Phosphorite Deposits of North America

Phosphorite Deposits of North America


a group of phosphorite deposits located primarily in the western United States (Phosphoria Formation), as well as in Florida, North Carolina, and Tennessee.

In the western states, namely Utah, Idaho, Wyoming, Montana, and Nevada, phosphorite deposits are located within the geosyncline of the Rocky Mountains and the western edge of the North American Platform. They are confined to the Lower and Middle Permian Phosphoria Formation. The average thickness of the deposits, which are distributed over an area of approximately 180,000 km2, is 180 m. These deposits include two uniform zones of phosphorites, as well as blocks of silicon and dolomites. Bedded phosphorite deposits consisting of pellets and oolites cemented with carbonate matter are common. The P2O5 content varies between 19 and 36 percent. The phosphorites contain organic matter and small concentrations of U, V, Se, and other metals. Within the basin region there are several dozen deposits with estimated reserves of 7.7 billion tons of phosphorite with an average P2O5 content of 25 percent. The total reserves are estimated at 23.4 billion tons (1971).

In Florida the phosphorite deposits within the platform are associated with the Middle Miocene Hawthorne Formation, which covers an area of 17,000 km2 and is composed of sands, dolomites, and clays containing disseminated phosphorite pellets. The phosphorite reserves are substantial, but concentrations are low and of no commercial value. Owing to the erosion and enrichment of the phosphorite-bearing Hawthorne strata, lenses and laminae of conglomerates containing an average of 15 percent P2O5 at a thickness of up to 7.5 m were formed during the Pliocene epoch in the Bone Valley Formation in central Florida; phosphorite reserves in these conglomerates exceed 1 billion tons. Deposits of hard phosphorites with an average P2O5 content of 33–36 percent are located in the Pliocene Alachua Formation in northwestern Florida; they were formed as a result of the leaching of rocks in the Hawthorne Formation and the karsting of Oligocene and Eocene limestones underlying the formation.

In North Carolina the phosphorite deposits, situated along the Atlantic coast on the eastern slope of the North American Platform (on the Coastal Plain and the shelf), are associated primarily with Neogene formations, such as the Duplin (Miocene), Yorktown (Pliocene), and Ladson (Pleistocene). These are composed of quartz sands, clays, and, more rarely, limestones and dolomites and rich in phosphate pellets. The reserves of phosphorite are estimated at 1.5–10 billion tons with a 16–18 percent P2O5 content. The phosphorites are rich in organic matter and require no processing for use as soil fertilizer.

The deposits in Florida and North Carolina account for approximately 80 percent of the total phosphate raw material in the United States; the deposits in the western states account for 10–12 percent.


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