The geology of an area northwest of Pretoria

Abstract

The conclusion is upheld that the two zones of quartzite xenoliths in the area represent detached fragments of the Magaliesberg horizon separated from the floor of the Bushveld Igneous Complex by the intrusion of the latter. Rocks which occupy a similar stratigraphical position in the Bushveld Igneous Complex to the leptites from the present area, have been described from elsewhere by Vermaas (1949, p. 60) and others. In most cases these rocks are described as being undoubtedly of sedimentary origin, as remnants of the original bedding planes serve to indicate. The writer could find no evidence of any sedimentary structures in the leptites from the area under discussion. These sediments are nevertheless considered to have originated from argillaceous rather than arenaceous sediments. Only part of the normal zonal succession of the gabbroidal rocks of the Bushveld Igneous Complex is present in the area surveyed, namely the Critical, Main and Upper Zones. The Critical Zone which is represented by various types of layered rocks, exhibits a chill phase at its base and is noticeably thinner than is normally the case. This zone, furthermore, does not contain either the Chromitite Horizons nor the Merensky Reef in this area. The Main Zone is characterized by a uniformity of rock-types except near the top of the pseudostratigraphical succession where spotted norite and mottled anorthosite make their appearance. Three main bands of magnetic iron ore are present in the Upper Zone, namely the Upper, Middle and Lower Bands. This zone also exhibits a granitic phase in its uppermost horizons. Field evidence indicates that the dunite occurrences, which are all situated stratigraphically in the lower part of the Main Zone below the Pyramid Hills Horizon, are transgressive structures. They are also considered to represent a final, pegmatitic phase in the Bushveld Ignoous activity. Relatively small bodies of gossan and of iron ore indicated on the map, arc associated with several of the dunitos. The suggestion is advanced that the Gro.nophyro attained its present pnsition by intrusion. A similar mode of emplacement, namely that of intrusion, is envisaged for the rod BushvoldGranito in the area, which forms an isolated occurrence surrounded on all sides by gabbro, on the horizon of the Louer Iron Ore Band. In conclusion, evidence is given of contemporaneous deformation within the Ecca Grit.