What is Geochemistry?
When a mineral deposit forms, higher that normal concentrations of metals and other elements are often dispersed in the rocks surrounding the deposits. Similar dispersion may occur when a deposit weathers in a near-surface environment. This dispersion feature known as a geochemical halo, can lead the explorer towards a possible deposit.
Geochemistry is the scientific process of locating these dispersion haloes by the systematic sampling and chemical analysis of rocks, soils, water or vegetation. The results of the sampling are used to assist the possible presence of concealed mineral deposits.
A survey typically follows a series of steps from examination of large areas to progressively smaller and smaller areas, until analysis and geological assessment has located the deposit. Further geochemical studies may continue to develop and improve understanding of the chemistry of the deposit.
The type of survey implemented depends on the current state of geochemical and geological knowledge, the nature of the land being explored and the exploration philosophy of the organisation involved.
Stream Sediment Surveys
Stream sediment sampling involves the systematic collection of small samples of sediment from the active beds of rivers and creeks. In combination with geological mapping and reconnaissance remote sensing methods, it can provide insight into the mineral potential of a large area.
In rugged areas, locating sampling sites can be difficult but explorers are aided by using high quality aerial photos, satellite-based global positioning systems and experienced ground crews.
Sediments at a sample point may be sieved to obtain the most sensitive size fraction or panned in the traditional way to determine if gold or other heavy minerals are present. However, it is not advisable to rely on panning alone as it is easy to miss very fine grained gold using such methods.
Sample size ranges from 100grams to five kilograms. The number of stream sediment samples taken can vary substantially but one to four samples per square kilometre is typical.
The environmental impact of stream sediment surveys is minor and temporary. No special access developments are required. The small samples holes in stream beds are self-repairing through normal river action. Flagging tape used to temporarily mark sample sites is biodegradable.
The geochemical halo associated with some mineral deposits may be detected by careful chemical analysis of rocks. Such halos are typically more extensive than the actual ore deposit, making them an easier target to locate.
Rock sampling involves the taking of small hand-sized samples generally of less that one kilogram. Once an anomalous concentration of the target elements is detected, more detailed sampling and careful geological mapping is required to locate the actual deposit.
Some elements associated with mineral deposits may exhibit variations in isotopic characteristics. The study of these variations, known as isotope geochemistry, is becoming more common in routine mineral exploration.
A geochemical indication of a nearby mineral deposit may be obtained by sampling and analysing the soil. Under some weathering conditions, elements present in the orebody may be widely dispersed in the spoil from a geochemical halo which is much larger than the source deposit. It is this enlarged halo that is the target for many soil sampling programs.
Geochemical soil sampling programs are usually conducted over relatively small areas and the location of the sample sites is controlled by a surveyed grid established on the ground or by temporary location techniques.
Soil samples are typically collected by hand from a small hole dug to a depth of about ten centimetres. They are generally less that one kilogram in weight in some flat, open environments, a small hand or vehicle mounted auger drill my be used to obtain a deeper sample. Appropriate rehabilitation of sample sites is carried out during the survey. Soil samples are transferred directly to a laboratory for geochemical analysis.
Trenching and Pitting
Trenching or costeaning and pitting are traditional methods of exposing concealed bedrock or taking samples for analysis. In the early days they were usually undertaken with hand tools. The practice is still carried out today but to a much lesser extent. Costeaning to map and sample the bedrock is normally undertaken with an excavator. Trenching is generally used for taking bulk samples of known deposits for detailed testing prior to mining.
Approval must be obtained prior to any excavation work of this nature, with strict requirements governing the back filling and surface rehabilitation of any such approved works.
Other Geochemical Methods
A variety of other geochemical methods has been developed in recent years, often in response to specific needs.
Minerals deposits may emit a variety of gases that leak to the surface and either accumulate in soils or are emitted to the atmosphere in extremely low concentrations. Attempts have been made to collect and analyse these gases, both with ground and airborne collection systems. Results to date have been variable.
Techniques employing the direct analysis of water samples have similarly been developed with mixed success. Certain plant species are capable of taking up metals from the soil through their root systems and concentrating them in various parts of the plants, such as the leaves or bark. Geo-botanical surveys based on direct sampling of plants or ground litter have been undertaken with some success around the world.
Recent advances in geochemical analysis have greatly assisted the exploration for concealed mineral deposits. The ability to continue developing new and sophisticated analytical techniques will provide the explorer with additional opportunities for recognising even more mineral deposit characteristics.
Reference : Mineral Exploration "The future of the Mining Industry in Western Australia" by DR M.J. DONALDSON