Roles of Organic Matter in Shale- and Carbonate-Hosted Base Metal Deposits
Base metals are incorporated into diverse sedimentary rocks as a result of routine biogeochemical processes during primary production, heterotrophic consumption, bacterial diagenesis, burial maturation, and hydrothermal alteration. In most cases, concentrations of organic carbon and base metals in sedimentary strata are below minimum levels for economic development of energy and/or mineral resources. Major ore deposits are linked, in many cases, to unusual basinal conditions that focus metal-bearing solutions into sedimentary facies containing reactive organic constituents inherited from the original sedimentary inventory or enriched by secondary fluid migration. This chapter examines the chemical and physical role of reactive organic matter in base metal deposits associated with carbonate and shale sequences. The discussion of selected well-studied deposits is organized on the basis of the dominant metals and the sedimentary facies hosting the orebody. Three groups of ore deposits are discussed in this chapter: (1) carbonate-hosted lead-zinc-barium deposits; (2) shale-hosted zinc-lead deposits; and (3) shale-hosted copper deposits. Diverse roles of organic materials in the genesis of sediment-hosted base metal deposits are evident from recent publications on the Mississippi Valley Viburnum trend, Canadian Pine Point, French Massif Central, Canadian Selwyn basin, Australian McArthur River region, Alaska Red Dog mine, European Kupferschiefer, and Michigan Nonesuch shale. Although several important types of sediment-hosted base metal ores are not considered in this paper, the selected examples provide a framework for understanding how modern organic geochemical techniques can be applied in an interdisciplinary approach to exploration and development of a wide range of base metal deposits. Additional discussions of shale- and carbonate-hosted base metal deposits can be found in Coveney (2000), Giże (2000), Kettler (2000), Leventhal and Giordano (2000), and Simoneit (2000).
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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