In high-nugget gold ore bodies, samples taken from drill core...

In high-nugget gold ore bodies, samples taken from drill core for gold assay are typically too small to allow for the extreme spatial variability in grade and are a poor representation of the underlying distribution of mineralisation. The GQ North lode at the Sunrise Dam Gold Mine in Western Australia is a good example of an orebody which has a very strong nugget effect (coefficient of variation >20) and that has proved very problematic to model.

The screen fire assay method is typically used to analyse samples containing coarse Gold. A nominal 1kg of sample is split from the bulk pulp and re-pulverized to ensure the oversize portion is not excessive. The sub-sample is then screened through the nominated screen cloth. The entire coarse fraction (including the screen cloth) is fired. The fine fraction is analysed in duplicate by fire assay. A weighted average is then calculated to determine the total Gold content for the sample.