Prospecting Techniques - Geochemical Prospecting
Many mineral deposits are not exposed at the earth's surface. They may either be concealed by thick soil cover, or they may lie buried beneath layers of rock. To find these deposits more complex techniques—based on geochemistry, geophysics, and geobotany can be very helpful. Most of these techniques require specialized training and, in some instances, expensive equipment. Geochemical prospecting is based on systematic measurement of one or more of the chemical properties of rock, soil, glacial debris, stream sediment, water, or plants. The chemical property most commonly measured is the content of a key “trace” element.
The purpose is to discover zones in the soils or rocks that contain comparatively high concentrations of particular elements that will guide the prospector to a hidden deposit. Such concentrations of indicator elements in rocks or soils constitute a geochemical “anomaly”. The actual amount of the key element in a sample may be very small and yet constitute an anomaly if it is high relative to the surrounding area. For example, if most samples of soil were found to contain about 0.00001 percent (0.1 part per million) silver, but a few contained as much as 0.0001 percent (1 part per million), the few “high” samples would be geochemical anomalies. Plots of analytical results on a map may indicate zones to be explored further.
Geochemical anomalies are classified as primary or secondary. Primary anomalies result from outward dispersion of elements from mineral-forming solutions. The “high” concentrations of metals surround the deposit and the dispersion of metals laterally or vertically along fractures may result in a leakage “halo” that extends hundreds of feet away from the deposit. Halos of this type are especially useful in prospecting because they may he hundreds of times larger than the deposit they surround and hence are easier to locate. Secondary anomalies result from dispersion of elements by weathering. Some primary minerals, such as cassiterite, are resistant to chemical weathering and are transported by the streams as fragmental material. Other minerals may be dissolved and the metals may be either redeposited locally or carried away in solution in ground and surface waters. Some of the metal in solution may be taken up by plants and trees and can be concentrated in the living tissue. A great many studies have been made of the metal content of residual soils over sulfide deposits, and in general the distribution of anomalous amounts of metal in the soil has been found to correspond closely with the greatest concentration of metals in the underlying rock. All the products of weathering in a drainage basin funnel through the system of streams and rivers that flow out of the area. The weathering products are partly in the form of chemicals in solution and partly in the form of sediments. Either or both can be sampled and tested, and their composition will reflect the chemical nature of the rocks in the drainage basin.
The presence of an ore deposit in a drainage basin may thus be detected by analyzing for metals removed by weathering and either incorporated with the stream alluvium or carried in solution. The deposit can be located within the basin by sampling water and sediment from each successive tributary and determining which contain higher than normal amounts of metals. This procedure narrows the search to favorable areas. Contamination of surficial material is an ever-present hazard in geochemical surveys. The most common sources of contamination are materials derived from mine workings. Such materials may become scattered widely over the ground, and the ore minerals in them may oxidize and go into solution, contaminating the soil, stream sediment, and water, and masking natural anomalies. Similarly, smelter fumes, windblown flue dust, and metallic objects introduced to the natural environment by man may also contaminate the soils and rocks. Analytical methods used in geochemical prospecting must be sensitive enough to determine minute amounts of key elements, accurate enough to show small differences in concentration, fast enough to permit large numbers of samples to be analyzed in a day, and inexpensive.
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NB Prospecting
