Uraninite, or UO2, is a radioactive mineral. In the presence of oxygen, uraninite is unstable and readily oxidizes to form UO3. In the presence of water, uranium trioxide goes on to readily form hydrates, essentially dissolving itself. In the 1950's, scientists at the Witwatersrand basin inferred that the substantial uraninite deposits in the basin were indicative that the Earth’s atmosphere had reducing properties (in the redox sense) during the Archaean eon because the grains appeared to be eroded by a stream. That is, uraninite present in an Archaean stream under modern, oxidative atmospheric conditions would have been dissolved and carried away, not deposited.
However, like many hypotheses regarding prehistory, this one is rife with challenges. There are a number of proposed problems with the so-called “placer hypothesis”, in part because of post-deposition uranium mobility. Factors such as aqueous carbonate presence can enhance movement of uraninite deposits, which raises skepticism about the significance of detrital indicators. The Witwatersrand uraninite placer hypothesis is significant for historical reasons, but current research has caused a number of scientists to question the soundness of the argument.

Page by Tory Grimm-Oropesa

Janeczek, J., and R. C. Ewing. 1993. Oxidation of Uraninite. Technical Report. Swedish Nuclear Fuel and Waste Management Co.