The Interaction of Organic Matter and Fluids during the Genesis of Some Precious Metal and Volcanogenic Massive Sulfide Deposits
Richard M. Ketiler, 1997. "The Interaction of Organic Matter and Fluids during the Genesis of Some Precious Metal and Volcanogenic Massive Sulfide Deposits", Ore Genesis and Exploration: The Roles of Organic Matter
Download citation file:
Despite the near-ubiquity of reduced carbon in many different types of epigenetic and exhalative ore deposits, it has been difficult to relate the reduced carbon to ore transport and deposition in many of these systems. This is particularly true with most precious metal deposits. Whereas many base metal deposits (e.g., Mississippi Valley-type or sedimentary exhalative, sedex, deposits) are restricted to sedimentary rocks and appear to be related to the evolution of sedimentary basins, many types of precious metal deposits are hosted in both organic carbon-rich sedimentary rocks and in volcanic rocks that contain little organic matter. If there are sufficient similarities in the mineralization and alteration observed in the igneous and sedimentary rocks, then any model of ore deposition that appeals to organic matter seems unnecessarily ad hoc. Efforts to produce simple and robust models of ore deposition combined with the refractory nature of the organic matter in many systems have led various workers to underrate the significance of organic matter in many epigenetic precious metal deposits. This situation is changing: improved analytical techniques can be used to document the presence of organic species in hydrothermal systems, the importance of bulk-minable ore deposits requires that wall-rock chemistry receive greater consideration, and continued interest in the role of organic matter in ore mineralization has prompted closer scrutiny of those systems that contain organic matter. This review will use a number of epigenetic precious metal deposits as examples and consider the significance of organic matter to ore deposition for each (see also Leventhal and Giordano, 2000; Giże, 2000; and Pratt and Warner, 2000). The review will also include a short discussion of volcanogenic massive sulfide mineralization and the significance of organic matter to these exhalative systems (see also Simoneit, 2000).
Figures & Tables
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