Assaying

The following is an excerpt from the ninth edition of Mining Explained, published by The Northern Miner. The book sells for $30 plus $3 for shipping and is available by calling 1-800-668-2374. E-mail: northernminer2@northernminer.com.

The most promising hand samples and core invariably wind up in a laboratory 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 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, in 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.

Often in 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 ft. 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.

Resource calculations

The quantity, or tonnage, of mineralized material in a deposit can be calculated if the volume of the deposit is known. The volume can be measured by the widths and depths of the drill holes, and the distance between them. It is now usual to make such calculations using sophisticated computer software that also allows a three-dimensional view of the deposit on screen.

Multiplying the volume by the average density of the mineralization gives the number of tonnes in the deposit. Although this sounds simple, orebodies can be quite complex: densities change, grades are often discontinuous, and the shape of an orebody is often highly irregular.

The grades determined by sampling the drill cores are plotted, and a weighted average of the grades is calculated, giving a resource figure. This estimate gives the grade and tonnage of the deposit, so far as it is known by drilling. More tonnage can be added by drilling areas that have not yet been drilled but extend along the strike and down the dip of the mineralized zones.

Resource estimates are classified as:

— measured — if the drill holes are closely spaced and the geologist is satisfied that the tonnage is reasonably certain;

— indicated — if a significant amount of drilling has been done but some zones not fully tested; and

— inferred — if the estimate is based on information from widely spaced drill holes.

Reserve calculations

Although a resource estimate gives some idea of the amount of mineralized material in the ground, it does not imply anything specific about the practical question of mining the deposit. A reserve estimate, on the other hand, refines the resource estimate by placing economic constraints on the size and grade of the material that is brought into the calculation.

The cutoff grade is the grade below which the rock is assumed to be uneconomic to mine. This grade will vary according to such factors as mining costs, metallurgical recoveries, and possible credits from associated minerals that can be recovered as byproducts. In a reserve calculation, any parts of the mineralized zone that have grades lower than the cutoff are not counted into the estimate.

If a mining professional is calculating a reserve estimate for a possible underground mine, it will be essential to include only the mineralized zones that are wide enough to mine. Small zones of waste between and along the sides of mineralized zones will be included in the estimate as dilutive material.

If the estimate is being made for an open-pit mine, a pit must be designed to show the limits of the mineralization, plus the waste rock that must be removed. The stripping ratio is the mass of waste rock that must be removed to mine a unit mass of ore. For example, if a 20-million-tonne orebody is mined from a pit with a total of 80 million tonnes of rock, the 60 million tonnes of waste give the pit a stripping ratio of 3 to 1.

It is now common to make several reserve calculations, each one based on a different cutoff grade and showing different mine or pit designs. The company can then choose from several different possibilities when it develops a final feasibility study on the project.

Like resource estimates, reserve estimates are labelled to show how reliable they are — proven, probable and possible. Only proven and probable reserves are considered in a feasibility study.