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Airborne geophysics in combination with geochemistry is helping to unveil more deposits in the Yukon's White Gold district.
By Virginia Heffernan
The discovery of the White Gold deposit by Underworld Resources in 2009 touched off a massive exploration rush in central Yukon that continues to grow in tandem with new discoveries of epithermal style lode gold and copper-gold-molybdenum porphyries.
Until recently, geochemistry has been the exploration technique of choice, specifically deep auger soil sampling successfully introduced to the area by prospector Shawn Ryan (see accompanying profile).
But geophysics is starting to play a more significant role in the minefinding process for the same reason soil sampling works so well: bypassed by the glaciers, the region's bedrock has had millions of years to weather undisturbed. Airborne geophysics sees through the resulting overburden to identify underlying structures or geological contacts that control mineralization, and can help the geochemistry teams narrow down sampling locations.
One of the companies flying extensive airborne geophysical surveys this year is Taku Gold Corp., a major claimholder in the White Gold district with a total of 76,238 hectares.
Having staked such a large land package, Taku needed a way to focus its exploration efforts. So the Vancouver-based company hired Precision Geosurveys to fly airborne magnetics and radiometrics in order to collect basic information about the underlying bedrock before launching soil sampling surveys.
Precision used a helicopter to fly 2,500 line kilometres at a 100 metre line spacing over Taku’s properties. Its high resolution magnetometer system is designed to detect bedrock units, structure and alteration. The radiometric system, consisting of a multichannel spectrometer with state-of-the-art design and large gamma ray sensing crystals, helps determine lithologies and alteration.
Using Geosoft’s Oasis montaj, Precision converted the resulting data from a binary file to a GDB file to display all the data channels. After the conversion, Precision ran Geosoft scripts and filters to eliminate noise and enhance the data to identify potential anomalies.
For instance, radiometric data measure the energy of gamma rays being emitted to infer the presence of particular radioisotopes. Different rock types and alteration patterns produce different radioactive signatures, but features such as swamps and lakes can attenuate these signatures. Precision used Geosoft to generate maps with different radiometric isotope ratios to filter out the effect of attenuation.
Using a leveling tool - an extension of Oasis montaj - Precision leveled the survey and tie line data to create the final processed data, and then created grids to display the gridded data. For more advanced analysis, Precision used MAGMAP, a filtering extension, to enhance the gridded dataset. To produce the final product, Precision created basemaps on top of the grids and imported topography into the background.
“The mathematical algorithms developed by Geosoft speed up our processing and are fully compatible with other GIS programs,” says Precision geophysicist Jenny Poon. “We also use Geosoft to generate the flight grid and the kilometers per line. We can calculate a quote for the client from that information”.
In the White Gold district, mineralization appears to be concentrated along northwest-trending fault structures at the contact between gneisses and ultramafic rocks. There is also an association with Cretaceous intrusions. Precision found indications of both on Taku’s properties.
“In the surveys completed for Taku, Precision identified numerous linear structures that may represent faults as well as numerous rounded features that may represent buried intrusions,” says Mark Feneke, a Taku director and professional geologist guiding the exploration program.
Taku is using the geophysical results to plan deep-auger soil geochemical surveys, targeting both the prospective linear features that may be faults and the rounded features that may be buried intrusions. This year, the company plans to collect 30,000 soil samples and fly several thousand kilometres of geophysics, then follow-up the results with 7,500 m of drilling.
The White Gold District lies within the Tintina Gold Belt, a 200-km-wide, 1,200-km-long arc extending from northern British Columbia into southwest Alaska. This metallogical province has produced 30 million ounces of gold in the past, but within the White Gold district exploration is just beginning.
Poon estimates that only about 10% of the area has been flown so far, suggesting more deposits are yet to be unveiled by airborne geophysics in combination with geochemistry in this extensively overburden-covered area.