Roll-Front Uranium Deposits  

by Edgar B. Heylmun, PhD

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If the uranium industry stages a comeback, it will not likely be a mad rush like it was in the 1950s. Except for non-radioactive uranium, most surface and near-surface deposits have been found. Some deposits may be mined by open-pit or underground methods, but chances are, most future mining will be done by in-situ leaching.

For a uranium deposit to be leached, certain requirements must be met. The ore body must be porous and permeable, most likely in a tuffaceous or arkosic sandstone or conglomerate. The beds must be flat-lying or only gently dipping, and the permeable beds must be sealed above and below by impervious mudstone or shale. The beds must be below the water table or in a confined aquifer. Uplift and erosion brings about a lowering of the water table, so many uranium deposits are now high and dry. Most deposits are in porous and permeable channelways within the sandstone or conglomerate beds.

Origin of Uranium

It has been demonstrated that most uranium has been derived from the alteration of volcanic ash or tuff. Volcanic ash and tuff are unstable under atmospheric conditions and will eventually alter to clay or mudstone. Upon alteration, uranium will be released into the groundwater. The uraniferous solutions will circulate through permeable beds until a reductant is encountered. Reductants include disseminated pyrite and organic material like plant remains or hydrocarbons, which give off hydrogen sulfide gas as they decompose. Hydrogen sulfide gas is a strong reductant, so uranium will precipitate from solution upon encountering the reducing conditions created by H2S gas. Sometimes, wood, peat, lignite, and hydrocarbons are completely replaced by black uranium oxides, usually as uraninite (pitchblende) or coffinite. Organic trash pockets in sandstone can result in the formation of rich ore bodies.

           
Roll-Front Deposits

Roll-front uranium deposits are formed where groundwater in permeable sandstone or conglomerate encounters the interface between oxidizing and reducing conditions. Uranium in solution is precipitated at the interface, often forming a crescent-shaped roll-front ore body (see illustration). Over the years, the reduction front will migrate in the direction of groundwater flow, thus creating an ore body that may extend for hundreds of feet. The crescent tips will often string-out and create tabular blanket deposits which may contain black and yellow uranium oxides. Oxidized zones are often distinctive features of uranium deposits, as shown on the illustration. Some deposits can be found just by noting the colors that are usually present. However, the deposits are usually found with radiation detectors long before other details are noted. Exceptions could be where uraninite or coffinite is so newly-formed that radioactive daughter products have not yet formed. These non-radioactive uranium deposits have to be discovered by means of chemical analyses. 

There are a lot of uranium deposits that are tabular and are not roll-fronts. Such beds often adjoin organic mudstones or shales, or occur where there are organic trash pockets in the sandstones. The rich deposits at Lisbon, Utah, and Grants, New Mexico, are tabular.

       
In-Situ Leaching    

If a uraniferous sandstone is confined by impervious mudstone or shale, and is in an aquifer, it could be a candidate for in-situ leaching. The leaching process involves benign chemicals, and is comparatively simple. Sodium bicarbonate is injected as a lixiviant, by means of drillholes, into the uraniferous sandstone bed, along with oxygen. These are benign substances that are commonly found in nature. The solutions circulate and are recovered, along with uranium, by means of a recovery well. Injection wells usually surround the recovery well. Yellowcake (U3O8) is recovered by an ion-exchange process, and the solutions are recycled. Roll-front deposits are especially amenable to in-situ leaching.

If the mudstone or shale bounding the sandstone or conglomerate is organic, yellow uranium oxides such as carnotite or uranophane may be present, and form blanket deposits. Carnotite is not readily leached, but many other yellow oxides, like uranophane, are easily leached. Kasolite and gummite are sometimes present where there is sulfide mineralization. They are formed by circulating groundwater, often in a permeable shear zone, not by hydrothermal solutions.

       
Summary    

Roll-front uranium deposits remain to be leached, especially those that are too deep for surface mining methods. It could be that all future recovery of uranium will be by in-situ leaching. The subject has been covered in trade journals, and further information is available in numerous Dept. of Energy reports. US Geological Survey Circular 1141 is a good summary that is available from the USGS at no cost.

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