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The cyclic unit beneath the UG1 chromitite (UGIFW unit) at RPM Union Section Platinum Mine—Rosetta Stone of the Bushveld Upper Critical Zone?

Published online by Cambridge University Press:  05 July 2018

H. V. Eales ,
W. J. de Klerk  and
A. R. Butcher
H. V. Eales
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
W. J. de Klerk
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
A. R. Butcher
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
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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 (Sric. 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 complexmagma mixingchromititecompositional convection
Type
Geochemistry and Petrology
Information
Mineralogical Magazine , Volume 54 , Issue 374 , March 1990 , pp. 23 - 43
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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