Organic Matter as a Transport Agent in Ore-Forming Systems
The formation of all ore deposits can be linked to three fundamental processes (Fig. 1): (1) mobilization of elements in one or several source regions; (2) transport of ore and non-ore constituents from a source region to the site of deposition and beyond; and (3) concentration of ore constituents, normally at the site of deposition. All three processes are interrelated, particularly in the case of hydrochemical deposits in which specific chemical mechanisms responsible for keeping metals in the ore fluid (chemical transport) also play a key role in the processes of mobilization and deposition. For example, a metal may be leached from a source rock and enter the ore solution as a metal-organic complex (in this case, metal-organic complexing is the chemical transport mechanism). It can then be transported physically by the ore fluid as a metal-organic complex, and finally, the metal can be precipitated by the breakdown of the metal-organic complex. Thus, for many hydrothermal, residual, and chemical sediment deposits, an understanding of the chemical transport mechanism is a key to understanding the related processes of mobilization and deposition. In this chapter, the focus is on the role of organic matter as a chemical transport agent in aqueous ore-forming systems (i.e., hydrothermal, residual, and chemical sediment deposits). In these systems, ore and nonore constituents are carried primarily by aqueous fluids. The transport of ore metals in ore fluids can take place as dissolved aqueous species (e.g., metal-organic complexes), or within a suspension in which metals are bound to organic particles, or within a crude oil phase (liquid petroleum). Of these possibilities, only chemical transport in solution will be considered in the following sections. First, the role of dissolved organic matter as a metal-transport agent in aqueous ore fluids will be discussed at some length. Next, our current understanding of ore-metal transport via petroleum is reviewed. Finally, the relative importance of inorganic and organic mechanisms of ore-metal transport will be briefly commented upon.
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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