Uranium ore deposits

Uranium ore deposits are ore deposits which constitute economically recoverable concentrations of uranium within the Earth's crust.

Distribution

The distribution of uranium ore deposits is widespread, with sizeable deposits and mines in Australia (40% of EDR [cite web|url=http://www.ga.gov.au/image_cache/GA7518.pdf|title=Geoscience Australia Uranium factsheet|accessdate=2007-08-14] ), Canada, former Soviet republics, Africa and South America.

Uranium ore deposits can be divided into several broad classes:
* Roll-front uranium deposits
* Palaeochannel hosted deposits
* Pegmatite-hosted deposits
* Alkaline granite intrusive hosted
* Carbonatite hosted
* Phosphorite and/or evaporite hosted
* Organic or carbonaceous hosted sedimentary deposits

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 heavy metal, as well as a radioactive heavy metal. Uranium can be easily dissolved, transported and reprecipitated within ground waters by very subtle changes in oxidation conditions. Uranium also 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.

Roll front uranium deposits

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'.

Palaeochannel-hosted deposits

The model for formation of palaeochannel deposits is similar to that for roll-front deposits, above, except that the source of uranium may be in the watershed leading into a stream, or the bed load of the palaeochannel itself. This uranium is transported through the groundwaters and is deposited either at a reduced boundary, or in ephemeral drainage systems such as those in deserts of Namibia and Australia, it is deposited in calcretised evaporation sites or even in salt lakes as the ground water evaporates.

Some particularly rich uranium deposits are formed in palaeochannels which are filled in the lower parts by lignite or brown coal, which acts as a particularly efficient reductive trap for uranium. Sometimes, elements such as scandium, gold and silver may be concentrated within these lignite-hosted uranium deposits [cite web|url=http://crcleme.org.au/RegExpOre/MulgaRock.pdf|title=Douglas, Butt & Gray. 2003, CRC LEME|accessdate=2006-03-07] .

Pegmatite hosted deposits

Uranium is hosted in uranium-bearing minerals such as pyrochlore, zircon and apatite in pegmatite intrusions.

See also

* Palaeochannel
* Pegmatite
* Carbonatite
* Ore genesis
* Uranium
* Nuclear reactor
* List of uranium mines

References

External sources

* [http://crcleme.org.au/RegExpOre/MulgaRock.pdf Mulga Rock Palaeochannel uranium deposit, Officer Basin W.A.]