Uranium — Where Is It Found?
Uranium is a naturally occurring element that has the highest atomic weight (~238 g/mole) and is slightly radioactive. It can be found in minute quantities in most rocks, soils and waters (normally < 5 ppm), but the real challenge is to find it in high enough concentrations to make it economically feasible to mine. Uranium is easily oxidized and forms a number of common uranium oxides and oxy-hydroxide like uraninite (or pitchblende) and schoepite (including meta- and para-).
|Material||Concentration (ppm U)|
|High-grade orebody (>2% U)||>20,000|
|Low-grade orebody (0.1% U)||1, 000|
|Average volcanic rock||20 - 200|
|Average sedimentary rock||2|
|Average black shale||50 - 250|
|Average earth's crust||2.8|
|Groundwater||>0.001 - 8|
Uranium can be found in soils and waters due to the breakdown (weathering) of rocks containing it. Once it is in the soil and water, it can be taken up by plants and consumed by people or grazing animals, or it can dissolve in the water to be consumed by any organism.
Types of Uranium Deposits
Uranium deposits occur in many different rock types from sedimentary to volcanic. One thing almost all economic uranium deposits have in common is that the uranium is remobilized from one area (ie., leached from a source rock containing minute quantities of U or as mineral grains with elevated U concentrations) and reprecipitated in a host rock where chemical conditions (reducing) are conducive to concentrating the uranium in higher concentrations or redeposited due to water action (waves on beaches or water flow in rivers) in placer deposits.
Common uranium deposits are
An unconformity is time gap in the rock record between two rock units where the lower unit may be deformed, brecciated or altered and the overlying units are less deformed. Uranium deposits can occur in the underlying or overlying units. In the underlying units, there may be a weathering zone, fault zone or some other feature that increases the rocks porosity and permeability. In the overlying units, it maybe the sandstones or some other features that allows the concentration of uranium. Deposits of this type are common in Australia,Canada and India.
- Breccia uranium deposits.
Breccias are pre-existing rocks that have be broken-up into pieces by either weathering and collapse or fracturing (hydraulic or tectonic). The blocks form a high porosity and permeability framework for U precipitation. Deposits of this type are common in Australia, United States and India.
Sandstone & Conglomerate deposits.
Normally in the coarser fraction of sandstones and conglomerates, these units are typically deposited in marginal marine to terrestrial environments. The best deposits are found between impermeable units and contain abundant organic debris or other material to promote the reducing conditions to cause the U to precipitate out of solution. Deposits of this type are common in United States, Niger, Kazakhstan, Uzbekistan, Gabon, South Africa, Canada, India and Australia. Types of deposits include:
- Roll-front Deposits
Roll-front deposits cut across bedding. Uranium-bearing ground waters precipitate uranium oxide minerals when they come in contact with reducing conditions in porous and permeable rocks.
Tabular or Trend
Uranium deposits form tabular bodies that may or may not cross bedding. They are usually associated with organic debris or pyrite. Some uranium deposits follow paleochannels or some other depositional trends. The ore can occur either as reprecipitated deposits in reducing zones associated with pyrite or organic debris (like roll-front deposits) or as placer deposits (heavy mineral deposits) concentrated in a beach, bar or channel due to water movement. Deposits found in USA, Japan, Niger and Canada.
- Roll-front Deposits
Uranium is remobilized and precipitates adjacent to permeable fault and/or fracture zones. See figure in unconformity deposits which also shows mineralization adjacent to fault zones.
- Limestone deposits.
Units that have high porosity and permeability (due to tectonic or diagenetic alteration) as well as organic carbon contents form good sites for uranium precipitation. Deposits of this type are rare, but can be found in United States (Grants Mineral Belt, New Mexico).
- Surficial deposits.
The U is concentrated in young sediments or soils near the earth's surface. Uranium minerals precipitate out onto the finer-grained particles or are transported particles. Associated with soil formation. Deposits of this type are found in United States, Australia, Canada and Namibia.
- Volcanic deposits.
Deposits are associated with fault, fracture and shear zones in acidic volcanic rocks. Deposits of this type are found in China, Russia, Kazakhstan, Mexico, Namibia, Greenland, South Africa, United States, Canada and Australia.
- Vein deposits.
Uranium ore is associated with veins or other lenses in igneous, metamorphic or sedimentary rocks. Deposits of this type are found in Australia, France, Czech Republic, Germany and Zaire.
- Intrusive deposits.
In intermediate to acidic igneous rocks and pegmatites, the uranium-rich minerals are direct precipitates (no dissolution and remobilization. Deposits of this type are found in USA, Namibia, Greenland, Canada and South Africa.
- Metasomatic deposits.
Hydrothermal alteration of deformed basement rocks. Deposits of this type are found in Brazil, Ukraine and Australia.
- Phosphorite and Lignite deposits.
The uranium occurs with organic-rich marine-deposited phosphorites (within the apatitie) or in lignites (low-grade coal). Fly ash, the result of burning coal, can increase the U concentration by burning off the carbon. Deposits of this type are found in the United States.
Uranium can be found in a large number of minerals (WebMineral has an excellent listing of them in order of uranium concentration). The most common economic minerals are listed below (click on the links to see photos and additional information on these minerals):
Uranium in New Mexico ranks second in the United States in U reserves, behind Wyoming. The map below shows the different mining districts in the state of New Mexico and can be downloaded from the New Mexico Bureau of Geology and Mineral Resources. The largest deposits can be found in the northwest corner of the state in the Grants Mineral Belt. The Jurassic-age Morrison Formation sandstones are the principle host for these deposits. The deposits in these sands are roll-front or tabular/trend deposits. Other units that contain uranium ore in New Mexico are: the Cretaceous Dakota Sandstone, the Triassic Chinle Group sandstones and small deposits in other Cretaceous and Tertiary units. New Mexico is one of the few areas that hosts significant uranium deposits in limestones (2% of the total uranium production to date). The Jurassic Todilto Limestones are unique because of their high-organic content and their relatively high porosity and permeability due to post-deposition diagenesis.
Search our website for more detailed articles concerning uranium deposits in the Grants District and elsewhere in New Mexico.
Links to more information
Any mention or link regarding a product, organization, company, or trade name is for information only and does not imply endorsement by the Bureau, NMT, or the State of New Mexico (see more).
- Uranium Minerals
- Uranium Deposits
- World Nuclear Assoc. - Geology of Uranium
- Australia Uranium Association
- Natural Resource Canada - Athabasca Uranium
- Uranium in New Mexico
- Wikipedia - Uranium
- World Nuclear Assoc. - Mining links
- Wise Uranium Project - Maps
- World Nuclear Assoc. - Production Figures
- Wyoming Geological Survey
- For Kids & Teachers: