PHOSPHATE DEPOSITS 



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is a common textural type. Individual beds tend to 

 be thinner than in the geosynclinal environment, but 

 some beds thicken locally. P2O5 contents are gen- 

 erally betvi^een 20 and 30 percent. 



Examples in the United States of phosphorite 

 deposited from upwelling vi^ater caused by diver- 

 gence include the Permian Phosphoria Formation 

 of the Western United States, the Miocene Mon- 

 terey Formation of California, the Mississippian 

 and Triassic deposits of Northern Alaska, and the 

 Mississippian deposits of Utah. The very large de- 

 posits of Kara Tau in the U.S.S.R., the Miocene de- 

 posits of the Sechura Desert in Peru, the Cretaceous 

 of Colombia, the Cambrian of Australia, and the 

 extensive deposits of Cretaceous and Eocene age in 

 Western and Northern Africa and the Middle East 

 are also of this type. 



DEPOSITS FORMED IN WARM CURRENTS 

 ALONG EASTERN COASTS 



Phosphorites formed in warm currents along the 

 eastern coasts of continents consist of phosphatic 

 limestone or sandstone. Black shale and chert are 

 not found with these deposits, although some diato- 

 maceous material may be present. The deposits are 

 generally lower in amount of phosphate per unit 

 area than are the miogeosynclinal deposits. The 

 most important deposits of this type are those of 

 the Eastern United States — Florida, Georgia, and 

 North Carolina. Phosphate in the Upper Cretaceous 

 Gramame Formation near Recife in Brazil is an- 

 other example. 



Deposits of this type are economic only when they 

 have been reworked by submarine currents, sub- 

 jected to weathering, or both. Most deposits of this 

 type must be further upgraded by some method of 

 b3neficiation after mining to provide an economic 

 product. 



The deposits along the eastern coast of the United 

 States are of middle Miocene age and extend from 

 southern Florida to southern Virginia. The richest 

 deposits are found in basins of structural lows on 

 the flanks of domes that were rising at the time of 

 phosphate deposition. The phosphorite may have 

 been deposited by dynamic upwelling or by the cool 

 countercurrent associated with the warm density 

 current. Turbulence caused by the divergence of the 

 cool coastal current due to the rising positive area 

 may have assisted in deposition. 



Concentrations of organic growth and nutrients 

 are found near the mouths of large rivers or in 

 estuaries in which the outflow of surface waters is 

 replaced by a countercurrent of sea water. Decay- 

 ing organic material sinks and is trapped in the 



inflowing sea water. Nutrients may accumulate in 

 the system, and the estuaries may thus have a 

 higher phosphate content than adjacent river or 

 sea water. 



Such a process may account for the fact that the 

 richer phosphate deposits in the Eastern United 

 States are in the bays or basins (some of which may 

 have been estuaries). At least this process may 

 initiate precipitation. A combination of processes 

 probably was operative during the formation of the 

 phosphate deposits of the Eastern United States. 



Deposits of phosphate in eugeosynclinal sedi- 

 ments, associated with chert, shale, and limestone, 

 have been found in Nevada. The phosphate beds are 

 thin and low in P2O5 content and are not likely to 

 be an economic source of phosphate rock in the 

 forseeable future. 



DEPOSITS FORMED ON STABLE SHELVES 

 OR IN CONTINENTAL INTERIORS 



The marine phosphate deposits that form on the 

 stable shelf or in the continental interior are asso- 

 ciated with limestone, dolomite, shale, and glau- 

 conitic sandstone. The phosphate is in the form of 

 nodules or grains that are large with respect to the 

 matrix in which they are found. Individual beds are 

 only a few feet thick, but commonly several beds 

 contain phosphate. The phosphate beds contain from 

 5 to 15 percent P2O5, but the nodules may contain 

 as much as 35 percent. 



Examples of deposits of this type in the United 

 States include beds in the Maquoketa Shale of Ordo- 

 vician age in Illinois, Iowa, and Wisconsin; several 

 formations of Ordovician age in Minnesota and 

 Arkansas; the Oriskany Sandstone of Devonian age 

 in New York, Pennsylvania, and Virginia; sand- 

 stone of Mississippian age in Tennessee and Arkan- 

 sas; black shale of Pennsylvanian age in Kansas, 

 Missouri, and Oklahoma; shale of Pennsylvanian 

 age in Arkansas; several beds of Cretaceous age in 

 Alabama, Mississippi, Georgia, and Texas, associ- 

 ated with limestone; and beds of Cretaceous and 

 Paleocene age in Texas, Tennessee, Alabama, and 

 Mississippi, and the northern Atlantic Coastal Plain, 

 associated with glauconite. 



Some of the beds have been mined — for example, 

 Mississippian sandstone of Tennessee, called "blue- 

 rock," and the Ordovician Cason Shale and Penn- 

 sylvanian Hale Formation in Arkansas — but pro- 

 duction was small. Reserves are small and resources 

 are at best moderate, but these types of deposits 

 could serve as local sources of phosphate rock in the 

 future. 



