Diamonds fall into one of four categories. They are, in order of decreasing value: gem, near-gem, industrial and boart. Individual diamonds are measured in carats (1 carat equals 0.2 gram), whereas the grade of diamondiferous rock is expressed in carats per tonne (or carats per 100 tonnes).
World diamond production is in the order of 100 million carats per year. In 1997, the Argyle deposits of Western Australia (the world’s largest producer) produced 40.2 million carats from ore grading 3.7 carats per tonne. Highly variable grades can make the value of ore in US dollars per carat quite unpredictable.
The major diamond producing nations are South Africa, Botswana, Australia, Russia and Zaire. In Canada, production from the BHP Diamonds-Dia Met Minerals operation at Lac de Gras, N.W.T., will begin later this year.
The vast majority of diamond production is from kimberlites, with only the Argyle deposits providing substantial production from lamproite sources.
About 3% of kimberlite pipes, which can occur in clusters of up to 50, contain diamonds, and only 1% of those occurrences are economically exploitable.
Exploration for diamonds, which occur as xenolithic crystals or fragments within kimberlites or lamproites (both of which are intrusive ultramafic igneous rock types), in heavily glaciated areas is difficult because kimberlites and lamproites are soft compared with other rock types, and are likely to be preferentially eroded as a result. The most useful exploration technique, therefore, is geochemical surveying of till and other alluvium.
Positive identification of intrusive rocks from a series of samples collected during an exploration program requires detailed petrographic examination and evaluation of the constituent minerals.
Indicator minerals within kimberlite or lamproite need to be geochemically analyzed and classified to determine the intrusion’s potential to contain diamonds. The precious stones are a relatively minor mineralogical constituent in those intrusives, though the indicator minerals are sufficiently abundant to be readily evaluated.
Similarly, the composition of indicator minerals in soils, tills and stream sediments can be analyzed to determine if such detrital material was eroded from an area that contained diamondiferous rocks. Indicator minerals include chromite, garnet and ilmenite, each of which has a distinct geochemical signature in diamondiferous rocks.
Critical to the evaluation of diamond potential is the precise analysis of rocks or detrital material, and their indicator minerals, to define their petrological and geochemical compositions. Based on the composition of the sample analyzed, different preparation techniques are required. In order to evaluate a particular kimberlite or lamproite intrusive, bulk samples of more than 30 kg are usually collected. Indicator minerals and diamonds are separated from the sample, producing a heavy mineral concentrate (HMC) for analysis. Heavy minerals will be similarly separated from large bulk samples of detrital material for analysis. The evaluation of diamond prospects is time-consuming owing to the exacting concentration of the minor constituents from such large samples and the precision required to analyze the HMC.
Kimberlite and lamproite intrusives often exhibit circular magnetic (mag) or electromagnetic (EM) geophysical anomalies that reflect the elevated mag or EM properties of the intrusives compared with the country rock, usually returning a bull’s-eye pattern. The problem with these surveys is that the craters or pipes cover such a small area that it may be difficult to distinguish the anomalies from regional gradients.
Kimberlite and lamproite craters and kimberlite diatremes are initially mined as open-pit operations because the host rocks are usually friable.
Underground production is frequently initiated with increases at depth, ely in diatremes. Alluvial sources are mined as open-pit operations. The ore is crushed and diamonds, because of their hardness, are readily separable.
— The author is a professor of geology at Memorial University in St. John’s, Nfld.
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