Uranium roll-front

Mobility of Uranium and Radon associated with uranium roll front occurrences in the Horton Group of the Windsor area, Nova Scotia,... 

Uranium mineralization in the Windsor area is a typical sandstone roll-front type. The Green Street occurrence that is the focus of this study occurs in thick arkosic sandstones, siltstones, and pebbly conglomerates of the Cheverie Formation (Fig. 2). The occurrence is a remnant tail of a uranium roll front that has subsequently migrated down dip from the outcropping (Ryan O Beirne-Ryan 2007). 

Rubin, B. 1970. Uranium roll front zonation in the southern Powder River Basin, Wyoming. Earth Science Bulletin [M], Wyoming Geological Association, 5-12. 

Figure 13.15 Schematic cross-section of an idealized uranium roll-front orebody showing the zonation of elements and primary hydrologic and geochemical features. Oxidized groundwaters flow from left to right. The roll front and associated redox interface moves in the same direction. After Larson (1978).

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

There is no doubt of the presence of roll fronts in the Horton Group sandstones, however the source of the uranium within the system may be related to the weathered horizons beneath the Horton Group rocks and not exclusively the result of diagenetic change within the sandstones (Ryan O Beirne-Ryan 2007). 

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 ... 

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... 

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 ...

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. 

Roll-front uranium Texas coastal plain Shallow groundwater Leaching of ash, transport, and precipitation at redox front... 

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

A roll front is a subterranean surface between reduced and subsequently oxidized rock. It may be essentially unmineralized. Alternatively, it may concentrate metals other than uranium, such as iron, vanadium, molybdenum, silver or heavy metals, In addition, these metals may concentrate in any combination, but invariably in overlapping shells or zones in order of electromotive force (Fig. 6). 

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. 

The distance of uranium transportation from source rock to present mineralized location is ordinarily between 1 and 10 km. For the most part a roll front will have passed through the most... 

When a proper aquifer has hosted a roll front over a fetch measured in kilometres, the source rock may well be the same stratigraphic horizon as the present host of an ore trend. Forthis condition to obtain the host must have originally contained the few ppm uranium required for subsequent leaching. 

Ju.st as roll fronts may be. seen to drape around permeability barriers in a uranium mine, so the colourbands follow the same... 

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