Supergene enrichment and dispersion involve similar chemical and physical processes operating near the surface of the Earth. In both cases, elements are mobilized from their source, transported and fixed at some new site. In supergene enrichment, elements are concentrated by these processes. Supergene enrichment is of importance because it can upgrade otherwise uneconomic primary deposits to ore grade. During dispersion, elements are spread over a greater volume of space and diluted. In this chapter, use of the term dispersion is restricted to what Rose et al. (1979, p. 17) referred to as “secondary dispersion,” i.e., the redistribution of elements by processes occurring after the main ore-forming event, usually in the surficial environment. Dispersion often results in anomalous elemental concentrations in rocks, soils, lake and stream sediments, plants, or natural waters in the vicinity of ore deposits, and thus impacts on geochemical exploration by increasing the probability that a geochemical survey will uncover the anomaly. Giordano (2000) summarized the various roles of organic matter as transport agents in ore-forming and related systems.
The inorganic geochemical processes which govern supergene enrichment and dispersion have been given considerable attention. Somewhat less attention has been paid to the role of organic matter. However, the potential roles for organic matter in supergene enrichment and dispersion are numerous and include (see also Schnitzer and Khan, 1972, 1978; Reuter and Perdue, 1977; and Wood, 1996): (1) increasing the solubility of minerals or decreasing the amount of sorption of ions onto mineral surfaces as a result of the formation of aqueous metal-organic complexes and/or increased acidity; (2) increasing metal mobility via coating and protection of colloids from coagulation; (3) metal fixation, either by reduction or through sorption onto solid organic material; (4) modification of the sorption-ion exchange properties of mineral surfaces; and (5) alteration of the rates of sorption, dissolution, and precipitation.
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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