692 



UNITED STATES MINERAL RESOURCES 



lipsite in alkaline, saline soil profiles at Olduvai 

 Gorge, Tanzania. Surface concentrations of analcime 

 were also reported in reddish, root-marked claystones 

 on the Luboi Plain, Kenya (Hay, 1970) . 



The most voluminous and potentially valuable zeo- 

 lite deposits belong to the open- and closed-system 

 types. The terms, "open system" and "closed sys- 

 tem," are used in a hydrologic sense rather than in 

 a thermodynamic sense. Deposits of the open-system 

 type formed by the reaction of volcanic glass w^ith 

 subsurface vi^ater that originated as meteoric water. 

 The original volcanic material commonly was de- 

 posited in marine or fluviatile environments or was 

 air laid on the land surface. Deposits of the closed- 

 system type formed by the reaction of volcanic glass 

 with the connate water trapped during sedimenta- 

 tion in a saline, alkaline lake. 



Deposits of the open-system type commonly 

 formed in thick tuffaceous strata and show a ver- 

 tical zonation of authigenic silicate minerals. Hay 

 (1963b) proposed hydrolysis and solution of silicic 

 glass by subsurface water to account for the forma- 

 tion of clinoptilolite in tuff and tuffaceous claystone 

 in the lower part of the Tertiary John Day Forma- 

 tion in central Oregon. The upper part of the forma- 

 tion contains unaltered glass or montmorillonite. 

 The authigenic mineral zonation is more complex 

 in the Tertiary tuffs at the Nevada Test Site (Hoo- 

 ver, 1968). An upper zone consists of unaltered 

 glass with local concentrations of chabazite and clay 

 minerals. Zeolitic tuff continues downward for as 

 much as 6,000 feet and is characterized by a down- 

 ward succession of zones rich in clinoptilolite, mor- 

 denite, and then analcime. The zeolite zones of the 

 open-system type commonly cut across stratigraphic 

 boundaries. 



Zeolite deposits of the closed-system type formed 

 during diagenesis in alkaline, saline lakes, com- 

 monly of the sodium carbonate-bicarbonate variety. 

 Brine of this composition generally has a pH greater 

 than 9, which probably accounts for the relatively 

 rapid solution of vitric material and precipitation of 

 zeolites. The authigenic silicate mineralogy can be 

 correlated with the salinity in deposits of the closed- 

 system type. The Pleistocene deposits of Lake Te- 

 copa, Calif. (Sheppard and Gude, 1968), are char- 

 acteristic of the closed-system type. Vitric material 

 is unaltered or partly altered to clay minerals in 

 tuff deposited in fresh water near the lake shore and 

 inlets; however, the tuffs consist of zeolites where 

 deposited in moderately saline water and of potas- 

 sium feldspar where deposited in the highly saline 

 and alkaline water of the central part of the basin. 

 Thus, individual tuffs show a lateral zonation in a 



basinward direction of unaltered glass to zeolites 

 and then to potassium feldspar. Zeolitic tuffs at 

 Lake Tecopa consist chiefly of phillipsite, clinop- 

 tilolite, and erionite. Chabazite is a minor consti- 

 tuent of tuffs at Lake Tecopa, but it is locally the 

 major constituent in zeolitic tuffs of other saline 

 lacustrine deposits. In some deposits of the closed- 

 system type, such as the Miocene Barstow Forma- 

 tion of California (Sheppard and Gude, 1969) and 

 the Eocene Green River Formation of Wyoming 

 (Surdam and Parker, 1972) a zone of analcime sepa- 

 rates the other zeolites from the zone of potassium 

 feldspar. In addition to clay minerals, zeolites, and 

 potassium feldspar, deposits of the closed-system 

 type locally contain opal or chalcedony, searlesite 

 (NaBSioOe'H.O), fluorite (CaF,), or dawsonite 

 (NaAl(C03)(0H),) of authigenic origin. 



PROSPECTING TECHNIQUES 



Prospecting for bedded zeolite deposits is difficult 

 because the zeolites are finely crystalline and resem- 

 ble bedded diatomite, feldspar, or bentonite in the 

 field. Zeolitic tuffs generally have an earthy luster 

 and are resistant. Although some zeolitic tuffs are 

 pastel shades of yellow, brown, red, or green, many 

 are white or pale gray. If the zeolitic tuff is nearly 

 monomineralic, certain gross physical properties of 

 the rock may aid field recognition (Sheppard and 

 Gude, 1969, p. 17-18). Utilizing the ion-exchange 

 and molecular-sieve properties of zeolites, Helfferich 

 (1964) designed a "field" test for the recognition 

 of zeolites. Helfferich's test distinguishes zeolites 

 from clay minerals, feldspars, and volcanic glass, 

 but the test will not identify the zeolite species. My 

 experience with Helfferich's test suggests that it is 

 better suited to the laboratory than to the field. 



X-ray powder diffraction analysis of bulk samples 

 is the technique generally used for identification of 

 the zeolites in sedimentary rocks. This method also 

 permits a semiquantitative estimate of the abund- 

 ance of zeolites and associated minerals in the sam- 

 ples. Tuflfaceous strata are sampled, and then the 

 samples are brought to the laboratory for examina- 

 tion by X-ray diffraction. Fresh tuff is generally 

 distinguishable from altered tuff in the field, so only 

 the altered parts of the tuffaceous rocks are sampled 

 in both vertical and lateral directions. Once zeolites 

 have been identified by X-ray diffraction, an addi- 

 tional detailed sampling is necessary to ascertain 

 the distribution and abundance of the zeolites and 

 associated authigenic minerals. Inasmuch as the 

 samples must be returned to the laboratory for X-ray 

 study, the availability of a truck-mounted X-ray 

 diffractometer unit, suitable for field use, would 



