Organic Maturation Modeling Applied to Ore Genesis and Exploration
One aspect of ore deposit studies is their potential application in prospecting for new deposits. That application has been considered in this volume (e.g., Leventhal and Giordano, 2000; Wood, 2000) and elsewhere. Heroux et al. (1996), for example, integrated organic reflectance and clay mineralogy to delineate alteration zones associated with mineralization. Both ore and petroleum deposits are a result of fluid migration, and from this viewpoint it can be expected that organic and inorganic fluids may use similar lithological pathways (Giże and Barnes, 1994). This chapter will briefly introduce organic maturation modeling as an approach to predicting both the age and relative timing of ore and petroleum fluids.
The thermal maturation of sedimentary organic matter is primarily a function of time and temperature. Simplistically, if two of the parameters (organic maturity, temperature, and time) are known, then the third can be derived. If organic maturity can be determined (using optical properties such as vitrinite or bitumen reflectance, or using geochemical parameters such as isomer ratios or elemental ratios), as well as temperature (fluid inclusions), then time (e.g., duration of heating event) can be estimated. A close association between organic matter and some ore deposits has been noted throughout this and other volumes. The association may reflect genetic links (e.g., reduction or complexing), or maysimply reflect genetically unrelated aqueous and hydrocarbon fluids using the same aquifer. Petroleum, or petroleum-derived bitumens, have been reported as inclusions in ore minerals from many ore deposits (Roedder, 1984). If the time-temperature dependence of organic matter can be used to estimate when the petroleum stage of organic maturation occurred, then a potential dating method for the age of the ore deposit is also established.
The use of organic modeling of the petroleum stage of organic maturation is shown for the Carlin (Nevada) disseminated gold deposit and the Bowland basin, United Kingdom, an historical district of renewed interest following the discovery of the Irish base metal deposits. The Carlin deposit provides an example of the use of organic modeling as a means of ascertaining whether or not organic matter was mobile at the time of mineralization, thus providing evidence to support or refute specific genetic concepts. The Bowland basin example will show that the integration of modeling, fluid inclusion data, and field observations can provide constraints on the probable age of mineralization.
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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