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The three main types of sandstone deposits (in black): Roll Front, Tabular and Tectonological.
The three main types of sandstone deposits (in black): Roll Front, Tabular and Tectonological.

An Introduction to Uranium Deposits

Although there are between 12 and 15 different typestyles of uranium deposits, almost all economic uranium deposits come from just two types:

  • unconformity-related deposits,and
  • sandstone uranium deposits.

Unconformity-related deposits account for around on third of the world’s uranium production, while sandstone uranium deposits account for another third. The remaining third of production comes from a variety of the remaining 10 or so deposit types.

The Athabasca basin deposits grade about 2% U, with exceptionally high-grade areas such as Cigar Lake (almost 20% uranium). The Australian Kombolgie basin. average grade is lower at 0.4%.

Ore Minerals

Autunite crystals. Image CC

Autunite crystals. Image CC

The most common uranium mineral is uraninite or pitchblende. There are a range of other uranium minerals including camotite, branmerite and euxenite.  Uranium is reactive and forms a large number of colourful secondary minerals including autunite (with calcium), saleeite (magnesium) and torbernite (copper). Uranium is only weakly radioactive, but is highly soluable, which is used to advantage with some mining methods.

Unconformity-Related Uranium Deposits

An unconformity is a boundary between two rock units that reflects a time gap. Uranium unconformity deposits are generally associated with structures in sedimentary rocks that reflect the erosion surface rocks, and then the later subsequent deposition of younger sedimentary rocks above. Uranium deposits occur in sedimentary basins deposited on top of very old basement metamorphic rocks. The source of uranium is either from the basin or the basement rocks. Groundwater circulating in the basin results in the concentration and deposition of the mineralisation.

 

Uranium deposits occurring at or near an unconformity between basin sedimentary rocks and basement metamorphic rocks.

Uranium deposits occurring at or near an unconformity between basin sedimentary rocks and basement metamorphic rocks.

Uranium deposits occurring at or near an unconformity between basin sedimentary rocks and basement metamorphic rocks.

Uranium is highly reactive and will deposit and accumulate in response to the changing chemical environment – particularly when moved from an oxidising environment to a reducing one. As the fluids move through the basin and basement rocks, they convert uranium to a highly oxidised state.. Eventually this fluid reaches an area which is less oxidative such as graphitic or volcanic rocks. This causes the oxidized fluid to become reduced and the uranium it’s carrying will be deposited as either uraninite or coffinite. These deposits can occur in the basin rocks, at the unconformity itself or even lower down in the basement rocks .

The most notable examples of these types of deposits include:

  • in Canada’s Athabasca basin and
  • Australia’s Kombolgie basin.
  • Canada’s Thelon basin

The Athabasca basin deposits grade about 2% U, with exceptionally high-grade areas such as Cigar Lake (almost 20% uranium). The Australian Kombolgie basin. average grade is lower at 0.4%.

Unconformity deposits are desirable because of their high grades and because they generally occur at shallow depths, allowing for low-cost mining techniques such as open pit.

Paleao-Rollfront at Dead Tree Creek, Mt. Painter Gebiet, South Australia

Paleao-Rollfront at Dead Tree Creek, Mt. Painter Gebiet, South Australia Image CC

Sandstone Uranium Deposits

Sandstone-hosted uranium deposits occur in sandstones that are contained above and below  impermeable shale units. The permeable sandstone allows fluids to move freely between the impermeable shale units. Sometimes the sandstone units are associated with concentrations of organic matter, sulphides or hydrocarbons. These impurities create the reducing environment with  the circulating uranium-rich fluids, to form the uranium ore bodies. Uranium mineralization, in the form of uraninite, coffinite and sometimes brannerite, forms along changes in the sandstone chemistry..

These deposits only occur in rocks that are around 400 million years old. These deposits are associated with the Carboniferous Period when the evolution of land plants, resulted in large amounts of organic matter (a reducing agent) to be incorporated into sediments. Exploration targets for sandstone uranium deposits therefore focus on appropriated aged formations which rich in organics and hydro-carbons as well as shale beds. There are three main structures to sandstone-hosted uranium deposits. They are:

  • roll front: deposits are arc-like and crosscut the sandstone bedding;.
  • Tabular: ore bodies occur parallel to the bedding of the sandstone
  • tectonological.: occur along fault zones that allow fluid to flow freely.
 The three main types of sandstone deposits (in black): Roll Front, Tabular and Tectonological.

The three main types of sandstone deposits (in black): Roll Front, Tabular and Tectonological.

Sandstone deposits usually have low to medium grades that range from 0.01 – 0.4% uranium. Ore bodies are also only a small to medium size, but it’s the fact that most of these deposits can be mined by in situ leaching that makes them highly prospective. This is because the in situ leaching technique can be carried out at low cost

Important Uranium Deposits

The countries with the largest reserves of uranium are Australia (with around a third of the world’s known reserves), Kazakhstan, and Canada.

Cigar Lake Mine, Athabasca Basin, Saskatchewan, Canada

Not yet in production, after flooding issues, Cigar Lake has reserves of over 500,000 tonnes of 17% uranium. The mine was recently granted a mining licence and production is planned to start in 2013. Underground mining will use a high-pressure water jets to carve the ore into a slurry, allowing mining to occur with minimal exposure of personnel to radiation.

McArthur River Uranium mine, Canada Image: CC

McArthur River Uranium mine, Canada Image: CC

McArthur River Uranium Mine, Saskatchewan, Canada

This unconformity style deposit is the world’s largest producing uranium mine, has a regulated production of 18.7 million pounds of yellowcake/year.  Current reserves are 870,000 tonnes of 17% uranium oxide. This mine is also owned and operated by Cameco.

As reserves of fossil fuels decrease and concerns over CO2 emissions grow, it is likely that nuclear power will take on a greater roll in providing our world’s ever increasing need for power. As uranium prices increase and so too does exploration and production. Familiarizing yourself with the main types, and most prospective, of uranium deposits will allow you to make the right decisions when investing in Uranium prospects.

Companies Mentioned

Cameco – McArthur River Uranium Mine

About Staff @ Geology for Investors

Geology for Investors seeks to demystify mineral exploration and mining projects for mining company investors. All of our writers and contributors are experienced and educated in geology and the geosciences.