Kansas Geological Survey, Bulletin 211, Part 4, p. 1-3
Fourteen samples of unaltered Pliocene and Pleistocene volcanic ashes contain an average of 7.1 ppm uranium and 33.2 ppm thorium; four devitrified ashes contain an average of 5.2 ppm uranium and 21.6 ppm thorium.
Dissolution and/or leaching of uranium from the abundant ash beds may account for the high regional background values of uranium (5-20 ppb) found in groundwaters of western Kansas. The rhyolitic ashes are also the likely source of silica found in the widespread uraniferous (10-200 ppm) silcrete deposits in the lower Ogallala Formation.
Fluvial sandstones with organic reductants that could have been in hydrologic contact with uraniferous waters include the Lower Cretaceous Cheyenne, Kiowa, and Dakota Formations of north-central and southwestern Kansas.
The close association of tuffaceous rock units with many sedimentary uranium deposits has led several investigators [e.g. 1,2] to postulate leaching of these types of rocks as a likely source of uranium which can be mobilized by meteoric or groundwaters, and subsequently fixed by interaction with organic reductants. Although there is only limited data available in the literature to support the leaching concepts [3,4], experimental work in progress  indicates substantial percentages of volcanic glass may go totally into solution, or that selective leaching of uranium may take place, depending on the degree of silica saturation of the leach solutions.
Lenticular deposits of volcanic ash in the Miocene, Pliocene, and Pleistocene in the fluvial and alluvial sediments of the central Great Plains have been described as relatively unaltered vitric tuffs with rhyolitic chemical compositions; attain maximum thicknesses of 30 feet; and constitute an estimated 3% of the volume of the sediments in portions of the Ogallala Formation [6,7,8]. Dissolution and/or leaching of the Great Plains ashes could have easily mobilized several millions of pounds of uranium which might be available for the formation of uranium deposits, given suitable mechanisms of fixation. That this has taken place is indirectly supported by (1) the anomalously high regional background values of uranium (5-20 ppb) found in groundwaters of western Kansas [9,10], (2) the widespread occurrences of uraniferous (10-200 ppm) silcrete deposits found in close proximity to the abundant ash beds in the lower portion of the Ogallala Formation [11,12,13], and (3) the occurrences of anomalous concentrations (10-15 ppm U) found in limonitic and hematitic sandstones of the Lower Cretaceous .
The purpose of this report is to present the results of preliminary investigations of the distribution of uranium and thorium in volcanic ashes in Kansas and to discuss the possible implications for uranium exploration in the central Great Plains.
The uranium and thorium contents of eighteen Pliocene and Pleistocene volcanic ashes were determined by x-ray spectrometry [14,15]; the results of these analyses are presented in Table 1. The uranium values range from 3.9 to 9.1 ppm U and average 6.6 ppm; the thorium values range from 17.8 to 39.6 ppm Th and average 30.6 ppm. The Th/U ratios range from 3.8 to 5.3 and average 4.6. The estimated standard errors at the 95% confidence level for these determinations are 1.2 ppm U and 1.6 ppm Th.
Table 1--Uranium and Thorium Contents of Volcanic Ashes.
|Sample||Ash Bed||U ppm||Th ppm||Th/U||Devitrification
|Pliocene--Ogallala Fm.--Ash Hollow Mbr.|
|Pliocene--Ogallala Fm.--Valentine Mbr.|
Most of the ash samples analyzed can be classified as virtually unaltered glasses, and have the petrographic and optical properties typical of most ashes previously described and illustrated [e.g. 8]. X-ray diffraction examinations of these samples reveal a few in moderate to advanced stages of devitrification (samples 5, 8, 13, & 17), although none have been mineralogically altered to expanding clays. A semiquantative devitrification index, based on the ratio of the crystalline diffraction responses of quartz and feldspars to the amorphous silica glass background, is also presented in Table 1. The four devitrified ashes contain an average of 5.2 ppm uranium and 21.6 ppm thorium; the fourteen fresh ashes contain an average of 7.1 ppm U and 33.2 ppm Th.
The devitrified ash samples appear to have significantly lower uranium and thorium contents; whether this is due to devitrification effects or differences in source composition cannot be resolved with the present data. Because of presumed redeposition by fluvial systems, the detailed stratigraphic correlations of most Great Plains ashes are tenuous, unless they have been correlated by absolute geochronological dating. Fission-track dating  indicates the Pearlette ash beds actually consist of four significantly different ages of ash, and preliminary age data on the late Tertiary ashes suggest alternative interpretations of the stratigraphic assignments. Although some of the ash samples are thought to be the same ash beds, interpretation based on the possible loss of uranium and thorium with devitrification cannot be conclusive until absolute age dating techniques are utilized to define co-existing ash sample suites.
It should be noted, however, that studies of uranium in suites of co-existing obsidian-perlite-felsite  indicate no loss of uranium in the hydration of obsidian to perlite, but with devitrification show uranium depletions up to 80% in felsites, relative to the co-existing obsidians and perlites.
The high levels of uranium found in the groundwater and the occurrences of uraniferous silcretes gives indirect support for the mobilization of uranium by dissolution and/or leaching of the volcanic ashes. Considering the prevalent concepts of the formation of uranium ore bodies in sandstones leads one to think in terms of potential host-rocks with suitable organic reductants. The Ogallala Formation does not appear to contain the required association of organic debris . The pyritic and organic-debris containing fluvial sands of the Lower Cretaceous of the Great Plains that are now, or have been, in hydrologic contact with Pliocene, Pleistocene, and/or Recent uraniferous groundwaters, either through fractures or erosional contacts, should be considered potential host facies for uranium ore bodies. The depositional environments and the petrographic characteristics of the deltaic and fluvial systems of the Cheyenne, Kiowa, and Dakota Formations of Kansas are reasonably well known [e.g. 18,19,20], and the variety of types of cementation in the sandstones (limonitic, hematitic, siliceous, and calcareous) indicates different interactions between the rocks and secondary solutions have taken place.
The Late Tertiary and Pleistocene volcanic ashes of the central Great Plains should be considered a uranium source facies from which uraniferous Pliocene, Pleistocene, and Recent waters have been derived. Potential uranium-bearing fluvial sandstones with organic reductants that could have been in hydrologic contact with these groundwaters include the Lower Cretaceous sands of north-central and southwestern Kansas.
1. H. H. Adler, "Formation of Uranium Ore Deposits," Int'l. Atomic Energy Agency, Vienna, 1974, pp. 141-168.
2. A. R. Dahl and J. L. Hagmaier, "Formation of Uranium Ore Deposits," Int'l. Atomic Energy Agency, Vienna, 1974, pp. 201-218.
3. K. A. Dickinson, U.S. Geol. Surv. Open File Report 75-595 (1975).
4. R. A. Zielinski, U.S. Geol. Surv. Open File Report 75-595 (1975).
5. R. A. Zielinski, personal communication.
6. A Swineford and J. C. Frye, Kan. Geol. Surv. Bull. 64, pt. 1 (1946). [available online]
7. J. S. Carey et al., Kan. Geol. Surv. Bull. 96, pt. 1 (1952). [available online]
8. A. Swineford et al., Jour. Sed. Petr., 25, 4, 243 ( 1955).
9. L. R. Hathaway and G. W. James, Anal. Chem., 47, 12, 2035 (1975).
10. L. R. Hathaway and G. W. James, unpublished data.
11. G. W. James, unpublished data.
12. P. Berendsen, unpublished data.
13. J. C. Frye and A. Swineford, Kan. Geol. Surv. Bull. 64, pt. 2 (1946). [available online]
14. G. W. James and L. R. Hathaway, "Exploration for Uranium Ore Deposits," Int'l. Atomic Energy Agency, Vienna, 1976, pp. 311-320.
15. G. W. James, Anal. Chem. 49, 967-969 (1977).
16. J. Boellstorff, Kan. Geol. Surv. Guidebook Ser, 1, 37 (1976).
17. J. C. Frye et al., Kan. Geol. Surv. Bull. 118 (1956).
18. A. Swineford, Kan. Geol. Surv. Bull. 70, pt. 4 (1947). [available online]
19. P. C. Franks, "Cretaceous Systems in the Western Interior of North America," Geol. Assoc. Canada, Sp. Paper 13, 1975, pp. 469-521.
20. C. T. Siemers, Jour. Sed. Petr., 46, 1, 97 (1976).
|Sample Number||Sample Locations
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Web version updated June 18, 2010. Original publication date April 1978.