Geochemistry Prospecting

A mineral deposit could be described as an area where certain substances — metals or minerals — exist in concentrations that are much higher than normal. In other words, the deposits themselves are “anomalies”. Finding anomalous concentrations of elements is the aim of geochemical exploration.

Exploration geochemists have traditionally distinguished between “primary” and “secondary” distribution of chemical elements. Primary distribution processes are the ones that form the ore deposits in the first place, and disperse metals and other chemical elements through the surrounding bedrock. Secondary processes like weathering, glaciation and the movement of ground and surface water move the elements around more — into soils, stream and lake sediments and waters, out to sea, and even into plants and the atmosphere.

A detailed knowledge of the geological processes that cause dispersion is required to trace the metals back to their source. This is the realm of geochemistry. In typical geochemical surveys, the prospector takes samples of a particular material, perhaps bedrock, soil, water or something else entirely. The samples are then analyzed chemically for the elements of interest, and the results plotted on a map.

In the simplest case, the prospector will follow up on samples that have the highest concentrations of the metals he is looking for. More often, the geochemist has to interpret the patterns of distribution to reconstruct the path the elements have followed. For example, glacial action can “smear” soils along the direction the glacier travels, and the geochemical pattern will be moved away from the mineral deposit that created it.

Geochemistry is used routinely in most exploration programs. In temperate areas that have residual soil, it is frequently the most important exploration technique. In areas that have been glaciated, the use of geochemistry is often more complex, depending on the exact nature of the glacial history of the area. In the tropics, deep weathering of soils can make geochemical patems very obscure.