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Mineralium Deposita

, Volume 40, Issue 5, pp 550–575 | Cite as

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

  • Nicholas Arndt
  • George Jenner
  • Maryse Ohnenstetter
  • Etienne Deloule
  • Alan H. Wilson
Article

Abstract

Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.

Keywords

Bushveld Merensky reef Orthomagmatic contamination modelling 

Notes

Acknowledgements

We thank Chris Lee for providing the samples and for many useful discussions. M. Cheadle, R. Cawthorn and A. Boudreau reviewed an earlier version of the manuscript and we benefited from useful discussions with J. Bédard. C. Ballhaus and W. Maier reviewed this manuscript and provided many worthwhile comments and suggestions. The work was supported by EarthISE a bilateral program of the French Ministry of Education and the South African Research Council by the French CNRS through the Groupement de Recherche de Métallogénie and by an NSERC grant to GAJ.

References

  1. Ballhaus C, Sylvester P (2000) Noble metal enrichment processes in the Merensky Reef Bushveld Complex. J Petrol 41:545–561CrossRefGoogle Scholar
  2. Ballhouse C, Stumpfl EF (1986) Sulphide and platinum mineralization in the Merensky Reef: evidence from fluid inclusions. Contrib Mineral Petrol 94:193–204CrossRefGoogle Scholar
  3. Barnes SJ (1989) Are Bushveld U-type parent magmas boninites or contaminated komatiites? Contrib Mineral Petrol 101:447–457CrossRefGoogle Scholar
  4. Barnes S-J and Maier WD (2002) Platinum-group elements and microstructures of normal Merensky Reef from Impala Platinum Mines Bushveld Complex. J Petrol 43:103–128CrossRefGoogle Scholar
  5. Bédard JH (2001) Parental magmas of the Nain Plutonic Suite anorthosites and mafic cumulates: a trace-element modelling approach. Contrib Mineral Petrol 141:747–771CrossRefGoogle Scholar
  6. Boudreau AF and Meurer WP (1999) Concentration of platinum-group elements by magmatic fluids in layered intrusions. Econ Geol 87:1830–1848Google Scholar
  7. Campbell IH (1985) The difference between oceanic and continental tholeiites: a fluid dynamic explanation. Contrib Mineral Petrol 91:37–43CrossRefGoogle Scholar
  8. Campbell IH and Barnes S-J (1984) A model for the geochemistry of the platinum-group elements in magmatic sulphide deposits. Can Mineral 22 :151–160Google Scholar
  9. Campbell IH, Naldrett AJ, Barnes S-J (1983) A model for the origin of the platinum-rich sulfide horizons in the Bushveld and Stillwater complexes. J Petrol 24:133–165Google Scholar
  10. Carignan J, Hild P, Mevelle G, Morel J, Yeghicheyan D (2001) Routine analyses of trace elements in geological samples using flow injection and low pressure on-line liquid chromatography coupled to ICP-MS : a study of geochemical reference Materials BR DR-N UB-N AN-G and GH. Geostandards Newslett 25:187–198CrossRefGoogle Scholar
  11. Cawthorn RG (1996) Models for incompatible trace-element abundances in cumulus minerals and their application to plagioclase and pyroxenes in the Bushveld Complex. Contrib Mineral Petrol 123:109–115Google Scholar
  12. Cawthorn RG (2002) Delayed accumulation of plagioclase in the Bushveld Complex. Mineral Mag 66:881–893CrossRefGoogle Scholar
  13. Cawthorn RG, Davies GF (1983) Experimental data at 3 kbars pressure on parental magma to the Bushveld Complex. Contrib Mineral Petrol 83:128–135CrossRefGoogle Scholar
  14. Cawthorn RG, Spies L (2003) Plagioclase content of cyclic units in the Bushveld Complex South Africa. Contrib Mineral Petrol 145:47–60CrossRefGoogle Scholar
  15. Chabiron A, Alyoshin AP, Cuney M, Deloule E, Golubev VN, Velitchkin V, Poty B (2001) Geochemistry of the rhyolitic magmas from the Streltsovka caldera (Transbaikalia Russia): a melt inclusion study. Chem Geol 175:273–290CrossRefGoogle Scholar
  16. Eales HV (2002) Caveats in defining the magmas parental to the mafic rocks of the Bushveld Complex and the manner of their emplacement: review and commentary. Min Mag 66:815–832CrossRefGoogle Scholar
  17. Eales HV, Botha WJ, Hattingh PJ, de Klerk WJ, Maier WD, Odgers ATR (1993) The mafic rocks of the Bushveld Complex: a review of emplacement and crystallization history and mineralization in the light of recent data. J S Afr Earth Sci 16:55–73Google Scholar
  18. Eales HV, Cawthorn RG (1996) The Bushveld Complex. In: Cawthorn RG (ed) Layered intrusions. Elsevier, Amsterdam, pp 181–230Google Scholar
  19. Ghiorso MS, Sack RO (1995) Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid–solid equilibria in magmatic systems at elevated temperatures and pressures. Contrib Mineral Petrol 119:197–212Google Scholar
  20. Hamilton PJ (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. J Petrol 18:24–52Google Scholar
  21. Harmer RE, Sharp MR (1985) Field relations and strontium isotope systematics of marginal rocks of the eastern Bushveld Complex. Econ Geol 80:813–8378CrossRefGoogle Scholar
  22. Hart SR, Dunn T (1993) Experimental clinopyroxene/melt partitioning of 24 trace elements. Contrib Mineral Petrol 113:1–8CrossRefGoogle Scholar
  23. Hofmann AW (1988) Chemical differentiation of the Earth the relationship between mantle continental crust and oceanic crust. Earth Planet Sci Lett 90:297–314CrossRefGoogle Scholar
  24. Irvine TN (1977) Origin of chromite layers in the Muskox intrusion and other stratiform intrusions: a new interpretation. Geology 5:273–277CrossRefGoogle Scholar
  25. Irvine TN (1980) Magmatic infiltration metasomatism double-diffusive fractional crystallization and adcumulus growth in the Muskox intrusion and other layered intrusions. In: Hargraves RB (ed) Physics of magmatic processes. Princeton University Press, Princeton, pp 325–384Google Scholar
  26. Irvine TN, Sharpe MR (1986) Magma mixing and the origin of stratiform oxide ore layers in the Bushveld and Stillwater Complexes. In: Gallagher MJ, Ixer R, Neary CR, Pritchard HM (eds) Metallogeny of basic and ultrabasic rocks. Institution of Mining Metall, London, pp 183–198Google Scholar
  27. Jenner GA, Foley S, Jackson SE, Green T, Fryer BJ, Longerich H (1993) Determination of partition coefficients for trace elements in high pressure—high temperature experimental run products by laser ablation microprobe—inductively coupled plasma mass spectrometry (LAM-ICP-MS). Geochem Cosmochem Acta 58:5099–5103CrossRefGoogle Scholar
  28. Kruger FJ (1992) The origin of the Merensky cyclic unit: Sr-isotopic and mineralogical evidence for an alternative orthomagmatic model. Aust J Earth Sci 39:255–261CrossRefGoogle Scholar
  29. Kruger FJ (1994) The Sr-isotopic stratigraphy of the western Bushveld Complex. S Afr J Geol 97:393–398Google Scholar
  30. Lee CA (1996) A review of mineralization in the Bushveld Complex and some other layered mafic intrusions. In: Cawthorn RG (ed) Layered Intrusions. Elsevier, Amsterdam, pp 103–145Google Scholar
  31. Lee CA and Butcher AR (1990) Cyclicity in the Sr isotope stratigraphy through the Merensky and Bastard Reefs Atok Section eastern Bushveld Complex. Econ Geol 85:877–883Google Scholar
  32. Longhi J, Wooden JL, Coppinger KD (1983) The petrology of high-Mg dikes from the Beartooth Mountains Montana: a search for the parent magma of the Stillwater Complex. J Geophys Res Suppl 88:B53-B69Google Scholar
  33. Maier WD, Arndt NT, Curl EA (2000) Progressive crustal contamination of the Bushveld Complex: evidence from Nd isotopic analyses of the cumulate rocks. Contrib Mineral Petrol 140:316–327CrossRefGoogle Scholar
  34. Mathez EA (1995) Magmatic metasomatism and formation of the Merensky reef Bushveld Complex. Contrib Mineral Petrol 119:277–286Google Scholar
  35. Mathez EA, Hunter RH, Kinzler R (1997) Petrological evoution of partially melted cumulate: the Atok section of the Bushveld Complex. Contrib Mineral Petrol 129:20–34CrossRefGoogle Scholar
  36. Naldrett AJ, Cameron G, Von Gruenewald G, Sharpe MR (1987) The formation of stratiform PGE deposits in layered intrusions. In: I ParsonsI (ed) Origins of igneous layering. Reidel, Dordrecht, pp 313–398Google Scholar
  37. Naldrett AJ, Gasparrini EC, Barnes SJ, von Gruenewaldt G, Sharpe MR (1986) The Upper Critical Zone of the Bushveld Complex and the origin of Merensky-type ores. Econ Geol 81:1105–1117CrossRefGoogle Scholar
  38. Nicholson DM, Mathez EA (1991) Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex. Contrib Mineral Petrol 107:293–309CrossRefGoogle Scholar
  39. Ohnenstetter M, Arndt N, Lee CA (1998) Occurrence and compositional variation of HFSE- and LILE-bearing oxides in the Merensky Reef Rustenburg area. In: 8th International Platinum Symposium Johannesburg, South African Institute of Mining & Metallurgy, pp 297–300Google Scholar
  40. Schoenberg R, Kruger FJ, Nagler TF, Meisel T, Kramers JD (1999) PGE enrichment in chromitite layers and the Merensky Reef of the western Bushveld Complex: a Re–Os and Rb–Sr isotope study. Earth Planet Sci Lett 172: 49–64CrossRefGoogle Scholar
  41. Sharpe MR (1981) The chronology of magma influxes to the eastern compartment of the Bushveld Complex as exemplified by its marginal border group. J Geol Soc 138:307–326CrossRefGoogle Scholar
  42. Sharpe MR, Evensen NM, Naldrett AJ (1986) Sm/Nd and Rb/Sr evidence for liquid mixing magma generation and contamination in the Eastern Bushveld Complex. In: Geocongress Conference Abstract, University of the Witwatersrand, Johannesburg, pp 621–624Google Scholar
  43. Sparks RSJ (1986) The role of crustal contamination in magma evolution through geological time. Earth Planet Sci Lett 78:211–223CrossRefGoogle Scholar
  44. Tredoux M, Lindslay NM, Davies GF, McDonald I (1995) The fractionation of platinum-group elements in magmatic systems with the suggestion of a novel causal mechanism. S Afr J Geol 98:157–167Google Scholar
  45. Vermaak CF (1995) The Platinum Group Metals—a global perspective. Mintek, Randburg, South AfricaGoogle Scholar
  46. Wilson AH, Lee CA, Brown RT (1999) Geochemistry of the Merensky Reef Rustenburg Section Bushveld Complex; controls on the silicate framework and distribution of trace elements. Mineralium Deposita 34:657–672CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Nicholas Arndt
    • 1
  • George Jenner
    • 2
  • Maryse Ohnenstetter
    • 3
  • Etienne Deloule
    • 3
  • Alan H. Wilson
    • 4
  1. 1.Laboratoire de Géodynamique des Chaînes Alpines UMR 5025CNRS Université de GrenobleGrenoble CedexFrance
  2. 2.Department of Earth SciencesMemorial UniversitySt John’sCanada
  3. 3.Centre des Recherches Pétrographiques et GéochimiquesVandœuvre-les-Nancy CédexFrance
  4. 4.Department of Geology and Applied GeologyUniversity of NatalDurbanSouth Africa

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