Dilution and geostatistics

CIM Bulletin, Vol. 81, No. 914, 1988

MICHEL DAVID,Professor, Ecole Polytechnique de Montreal,President, Geostat Systems International, Montreal, Quebec

The usual complaint of mines is that grade does not match exploration expectations. Quite often, more tons are mined than expected but at a lower grade. This can be seen as dilution. Geostatistics can help predict which dilution can be expected both in terms of grade and tonnage for different mining methods. The usual vocabulary of geostatistics speaks of block distribution, block variance, size effect, information effect, all terms which say very little to the practitioner. In fact, all this can be rephrased in terms of dilution. In this paper, the following is presented — an example which shows how reserves change with the size of selection units and number of samples taken, and tables which show the dilution that are bound to happen when dealing with the commonly encountered log-normal distribution. Finally, the paper shows how grade tonnage computations should be made in the case of an arbitrary distribution.
Keywords: Geostatistics, Dilution, Grade control, Block distribution, Block variance, Ore reserve estimation.