- © 2006 Geological Society of South Africa
The liberalization of the diamond marketing system led to increased diamond prices and this, coupled with the devaluation of the Rand against the US$ during late 2001 and 2002, resulted in a significant increase in onshore alluvial diamond production in South Africa. Inland, alluvial diamond production reached a recent peak of 1.23 million carats in 2003, providing employment for an estimated 25,000 people which then dropped to a disappointing 6,000 people in 2005. The discovery of the Lichtenburg diamond field in 1926 made a huge, short term impact on South Africa’s diamond production but this impact was short-lived due to the global decline in diamond sales that resulted from the Great Depression.
In recent years, there have been important advances in the techniques used in the exploration for alluvial diamonds, enhancing the ability to locate new deposits. These include improvements in the remote sensing techniques of aerial photography and satellite image interpretation, as well as various airborne and ground geophysical methods. The extremely low concentrations of diamond in most alluvial deposits and the almost random distribution of the diamonds within the gravels, necessitate the taking of large samples in order to evaluate the commercial potential of a deposit.
In the southern, Schweizer-Reneke diamond field, five ages of diamondiferous gravels occur, ranging from late Cretaceous to Pleistocene. Post-depositional modification has resulted in the formation of colluvial and eluvial “Rooikoppie” deposits, which were preferentially mined by the artisanal diggers of the previous century.
The northern Lichtenburg, and eastern Ventersdorp diamond fields are largely underlain by dolomitic horizons that have undergone at least four phases of karst development. Even though several phases of gravel deposition have occurred, the distribution of the diamondiferous gravels is influenced by weathering features in the dolomite and these now occur as pothole-fill, sinuous, discontinuous, narrow, steep-sided alluvial channels or laterally more continuous, yet thin sheet-like gravels, assumed to be younger reworked material. The pot holes (or sinkholes) have formed excellent trap sites for diamonds and enormous quantities of diamonds have been recovered from some of these.