Simultaneous precipitation of U4+ and U6+ in quartz-pebble conglomerate in the Singhbhum Craton, India.

Uranium is generally mobilized in solutions in U6+-complexes and gets precipitated as compounds containing the U4+ ion. Here we report hydrothermal remobilization of U, and the simultaneous precipitation of U6+ and U4+ in the Paleoproterozoic quartz-pebble conglomerate (QPC) from the Mankarchua basin, Singhbhum Craton, India. Uranium occurs associated with vein pyrite in the basal QPC of the Mankarchua basin, suggesting a clear hydrothermal origin of the mineralization. The pyrites occur linearly in veins and are replaced U-bearing minerals. Detrital monazites are the source of U. Newly formed fine-grained U-free monazites are observed along with crandallite series minerals (Ca,Ba,Sr,Pb)Al3(PO3.5(OH)0.5)(OH)6., suggesting alteration of detrital monazite by moderately oxidising acidic fluids. The remobilized U precipitates when it encounters pyrite and apatite. Brannerite (UTi2O6, containing U4+) forms concomitantly with the oxidation of pyrite, while autunite (Ca(UO₂)₂(PO₄)₂·10–12H₂O, containing U6+) forms by replacement of apatite. Thus, detrital or hydrothermal apatite enriched zones with or without pyrite may serve as U-traps for potential mineralization.