PHOSPHATE DEPOSITS 



523 



Truly speculative resources may be present in 

 Precambrian rocks as metamorphosed marine phos- 

 phorite. Precambrian phosphorite deposits are 

 known in several parts of the vi^orld and a study of 

 the Precambrian should be made to determine the 

 location of possible sedimentary basins. These basins 

 should then be studied to determine if phosphorite 

 deposits are present. 



PROSPECTING TECHNIQUES 



The geologic relations discussed earlier are the 

 basis for prospecting, particularly in the outlining 

 of large areas that are worthy of further examina- 

 tion. 



Exploratory work should be concentrated on the 

 search for marine phosphorite deposits. Deposits are 

 very large; for example, it is estimated that phos- 

 phate in the Phosphoria Formation was deposited 

 over an area of 135,000 square miles, and the po- 

 tential resources, therefore, are correspondingly 

 vast. 



Most marine phosphorites were deposited in ba- 

 sins of deposition in miogeosynclines or in subsid- 

 ing areas on the adjacent relatively stable platform, 

 and deposits have been classified as either geosyn- 

 clinal or platform type. The first step in prospect- 

 ing is to identify the large marine sedimentary 

 basins of deposition. 



The second step is to determine whether any 

 phosphate is present in the rocks of the basin. The 

 typical suite of rocks associated with the deposits 

 of phosphorite in the miogeosyncline is an invalua- 

 ble aid to prospecting. The recognition of the black 

 shale-chert-carbonate sequence in samples collected 

 from oil exploration is a valuable clue to the loca- 

 tion of a phosphorite deposit. Sandstone and light- 

 colored shale and siltstone are much more abundant 

 in the platform facies, but chert, calcareous chert, 

 siliceous shale, and some carbonate rock are also 

 present. 



Phosphorite is not a very distinctive rock and can 

 be overlooked or misidentified. Most phosphorites 

 contain rounded to ovoid structureless sand-size 

 pellets. Phosphatized bone fragments, fecal pellets, 

 and fish and sharks' teeth are frequently found in 

 phosphorites, and glauconite is a common accessory 

 mineral. Any pelletal rock, particularly one that 

 contains bone or teeth fragments, is worthy of 

 further examination. Higher grade phosphorites 

 may have a bluish-gray coating (phosphate bloom) 

 on weathered surfaces, and high-grade phosphorite 

 may also be distinguished by a fairly high specific 

 gravity. 



Phosphorite deposits are found where the rate of 

 deposition of other clastic material is at a minimum. 

 The phosphorite sequence, therefore, tends to be 

 thin when compared with sedimentary sequences 

 deposited elsewhere at the same time. 



In a given basin of deposition, the phosphorite 

 tends to be concentrated at one stratigraphic posi- 

 tion, although the phosphorite facies may transgress 

 time. 



Phosphorite deposits may be structurally con- 

 trolled. For example, phosphorite of the platform 

 facies is deposited in structural basins on the flanks 

 of domes or anticlines that were rising at the time 

 of deposition. Structural controls, other than the 

 broad geosynclinal basin, may determine the site of 

 deposition in the deeper parts of the basin. 



All marine phosphorites contain uranium in 

 amounts that are high enough so that these rocks 

 can be identified by gamma-ray well logging and 

 even by aerial radiometric reconnaissance. Anoma- 

 lous amounts of uranium are present in acid streams 

 that drain phosphatic terranes, and in covered areas, 

 such as the coastal plains of the United States, the 

 presence of uranium in stream waters may be a 

 clue to the presence of a phosphate deposit. 



Large deposits of igneous apatite are related to 

 alkalic rocks, particularly to carbonatites, and these 

 rocks are related to the large rift zones. Many car- 

 bonatite deposits are now known; several contain 

 large apatite deposits. All carbonatites and areas 

 where they may be present should be examined for 

 phosphate. 



The only guano deposits that remain to be found 

 are probably very small bat guano deposits in caves. 

 Guano-related deposits, particularly those on arid 

 coasts where there is modern dynamic upwelling of 

 water, may remain to be found, and the possibilities 

 should not be overlooked. 



PROBLEMS FOR RESEARCH 



Research is needed on a variety of problems that 

 can be classified as recovery of byproducts, waste 

 disposal, mining and processing, and geological. 



Minor elements that might be recovered as by- 

 products include fluorine, vanadium, uranium, scan- 

 dium, and the rare earths. Marine phosphorites, 

 because of their vast tonnages, constitute signifi- 

 cant resources of these elements. Pure apatite con- 

 tains from 3.5 to 4 percent F; phosphorites prob- 

 ably average about 3 percent. All marine phosphor- 

 ites contain uranium, probably substituting for 

 calcium in the apatite lattice, in amounts ranging 

 from 0.005 to as much as 0.05 percent; the average 

 is about 0.01 percent. The rare earths (lanthanides 



