Making the grade: Diamonds take forever

Finding a kimberlite pipe, the peculiar rock type and formation that hosts most of the world’s significant diamond deposits, is a tall order; finding one that actually contains diamonds, even more so.

However difficult these tasks may be, they represent just the beginning of the long, and in most cases insurmountable, obstacle course which involves: finding, quantifying and appraising diamonds in their natural setting, be it lode or placer; calculating a resource, then a reserve; planning, permitting and financing, and then developing, a diamond mine; and finally, selling the rough stones to a selling organization (of which there is only one . . . and that is a story in itself) or directly to a buyer, who will then cut and polish the stones and either sell them to a jeweler or directly to the public.

Finding diamonds is a good start, but it isn’t good enough unless there are enough of them that are good enough. Get it? Let me explain. Compared to a metallic deposit (gold, for example), the deposit itself might host a respectable grade (meaning concentration of metal), but if the ore is refractory (in other words, locked up in another mineral such as pyrite, as is often the case), then the gold may not be easily recoverable and the deposit is thus rendered uneconomic. However, if the gold occurs in an easily recoverable form, then a deposit of the same grade could be quite economically significant indeed. The same holds true in the case of diamonds, but with a slight twist. In this case, it is not metallurgy or ease of recovery that is the concern; rather, it is the actual optical and physical properties of the diamonds themselves that have an overwhelming impact on the economic value of the deposit. Whereas an ounce of gold has a standard price around the world, regardless of its original form, the value of a similar quantity of diamonds can vary by more than 1,000%! In other words, if a diamond deposit were to host a grade of as low as 0.05 carat (0.01 gram) per tonne but all the diamonds were five carats in size or more, perfectly colourless and flawless sawable octahedra, then, hey, it’s time to retire! On the other hand, if the deposit were to host a grade of 1 carat (0.2 gram) per tonne or better but all of the diamonds were under 0.5 mm in diameter (that is, microdiamonds) or barely larger, full of inclusions, brown, and flats, well, . . . better keep exploring.

Therefore, once a diamondiferous occurrence is found, obtaining an estimate of the average value of the contained rough diamonds becomes of paramount importance, along with ongoing drilling to determine the deposit’s ultimate size. This assessment process must be repeated numerous times as exploration continues, depending on the variability of the contained diamonds across the length, width and depth of the deposit. The first step in this process is a comprehensive sampling campaign to determine the average grade of the deposit (total weight of stones divided by the total weight of the sample), the average weight of the diamonds themselves (total weight of stones divided by the total number of stones), and the average stone density (the total number of stones divided by the total volume of the sample).

Statistically reliable data are not easy to obtain, because there are usually only a small number of small stones in a typical tonne of rock (average deposits grade in the range of 0.5-1 carat per tonne, or 50-100 carats per 100 tonnes, the latter being a more typical frame of reference, referred to as “cpht”), an amount that, in itself, exceeds what would normally constitute an average sample size (a few kilograms). The larger the sample, the better the grade estimate, but the higher the cost. A balance must be struck between the cost of sampling, and the accuracy of the result. The ideal sample size can be estimated by statistics: a sample size that returns about 30 diamonds will prove reliable; however, early on it may be impractical, and expensive, to collect a sample that is large enough.

At the early stage, when there is not yet any grade data for the occurrence/deposit, average figures for commercial diamond operations can be utilized, these being: an average grade of 50 cpht (or 0.1 gram per tonne), an average diamond size of 0.1 carat per stone, and an average value of US$50 per carat. What follows is that the average stone density is five stones per tonne (a grade of 0.5 carat per tonne divided by an average stone size of 0.1 carats per stone). Therefore, in order to recover a total of 30 diamonds, a sample size of six tonnes would be required. As we can see, a deposit with a higher stone density (regardless of the stones’ quality) is going to be easier to evaluate than one with a lower density, because the sample size necessary to recover a statistically significant number of stones will be smaller.

The number of diamonds recovered depends not just on the factors summarized above but, as well, on the smallest size of particle recovered in the crushing and screening process. Microdiamonds are not generally considered to be commercially significant, even though they are usually recovered during the early stages of sampling, since they may be indicative of the possible presence of macrodiamonds (those greater than 0.5 mm in diameter) elsewhere in the deposit. Plus, they make for better news releases! As a rule, the number of diamonds recovered will increase as the minimum size of diamond decreases. Since diamond size distributions tend to follow a “lognormal” distribution, this means that the number of diamonds recovered in any given sample is also a function of the recovered size range.

Ensuring thorough macrodiamond recovery is critical to the accurate estimation of grade, and the choice of sample collection and processing methods determines how thorough that recovery will be. Security is also critical, as the theft of as few as one or two high-value stones could radically alter the valuation of the deposit; of course, adding one or two stones to the sample could have an equal and opposite effect. The method of sample collection will depend on local conditions, with several options available, including manual or mechanized trenching (first carefully stripping away overburden, if necessary), drilling, pitting, or even shaft-sinking. Contamination from non-diamond-bearing units must be strictly avoided, and the location of the sample within the diamond-bearing unit must be accurately recorded.

In order to ensure total recovery of any diamonds that may occur within a given sample, the sample must be properly processed. In many cases, this will require crushing of the host rock, and if the rock is not crushed to a small enough size, then contained diamonds may be lost in the coarse reject material. Alternatively, if the host rock is crushed too fine or too violently, then contained diamonds may be broken or otherwise damaged, thus greatly reducing their value. Once the host material has been prepared in such a way that any diamonds occurring within can be physically separated, then there are various methods available to do so, including density separation, jigging, grease tables, hand-sorting and X-ray sorting.

After a sufficient number of macrodiamonds have been recovered, an estimation of the average grade, average stone size, and average density can be made. Next comes the assessment of their actual value, a process that requires many more sampling campaigns in order to obtain the minimum necessary number of rough diamonds. This will be the subject of the next article in this series.

— Hy Grader, the author, is the pen name of a Toronto-based geologist, gemologist and vice-president of a Canadian junior mining company.