466 



UNITED STATES MINERAL RESOURCES 



Although that is a low concentration, the total 

 amount of uranium in sea water is very large (Euro- 

 pean Nuclear Energy Agency, 1965). It seems rea- 

 sonable to suppose that a small amount of byproduct 

 uranium will be recovered in the near future from 

 desahnation plants or from plants recovering mag- 

 nesium from sea water. 



SPECUL-^TIVE RESOURCES 



Domestic speculative resources consist of (1) new 

 districts containing types of uranium deposits found 

 elsewhere in the United States, (2) types of deposits 

 known only elsewhere in the world, and (3) new 

 types of deposits. 



The margins and locally the axial parts of sedi- 

 mentary basins, particularly those containing sedi- 

 ments derived from either granitic terranes or vol- 

 canic ash, are favorable targets in the search for 

 new uranium districts. These basins may be either 

 intermontane such as in Wyoming, or of the gulf 

 type as exemplified by the coastal plain in Texas 

 where shoreline-facies rocks contain major deposits. 

 Although both of these basins are Tertiary in age, 

 analogs in older rocks, especially Cretaceous, ought 

 to be equally favorable. 



Unconformities related to paleokarst topography 

 may be favorable sites for occurrence of large urani- 

 um deposits. Two types of deposits are known in this 

 setting. A large deposit of Cretaceous uranium- 

 bearing phosphatic rock reportedly fills paleokarst 

 developed in Precambrian dolomite in Bakouma, 

 Central African Republic, and small deposits of 

 tyuyamunite-bearing breccia occur in cavities re- 

 lated to karst developed on the Madison Limestone 

 in the Pryor Mountains, Mont. Exploration here and 

 elsewhere in this environment may be fruitful. 



Ancient, 2.0-2.4 b.y. (billion years) old, Precam- 

 brian quartz-pebble conglomerates analogous to 

 those that contain the Witwatersrand and Blind 

 River-Elliot Lake uranium deposits do not crop out 

 in the United States. This type of rock might be 

 present, however, somewhere in the deep subsur- 

 face of North and South Dakota and the east half of 

 Montana, where, according to Goldich, Lidiak, 

 Hedge, and Walthall (1966), the ancient continental 

 crust was formed more than 2.5 b.y. ago. 



Theoretical considerations and the description of 

 the Rossing deposit in South West Africa suggest 

 that there may be "porphyry" uranium deposits 

 similar to porphyry copper and molybdenum de- 

 posits. Porphyry uranium deposits would be late 

 magmatic differentiates — for example, alaskites and 

 associated pegmatites — and would be easiest to 

 recognize in arid or semiarid regions. 



PROSPECTING TECHNIQUES 



Radiation counters are the commonest tools used 

 in prospecting for uranium. They are of two main 

 types: (1) Geiger counters, which are normally 

 portable and are designed to count y radiation alone, 

 and 13 plus y radiation, and (2) scintillometers, 

 which count only y radiation and which may be port- 

 able, carborne, or airborne. Scintillometers are more 

 sensitive and respond faster than Geiger counters. 

 If uranium only is present and is in equilibrium with 

 all the daughter products of its radioactive decay, 

 both types of instruments may be calibrated to indi- 

 cate uranium content. However, if the uranium is 

 not in equilibrium, they may indicate either more or 

 less uranium than is present. In places where urani- 

 um has been leached but its daughter products re- 

 main, it is possible to detect radiation where there is 

 little or no uranium. Conversely, recently deposited 

 uranium that is too young (less than 500,000 years 

 old) to be in radioactive equilibrium, emits so little 

 /3 and y radiation that a deposit may contain more 

 uranium than is indicated by its p and y count. 

 Standard Geiger counters and scintillometers cannot 

 distinguish uranium from thorium, or from natural- 

 ly occurring radioactive potassium-40. 



Radiation counters are also used in drill-hole ex- 

 ploration. Only y radiation can be detected in drill 

 holes because p particles lack the energy to pass 

 through the water which is usually in the hole or 

 through the walls of the detecting instrument. Scin- 

 tillometers are used to probe drill holes because of 

 their greater sensitivity and faster response. Resis- 

 tivity and spontaneous potential surveys of drill 

 holes are also customarily run. These two geophysi- 

 cal methods do not indicate uranium directly, but 

 can be interpreted to distinguish sandstone from 

 shale and to indicate relative porosity. The identifica- 

 tion of shale at the site of a small y anomaly strong- 

 ly suggests that the anomaly is caused by potassium- 

 40 in the shale. 



Airborne radiometric surveys using the total count 

 method have proved useful in prospecting for urani- 

 um. In this method the scintillometer records total y 

 radiation and does not discriminate between urani- 

 um, thorium, and potassium-40 sources. Total count 

 anomalies have to be ground checked to determine if 

 uranium causes the anomaly. Airborne gamma-ray 

 spectrometry, which has been developed in recent 

 years, distinguishes between uranium, thorium, and 

 potassium-40 by measuring y radiations at particu- 

 lar energy levels. Thus, the strong y-emitting 

 daughters of U"^ and Th"% Bi"^ and TP°S respec- 

 tively, are used to distinguish uranium and thorium ; 



