Due to its solubility in oxidising conditions, any uranium
mineral exposed at the Earths surface will undergo chemical
weathering, be dissolved and taken into solution in the surface and
groundwater.
Slide 4
We know that groundwater flows through aquifers. Buried fossil
river channels containing porous and permeable sandstones and
conglomerates make good aquifers. Buried fossil river channels
Slide 5
When the groundwater meets a change in conditions from
oxidising to reducing, uranium ore minerals are reprecipitated in
the palaeo- channel sediments.
Slide 6
Roll-front deposits cut across bedding planes. Uranium-bearing
ground waters precipitate uranium oxide minerals when they come
into contact with reducing conditions in porous and permeable
rocks. Curved roll-type uranium ore deposits form at the
oxidation-reduction (redox) boundary. Often the redox boundary is
at the water table.
Slide 7
In many sandstone hosted uranium ore deposits there is a close
association between the uranium ore and organic matter. It is
highly likely that sulphur- reducing bacteria present in decaying
vegetation are involved in the ore forming process. There are
examples of individual fossil logs that have been entirely replaced
by uranium minerals, making small but high grade ore deposits.
Uranium ore deposits of this type are found in the Colorado Plateau
areas of the western USA.