UBC Reports | Vol. 49 | No. 12 | Dec. 4, 2003

A New Way to Search for Diamonds in the Rough

Breaking new ground

By Michelle Cook

Thanks to an invitation to study an unusual deposit of kimberlite in Canada’s Northwest Territories, UBC researchers have discovered a new, cost-effective way to help diamond mining companies search for the valuable minerals and develop mine operations.

Kimberlite is a rare type of rock that sometimes, but not always, contains diamonds.

While their findings may be new, researchers Ron Clowes and Phil Hammer used an existing method of underground exploration -- seismic reflection -- that is commonly used by the petroleum industry to produce subsurface images but until now, has not been considered effective for finding diamonds.

“There were a lot of unknowns from a research point of view and we wanted to answer two questions,” says Hammer, a research associate in the Dept. of Earth and Ocean Sciences. “Is seismic reflection a cost-effective way to explore for shallow kimberlite dykes and sills in a hard rock environment like the Northwest Territories, and can seismic reflection produce images of the kimberlite that would be useful for mine planning?”

In a business where it can cost up to $300,000 to drill one sample hole to reach certain types of kimberlite deposits, mining companies want to know as much as possible about a mineral deposit before breaking the earth’s surface. Hammer says the work done at UBC proved seismic reflection could be a very good exploration tool for some kimberlite structures.

“The results suggest that we can use it to find the thin, kimberlite sheets at depths of more than 1,000 metres. In addition, the technique can show where kimberlite sheets are really complex. That is of interest to mining companies because it’s the complex areas that are going to cost them extra money to mine.”

Hammer and Clowes became involved in Canada’s diamond hunting race in 1999 when Clowes was approached by Vancouver-based company, Diamondex Resources Ltd. A professor of earth and ocean sciences and director of Lithoprobe, a 20-year national earth sciences research project, Clowes has been working with hundreds of geosciences researchers to explore Canada’s geological history, including the development of seismic reflection technologies needed to see what lies beneath the country’s surface.

Diamondex Resources was interested in whether seismic reflection could be used to explore a field of kimberlite at Snap Lake. Seismic reflection, like sonar, uses sound waves to “map” subsurface terrain. From previous geology work, the company already knew that the Snap Lake kimberlite, located 100 km south of the rich kimberlite deposits of Lac de Gras where the Ekati and Diavik diamond mines now operate, contained high-quality diamonds. The problem was that the kimberlite at Snap Lake is unlike most of the world’s other kimberlite deposits.

Kimberlite formations are created when kimberlite magma travels rapidly to the earth’s surface in massive explosive eruptions. The eruptions typically form carrot-shaped pipes with surface diameters hundreds of metres wide that taper down into the earth for thousands of metres. Diamonds lie buried in these long, vertical pipes.

Unlike the more common vertical pipes, the Snap Lake kimberlite deposit is a thin, flat-lying sheet, two to three metres thick, spread out over 25 square kilometres. It gently dips from the surface to depths of 1,300 metres or more. It also feathers, in places, into multiple strands.

The kimberlite’s unusual structure made it difficult to detect with traditional exploration methods such as magnetic and electromagnetic surveys done from the air. Without more specific information about the type of deposit, drilling over such a vast area would be prohibitively expensive, and mapping the sheet for mine development purposes would be difficult. Diamondex Resources, along with international diamond giant DeBeers, which owns 70 per cent of the Snap Lake property through its Canadian subsidiary, needed to find a different way to gather detailed data. The companies asked Clowes for help.

“Knowing what we’ve been able to achieve with helping base metal companies in applying seismic reflection technologies to exploration, they wondered whether this method would be feasible at Snap Lake,” Clowes says, adding that, at first, even he was sceptical.

“Seismic reflection techniques are well suited for mapping sub-horizontal structures so kimberlite dykes and sills have the potential to be good seismic targets,” Clowes explains. “But I didn’t know whether the technique would work at Snap Lake. If you took a straight rule-of-thumb analysis, it shouldn’t work because the kimberlite is so thin relative to the depths we wanted to look at -- depths ranging from near the surface down to 1,500 metres, and you would not normally be able to see it without extremely high frequencies which are quickly absorbed with depth.”

With funding from the two companies and a collaborative research grant from the Natural Sciences and Engineering Research Council of Canada, Clowes and Hammer first ran a computer-based feasibility study using data taken from drill hole samples from the site. Based on these results, they headed into the field in April 2001 to see if they could use seismic reflection to map the underground kimberlite field beneath the tundra and the icy surface of Snap Lake.

With more than 800 tiny geophones planted along a straight line in the frozen ground to pick up subsurface sound waves, the researchers ran seismic surveys using two methods to produce vibrations -- explosive charges and vibroseis, a way of pounding the earth with a device mounted on a pickup-truck-sized vehicle. The results surprised them. Emerging from the data was a striking image of the kimberlite curving deep down into the earth.

“We thought we’d probably have success at shallower depths, but we imaged the kimberlite to well over 1,500 metres and that was very exciting and rewarding,” Hammer says.

The companies that sponsored the research are also pleased with the results. Diamondex has since carried out a second successful survey using vibroseis. DeBeers has plans to begin mining the property, and has consulted with Clowes and Hammer about running marine seismic surveys on Snap Lake with the goal of further identifying the kimberlite’s characteristics. The company has indicated that it wants to continue seismic exploration work.

“If a 2D marine survey is successful, they can also apply this technique in 3D fashion, and that’s what they’re really interested in -- mapping in 3D,” Clowes says.