Preliminary testing along a local road has detected conductors and areas of high induced polarization (IP) in both conductive (shales) and resistive (sandstones and volcanic rocks) zones, suggesting the presence of disseminated sulphides, said Marc Richer-Lafleche, a professor at the Institute National de la Recherche Scientifique (INRS) in Quebec City, who is leading Beauce's geophysical team.
More recent discoveries of gold in the saprolite -- formed by deep weathering of the bedrock surface -- suggest that the source lies nearby, but a one- to 25-metre thick cover of quaternary till and dense forest have made prospecting in the area a challenge.
Exploring with electromagnetism. New surveying technology to find a historic gold deposit
Canadian Institute of Mining, Metallurgy and Petroleum.
The time-domain electromagnetic (TDEM) system can identify the conductive black shales that stymied previous exploration attempts. Courtesy of Beauce Gold Fields.
Montreal-based Beauce Gold Fields is using a time-domain electromagnetic (TDEM) system developed in Russia to find the hard rock source of the Beauce placer gold deposits in southern Quebec.
The company hopes that by separating out induced polarization (IP) and conductivity measurements, the high-resolution system will be able to see past the conductive black shales – commonly mistaken for sulphides – that have stymied previous geophysical attempts to detect the source of the gold in the Gilbert River Valley near St-Simones-les-Mines, about 100 km south of Quebec City.
Richer-Lafleche decided to give the mobile TDEM system a try after reading several articles written by Professor Georgy Trigubovich and Dr. Anton Chernyshev of Aerogeophysical Surveys, a leading airborne geophysical company in Russia. Trigubovich, the deputy director of science in geophysics at the Siberian Research Institute of Geology, Geophysics and Mineral Resources, has developed a number of innovative techniques to improve the reliability of resource predictions. His academic colleague Chernyshev specializes in using mathematical methods to solve geophysical problems.
"This demonstrates the importance of being able to separate the electrical conductivity and induced polarization components," he said. Added to that feature is a horizontal resolution of about 15 centimetres, allowing the team to detect relatively narrow sulphide mineralized bodies, such as gold veins.
The Beauce ground survey follows an audio magnetotelluric, gravimetric and geological survey conducted by INRS to detect discontinuities that might represent faults or shear zones that could be conduits for gold mineralization. The comprehensive survey detected a 1,500-metre-deep structure underneath the placer deposits and multiple conductors that could be either graphite or sulphide mineralization. The TDEM survey will better characterize the location and physical characteristics of these anomalies.
In 1846, the discovery of a 2.5-ounce nugget in the St. Gilbert River near St-Simon-Les-Mines triggered Canada's first gold rush. Beauce, formerly HPQ Silicon Resources, holds claims covering a six-kilometre-long unconsolidated sedimentary unit that hosted placer gold mines from the 1860s to the 1960s. Although the mines produced an estimated 1.5 short tons of gold, the source of the placer deposits has never been found.
According to Richer-Lafleche, the Russian technology has located both gold and diamondiferous kimberlites in Siberia. It has been adapted to operate on relatively narrow Canadian forest roads and can be towed behind a snowmobile on land and behind a boat on lakes and rivers.
The system uses an induction loop powered by currents of 10 to more than 50 amps and two receiving antennas positioned to allow optimal decoupling of conductivity and IP. Depending on the power of the current, the number of turns of the induction loop and the conductivity of the ground, the system can probe depths of up to 250 metres.
"The critical point of the method is the configuration of the transmitter-receiver system and the quality of the algorithms used to separate the electrical conductivity and IP components," said Richer-Lafleche, who is also testing the technology on Falco Resource's Noralex project in the Rouyn-Noranda mining camp, where historical drilling returned low-grade gold values over wide intercepts.
Beauce is currently using the system on country roads and will expand the survey to agricultural fields and private forest roads at the end of the year. "Using this new state-of-the-art exploration technology will add to our understanding of the geology and help us reach our goal of finding a hard rock source of the famous Beauce placer gold deposits," said Beauce president Patrick Levasseur.
By Virginia Heffernan
May 09, 2019