SECT. 3] BASIN SEDIMENTATION AND DIAGENESIS 611 



C. Uranium and Transition Metals 



Uranium, cobalt, molybdenum, nickel and vanadium are known to be en- 

 riched in bituminous marine shales, and vanadium and nickel are present in 

 some crude oils. There is no doubt that these trace metals are authigenic; the 

 mechanism of their deposition is, however, still ambiguous. Three processes can 

 be suggested, but their relative importance must await further studies. 



The first is the active concentration of the trace metals by living organisms. 

 It is well know^n that the transition metals are important constituents of a 

 number of biological substances. They are able to form chelate compounds 

 which perform the important function of solubilizing particulate matter. 

 Goldberg (1957) reviewed the literature and found ample evidence for their 

 selective concentration by plankton, sessile algae, sponges and other organisms. 

 Vanadium appears to be concentrated the most, by a factor of > 280,000 over 

 its concentration in sea-water. 



Scavenging by clays, colloids and settling organic matter is another process 

 that has been suggested (Krauskopf, 1955) for the concentration of trace metals. 

 Krauskopf found that hydroxides of manganese and iron were the most effici- 

 ent in the adsorj^tion of ions from sea-water. 



The third method, and according to Krauskopf (1955) the least important, 

 is the chemical precipitation of oxides, carbonates or sulfides. In areas of the 

 ocean where upwelling results in high productivity and later in stagnation in 

 bottom waters, or in restricted stagnant basins discussed in this chapter, the 

 Eh may drop sufficiently to transform the soluble complex ions into the 

 lower valence insoluble oxides. The theoretical calculations by Garrels (1955) 

 show that the reducing conditions produced by sulfate reduction create an 

 environment in which the oxides of vanadium III and IV, uranium IV and 

 sulfides of iron, copper, zinc and lead are stable. The isotoiDic studies by Jensen 

 (1958) on the metal sulfides associated with the uranium deposits of the Colo- 

 rado Plateau (freshwater deposits) give extra weight to the hypothesis that 

 bacterial sulfate reduction may be directly involved in the deposition of 

 uraninite and montroseite. 



It is well known that primary uranium occurs in ancient marine sediments, 

 principally those rich in phosphate and poor in lime, and the carbonaceous or 

 black bituminous shales. The most favorable environments for its deposition 

 seem to have been areas near margins of platforms during the Paleozoic. It 

 appears that the uranium may substitute for calcium in the crystal lattice of 

 the carbonate-fluorapatite, which is the predominant phosphate mineral in 

 phosphatic sediments. In the organic shales there appears to be a direct rela- 

 tionship with the organic matter (McKelvey and Nelson, 1950). Thin layers 

 particularly rich in organic matter can attain uranium concentrations of 0.7%. 

 In general, the uranium content of these shales varies from a few thousandths to 

 0.02% (Klepper and Wyant, 1957). The shale beds are often several feet thick 

 and cover areas of tens or hundreds of thousands of square miles. The uranium 

 is preferentially concentrated in the fine-grained sediments, especially where the 

 21— s. Ill 



