Roll-front uranium deposit

Uranium in Ground Water Associated with Roll-Front Deposits... 

The uranium that is found in roll-front deposits is generally believed to be derived from the dissolution and leaching of host minerals by soil water and ground water (2 3). Typical source rocks for the uranium are granites, tuffs, and tuffaceous sandstones that have relatively high concentrations of uranium in... 

The concentration of uranium in the sediments of roll-front deposits is typically in the 1000 to 2000 ppm range (8). The uranium occurs as coatings on grains and as interstitial material the predominant uranium mineral is uraninite Coffinite is often... 

Uranium is a trace constituent in most ground waters. In typical aquifers with oxidizing ground water, uranium s concentration in solution appears to be limited by its abundance in source rocks. The uranium in ground water is removed from solution and deposited at a redox interface between sediments with reducing minerals and sediments without. In the resultant roll-front deposit, uranium is concentrated in the sediment, and its ground-water concentration remains low because of the low solubility of the uranium minerals that compose the deposit. Consequently, it is possible to have localized high concentrations of uranium in the earth s crust that are stable. 

One popular ettneept holds that the metals are progressively leached from the host rock as the roll front passes through it. Uranium districts are found in areas where the background levels of uranium are elevated. Based on the nature of the alteration, leaching trf the host rock on the oxidized side of a roll can be extremely inten.se. With the remarkable efficiency of the precipitating mechanism only a few parts per million of uranium have to be removed from the volume of oxidized rock associated with most roll-front deposits to account for the amount of uranium in the orebody. 

A consideration of other elements associated with roll-front deposits improves the accuracy and confidence in locating favourable trends (Fig. 9). Sulphate concentration and conductivity increase toward the redox front and then decrease abruptly owing to precipitation of iron sulphide, followed by calcium carbonate. High values of bicarbonate and selenium also contribute to identification of the zone of uranium mobilization on the oxidized side of the front. Molybdenum concentrations are normally associated with sandstone deposits, but haloes in the groundwater may be to the side of or farther down-dip than the centre of the geochemical cell. Arsenic is most valuable for its regional halo around areas of mineralization. 

Fig. 3. Elemental and mineral zoning exhibited by selected roll-front uranium deposits. From Adams Smith (1981) adapted from Harshman (1974).

Data from in-situ leach mining and restoration of roll-front uranium deposits also provide information on the potential mobility of the waste if oxidizing ground water should enter the repository. Uranium solids probably will be initially very soluble in carbonate ground water however, as reducing conditions are re-established through water/rock interactions, the uranium will reprecipitate and the amount of uranium in solution will again equilibrate with the reduced uranium minerals ... 

Table I. Concentration of Uranium in Parts per Billion in Ground Water Associated with Roll-Front Uranium Deposits...

Roll-front uranium deposits in confined aquifer systems are amenable to extraction by in situ leach techniques. This method of mining was first tested in Wyoming approximately twenty years... 

Figure 1. Schematic representation of a roll-front uranium ore deposit ...

Hansink, J.D., 1976. Equilibrium analysis of sandstone roll-front uranium deposits. In Exploration for Uranium Ore Deposits. IAEA, Vienna, pp. 683-693. 

Goldhaber M. B. and Reynolds R. L. Origin of marcasite and its significance for understanding the genesis of roll-front uranium deposits. Open File Rep. U.S. geol. Surv. 79-1696, 1979, 38 p. 

The uranyl molybdate minerals are relatively poorly characterized. The first uranyl molybdate mineral was recognized by Brophy and Kerr, but since then a number of minerals that belong to this group have been described from many different deposits (Table 14). An affinity of uranium and molybdenum, especially in roll-front type deposits, has been known for some time. Initially, only minerals with U were recognized, but the occurrence of such minerals as sedovite shows that U compounds also exist. 

Q ROLL FRONT SUBTYPE Fig. 3 Sandstone-type uranium deposits... 

Mesozoic and Caenozoic strata, especially in the western United States, contain many important sandstone-hosted uranium deposits of blanket, stack and roll-front sub-types, with fewer volcanic-hosted types. As may be expected, the dynamic, migrating roll-front sub-type predominates in the youngest Tertiary rocks of the Wyoming basins, where final fixation of the migrating uranium is yet to be attained. In con-... 

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