Mechanism of U-reduction and concentration by sulphides at sandstone type uranium deposits

The concentration of uranium at the redox front of sandstone type U-deposits are commonly interpreted as a result of the reduction of U6+ in ground water to U4+ to form uranium minerals and their precipitation, and reductants of uranium are considered to be carbonized plant debris remained in sandstone and sulphides, of them, particularly the pyrite. Having reviewed a large number of relevant literature in detail, combined with some research achievements at sandstone type uranium deposits in China and abroad. The authors try to make an overview of the role of sulphides, particularly the pyrite, in the reduction, precipitation and concentration of uranium at the redox front of sandstone type uranium deposits. Experimental data and geologic observation of the ore-concentrated zone at the redox front indicate that pyrite could be a good reductant of U6+ only in case when the medium is lacking of free oxygen, and the reduction mechanism of pyrite lies in the formation of H2S as a result of the reaction between FeS2 and H2O. However, the FeS is a stronger reductant as the reaction between FeS and H2O could produce H2 which is more effective reductant of U6+. The commonly observed phenomenon- uranium minerals (UO2 and USiO4) which precipitate around newly-formed pyrite grains in many cases shows a precipitation sequence of the Eh value or pH value dropping, and the pyrite is always formed earlier than uranium minerals. (authors)