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F E B R U A R Y / M A R C H 2 0 1 7 www.resourceworld.com 15 gest that an open pit target that grades 2.2 pounds/tonne, or approximately 0.1% would be an economic intercerpt over 100 metres. For high-grade, underground deposits, a grade of 1% would be a signifi - cant intercept over thicknesses of 2 metres or more. DIAMONDS: Economic diamond mines are generally small even though they are commonly shallow deposits mined by open pits. The contained value per tonne can be extremely high, but var - ies from deposit to deposit, depending on the quality and size distribution of the contained diamonds. Larger diamonds are much more valuable than smaller ones, and consequently, two diamond deposits with the same grade, which contain different proportions of large stones, will vary sig - nificantly in their value per tonne of ore. As a general rule of thumb, 1 carat/ tonne of ore is viewed as high-grade. The geometry is important. Diamond pipes are carrot-shaped, vertically inclined bod - ies that come to surface and can be mined by open pits, and then, if the grade is high enough, can also be mined from underground. Diamond pipes are usu- ally comprised of a rock called kimberlite which gushed up from deep in the earth carrying diamonds. The diamonds were not 'born' in the kimberlite; it is only a medium of transport. The diamonds that survived the long voyage to near surface are found in what is called the diamond stability zone. Diamonds can often occur in dykes, and these are much less prefera - ble for mining due to the limited thickness of the bodies. So look for grades approach- ing 1 carat/tonne and a description that the sample comes from a pipe rather than a dyke. In early stage diamond exploration, values are presented as a diamond count rather than a grade. Here the rule of thumb is to have a minimum of one dia - mond per kilogram sample. One example was Diamonds North Resources, which reported results of 551 diamonds in an 81.75 kilogram sample, for a diamond count of approximately seven diamonds per kilogram sample. This is seven times greater than what we hold as our rule of thumb, and so it's not surprising that the stock went up over 100% from $0.75 to $1.80 in the day following the release of this result. However, there were not enough commercial-sized diamonds and the project is now on hold. UNDERGROUND MINING OR SMALL TONNAGE SCENARIOS Now consider smaller tonnage scenarios, where the thicknesses are much less than 100 metres, but still at least 2 metres thick. These geometrics can be mined by under- ground mining technologies, and the costs are considerably higher, say $25 to $50 per tonne as a rule of thumb. In this style of mining $100 per tonne gross metal value is interesting and $500 per tonne is consid - ered to be high-grade.