LETTER TO THE EDITOR — Determining gold content in samples

During the past year, your paper has covered, with appropriate caution, reports of unusual gold mineralization from Fort MacKay in Alberta, Brookfield in Nova Scotia and a property in Arizona. The reported gold phases include tellurides and alloys.

At Fort MacKay and Arizona, they were sought to explain high gold assays obtained by unconventional means and not attainable by conventional fire assaying. The unusual mineral phases at Fort MacKay appear to have been determined mainly by examinations, using an ultra-high-magnification scanning electron microscope (SEM) or electron microprobe, of small specimens — in some cases, heavy mineral concentrates rather than raw rock. SEMs and electron microprobes are marvelous instruments for studying minute gold particles. However, they cannot produce gold assays. Identification of a few minute particles of native gold, gold tellurides or gold alloys does not necessarily indicate significant gold in the sample.

We have been conditioned to worry about the nugget effect, but samples with a low gold content may actually contain a surprisingly large number of gold particles. Fine gold particles can also be introduced accidentally during sample preparation.

Determining whether an identified gold mineral phase is quantitatively significant requires a mass balance comparison of the number and size of observed gold particles versus reliable assays, as shown by the following example.

Our laboratory was asked to extract any particulate gold from material grading about 250 parts per billion (ppb) gold (0.007 oz. per ton). This material was thought to contain only micron-sized gold particles because the assays were reproducible. However, we were able to extract 20% of the gold value as native particles having an average width of 25 microns (half the width of a human hair). The number of particles of this size was about 250,000 per tonne, or 1,000 particles per ppb per tonne. If the remaining gold occurred as 1-micron particles of a similar shape, the total particles present would be a phenomenal 16 billion, or 64 million per ppb per tonne. It follows that similar, slightly anomalous rock grading only 25 ppb gold would still contain 1.6 billion gold particles per tonne. An ultra-high-magnification SEM or electron microprobe would probably have little difficulty locating a few of these particles in a small specimen, but the material would still grade only 25 ppb gold. This fact would be unalterable even if the particle identifications were made by a large government research body or a major gold mining company, as in the Fort MacKay case.

Stuart Averill

President

Overburden Drilling

Nepean, Ont.