612 KAPLAN AND RITTENBERG [CHAP. 23 



carbonate content is low, and, from the nature of the deposits, in areas that are 

 tectonically relatively stable allowing for a slow accumulation. The source can 

 sometimes be traced to overlying volcanic ash beds that have been leached 

 (Becraft, 1958). 



Because of the recent interest in nuclear energy, a considcraljle literature 

 exists on ancient marine basin sediments enriched in syngenetic uranium. No 

 reports have yet been published on the occurrence of uranium in recent neritic 

 basin sediments. As in the case of the metal sulfides, it is recommended that 

 lagoons or other reducing environments adjacent to volcanic areas or to 

 localities particularly rich in granitic rocks, that can act as sources, be studied 

 for enrichment of uranium as well as vanadium and the other trace metals 

 mentioned above. 



D. Phosphates 



Phosphate deposits may be classified into six geological types: apatite 

 deposits of igneous origin, residual deposits, phosphatized rocks, river-pebble 

 deposits, guano and marine phosphorites (Curtis, 1957). There are two types of 

 marine phosphates present in the geological column: (a) the phosphatic nodules 

 and (b) the stratified phosphates associated with shales or limestones; both of 

 these are authigenic. The former type is present along the continental shelves 

 of many continents, and is especially high in regions where upwelling is domi- 

 nant and sedimentation of detrital material low, e.g. the coasts of Peru and 

 Chile, the southwest coast of Africa and the coast of North America (Dietz, 

 Emery and Shepard, 1942). The second type has not yet been described in 

 recent marine sediments. 



The phosphorites associated with shales are widespread, often cover large 

 areal distances and tend to accumulate uranium. The Phosphoria Formation of 

 the Permian of Western United States of America is perhaps the best known 

 deposit of this type. It is thought that deposition occurred in the miogeosyncline 

 facies of the orthogeosyncline. The phosphate is often enriched in clastic or 

 limestone members of a dominantly shale formation. For example, the Ten- 

 nessee blue-rock phosphate occurs in the Hardin Sandstone member of the 

 Chattanooga Shale. 



Many authors have suggested origins for the phosi)horites present in shales 

 (Mansfield, 1931; McKelvey et al., 1953); these explanations are based largely 

 on field observations without giving much attention to the conditions prevailing 

 in present-day environments. Emery (1960) has attempted to explain phos- 

 phatic nodule formation on continental shelves as the result of precipitation 

 from supersaturated solution probably in the form of colloids. He concludes 

 that upwelling bottom waters rich in phos])hate may undergo changes in 

 environmental conditions which cause the tricalcium phosphate to exceed its 

 solubility product. 



We wish to draw attention to a further possibility which appears to have been 

 generally overlooked and could ])rofitably be investigated. The close association 

 of ancient marine phosphates with black shales indicates a reducing environ- 



