Uranium ore


Uranium ore deposits are economically recoverable concentrations of uranium within Earth's crust. Uranium is one of the most common elements in 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

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 and 235U. All uranium isotopes present in natural uranium are radioactive and fissionable, and 235U is fissile. Uranium, thorium, and one radioactive isotope of potassium 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 . 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, autunite, saleeite and torbernite ; and hydrated uranium silicates such as coffinite, uranophane and sklodowskite.

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 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.
  1. [|Unconformity-related deposits]
  2. [|Sandstone deposits]
  3. [|Quartz-pebble conglomerate deposits]
  4. [|Breccia complex deposits]
  5. [|Vein deposits]
  6. Intrusive deposits
  7. [|Phosphorite deposits]
  8. [|Collapse breccia pipe deposits]
  9. [|Volcanic deposits]
  10. [|Surficial deposits]
  11. [|Metasomatite deposits]
  12. [|Metamorphic deposits]
  13. [|Lignite]
  14. [|Black shale deposits]
  15. [|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

-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.

Athabasca Basin

The highest grade uranium deposits are found in the Athabasca Basin in Canada, including the two largest high grade uranium deposits in the world, Cigar Lake with 217 million pounds U3O8 at an average grade of 18% and McArthur River with 324 million pounds U3O8 at an average grade of 17%. These deposits occur below, across and immediately above the unconformity. Additionally, another high grade discovery is in the development stage at Patterson Lake with an estimated mineral resource identified as; "Indicated Mineral Resources" estimated to total 2,291,000 tons at an average grade of 1.58% U3O8 containing of U3O8. "Inferred Mineral Resources" are estimated to total 901,000 tons at an average grade of 1.30% U3O8 containing of U3O8.

McArthur Basin

The deposits of the McArthur River basin in the East Alligator Rivers region of the Northern Territory of Australia 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. There has been very little exploration in Australia to locate deeply concealed deposits lying above the unconformity similar to those in Canada. It is possible that very high grade deposits occur in the sandstones above the unconformity in the Alligator Rivers/Arnhem Land area.

Sandstone deposits

Sandstone deposits are contained within medium to coarse-grained sandstones deposited in a continental fluvial or marginal marine sedimentary environment. Impermeable shale or mudstone units are interbedded in the sedimentary sequence and often occur immediately above and below the mineralised horizon. Uranium is mobile under oxidising conditions and precipitates under reducing conditions, and thus the presence of a reducing environment is essential for the formation of uranium deposits in sandstone.
Primary mineralization consists of pitchblende and coffinite, with weathering producing secondary mineralization. Sandstone deposits constitute about 18% of world uranium resources. Orebodies of this type are commonly low to medium grade and individual orebodies 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:
  1. Wyoming basins
  2. Grants District of New Mexico
  3. Central Europe
  4. Kazakhstan
Significant potential remains in most of these centers as well as in Australia, Mongolia, South America, and Africa.
This model type can be further subdivided into the following sub-types:
  • tabular
  • roll front
  • basal channel
  • structurally related
Many deposits represent combinations of these types.

Tabular

Tabular deposits consist of irregular tabular or elongate lenticular zones of uranium mineralisation within selectively reduced sediments. The mineralised zones are oriented parallel to the direction of groundwater flow, but on a small scale the ore zones may cut across sedimentary features of the host sandstone. Deposits of this nature commonly occur within palaeochannels cut in the underlying basement rocks. Tabular sandstone uranium deposits contain many of the highest grades of the sandstone class, however the average deposit size is very small.

Roll front

Roll-front uranium deposits are generally hosted within permeable and porous sandstones or conglomerates. The mechanism for deposit formation is dissolution of uranium from the formation or nearby strata and the transport of this soluble uranium into the host unit. When the fluids change redox state, generally in contact with carbon-rich organic matter, uranium precipitates to form a 'front'.
The roll front subtype deposits typically represent the largest of the sandstone-hosted uranium deposits and one of the largest uranium deposit types with an average of 21 million lb U3O8. Included in this class are the Inkai deposit in Kazakhstan and the Smith Ranch deposit in Wyoming. Probably more significant than their larger size, roll front deposits have the advantage of being amenable to low cost in situ leach recovery.
Typical characteristics:
  • roll-front deposits are crescent-shaped bodies that transect the host lithology
  • typically the convex side points down the hydraulic gradient.
  • the limbs or tails tend to be peneconcordant with the lithology.
  • most ore-bodies consist of several interconnected rolls.
  • individual roll-front deposits are quite small but collectively can extend for considerable distances.