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Detrital uraninite bearing QPCs are restricted in pre-GOE Archean rocks (Cuney, 2010). U+4 state in these uraninites is highly unstable in surficial environment even at very low oxygen concentration. Uraniferous QPCs are preserved in strata below supergene profile. Our study from Mesoarchean siliciclastics of the Singhbhum craton India reveals significant variations in mineralogy, major and trace element composition from subsurface and supergene alteration levels. The QPCs from below the weathering profile still preserve detrital uraninite grains (Mukhopadhyay et al., 2016). In contrast the QPCs from the weathering profile mostly contain U in Ti-bearing phases such as brannerite and leucoxene. Uraniferous QPCs from below weathering profile show enrichment of Cr, Ni, Cu, Zn, LREE, Y, U, Th, Pb, depletion or low variation of V, Co, Rb, Ba, Sc, Sr, Cs, Eu anomaly, and flat HREE pattern with respect to non-uraniferous QPCs. Elemental correlation between U-Th with respect to other elements show positive correlation with Fe2O3, Sc, Co, Ni, Y, Ta, Pb and HREE and negative correlation with SiO2, Al2O3, MgO, MnO, Na2O, K2O, Zr, Hf, Rb, Ga, Cs, Zn and LREE. It is obvious that in surface samples within supergene weathering profile, chance of survival of uraninite is very rare. The correlation of other elements with U and Th indicate positive values with MnO, TiO2, Zr, Ti, Hf, Sr, Ba and LREE and negative correlation with respect to SiO2, Al2O3, Na2O, V, Co and Ta. There exists slight negative or no correlation with respect to Fe2O3, K2O, Sc, Cr, Cu, Ga, Y, Nb, Rb, Pb and HREE within QPC samples from supergene profile. The enrichment of redox elements (Cr, Cu and Ni) and their positive correlation with U-Th clearly point to the reduced nature of the uraniferous QPCs from subsurface beyond the supergene profile. The redox-sensitive trace elements indicate the presence of detrital pyrites. On the other hand the QPCs from the supergene zone show definite U mobilization and complexation mainly with Ti. Such mobilization may have taken place during diagenesis or during surficial oxidative weathering at low atmospheric oxygen level below the threshold of complete oxidation of U+6 state. A low positive correlation of U with redox sensitive elements (V, Cr, and Cu) may indicate that U- here is not associated with redox sulphides. U also reveals positive correlation with LREEs indicating preferential U-concentration in supergene phosphate phases. The elemental correlations of U with Ti and LREE are likely to suggest that the U-concentration in QPCs took place in the diagenetic to supergene weathering profile through alteration of Ti-bearing phases (Pronto Reaction) and phosphates. Such signature may be important in understanding hidden redox U-paleoplacers below supergene weathering profile. ReferencesCuney, M., 2010. Evolution of Uranium Fractionation Processes through Time: Driving the Secular Variation of Uranium Deposit Types. Economic Geology, 105, 553–569.Mukhopadhyay, J., Mishra, B., Chakrabarti, Kalyan., De, S., and Ghosh, G., 2016. Uraniferous paleoplacers of the Mesoarchean Mahagiri Quartzite, Singhbhum craton, India: Depositional controls, nature and source of >3.0 Ga detrital uraninites. Ore Geology Reviews, 72, 1290-1306. [ABSTRACT FROM AUTHOR]

The quartz-pebble conglomerate (QPC)-hosted detrital uranium mineralization is unique in character in terms of their restricted distribution before 2.2 Ga atmosphere during pre-Great Oxidation Event (pre-GOE). Such QPC paleoplacer deposits over the world are good targets for moderate to high tonnage and low grade uranium deposits and more importantly for their gold content. The Mahagiri Quartzite, dated c. 3.02 Ga for their youngest detrital zircon population, is developed unconformably over the Mesoarchean Singhbhum Granite (3.44 Ga to 3.1 Ga). The Mahagiri Quartzite includes a conglomerate-pebbly sandstone dominated subaerial alluvial fan to coastal braided plain sequence in the lower parts and shallow marine mature quartz arenite in the upper parts. The alluvial fan-braided plain deposits in the lower parts host a number of pyritiferous and uraniferous conglomerate and pebbly sandstone beds. The uraninite grains are rounded to subrounded in outline suggesting mechanical transport and detrital origin. Together with detrital pyrite and uraninite constitute the example of > 3.0 Ga paleoplacer closely comparable to the Witwatersrand Au–U deposits. EPMA and SEM-EDS studies suggest that the uraninite grains are rich in Th (> 4 wt.%), S and REE-Y. Chemical formula calculations from EPMA analyses suggest uraninite grains belong to two populations with different oxidation states as revealed from Y/REE and cation U 4 + : U 6 + [apfu] ratios. The U contents of the detrital uraninite grains from Mahagiri are significantly lower than that of the ideal stoichiometric composition of UO 2 . This is mainly due to higher amount of heterovalent cationic substitution by Th, REE, Y, Pb, and Ca in Mahagiri QPC uraninite structures, and partial alteration and metamictization of uraninites. Alteration due to metamictization resulted in elevated concentration of Si, Al, P, and Ca in more altered and metamict uraninite grains. The REE pattern is typically flat with comparable LREE–HREE concentration. The high Th content flat REE-pattern suggests that the uraninitere presents high temperature phases (> 350 °C) and are magmatic in origin. The Mahagiri detrital uraninite grains suggest existence of highly felsic and K-rich (richer than TTG) granodiorite–granite–monzogranite suites (GGM) of rocks older than 3.1 Ga in the Singhbhum craton. [ABSTRACT FROM AUTHOR]