The Cyclic Unit Beneath the UG1 Chromitite (UG1FW Unit) at RPM Union Section Platinum Mine—Rosetta Stone of the Bushveld Upper Critical Zone?

H. V. Eales, W. J. de Klerk, A. R. Butcher and F.J. Kruger
Department of Geology, Rhodes University, Grahamstown, South Africa
Bernard Price Institute, University of the Witwatersrand, Johannesburg, South Africa

Abstract: The UG1 Footwall unit is a layered pyroxenite-norite-leuconorite-anorthosite sequence between the Middle Group 4 and Upper Group 1 chromitites of the Upper Critical Zone, and is c. 300 m thick at Rustenburg Platinum Mines, Union Section, where it shows an oscillatory fluctuation in whole-rock Mg/(Mg + Fe), Cr/Co, Ni/V and Fe/Ti ratios with stratigraphic height. This permits subdivision into 8 sub-cycles which match a subdivision based on cyclical variations in orthopyroxene and feldspar compositions. Constituent pyroxene grains of pyroxenites, norites and leuconorites alike contain rounded and embayed plagioclase inclusions in abundance. Sr-isotope disequilibrium prevails in some samples between the orthopyroxene and feldspar populations. Chemical and isotopic data support a model of pulsatory injection of limited volumes of a more primitive, mafic liquid into a resident column of depleted residua, from which sodic labradorite and Mg-poor bronzite were crystallizing. The depleted liquid is equated with the supernatant liquid residuum of buried cumulates (Sri c. 0.7054) and the primitive liquid with magma parental to the UG1-UG2 lineage (Sri ⩾ 0.7068). The increase in leucocratic character of the 300 m column, with height, is attributed to the rising of low-density liquids enriched in the components of feldspar during separation of the pyroxenites. Deposition of the UG1 chromitite layers is attributed to mixing of a major influx of primitive liquid with a feldspathic residuum at the top of the UG1 Footwall unit. There is no evidence to indicate the participation of a discrete A-type liquid (Irvine and Sharpe, 1982) in this process.

Keywords: Bushveld complex • magma mixing • chromitite • compositional convection

Mineralogical Magazine; March 1990 v. 54; no. 374; p. 23-43; DOI: 10.1180/minmag.1990.054.374.03
© 1990, The Mineralogical Society
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