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Geochemical Prospection
Basic knowledge, definition of geochemical prospection
Geochemical prospection consists on the systematic measure of the content of one or more element traces in the rocks, grounds, or in the sediments of the courses of water, in the water or in gasses. The objective of these measures is to put into evidence the “geochemical abnormalities”, in other words, the abnormal concentrations of certain elements that clearly go against the adjacent areas that represent the chemical ground or background. The formation of the abnormalities comes about from the mobility and the dispersion of elements that have mineral concentration. The “normal” value is not a value that is exactly defined but that has more of a variable range, which is characteristic for a certain area. The abnormality, by definition, is the deviation of a normal value. The threshold is the concentration of an indicating element over which the sample is considered abnormal. What is an indicating element? There are elements that are difficult to analyze because they are immovable or provide data that are difficult to interpret, but there are other elements commonly associated with mineralization, that can be more useful. These elements are known as indicating elements or pathfinder. These are elements with better geochemical or analytical characteristics than the main metal. For example, Mo is more mobile than Cu in most of the superficial environments and that is why it can be used as Pathfinder for copper deposits that usually have a little bit of Mo. The same is done with As in the search for deposits with Au. Some indicating elements are known of in the following table:
Indicating element: As
Material: fitting rock, residual ground, active sediments
Type of deposit: Au veins
Indicating element: Hg
Material: fitting rock, ground
Type of deposit: deposits of complex Pb, Zn, Ag
Indicating element: Se
Material: iron layers
Type of deposit: epigenetic sulphurs
Indicating element: Ag
Material: residual grounds
Type of deposit: deposits of Au with Ag content
Indicating element: Mo
Material: water, fluvial sediments, grounds
Type of deposit: deposits of Au with Ag content
Indicating element: SO4
Material: water
Type of deposit: sulphate deposits
The origin of an abnormality is not only exclusively metal but it can derive from a concentration due to man (fertilization, industrial waste, etc.), or also because of the discharge of an ancient mine or recent. There are also “formational” abnormalities due to the presence of a constantly united element to a determined geological formation (Cu in the basic rocks, Pb-Zn in dolomite), or in a finely dispersed mineral in a rock but which is not economically recoverable. The distinction between a “real” abnormality and a “formational” or one of “contamination” abnormality is one of the greatest responsibilities of the geochemist.
