Organic matter provenance
Organic matter in sedimentary basins, usually marine and either of Recent or geologically old origin, is derived from the syngenetic residues of posthumus biogenic debris (Simoneit, 1982a; Tissot and Welte, 1984; Hunt, 1996). This material is composed of both autochthonous detritus and allochthonous residues derived from continental sources (Simoneit, 1982a). Aquatic sediments receive allochthonous organic detritus primarily by river wash-in and eolian fallout particles, with ice-rafting and sediment recycling as minor contributing processes (Simoneit, 1975, 1978). Organic matter that accumulates in contemporary sediments represents the residues from primary biological carbon fixation and its degradation (remineralization; Table 1). The nature of this immature organic material is described below, followed by a general description of more mature organic matter formed as a result of maturation in subsiding basins.
Nature of immature organic matter in sedimentary basins
Gas: Interstitial gas in recent sedimentary environments consists primarily of methane, carbon dioxide, and sometimes hydrogen sulfide (Claypool and Kaplan, 1974; Claypool and Kvenvolden, 1983). The biogenic hydrocarbon gases usually have CH4/(C2H6 + C3H8) ratios greater than 1,000, while those of a thermogenic origin have ratios less than 50 (Bernard et al., 1976). For example, the C1/(C2 + C3) ratios for shallow sediment gases from Guaymas basin, Gulf of California range from 41 to 150 and thus indicate a mixed origin of biogenic (CH4) and thermogenic (C1-C8) hydrocarbons (Simoneit et al., 1979). The depth range where biogenic gas can be found is variable but generally shallow (∼100 m) and depends on microbial production and environmental conditions in the sediments.
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