As discussed in our first installment, orientation studies should enable the selection of optimum field, laboratory and interpretation procedures. Ideally, these procedures should allow for the clear resolution of significant anomalous patterns, with as little overlap as possible between anomalous and background populations.
Below is a list of some of the most important factors that need to be determined during a residual soil geochemistry orientation study:
Understanding the study area
- The nature of the surficial environment, e.g., the influence of topography, drainage, vegetation, and rock type on soil profile development and geochemistry
- Optimal soil horizon(s) for sampling
- Optimal sample fraction for analysis (based on size and/or magnetic, specific gravity or organic properties)
- Sample collection procedures
- Sample size requirements
- Reproducibility of sampling
- Optimal sampling interval
- Field observations that need to be recorded
- Potential sources of contamination
- Geochemical suite for analysis that highlights the best indicator elements (ore, pathfinder and/or lithological)
- Sample preparation procedures
- Sample digestion methods (strong acid, weak acid, fusion, etc.)
- Analytical methods (consider detection limits, precision, accuracy, interferences, etc.)
Data interpretation considerations
- Background ranges of indicator elements in local rock types, as well as anomaly threshold level(s)
- Using traverses across the presumed mineralized area to better understand the intensity, shape, extent, and homogeneity of anomalies
- Interpretation procedures
- Data presentation formats
While not an exhaustive list for soil orientation surveys in residual terranes, the above can seem overwhelming, especially when first starting out. How does one even begin to start to answer these questions when perhaps you have not ever considered sampling anything other than the B horizon (what is the B horizon?!) or using any size fraction other than the the standard 80# (# = mesh, which is the 180 µm particle size fraction).
Relax. Information to help you on your journey can be gathered by reading case histories of geochemical programs in either your area of interest or other areas with similar geological and surficial environments. If there is nothing to be found… start with the basics, by designing a program that collects a sufficient amount of sample to try different soil horizons and particle fractions to find out what the best combination is for your program.
Field areas selected for orientation studies should contain the full range of geological and surficial conditions and types of mineralization anticipated in the proposed project area. To ensure effective definition of optimum survey parameters, samples of the relevant sample media should be collected at sufficiently close intervals and of sufficient size to permit the full evaluation of the parameters of potential interest.
Remember: background / non-mineralized samples must be collected as they are essential to identify which combinations of size fraction, soil horizon, analytical digest, and finish amplifies the mineralization signal.
Finally, keep your geological and regolith maps handy! Working in tandem with these pieces of information is essential towards preventing the identification of false anomalies, for example, areas with a basalt basement will have higher background base metals than a carbonate or rhyolite. Keep your geology hat on and when in doubt – ask for help!