SAMPLING AND STATISTICAL EVALUATION OF DIAMOND DEPOSITS

To evaluate a diamond deposit, a sampling programme needs to be designed to obtain a reliable estimate of the average grade of the deposit, expressed as carats per tonne or cubic metre, and of the average value of the diamonds, traditionally expressed as US dollars per carat. An analysis of the statistical distribution of the stone sizes, occurrences and values is essential for calculating the confidence limits on the estimated averages. Too wide confidence limits will indicate the need for more sampling. If variables are lognormally distributed (such as is often the case with stone sizes and stone values), a more efficient estimator, such as the t-estimator, can be used, instead of the arithmetic mean. Knowledge of the statistical distribution is also useful when microdiamond counts from early stage exploration drilling are used to make preliminary estimates of potential grades of commercial-sized diamonds. Stone occurrences (density distribution) are more homogeneous in kimberlites and lamproites than in alluvial or beach deposits. Grade distributions of secondary deposits are very skew, with often a large proportion of barren samples.