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 Diamond Grade Resource Calculation
    Author: Randal Cullen - Chief Geologist

 Question: Why are grades quoted as carats per square meter, carats per cubic meter and carats per ton, how are these different factors calculated, when are they used?

Carats per Square Meter (Cts/sqm or Cts/m2)

Grades for marine placer deposits of diamonds are generally described in terms of cts/sqm. The deposits are generally visualized as occurring as a thin layer on the surface of bedrock, below sediment cover, rather than being dispersed throughout the sediment overburden. The deposit is thus regarded as being infinitely thin and therefore occupies an area but no appreciable volume.

Representing the grade of a deposit in cts/sqm allows easy conversion to the potential contained resource if the area of sediment below which diamonds are thought to occur is known. For instance, our Marshall Fork feature measures in excess of 1.1 million square meters. A projection of mineralization within the feature of 1 ct./sqm converts easily to a contained resource of 1.1 million carats of diamonds.

The measure of the competence of a marine diamond miner is their ability to effectively clean an area of bedrock and thereby recover a large percentage of the diamonds from the deposit.

Carats per cubic meter (cts/cbm or cts/m3)

Indicating grade in cts/sqm disregards a number of important factors affecting the mineability or economic recovery of the diamonds in the deposit. One key factor affecting mineability is the volume of sediment that must be removed to get to bedrock. Current technology allows economic recoveries to be made at sediment thicknesses of up to 5 meters, if grades are high enough. Thickness can vary radically within a deposit but on the level of the mining block (mine planning unit) can be reduced to near uniformity. In this case calculation of carats per cubic meter (cts/cbm or cts/m3) is a useful indication of grade.

For instance, in some portions of the Marshall Fork feature the sediment is 4.5 meters thick. Using the figure of 1 ct/m2 and multiplying by 4.5m gives a new volumetric grade of 1 ct/4.5 m3 or 0.22 cts/m3. Further north within the same channel, sediment thickness drops to 1 meter which converts to a volumetric grade of 1 ct/m3. Although these are anticipated to be similar grades in terms of area mined, consideration of volume shows portions of the channel are high grade, other are medium grade and where sediment is thickest, the area could be considered low grade. In open pit mining this aspect of mining is referred to as the stripping ratio, but in that case, the ore body has some appreciable volume.

Diamonds lying beneath up to 5 meters of sediment can be mined if grades are high enough. A deposit that is viable beneath 1m of sediment may not be mineable where thickness of sediment exceeds 1m, as mining rates will be too slow to guarantee profitability.

Other mineability factors include water depth at which the diamonds lie, thickness of overburden which must be removed to get to bedrock, geological and engineering characteristics of the overburden like particle size and shear strength, potential for diamond deposits on 'false bedrock' layers within the overburden, secondary trap site (pothole and gulley) size and so on.

Carats per tonne (Cts/tne)

Shipboard processing on DFI's programs consists of three sub processes: screening, to separate the diamondiferous size fraction (sized) from the larger rocks (oversize) and the mud, clay and sand fraction (undersize); dense media separation (DMS). to float off the sized material that is too light (low density) to be diamond and xray extraction, where material that fluoresces in an xray beam is ejected into a diamond concentrate container and canned for transport to a secure shore based sorthouse for the final extraction of diamonds.

Sediment mined from the seabed is processed in real time to separate diamondiferous concentrate at a rate that makes the operation profitable. The rate of processing through the DMS plant is measured in tons of sized gravel that can be processed per hour. Marine processing plants are in the range of 50 tonnes per hour capacity. For this calculation, data from the sampling program contributes to an estimate of tonnes of sized gravel within the deposit.

For example, in the Marshall Fork feature at a grade of 1 ct/m2 in an area where the sediment is 4.5m thick, only 0.5m of that thickness contains gravel, the rest is mud, clay and sand (undersize) which is rejected in screening and does not enter the DMS circuit. Within the gravel layer, 30% of the material is oversize; it rides across the top screen and goes back overboard to fall in the mined out area on the seabed. The inputs to the process tonnage calculation are that within 1 square meter of the deposit 70% of 0.5m3 is sized material. Using a density of quartz (2.66gms/cm3) as an estimate for the average density of the sized gravel fraction we arrive at 0.93 tons of sized material per square meter and 1 carat per 0.93 tons of sized material processed, or 1.07 carats per tonne.

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