Abstract
Sampling, sample preparation, and assay protocols aim to achieve an acceptable estimation variance, as expressed by a relatively low nugget variance compared to the sill of the variogram. With gold ore, the typical heterogeneity and low grade generally indicate that a large sample size is required, and the effectiveness of the sampling protocol merits attention. While sampling protocols can be optimised using the Theory of Sampling, this requires determination of the liberation diameter (dℓAu) of gold, which is linked to the size of the gold particles present. In practice, the liberation diameter of gold is often represented by the most influential particle size fraction, which is the coarsest size. It is important to understand the occurrence of gold particle clustering and the proportion of coarse versus fine gold. This paper presents a case study from the former high-grade Crystal Hill mine, Australia. Visible gold-bearing laminated quartz vein (LV) ore was scanned using X-ray computed micro-tomography (XCT). Gold particle size and its distribution in the context of liberation diameter and clustering was investigated. A combined mineralogical and metallurgical test programme identified a liberation diameter value of 850 µm for run of mine (ROM) ore. XCT data were integrated with field observations to define gold particle clusters, which ranged from 3–5 mm equivalent spherical diameter in ROM ore to >10 mm for very high-grade ore. For ROM ore with clusters of gold particles, a representative sample mass is estimated to be 45 kg. For very-high grade ore, this rises to 500 kg or more. An optimised grade control sampling protocol is recommended based on 11 kg panel samples taken proportionally across 0.7 m of LV, which provides 44 kg across four mine faces. An assay protocol using the PhotonAssay technique is recommended.
Highlights
Example X-ray computed micro-tomography (XCT) Results from Samples B5 and B6 4.3.1
The approach presented can be applied to fine- or coarse-gold mineralisation to determine the nature of the gold particle sizing and make inferences about the mineralisation
Characterisation of gold mineralisation should be undertaken from the earliest opportunity, whether from diamond drill core or exposure sampling
Summary
The importance of fit-for-purpose sampling throughout the mine value chain; from exploration, through to evaluation and exploitation, has been emphasised by many authors [1,2,3,4,5,6,7]. Field sample collection is followed by size reduction in both the sample mass and particle size to provide an assay charge or test sample. This process is challenging when sampling gold ore which contains coarse gold [8,9,10,11,12]
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