By Dave Brown – Exclusive to Uranium Investing News
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. The distribution of uranium ore deposits are widely dispersed globally, with the largest known recoverable resources found in Australia, Kazakhstan, Russia, South Africa and Canada. It can be found almost everywhere in soil, rivers, oceans and rocks. The challenge is to find those areas where the concentrations are adequate to form an economically viable deposit and better yet a world class deposit.
With about 63 percent of the world’s production of uranium from mines sourced from Kazakhstan, Canada and Australia in 2009, Kazakhstan produced the largest global share of uranium from mines (27 percent of world supply from mines), followed by Canada (20 percent) and Australia (16 percent). To date, high-grade deposits have been found only in the Athabasca Basin region of Canada. Founded in 1988, the largest uranium producing mine in the world is the McArthur River Mine, which is an underground mine owned by Cameco (TSX:CCO) that produced approximately 7,339 tonnes of uranium ore, equal to about 15 percent of the world’s total in 2009.
Uranium ore is mined in several ways, depending on the geological conditions: by open pit, underground, in-situ leaching, and borehole mining. 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 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 deposits
Unconformity-type uranium deposits host high grades relative to other uranium deposits and include some of the largest and richest deposits known. 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, and the McArthur Basin in the Northern Territory, Australia.
The highest grade uranium deposits are found in the Athabasca Basin, including the two largest high grade uranium deposits in the world, Cigar Lake with 105,000 tonnes of U3O8 at an average grade of 19 percent and McArthur River with 221,000 tonnes U3O8 at an average grade of 24 percent. These deposits occur below, across and immediately above the unconformity.
The deposits of the McArthur River basin in the Alligator Rivers region of the Northern Territory are below the unconformity and are at the low-grade end of the unconformity deposit range but are still high grade compared to most uranium deposit types. This river basin also hosts a world-class McArthur River mine, sharing the same name as the world’s largest uranium mine operated by Cameco, although the Australian facility produces zinc, lead and silver. It is possible that the area has very high grade deposits in the sandstones above the unconformity as there has been relatively little exploration to locate deeply concealed deposits similar to those in Canada.
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” (”Pechblende”), 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.
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 deposts 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 mineralisations 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.
13. Other types of deposits
In China and Hungary trials are presently underway to extract uranium from fly ash, a residue generated from the combustion of coal. Other types of uranium deposits exist most significantly in New Mexico within the United States and Eastern Germany.
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