Influence of selective mining on optimum pit design

CIM Bulletin, Vol. 77, No. 867, 1984

ALEXANDRE DESBARATS and MICHEL DAVID, Departement de Genie mineral, Ecole Polytechnique

All optimum pit design studies start from the geological ore reserves inventory, the estimated grades of all blocks of potential ore within the interpreted limits of the deposit. The actual optimization procedure is then carried out on block valuations based on these estimated grades. Thus, the relevance of any pit design can be seen to depend directly on the ore reserve estimation. To insure a proper pit design, a first step would be to use a geostatistical reserve estimation technique such as kriging, however, this is not enough. A frequently neglected fact is that the blocks of ore estimated during the planning stage of the mine are usually considerably larger than those on which the ore-waste selection will be made at the time of mining. Experience in the exploitation of large low-grade deposits has shown that large blocks, estimated to be ore, when mined selectively in smaller blocks, often yield significant amounts of waste material. This phenomenon, known as internal waste, is a function of the variability of the grades, the size of mining unit and the amount of sampling information on which ore-waste decisions are made. In this study, it was proposed to consider the effect of internal waste caused by selective mining, on pit design, using geostatistical methods to predict the tonnage and grade of recoverable reserves within large blocks. Optimum pits obtained using a block valuation formula taking into account the recoverable reserves within large blocks were compared with optimum pits obtained using a conventional block valuation formula. It was found that the optimum pits obtained by the proposed method had the highest total valuations and thus were, in fact, "more optimum". These pits were characterized by greater ore tonnages at a lower average grade, the additional ore more than compensating for the drop in grade. It was also found that the consideration of internal waste in pit design could considerably alter pit limits in marginal areas of the deposit. It was concluded that selective mining had a significant, potentially critical, impact on the design of optimum pits especially in the case of large low-grade deposits.
Keywords: Open-pit mining, Geostatistics, Optimum pit design, Selective mining, Kriging, Parametrization