Uranium is one of the more common elements in the Earth’s crust, and 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, including soil, rivers, oceans and rocks. The challenge, however, is to find areas with concentrations adequate enough to form an economically viable deposit — or, better yet, a world-class deposit.
Uranium ore is mined in several ways, depending on the geological conditions; processes include open-pit mining, underground mining, 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 oxide. Yellowcake is then calcined to remove impurities from the milling process before refining and conversion.
To date, the highest-grade uranium deposits have been found in Canada’s Athabasca Basin in Northern Saskatchewan. 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 rock, structural setting and deposit mineralogy. The most widely used classification scheme was developed by the International Atomic Energy Agency with the objective of subdividing deposits into 15 categories arranged according to their approximate economic significance:
1. Unconformity-related uranium deposits
Unconformity-type deposits are one of the most-discussed uranium deposit types, and for good reason — they can be some of the largest and richest deposits on earth. Relative to other uranium deposits, 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 area for this style of deposit is currently the Athabasca Basin.
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 an 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 the world’s uranium resources and are commonly classified as low to medium grade, in the range of 0.05 to 0.4 percent; 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 those 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 II. 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 exist where uranium minerals have filled in cavities such as cracks, veins, fractures, breccias and stockworks associated with steeply dipping fault systems. These deposits are responsible for the term “pitchblende,” which originates from German vein deposits 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 is known to contain economically significant quantities of uranium. 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 meters 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 from 0.01 to 0.015 percent U3O8. Very large phosphorite deposits occur in Florida and Idaho in the US and Morocco, as well as in 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 reach up to 2,500 tonnes U3O8 at an average grade of 0.3 to 1 percent U3O8. The best-known examples of this deposit type are in Arizona, where several of these deposits have been mined.
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 to 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 in 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 geochemical 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 deposits include large, low-grade uranium resources. They form in submarine environments under oxygen-free conditions. Because of their low grade, almost no black shale deposit has ever produced significant amounts of uranium. However, there is one exception: the Ronneburg deposit in Germany. Production between 1950 and 1990 was about 100,000 tonnes of uranium at an average grade of 0.07 to 0.1 percent. Measured and inferred resources containing 87,000 tonnes of uranium grading between 0.02 and 0.09 percent remain.
This is an updated version of an article originally published on Uranium Investing News on June 8, 2010.