Thermodynamic Response of Organic Compounds in Geochemical Processes of Sedimentary Basins
Organic compounds generated by biological processes, either at the surface or within the Earth' s crust, are incorporated into many types of geologic materials and undergo numerous transformations driven by changes in temperature, pressure, and composition. These transformations reflect the energetic response of compounds that are transported by geologic processes into conditions where they are far from equilibrium. The active geochemical processes and the geologic variables that influence the course of organic alteration can be identified by evaluating the energy differences between the starting compounds and their alteration products in deeply buried sedimentary rocks, ore deposits, petroleum, and elsewhere. Thermodynamic calculations provide quantitative assessments of these energetic differences, and it is the purpose of this review to illustrate how such calculations can reveal the driving forces of organic transformations. This type of approach can be useful in the study of ore deposits because oxidation-reduction reactions dictate the course of organic alteration. These redox reactions can couple to inorganic redox processes that enhance metal transport or trigger ore deposition. As always, thermodynamics indicates the possible, and cannot, on its own, reveal the mechanisms through which the transformations may occur. Nevertheless, thermodynamics provides the means to assemble plausibility arguments based on geologic information and to test those arguments with an independent set of data.
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The assooatwn of organic matter with ore minerals, gangue, and host rock in many low-temperature ( 120C) o moderate-temperature (120-350C) ore deposits is a well-known phenomenon (Saxby, 1976; Leventhal, 1986; Parnell et al., 1993; Giordano, 1996; Gize, 1999) and was recognized early in the twentieth century (Siebenthal, 1915; Harder, 1919; Schneiderholm, 1923; Bastin, 1926; Fowler, 1933). The study of organic constituents in ores, particularly if coupled with studies of other ore components and conditions, can provide much information on both active and passive roles of organic matter before,during, and after ore genesis, and in some cases can leadto the development of valuable exploration techniques.By the 1950s, it was recognized that biological sequesteringof metals, sulfide production by sulfate-reducing bacteria,biological precipitation of metals, sorption of metals byorganic colloidal particles, modification of geochemicalenvironments by organic processes, and the mobilizationof metals by metal-organic complexes were all potentiallyimportant roles played by organic matter in the concentrationof metals to form metalliferous shales and certaintypes of ore deposits (Berger, 1950; Krauskopf, 1955). Bythe 1960s, it was recognized that dead organic matter(organic matter not in living organisms) may be a powerfulreducing agent for sulfate and thus may provide asource of sulfide for ore-forming systems (Barton, 1967;Skinner, 1967). Roedder (1967) reported the presence ofhydrocarbons and sulfate in fluid inclusions from oredeposits. This observation was cited by Barton (1967) asstrong evidence that organic matter was present at thetime of ore formation and that thermodynamic equilibrium(which predicts hydrogen sulfide and carbon dioxide)was not attained in the ore fluid because of sluggishkinetics at the low temperature of ore formation. Hoering(1967) summarized his pioneering work on organic matterassociated with gold and uranium in the Carbon LeaderFormation of the Witwatersrand district, South Mrica.Because it was relatively immature Precambrian organicmatter (rather than graphite), it was suitable for analysis ofsimple and complex chemical compounds and led the wayfor future studies of organic matter in ore deposits andPrecambrian rocks (Leventhal et al., 1975).It was not until the 1970s and early 1980s that majorefforts on a worldwide scale were initiated to study theroles of organic matter in ore genesis (Breger, 1974; Leventhalet al. 1975; Connan and Orgeval, 1976; Saxby,1976; Giordano, 1978; Connan, 1979; Estep eta!., 1980).As a consequence of this major