Stewart R. Wallace — 1919–2009 - Society of Economic Geologists

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18 SEG NEWSLETTER No 78 JULY 2009 ... from 17 (Mitchell and Scoon, 2007), an important consideration if, as suggested by Eales et al. (1986), the lowermost part of the Main zone was emplaced after the reef. A similar hypothesis is applicable to the Platreef: U-type magma intruded earlier-formed gabbronorite and websterite, forming mineralized harzburgite and pegmatoid, which in turn was emplaced prior to the Main zone. Chromitite layers formed by mixing of U-type and more evolved magma (Atype), a hypothesis presented by Sharpe and Irvine (1983), albeit we have suggested the latter, at least in the Upper Critical zone was derived by partial melting of plagioclase cumulates under the influence of the new influxes of Utype magma. The spectacular grade of some chromitite stringers, as well as PGE-bearing chromitites with low sulfide contents, suggests two processes— nucleation of PGM triggered by crystallization of copious amounts of chromite and S-saturation—were necessary to generate economic deposits. U-type magmas also intruded the complex in vertical conduits (Scoon and Mitchell, 2004b). Partial melting of earlierformed reefs, triggered by heat associated with the magnesian dunite, generated a PGE- and iron-rich melt which drained downward into the cores of specific pipes, a hypothesis supported by similarities with so-called replaced Merensky reef (reef invaded by discordant iron-rich ultramafic pegmatite: Scoon and Mitchell, 2004a). In summary, we find that hydrothermal fluids are of little importance in formation of PGE deposits in the Bushveld, as discussed by Barnes and Campbell (1983), and recognition of orthomagmatic processes is fundamental. The uniquely layered Upper Critical zone, containing the world’s premier PGE ores, resulted from episodic replenishment by U-type magmas that persisted long after the onset of the crystallization of noriteanorthosite. ACKNOWLEDGMENTS Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued) We acknowledge reviews by Wolfgang Maier and an anonymous reviewer, and are particularly grateful to the former for his constructive comments. We pay tribute to Hugh Eales for his mentorship early on in our careers, and for his ongoing interest in our research. REFERENCES Barnes, Stephen J., and Campbell, I.H., 1983, Role of late magmatic fluids in Merenskytype platinum deposits: Geology, v. 16, p. 488–491. Bettel, W., 1925, Occurrences of platinum in the Transvaal: South African Mining and Engineering Journal, no. 36 (March 14), p. 35. 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JULY 2009 No 78 SEG NEWSLETTER 19 Scoon, R.N., and Mitchell, A.A., 2004a, The platiniferous dunite pipes in the eastern limb of the Bushveld Complex: review and comparison with unmineralized discordant ultramafic bodies: South African Journal of Geology, v. 107, p. 505–520. ——2004b, Petrogenesis of discordant magnesian dunites from the central sector of the Eastern Bushveld Complex with em - phasis on the Winnaarshoek pipe and disruption of the Merensky Reef: Economic Geology, v. 99, p. 517-541. Scoon, R.N., and Teigler, B., 1994, Platinumgroup element mineralization in the Criti - cal zone of the western Bushveld Complex: I. Sulfide-poor chromitites below the UG-2: Economic Geology, v. 89, p. 1094–1121. Sharpe, M.R., 1981, The chronology of magma influxes to the eastern compartment of the Bushveld Complex as exemplified by its marginal border groups: Geologi - cal Society of London Journal, v. 138, p. 307–326. Sharpe, M.R., and Irvine, T.N., 1983, Melting relations of two Bushveld chilled margin rocks and implications for the origin of chromitite: Carnegie Institution of Wash - ington Yearbook, v. 82, p. 295–300. South African Committee for Stratigraphy, 1980, Stratigraphy of South Africa, Part I, in Kent, L.E., comp: Geological Survey of South Africa Handbook 8, 690 p. Tankard, A.J., Jackson, M.P.A., Eriksson, K.A., Hobday, D.K., Hunter, D.A., and Minter, W.E.L., 1982, Crustal evolution of PAID ADVERTISEMENT Southern Africa, 3.8 billion years of earth history: New York, Heidelberg, Berlin, Springer Verlag, 523 p. Uken, R., and Watkeys, M.K., 1997, Diapirism initiated by the Bushveld Complex, South Africa: Geology, v. 25, p. 723–726. Vermaak, C.F., 1985, The UG2-layer—South Africa’s slumbering chromitite giant: Chromium Review, v. 5, p. 9–22. Viljoen, M.J., and Hieber, R., 1986, The Rusten - burg Section of RPM Ltd, with reference to the Merensky reef, in Anhaeusser, C.R., and Maske, S., eds., Mineral deposits of South - ern Africa: Johannesburg, Geological Society of South Africa, v. 2, p. 1107–1134. Viljoen, M.J., and Schurmann, L.W., 1998, Platinum-group metals, in Wilson, M.G.C., and Anhaeusser, C.R., eds., The mineral resources of South Africa: Handbook, Council for Geoscience, v. 16, p. 532–568. 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, v. 80, p. 1109–1128. Viljoen, M.J., De Klerk, W.J., Coetzer, P.M., Hatch, N.P., Kinloch, E. and Peyerl, W., 1986a, The Union Section of RPM Ltd with reference to the Merensky reef, in An - haeusser, C.R., and Maske, S., eds., Mineral deposits of Southern Africa: Johannesburg, Geological Society of South Africa, v. 2, p. 1061–1090. Viljoen, M.J., Theron, J., Underwood, B., Walters, B.M., Weaver, J. and Peyerl, W., 1986b, The Amandelbult Section of RPM Ltd, with reference to the Merensky reef, in Anhaeusser, C.R., and Maske, S., eds., Mineral deposits of Southern Africa: Johannesburg, Geological Society of South Africa, v. 2, p. 1041–1060. Von Gruenewaldt, G., Sharpe, M.R., and Hatton, C.J., 1985, The Bushveld Complex: Introduction and review: Economic Geology, v. 80, p. 803–812. Viring, R.G., and Cowell, M.W., 1999, The Merensky reef on Northam Platinum Mine Limited: South African Journal of Geology, v. 102, p. 192–208. Wager, L.R., and Brown, G.M., 1968, Layered igneous rocks: Edinburgh, Oliver and Boyd, 588 p. Wagner, P.A., 1929, The platinum deposits and mines of South Africa: Edinburgh, Oliver and Boyd, 326 p. White, J.A., 1994, The Potgietersrus prospect —geology and exploration history: Council of Mining and Mineral Institute, XVth CMMI Congress, Johannesburg, South African Institute of Mining and Metallurgy, Proceedings, v. 3, p. 173–181. Willemse, J., 1969, The geology of the Bush - veld Igneous Complex, the largest repository of magmatic ore deposits in the world: Economic Geology Monograph Series, v. 4, p. 1–22. Wilson, M.G.C., and Anhaeusser, C.R., 1988, The Mineral resources of South Africa: Handbook, Council for Geoscience, v. 16, 740 p. 1 PAID ADVERTISEMENT
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