Air prospecting less in demand

The era of airborne geophysics (aero-magnetics and aero-electromagnetics) is far from over but its heyday in Canadian mineral exploration is certainly passed.

That time was in the late 1950s to the early 1970s when a host of base metal deposits, if not found directly from airborne work, were initially identified by it. It was a remarkable period. It was the expression of the work of dozens of dedicated and gifted individuals who had been working on airborne systems since the end of The Second World War.

Most of these individuals sensed Canada to be the ideal country for airborne work. There was an enormous area to be prospected and the airplane was the only practical way to do it.

The end of the war had a major impact, too. The U.S. Navy had successfully developed the airborne magnetometer for locating enemy submarines. The question was asked, if aircraft could use magnetometers, surely it should be possible to package electromagnetic systems for aircraft as well. The challenge was there and the response was taken up by an improbable assemblage of theoretical geophysicists, electronic tinkerers and diehard entrepreneurs (sometimes all three wrapped up in the same person). The results were dramatic. Among the host of new orebodies uncovered were many of Canada’s major orebodies, including Thompson, Mattagami, Sturgeon Lake and Kidd Creek.

Like so much in the technical world, the basics of airborne geophysics are simple to describe but extraordinarily difficult to resolve in practice. Briefly, a body of massive sulphide in the earth’s crust is an electrical conductor. If a flow of electrical current can be made to flow through the sulphide then the sulphide will create its own distinctive electrical field. If this distinctive secondary field can be separated from a mass of extraneous electrical responses (noise), then the observer can say, “there’s an electrical conductor down there and it warrants detailed investigation on the ground.”

In practice, the electrical wizardry is housed in a towed “bird” about 20 ft. long and 2 ft. in diameter. It is towed by a helicopter or fixed-wing aircraft and maintained 100-250 ft. above treetop level. Obviously, the closer to the ground the better. Even under ideal conditions, the depth to which the system will penetrate below surface and register a conductor is less than 150 ft.

In addition, for every electrical conductor that is composed of sulphide there will be several score resulting from other causes. Graphite is the main spoiler in that category. Also to be remembered, only a few bodies of sulphide carry commercial values so there is a long, expensive trail to follow once a series of anomalies has been detected.

To give some idea what the modern prospector is faced with, consider a recently completed airborne geophysical survey carried out for the Ontario Ministry of Northern Development and Mines. This survey comprised 13,640 line miles of survey in the Shebandowan area, Thunder Bay, Ont., and detected 20,740 electromagnetic anomalies. Some trail!

In this case, parallel lines 650 ft. apart were surveyed and this is a fairly common spacing.

The reason air surveys of this kind are no longer carried out in the volume or with the intensity that characterized the late 1950s to the early 1970s is to be found in metal prices and the nature of airborne prospecting itself. In the first case, gold has been the target of choice since the late 1970s and massive sulphides host base metals rather than gold. The choice of target has altered.

Second, airborne surveys detect only the shallowest of orebodies. Conventional thinking is that Canada has reaped most of its near outcropping orebodies. The future of mining lies at depth. Logic gives some support to such hypothesizing but long-term practitioners will claim with some force that this is anything but a predictable industry.