Gold losses in tailings present a particular challenge to traditional 2D based petrography and automated mineralogy techniques due to a number of factors.
- Grades tend to be very low (<0.5 g/t).
- Gold mineralisation is often nuggety and unevenly dispersed.
- Gold particles have high density and high tenor (Au minerals are proportionally very rich in gold content – i.e. native gold at >90% Au). The issue being that only a few grains will carry the entire grade of the sample.
All these factors combined mean there are often very few particles to aim for in polished section preparation. Automated mineralogy techinques would require the preparation of multiple polished blocks and the analysis of large particle populations which is expensive and time consuming, often with a low chance of intersecting a fine-grained gold particle. An additional problem would be the potential to simply obliterate very fine-grained gold occurrences during grinding and polishing. X-Ray microscopy (XRM) techniques have the potential to overcome many of these problems.
Work exploring the 3D (XRM) and 2D (automated mineralogy) based analysis of low-grade gold tailings has been carried out by Zeiss Natural Resources and Petrolab Limited respectively and was recently presented at the conference of metallurgists by Shaun Graham of Zeiss. The abstract is below and the full paper can be found here.
An evaluation of the application of X-Ray Microscopy in understanding gold losses in tailings.
Graham, S*. & Brough, C**.
*Zeiss Natural Resources Lab, 509 Coldhams Lane, Cambridge, CB1 3JS
**Petrolab Limited, C Edwards Offices, Gweal Pawl, Redruth, Cornwall, TR15 3AE
The extractive metallurgy of gold is largely controlled by mineralogical factors such as the presence of refractory gold, particle size, gangue mineral associations, presence of preg-robbers, oxygen and cyanide consumers, and locking characteristics in base metal sulphides.
Light microscopy and automated mineralogy techniques are regularly used to characterise the effect of these variables on gold recovery. However, despite their widespread application there are a range of limitations when grades are low. Typically, the analysis of low-grade samples requires the preparation and analysis of numerous polished blocks. This is often costly and time consuming, with complexities regarding analysing statistically representative particle quantities and problems overcoming stereological bias. This is compounded by the “nugget” effect and high tenor nature of gold mineralisation.
X-ray Microscopy techniques have the potential to overcome many of these problems, in particular the statistical representivity of analysis, removing stereological bias and effectively locating phases that are present in ultra-trace proportions. In this paper we outline the potential benefits of employing X-ray Microscopy (XRM) in characterising gold losses from a typical Cu-Au porphyry project, which were illusive in 2D based analysis. The talk will also outline additional application areas of XRM within the extractive value chain.