Geochemical Maps

maps showing the principles of the spatial distribution of chemical elements in rock. They show regions of distribution and zones of concentration of elements in various types of rock (igneous, sedimentary, and metamorphic), as well as inside the boundaries of the various structural zones of a region.

The classification developed by A. E. Fersman differentiates general and special geochemical maps. General maps are compiled using qualitative and semiquantitative analytical data, which are plotted on a generalized geological or tectonic basis as chemical symbols of varying value and form and which show areas of the presence or high concentration of individual elements and their groups. In compiling special geochemical maps (that is, by element), quantitative determinations characteristic of the elements of a particular region are used. Special geochemical maps are usually compiled for the elements that determine the metallogenic and industrial specialization of a region (for example, copper, lead, zinc, nickel, and uranium) or for accompanying indicator elements (such as sulfur, arsenic, antimony, fluorine, and chlorine), which are of great significance in prospecting. The changes in absolute or relative content (compared with the abundance ratio) of each element in the rocks over the area of the region are shown by a change in coloration or by isographs.

The compilation of lithological-geochemical maps is most effective when mapping territories that are composed of sedimentary or sedimentary-volcanogenic rocks and are well provided with drilling data. On lithological-geochemical maps, isographs represent a qualitative measurement of the content of any one characteristic element or values of a ratio of geochemically alike pairs of elements in stratigraphically coeval strata deposited inside the boundaries of an ancient sedimentation basin. The lithological-paleogeographic basis of such a map makes it possible to examine the concentrations of elements (such as aluminum, iron, manganese, phosphorus, and uranium) against the background of reconstructed environmental and climatic conditions of formation of the sediments of the particular age, taking into account the arrangement of ancient shorelines and regions of removal, their petrographic compositions, and, given sufficient initial data, also the physicochemical conditions that existed in the weathering region and in the strata of sediments at the bottom of the basin.

Geochemical maps, along with attached sections, histograms, tables of chemical and mineralogical analyses, and other geochemical materials, assist in the interpretation of the causes of anomalous (commercial) concentrations of elements compared with their background content in the surrounding rock of the region. Geochemical maps substantially supplement the data of prediction-metallogenic maps, facilitating the location of promising areas when exploring for deposits of endogenic and exogenic minerals. The portrayal on geochemical maps of regions of high and particularly low concentrations of some elements (such as iodine, boron, strontium, and copper) is also of special interest in medicine and agriculture, since illnesses of humans and animals are associated with them. The peculiarities of migration of chemical elements under present-day conditions are shown by compiling landscape-geochemical maps.