Uranium ore deposits are economically recoverable concentrations of uranium within Earth's crust. Uranium is a fairly common element in the Earth's crust, being 40 times more common than silver and 500 times more common than gold. It can be found almost everywhere in rock, soil, rivers, and oceans. The challenge for commercial uranium extraction is to find those areas where the concentrations are adequate to form an economically viable deposit. The primary use for uranium obtained from mining is in fuel for nuclear reactors. Globally, the distribution of uranium ore deposits is widespread on all continents, with the largest deposits found in Australia, Kazakhstan, and Canada. To date, high-grade deposits are only found in the Athabasca Basin region of Canada. 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 and subdivides deposits into 15 categories.
Uranium
Uranium is a silvery-gray, weakly radioactive metallic chemical element. It has the chemical symbol U and atomic number 92. The most common isotopes in natural uranium are 238U (99.274%) and 235U (0.711%). All uranium isotopes present in natural uranium are radioactive and fissionable, and 235U is fissile (will support a neutron-mediated chain reaction). Uranium, thorium, and one radioactive isotope of potassium (40K) as well as their decay products are the main elements contributing to natural terrestrial radioactivity. Cosmogenic radionuclides are of less importance, but unlike the aforementioned primordial radionuclides, which date back to the formation of the planet and have since slowly decayed away, they are replenished at roughly the same rate they decay by the bombardment of Earth with cosmic rays. Uranium has the highest atomic weight of the naturally occurring elements and is approximately 70% denser than lead, but it is not as dense as tungsten, gold, platinum, iridium, or osmium. It is always found combined with other elements. Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernova explosions.
Uranium minerals
The primary uranium ore mineral is uraninite (UO2) (previously known as pitchblende). A range of other uranium minerals can be found in various deposits. These include carnotite, tyuyamunite, torbernite and autunite. The davidite-brannerite-absite type uranium titanates, and the euxenite-fergusonite-samarskite group are other uranium minerals. A large variety of secondary uranium minerals are known, many of which are brilliantly coloured and fluorescent. The most common are gummite (a mixture of minerals), autunite (with calcium), saleeite (magnesium) and torbernite (with copper); and hydrated uranium silicates such as coffinite, uranophane (with calcium) and sklodowskite (magnesium).
Ore genesis
There are several themes of uranium ore deposit formation, which are caused by geological and chemical features of rocks and the element uranium. The basic themes of uranium ore genesis are host mineralogy, reduction-oxidation potential, and porosity. Uranium is a highly soluble and radioactive heavy metal. It can be easily dissolved, transported and precipitated within groundwater by subtle changes in oxidation conditions. Uranium does not usually form very insoluble mineral species, which is a further factor in the wide variety of geological conditions and places in which uranium mineralization may accumulate. Uranium is an incompatible element within magmas, and as such it tends to become accumulated within highly fractionated and evolved granite melts, particularly alkaline examples. These melts tend to become highly enriched in uranium, thorium and potassium, and may in turn create internal pegmatites or hydrothermal systems into which uranium may dissolve. While it was previously assumed that biological processes played little to no role in the formation of uranium ores, more recent research posits that some ore-bodies may have been significantly influenced by metal-reducing microorganisms such as Geobacter metallireducens, Geobacter uraniireducens or Shewanella oneidensis. These species are capable of using uranium-species as a terminal electron acceptor.
Classification schemes
IAEA Classification (1996) The International Atomic Energy Agency (IAEA) assigns uranium deposits to 15 main categories of deposit types, according to their geological setting and genesis of mineralization, arranged according to their approximate economic significance.
Unconformity-related deposits Sandstone deposits Quartz-pebble conglomerate deposits Breccia complex deposits Vein deposits Intrusive deposits (Alaskites) Phosphorite deposits Collapse breccia pipe deposits Volcanic deposits Surficial deposits Metasomatite deposits Metamorphic deposits Lignite Black shale deposits Other types of deposits
Alternate scheme The IAEA classification scheme works well but is far from ideal, as it does not consider that similar processes may form many deposit types, yet in a different geological setting. The following table groups the above deposit types based on their environment of deposition.
Deposit types (IAEA Classification)
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 unconformities between relatively quartz-rich sandstones comprising the basal portion of relatively undeformed sedimentary basins and deformed metamorphic basement rocks. These sedimentary basins are typically of Proterozoic age, however some Phanerozoic examples exist. Phanerozoic unconformity-related deposits occur in Proterozoic metasediments below an unconformity at the base of overlying Phanerozoic sandstone. These deposits are small and low-grade (e.g., Bertholene and Aveyron deposits in France).
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