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(PO_3.5(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.