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by Virginia Heffernan on MaRCH 8, 2013 expertise
Matching an exploration target with a geophysical technique can be tricky for an explorer, partly because there are so many options available and partly because there can be significant differences in rock properties even among the same deposit types.
Even airborne electromagnetics (EM), which has contributed to a diverse array of major Canadian discoveries over the past half-century, including the Thompson nickel camp in Manitoba, the Athabasca uranium complex in Saskatchewan, the Lac de Gras diamond deposits of The Northwest Territories and more recently, the Ring of Fire base metal discoveries in northern Ontario, is ineffective in some environments.
Most gold deposits, for instance, are poor conductors. Diamond-bearing kimberlite deposits such as the Ekati and Diavik pipes at Lac de Gras can also elude EM instruments because they range from relatively conductive to resistive depending on the pipe geology and host rock.
At the Diavik mine, for instance, half of the diamondiferous kimberlite pipes are detectable using EM while the other half respond better to magnetics because of the differences in pipe geology. And about a decade after the initial discoveries, a gravity gradiometry survey overthe Ekati property picked up a diamondiferous pipe that had been previously undetected by either EM or magnetics.
“It is very important to be able to optimize the EM system for the target and environment, from the high frequency, high resolution of DIGHEM to the high-powered, deep detection of HELITEM or MEGATEM,” says Greg Hodges, Chief Geophysicist for Fugro Airborne Systems, which offers a diversity of geophysical solutions. “And sometimes magnetic, radiometric, and/or gravity techniques will be better able to directly detect the deposit.”
Another classic example of how a deposit can confound geophysical assumptions is the Kidd Creek copper-zinc mine in northern Ontario, one of the world’s largest VMS deposits. When the deposit was discovered in 1963, most explorers assumed all sulphide deposits carried magnetic signatures. But because Kidd Creek has little to no pyrrhotite, it proved to be a significant exception to the rule that forever changed the way explorers looked for VMS deposits.
Hodges urges explorers to cast aside assumptions such as this and instead consider the geology of the target area on a case-by-case basis before deciding which system to use.
“It always grieves me when I see companies, without thought, use the same survey system they’ve always used for a new deposit, or automatically use whatever is most popular at the time.”
He says a common complaint in gold exploration, for instance, is that the magnetic highs do not correspond with the gold mineralization. But by considering the process of alteration around a gold deposit and how that alteration would have reduced the magnetite in the host rock, it becomes clear that magnetic lows are more likely to indicate gold mineralization in most cases.
“You know there is a certain geological change, so you have to predict the geophysical change and then pick the best system to use and the best way to interpret the data, ” says Hodges, who teaches a short course on geophysics for geologists and wishes more educators taught geophysics from a geological perspective.
As economic targets become deeper and/or subtler and therefore harder to find, the way geophysics is used to find mineralization is changing. Often it makes more sense to use a mapping system that, rather than trying to find the needle in the haystack, maps subtle changes indicating alteration zones as a means of narrowing in on the target.
“It is very important to be able to optimize the EM system for the target and environment, from the high frequency, high resolution of DIGHEM to the high-powered, deep detection of HELITEM or MEGATEM,” says Hodges. “And sometimes magnetic, radiometric, and/or gravity techniques will be better able to directly detect the deposit.”
In those cases, fixed-wing systems that survey large areas relatively quickly and cheaply might be a better choice than a helicopter system that provides higher sensitivity and resolution but at a higher cost.
“The more rock properties you can measure, including conductivity, magnetic susceptibility, density, and radio-element concentration, the better chance you have of mapping changes in geology and alteration zones,” says Hodges.“The low altitude of helicopter surveys adds resolution to all the rock properties measured, but the big fixed-wing aircraft like Gryphon can measure more properties over big areas faster.”
Hodges describes Fugro’s new Gryphon system as a flying laboratory. The aircraft can carry an array of geophysical tools including the huge MEGATEM EM system, a magnetometer, radiometric crystals, and the Falcon gravity gradiometer. The resulting information helps narrow down targets over large claim blocks by recording several different physical properties at once.
Gryphon is currently making its debut over a land package in South America. The details of the survey and its target remain confidential, but for the epithermal or porphyry-related gold systems commonly found in that area of the world, a combined geophysical package will allow explorers to pick up on the subtleties of geology and alteration that might otherwise be missed.
“There is no single airborne geophysical system that is the answer to all exploration needs,” says Hodges. “Explorers need to analyze the target and host to choose the best geophysical method, then select the optimum system for that method.