COMMENTARY — Quality control, part 2

The best geological standards are property-specific, meaning they contain the same geological matrix as the samples being routinely analyzed.

Standards are prepared by collecting material representative of the mineral occurrence being explored. Usually, this material is collected from coarse reject following preliminary drilling or sampling. Composites weighing between 30 and 50 kg are made. A common procedure is to prepare three standards — one for each of the resource’s expected cutoff, average grade and upper concentration limit.

These composites should be forwarded to a laboratory specializing in the preparation of standards. Each composite must be crushed, pulverized and wet-screened through at least 200 mesh. The finely pulverized pulp is dried and placed in a large mixer for several days to a week. Once homogenization tests are performed, at least 50 portions of the pulp are sent for “round robin” analysis to various laboratories. Results from this analysis are compiled and treated statistically to determine the “accepted value” and error limits on that value. This process sometimes must be repeated, as laboratories may report unacceptably poor precision, or have significant bias in comparison to other laboratories.

Costs for the manufacturing and characterizing of standards average $120 per kg. The cost of three standards, each of which weighs 50 kg, may exceed $18,000. The standards are packaged either in 1-kg, heat-sealed plastic bags or in individual paper pulp bags (which are heat-sealed in plastic), then forwarded to the exploration company. Quantities prepared are normally sufficient to last the duration of an advanced exploration and feasibility program, and may well be used to “tune up” a mine laboratory. The use of the same standards will ensure that any changes in the laboratory are closely monitored, and that the numbers from a future laboratory will be similar to historical data. As with blanks and duplicates, at least one standard per batch of 20 samples should be inserted in a sample stream to monitor accuracy.

Currently, the most common method of measuring accuracy is to submit periodically a sub-sample of pulp or reject to a second laboratory for confirmatory analysis. This method does not, of course, measure accuracy, but it does establish the reproducibility of analysis and the presence or absence of bias between the laboratories.

The sampling and analysis of drill core or cuttings that are suspected of containing coarse gold must be treated carefully. However, analyzing whole core rather than half core, or taking a large sub-sample for analysis, does not solve the sampling problem. The following simple calculation is revealing: if a drill core is 10 cm in diameter and drilled to a depth of 100 metres, its total volume will be about 8 cubic metres. If the drill hole spacing is 100 metres, then each hole will represent half the distance between holes, or 100 metres. The volume between holes to a depth of 100 metres is therefore 1 million cubic metres. The drill core is 0.00008% of the total volume it is attempting to represent. That the whole core is a great deal better than half of the core does not make sense. It is more meaningful to quantify the precision so that the total sampling error for the entire volume of rock is known. This precision can be quantified and the inherent error in removing the core from the ground can be estimated by undertaking the statistical analysis from the duplicates as outlined above.

Chain of custody in the case of mineral exploration is an assurance that geological samples have been transported in such a manner as to be secure and completely traceable from the field site to a commercial laboratory. It is hoped that a proven chain of custody will prevent unusual sampling handling or tampering. A well-designed chain of custody will certainly add credibility to an exploration program, but it cannot prevent a determined effort to tamper with samples.

Quality control in mineral exploration must be supported by senior management. A series of corporate guidelines must be instituted for each stage of exploration and adhered to. This process costs money and time, and the temptation to cut corners is great, especially if the budget is tight and the will is weak. It is important for each major project to assign a quality control technician to compile and monitor data within 24 hours of receipt of results from a laboratory. Suspect analytical data must be identified immediately, before it is entered into a database or released to the public.

This in-house technician must report to an independent specialist who can anticipate problems, draw on experience, and independently audit the data. An audit of a mineral resource that does not include comprehensive quality control to prove sampling accuracy and define the precision characteristics of the mineralization is essentially worthless for preparing a bankable feasibility study.

— The preceding is from a presentation given by the author, a consulting geologist, to mining analysts, brokers and media.