Unconformity-Related Uranium Deposit

Unconformity-Related Uranium Deposits

definition:

Unconformity [ŭn′kən-fôr′mĭ-tē]
A surface between successive strata representing a missing interval in the geologic record of time, produced either by an interruption in deposition or by the erosion of depositionally continuous strata followed by renewed deposition. An unconformity is a type of discontinuity. 
or:
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 may be the sandstones or some other features that allows the concentration of uranium. 
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 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 Image CC



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