Assessing the diamond potential of kimberlites from discovery to evaluation bulk sampling

The economic evaluation of diamond-bearing kimberlites is usually carried out in four stages. Expenditure tends to increase by an order of magnitude at each successive stage. At the end of each stage, the sample results should be critically appraised before deciding to proceed to the next phase. In the first stage, even before individual kimberlite bodies have been discovered, the indicator mineral geochemistry will give a first rough idea of the diamond potential. The relative abundance of harzburgitic pyropes (subcalcic chrome-rich) is often directly correlated with the diamond grade. In the next stage, when the kimberlite body has been discovered, a relatively small sample of a few hundred kilograms will be enough to recover sufficient microdiamonds to allow an extrapolation of the size distribution towards the commercial-sized diamonds and a rough estimate of their grade. If positive, the third stage should be a limited bulk sampling programme (order of 200 tonnes) to determine the commercial-sized diamond grade, expressed as carats per tonne. The aim of the final stage is to obtain a parcel of the order of 1,000 carats to estimate the average commercial value of the diamonds. The robustness and reliability of the grade and value estimates can be verified with extreme value analysis and by obtaining the confidence limits with bootstrapping.