The theory behind the use of heavy mineral concentrates relies on the fact that by concentrating the heavy mineral fraction from the sample, the dilutant light minerals which may obscure an anomaly derived from a deposit are eliminated, providing a clearer picture. This allows the minerals to be examined under a binocular microscope, SEM or electron microprobe for particle shape or composition. Particle shape of gold grains will commonly identify distance to source. Very abraded grains may have travelled a long distance whereas delicate angular grains are probably more local. Similarly for diamond exploration, the presence of certain mineral phases may indicate the distance to the source kimberlites. Bulk analysis of heavy minerals provides a rapid, low cost method to screen large volumes of samples. Samples with appropriate chemistry can then be further evaluated as the INAA technique is non-destructive.
Heavy mineral concentrates are usually prepared by tabling or wet sieving a very large sampleof till or stream sediments (up to 20 kg may be routine). The heavy mineral concentrate collected at this stage is then further processed with heavy liquids using methylene iodide (SG = 3.3). The resultant concentrate then is separated into magnetic and non-magnetic fractions and it is the non-magnetic fraction which is usually analyzed. In some cases more elaborate separations include separating paramagnetic fractions and sieving to various particle sizes before analysis. This may provide further insight into identifying the source of the heavy minerals. Both gold and diamond mines have been discovered using this technology.