214 



UNITED STATES MINERAL RESOURCES 



Almost every subdivision of earth sciences has 

 contributed techniques useful in prospecting for 

 evaporites. It is, in fact, this variety in disciplines 

 that makes it diiUcult for exploration groups to 

 utilize known techniques effectively because so few 

 groups include members that have facility in all 

 fields. All subdivisions of the sciences also have 

 great promise of developing new techniques. New 

 geochemical methods, for example, may well en- 

 hance our ability to reconstruct salinity gradients 

 within evaporite-bearing basins. Variations in the 

 bromine content in halite of marine origin provide 

 an established technique, but far less work has 

 been done on the distribution of other elements that 

 reflect salinity as they change concentration in 

 minerals associated with evaporite bodies. Such 

 changes might be indicated by variations in the con- 

 centration of strontium as it proxies for calcium in 

 minerals such as limestone, dolomite, gypsum, and 

 anhydrite; by shifts in the isotopic ratios of both 

 hydrogen and oxygen in the water of liquid inclu- 

 sions which may reflect changes in evaporation- 

 induced salinity of crystallizing brines ; or by varia- 

 tions in the O'"* in the sulfate ion of gypsum and 

 anhydrite which may change during evaporation of 

 the parent brine yet be less affected by diagenesis 

 than most other ions or radical components. 



Also needed are more complete studies of evapo- 

 rite deposits in the process of being formed. Few 

 marine deposits are forming today, and none has 

 been examined in all ways that are desirable. More 

 nonmarine deposits than marine are being formed 

 today, and immediate studies of many are needed 

 because these evaporites may not repeat their depo- 

 sitional cycle. 



Perhaps the least understood geologic phenomena 

 related to evaporites and brines are the roles and 

 mechanisms of diagenesis. Almost immediately after 

 deposition, some changes in mineral composition 

 and form occur that stem from changes in the sur- 

 rounding chemical and physical conditions relative 

 to those extant at the site of original crystallization. 

 Other changes occur later — for example, as deeper 

 burial increases load pressures, as tectonic and other 

 forces alter the hydraulic regimes that control the 

 migration of interstitial brines, as time allows meta- 

 stable assemblages of minerals to convert to ther- 

 modynamically stable assemblages, and as clay, 

 organic, and other substances that act as ionic filters 

 change the compositions of brines passing through 

 them. Field studies of these problems have been 

 made, but progress is difficult because so much of 

 the evidence has been destroyed or is not available 

 from existing drill cores. Laboratory studies are 



difficult because so many of the reactions are slow. 

 Future research should be more fruitful because the 

 problems are becoming better defined and new tech- 

 niques are becoming available, but much remains 

 to be done. 



REFERENCES CITED 



Ashizawa, R. V., 1971, Gypsum, in U.S. Bur. Mines Minerals 

 Yearbook 1969 : v. 1-2, p. 547-553. 



Babcock, C. O., 1964, Calcium and calcium compounds, in 

 U.S. Bur. Mines Minerals Yearbook 1963: v. 1, p. 341- 

 346. 



Barnard, R. M., and Kistler, R. B., 1966, Stratigraphic and 

 structural evolution of the Kramer sodium borate ore 

 body, Boron, California, in Rau, J. L., ed., 2d Symposium 

 on salt: Cleveland, Northern Ohio Geol. Soc, Inc., p. 

 133-150. 



Bentor, Y. K., 1961, Some geochemical aspects of the Dead 

 Sea and the question of its age: Geochim. et Cosmochim. 

 Acta, V. 25, no. 4, p. 239-260. 



Bixler, G. H., and Sawyer, D. L., 1957, Boron chemicals 

 from Searles Lake brines: Indus, and Eng. Chemistry, 

 V. 49, p. 322-333. 



Bradley, W. H., and Eugster, H. P., 1969, Geochemistry and 

 paleolimnology of the trona deposits and associated 

 authigenic minerals of the Green River Formation of 

 Wyoming: U.S. Geol. Survey Prof. Paper 496-B, 71 p. 



Braitsch, Otto, 1971, Salt deposits, their origin and com- 

 position : New York, Heidelberg, and Berlin, Springer- 

 Verlag, 297 p. 



British Sulphur Corporation, 1966, World survey of potash: 

 Wrexham, Wales, Cambrian Press Ltd., 93 p. 



1971, Statistical supplement, raw materials supply/ 



demand, 1970 — fertilizer supply/demand 1970/71: Lon- 

 don, Stat. Suppl. no. 4, p. 14-15. 



Buzzalini, A. D., Adler, F. J., and Jodry, R. L., spec, eds., 

 1969, Evaporites and petroleum: Am. Assoc. Petroleum 

 Geologists Bull., v. 53, no. 4, p. 775-1032. 



Collins, A. G., Zelinski, W. P., and Pearson, C. A., 1967, 

 Bromide and iodide in oilfield brines in some Tertiary 

 and Cretaceous formations in Mississippi and Alabama: 

 U.S. Bur. Mines Rept. Inv. 6959, 27 p. 



Colorado School of Mines, 1967, Potash: Mineral Indus. 

 Bull., v. 10, no. 3, 18 p. 



Comstock, H. B., 1963, Magnesium and magnesium com- 

 pounds: A materials survey: U.S. Bur. Mines Inf. Giro. 

 8201, 128 p. 



Culbertson, W. C, 1971, Stratigraphy of the trona deposits 

 in the Green River Formation, southwest Wyoming, in 

 Contributions to Geology, v. 10, no. 1, Trona Issue: 

 Laramie, Wyo., Wyoming Univ., p. 15-23. 



Durrell, Cordell, 1953, Geological investigations of strontium 

 deposits in southern California: California Div. Mines 

 Spec. Rept. 32, 48 p. 



Dyni, J. R., Hite, R. J., and Raup, 0. B., 1970, Lacustrine 

 deposits of bromine-bearing halite, Green River Forma- 

 tion, northwestern Colorado, in Rau, J. L., and Dellwig, 

 J. L., eds., 3d Symposium on salt: Cleveland, Northern 

 Ohio Geol. Soc, Inc., p. 166-180. 



Eilertsen, D. E., 1971, Potash, in U.S. Bur. Mines Minerals 

 Yearbook, 1969: v. 1-2, p. 933-946. 



Eugster, H. P., 1970, Chemistry and origin of the brines of 



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