As one of the more common elements in earth’s crust, uranium is often said to be some 40 times more common than silver and 500 times more common than gold. Uranium ore deposits are found globally, with the largest known recoverable resources found in Kazakhstan, Canada, Australia and South Africa. The metal is so abundant that it can be found almost everywhere in soil, rivers, oceans and rocks. The challenge, however, is to find areas where the concentrations are adequate to form an economically viable deposit and better yet, a world class deposit.

Uranium ore is mined in several ways, depending on the geological conditions: by open pit, underground, in-situ leaching and borehole mining. Once extracted, uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting enriched mixture, called yellowcake, contains at least 75 percent uranium oxides. Yellowcake is then calcined to remove impurities from the milling process before refining and conversion.

Uranium deposits

To date, the highest grade uranium deposits have only been found in Canada’s Athabasca Basin region in northern Saskatchewan. Furthermore, the largest uranium producing mine in the world is Cameco‘s (TSX:CCO) McArthur River, an underground mine located in the Athabasca Basin.

Uranium deposits are generally classified based on host rocks, structural setting, and mineralogy of the deposit. The most widely used classification scheme was developed by the International Atomic Energy Agency (IAEA) with the objective of subdividing deposits into 15 categories arranged according to their approximate economic significance:

1. Unconformity-related uranium deposits

Probably one of the most talked about type of uranium deposit, unconformity-type deposits can be some of the largest and richest deposits on earth. Relative to other uranium deposits, the unconformity-type deposits tend to be the most high grade. They occur in close proximity to major unconformities between relatively quartz-rich sandstones and deformed metamorphic basement rocks. The most significant areas for this style of deposit are currently the Athabasca Basin in Saskatchewan, Canada.

Cameco owns and operates two of the highest grade mines in the world, McArthur River and Cigar Lake. Also, working on an unconformity project is Fission Uranium (TSX:FCU) with the Paterson Lake South project. Paterson Lake, while still in the exploration phase, has returned and indicated resource of 79.6 million pounds, including 44.3 million pounds indicated at  18.21 percent U3O8 at the Triple R deposit.

2. Sandstone deposits

Sandstone deposits are contained within medium to coarse-grained sandstones located in a continental fluvial or marginal marine sedimentary environment. Sandstone deposits constitute about 18 percent of world uranium resources and are commonly classified as low to medium grade, in the range of 0.05–0.4 percent and individual ore bodies are small to medium in size.  Sandstone hosted uranium deposits are widespread globally and span a broad range of host rock ages. Some of the major provinces and production centers include: Wyoming, New Mexico, Central Europe and Kazakhstan. Significant potential for sandstone deposits remains in the regions as well as in Australia, Mongolia, South America, and Africa.

3. Quartz-pebble conglomerate deposits

Uranium deposits of quartz-pebble conglomerate were historically significant as the major source of primary production for several decades following World War 2. This type of deposit has been identified in eight jurisdictions around the world; however, the most significant deposits are in the Huronian Supergroup in southern Ontario, Canada and in the Witwatersrand Supergroup of South Africa. These deposits make up approximately 13 percent of the world’s uranium resources.

4. Vein deposits

Vein deposits consist of uranium minerals filling in cavities such as cracks, veins, fractures, breccias, and stockworks associated with steeply dipping fault systems. These

deposits are responsible for the term “pitchblende”, as it originates from German vein deposits when they were mined for silver in the 16th century. F.E. Brückmann made the first mineralogical description of the mineral in 1727. The first industrial production of uranium was made from a vein deposit in the Czech Republic and Marie and Pierre Curie used the tailings of the mine for their discovery of polonium and radium.

5. Breccia complex deposits

Only one iron-ore-copper-gold (IOCG) deposit of this type is known to contain economically significant quantities of uranium. The Olympic Dam, currently operated by BHP Billiton (NYSE:BHP) is the world’s largest resource of low-grade uranium and accounts for about 66 percent of Australia’s reserves plus resources. Uranium occurs with copper, gold, silver, and rare earth elements in a large hematite-rich granite breccia complex in the Gawler Craton overlain by approximately 300 metres of flat-lying sedimentary rocks of the Stuart Shelf geological province.

6. Intrusive associated deposits

Intrusive deposits make up a large proportion of the world’s uranium resources and are associated with rock types including alaskite, granite, pegmatite and monzonites. Major world deposits are found in Namibia, Greenland and South Africa.

7. Phosphorite deposits

Low grade concentrations of notably large uranium deposits can be found in marine sedimentary phosphorite structures, ranging up to 0.01–0.015 percent U₃O₈. Very large phosphorite deposits occur in Florida and Idaho in the United States, Morocco, as well as some Middle Eastern countries.

8. Collapse breccia pipe deposits

Collapse breccia pipe deposits occur within vertical, circular solution collapse structures, formed by the dissolution of limestone by groundwater. Resources within individual pipes can range up to 2500 tonnes U₃O₈ at an average grade of between 0.3-1.0 percent U₃O₈. The best known examples of this deposit type are in Arizona, USA, where several of these deposits have been mined.

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9. Volcanic deposits

The principal uranium mineral in volcanic deposits is pitchblende, which is usually associated with molybdenum sulfide and minor amounts of lead, tin and tungsten mineralization. The average deposit size is rather small with grades of 0.02-0.2 percent U₃O₈. These deposits make up only a small proportion of the world’s uranium resources. Currently, the only volcanic hosted deposits being exploited are those of the Streltsovkoye district of eastern Siberia.

10. Surficial deposits (calcretes)

Surficial deposits account for approximately 4 percent of world uranium resources and are interbedded with sand and clay, usually cemented by calcium and magnesium carbonates. These deposits may also occur in peat bogs, karst caverns and soils. The Yeelirrie deposit in Western Australia is by far the world’s largest surficial deposit, averaging 0.15 percent U₃O₈.

11. Metasomatite, metamorphic and lignite deposits

Metasomatite deposits consist of disseminated uranium minerals within structurally deformed rocks formed from geo chemical processes. The uranium content is very low, on average less than 0.005 percent U₃O₈, deposits are typically small in size containing less than 1,000 tonnes and currently do not warrant commercial extraction.

12. Black shale deposits

Black shale mineralizations are large low-grade resources of uranium. They form in submarine environments under oxygen-free conditions. Because of their low grades, no black shale deposit ever produced significant amounts of uranium with one exception: the Ronneburg deposit in Germany. Production between 1950 and 1990 was about 100,000 tonnes of uranium at average grades of 0.07-0.1 percent. Measured and inferred resources containing 87,000 tonnes of uranium at grades between .02-.09 percent remain.

This is an updated version of an article originally published on June 8, 2010. 

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