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Modern exploration methods lead to significant discoveries
by Virginia Heffernan on August 15, 2008 expertise
Diamonds North Amaruk Camp in Nunavut
Diamond exploration and discovery is undergoing a renaissance in Canada's north that rivals the first wave of successful exploration in the early nineties, though the activity is going largely unrecognized.
The Ekati and Diavik mines, products of the original rush and investor frenzy that followed the discovery of diamonds near Lac de Gras, NWT in 1991, continue to churn out billions of dollars worth of diamonds each year and a third mine, Snap Lake, is gearing up for full production in mid-2008.
Meanwhile, mining companies are spending more than ever on diamond exploration in Canada and using modern exploration techniques to get some spectacular results. Last year more than $50 million was spent in Nunavut alone on 41 different projects, though few - even within the mining industry - are aware of the scope of the activity.
"There are some very high grade discoveries going under the radar screen," Pamela Strand, president and CEO of Edmonton-based Shear Minerals, told members of the Canadian Institute of Mining (CIM) and Women in Mining (WIM) at a joint luncheon in Toronto.
Some companies are investigating areas that have never been explored for diamonds before but show potential for the diamond host rock, kimberlite, while others are re-evaluating old prospects by revisiting data with faster and more sophisticated processing techniques.
In both cases, modern exploration methods, especially improvements in the ability to trace and analyze kimberlite indicator minerals such as pyrope garnet, chromite and chrome-diopside, have led to significant discoveries.
The main reason Canada attracts more diamond exploration spending than any other part of the world is because the county is blessed with large cratons, ancient stable parts of the earth's crust that are prospective for kimberlites. Exploration over the years has confirmed that many of these cratons have indicator mineral chemistry in the diamond stability field.
In northern Canada, it takes a combination of mineral chemistry, knowledge of regional ice advance and retreat patterns, and geophysical sleuthing to find potentially diamond-bearing kimberlites. The process is slow, pain-staking and expensive because in the early days, prospective areas can span hundreds of millions of acres.
Take Vancouver-based Diamonds North, for instance, one of the most active diamond explorers in Nunavut and the Northwest Territories. In 2002 the company identified more than 300 million acres that were prospective for diamonds and spent $9 million on till sampling over the ensuing years. Based on the resulting mineral geochemistry, the junior narrowed its focus to eight million acres encompassing four district-scale projects, including the flagship Amaruk project in eastern Nunavut.
"A lot of the previous exploration was on the Slave craton so we stepped out of that area into another area we thought would have the old Archean signature," says Mark Kolebaba, president of Diamonds North. "One of the early samples had 1,400 indicator minerals and that's what kicked off the staking rush."
The next step was to combine the anomalous mineral chemistry with coincident geophysical targets. As a result of that work, the Amaruk project alone has produced 500 drill targets, 22 kimberlites (90% of which are diamondiferous) to date, and some promising diamond counts. By comparison, the Ekati field contains 160 known kimberlites, including eight in the mine plan.
Earlier this year, an 82 kg sample from an Amaruk kimberlite returned 550 diamonds for an unusually high average of about seven diamonds per kilogram. Recent processing of a much larger sample produced similar grades, as well as the hoped for larger stones that were lacking in the first sample.
"At Amaruk, the chemistry is every bit as good as the Ekati area. If we have an economic deposit among our 500 targets, we'll find it. It becomes a prioritization game," says Kolebaba, who will test another 60 targets this year with a reverse circulation (RC) drill.
Percussion Drilling on the Amaruk Property in Nunavut
RC drilling is a unique aspect of the Diamonds North program that allows the company to move a drill rig by helicopter from site to site to test multiple targets much more quickly and cost-efficiently than using core drilling. The best targets are then earmarked for further drilling with a diamond rig.
Applying innovative exploration techniques is also paying off for Shear Minerals at its Churchill project in eastern Nunavut. Until recently, Shear and its partner Stornoway Diamonds Corporation were having a lot of luck finding kimberlite bodies using geophysics, but not much luck finding diamonds.
"It was like shooting fish in a barrel," says Strand of the geophysical target testing. But the diamond counts within the newly discovered kimberlites– if they carried diamonds at all - were discouragingly low.
A closer look at the chemistry of the indicator minerals explained why. Though the company was finding oodles of G10 garnets - the best indictor of diamonds - in the mineral trains at Churchill, further analysis revealed that the garnets were created at temperatures and pressures that were not conducive to diamond preservation.
Armed with these results, Shear determined that there are two distinctly different types of kimberlite co-existing on the Churchill project.
The first type – what the company was finding in the early years of exploration - is strongly magnetic with low indicator mineral counts, poor mineral chemistry and a warm geotherm indicating low diamond carrying capacity. The second type has a more subtle geophysical signature, a high abundance of indicator minerals, good mineral chemistry and a cold geotherm indicating moderate to higher diamond carrying capacity.
Partly as a result of this new thinking, the company has pinpointed four diamond-bearing kimberlite dykes that have returned grades in the range of 0.5-2.2 carats per tonne (cpt) in mini-bulk samples and is following up with much larger samples this year.
"The advanced filtering techniques have allowed us to focus our exploration," says Strand.
Age dating is another method that Shear uses to determine the diamond potential of its kimberlites. The oldest kimberlites at Churchill (approximately 240 million years) have the most diamonds. As volcanism waned, so did the tendency for the kimberlites to bear diamonds.
Age also determines the economic viability of pipes at Ekati, the richest of which erupted during a relatively short geological time window 55-56 million years ago. "The diamond content of Ekati kimberlites apparently is related more to the age of eruption than to any other parameter investigated in this work," wrote Grant Lockhart, Herman Grütter and Jon Carlson in Temporal, Geomagnetic and Related Attributes of Kimberlite Magmatism at Ekati, Northwest Territories, Canada, a paper published in the September, 2004 edition of Lithos.
The authors suggest that magneto-stratigraphic correlation of kimbetlite pipes could provide a powerful exploration tool for further economic discoveries in the area because older kimberlites present as aeromagnetic low anomalies while the younger pipes are characterized as aeromagnetic highs.
Interpretation of Airborne Geophysical Data for Shear Mineral's Churchill Diamond Project in Nunavut. Created in Geosoft Oasis montaj.
Though the geophysical methods used to find diamonds have been refined since the early nineties, the basic techniques – airborne magnetics and electromagnetics and ground magnetic and horizontal loop electromagnetics – have not changed.
In recent years, however, capacitive coupled resistivity (CCR) and gravity have been added to the toolbox. CCR can be used as ground check for airborne resistivity values, while gravity helps delineate subtle density contrasts between kimberlites and the surrounding country rock.
The use of seismic methods to find kimberlites has also developed over the last decade. The method works because kimberlite is generally less dense and more porous than the rock that surrounds it. Seismics are particularly useful in finding kimberlite dykes in Canada's north, which tend to be gently dipping, because the sound waves can more easily delineate bodies that have shallow orientations.
Diamond prospects that were abandoned by previous explorers are also taking on new life as modern explorers revisit historical data. Vancouver-based Peregrine Diamond's WO property in the Northwest Territories, for example, is benefiting from a re-evaluation of historical data combined with recent data to outline a significant diamond deposit.
The DO-27 kimberlite pipe on the property was discovered in 1993 and returned promising microdiamond results through core drilling, which encouraged Kennecott – the major partner in the project at the time - to proceed directly to an underground bulk sampling program. But the average grade in the bulk sample - 0.36 carats per tonne – proved disappointing and Kennecott subsequently dropped the property.
Peregrine returned to the WO property in 2005 to poke six reverse circulation drill holes into the vent. The resulting 151-tonne sample returned an average grade of 0.98 carats per tonne.
Since then, Peregrine has completed statistical modeling and normalization of the data and produced a 3-D density model that demonstrates grade consistency of about 0.91 cpt and diamond values ranging from US$43-70 per carat in the main vent, indicating the potential for an economic deposit.
"This is a good example of a kimberlite that has been reevaluated with modern exploration techniques,' says Strand.
Whether companies are investigating grassroots prospects or re-evaluating established diamond deposits, the ingredients are in place for another Ekati-like find in Canada's north.