Incorporating the uncertainty in geological boundaries into mineral resources evaluation

Abstract

Geological modelling is a key step prior to grade estimation in a mineral deposit and consists of a deterministic partitioning of the deposit into sub-domains called 'geological units', each with relatively homogeneous properties. Although this partitioning allows a better characterisation of the grade distribution in space, it fails at accounting for the uncertainty in the layout of the geological unit boundaries. This paper proposes a methodology for incorporating the geological uncertainty into mineral resources evaluation, through an application to a porphyry copper deposit located in the Central Andes, Chile. Geostatistical simulations are used to obtain a probabilistic description of three lithotypes in this deposit, viz.: granodiorife, tourmaline breccia and other breccias. Two kinds of conditioning information are considered for constraining the lithotype realisations: hard data available at exploration drill hole samples and soft information on geological continuity assumed by the mining geologist, consisting of interval constraints on the probabilities of occurrence of the lithotypes at locations where drill holes are scarce. The conditional realisations so made are used to derive the posterior probabilities of occurrence of the lithotypes over the de posit. The mineral resource model is finally obtained by weighting the copper grade estimates associated with the different lithotypes by their probabilities of occurrence. The proposed approach allows one to construct a grade model that incorporates the uncertainty in the rock types, avoiding artefacts caused by the assumption of deterministic boundaries between high-graded and low-graded lithotypes, and is shown to improve the grade estimates near the lithotype boundaries.