The Ventersdorp Contact Reef (VCR) at the base of the >4.5 km-thick volcanosedimentary Ventersdorp Supergroup unconformably overlies the Witwatersrand Supergroup and is the second youngest mineable reef in the Witwatersrand Goldfields. The volcanic rocks of the Ventersdorp Supergroup are predominantly mafic, affected by low-grade thermal metamorphism and difficult to date. Only the Makwassie Formation in the upper Platberg Group of the main Ventersdorp repository has been reliably dated on four felsic volcanic samples at 2720 ± 2 Ma. The actual timing of Ventersdorp volcanism and the duration of the three recognised lithostratigraphic groups remains enigmatic, despite much research and heroic attempts to synthesize the available data.In this work detrital zircon grains from VCR conglomerates were U-Pb dated in order to improve the time constraints on the Klipriviersberg Group at the base of the Ventersdorp Supergroup. The six youngest grains in VCR samples were reliably dated at 2799 ± 9 Ma. The Klipriviersberg Group and the Ventersdorp Supergroup is thus younger than 2808 Ma and the supergroup is older than the 2642 Ma Vryburg Formation at the base of the Transvaal Supergroup.Comparisons of detrital grain dates confirm that the VCR was largely derived from erosion products of the underlying Witwatersrand Supergroup, however the youngest VCR grains are ~20 Ma younger and may have been derived directly from magmatic rocks in the provenance or a felsic facet of the synchronous komatiitic Klipriviersberg volcanism.Multi-grain analyses of discordant grains show that recent lead loss is predominant. However about 5% of the data show the effect of complex Mesoproterozoic lead loss, which can yield ages as much as 150 Ma too young in 10% discordant data. This was found in grains with high Th-induced radiation damage, providing a criterion for data rejection.The proposed large igneous provinces dated between 2791 and 2683 Ma, based mainly on dated mafic dykes, which are not in contact with supracrustal Ventersdorp rocks, do fit the established time constraints and might provide a key to Ventersdorp chronostratigraphy. However only the proposed 2754-2709 Ma Platberg volcanic province is based on reliably dated Platberg Group volcanic rocks.
The Neoarchaean Ventersdorp Supergroup represents a fully evolved bimodal volcano-sedimentary succession on the Kaapvaal Craton. Stratigraphic information was mainly derived from exploration drilling for gold in the underlying Witwatersrand Supergroup. Following the basal flood basalts of the Klipriviersberg Group, the Platberg Group mostly comprises extensive and voluminous intermediate to felsic volcanics of the Goedgenoeg and Makwassie Formations. Detailed stratigraphic and geochemical reports of these two formations were previously mostly restricted to the so-called Bothaville gap, between the Free State and Klerksdorp Gold Fields. The lower part of the Goedgenoeg Formation succession consists of non-porphyritic andesitic lava flows, followed upwards by dacitic phenocryst-rich feldspar porphyries. The basal lava flows are frequently interbedded with sedimentary rocks of the underlying Kameeldoorns Formation. Andesitic lava-flow units persist upwards, occurring interbedded with the Goedgenoeg Formation feldspar porphyries. The feldspar porphyries originated as ash flows; most are highly welded. The upper feldspar porphyries in the succession are absent in large parts of the Bothaville gap. The overlying Makwassie Formation consists of quartz porphyries, also with intermittent interbedded mafic lava units throughout the succession. The upper part of the succession is composed of phenocryst-rich crystal tuffs, only preserved over limited areas of the Bothaville gap. The quartz porphyries are highly welded, high-temperature ash flows with individual flows of tens of metres in thickness, typically separated by thin tuffs and sediments. The Goedgenoeg and Makwassie interbedded andesitic lavas have similar geochemical composition (the geochemical composition of the overlying Rietgat Formation lavas is also similar to that). The Goedgenoeg dacitic feldspar porphyries are separated into two geochemical groups: a lower high-Zr group and an upper low-Zr group. The Makwassie quartz porphyries are of dacitic compositions, while the upper part is rhyolitic. Two geochemical groups can be distinguished in the rhyolites, based on the Zr/Cr ratio. Although each of the two groups occur as distinct stratigraphic units, in some boreholes the low-Zr/Cr group form the lower part of the rhyolite succession overlain by the high-Zr/Cr group, while in other areas the sequence is reversed. A significant hiatus may have existed between eruption of the Goedgenoeg and Makwassie volcanics. The andesitic mafic lavas persistently co-erupted with the Goedgenoeg dacites and with the Makwassie dacites and rhyolites, indicating a stable mafic magma source that was re-mobilised from time to time to initiate eruption of the more viscous dacite and rhyolite crustal melts. The Goedgenoeg and Makwassie dacites are mixed magmas of the mafic andesitic source with crustal melts, while the Makwassie Formation rhyolites are a distinct crustal melt.
During the late 1980s a deep exploration borehole (LLE1) was collared on the Makwassie Formation outcrop at Makwassie Hills, near Leeudoringstad. The borehole intersected a 2420 m thick succession of rhyolitic quartz porphyries and interbedded andesitic mafic lavas of the Makwassie Formation, with a further 1301 m of dacitic feldspar porphyries and andesitic mafic lavas of the Goedgenoeg Formation, before it was stopped at 3721 m, still in the Goedgenoeg Formation. The mafic lavas in the Goedgenoeg Formation are low viscosity basaltic flows, as are the mafic lavas interbedded in the Makwassie Formation. The Goedgenoeg Formation feldspar porphyries and the Makwassie Formation quartz porphyries are of extrusive origin, emplaced as ignimbrites and ash-flow tuffs, ascertained by macroscopic and petrographic characteristics. Individual ignimbrite or ash-flow units are several tens of metres to hundreds of metres thick, regularly separated by volcanoclastic sandstone and calcareous shales. Geochemically the lower part of the Goedgenoeg Formation feldspar porphyries in borehole LLE1 correlates with the low-Zr dacite group of the Bothaville area, and the upper part with the high-Zr dacites. The lower part of the Makwassie Formation quartz porphyries correlates with the high-Zr/Cr rhyolite group of the Bothaville area, and the upper part with the low-Zr/Cr rhyolite group. The Bothaville area’s Makwassie Formation dacite unit is absent from the LLE1 profile. The interbedded andesitic mafic lavas are similar in composition to those in the Bothaville area. There is no structural or stratigraphic indication (through lithological or geochemical means) of duplication in this borehole’s stratigraphic profile. The borehole drilled into a caldera succession, part of a cluster of calderas associated with half-graben structures aligned from west-northwest to east-southeast, from the Makwassie Hills towards Bothaville. These and other Platberg caldera clusters are developed outside the Central Rand Basin, mainly to the west of it.
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