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.