The transition between the main and the upper zone of the bushveld complex, in the Western Transvaal

Abstract

The concept which incorporates the term "unit" is suggested for the grouping of the mafic rocks of the Bushveld Igneous Complex. Each unit has definite boundaries. The upper surface of an anorthosite band forms such a boundary in two instances. The contact between the Main and the Upper Zone of the Bushveld Complex is interpreted as representing a hiatus in crystal deposition from the magma. A magma of slightly different composition is required for each unit. One complete and two incomplete units are described, one following the other. They are designated in order: "The Bon Accord Unit" which represents the uppermost portion of the Main Zone, "Unit 111 which forms the lowermost unit of the Upper Zone, and unit 2" which follows on Unit 1. Two rock types belonging to the lowest unit exhibit a most extraordinary texture, which has been termed the texture of the Bon Accord Hyperite. It consists of plagioclase as ctm1ulate and interstitial orthopyroxene crystals. The latter are in groups orientated in the same way. Seven distinct rock types comprise Unit 1 of the Upper Zone and it is 300 feet thick. This unit, in contrast to the Bon Accord Unit which is free from solid phases of iron oxide, contains 11.6% oxide minerals by volume calculated over its total thickness. The lower portion of Unit 2 of the Upper Zone consists of three rock types, which have an average content of 19.3% iron oxides. Two major and numerous minor magnetitite bands are interbedded in Units 1 and 2. Crystal settling from the magma played a dominant role in regard to the petrogenic history of the various rock types. Only one rock type, the magnetite troctolite of Unit 1 of the Upper Zone, is considered to have originated as a result of an increase in confining pressure. "Fresh" and weathered magnetite specimens from the same magnetitite band consist of different minerals. The microscope reveals "fresh" unweathered magnetite ore as consisting of magnetite, ilmenite and some minor accessories. The texture of this rock, which has originated through exsolution, consists of interlocking anhedral grains of magnetite and ilmenite. Weathered surface samples of magnetite ore consist predominently of maghemite, ilmenite, an unknown reddishbrown mineral and some minor accessories. The reddishbrown mineral is tentatively regarded as a further alteration product of maghemite and has not been positively identified. Phenomena, depending on the bireflectance of "fresh" and weathered magnetite ore, are classified into five patterns and each pattern is fully described.