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Re-evaluation of magma compositions and processes in the uppermost Critical Zone of the Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

R. Grant Cawthorn
R. Grant Cawthorn*
Affiliation:
Department of Geology, University of the Witwatersrand, Wits, 2050, Republic of South Africa
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Abstract

A detailed geochemical study is presented of the uppermost Critical Zone, especially of the footwall and hanging wall to the Merensky Reef, at Impala Platinum Mines in the Bushveld Complex. The approximately 100 m-thick sequence below the Merensky Reef consists of 13 distinct layers which have sharp boundaries. They are adcumulates with varying proportions of cumulus plagioclase, orthopyroxene and chromite.

Experimental studies on the composition of coexisting orthopyroxene liquid indicate that the magma which produced this sequence contained between 4 and 6% MgO. The magma from which the Merensky Reef formed was more evolved than the footwall magma.

Significant variations exist for both the En content of orthopyroxene and mg# number of whole-rock analyses in short vertical sections. Pyroxenite and norite always have higher values than anorthosite. Extremely sharp breaks in these values correlate with changes in modal proportions, and argue against both significant fractionation within the studied interval, and infiltration metasomatism. Quantitative modelling shows that the entire footwall section could have contained pyroxene with a uniform primary composition of En82, and that all the variation now observed reflects the effect of reaction with trapped magma.

Two independent methods for determining the proportion of trapped liquid are presented, based on mg# number and incompatible element abundances. Both yield a uniform proportion in all samples of approximately 10%. Immiscible sulphide liquid from the Merensky Reef can be shown to have infiltrated downwards for <5 m, despite its high density contrast with silicate magma, very low viscosity and low crystallization temperature. Residual silicate magma would have had even more restricted mobility. The migration of residual liquid or fluid through pothole structures in the floor of the Merensky Reef is not supported by the present data.

Keywords

Bushveld Complexcyclic unitsfractionationtrapped liquid
Type
The 1995 Hallimond Lecture
Information
Mineralogical Magazine , Volume 60 , Issue 398 , February 1996 , pp. 131 - 148
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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References

Barnes, S.J. (1986a) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contrib. Mineral. Petrol., 93, 524–31.CrossRefGoogle Scholar
Barnes, S.J. (1986b)The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure. Geochim. Cosmochim. Acta, 50, 1889–909.CrossRefGoogle Scholar
Boudreau, A.E., (1992) Volatile fluid overpressure in layered intrusions and the formation of potholes. Austral. J. Earth Sci., 39, 277–87.CrossRefGoogle Scholar
Campbell, I.H. (1986) A fluid dynamic model for the potholes of the Merensky Reef. Econ. GeoL, 81, 1118–25.CrossRefGoogle Scholar
Cawthorn, R.G. and Biggar, G.M. (1993) Crystallization of titaniferous chromite, magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province. Contrib. Mineral. Petrol., 114, 221–35.CrossRefGoogle Scholar
Cawthorn, R.G. and Davies, G. (1983) Experimental data at 3 kbars pressure on parental magma to the Bushveld Complex. Contrib. Mineral Petrol., 52, 81–9.Google Scholar
Cawthorn, R.G. and Poulton, K.L. (1988) Evidence for fluid in the footwall beneath potholes in the Merensky Reef of the Bushveld Complex. In Geoplatinum 87(Prichard, H.M., Potts, PJ., Bowles, J.F.W. and Cribb, S.J., eds.) Elsevier, London, 343–56.CrossRefGoogle Scholar
Eales, H.V., Marsh, J.S., Mitchell, A.A., deKlerk, W. J., Kruger, F.J. and Field, M. (1986) Some geochemical constraints upon models for the crystallization of the upper Critical Zone — Main Zone interval, northwestern Bushveld Complex. Mineral. Mag., 50, 567–82.CrossRefGoogle Scholar
Eales, H.V., Field, M., de Klerk, W.J. and Scoon, R. (1988) Regional trends of chemical variation and thermal erosion in the upper Critical Zone, western Bushveld Complex. Mineral. Mag., 50, 63–79.CrossRefGoogle Scholar
Eales, H.V., Botha, W.J., Hattingh, P.J., De Klerk, W.J., Maier, W.D. and Odgers, A.T. (1993a) The mafic rocks of the Bushveld Complex: a review of emplacement and crystallization history, and mineralization, in the light of new data. J. Afr. Earth Sci., 16, 121–42.CrossRefGoogle Scholar
Eales, H.V., Teigler, B. and Maier, W.D. (\993b)Cryptic variations of minor elements Al, Cr, and Mn in Lower and Critical Zone orthopyroxenes of the western Bushveld Complex. Mineral. Mag., 57, 257–64.Google Scholar
Fleet, M.E., Chryssoulis, S.L., Stone, W.E. and Weisener, C.G. (1993) Partitioning of platinum- group elements and Au in the Fe-Ni-Cu-S system: experiments on the fractional crystallization of sulphide melt. Contrib. Mineral. Petrol., 115, 36–44.CrossRefGoogle Scholar
Harmer, J. and Sharpe, M.R. (1985) Field relations and strontium isotope systematics of the marginal rocks of the Eastern Bushveld Complex. Econ. GeoL, 80, 813–37.CrossRefGoogle Scholar
Irvine, T.N. (1980) Magmatic infiltration metasomatism, double-diffusive fractional crystallization, and ad- cumulus growth in the Muskox intrusion and other layered intrusions. In Physics of Magmatic Processes.(Hargraves, R.B., ed.). Princeton University Press, Princeton, New Jersey. 326—83.Google Scholar
Irvine, T.N., Keith, D.W. and Todd, S.G. (1983) The J-M platinum-palladium reef of the Stillwater Complex, Montana. II. Origin by double-diffusive convective magma mixing and implications for the Bushveld Complex. Econ. GeoL, 78, 1287–334.CrossRefGoogle Scholar
Kruger, F.J. (1992) The origin of the Merensky Reef cyclic unit: Sr-isotopic and mineralogical evidence for an alternative orthomagmatic model. Austral. J. Earth Sci., 39, 255–61.CrossRefGoogle Scholar
Kruger, F.J. and Marsh, J.S. (1982) Significance of 87Sr/86Sr ratios in the Merensky Cyclic Unit of the Bushveld Complex. Nature, 298, 53–5.CrossRefGoogle Scholar
Leeb-du Toit, A. (1986) The Impala platinum mines. In Mineral Deposits of Southern Africa, vol. 2(Anhaeusser, C.R. and Maske, S., eds.) Geol. Soc. S. Afr., Johannesburg, 1091—106.Google Scholar
McKenzie, D.P. (1984) The generation and compaction of partially molten rock. J. Petrol., 25, 713–65.CrossRefGoogle Scholar
Naldrett, A.J., Gasparrini, E.C., Barnes, S.J., von Gruenewaldt, G. and Sharpe, M.R. (1986) The upper Critical Zone of the Bushveld Complex and a model for the origin of Merensky-type ores. Econ. Geol, 81, 1105–18.CrossRefGoogle Scholar
Reid, D.L., Cawthorn, R.G., Kruger, F.J. and Tredoux, M. (1993) Isotope and trace element patterns below the Merensky Reef, Bushveld Complex, South Africa: evidence for fluids? Chem. Geoi, 106, 171–86.CrossRefGoogle Scholar
Robins, B. (1982) Finger structures in the Lille Kufjord layered intrusion, Finmark, northern Norway. Contrib. Mineral. Petrol, 81, 290–5.CrossRefGoogle Scholar
Schurmann, L.W. (1993) The geochemistry and petrology of the upper Critical Zone of the Boshoek section of the western Bushveld Complex. Geol. Surv. S. Afr. Bull., 113, 88 pp.Google Scholar
Tait, S.R. and Jaupart, C. (1992) Compositional convection in a reactive crystalline mush and melt differentiation. J. Geophys. Res., 97, 6735–56.CrossRefGoogle Scholar
Tait, S.R., Huppert, H.E. and Sparks, R.S.J. (1984) The role of compositional convection in the formation of adcumulate rocks. Lithos, 17, 139–46.CrossRefGoogle Scholar
Wager, L.R., Brown, G.M. and Wadsworth, W.J. (1960) Types of igneous cumulates. J. Petrol., 1, 73–85.CrossRefGoogle Scholar
Walker, D., Jurewitz, S. and Watson, E.B. (1988) Adcumulus growth in a laboratory thermal gradient. Contrib. Mineral Petrol., 99, 306–19.CrossRefGoogle Scholar