Optimisation of the grade control procedures at the Yandi iron-ore mine, Western Australia: geostatistical approach

Abstract

Effectiveness of ore grade control at operating mines depends on both the quality and quantity of the samples used. Therefore, optimisation of grade control procedures requires analysis of sample quality and their spatial distribution. This approach, implying quantitative estimation of both these factors and quantification of their contribution to the grade control errors, was used for comparing two different grade control procedures at the Yandicoogina (Yandi) iron-ore open pit mine, located in the eastern part of the Pilbara region of Western Australia. At Yandi, pisolitic iron oxide mineralisation is distributed within a meandering palaeoriver channel, characterised by abundant clay pods that contain the deleterious components; in particular Al2O3 and SiO2. The irregular shapes and highly erratic distribution of the clay pods are a serious challenge for grade control at this mine. Current grade control procedures at the Yandi open pit are based on sampling 10 m deep blast holes on a 6 × 6 m grid. An alternative approach was considered of using reverse circulation (RC) holes. However, study of the sample duplicates and direct comparison of the blast holes and RC holes drilled at distances approximately 1 m apart (i.e. twin holes) have revealed that use of RC drilling does not guarantee an improvement in the quality of the samples. Reverse circulation samples exhibit precision errors similar to that of the blast holes and both sets of data are characterised by large nugget effects on the variograms of Al2O3 values. It was also noted that RC samples were biased, underestimating Al2O3 and SiO2 grades, and overestimating Fe grades. The Sequential Gaussian conditional simulation model was used for estimation of grade control errors associated with both grade control techniques. This model has shown that using a grid of 25 × 25 m for grade control leads to 12·3% of the selective mining unit sized blocks being misclassified, which is more than a two-fold increase in comparison with current procedures based on sampling blast holes, when approximately 5·8% of SMU blocks are misclassified. In general, comparative analysis of the two grade control approaches at the Yandi mine shows that using the RC holes drilled as the 25 × 25 m grid will not reduce the grade control errors. In fact, it can produce the opposite effect, significantly increasing the number of misclassified ore and waste blocks.