Searching for Kimberlite
Diamonds are found in kimberlite, a volcanic rock that begins its ascent from well over 100 kilometers beneath the earth’s surface.
Upon eruption, kimberlite forms very small features with little to no surface expression. As such they are very difficult to discover. The most effective geological tool for discovery is looking for certain minerals that occur uniquely in kimberlite – kimberlite ‘indicator minerals’. These minerals can be identified in till and stream sediment samples and ultimately tracked up-ice or upstream back to their kimberlite source.
Diamond-rich kimberlite pipes, however, are difficult to find. Hundreds of kimberlites have been discovered in Canada but only a few have diamond in sufficient quantity to be considered for mining. To determine whether the kimberlite has economic potential, special testing for diamond must be implemented.
Testing for Diamond Presence
Diamond in kimberlite occurs in very low abundance – 10 to 100 times lower than gold in gold mines. To determine the presence of diamond, the most common approach is to test for microdiamonds. Microdiamonds are so small that they are not economic, but they are much more abundant than bigger, commercial-size diamonds. This is an advantage as small samples of kimberlite can be treated for microdiamonds, but getting all of the microdiamonds from a sample of a few tonnes or less requires highly specialized methods. The most accepted method is caustic dissolution, which dissolves nearly all of the kimberlite in order to recover the microdiamonds. The microdiamonds can then be sieved into different size classes and their relative abundance in each size class indicates the potential for larger diamonds in the kimberlite.
Commercial Diamond Potential
When the microdiamond data support the potential for commercial-size diamonds, larger samples must then be collected – typically 100’s to 1,000’s of tonnes. On rare occasions enough kimberlite is exposed at the surface that trenching or pitting is possible, but most kimberlites in Canada are not well-exposed. Large drill rigs are the most common method used to recover the kimberlite, with the holes drilled systematically in each body to get the most representative sample.
These ‘bulk’ samples are then treated through specialized diamond recovery plants that are small-scale versions of actual diamond mines. In these plants, the kimberlite is systematically crushed and then treated through machines that separate the diamond based on its physical properties, namely its density, fluorescence, lack of magnetism, and hydrophobic nature.
To put a diamond-bearing kimberlite into production three things must be known; the total tonnes present in the kimberlite, the diamond content or grade of the kimberlite, and the value of the commercial-sized diamonds. If enough carats are recovered, they can be valued by individuals or organizations that are trained in this specialty and familiar with market conditions. Each of these parameters is then incorporated into mining and marketing models to determine the economic potential of the kimberlite. Once a positive potential is established, a mine that enhances the unique characteristics on the diamonds in the kimberlite can be built.