MINERAL EXPLORATION — Consortium investigates mobile-ion geochemistry

Trying to aim the drill at the needle, and not the haystack, is the goal of researchers working on mobile metal ion (MMI) geochemistry.

Zones of metal enrichment in soils often do not occur right on top of the primary mineralized zone. The metals, or the soils themselves, can often be transported away from their primary source, leaving an anomaly that can be tens or even hundreds of metres off the mineralization. The MMI technique seeks out the “secondary dispersion” of chemical elements from a mineralized zone, the dissolved species that migrate in the soil profile. It uses weak solvents to extract only the least strongly bound metal ions from soils.

Typically, a suite of elements is analyzed and found to contain, for example, copper, lead, zinc, cadmium, nickel, cobalt, gold, silver and palladium. The multi-element data give a surer interpretation of the results. They also provide a sample “fingerprint” useful in ensuring quality-control in the laboratory. The analyst and explorationist cannot merely be satisfied with duplicate analyses of one element that check; the relative amounts of each element should be similar in the check samples as well.

Researchers at MMI Technology, who are supported by 16 Australian mining companies and the state government of Western Australia, claim accurate results in transported overburden, as well as in soil formed in place. The principal application of the method has been in shrinking large drill targets that have already been outlined in regional geochemical, geophysical or geological surveys.

One discovery already credited to the technique is the Golden Web deposit in Western Australia, held by St. Francis Mining. High-resolution magnetic surveys, soil geochemistry and geological mapping had pointed to a target area, but drilling had come up dry and it was concluded that the “anomalous” gold at surface was actually preferentially concentrated in iron-rich residual soils.

An MMI survey identified a smaller target where gold values were about 30 times the background level. In an 8-hole drill program centred on the MMI anomaly, five holes cut mineralized intersections. MMI has licensed several commercial laboratories to use the technique.