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-).
Table 1: Average uranium concentrations
in ores, rocks and waters (ppm - parts per million).
Material
Concentration (ppm U)
High-grade orebody (>2% U)
>20,000
Low-grade orebody (0.1% U)
1, 000
Average granite
4
Average volcanic rock
20 - 200
Average sedimentary rock
2
Average black shale
50 - 250
Average earth's crust
2.8
Seawater
0.003
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.
Unconformity-related deposits.
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.Schematic Cross Section of a "Typical" Breccia Pipe
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.
Tectonic 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
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trade name is for information only and does not imply endorsement by
the Bureau, NMT, or the State of New Mexico (see
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