Phosphorite deposits formation

Phosphorite deposits are currently forming in areas of high organic productivity and low detrital input. These are typically coastal upwelling zones adjacent to arid continental lands. Phosphorites form at slow rates, so low detrital input is important to prevent dilution or burial. As shown in Figure 18.10, sites of formation include the continental margins of Peru, Chile, and southwest Africa. 

Further evidence that Kazakov s (1937) inorganic hypothesis is quite inadequate to account for the formation of modern phosphorite deposits off... 

Figure 6 Schematic figure of the western South American shelf and slope, showing contours of dissolved phosphate and oxygen in the bottom waters and the locations of aerobic and anaerobic sediments, phosphorite and Recent phosphorite deposits. (After Burnett WC, Veeh HH and Soutar A (1980) U-series, oceanographic and sedimentary evidence in support of recent formation of phosphate nodules off Peru. In Marine Phosphorites, SEPM Special Publ. no. 29, pp. 61-72.)...

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Minerals of economic importance within sedimentary formations include, hut are not limited to fluorite, barite, phosphorite, and oolitic hematite. Fluorite is utili/ed us a flux in steelniakiitg and when of high quality as lenses and prisms in the optical industry. Barite is an essential mineral used m gas- and oil-well drilling. Phosphorite, a product of chemical precipitation from seawater, when ircaled with sulfuric acid, produces superphosphate fertilizer, (.killtic hematite deposits of extensive size are important sources of iron ore. 

The Permian Phosphoria Formation in the northwestern Interior United States contains two phosphatic and organic-ncarbon-rich shale members, the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member. Ihese rocks were formed at the periphery of a foreland basin between the Paleozoic continental margin and the North American cratonic shelf. The concentration, distribution, and coincidence of phosphorite, organic carbon, and many trace elements within these shale members probably were coincident with areas of optimum trophism and biologic productivity related to areas of upwelling. In the Phosphoria sea upwelling is indicated to have occurred by sapropel that was deposited adjacent to shoals near the east flank of the depositional basin. 

For large-scale phosphorites to form, two major obstacles must be over come. First, phosphorous must be concentrated at levels from 1 to 2 million times that of average seawater. Secondly, once phosphate (P04 ) is accumulated, the stable CFA phase must crystallize. For much of the history of phosphorite study, the main objective has been to discover the unique method for the formation of these deposits. However, through analysis and reanalysis of the many deposits being formed today and those formed throughout the geologic record, it has come to be accepted that phosphorites have formed in a variety of ways throughout Earth s history. However, there are important processes that connect all, or at least most, of the known deposits. 

Very slow deposition rates are necessary to create economic-grade phosphorites. Slansky (1986) estimates sediment accumulation rates of economic-grade phosphorites between 2x10 and 1x10 years per meter of deposit. This slow deposition rate is seen in Permian sediments of the Rocky Mountains where the Phosphoria Formation is about 30 m thick in the most phosphatic zone, whereas the same time period is represented by over 1000 m in non-phosphatic sections of the Permian in the Rocky Mountains. 

Inlet, Santa Barbara Basin, and the Miocene Monterey Formation. Mar Geol 174 159-176 Jnrinak JJ, Dndley LM, Allen MF, Knight WG (1986) The role of calcium oxalate in the availability of phosphoras in soils of semiarid regions A thermodynamic study. Soil Sci 142 255-261 Kaz ov AV (1937) The phosphorite facies and the genesis of phosphorites. Soviet Geol 8 33-47 Kemp AES, Baldanf JG (1993) Vast Neogene laminated diatom mat deposits from the eastern equatorial Pacific Ocean. Natnre 362 141-144... 

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