There's gold in them thar ... trees!?: Understanding Biogeochemical Data in Mineral Exploration
Updated: Mar 1, 2019
Thanks to Mark Twain's iconic character Mulberry Sellers, from the 1893 novel "The American Claimant", we get the iconic phrase, based on Twain's interview with Georgia assayer Dr. Matthew Stephenson, that sums up the excitement and popularity of the California Gold Rush (1848-1855): "There's gold in them thar hills" and "there's millions in it". During the late 1890's, the search for improved methods of finding ore blossomed and determining the relationship between plants and mineral deposits is thought to have begun. There was a slow start to this research and even in present-day there tends to be a lot of skepticism as to whether biogeochemical mineral exploration is reliable.
Similar to geochemical data, biogeochemical data is the study of the relationship between the compositions of biological materials and geological processes. The resulting data are compositional (not free to vary independently) and are defined in terms of quantities and ratios. The primary differences between geological and biological sampling materials are that biological materials are not limited to what's in the soil (they access the water table too!) and there are filtering and uptake mechanisms that may enhance or subdue a chemical signature.
What does a biogeochemical survey look like?
Planning a biogeochemical survey is the same as to setting up a geochemical survey: an area is targeted, sample spacing is determined and an appropriate assay method is chosen. Akin to geochemical sampling, the type of material to be sampled must also be consistent; for example a specific horizon in soil samples or basal material for till samples. Selecting biological tissues to sample requires previous knowledge of the area to ensure that the frequency and distribution of a particular plant species to sample is adequate based on your chosen survey density as well as ensuring the type of tissue collected will be unlikely to change during the course of your survey. For example, the chemical composition will differ between leaves collected in the Spring and leaves collected in the Fall, but outer bark material will remain constant. Multiple species of plants may be collected while surveying but the results should never be directly compared as many species have different tolerances and uptake mechanisms which could affect element concentrations.
I have biogeochemical results, now what do they mean?
Well, first off, your results will likely show considerably more subdued concentrations than rock samples, especially if the samples were macerated prior to assaying as compared to ashing (concentrating) the samples, and will likely be more similar to the concentration levels expected in soil samples. The samples, however, will not correspond with soil or till samples taken in the same spot because the plant's uptake is not limited to the soil's composition and is relative to the area of the roots (rather than one small hole in the ground). For example, work in the Arizona desert has shown that soil had less contrast than the overlying plants and the underlying water table while work at the Mt. Milligan Mine in BC showed distinct biogeochemical anomalies where till samples were subdued or displaced down-ice.
Sample collection is quick and the samples are much lighter, ~30 grams each
Sampling may be conducted year-round even in temperate areas (e.g., outer bark samples of some conifer trees)
Effective in areas with deep till or organic cover where outcrop is limited and soil or till sampling would be difficult
Anomalies tend to be distinct and directly correspond to underlying mineralization
Sample survey set-up requires prior knowledge of the area
Samplers require basic species identification and tissue recognition training
In-depth data interpretations may require an experienced biogeochemist
For further information: Colin Dunn's text on Biogeochemistry in Mineral Exploration (2007)
About the Author:
Diana has over 20 years of experience working in the mineral exploration industry searching for diamonds and metals in a range of roles: from heavy minerals lab technician to till sampler, rig geologist, project manager and business owner. Following a Master of Science degree in diamond indicator mineral geochemistry, Diana has conducted field work in BC, NWT, YT, ON (Canada) and in Greenland. She has also been involved, remotely through a BC-based office, on mineral exploration projects located in South America, Africa, Eurasia, and the Middle East. Diana finished a Ph.D. at UNBC in 2017 researching geochemical multivariate statistical analysis and interpretation. Currently, Diana is the owner of Takom Exploration Ltd., a small geological and environmental consulting firm focused on metal exploration in BC and the Yukon.