Origin of Life
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ORES, MICROBIAL PRECIPITATION AND OXIDATION Beda A. Hofmann Natural History Museum Bern, Bern, Switzerland
Synonyms Gossan; Ore deposits; Oxidation zone; Supergene enrichment Definition Ore deposits are natural enrichments of chemical elements of economic interest. While all natural elements are present in certain background concentrations in rocks and minerals, they are typically not economically extractable at these levels. Geological processes may lead to enrichments of elements in such a way that orebodies are formed from which large quantities of elements may be extracted at a much lower cost. The formation of orebodies generally occurs in three steps: (1) element extraction from a large volume of rock or melt; (2) transport of elements; (3) deposition of elements in a volume of rock much smaller than the extracted volume. These steps may occur in magmatic melts, hydrothermal systems, diagenetic environments, and at the Earth’s surface. Melts, solutions, gases, and solid phases (minerals) may be involved in these processes. In many cases, ore forming processes occur in magmatic and hydrothermal systems well beyond the temperature limit of microbial growth. However, important ore forming systems are also known within the temperature realm of microbial life (maximum 121 C). These are mainly confined to sedimentary and diagenetic environments and the low-temperature end of hydrothermal systems. A microbial involvement in the different
steps of ore formation may be due to direct interaction, i.e., ore precipitation as a result of microbial activity. On the other hand, indirect influences of life may include the conditioning of the environment (e.g., oxidizing atmosphere), allowing transport of certain elements in a soluble form, and so influencing ore forming processes in systems where microbes cannot live. During the weathering of ore deposits under near-surface conditions, microbes may also play an important role. In general, the direct involvement of microbes in ore formation is one of catalyzing redox reactions such as the oxidation of Fe and Mn and the reduction of S, Cr, and U. The role of lowtemperature redox fronts as sites of ore formation and availability of energy for microbes have been reviewed by Hofmann (1999). The possible role of microbes in ore precipitation has long been recognized (Macqueen and Coope, 1985; Southam and Saunders, 2005). Precipitation of ore minerals as a direct result of microbial activity can be inferred from the fact that many microbes can harvest energy from redox reactions, leading to immobilization of elements. Such reactions include the reduction of U(VI), Cr(VI), and Au(I) (Kashefi and Lovley, 2000; Kashefi et al., 2001; Labrenz et al., 2000; Moreau et al., 2004), the reduction of As and Se (Stolz and Oremland, 1999), and possibly also the indirect reduction of oxidized elements by H2S (Mohagheghi, 1985). Arsenic is an important element in ore deposits. Reduced arsenic (As-I) leads to the precipitation of Ni, Co, Fe, and other elements as arsenides. Native As, As(0), al
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