Specific survey design & analytical method services within this area include:
Reviews of potential sampling media, such as rock/drill core, soil, drainage sediments, water and biological material
Design of soil sampling surveys to optimize spacing (sample and line spacing) and orientation
Design of and stream sediment sampling programs using a catchment analysis approach
Preparation of Standard Operating Procedures (SOP) for the collection of soil, stream sediment, rock, vegetation and water samples for mineral exploration and recommendations of metadata to collect during sampling
Recommendations on suitable laboratories or use of field portable equipment
Contracting arrangements with experienced sampling crews
Conduct and interpret orientation surveys to optimize:
Soil grids were used in areas of till. Contour sampling was used in areas of colluvium. Ridge & spur sampling was used in areas of residual soil.
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.
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.
Pre-existing Au Analyses From Field Duplicates (left) Compared to Orientation Survey Results (right)
The orientation survey has identified the best grain size fraction (clay) to sample in order to produce repeatable data (from Arne & MacFarlane, 2014)