The metamorphosed sediments of the Pretoria group and the associated rocks northwest of Zeerust, Western Transvaal

Abstract

Part of the Transvaal Supergroup, viz. the upper formations of the Chuniespoort Group and the Pretoria Group between Zeerust and the border with Botswana, was mapped. The metamorphic aureole of the Bushveld Complex as developed in the Transvaal Supergroup in this area decreases in metamorphic grade both along and across the strike as a result of an apparent reduction in the thickness of the mafic rocks towards the west. The presence of the metamorphic index minerals tremolite, anthophyllite, cummingtonite, cordierite and almandine indicate rocks of the hornblende-hornfels facies in the east. West of long. 25°53'E (which transverses the strata), the metamorphic assemblages are typical of the albite-epidote-hornfels facies and contain minerals such as talc and epidote. Total fluid pressure in the mapped area is unlikely to have exceeded 4 kb. Temperatures are estimated to have been between 300 and 5oo 0 c for the albiteepidote- hornfels facies and between 500 hornblende-hornfels facies. 0 and 600 C for the Few of the complex structures present in adjoining Botswana enter South Africa. A low-angle strike fault zone is an exception, however, and enters South Africa to cut out a large part of the Malmani Subgroup. The port Bushveld linear structures, which are predominant in most-. of the mapped area as well as in a large part of the Kaapvaal province, viz. the NNW faults and the ENE photo-lineation’s, were formed under tensional conditions. EW faults are rare. Several lateral lithofacies changes in the sedimentary layers of the area indicate a higher energy environment in the west. This, together with an increase in the amount and size of detrital chert near Botswana, suggests a nearby original margin of the Transvaal basin in Botswana. Uplift and erosion of the Chuniespoort Group after its deposition did not remove the Penge Formation, with the result that the basal layer of the Pretoria Group is a chert conglomerate and not a residual chert breccia as elsewhere in the Transvaal basin. The sequence of the Rooihoogte Formation at Skilpadhek,as indicated by the cores of four boreholes, represents a delta partly destroyed by strong currents and superseded by linear elastic shore deposits. Lava of the Hekpoort Andesite Formation was extruded subaerially and has an upper, metamorphosed, residual paleosoil cover. Nsutite, a battery-active manganese dioxide, is mined between Skilpadhek and Livingstone's Poort. The ore body originated by replacement of a layer of stromatolitic1 dolomitic limestone in the Polo Ground Member of the Rooihoogte Formation and 0 has a dip of 15 to the north or northeast, a strike length of 10 km and a thickness of between O and 7 m. Ore formation could still be in progress in the oxygen-rich zone of the ground water, which would curtail the extent of the ore to between 30 and 40 groundwater level.