Abstract:
This geophysical and geological study was undertaken to determine the structural control and mode of deposition of the diamondiferous gravel runs north and east of Lichtenburg, especially on the farms Ruigtelaagte 353 JP, Welverdiend 361 JP and vicinity, which lie on the WelverdiendGrasfontein run. This study formed part of a larger experimental geophysical investigation conducted by the Geological Survey to determine the present potential of the runs and whether gravel-filled sinkholes and channels could be found by means of geophysical techniques. The geophysical investigation comprised detailed gravity, magnetic and electromagnetic surveys on parts of the above-mentioned farms. Geological mapping coupled with a sedimentological study and a heavy mineral analysis of the gravels were undertaken to reach a better understanding of the origin and mode of deposition of the gravels. The gravel runs are situated on the Lyttelton and Monte Christo Forma= tions of the Chuniespoort Group and the deposition and preservation of the gravels were controlled by karst areas which formed on or along dykes and quartz veins and in the immediate vicinity of the contact between the Lyttelton and Monte Christo Formations. Possible post-Transvaal tension fractures and faults striking north-east and north-north-west, and along which sinkhole formation occurred are visible on the Bouguer and residual gravity maps as gravity lows. Filled palaeosinkholes also occur along three dolerite dykes that cross the study area. These dykes appear as gravity lows, indicating that they are deeply weathered. Iterative, three-dimensional computer modelling of gravity data was undertaken to interpret parts of the residual gravity field over gravel-filled sinkholes ("potholes") and gravel runs. From the geological and sedimentological information it is evident that no ordinary fluvial model can be proposed to explain the deposition of the gravels. It is concluded that the palaeoriver courses were governed by the exploitation of s·inkholes and that flash floods and different stages of sinkhole collapse caused the palaeorivers to frequently change their course, resulting in the wide-spread gravel occurrences. The source of the diamonds is believed to be locally present, yet undiscovered, kimberlites.
