Journal of Geology, 105, 1524.
Raedeke, L.D. and McCallum, I.S. (1984) Investigations in the Stillwater Complex: Part II: Petrology and petrogenesis of the Ultramafic Series. Journal of Petrology, 25, 395420.
SACS (South African Committee for Stratigraphy) (1980) Stratigraphy of South Africa (Kent, L.E., compiler). Handbook, 8, Geological Survey of South Africa, Pretoria, 690 pp.
Schümann, L.W. (1993) The geochemistry of the Upper Critical Zone of the Boshoek section of the western Bushveld Complex. Geological Survey of South Africa, Bulletin, 113, 88 pp.
Schümann, L.W., Grabe, P.-J. and Steenkamp, C.J. (1998) Chromium. The Mineral Resources of South Africa (Wilson, M.G.C. and Anhaeusser, C.R., editors) Council for Geoscience, Pretoria, South Africa, pp. 90105.
Sharpe, M.R. and Hulbert, L.J. (1985) Ultramafic sills beneath the eastern Bushveld Complex. Economic Geology, 80, 849871.
Teigler, B. and Eales, H.V. (1996) The Lower and Critical Zones of the western limb of the Bushveld Complex as intersected by the Nooitgedagt boreholes. Geological Survey of South Africa, Bulletin, 111, 126 pp.
Teigler, B., Eales, H.V. and Scoon, R.N. (1992) The cumulate succession in the Critical Zone of the Rustenburg Layered Suite at Brits, western Bushveld Complex. South African Journal of Geology, 95, 1728.
Wager, L.R. and Brown, G.M. (1968) Layered Igneous Rocks. Oliver and Boyd, Edinburgh, 588 pp.
Walraven, F. (1987) Geochronological and isotopic of Bushveld Complex rocks from the Fairfield borehole at Moloto, northeast of Pretoria. South African Journal of Geology, 90, 352360.
Willemse, J. (1959) The floor of the Bushveld Igneous Complex and its relationships with special reference to the eastern Transvaal. Transactions of the Geological Society of South Africa, 62, 2183.
Wilson, A.H. (1982) The geology of the Great Dyke, Zimbabwe: the ultramafic rocks. Journal of Petrology, 23, 240292.
Wilson, A.H., Lee, C.A. and Brown, R.T. (1999) Geochemistry of the Merensky Reef, Rustenburg section, Bushveld Complex: controls on the silicate framework and distribution of trace elements. Mineralium Deposita, 34, 657672.

Keywords

The extension of the western limb, Bushveld Complex (South Africa), at Cullinan Diamond Mine

  • R.G. Cawthorn (a1) and N. McKenna (a1)

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