212 



UNITED STATES MINERAL RESOURCES 



tends to support the inference that most of the 

 earth's evaporite and potash deposits have already 

 been discovered. No doubt some deposits remain un- 

 known, but the above calculation strongly implies 

 that the number is not large. 



In the United States, all marine evaporite de- 

 posits represent possible sources of potassium com- 

 pounds, salt, and gypsum. Figure 24 shows that 

 about 35 percent of the 3 million square miles that 

 make up the 48 adjoining States is underlain by 

 evaporites, and calculations like those used for 

 estimating world resources indicate that about 8 

 billion tons of K2O equivalent could be contained in 

 these deposits. This amount is about 20 times 

 greater than estimates of known U.S. resources and 

 suggests that several large undiscovered deposits 

 may exist. 



In summary, it is not likely in the foreseeable 

 future that potassium compounds, salt, or gypsum 

 will become the sole target of expensive explora- 

 tion programs because of dwindling world supplies. 

 Enormous quantities are now known although their 

 global distribution is uneven. However, there is 

 strong incentive to keep costs of these substances 

 low because their costs influence the price of a very 

 large number of other products. Furthermore, stra- 

 tegic reasons, balance of payment problems, differ- 

 ences in quality of the product from different 

 sources, and potential profits stemming from attrac- 

 tively priced transportation to large or growing 

 markets remain as strong incentives for exploration. 

 New and exploitable resources may well be discov- 

 ered as a result of these stimuli. 



NONMARINE EVAPORITES 



Ultimate resources of commodities derived from 

 continental evaporites — deposits formed by the 

 evaporation of inland bodies of water — are less 

 easily evaluated than are resources of marine 

 evaporites. Requirements for an environment favor- 

 able for the deposition of continental evaporites are 

 a semiarid to arid climate that will produce a high 

 evaporation rate, a closed basin having internal 

 drainage, and a source of large quantities of water 

 having the proper ratio of dissolved components. 

 Evaporation rates high enough to keep closed basins 

 from filling and overflowing exist today in areas 

 that mostly lie between lats 45° N. and 45° S. ; the 

 sizes and locations of basins within such areas, 

 however, are variable. Closed basins receive large 

 inflow where they adjoin high mountains or upland 

 areas ; the high mountains or uplands extract most 

 of the moisture from passing airmasses, and the 

 closed basins downwind from the mountains are 



arid because they have been deprived of this mois- 

 ture. Extensive faulting and warping are the geo- 

 logical processes required to create both the basins 

 and the mountains. 



Areas known to contain large volumes of non- 

 marine sedimentary rocks that were deposited in 

 closed basins between the 45° parallels represent a 

 subordinate part of the world's surface. Most known 

 continental evaporite deposits are Cenozoic, and 

 some evaporite assemblages are favored by drain- 

 age from a volcanic rock environment whereas 

 others are favored by drainage from other types of 

 terrane; these factors provide further guidance to 

 areas that should be considered during exploration 

 for deposits and in estimating undiscovered re- 

 sources. 



In the United States, virtually all areas that have 

 had a long history of internal drainage are west of 

 the Rocky Mountains, although several basins hav- 

 ing more brief histories of interior drainage lie in 

 the Great Plains. In the area between the Wasatch 

 Range and the Sierra Nevada, more than 100 closed 

 basins are large enough to have contained lakes 

 during the Pleistocene, and probably 5-10 times 

 this number of closed basins existed during one part 

 or another of Cenozoic time. Between 500 and 1,000 

 such basins may therefore have formed in that area, 

 and the sediments deposited in perhaps half are now 

 preserved. Of these basins, less than 10 percent have 

 been adequately prospected. If remaining basins 

 were tested and a 1-percent success ' resulted, two 

 to five basins would be found to contain evaporite 

 deposits. Although such deposits might be profitable, 

 the degree of increase to resources would depend on 

 the minerals they contain ; salt, potash, sodium car- 

 bonate, or magnesium deposits would probably not 

 greatly change the resource outlook, but deposits 

 containing other possible components might. Dis- 

 covery of new deposits in favorable areas in other 

 parts of the world is even more likely because these 

 parts have been less explored ; western China and 

 adjacent parts of Afghanistan and the U.S.S.R., 

 Iran, Turkey, western Central and South America, 

 and eastern Africa all have probably had Cenozoic 

 geologic histories similar to the Western United 

 States. If those areas together contain 10 times as 

 many basins as the Western United States, it seems 

 likely that 20-50 major deposits of continental 



' A basis used by some mining companies for budgeting mineral ex- 

 ploration programs in geologically favorable areas is to spend up to 

 1 percent of the value of the anticipated ore body. This figure is com- 

 monly justified by citing past experience, which shows that there is about 

 a 10 percent chance of success in finding an ore body that would allow 

 at least a 10 percent profit. The 1 percent chance of success used in the 

 present calculation results from an estimate that only 10 percent of the 

 basins in this area are geologically favorab'e, and that 10 percent of 

 them — 1 percent of the total number — would contain a deposit. 



