The Geobiology of Sediment-Hosted Mineral Deposits
The role of biological processes in the formation of sediment-hosted ore deposits has long been recognized. In this review, we focus on the biogeochemical cycling of C, Mn, Fe, and S as they relate to the formation of sediment-hosted Mn and Fe deposits, metalliferous black shales, clastic-dominated (CD) Pb-Zn deposits, and phosphorites. Biological mediation of ore-forming processes occurs over large spans of space and time. The most important step is oxygenic photosynthesis, a biological innovation dating from the Archean Eon that releases free oxygen into the surface oceans and atmosphere and delivers chemical potential, in the form of reduced carbon, to the seafloor. Photosynthetic oxygen is available to precipitate dissolved Fe2+ and Mn2+, and therefore it augments the formation of sedimentary Mn and Fe deposits, and drives oxidative weathering of exposed crust, thereby delivering sulfate and transition metals to the ocean. Where reduced carbon accumulates in the deep oceans and on the seafloor, bacterial sulfate reduction produces hydrogen sulfide thereby facilitating the formation of metalliferous black shales, sediment-hosted Pb and Zn sulfide deposits, and phosphorites. Thus, an understanding of major biogeochemical processes and how they have evolved over time is required in order to refine genetic models for sediment-hosted ore deposits and to guide future mineral exploration.
A close secular relationship between deposit formation and trends in major biogeochemical cycles provides a potentially powerful tool for mineral resource assessment. Sedimentary basins that formed during a time that is known to lack deposits of a particular metal can be eliminated during exploration programs, whereas others of permissive ages should be considered priorities. For example, sedimentary basins older than ca. 1.8 Ga are unlikely to contain large CD Pb-Zn deposits, and basins that formed between 1.6 and 0.6 Ga are not prospective for phosphorites. Recent technological advances in the application of nanometer-, micron-, and bulk-scale analytical techniques allow for imaging of complex biological structures and have provided new insights into the role of bacteria, not only in direct formation of mineral deposits, but also in leaching of metals from ore and mineralized rocks. Future exploration for, and exploitation of, mineral deposits may include offshore or land-based, low-grade, high-tonnage targets; understanding the role of bacteria in mineral growth, mineral dissolution, and redox transformations will aid in predicting where such deposits exist, and how metal extraction from ores can be enhanced.
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Building Exploration Capability for the 21st Century
Earth’s near-surface mineralogy has diversified over more than 4.5 b.y. from no more than a dozen preplanetary refractory mineral species (what have been referred to as “ur-minerals” by Hazen et al., 2008) to ~5,000 species (based on the list of minerals approved by the International Mineralogical Association; http://rruff.info/ima). This dramatic diversification is a consequence of three principal physical, chemical, and biological processes: (1) element selection and concentration (primarily through planetary differentiation and fluidrock interactions); (2) an expanded range of mineral-forming environments (including temperature, pressure, redox, and activities of volatile species); and (3) the influence of the biosphere. Earth’s history can be divided into three eras and ten stages of “mineral evolution” (Table 1; Hazen et al., 2008), each of which has seen significant changes in the planet’s near-surface mineralogy, including increases in the number of mineral species; shifts in the distribution of those species; systematic changes in major, minor, and trace element and isotopic compositions of minerals; and the appearance of new mineral grain sizes, textures, and/or morphologies. Initial treatments of mineral evolution, first in Russia (e.g., Zhabin, 1979; Yushkin, 1982) and subsequently in greater detail by our group (Hazen et al., 2008, 2009, 2011, 2013a, b; Hazen and Ferry, 2010; Hazen, 2013), focused on key events in Earth history. The 10 stages we suggested are Earth’s accretion and differentiation (stages 1, 2, and 3), petrologic innovations (e.g., the stage 4 initiation of granite magmatism), modes of tectonism (stage 5 and the commencement of plate tectonics), biological transitions (origins of life, oxygenic photosynthesis, and the terrestrial biosphere in stages 6, 7, and 10, respectively), and associated environmental changes in oceans and atmosphere (stage 8 “intermediate ocean” and stage 9 “snowball/hothouse Earth” episodes). These 10 stages of mineral evolution provide a useful conceptual framework for considering Earth’s changing mineralogy through time, and episodes of metallization are often associated with specific stages of mineral evolution (Table 1). For example, the formation of complex pegmatites with Be, Li, Cs, and Sn mineralization could not have occurred prior to stage 4 granitization. Similarly, the appearance of large-scale volcanogenic sulfide deposits may postdate the initiation of modern-style subduction (stage 5). The origins and evolution of life also played central roles; for example, redox-mediated ore deposits of elements such as U, Mo, and Cu occurred only after the Great Oxidation Event (stage 7), and major Hg deposition is associated with the rise of the terrestrial biosphere (stage 10; Hazen et al., 2012).