Integrating the Theory of Sampling into Underground Mine Grade Control Strategies

Abstract

Grade control in underground mines aims to deliver quality tonnes to the process plant via the accurate definition of ore and waste. It comprises a decision-making process including data collection and interpretation; local estimation; development and mining supervision; ore and waste destination tracking; and stockpile management. The foundation of any grade control programme is that of high-quality samples collected in a geological context. The requirement for quality samples has long been recognised, where they should be representative and fit-for-purpose. Once a sampling error is introduced, it propagates through all subsequent processes contributing to data uncertainty, which leads to poor decisions and financial loss. Proper application of the Theory of Sampling reduces errors during sample collection, preparation, and assaying. To achieve quality, sampling techniques must minimise delimitation, extraction, and preparation errors. Underground sampling methods include linear (chip and channel), grab (broken rock), and drill-based samples. Grade control staff should be well-trained and motivated, and operating staff should understand the critical need for grade control. Sampling must always be undertaken with a strong focus on safety and alternatives sought if the risk to humans is high. A quality control/quality assurance programme must be implemented, particularly when samples contribute to a reserve estimate. This paper assesses grade control sampling with emphasis on underground gold operations and presents recommendations for optimal practice through the application of the Theory of Sampling.

Towards Representative Metallurgical Sampling and Gold Recovery Testwork Programmes

“…Dominy et al [37,80] applied whole HQ 2-m composites (approximately 17 kg samples) to a coarse gold orebody. A single-stage GRG test with tails leach (and some flotation testwork) was used to provide data for GRG and tails leach recovery, and head grade.…”

“…This option involves taking half or whole core samples and processing the entire sample by, for example, the single-stage GRG test [18], large-scale screen fire assay and/or bulk leaching. Dominy et al [37,80] applied whole HQ 2-m composites (approximately 17 kg samples) to a coarse gold orebody. A single-stage GRG test with tails leach (and some flotation testwork) was used to provide data for GRG and tails leach recovery, and head grade.…”

“…Analytical (measurement) error generally accounts for between 1% and 13% of the relative error [48]. It is generally recognised that the total sampling value chain error is dominated by the sampling process, which can be in the range of 15% to 60% [37,48,51].…”

“…In another case study, moderate-to high-grade mineralisation required a theoretical sample mass of 200 kg for high-grade core zone (21 g/t Au) mineralisation [37,80]. Whole diamond core (17 kg) and development bulk (140 t each) sampling was used to determine gold grade, and GRG and leach recovery parameters.…”

“…It does not strictly include sampling of in-situ material such as drill core or linear samples, as the sample does not have an equiprobable chance of being collected. However, its application is relevant and highlights some of the challenges of field sampling through analysis of the so-called incorrect sampling errors (ISE ; Table 3) during the sample collection process [36][37][38][39].…”