Assaying Methods

The most promising hand samples and core invariably wind up in a lab oratory to be analyzed. This process, in which the precise constituents of the rock can be measured and catalogued, is called assaying.

The chemist chooses an assay method that best determines the concentration of the metal of interest. Among the methods commonly used today are:

• fire assaying, in which the sample is melted and the unwanted elements are chemically removed;
• wet assaying, in which the sample is dissolved and metals are recovered chemically using reagents; and
• instrumental analysis, in which the metals’ atomic properties are detected — for example, their response to x-rays or visible light.

After surface assay results are returned from the lab, the location of the samples and their corresponding assay values are plotted on a map to give a two-dimensional picture of the potential ore zone.

In later stages of exploration, core assays are similarly plotted on maps, adding a third dimension that enables the geologist to visualize the entire orebody.

In the case of gold exploration, any unusually high values are cut; that is, they are not included in the average. Isolated high values are frequently found not to reflect the grade around the sample location.

If the assay result is from a channel sample or a length of core, it is written in terms of metal concentration over a given length — for example, 5 grams gold per tonne (0.15 oz. per ton) over 8 metres, or 10 feet of 3% nickel. While these are only two-dimensional snapshots of a mineralized zone, enough of them in one vicinity combine to form a three-dimensional picture of the tonnage and grade of a deposit.