Jan and 8 coauthors reported in late 2023 that:
- Detrital phosphate provides a primary U source in sandstone-hosted U deposits.
- Porous titania is specifically suited for U adsorption leached from phosphate.
- Trace-element signature in hematite is a reliable fingerprint in sandstone.
Economic U mineralization in sandstone was commonly attributed to oxygenated-fluid infiltration in reduced sandstone, leading to U precipitation in redox front during late-diagenetic stage. It was proposed that U can be pre-concentrated before this period based on trace amounts of U in altered products of Fe-Ti oxides, as revealed by electron microprobe analysis (EMPA). However, the obtained trace elements concentrations in altered Fe-Ti oxides (<1 wt%) is generally accompanied with relatively low analytical precision, leading to a skeptical explanation under this situation.
The authors used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine trace elements concentrations in hematite from the uraniferous Lower Yaojia Formation (Hailijin ore field, SW Songliao Basin, NE China), aiming to fingerprint this proposed U pre-concentration in details. Some trace elements contents, such as Mo, Ge, Ca, P, Y, Pb, Th, Sn, and W, are positively correlated with U content in hematite.
Given that uraniferous oxygenated groundwater with a near-neutral pH is unable to efficiently dissolve detrital phosphate (i.e., apatite and monazite) in the Lower Yaojia Formation, phosphorous must have been released from these P-rich phases before late diagenesis. Such a conclusion is supported by our observations that monazite from the reduced sandstone has a corrosive morphology, and exhibits a corresponding U enrichment in the corrosive rim.
The dissolved P and U can be adsorbed by porous titania up to several thousand ppm during early diagenesis, providing a pre-concentrated metal source for remobilization during late diagenesis. It can not only account for a trace-element signature derived from monazite (i.e., Ca, P, Y, Pb, and Th) in hematite geochemistry, but also fingerprint Mo and Ge migrations resulted from oxidation of pyrite and organic matter, respectively.
In addition to U, tungsten and Sn would be re-equilibrated with infiltrated oxidized fluid during the recrystallization of porous titania, and partially incorporated into simultaneously formed hematite grains due to high compatibility. The remobilization of pre-concentrated U induced by oxygenated fluid incursion led to U accumulation in a redox front during late diagenesis, forming uraninite nanoaggregates with relatively high concentrations of P, Y and Ti.
The authors concluded that trace-element signature in hematite from sandstone is a reliable fingerprint to reveal ore-forming processes, and they proposed for the first time that titania and phosphate play a crucial role for U pre-concentration in sandstone-hosted U deposits.
Original URL: https://www.sciencedirect.com/science/article/pii/S0169136823003773?via%3Dihub
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