Numerous models have been presented over the years to explain the nature and distribution of the diamondiferous alluvial deposits on the dolomite plain in the Ventersdorp district. These have, typically, highlighted the importance of karst processes both prior to, and coincidental with, deposition of the gravels. Karst has a particularly distinctive method of landscape sculpturing arising from greater rock solubility in natural waters than elsewhere. Solution may not always be the prevalent process, or even the dominant one, but it does play a greater role than in other kinds of landscape. It’s most critical effect lies in the enlargement of underground voids, causing increased permeability of the rock. As more water passes through the rock, the voids grow to become caves. Greater solubility of a substance is accompanied by a greater liability for the dissolved load to come out of solution, so chemical deposition can contribute in an important way to landform building in karst terranes, both on the surface and underground. In karst landscapes, the generally harmonious pattern of surficial drainage is broken up and closed depressions take over the landscape to varying degrees. Superficially, it lacks organisation and the drainage system has to be sought underground.
The general stratigraphic sequence of the alluvial gravel deposits of the Ventersdorp district is shown to comprise an economic lower gravel (Lower Gravel Package and the Transition Zone Gravels), a sub-economic clay-rich unit and an economic upper gravel unit (Upper Gravel Package), all of which have been overlain by Kalahari clays/sands and the Hutton soil profile (including colluvial diamondiferous gravel units). It is fundamental to the model to appreciate that the gravels were deposited in a karst system where:
The dolomitic bedrock contacts with the gravels may be steeply dipping to vertical
The mode of gravel deposition is not typical fluvial alluvial
Periodic subsidence has taken place during deposition
Deposition has taken place over a long time (at least since the Mesozoic), resulting in the build-up of a very thick gravel sequence.
Furthermore, the runs are often controlled by cave formation along linear, fracture or fault zones which allowed the increased flow of ground water that led to higher than normal levels of dissolution of the dolomites and resulted in the formation of sinkholes and channels. Some of the sinkholes are more than 100 m deep. The sedimentary infill of the sinkholes is variable and depends on the depth and subsidence history of each individual sinkhole.
Numerous issues have been raised with respect to the potential sources of both the diamonds and the gravels. Among the major problems is the limited variation in grade and quality of the diamonds over a significant area and the relatively high grade of the lower gravels that show a distinct lack of reworking and re-concentration throughout their vertical extent.