Techniques of Geochemical Analysis
By Tirumala Prasad
The chief aim of geochemical analysis is to locate the prospecting areas of mineral deposits. The process involves several steps such as collection of samples, grinding, pulverising, and laboratory analysis.
The pattern of grid to collect samples does depend upon strike and dip of the exposed rock types. The spacing between the collection sites does depend upon the depth and type of the mineral deposit. Similarly, for fine grained rocks, the amount of sample should be from one hundred grams to about five hundred grams. The size of sample for medium grained rocks does range from a thousand grams to two kilograms. Coarse grained rocks samples should range from five kilograms to ten kilograms. The geochemical samples include, rock, drainage, organic matter, gas, soil, and water. The type of rock samples include, channel, chip, drill core, drill mud, and grab. The sample collected should represent the whole rock, hence samples are prepared for the chemical analysis by riffle cutting, cone and quartering, etc.
Samples can be analysed on the spot. For example, electrical conductivity, chemical reactivity, hydrogen ion concentration, etc can be done in the field itself. Date, description of the sampling area, latitude, longitude, elevation, land use pattern of the study area, possible contamination of water, etc should be recorded in the field note book.
Knowledge of soil formation processes is required to collect soil samples. Generally, the soil samples can be collected by augering (manual or mechanical), pitting, and trenching. The collected samples should be dried, and should also be converted to uniform size, I.e, removal of any high grade fragments.
Plants and animals can also yield certain clues about the elemental concentrations. For instance, the excessive presence of some chemical elements can be seen in the form of chlorosis, and genetic dwarfism. Similarly, dogs can sniff off sulfide gases released during the process of oxidation of some mineral deposits.
Stream channel samples do contain various types of chemical elements such as trace elements, minor elements, fine particles of the rocks, etc. This type of samples do contain the products of several years of weathering and erosion.
The method of sample collection does depend upon several factors such as the type of deposit, goal of the survey, availability of time, and other resources.
The processed samples should be analysed by several techniques such as atomic absorption spectrometry, mass spectrometry, neutron activation analysis, emission spectrometry, etc to know the elemental concentrations. The above mentioned techniques utilise the wavelengths of the electro magnetic spectrum. In other words, these techniques are the reactions between samples and electro magnetic radiations.
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