Hostname: page-component-78c5997874-lj6df Total loading time: 0 Render date: 2024-11-10T07:49:33.806Z Has data issue: false hasContentIssue false
  • English
  • Français

Iron-rich ultramafic pegmatite replacement bodies within the Upper Critical Zone, Rustenburg Layered Suite, Northam Platinum Mine, South Africa

Published online by Cambridge University Press:  05 July 2018

D. L. Reid  and
I. J. Basson
D. L. Reid*
Affiliation:
Department of Geological Sciences, University of Cape Town, Private Bag Rondebosch, 7700, South Africa
I. J. Basson
Affiliation:
Department of Geological Sciences, University of Cape Town, Private Bag Rondebosch, 7700, South Africa
*
*E-mail: dlr@geology.uct.ac.za
Get access Rights & Permissions [Opens in a new window]

Abstract

Discordant veins, pipes and occasionally subconcordant sheets of iron-rich ultramafic pegmatite disrupt the layered cumulate sequence of the Upper Critical Zone, Rustenburg Layered Suite, Bushveld Complex. These pegmatite bodies have been studied where they replace the Merensky Reef footwall at Northam Platinum Mine, situated in the Swartklip Facies of the western lobe of the Rustenburg Layered Suite. Composed chiefly of ferroaugite and fayalitic olivine, the pegmatites appear to be formed by the preferential replacement of plagioclase-rich cumulates within the layered sequence. Fe-Ti oxides, sulphide (pyrrhotite and chalcopyrite) and plagioclase also occur in variable quantities. Differentiation within the pegmatite is observed where it has spread laterally beneath the impervious Merensky chromitite layer, with the development of subparallel cm-scale layers of massive magnetitite, massive sulphide and sulphide pegmatite. While some Fe-rich mobile phase must have been responsible for the pegmatites, it is concluded that the pegmatite bulk composition does not represent the original liquid. Furthermore the mode of occurrence precludes the injection of a crystal mush. Rather it is argued, mainlyon geochemical and isotopic grounds, that Fe-rich residual melts derived from the Upper Zone in the downward crosscutting gap areas migrated laterallyand upwards into the adjacent Upper Critical Zone. Variable reaction with the layered cumulates produced the anastomosing pegmatite bodies.

Keywords

iron-rich ultramafic pegmatitesUpper Critical ZoneRustenberg Layered SuiteBushveld ComplexSouth Africa
Type
Research Article
Information
Mineralogical Magazine , Volume 66 , Issue 6 , December 2002 , pp. 895 - 914
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2002

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Cawthorn, R.G. (1999) The discovery of the platiniferous Merensky Reef. South African Journal of Geology, 102, 178–183.Google Scholar
Eales, H.V., Botha, W.J., Hattingh, P.J., De Klerk, W.J., Maier, W.D. and Odgers, A.T.R. (1993) The mafic rocks of the Bushveld Complex, a review of emplacement and crystallization history, and mineralisation, in the light of recent data. Journal of African Earth Sciences, 16, 121–142.CrossRefGoogle Scholar
Hamilton, J. (1977) Sr isotope and trace element studies of the Great Dyke and Bushveld Mafic Phase and their relation to Early Proterozoic magma genesis in Southern Africa. Journal of Petrology, 18, 24–52.CrossRefGoogle Scholar
Irvine, T.N. and Baragar, W.R.A. (1975) A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8, 523–548.CrossRefGoogle Scholar
Kruger, F.J. (1994) The Sr-isotopic stratigraphy of the western Bushveld Complex. South African Journal of Geology, 97, 393–398.Google Scholar
SACS (1980) Stratigraphy of South Africa. Handbook 8, Geological Survey of South Africa, Pretoria, 690 pp.Google Scholar
Scoon, R. and Mitchell, A. (1994) Discordant iron-rich ultramafic pegmatites in the Bushveld Complex and their relationship to iron-rich intercumulus and residual liquids. Journal of Petrology, 35, 881–917.CrossRefGoogle Scholar
Tanaka, T. and Masuda, A. (1982) The La-Ce geochronometer: A new dating method. Nature, 300, 515–517.CrossRefGoogle Scholar
Taylor, S.R. and McLennan, S.M. (1985) The Continental Crust: its Composition and Evolution. Blackwell Scientific, Oxford, UK, 312 pp.Google Scholar
Viljoen, M.J. (1999) The nature and origin of the Merensky Reef of the western Bushveld Complex based on geological facies and geophysical data. South African Journal of Geology, 102, 221–239.Google Scholar
Viljoen, M.J. and Scoon, R.N. (1985) The distribution of main geologic features of the discordant bodies of iron-rich ultramafic pegmatite in the Bushveld Complex. Economic Geology, 80, 1109–1128.CrossRefGoogle Scholar
Viljoen, M., Theron, J. and Underwood, B. (1986 a) The Amandelbult section of Rustenburg Platinum Mines Limited with reference to the MerenskyReef. Pp. 1041–1060 in: Mineral Deposits of Southern Africa II (Anhauesser, C.R. and Maske, S., editors). Geological Societyof South Africa, Johannesburg.Google Scholar
Viljoen, M.J., De Klerk, W.J., Coetzer, P.M., Hatch, N.P., Kinloch, E.D. and Peyerl, W. (1986 b) The Union Section of Rustenburg Platinum Mines Limited, with reference to the Merensky Reef. Pp. 1061–1090 in: Mineral Deposits of Southern Africa II (Anhauesser, C.R. and Maske, S., editors). Geological Societyof South Africa, Johannesburg.Google Scholar
Viring, R.G. and Cowell, M.W. (1999) The Merensky Reef on Northam Platinum Limited. South African Journal of Geology, 102, 192–208.Google Scholar
Wagner, P.A. (1929) The Platinum Deposits and Mines of South Africa. Oliver and Boyd, Edinburgh and London, 588 pp.Google Scholar
Walraven, F. and Hattingh, P. (1993) Geochronologyof the Nebo granite, Bushveld Complex. South African Journal of Geology, 96, 31–41.Google Scholar
Walraven, F., Armstrong, R.A. and Kruger, F.J. (1990) A chronostratigraphic framework for the north-central Kaapvaal Craton, the Bushveld Complex and Vredefort structure. Tectonophysics, 171, 23–48.CrossRefGoogle Scholar
White, W.M. (1997) Geochemistry. Cornell University Press, New York, 560 pp.Google Scholar
Wilson, J.R., Cawthorn, R.G., Kruger, F.J. and Grundvig, S. (1994) Intrusive origin for the unconformable UZ in the Northern Gap, western Bushveld Complex. South African Journal of Geology, 97, 462–472.Google Scholar