18.4 Diamonds

Although Canada’s diamond mining industry didn’t get started until 1998, diamonds are currently the sixth most valuable product mined in the country (Figure 18.3), and Canada ranks sixth in the world in diamond production. Diamonds form deep in the mantle, at approximately 200 km to 250 km depth. Under very specific pressure and temperature conditions, carbon that’s naturally present in mantle rock (not coal) can be transformed into diamonds. If pressures and temperatures change to be outside the ideal window for diamond formation, diamonds can be turned into graphite.

Diamond-bearing rock is brought to the surface by kimberlite volcanism. This type of volcanism is extremely rare. The most recent kimberlite eruption is thought to have been 10,000 years ago and prior to that at around 30 Ma. You can read more about the volcanology of kimberlites in Section 11.3 Types of Volcanoes. All of the world’s kimberlite diamond deposits are situated within ancient shield areas called cratons in Africa, Australia, Russia, South America, and North America.

It has long been known that diamonds could exist within the Canadian Shield, but up until 1991, exploration efforts had been unsuccessful. In 1980 two geologists, Chuck Fipke and Stu Blusson, started searching in the Northwest Territories by sampling glacial sediments looking for some of the minerals that are normally quite abundant within kimberlites: chromium-bearing garnet, chromium-bearing pyroxene, chromite (Cr2O3), and ilmenite (FeTiO3). These distinctive minerals are used for this type of exploration because they are many times more abundant in kimberlite than diamond is.

After more than a decade of exploration, Fipke and Blusson finally focused their search on an area 250 km northeast of Yellowknife, and in 1991, they announced the discovery of a diamond-bearing kimberlite body at Lac de Gras. That discovery is now the Diavik Mine, and there is another diamond mine—Ekati—25 km to the northwest (Figure 18.25). There are two separate mines at Diavik accessing three different kimberlite bodies, and there are five at Ekati. There are six operating diamond mines in Canada: four in the Northwest Territories (including Diavik and Ekati), and one each in Nunavut and Ontario.

Figure 18.25 Diamond mines in the Lac de Gras region, Nunavut. The twin pits of the Diavik Mine are visible in the lower right on an island within Lac de Gras. The five pits of the Ekati mine are also visible, on the left and the upper right. The two main mine centres are 25 km apart. Source: NASA (2013), Public Domain. Image source.

Putting It Together

Why don’t diamonds form from coal? Fill in the blanks to find out.Diamonds require special pressure and temperature conditions deep within the               (hint: crust, mantle, or core?).

Coal forms from plant matter in               (hint: a type of wetland), which are largely terrestrial ecosystems, and therefore on               (hint: continental or oceanic?) lithosphere.

Because of               (hint: the balance between the weight of lithosphere and its buoyancy in the mantle), there’s no way to get the coal down to where diamonds form.

To check your answers, navigate to the below link to view the interactive version of this activity.


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