Assay salting scams are few and far between each other, and are usually detected very fast because assay results are abnormally high or do not conform to what could be expected from the geological setting of the region. Nonetheless, the dubious art is always being refined, and sophisticated methods have been used in some recent salting swindles, including that of Bre-X Minerals which is at its Busang property in Indonesia.
Salting is the deliberate introduction of metal into a sample, intended to produce a false assay result. There is a long and ignoble history of salting scams in mineral exploration, most of the time in the exploration of precious metals, which lends itself to sample tampering. The small quantity of gold, silver or platinum needed to affect the result of an assay makes the practice pretty easy. Also, because genuine precious metal mineralization is most of the time invisible to the naked eye, barren and mineralized samples may look similar, making it harder to detect salting.
Generally coarse, placer gold is included to barren samples, which results in erratic assay results and supposedly "reproducibility problems" far surpassing what might be expected from a naturally occurring deposit with coarse gold. The salting is usually detected through testwork which demonstrates that all of the gold found in the samples is coarse-grained, with little or no fine grains. Normally, if coarse gold happens in a deposit, there is always a large amount of fine gold along with it.
In base metal exploration salting scams are not as common, even though they are not unknown. One reason is that base metal mineralization is generally visible in drill cores and hand specimens, and it is easy to differentiate salted barren samples. There could be another purpose: base metals just are not as romantic as precious metals, and do not stir the imagination of naive investors. However, even in late years, the share prices of some juniors have advanced to great heights based on visual estimates of the base metal content in drill core. "Eyeball assays" are now looked on with displeasure by securities regulators because, in most cases, the subsequent assays show little or no mineral content, but only a need for the geologists to spend money in corrective eyewear.