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.

Northern Miner, by Derek Wilton, June 1, 1998

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