Suggestion Box THREE SOLITUDES

— The geologist explores and discovers mineral deposits based on his understanding of their geological controls. Thus, to explain the behavior of the mineralization, the geologist uses the known geological controls in addition to the sample grades.

— The mining engineer considers that the grades are the only objective and quantitative observations obtained by the geologist. To him, geological controls represent subjective and qualitative data.

— On the other hand, the geostatistician considers that sample grades contain much more information about the deposit than just the actual raw numbers.

These three conflicting views are well-illustrated by the estimation of a gold deposit which typically has a large number of low-grade samples and a few rich ones. The rich samples have a large influence on the estimated average grade, and its determination is very sensitive to them. Without the richest samples, the average grade may drop by 50%.

The three respective views on this are as follows:

* Providing no sampling or analytical problems are present, the geologist believes that all the sample grades are correct and should be included in the average grade estimation.

* The mining engineer knows by experience that high grades should be given less weight than the lower grades. The question is: how much less? Cutting the high grades to a lower arbitrary grade value or simply removing them are the two solutions commonly adopted by the mining engineer.

* The geostatistician admits that the influence of the richest samples should be reduced but also states that these samples represent a geological reality. Cutting or removing the high grades is equivalent to a loss of information. The solution proposed by the geostatistician is to make a statistical assumption about the grade distribution, and work with grades transformed to fit that distribution.

Three different views have been expressed on the method of calculating the average gold grade. Who has the correct solution? Probably any of the three, because each specialist has not considered all the elements necessary to provide an answer to this complex problem.

Clearly, the geologist, the mining engineer and the geostatistician must work as a team. The team’s objective should be to design an ore reserve estimation approach that provides credible results for management and financing organizations. The approach should include:

— a high standard of geological interpretation (for example, when a geological contact occurs with an important variation of grade across it, then the boundary must be drawn by the geologist before estimating the reserves); — consideration of the mining and economic parameters (the mining method and the cutoff grade, respectively) as early as possible in the reserve calculation; and

— a knowledge of the geostatistical methods available, and the ability to select a method appropriate to the geological and mining characteristics of the deposit.

Designing the ore reserve estimation approach is as important as its execution: a poor design will give poor and unreliable reserves. After agreeing on the reserve calculation procedure, the team should review critically the results at each stage of the ore reserve estimation.

To perform a geostatistical reserve estimate, computers and software are essential. During the actual calculation, it is dangerous to assume that the computer-generated results are better than the intuition of the geologist, the practical experience of the mining engineer and the systematic approach of the geostatistician. Computer software helps enhance the presentation of the results, but efforts are required by the professional geologist and mining engineer in order for these results to be fully beneficial. The problem of “computer-driven” reserves still exists but can be avoided with a strong professional team.

An integrated group composed of a geologist, a mining engineer and a geostatistician is needed to design an ore reserve estimation approach that is realistic from a geological and mining point-of-view. The design of the methodology is as important as its execution. The role of the geostatistician has evolved from the sole execution of the reserve estimation to:

* advising on the geostatistical method to choose;

* training geologists and mining engineers;

* creating geostatistical educational software; and

* being aware of new geostatistical approaches. Normand Champigny is a consultant with The Coopers & Lybrand Consulting Group, in Toronto.

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