Quantification of uncertainty associated with 4E assay results from reverse circulation sampling of the middlegroup seams at Tharisa mine

Abstract

This research explores the sources of discrepancy between platinum, palladium, rhodium and gold (4E) g/t in Reverse Circulation (RC) drilling samples and Diamond Drillhole (DDH) samples, ultimately defining the applicability of RC drilling on the narrow seams of the Bushveld Middle Group at Tharisa Mine. The research also quantifies the sampling and analytical uncertainty associated with the 4E g/t grades in RC samples through measurement and statistical approach. Twin-hole drilling is used where RC holes are drilled within a 10m radius of the DDH to understand and to demonstrate the difference between the seam intersections of RC sampling in relation to DDH. Sampling uncertainty was quantified by using replicate samples. Analytical method uncertainty was quantified by using Quality Assurance Quality Control (QAQC) data, blank samples, certified reference materials (CRMs) and duplicate samples. Two-way laboratory diamond drillholes (DDH) check samples and two-way laboratory RC check samples were used to determine the accuracy and repeatability of the analytical methods. The research identified two main sources of uncertainty in the 4E analyses; • Sampling method errors as the highest contributor to the uncertainty in the 4E assay results. • The difference in the analytical methods contributes to the difference in the 4E grades in RC and DDH. The identified issues can be resolved by re-designing the sampling method and procedures under the guidance of Theory of Sampling (TOS) principles. Analytical methods and procedures need to be improved for accurate reporting of assay results.