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

Abstract

English: 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 indicates 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 soo 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 near Skilpadhek to cut out a large part of the Malmani Subgroup. The postBushveld 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-lineations, were formed under tensional conditions. En echelon NNW faults appear to be a result of crustal adjustments in response to an isostatic rise of the underlying Gaberone basement granite dome, which is part of the Vryburg arch. ENE and EW photolineations appear to be the surface manifestations of diabase dyke systems. 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 stromatolitic,dolomitic limestone in the Polo Ground Member of the Rooihoogte Formation and 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 m below ground level. Lenses of magnetite occur in a thick noritic sill above the Daspoort Quartzite Formation. Many small iron ore bodies exist in the Transvaal Supergroup. Carbonaceous slates are usually rich in sulphides, especially pyrite.Afrikaans: 'n Gedeelte van die Transvaalsupergroep nl. the boonste forrnasies van die Chuniespoortgroep en die Pretoriagroep tussen Zeerust en die Botswanagrens, is gekarteer. Die rnetarnorfe oureool van die Bosveldkompleks wat ontwikkel is in die Transvaalsupergroep, verrninder in rnetarnorfe graad langs sowel as dwarsoor die strekking weens 'n blykbare afnarne in die dikte van die rnafiese gesteentes na die weste. Die teenwoordigheid van die rnetarnorfe indeksrninerale trernoliet, antofilliet, curnrningtoniet, cordieriet en alrnandien in rnineraalassosiasies dui op gesteentes van die horingblendehoringfelsfasies in die ooste. Wes van die lengtegraad 25°53'0 wat dwarsoor die gelaagdheid sny, is rnetarnorfe assosiasies wat tipies is van die albiet-epidoot-horingfelsfasies en rninerale soos talk en epidoot bevat. Dit is onwaarskynlik dat die totale vloeistofdruk ooit 4 kb oorskry het. Die ternperatuur het blykbaar gewissel tussen 300 en 50o0cvir die albiet-epidoot-horingfelsfasies en tussen 500 vir die horingblende-horingfelsfasies. 0 en 600 C Min van die ingewikkelde strukture in die aangrensende gebied in Botswana dring Suid-Afrika baie ver binne, behalwe vir 'n sone van vlakhoekige strekkingsverskuiwings wat 'n groot gedeelte van die Malrnanisubgroep afsny in die Skilpadhekorngewing. Na-Bosveldse lineere strukture is dominant in die grootste gedeelte van die gebied asook in 'n groot gedeelte van die Kaapvaalprovinsie. Die linearnente bestaan uit NNW-verskuiwings en ONO-fotolineasies, wat beide onder tensie toestande ontstaan het. Die en echelon, NNW-verskuiwings is skynbaar te wyte aan aanpassings van die aardkors wat genoodsaak is deur 'n isostatiese styging van die onderliggende Gaberone-koepel van Argeiese graniet, wat deel uitmaak van die Vryburgboog. Die ONO-en OW-fotolineasies is waarskynlik oppervlakverskynsels van diabaasgangsisteme. Daar is min OW-verskuiwings. Etlike laterale litofasiesveranderinge in die sedimentere lae van die gebied dui op 'n hoer energie omgewing in die weste as in die ooste. Daar is ook 'n styging in die hoeveelheid en grootte van detritale chert naby Botswana. Hierdie verskynsels dui op 'n nabygelee oorspronklike grens van die Transvaalkom in Botswana. Opheffing en erosie van die Chuniespoortgroep na afsetting het nie die verwydering van die Pengeformasie tot gevolg gehad nie, met die gevolg dat die basale laag van die Pretoriagroep 'n chertkonglomeraat is en nie 'n residuele chertbreksie nie soos elders voorkom in die Transvaalkom. Die opeenvolging van die Rooihoogteformasie in die Skilpadhekomgewing soos aangedui deur die kerne van vier boorgate, stel 'n delta-afsetting voor wat gedeeltelik vernietig is deur sterk seestrome. Lineere klastiese kusafsettings volg bo-op die oorblyfsels van die delta. Lava van die Hekpoort Andesiet Formasie het op land uitgevloei. Die boonste laag lava is verander na 'n gemetamorfoseerde residuele paleogrond. Nsutiet, 'n battery-aktiewe mangaandioksied word in die Skilpadhek- Livingstone's Poortgebied ontgin. Die ertsliggaam is 'n vervanging van 'n laag strornatolietiese dolornitiese kalksteen wat voorkorn in die Polo Groundlid van die Rooihoogteforrnasie en het 'n helling van 15 o noord of noordoos, 'n strekkingslengte van 10 km en 'n dikte van tussen Oen 7 rn. Vorrning van die erts kan rniskien nog aan die gang wees in die suurstofryke sone van die grondwater, wat die diepte van die erts sou beperk tot tussen 30 en 40 rn onder die grondoppervlak. Magnetietlense korn voor in 'n dik noritiese plaat bokant die Daspoortkwartsietforrnasie. Baie ander klein ysterertsafsettings is in die studiegebied opgernerk. Koolstofryke skalie bevat gewoonlik heelwat sulfiede,veral piriet.