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UNITED STATES MINERAL RESOURCES 



sociated with alkaline igneous rocks including car- 

 bonatite, ijolite, nepheline syenite, and pyroxenite. 

 The larger deposits in the world are in the Kola 

 Peninsula (U.S.S.R.), eastern Uganda and Palabora 

 in Africa, and Araxa and Jacupiranga in Brazil. 



Reserves in individual deposits are measured in 

 scores to hundreds of millions of tons. The phosphate 

 mineral is a fluorapatite. P2O5 contents are as high as 

 36 percent, and weathering may form apatite-rich 

 residual deposits overlying the surface of calcitic or 

 dolomitic source rocks. 



Other forms of igneous apatite deposits are small- 

 er and are lower in P2O5 content. Examples in the 

 United States include the ilmenite-apatite deposits 

 of Virginia and the apatite-magnetite marginal 

 deposits of the eastern Adirondacks. None of the 

 deposits in the United States are being mined, and 

 reserves and resources of this type of material are 

 very small as compared with total resources of phos- 

 phate rock. 



About 17 percent of the world's production of 

 phosphate is from deposits of igneous apatite; most 

 of this is from the Kola Peninsula and Palabora. 



GUANO AND RELATED DEPOSITS 



All large accumulations of guano are formed at the 

 surface by sea birds. Smaller quantities are formed 

 in caves principally by bats. Reserves in most 

 deposits are measured in thousands or hundreds of 

 thousands of tons. The mineralogy of guano deposits 

 is complex inasmuch as it depends on the amount of 

 leaching by surface waters. Slightly decomposed de- 

 posits consist of ammonium and alkali oxalates, 

 sulfates, and nitrates, and magnesium and ammoni- 

 um-magnesium phosphates. Decomposed guano con- 

 sists chiefly of calcium phosphates — monetite and 

 whitlockite. 



In areas of even slight rainfall, the soluble phos- 

 phate of guano is carried downward and replaces or 

 alters the underlying rock. Apatite is the mineral 

 formed where the underlying rock is limestone, and, 

 over geologic time, deposits measuring scores of 

 millions of tons have been formed on some of the 

 islands of the western Pacific. Aluminum or alumi- 

 num-iron phosphates (strengite and variscite) are 

 formed where the underlying rock is volcanic. De- 

 posits are known in several parts of the world (Mal- 

 pelo Island, Colombia) , but reserves are limited and 

 processing costs are high. 



Guano has been mined from caves in various parts 

 of the United States, but reserves and resources are 

 negligible and world resources are small. Production 

 of guano, largely from renewable sources, measured 

 in terms of its phosphate content, is less than 1 per- 



cent of the total production of phosphate in the 

 world. 



Guano-derived deposits, particularly those in the 

 western Pacific account for almost 5 percent of the 

 world's production of phosphate. Reserves are meas- 

 ured in millions of tons. 



SEDIMENTARY PHOSPHORITE 



About 75 percent of the world's production of 

 phosphate rock is from deposits of marine phos- 

 phorite. The richest and largest of these deposits 

 form in warm latitudes, mostly between the 40th 

 parallels and in areas of upwelling caused by diverg- 

 ence. These areas are in the tradewind belts on the 

 west coasts of the continents or in mediterranean 

 seas along the equatorial side of the basin. 



Significant concentrations also form along the 

 west sides of poleward-moving warm currents along 

 the eastern coasts of continents. In these areas 

 there are cool coastal countercurrents, such as along 

 the eastern coast of the United States. 



DEPOSITS CAUSED BY DIVERGENCE UPWELLING 

 OF SEA WATER 



Deposits caused by divergence upwelling are 

 characterized by a suite of rocks that consists, in a 

 shoreward direction, of black shale, phosphatic shale, 

 phosphorite, dolomite, chert or diatomite, and saline 

 deposits and red or light-colored sandstone or shale. 

 The rocks grade laterally into one another, and be- 

 cause of lateral shifts in environment due to epeiro- 

 genic movements, the sequence may also be repeated 

 vertically. The thickest accumulations of phosphor- 

 ite form in areas of geosynclinal subsidence where 

 clastic sedimentation is at a minimum; that is, a 

 miogeosynclinal environment. Phosphorites are also 

 found in the eugeosynclines, but are so mixed with 

 the thick shale, chert, and volcanic materials of the 

 eugeosyncline that thick high-grade deposits are not 

 known and are unlikely. 



The phosphate in these rocks is carbonaceous and 

 consists of pellets, some nodules, and minor phos- 

 phatized bone material and shell. The pellets may 

 have formed by direct precipitation or by diagenesis, 

 and some are certainly replaced calcite (as for ex- 

 ample, the phosphatized shell material). Individual 

 beds may be several feet thick and contain 30 per- 

 cent or more P2O5, and the beds may extend over 

 hundreds of square miles. Phosphorite that was 

 formed on the adjacent stable continental shelf area 

 is generally associated with chert, calcareous chert, 

 and light-colored sandstone and shale. The phosphate 

 consists of nodules, pellets, and abundant phospha- 

 tized shell and bone material; phosphatic sandstone 



