Soil Sampling Strategies for Mineral Exploration

Adequate planning is required in order to ensure that soil geochemical surveys provide the information necessary to allow exploration management decisions to be made with confidence. An understanding of the material to be sampled, whether it's surficial material, rock samples or drill core, and the elements of interest are required in order to design the most cost-effective and informative program. An estimate of the expected size (or footprint) and anticipated geochemical signature of the mineral deposit being siought are also desirable. The planning stage will include the compilation of all relevant information, and may involve the use of specialist expertise in surficial geology and/or remote sensing.

The adjacent example to the shows the complexity that may be required in soil survey design where surficial materials having a variety of origins are encountered within a single project area. Soil surveys in this particular project area included ridge & spur traverses to sample residual soils, contour and toe of slope surveys to sample down-slope dispersion in colluvial soils, and grids to sample high-level, remnant till. Sampling crews must be trained to recognize the different soil types in the field and are encouraged to avoid sampling the incorrect material for a particular survey. Correct survey design allows specific questions regarding both mechanical and chemical dispersion to be addressed and removes ambiguity in the interpretation of the results.

Stream Sediment Sampling

Analytical methods are selected on the basis of whether partial (or selective) analyses are required, the lower detection limits needed to define sufficient anomaly to background contrast, and potential interferences related to the proposed sample matrix. In some instances it is preferable to undertake an orientation survey to:

• Determine the optimal sampling media for defining anomaly to background contrasts;
• Trial various sampling methods and approaches;
• Identify the geochemical signature and areal extent of either primary or secondary dispersion;
• Select appropriate digestion and instrumental finishes to provide necessary sensitivities; and
• Define background and anomalous thresholds that might be applicable to the main survey.

In the example provided below of stream sediment data from the Yukon, Canada, publicly available gold data show poor reproducibility in field duplicates. Making exploration decisions based on non-reproducible data is fraught with challenges! Instead, an orientation survey was conducted to compare the standard approach using -80 mesh sieved material, a Bulk (cyanide) Leach Extractable Gold (BLEG), and clay separates. Clearly the clay separates gave the most reproducible results of the three methods tested and would be the sample preparation method of choice going forward in this particular area.