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The diamondiferous sediments on the farm Nooitgedacht (66), Kimberley South Africa

De Beers Consolidated Mines Ltd. Africa Exploration. P.O. Box 7383, Centurion 0046, South Africa, E-mail: mike.dewit{at}debeersgroup.com

Diamonds were mined on the farm Nooitgedacht (66) in the Kimberley area between 1908 and 1930, 1948 and 1981, and from 1996 onwards. Although the records are incomplete, at least 80 000 carats of diamond had been recovered by 1963 with an average stone size of 0.9 ct/stone. It also produced at least 15 diamonds larger than 100cts in weight, including one of the biggest alluvial diamond ever found in South Africa, the Venter diamond (511.25cts).

The farm can be divided into the east-central and high level part where most of the digging occurred, and the western part associated with younger and lower level terraces of the Vaal River. The former area is characterised by near surface and surface outcrop of late Archaean Ventersdorp lava with typical corestone development. Between the corestones and the overlying Quaternary Hutton sands there is a thin diamondiferous gravel, composed mainly of resistant material derived from the Ventersdorp volcanics, and isolated well-rounded and extrabasinal clasts from remnants of Dwyka Group sedimentary rocks. This diamondiferous deposit on Nooitgedacht was part of a tributary that occupied a wide and shallow valley draining the Kimberley area into the palaeo-Vaal River. This tributary flowed over Ecca Group shales which underlie the area between Nooitgedacht and Kimberley, and which offer poor trapsite potential. In contrast, the exhumed pre-Karoo high of Ventersdorp on Nooitgedacht promoted the high concentration of (big) diamonds on the farm as a result of an increase in bed-roughness associated with corestone development of these lavas forming preferred trapsites. Most diamonds are unabraded and are sourced from the Kimberley cluster of kimberlites along with kimberlitic ilmenites that can also be matched to that population. The presence of highly abraded diamonds indicates that the palaeo-Vaal River was already transporting diamonds from other sources in its headwaters. Based on geomorphological evidence it appears that the Nooitgedacht deposit is associated with the African erosion cycle and is therefore probably Late Cretaceous or Early to Middle Tertiary in age.

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