The Organic Geochemistry of Gold, Platinum, Uranium, and Mercury Deposits
Gold, the platinum group elements, uranium, and mercury have been subjects of organic geochemical studies for the past few decades. This geochemical interest contrasts with their chemistry, as platinum and gold compounds were synthesized over 150 years ago. Gold and the platinum group elements are valued for their relative chemical inertness, in experimental bombs, for example. In organic and biochemical systems, however, they are reactive. Mercury occurs in ores as a native element or sulfide, but in the environment it is often a component of organic species.
Several approaches are combined in this review in order to highlight areas where there is good agreement between the perspective of a chemist and a geologist and areas where there are discrepancies. Initially, the concept of organometallic compounds is introduced, and a brief chemical or biochemical guide as to which metals would be expected to show strong or weak interactions with organic matter is presented. The ore deposits in which organic matter is present with each metal are then summarized in order to provide an initial comparison between chemical and biochemical predictions and geologic observations. To illustrate the stages during which metal-organic interactions take place, the occurrence of organosulfur and organonitrogen compounds is summarized, together with a brief summary of reduction mechanisms. Finally, the potential interactions between metals and organic matter are summarized in terms of the three general stages of ore deposit formation (source, transport, and precipitation).
In this paper, an unusual citation convention is used in reference to certain papers cited by Boyle (1987). This volume contains many key papers on gold, some of which were published in neither recent nor easily accessible original sources. Citations of papers considered difficult to obtain have been made therefore to Boyle (1987). The literature on the organic geochemistry of ore deposits has grown considerably in the past 20 years, especially with respect to uranium, so that a detailed review would be excessively long. As far as possible, therefore, references have been cited which permit the interested reader to follow a topic in more detail.
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