Metalliferous Shales and the Role of Organic Matter with Examples from China, Poland, and the United States
Raymond M. Coveney, Jr., 1997. "Metalliferous Shales and the Role of Organic Matter with Examples from China, Poland, and the United States", Ore Genesis and Exploration: The Roles of Organic Matter
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Some of the world' s most important mineral resources occur in shales. Deposits at Shaba in the Congo, White Pine in the United States, and Lubin in Poland each hold or originally contained greater than 500 million tons of high-grade Cu ore and those at Meggen in Germany, the Selwyn basin in Canada, and Red Dog in Alaska contain multimillion-ton Pb-Zn sedimentary-exhalative (sedex) ores hosted by black shales and related beds (e.g., Rentsch, 1974; Brown, 1978; Gustafson and Williams, 1981; Carne and Cathro, 1982; Lange et al., 1985; Goodfellow and Jonasson, 1987; Sangster, 1990; Maynard, 1991a, b; Giże, 2000; Pratt and Warner, 2000). Moreover, recent discoveries in China (Fan et al., 1973) have led to small-scale production of shale-hosted Ni and Mo from an entirely new sort of bedded sulfide ore containing values for precious metals totaling several hundred parts per billion Au, Pt, and Pd. Together with similar deposits from the Devonian of the Yukon, the Chinese ores raise hopes for discovery of important new resources for Pt, Au, and ferroalloy metals (Fan 1983; Coveney and Chen, 1991). On the other hand, despite high tenors, shales from some enriched deposits such as the Mo-rich Pennsylvanian Mecca Quarry shale of North America and the Proterozoic HYC McArthur River Pb-Zn-Ag prospect of Australia are presently unmineable because of inadequate thickness and difficulties with beneficiation of fine-grained host phases.
In addition to their economic value as ores of base metals, organic-rich shales have been important sources of solid fuels and hydrocarbons, clays, and uranium. Moreover, organic-rich shales have had adverse economic significance because of their tendencies to expand and disintegrate as a result of the growth of sulfate crystals or swelling of clays disrupting the foundations of buildings and the roofs of mines (Coveney and Parizek, 1977; Rudy, 1982). Metalliferous black shales also may have more general environmental significance. For example, they no doubt contribute heavy metals to soils, ground water, and surface waters (Bailey, 1995; Forseh, 1997) and therefore their presence may dictate the need for baseline geochemical studies in road construction projects, for example. Because of their high U contents, black shales have been suspected as sources of gas (e.g., Coveney et al., 1987).
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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