202 



UNITED STATES MINERAL RESOURCES 



Examples representing virtually all possible dis- 

 tributions of evaporite facies are found in evaporite 

 basins of the United States. Potassium minerals 

 typically occupy only a small part of a basin and are 

 relatively rare; only 7 of the 69 marine evaporite 

 deposits listed in table 42 are known to contain 

 potassium minerals. Halite facies are found in 28 of 

 the 69 deposits listed in that table. Gypsum or anhy- 

 drite facies are found in all. 



NONMARINE EVAPORITES 



Nonmarine evaporite deposits include near-surface 

 saline minerals (and associated brines) in Quater- 

 nary dry lakes, and bedded crystalline deposits in 

 continental Tertiary formations. Pre-Tertiary de- 

 posits are known, but are rare and much less im- 

 portant. All are geologically similar in that they 

 formed by evaporation of lakes in closed basins. The 

 geologic requirements for such deposits are: (1) 

 development of a closed basin that prevents the com- 

 ponents dissolved in the waters from escaping to the 

 sea, and (2) an original mixture of dissolved com- 

 ponents that evolves by means of successive crystal- 

 lizations into a concentrated brine that precipitates 

 valuable salts. 



Basins having no external drainage are mostly 

 products of faulting or warping. To persist, they 

 require a tectonic setting that will either lower the 

 floor of the basin faster than sedimentation can raise 

 it or elevate barriers across possible outlets faster 

 than erosion can destroy them. Most, but not all, 

 closed basins today are in arid or semiarid areas 

 where sedimentation rates are low. Comparable 

 basins in the geologic past probably had a similar 

 climatic setting. 



For valuable evaporite deposits to form in a closed 

 basin, however, enough water must have flowed into 

 the basin to introduce large quantities of salts. This 

 requirement greatly limits the number of highly 

 favorable environments because arid basins generally 

 receive large volumes of water only where they ad- 

 join high mountain ranges or are the terminus of 

 integrated drainages from adjacent less arid regions. 

 Even when these conditions are satisfied, most 

 waters will not evolve through natural evaporation 

 into a lake containing a valuable mix of components. 

 The most favorable source waters for a sodium car- 

 bonate deposit, for example, come from springs or 

 from rivers draining volcanic terrane, although met- 

 amorphic and some other igneous terranes are suit- 

 able. Apparently a sodium carbonate lake cannot be 

 formed by evaporation of calcium bicarbonate wa- 

 ters from limestone terrane, of sulfate waters from 



sulfide deposits, or of chloride waters from marine 

 evaporites (Eugster, 1971, p. B46). 



The minerals deposited depend upon the composi- 

 tion and temperature of the brine at the time of 

 crystallization. Normally, saline minerals are formed 

 in a sequence that is determined by their solubility 

 in the particular chemical and physical environment 

 provided by the lake. Studies of sodium carbonate 

 deposits, for example, find that these minerals gen- 

 erally crystallize after the calcium-bearing minerals 

 and before the sodium sulfate and chloride minerals, 

 although the sequence varies. The species of carbon- 

 ate mineral formed depends on the partial pressure 

 of dissolved CO2 in the brine. If the brine is in equi- 

 librium with the partial pressure of CO2 in the at- 

 mosphere, trona (NaaCOg-NaHCOa -21120) is formed; 

 if the partial pressure is higher because of organic 

 activity, nahcolite (NaHCO;,) is formed; if lower, 

 natron (NaaCOa-NaHjO) is formed (Bradley and 

 Eugster, 1969). Because of diagenesis after burial, 

 the carbonate mineral constituting the final deposit 

 tends to reflect the CO2 pressures in the accumulat- 

 ing bed of minerals rather than in the open lake 

 waters. 



BRINES 



Many of the brines that are exploited commercially 

 contain abnormally large amounts of bromine, iodine, 

 calcium chloride, and magnesium. The geologic 

 processes that result in such brines have long been 

 debated and are not well understood today. Probably 

 several processes were operative inasmuch as the 

 brines come from diverse geologic settings ; some of 

 the brines are pore waters in oil fields, some are 

 deep-well brines from marine salines or rocks per- 

 ipheral to them, some are from thermal springs or 

 wells, and some are surface or subsurface brines that 

 were associated with Quaternary salt lakes or seas. 

 The geologically old and deep-well brines were proba- 

 bly produced chiefly during diagenesis by the inter- 

 action of pore fluids with clays and possibly with 

 other components of clastic rocks that act as ionic 

 filters; some of the fluids may have originally been 

 sea water of normal salinity although most were 

 probably bitterns from marine evaporites (White, 

 1957, 1965; White and others, 1963; Rittenhouse, 

 1967; ColHns and others, 1967). Quaternary brines 

 that contain abnormally large amounts of bromine 

 and other substances are products of natural evapo- 

 ration of dilute river and spring waters that mostly 

 originally had unusually high ratios of these com- 

 ponents (Bentor, 1961 ; Livingstone, 1963 ; Jones, 

 1966; Hardie and Eugster, 1970). 



