MATERIALS WORLD December 2010 32 D iamonds are found in volcanic rocks in the oldest parts of the continental crust. Two types of diamond-bearing rocks are known – lamproites that form relatively shallow crater facies or kimberlites that are deep carrot-shaped bodies. There is only one lamproite diamond mine in Argyll in Western Australia, in contrast to the numerous kimberlite mines in Southern Africa. Releasing them from rock by crushing may damage stones, however, nature has found a way of releasing them through weathering. In Southern Africa, the greatest period of erosion occurred during the Cretaceous (154-65Ma) and diminished with time through the Tertiary (65-1.8Ma). It is estimated that the uppermost 1,400m of the kimberlite pipes in the Orange/Vaal river catchments were removed and transported into the westward drainage systems. In kimberlite pipes, grades are known to decrease towards the root of the intrusion and, as a result, the best and largest stones are found in alluvial deposits eroded from their upper regions. Exploring for alluvial diamonds, however, requires a different skill set to kimberlite exploration. The objective is to identify ancient river systems and their residual sands and gravels. One of the most useful methods is interpreting aerial photographs to illustrate the subtle changes that reflect the underlying topography and geology. They are particularly useful when taken after summer rains, showing standing water and/or greener vegetation. Infrared spectrum photos are useful here, while geophysical methods are Main image: Separation plant (14ft), used for trial mining. Below: Double 14ft separation plant with scrubber and vibrating grizzly feeder for clay gravels of limited help, and the final arbiter in any alluvial exploration is the drill bit. Ground force Parts of the Orange/Vaal river systems have been well explored, especially those in the lower reaches where ancient terraces abound. The middle Orange catch- ment poses a different problem. Here, exceptional quality diamonds are found in two gravel horizons – an upper wind-blown deposit that varies from a few centimetres to two metres thick, known as the Rooikoppie, and a deeper basal primary gravel that rests on bedrock shales. Overlaying these is a hard calcrete (a natural cement) bed some 8-10m thick. ‘In the past, miners using hand tools were unable to penetrate the calcrete. A few mining operations in the Rooikoppie yielded gemstones in the 50-100 carat range with typical stones in the one-to-two carat range with values above US$1,000 per carat’, states Martin Prinsloo of Dreamstone Mining, in London, UK. His company has a mining license over the Vaal River gravels that occur in three terraces 20, 60 and 105m above the present course. Gravels can be as much as 10km from today’s river channel with the 105m terrace being the oldest and containing the best diamond grade. ‘Ground observations are made difficult by farming and irrigation systems that mask the natural features’, he says. Exploration in this terrain for diamonds, found in concentrations grading 0.3-0.8 carat per hundred tonnes (cpht) of gravel, is not easy. It requires knowledge of the underlying relief and type of bedrock obtained through drilling, especially in those areas where there may be sediment traps. Bedrock riffles and rock bars cause energy and velocity fluctuations in a river system, resulting in variable hydraulic energy in the gravel bed. ‘This chaos, in an otherwise perfect river stream causes diamond enrichment within gravel pockets that is almost impossible to detect during a drilling