Abstract

Remnants of diamond-bearing fluvial gravels of Cretaceous age have been preserved on the Ghaap Plateau north-west of Reivilo at 1455 m above sea level. The alluvial deposit contains well preserved fossil woods representing at least four periods: Post-Permian (Upper Karoo), Early Cretaceous, Late Cretaceous and Tertiary. The Karoo specimen has been derived from Beaufort Group sediments, now eroded from the Ghaap Plateau. More significant are the presence of Early and Late Cretaceous woods, both periods believed to have been wetter in the interior of southern Africa. In addition to the diamonds, other kimberlite derived minerals such as ilmenite and garnet have also been recovered from these gravels. Detailed mineral chemical analyses of the ilmenites and those occurring in primary sources within a radius of 120 km of the Mahura Muthla area indicate that the most likely source are kimberlites occurring to the south-south-east, indicating that the palaeo-channel was flowing from the south-east to the north-west. This is supported by the presence of ironstone and red chert cobbles and pebbles derived from the Proterozoic Kanguru Member outcropping directly south of the channel. Further evidence comes from a palaeo-tributary on Laurika which joined the main Mahura Muthla trunk stream from the south-west; limited pebble imbrications; and a decrease in the sizes of wood, agate and ironstone cobbles and pebbles in a northerly direction associated with an increase of roundness of these clasts. At the start of the Cretaceous the area was covered by sandstones of the Beaufort Group with overlying flood basalts of the Drakensberg Group. First during the Early Cretaceous the Mahura Muthla channel was incising into an easterly retreating palaeo-escarpment of Karoo basalts. Group 2 kimberlites such as Finch, Darleston and Duivelskop, had already intruded these basalts around 120Ma. The north-westerly orientated drainage was directly linked to the drainage basin of the Kalahari River draining the northern part of the northern Cape and southern Botswana via the palaeo-Molopo and lower Orange River. This period of erosion would have released an abundance of agates from the basalts that are so abundantly present in the gravels and also diamonds from these Group 2 kimberlites. On comparing the mineral chemistry of the ilmenites the most likely source for these kimberlitic minerals is the area to the south east of Mahura Muthla. Kimberlites occurring in this area include the Duivelskop Group, X 154, Pienaarspoort, the Bellsbank Group, Bull Hill and Mayeng. At least some of these could have provided diamonds to the gravels. Later during Upper Cretaceous times the Karoo basalts had been removed and the drainage stripped the remaining sediments of the Beaufort Group before the sinuous channel became locked and preserved within the Transvaal dolomites. Subsequently the channel became calcretised during the development of the African Surface in the Early Tertiary. Mahura Muthla remained part of the Kalahari River basin which fed the Molopo River and the lower Orange drainage network and by the Early Tertiary the latter had captured the Vaal and the upper and middle Orange River drainage basin which had been part of the Karoo River drainage for most of the Cretaceous.

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