The Trials and Tribulations of "Soil" Sampling in Glaciated Terrain
Updated: Dec 18, 2020
During the last glacial episode, the vast majority of Canada was glaciated. As a result of this recent glaciation event, the parent materials of today's soils are predominately glacial in origin and relatively young (geologically speaking). In other words, the soil chemical composition is strongly influenced by unconsolidated material with transported origins (e.g., till) and some areas have very poor soil development.
Example site in BC's Central Interior near Fraser Lake: top layer is ash with few organics and ~0.5 metres of primarily cobblestones, silt loam (like flour) and minor gravel.
Example from the same project area as above, approximately 200 metres downslope. Sample was collected from under an overturned tree root ball in a wet area with approximately 0.5 metres of organics topped with a thin layer of charcoal. The sample consisted primarily of moist medium brown sand that dried to a pale green (60%), 15 % silt, 10 % clay, 5 % gravel and 10% cobblestone.
Cook and Dunn (2006) Till Profile from the Central Interior, 125 kilometres South of Vanderhoof, page 32
There are exceptions, of course, such as the Yukon where a large portion was free of ice during the last ice age (25,000 to 10,000 years before present). This lack of recent glaciation in the Yukon created areas that underwent long periods of weathering and erosion to form rounded summits suitable for soil sampling. Within British Columbia (BC), however, the erosion effects of the glaciers was widespread and most prevalent on high plateaus and mountains.
In the mountainous areas of BC, that were not completed covered by ice, we find jagged peaks in the form of horns, arêtes and cirques allowing for samples of outcrop, subcrop, talus, stream sediments and talus fines. On plateaus and plains, the landscape consists of grooved and fluted terrain where the most intense erosion took place in valleys aligned with the direction of ice flow: for example, areas within the Interior Plateau of BC. Geochemical sampling in plateaus is particularly challenging as outcrop exposure is limited; eskers, kames, drumlins, fluted till and meltwater channels are extensive; glaciolustrine silts are found within many valleys and basins; and valleys with major rivers have been dissected and terraced by post-glacial downcutting. An added complexity is the geologically recent basaltic flows that cover a large portion of BC's Interior.
A simultaneous biogeochemical and till sampling program was conducted by Dunn, et al. (1996) over the Mount Milligan Copper-Gold Porphyry Mine in central BC. It was found that both sampling medias showed spatial correspondence with known mineralized zones where the till cover was thin (~ 1 metre). In the areas with thin till cover, the biogeochemical samples were more spatially constrained over the known mineralization whereas the till geochemical anomalies showed some dispersion in the direction of the ice movement. In the areas with thicker till cover (~ 80 metres), the anomalous till samples were displaced ~ 2.5 kilometres down-ice whereas the biogeochemical samples were centred over the known mineralization. This discrepancy in sampling media results may be due to the vegetation drawing the chemical signature from the water table whereas till samples are representative of transported parent material.
While the Huckleberry Mine isn't located within the Interior Plateau of BC, but it is in the transition zone between the Nechako Plateau and the Coast Mountain, this study shows the challenges of working in glaciated terrain.
In 2000, the BC Geological Survey conducted a till geochemistry and ice-flow mapping project at the Huckleberry Copper-Molybdenum Porphyry Mine in west-central British Columbia. They were looking to model the dispersal plumes of mineralized zones using clast lithologies and trace element geochemistry to help provide guidance on the interpretation of till geochemistry in relation to the bedrock source of the anomalies. The results of the study concluded that till thickness is an important indicator of how long the till has been preserved and till thickness affects the conclusions that can be drawn regarding the net direction: multiple events affect the direction and thicknesses of the dispersal trains.
Ferbey, et al. (2012) Thick Till Dispersion Model for the Huckleberry Mine Area, page 42
Ferbey, et al. (2012) Thin Till Dispersion Model for the Huckleberry Mine Area, page 44
In general, the study found that the sub- and near-surface till anomalies were offset 500 to 1,000 metres from their associated mineralized bedrock source.
From a greenfields perspective, how would this information affect planning sampling programs?
Determine the regional Quaternary deposits for your area of interest: BC Geological Survey Surficial Mapping.
Conduct test pits or use auger/probe descriptions to discover the type of cover, composition and depth for the target area.
Develop a sampling plan appropriate to the surficial characteristics of the target area.
Interpret results in terms of the effects of primary, secondary, post-secondary and controlling factors.
Blaine, et al. (2012) Dispersion Effects on Soil Sample Copper Distribution at the Mount Milligan Porphyry Deposit, page 34.
Blaine, F.A., Hart, C.J.R (2012). Geochemical-Exploration Models for Porphyry Deposits in British Columbia. Geoscience BC Report 2012-1.
Cook, S.J. and Dunn, C.E. (2006). A Comparative Assessment of Soil Geochemical Methods for Detecting Buried Mineral Deposits - 3Ts Au-Ag Prospect, Central British Columbia. Geoscience BC Report 2007-7.
Dunn, C. E. (2007a). Biogeochemistry in Mineral Exploration, Handbook of Exploration and Environmental Geochemistry. Vol. 9. p. 462. Amsterdam: Elsevier.
Ferbey, T., Levson, V.M. and Lett, R.E. (2012). Till Geochemistry of the Huckleberry Mine Area, West-Central British Columbia (NTS 093E/11). British Columbia Geological Survey Open File 2012-02.
Don I. Cummings & Hazen A. J. Russell (2018) Glacial dispersal trains in North
America, Journal of Maps, 14:2, 476-485, DOI: 10.1080/17445647.2018.1478752.
Update December 18, 2020 Fact or Fiction Which tales do your surficial samples tell? by David Sacco, PECG
About the Author:
Dr. Diana Benz has 24 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. She has a Bachelor of Science in General Biology, a Master of Science in Earth Sciences researching diamond indicator mineral geochemistry and a Ph.D. in Natural Resources and Environmental Studies on using geochemical multivariate statistical analysis techniques to interpret biogeochemical data for mineral exploration. Diana has conducted field work in Canada (BC, NWT, YT and ON) as well as in Greenland. She has also been involved, remotely through a BC-based office, on mineral exploration projects located in South America, Africa, Eurasia, Australia and the Middle East. 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.