Mineral Exploration is the official publication of the Association of Mineral Exploration British Columbia.
Issue link: http://digital.canadawide.com/i/1319479
44 Mineral Exploration | amebc.ca SPONSORED CONTENT Fact or Fiction Which tales do your surficial samples tell? By DAVID SACCO, PECG E very surficial sample, be it soil, surface sediment or vegetation, tells a story. The challenge in mineral exploration is knowing whether that story belongs in the fiction or non-fiction aisle. As surficial geologists, our job is to help make that determination. An exploration program that properly considers the nature of the overburden – or surficial geology, to be more explicit – invariably saves money and expedites the path to successful drilling. Overburden is a dynamic tool for mineral exploration, yet many exploration geologists do not know how to use it effectively. Unfortunately, it's still all too common to conduct generic "soil" sampling along a grid without considering the surficial geology. In previously glaciated terrain, these soil grids have a high potential to mislead exploration. Surficial geologists are sometimes completely omitted from mineral exploration projects. But I guess this isn't that surprising, considering few geology programs offer courses that even touch on the application of Quaternary science to mineral exploration; many geologists finish school without learning how important and complex the implications of surficial geology can be. Without this surficial context, the line between fiction and non-fiction becomes blurred. Surficial samples are commonly used as a foundation to direct geophysics and drilling programs. My colleagues and I have spent a lot of time in the field and have seen firsthand how the quality of surficial exploration data can influence the success of a program. It is essential that the surficial exploration data are reliable or the project will be at risk of costly mistakes. We've had the pleasure to meet and learn from a lot of clever geologists. If we've done our jobs correctly, they've also learned from us. Working together, we've witnessed the "aha moment" when the importance of understanding the surficial setting really hit home. Some of these moments lead to exciting stories where an important tool was added to someone's mineral exploration toolbox, and others were unfortunate tales of deception. Let's start with a historical fiction about diamond exploration in the Lac de Gras region of the Northwest Territories. The client had been following a poorly defined dispersal of indicator minerals in till. There were subtle patterns apparent in the vintage, washed-out air photos, but it was difficult to determine if or how the dispersal definition was related to these patterns. So, off we went to investigate. After breaking one of our only two shovels, we cautiously proceeded to characterize gravelly diamicts, conspicuous mounds of poorly sorted sediment and outcrop-scale evidence of fluvial erosion. It became clear that predictable pathways were reworked by meltwater. The area was generally mantled by till, as indicated in the regional mapping, but this existing mapping did not identify the subtle processes that affected the till after deposition. Armed with field-verified calibration points, we re-examined the air photos, delineated the areas affected by meltwater and filtered out the reworked till samples. The resulting dispersal was better defined and provided a better understanding of the source area. Our clients learned that not all till is "good till" and that it's important to consider subtle differences in sample media that may affect their comparability. We learned that conventional surficial mapping protocols did not provide all the information necessary to inform exploration. Going forward, we would tailor our surficial geology interpretations to include the unique characteristics of each surficial environment that can affect exploration. Now a mystery story about soil sampling. To be clear, we're referring to sampling near-surface soil horizons with the expectation that the results are comparable and/or indicate mineralization in underlying bedrock. This common default approach to sampling and interpretation is a direct- detection method carried over from regions like Australia or Argentina where soils and soil anomalies have developed over hundreds of thousands of years. Unfortunately, the soils in previously glaciated terrain are typically too young, and the materials too complex, for this method to be effective. A recent example of this comes from a gold project in northern British Columbia. After a series of unsuccessful drilling campaigns, the client asked us to visit the site and review the soil sampling program results. We investigated several sample sites with the client and discussed the different materials and their varying potential for soil David Sacco Thick complex of glacial sediments exposed along a river in central British Columbia Helicopter drop-off on a diamond property in Northwest Territories