Contributions to Mineralogy and Petrology

, Volume 150, Issue 1, pp 119–130 | Cite as

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex, South Africa

  • Chusi LiEmail author
  • Edward M. Ripley
  • Arindam Sarkar
  • Dongbok Shin
  • Wolfgang D. Maier
Original Paper


About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe2+) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na2O (2.4 wt%) and H2O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ18O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ18O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.


Chromite Footwall Hangingwall Bushveld Complex Base Metal Sulfide 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



We thank Alan Boudreau and Edmond Mathez for their thoughtful reviews of the manuscript. Partial financial support for this study was provided through NSF grant EAR0335131 to EMR and CL, and through a Korean PDF fellowship to DS.


  1. Ballhaus CG, Stumpfl EF (1986) Sulfide and platinum mineralization in the Merensky Reef: evidence from hydrous silicates and fluid inclusions. Contrib Mineral Petrol 94:193–204CrossRefGoogle Scholar
  2. Barnes S-J, Maier WD (2002) Platinum-group elements and microstructures of normal Merensky Reef from Impala Platinum Mines, Bushveld Complex. J Petrol 43:103–128CrossRefGoogle Scholar
  3. Boudreau AE (1988) Investigation of the Stillwater Complex IV The role of volatiles in the petrogenesis of the J-M Reef, Minneapolis adit section. Can Mineral 26:193–208Google Scholar
  4. Boudreau AE, Mathez EA, McCallum IS (1986) Halogen geochemistry of the Stillwater and Bushveld Complexes: evidence for the transport of the platinum-group elements by Cl-rich fluids. J Petrol 27:967–986Google Scholar
  5. Cameron EN (1979) Titanium bearing oxide minerals of the critical zone of the Eastern Bushveld. Am Mineral 64:140–150Google Scholar
  6. Carman JH (1974) Synthetic sodium phlogopite and its two hydrates. Am Mineral 59:261–273Google Scholar
  7. Cawthorn RG, Merkle RKW, Viljoen MJ (2002) Platinum-group element deposits in the Bushveld Complex, South Africa. In: Cabri LJ (ed) The Geology, Geochemistry, Mineralogy, and Mineral Beneficiation of Platinum-group Elements. Canadian Institute of Mining, Metallurgy and Petroleum Special vol 54:389–429Google Scholar
  8. Clayton RN, Mayeda TK (1963) The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis. Geochim Cosmochim Acta 27:43–52CrossRefGoogle Scholar
  9. Deer WA, Howie RA, Zussman J (1992) An introduction to the rock-forming minerals 2nd edn. Longman, England, pp 696Google Scholar
  10. Eales HV, Cawthorn RG (1996) The Bushveld Complex. In: Cawthorn RG (ed) Layered Intrusions. Elsevier, Amsterdam, pp 181–229Google Scholar
  11. Eiler JM (2001) Oxygen isotope variations of basaltic lavas and upper mantle rocks. Rev Mineral 43:319–364Google Scholar
  12. Ford CE, Biggar GM, Humphries DJ, Wilson G, Dixon D, O’Hara MJ (1972) Role of water in the evolution of lunar crust: an experimental study of sample 14310; an indication of lunar calc-alkaline volcanism. In: Proceedings of 3rd Lunar Science conference, pp 207–229Google Scholar
  13. Gaetani GA, Grove TL, Bryan WB (1994) Experimental phase relations of basaltic andesite from hole 839B under hydrous and anhydrous conditions. Proc Ocean Drill Prog Sci Rep 135:557–563Google Scholar
  14. 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 temperature and pressure. Contrib Mineral Petrol 119:197–212Google Scholar
  15. Hall AL (1932) The Bushveld Complex in the central Transvaal. Geol Soc South Africa Memoir 28:544Google Scholar
  16. Harmer RE, Sharpe MR (1985) Field relations and strontium isotope systematic of the marginal rocks of the eastern Bushveld Complex. Econ Geol 80:813–837Google Scholar
  17. Harris C, Pronost JJ, Ashwal LD, Cawthorn RG, (2004) Oxygen and hydrogen isotope stratigraphy of the Rustenburg layered suite, Bushveld Complex: constraints on crustal contamination. J Petrol 46:579–601CrossRefGoogle Scholar
  18. Hiemstra SA (1986) The distribution of chalcophile and platinum-group elements in the UG2 chromitite layer of the Bushveld Complex. Econ Geol 81:1080–1086CrossRefGoogle Scholar
  19. Hulbert LJ, von Gruenewaldt G (1985) Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus. Econ Geol 80:872–895Google Scholar
  20. Irvine TN (1975) Crystallization sequences in the Muskox intrusion and other layered intrusions—II. Origin of chromitite layers and similar deposits of other magmatic ores. Geochim Cosmochim Acta 39:991–1020CrossRefGoogle Scholar
  21. Irvine TN (1977) Chromite crystallization in the join Mg2SiO4-CaMgSi2O6-CaAl2Si2O8-MgCr2O4-SiO2. Carnegie Institute of Washington Year Book 76:465–472Google Scholar
  22. 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, Prichard HM (eds) Metallogeny of basic and ultrabasic rocks. Institution of Mining and Metallurgy, London, pp 183–198Google Scholar
  23. Ito E, White WM, Gopel C (1987) The O, Sr, Nd, and Pb isotope geochemistry of MORB. Chem Geol 62:157–176CrossRefGoogle Scholar
  24. Kingston GA, El-Dosuky BT (1982) A contribution on the platinum-group mineralogy of the Merensky Reef at the Rustenburg Platinum Mine. Econ Geol 77:1367–1384Google Scholar
  25. Kroger FJ, Kinnaird JA, Nex PAM, Cawthorn GR (2002) Chromite is the key to PGE. In: 9th International Platinum Symposium. Abstract and Program, 21–25 July 2002, Billings, Montana, pp 211–213Google Scholar
  26. Kruger FJ, Marsh JS (1982) The significance of 87 Sr/86 Sr ratios in the Merensky cyclic unit of the Bushveld Complex. Nature 298:53–55CrossRefGoogle Scholar
  27. Lee CA (1983) Trace and platinum-group element geochemistry and the development of the Merensky unit of the western Bushveld Complex. Miner Deposita 18:173–190CrossRefGoogle Scholar
  28. Leeb-du Toit A (1986) The Impala Platinum mines. In: Anhaeusser CR, Maske S (eds) Mineral Deposits of South Africa. Geol Soc South Africa, Johannesburg, pp 1091–1106Google Scholar
  29. Legg CA (1969) Some chromite-ilmenite association in the Merensky Reef, Transvaal. Am Mineral 54:1347–1354Google Scholar
  30. Li C, Ripley EM, Merino M, Maier WD (2004) Replacement of base metal sulfides by actinolite, epidote, calcite and magnetite in the UG2 and Merensky Reef of the Bushveld Complex, South Africa. Econ Geol 99:173–184CrossRefGoogle Scholar
  31. Lorand JP, Ceuleneen (1989) Silicate and base-metal sulfide inclusions in chromites from the Maqsad area (Oman ophiolite, Gulf of Oman): a model for entrapment. Lithos 22:173–190CrossRefGoogle Scholar
  32. Lorand JP, Cottin JY (1987) Na-Ti-Zr-H2O-rich mineral inclusions indicating postcumulus chrome-spinel dissolution and recrystallization in the Western Laouni mafic intrusion, Algeria. Contrib Mineral Petrol 97:251–263CrossRefGoogle Scholar
  33. Luth WC (1967) Studies in the system KAlSiO4-Mg2SiO4-SiO2-H2O, I Inferred phase relations and petrologic implications. J Petrol 8:372–416Google Scholar
  34. MacNeil AM, Edgar AD (1987) Sodium-rich metasomatism in the upper mantle: implications of experiments on the pyrolite-Na2O-rich fluid system at 950°C, 20 kbar. Geochim Cosmochim Acta 51:2285–2294CrossRefGoogle Scholar
  35. Maier WD, Barnes S-J (1998) Concentrations of rare earth elements in silicate rocks of the Lower, Critical and Main Zones of the Bushveld Complex. Chem Geol 150:85–103CrossRefGoogle Scholar
  36. Maier WD, Arndt NT, Curl E (2000) Progressive crustal contamination of the Bushveld Complex: evidence from Nd isotopic analyses of the cumulus rocks. Contrib Mineral Petrol 140:328–343CrossRefGoogle Scholar
  37. Mathez EA (1995) Magmatic metasomatism and formation of the Merensky reef, Bushveld Complex. Contrib Mineral Petrol 119:227–286Google Scholar
  38. Mathez EA, Webster JD (2004) Partitioning behavior of chlorine and flurine in the system apatitite-silicate melt-fluid. Geochim Cosmochim Acta 69:1275–1286CrossRefGoogle Scholar
  39. Mathez EA, Agrinier P, Hutchinson R (1994) Hydrogen isotopic composition of the Merensky Reef and related rocks, Atok section, Bushveld Cmplex. Econ Geol 89:791–802Google Scholar
  40. Mathez EA, Hunter RH, Kinzler R (1997) Petrologic evolution of partially molten cumulate: the Atok section of the Bushveld Complex. Contrib Mineral Petrol 129:20–40CrossRefGoogle Scholar
  41. McDonald JA (1965) Liquid immiscibility as a factor in chromite seam formation in the Bushveld igneous complex. Econ Geol 60:1674–1685Google Scholar
  42. Mostert AB, Hofmeyr PK, Potgieter GA (1982) The platinum-group mineralogy of the Merensky Reef at the Impala Platinum Mines, Bophuthatswana. Econ Geol 77:1385–1394CrossRefGoogle Scholar
  43. 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
  44. Prichard HM, Barnes S-J, Maier WD, Fisher PC (2004) Variations of in the nature of platinum-group minerals in a cross section through the Merensky Reef at Impala Platinum: implications for the mode of formation of the Reef. Can Mineral 42:423–437CrossRefGoogle Scholar
  45. Prior DJ, Boyle AP, Brenker F, Cheadle MC, Day A, Lopez G, Peruzzo L, Potts GJ, Reddy S, Spiess R, Timms NE, Trimby P, Wheeler J, Zetterström L (1999) The application of electron backscatter diffraction and orientation imaging in the SEM to textural problems in rocks. Am Mineral 84:1741–1759Google Scholar
  46. Rosenbaum JM, Kyser TK, Walker D (1994) High temperature oxygen isotope fractionation in the enstatite-olivine-BaCO3 system. Geochim Cosmochim Acta 58:2653–2660CrossRefGoogle Scholar
  47. Schiffries CM, Rye DM (1989) Stable isotopic systematics of the Bushveld Complex: I. Constraints of magmatic processes in layered intrusions. Am J Sci 289:841–873CrossRefGoogle Scholar
  48. Sharp ZD, Atudorei V, Durakiewiez T (2001) A rapid method for determination of hydrogen and oxygen isotope ratio from water and hydrous minerals. Chem Geol 173:197–210CrossRefGoogle Scholar
  49. Sharpe MR (1981) The chronology of magma influxes to the eastern compartment of the Bushveld Complex as exemplified by its marginal border groups. J Geol Soc London 138:307–326CrossRefGoogle Scholar
  50. Sisson TW, Grove TL (1993) Experimental investigations of the role of H2O in cal-alkaline differentiation and subduction zone magmatism. Contrib Mineral Petrol 113:143–166CrossRefGoogle Scholar
  51. Stumpfl EF, Rucklidge JC (1982) The platiniferous dunite pipes of the eastern Bushveld. Econ Geol 77:1419–1431Google Scholar
  52. Valley JW (2003) Oxygen isotopes in zircon. In: Hanchar JM, Hoskin PWO (eds) Zircon. Rev Mineral Geochem 53:343–385Google Scholar
  53. Viljoen MJ, Hieber R (1986) The Rostenburg Section of Rustenburg Platinum Mines Limited, with reference to the Merensky Reef. In: Anhaeusser CR, Maske S (eds) Mineral deposits of South Africa. Geological Society of South Africa, Johannesburg, pp 1107–1134Google Scholar
  54. Willmore CC, Boudreau AE, Kruger FJ (2000) The halogen geochemistry of the Bushveld Complex: Republic of South Africa: implications for chalcophile element distribution in the lower and critical zones. J Petrol 41:1517–1539Google Scholar
  55. Zheng Y-F (1991) Calculation of oxygen isotope fractionation in metal oxides. Geochim Cosmochim Acta 55:2299–2307CrossRefGoogle Scholar
  56. Zheng Y-F (1993) Calculation of oxygen isotope fractionation in anhydrous silicate minerals. Geochim Cosmochim Acta 57:1079–1091CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Chusi Li
    • 1
    Email author
  • Edward M. Ripley
    • 1
  • Arindam Sarkar
    • 1
  • Dongbok Shin
    • 2
  • Wolfgang D. Maier
    • 3
  1. 1.Department of Geological SciencesIndiana UniversityBloomingtonUSA
  2. 2.Korea Institute of Geoscience and Mineral ResourcesDaejeonSouth Korea
  3. 3.Sciences de la TerreUniversité du Québec à ChicoutimiChicoutimiCanada

Personalised recommendations