VANCOUVER (miningweekly.com) – A Vancouver-headquartered technology firm CRM Geotomography Technologies has developed a technology that exploits cosmic ray muons to find mineral deposits hidden underground.
A spin-off company of TRIUMF Innovations, the commercialisation arm of TRIUMF – Canada's national particle accelerator laboratory – CRM has developed and built robust, field-deployable muon detectors, that have been tested successfully at multiple mines.
CRM CEO Don Furseth explains to Mining Weekly Online that the muon detectors work much the same way that a computed tomography (CT) scan constructs a three-dimensional (3D) model of an object from multiple two-dimensional X-ray images. The technology then combines the information from multiple detector locations to build a 3D model of geological structures within the surface of the earth.
Further, this information can be combined with geological context and other geophysical and geological data such as gravimetry measurements and drill core assays for geological interpretation.
According to Furseth, this technique has obvious applications in brownfield and greenfield mineral exploration. "In fact, any geological structure with a sufficient density contrast to the surrounding material may be amenable to this technique," he notes.
Furseth explains that muons are a type of elementary particle that is essentially a heavier version of an electron, and are naturally abundant from cosmic rays. "High-energy muons, which travel at nearly the speed of light, are virtually unaffected by the earth's magnetic field or by temperature and pressure variations in the atmosphere. Therefore, the muon flux at the surface of the earth is very nearly uniform, both spatially and temporally.
"Because muons are so massive, they penetrate matter much more effectively than other charged particles. The rate at which they lose energy depends on the amount of matter traversed by the muons, and to a lesser degree, on the nuclear properties of the medium they pass through," he says.
This means that muons are stopped quicker in denser material. For this reason, by measuring the rate of muons that penetrate to some depth underground along a particular direction, one can directly measure the average density towards the surface in that direction, he reveals.
"This is important, because density is an important geophysical property that can be associated with many valuable geological formations such as mineral deposits," according to Furseth.
"Given our knowledge of muon properties and propogation, we can develop a precise model for the expected flux of muons along each direction at an underground detection location. If we count the muons that are incident on a detector from each direction, and compare to the expected number of muons, we can determine the location of density anomalies within the field of view of the detector. Not only that, but we can provide a density measurement."
Furseth advises that CRM has successfully demonstrated muon geotomography in the field at existing mines for several types of deposits including For example, they have imaged volcanogenic massive sulphide (VMS), and Mississippi Valley Type (MVT) deposits for zinc and other metals, at sites including Nyrstar's Myra Falls Mine in British Columbia and Teck Resource's Pend Oreille mine, in Washington State. Results of successfully imaging a high-grade uranium deposit, at almost 600 m depth, are being presented at Prospectors and Developers Association of Canada Convention 2018.
The current version of muon detectors have a footprint of about 1 m by 2 m, and are about 1.5 m high. They are suitable in scenarios where there are suitable access drifts, Furseth says.
"Our detectors have been developed to handle the harsh and unpredictable environment of a mine, and have been deployed for months at a time without human intervention."
The brownfield work is currently focused on discovering deposits near existing mine operations, estimating the size, location and shape of dense mineralisation; developing better targeted drilling programmes; and helping mine geologists refine their geological models.
However, as the mining industry as a whole struggles with replacing mined-out reserves, the technology provides a promising solution in greenfield applications. CRM is developing a more compact, higher resolution next generation detector which will be in field trials later this year. This is an important step to developing a borehole detector for use in greenfield applications.
"We have made significant progress towards development of a borehole device and have simulated its capabilities, leveraging our expertise and experience from real-world brownfield applications. One important feature of borehole detectors is that they have the ability to image deposits from multiple depths underground, which provides a powerful handle to resolve 3D geometry, in contrast with brownfield where detectors can only be placed at fixed depths within existing mine drifts. However, the most important benefit is they can be deployed almost anywhere, for both brownfield and greenfield applications," he says.
In one example simulated scenario, a fence of muon detectors for PQ-size boreholes were situated in spacings of 400 m to image a VMS deposit with two lenses, that had a density contrast of 0.7 and 1 gram per cubic centimetre, respectively, Furseth states. "Within three months, the data from the detectors provided evidence of an ore body at a better than 99% confidence level. Combining the images yielded a 3D model that corresponds very well with the input VMS deposit," he says.
Further, block caving is another area where the muon detectors can play a critical role, according to CRM.
Furseth advises that an important safety concern of this method arises from air voids forming within the collapsing overburden. Such 'hang ups' can collapse suddenly and create strong, dangerous air blasts throughout the mine. An example of such a collapse is the event at Northparkes mine, in Australia, where on November 25, 1999, a void, estimated at four-million cubic meters, suddenly collapsed, resulting in multiple fatalities.
"Because of the large density contrast and volume of such hang ups, it is possible to discover and image such voids using muon tomography on a compressed timescale compared with mineral exploration applications. This timescale may be appropriate for monitoring for dangerous hang ups in block caving operations," he says.
Meanwhile, TRIUMF Innovations CEO Kathryn Hayashi tells Mining Weekly Online that CRM Geotomography Technologies has become a world pioneer in exploiting cosmic ray muons for mineral exploration.
"They have developed and built robust, field-deployable muon detectors, and have tested them successfully in multiple mines. CRM helps exploration geologists reduce cost and waste by providing new 3D insights into dense ore bodies using its field-proven muon detectors, tomographic imaging and integration with other geological data."