Sampling for resource evaluation and grade control in an underground gold operation: a case of compromise

Abstract

The foundation of any resource evaluation and/or grade control programme is the collection of high-quality samples within a geological context. The requirement for quality samples has long been recognised, where they must be representative and fit-for-purpose. Correct application of the Theory of Sampling reduces sampling errors across the collection to assay process. This contribution presents a case study where small-sample based assays understate grade in geometrically simple, coarse gold-dominated veins. Drilling with whole core sampling and assaying is applied to estimate Inferred Mineral Resources that are accepted to understate grade. Dominant gold particle clustering drives the application of bulk sampling. Development drives are sampled as bulk composites and processed via an on-site plant. Upper and lower development drive grades are assigned to stope blocks and reported in the Indicated Mineral Resource category. The case study illustrates the challenges and potential solutions to achieve representative sampling. Solutions ranging from individual bulk samples processed through a plant, to bulk composites and whole-core sampling and screen fire assaying are discussed. These approaches account for the nature of the mineralisation, where extreme gold particle-clustering effects render the analysis of small samples problematic.