Clay Minerals Suitable for Overpack in Waste Repositories: Evidence From Uranium Deposits
Various clay minerals have been proposed for overpack around radioactive waste canisters. The laboratory studies of the migration of elements from the nuclear waste through proposed overpack have resulted in some disagreement. As an alternative approach to the laboratory studies, a review of data for the uranium deposits of the Grants, New Mexico, mineral belt has been undertaken (1, 2). This paper reviews the evidence from the clay mineralogy, the Rb-Sr systematics and the trace element analyses of the clay size (<2μ) fraction of rocks from the uranium ore zones and the barren host rocks. The <2μ fraction is dominated by clay minerals, but small amountsof other minerals and organic matter are present as well. These deposits are well suited to such a study because (a) they are old, (b) some have been remobilized and reprecipitated, (c) some have been destroyed by subsurface waters and the uranium disseminated, and (d) each type of uranium deposit has a rather diagnostic clay mineral suite and characteristic suite of elements associated with it. The oldest ore is found in argillaceous sandstone in deeply buried sediments which, based on geochronologic study, have remained closed systems since about 135–145 m.y. ago. That the Rb-Sr systematics for these deposits are undisturbed also indicates that Cs, with a greater tendency to be retained in clay minerals than Rb, is also retained. In addition, pronounced enrichment of Y, the REE, Nb, Mo, Sb, Te, Ba and Cd is noted in the mixture of vanadiferous chlorite, an illite and mixed layer illite: montmorillonite with coffinite, pyrite, organic matter. While the organic acids derived from lignin-derived humates have been associated with ore formation, they have not evidently affected the ore in post-formational time due presumably to the low Eh and intermediate (6.5-7.5) pH values. It is interesting to note that this suite of elements, although in different proportions and of normal isotopic composition, contains many elements in the mass range produced by 235-U fission.
KeywordsClay Mineral Uranium Deposit Barren Rock Clay Mineralogy Mineral Belt
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