Piper and Perkins reported in 2014 that phosphorus is a critical element in many biologically active compounds (e.g., DNA, RNA, ATP, and bone apatite), an essential nutrient for all living things. As Isaac Asimov (1974) wrote, “we are able to substitute nuclear power for coal power, and plastics for wood, and yeast for meat, and . . ., but for phosphorus there is neither substitute nor replacement.” In the fullest sense, it is the limiting nutrient among the list of essential nutrients for plant and animal growth. Its major use today is as fertilizer to enhance crop yields. Its major source is marine sedimentary phosphate deposits, or phosphorites that provide approximately 80% of the world phosphate supply.
Sedimentary phosphate deposits are found in rocks of all ages, but the larger deposits are distributed unevenly both geographically and stratigraphically. Approximately 75% of the world demand comes from marine sedimentary rocks of (1) Precambrian to Cambrian age that occur in Australia, China, Mongolia, and Russia; (2) Cretaceous to Tertiary age that occur along the northern rim of Africa from Morocco to Tunisia, Egypt, Israel, and Jordan; and (3) Tertiary age that are present along the eastern seaboard of the United States from North Carolina to Florida. The igneous phosphate deposit on the Kola Peninsula (Luzin et al., 1993; Notholt and Sheldon, 1986) has supplied Russia with a major part of its phosphate demand for more than half a century, but it constitutes only about 10% of the world production.
Insular phosphate deposits, whose source was bird guano (Hutchinson, 1950), were important sources of phosphate during the latter part of the nineteenth century. On the Chincha Islands off the coast of Peru, the guano achieved a thickness of 45 m. The deposits on islands in the central Pacific Ocean, for example, Makatea, Baker, Nauru, and Ocean (Ellis, 1935; Elschner, 1913; Montaggioni et al., 1985), formed from guano leached of phosphate by rainwater and seawater. The water, on contact with the underlying coral, reacted to precipitate pelletal carbonate fluorapatite (CFA) coeval with the formation of a pinnacled karst platform, or karrenfeld (Piper et al., 1991).
These deposits were discovered approximately 100 years ago and provided a few percent of the world demand throughout much of the twentieth century. Mining on Nauru has only recently virtually ceased. A submerged phosphate deposit on Mataiva Atoll (Rossfelder, 1990) has not been exploited. The geochemistry of the igneous and insular deposits are extremely interesting, but the discussion in this chapter is limited to the geochemistry of the marine sedimentary deposits.
Phosphate occurs in these deposits primarily as the mineral francolite, a CFA with a stoichiometry adapted by Kolodny and Luz (1992) from McClellan and Lehr (1969). Kolodny and Luz noted several trace elements that substitute for the major elements in the lattice structure, notably the rare earth elements (REE), U, Sr, and the oxyion of SO4, and discussed the isotopes of C, S, O, Sr, and U (Figure 1). This rather complex isotopic composition and trace-element substitution enhances the use of the geochemistry of the CFA as a record of the marine environment within which phosphorites accumulate.
Full citation: Piper D.Z., and Perkins R.B. (2014) Geochemistry of a Marine Phosphate Deposit: A Signpost to Phosphogenesis. In:
Holland H.D. and Turekian K.K. (eds.) Treatise on Geochemistry, Second Edition, vol. 13, pp. 293-312. Oxford: Elsevier.
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