Using grade uncertainty to quantify risk in the ultimate pit design for the sadiola deep sulfide prefeasibility project, Mali, West Africa

Abstract

In order to quantify the uncertainty in the grade estimate for the Sadiola deep sulfide prefeasibility project, a conditional simulation model was generated using the direct block simulation methodology. Compared to conventional sequential Gaussian simulation, the direct block simulation algorithm produced a reliable model in significantly less time, lending its application to a production environment. Through application of a mining transfer function, risk pits were generated for comparison with the deep sulfide prefeasibility pit. The results of this study revealed that the prefeasibility pit is optimal at the applied gold price and cost parameters, and that the risk of not achieving the project grade profile is low. Should the gold price increase, or the operating costs of the project decrease significantly, the deep sulfide reserve tonnage would realise significant upside potential. Probability and uncertainty analysis revealed that the greatest risk to the project is the confidence in the footwall grade estimate. At a drill spacing of 50 m × 50 m and a sample interval of 1 m, the probability of the footwall grade exceeding the economic cut-off of 2.0 g/t is low, while the uncertainty in the grade estimate is high. Although significantly lower in grade than the main zone, which is the primary economic driver of the project, the footwall mineralisation is important in terms of reducing stripping ratio and delivering ore tonnes to optimise the treatment schedule. This zone is therefore a focus area for further drilling.