The geology of the Bushveld complex on the Sekhukhune Plateau, Eastern Transvaal

Abstract

This study comprises a general geological investigation of the rocks from an area 1 200 square km in size, situated about 50 km'north-east of Marble Hall. The area lies upon the Sekhukhune Plateau and is dominated by acid rocks of the Bushveld Complex. The oldest rocks in the area include hornfels and quartzite of the Pretoria Group which are present in the form of xenoliths in, the rocks of the Bushveld Complex, and also as part of the f uptiomed floor of the Complex in the Malope area. The quartzite which occurs in the form of xenoliths has been locally transformed to a pseudogranophyre by a process of feldspathtzation. Three suites of Bushveld rocks are present within the area investigated, namely the Rustenburg Layered Suite, the Rashoop Granophyre Suite and the Lebowa Granite Suite. The first out crop at the base of the Sekhukhune Escarpment which forms the eastern boundary to the area. They have also been discovered on the plateau around Malope to the south-east of the 0lifants River, where they appear to overlie the updomed floor of the Bushveld Complex. The rocks of the layered sequence exposed east of the Sekhukhune Escarpment belong to the Upper Zone, whilst the majority of those around Malope are typical of the Critical Zone, although rocks which typify the Upper Zone are also present. The latter seem to transgressively overlie the rocks of the Critical Zone. The Rashoop Granophyre Suite consists of granophyre and a fine-grained granophyric granite. These rocks are separated from the basic rocks of the layered sequence below by a thin layer of leptite which is considered to be metamorphosed Rooiberg felsite. The Lebowa Granite Suite consists of an older Nebo Granite and a younger Makhutso Granite. Four facies of the Nebo Granite have been recognized, viz. main, contact, granophyric and aplitic facies, and their textural, mineralogical and chemical differences are discussed. The Makhutso Granite also consists of four facies, namely the main facies, the contact facies and the aplitic facies, as well as the Koornkopje granite which is considered to be a variety of the Makhutso Granite. The Makhutso Granite which was previously only known to exist in the Dennilton area, has been found at seventeen separate localities on the Sekhukhune Plateau, and the intrusive relation= ships with the Nebo Granite are described. The Makhutso Granite differs from the Nebo Granite in its higher biotite content and its higher anorthite content of the plagioclase, and also in its trace element composition. It also differs from the main facies of the Nebo Granite in its abundance of zoned plagioclase phenocrysts. The Koornkopje granite differs from the main fadies of the Makhutso Granite in its persistent fine-grained nature, and in its lower biotite content and the lower anorthite content of the plagioclase.