Lithogenic Geochemistry

(geochemistry of sediments or geochemistry of sedimentary rocks), a branch of geochemistry that studies the chemical composition and physicochemical processes of formation of sedimentary rocks and ores, their evolution in the earth’s history, the laws governing abundance, and the distribution and migration of elements in the sedimentary mantle and hydrosphere. Through the common object of study, lithogenic geochemistry is closely connected with lithology. In reconstructing geochemical processes the data of stratigraphy, geotectonics, paleogeography, and oceanography, observations of contemporary weathering and sediment accumulation processes, and data from the experimental reproduction of equilibrium systems (carbonate, phosphate, salt, and other systems) are used as models of processes and reactions in the geological past, with the addition of the necessary corrections for evolutionary changes in physicochemical conditions of sedimentary rock formation. Lithogenic geochemistry investigates processes occurring at relatively low temperatures and pressures, restricted to the interval between those values that are characteristic of the earth’s surface and the upper boundary of the region of regional metamorphism.

Lithogenic geochemistry encompasses the study of all the stages of sedimentary rock formation, including the weathering and the mobilization of the initial substances in the region of denudation, their transport by rivers to the terminal basins of flow (intracontinental, marine, and ocean), accumulation in the sedimentary deposits being formed, and the subsequent redistribution in the course of diagenetic and epigenetic processes. It aims to establish quantitative relationships for the various modes of the transport of elements in the form of true and colloidal solutions, complex compounds, mechanical suspensions, and sorptions on clay and other minerals as well as to elucidate the quantitative laws of the spatial distribution of elements in an aqueous medium and in the sedimentary deposits. Of primary importance in lithogenic geochemistry are concepts regarding the equilibriums between atmospheric gases, the ionic composition of ocean waters, and bottom sediments (aluminosilicate and carbonate-bicarbonate equilibriums), and the study of the sedimentary differentiation of elements and their zonal distribution over basin areas. In this connection, the problem of the relationship between clarke (dispersal) and ore (concentration) processes is considered; its solution is of great practical interest in prospecting for hidden ore deposits.

The significance of the various types of chemical reactions in the formation of sedimentary ore deposits is not identical at different stages of lithogenesis. In the formation of weathered crustal deposits (bauxite, iron, and nickel ores), the leading role is played by oxidation and hydrolysis reactions and in the formation of salt deposits, by precipitation reactions (crystallization) from real solutions. In the formation of deposits of phosphorites, native sulfur, and iron, manganese, and uranium ores the leading part is played by chemical biological processes accompanied by reactions of reduction and diffusion redistribution of substances in interstitial solutions.

To a considerable extent, sedimentary rock and ore formation and the types of chemical reactions involved were determined by the physicogeographical conditions existing on the earth’s surface at various geological time periods, by the regimen of tectonic movements within a given region, by the intensity of volcanic activity, and many other factors.

Lithogenic geochemistry makes use of geochemical indicators to reconstruct facies and climatic conditions of sedimentation, particularly the salinity of the waters of ancient basins and their gaseous regimen, depth, and temperature. These indicators are the ratios of chemically similar pairs of elements and the isotopic ratios of oxygen, sulfur, carbon, and so forth. Special attention is given to the study of the geochemistry of organic matter, which is not only the source of fuel gases and petroleum but also a factor in determining the processes of reduction and migration of polyvalent elements and the study of the formation of mobile heteroorganic compounds and complexes.

Lithogenic geochemistry is directly related to the problem of the geochemical balance of chemical elements in the earth’s external shell. A basic feature of sedimentary rocks is the well-defined difference between their composition and the mean composition of the rocks of the “granitic” shell, which is the main source of sedimentary material during the last 2-3 billion years of the earth’s history. The difference is primarily in the higher average content of water, carbonic acid, and organic carbon as well as of S, Cl, F, B, and other volatile substances in the rocks of the sedimentary mantle. Other important characteristics of sedimentary rocks are their high content of calcium, a shift in the K/Na ratio towards K, a higher ratio of ferric iron to ferrous oxide, and a higher content of sulfate minerals than in crystalline rocks of the “granitic” shell. All these properties are most clearly evident in platform sediments, since they are the results of more intense weathering and of well-defined surface differentiation. Unlike platform sediments, geosynclinal sediments (particularly sands) have undergone less intense changes, and their composition is closer to that of the source rocks. Poor differentiability of the composition of sediments is, in geosynclinal regions, opposed by their intense epigenetic transformations, which are connected with the submersion of reaction-susceptible minerals in a region of higher temperatures and pressures.