Abstract

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.

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