Hostname: page-component-78c5997874-xbtfd Total loading time: 0 Render date: 2024-11-19T12:29:51.318Z Has data issue: false hasContentIssue false
  • English
  • Français

Unusual Fe-Ti-Cr spinels from discordant bodies of iron-rich ultramafic pegmatite at the Amandelbult Platinum mine, northwestern Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

R. N. Scoon  and
H. V. Eales
R. N. Scoon*
Affiliation:
PO Box 2461, Rivonia 2128, South Africa
H. V. Eales
Affiliation:
Department of Geology, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
*
*E-mail: [email protected]
Get access Rights & Permissions [Opens in a new window]

Abstract

Spinels associated with discordant bodies of iron-rich ultramafic pegmatite are described from the Amandelbult Platinum mine in the northwestern part of the Bushveld Complex. The spinels are divided into three groups, disseminated Ti-magnetite, disseminated Fe-Ti-Cr spinel and massive Fe-Ti-Cr spinel. The Fe-Ti-Cr spinels show a range of unusual compositions intermediate between chromite and Ti-magnetite. A relationship was found between stratigraphic height and spinel-type, with the Fe-Ti-Cr spinels restricted to pegmatites from the Upper Critical zone and Ti-magnetite to pegmatites from the Lower Main zone. Ilmenite is a ubiquitous component of all of the pegmatites examined here. The massive Fe-Ti-Cr oxide pegmatites are found only where earlier-formed chromitite layers are juxtaposed with sheet-like bodies of olivine-clinopyroxene pegmatite. A distinct thickening of the original chromitite layers in this situation, and compositional gradients within them, points to accretion of Fe-Ti-Cr spinels onto them prior to partial sub-solidus re-equilibration. Analytical data are presented for these spinels and for the Ti-magnetite.

The composition of the Fe-Ti-Cr spinels is not duplicated by cumulus spinels in the Bushveld Complex, but the compositions and microtextures of the disseminated Ti-magnetite are very similar to cumulus Ti-magnetite from the Upper zone. Accordingly, it is deduced that the Ti-magnetite in the pegmatites from the Lower Main zone, together with the ilmenite, crystallized at magmatic temperatures from a suitable Fe-Ti-rich silicate-oxide melt. No evidence has been found to link the pegmatites to hydrothermal fluids. The Cr-rich nature of the disseminated spinels in pegmatites from the Upper Critical zone suggests that the pegmatite melt was richer in chromium at this stratigraphic height, although re-equilibration with earlier-formed cumulus chromite also occurred. Formation of the Fe-Ti-Cr oxide pegmatites reflects a complex process that is incompletely understood and why new oxides plate onto pre-existing chromitite layers that are juxtaposed with Fe-rich ultramafic pegmatites is a matter of conjecture.

Keywords

spinelultramafic pegmatiteAmandelbult Platinum mineBushveld ComplexSouth Africa
Type
Research Article
Information
Mineralogical Magazine , Volume 66 , Issue 6 , December 2002 , pp. 857 - 879
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

Barnes, S.J. and Roeder, P. (2001) The range of spinel compositions in terrestrial mafic and ultramafic rocks. Journal of Petrology, 42, 2279–2302.CrossRefGoogle Scholar
Bateman, A.M. (1951) The formation of late magmatic oxide ores. Economic Geology, 46, 404–426.CrossRefGoogle Scholar
Bence, A.E. and Albee, A.L. (1964) Empirical correction factors for the electron microprobe analysis of silicates and oxides. Journal of Geology, 76, 382–403.CrossRefGoogle Scholar
Buddington, A.F. and Lindsley, D.H. (1964) Iron-titanium mineral s and synthetic equivalents. Journal of Petrology, 20, 327–354.Google Scholar
Cameron, E.N. (1971) Opaque minerals in certain lunar rocks from Apollo 12. Proceedings of the Second Lunar Science Conference, 1, 193–206.Google Scholar
Cameron, E.N. and Desborough, G.A. (1964) Origin of certain magnetite-bearing pegmatites in the eastern part of the Bushveld Complex, South Africa. Economic Geology, 59, 197–225.CrossRefGoogle Scholar
Cameron, E.N. and Glover, E.D. (1973) Unusual titanium-chromian spinels from the eastern Bushveld Complex. Economic Geology, 59, 197–225.CrossRefGoogle Scholar
Eales, H.V. (1979) Anomalous Karoo spinels along the chromite-titanomagnetite join. South African Journal of Science, 75, 24–29.Google Scholar
Eales, H.V. and Reynolds, I.M. (1986) Cryptic compositional variations within chromitites of the Upper Critical zone, northwe stern Bushveld Complex. Economic Geology, 81, 1056–1066.CrossRefGoogle Scholar
Eales, H.V., Reynolds, I.M. and Gouws, D.A. (1980) The spinel group minerals of the central Karoo tholeiitic province. Transactions of the Geological Society of South Africa, 83, 243–253.Google Scholar
Frankel, J.J. (1942) Chrome-bearing magnetic rocks from the eastern Bushveld. South African Journal of Science, 38, 152–157.Google Scholar
Haggerty, S.E. (1976) Opaque minerals in terrestrial igneous rocks. Pp. 101–300 in: Oxide minerals (Rumble, D., editor). MSA Short Course Notes 3. Mineralogical Society of America, Washington, D.C.Google Scholar
Irvine, T.N. (1967) Chromian spinel as a petrogenetic indicator. Part 2. Petrologic applications. Canadian Journal of Earth Science, 4, 71–103.CrossRefGoogle Scholar
Jones, J.P. (1976) Pegmatitic nodules in the layered rocks of the Bafokeng leasehold area. Transactions of the Geological Society of South Africa, 79, 312–320.Google Scholar
Norrish, H.R. and Hutton, J.T. (1969) An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochimica et Cosmochimica Acta, 33, 431–453.CrossRefGoogle Scholar
Reynolds, I.M. (1985) The nature and origin of titaniferous magnetite-rich layers in the Upper zone of the Bushveld complex: a review and synthesis. Economic Geology, 80, 1089–1108.CrossRefGoogle Scholar
Scoon, R.N. (1987) Metasomatism of cumulus magnesian olivine by iron-rich postcumulus liquids in the Upper Critical Zone of the Bushveld Complex. Mineralogical Magazine, 51, 389–396.CrossRefGoogle Scholar
Scoon, R.N. (1994) Comment on ‘Compositional convection in a reactive crystalline mush and melt differentiation’ by S. Tait and C. Jaupart. Journal of Geophysical Research, 99, 11, 913–11, 917.CrossRefGoogle Scholar
Scoon, R.N. and Mitchell, A.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
Spencer, K.J. and Lindsley, D.H. (1981) A solution model for co-exis ting iron-titanium oxides. American Mineralogist, 66, 1189–1201.Google Scholar
Stormer, J.C. (1983) The effects of recalculation on estimates of temperature and oxygen fugacity from analyses of multicomponent non-titanium oxides. American Mineralogist, 68, 586-94.Google Scholar
Stumpfl, E.F. and Rucklidge, J.C. (1982) The platiniferous dunite pipes of the eastern Bushveld. Economic Geology, 77, 1419–1431.CrossRefGoogle Scholar
Viljoen, M.J. and Scoon, R.N. (1985) The distribution and main geologic features of discordant bodies of iron-rich ultramafic pegmatite in the Bushveld Complex. Economic Geology, 80, 1109–1128.CrossRefGoogle Scholar
Viljoen, M.J., Theron, J., Underwood, B.M., Weaver, J. and Peyerl, W. (1986) The Amandelbult section of Rustenburg Platinum Mines, with reference to the Merensky reef. Pp. 1041–1060 in: Mineral deposits of southern Africa 2 (Anhaeusser, C.R. and Maske, S., editors). Geological Society of South Africa, Johannesburg.Google Scholar
Wager, L.R. and Brown, G.M. (1964) Layered Igneous Rocks. Oliver and Boyd, Edinburgh and London, 588 pp.Google Scholar
Wagner, P.A. (1929) The Platinum Deposits and Mines of Southern Africa. Oliver and Boyd, Edinburgh and London, 326 pp.Google Scholar
Wasserstein, B. (1936) Some notes on the Critical zone of the Bushveld gabbro at the Swartkop chrome mine in the Rustenburg district. Geological Society of South Africa Transactions, 39, 215–222.Google Scholar
Willemse, J. (1969 a) The geology of the Bushveld Igneous Complex, the largest repository of magmatic ore deposits in the World. Pp. 1–22 in: Magmatic Ore Deposits: A Symposium (Wilson, H.D.B., editor). Economic Geology Monograph 4. Economic Geology Publishing Co., Lancaster, PA, USA.Google Scholar
Willemse, J. (1969 b) The vanadiferous magmatic iron ore geology of the Bushveld Igneous Complex. Pp. 187–208 in: Magmatic Ore Deposits: A Symposium (Wilson, H.D.B., editor). Economic Geology Monograph 4. Economic Geology Publishing Co., Lancaster, PA, USA.Google Scholar