Post-Burn Mineral Exploration
The summers of 2017 and 2018 saw the displacement of thousands of British Columbians due to wildfires and affected more than 2.5 million hectares. The fires also resulted in the abrupt halt or delay to some exploration programs and changed the landscape on many mineral tenures. As a result, numerous areas and bedrock exposures were opened due to the fire, firefighting activities and, more recently, by salvage operations. Have you adapted your mineral exploration activities to the changed landscape?
Historically, prospectors have used fire as a tool for exposing outcrops and making the search for minerals less strenuous. In British Columbia (BC), however, there is very little evidence that this was a common practice as the majority of prospectors were in search of placer gold deposits making prescribed burns of very little benefit. Today, it is best that prescribed burns are left to the professionals and use modern tools to explore the areas affected by wildfires.
Satellite Imagery Enhancement
After the smoke has cleared, post-burn satellite imagery can provide a lot of useful information to help direct mineral exploration programs. The North American Space Agency (NASA) provides a large amount of free satellite data to view and/or download. EOSDIS Worldview is a browser viewer that has a lot of extra enhancement features built-in and can be used as a daily check for snow cover on mountain tops and the extents of areas affected by forest fires (if the clouds cooperate!).
If you are familiar with satellite imagery enhancement for mineral exploration, NASA's EarthExplorer is a searchable repository for their satellite imagery inventory. A lot of the data has already undergone some pre-processing such as geometric and radiometric corrections or crosstalk correction for ASTER scenes. The process of enhancing satellite imagery for BC is a bit different than the literature where deserts are typically their project areas.
After downloading the scene package, and the corresponding Digital Elevation Model (DEM), the same pre-processing algorithms used for desert areas can be applied to data that has already undergone geometric and radiometric corrections: calculate Top-of-Atmosphere-Reflectance (TOAR), perform atmospheric correction and then topographic correction. Next, an algorithm for cloud, water, shadow and snow cover can be used to devise a mask of these areas. At this point I would diverge from the methods used over desert areas and recommend masking out the areas with vegetation using the Normalized Difference Vegetation Index (NDVI). Since we are also dealing with burned zones, I also recommend masking the severely burned areas that may only represent charcoal using the Normalized Burn Ratio (NBR). Lastly, your desired band ratios or other techniques for mapping minerals can be created.
Satellite imagery can also be used to map the locations of potential outcrops. Unfortunately, the resolution of the free imagery tends to be quite coarse but using a true colour composite image in conjunction with a thermal band can assist in planning traverses to maximize the potential of finding outcrop exposures. If you are familiar with your project area, the NBR can also be used to help narrow down areas of potential outcrop.
*Please be aware of the safety measures associated with working in a recent post-burn environment.
As your project recovers from the fire, natural succession of vegetation begins and could lead to an opportunity for widespread vegetation sampling. In BC, fireweed typically returns in abundance the following Spring after a fire or other disturbance and has proven useful in target delineation. For example, fireweed surveys around the Endako Molybdenum Mine, both during early mining activities and decades later, showed high concentrations of molybdenum in ash ranging up to 17,000 ppm in 1965 and 2,950 ppm in 1998.
After a wildfire, small scale salvage operations may begin to clear standing dead trees as part of safety measures along well-used resource roads and/or to harvest trees that were damaged beyond the ability to live successfully by fire, insects or wind storms. Salvage activities begin shortly after the damage occurred and may include road building and/or create landings which are excellent areas to search for outcrop and take samples. Post-burn satellite imagery can assist with digitizing new roads and other areas of disturbance caused by firefighting activities and post-burn salvage operations.
While the summer of 2020 has had many challenges, wildfires haven't been as prominent so far this season. Within the first week of August 2020, however, BC wildfire fighters were dispatched to 111 fires . We should, therefore, not forget the hard work and bravery of our world-class firefighting organizations nor the people, places and animals affected by the fires. Please consider donating:
About the Author:
Dr. Diana Benz 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. 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 researching 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.
I am a big fan of using theses as references. There is a level of detail provided in theses that is considerably lacking in articles published in periodicals. The following thesis has an interesting collection of anecdotes from the late 1800's to early 1900's. Parminter, J. V. (1975). An Historical Review of Forest Fire Management in British Columbia. Master of Forestry Thesis. The University of British Columbia.