Roles of Multisourced Fluids in the Formation of Sandstone-Hosted Uranium Deposits in the SW Songliao Basin, NE China

The sandstone-hosted uranium deposits in the SW Songliao Basin differ from typical sandstone-hosted uranium deposits in terms of the geological features of the ore-deposits, including the geometry of the orebodies, mineral assemblage and petrography. Detailed drill core and microscopic observations, scanning electron microscopy (SEM), electron microprobe analysis (EMPA), heavy mineral concentrates, and fluid inclusion studies of the Upper Cretaceous Yaojia Formation, i. e., the uranium-bearing layer, were integrated to investigate the roles of hydrothermal fluids in the formation of these uranium deposits. We found that the kaolinite alteration is developed in the mineralized zones, but it is less common in the peripheral areas. The fluid inclusions are hydrothermal fluids with a medium-low temperature (67 to 179 degrees C) and a high salinity (5.9 wt.% to 20.1 wt.%). According to the analyses, three kinds of hydrothermal fluids, i.e., the acid fluid, the groundwater heated by the mafic magma, and the alkaline fluid rich in Ca2+ and CO32-, were identified. The fluids might have low U content, but they have participated in the formation of the uranium deposits successively. Kaolinite formed by the acid-hydrothermal fluid absorbed large amounts of uranium. Subsequently, the thermal energy from the hydrothermal fluids changed the intrastratal redox environment and increased the solubility of the uranium minerals in the fluid. The alkaline-hydrothermal fluid rich in Ca2+ and CO32- facilitated the formation of stable Ca-U(VI)-CO3 complex, which led to the enrichment of soluble uranium in solution, and final precipitation as pitchblende, brannerite and Ti-bearing uranium minerals in the uranium ores.

Automated summary

The sandstone-hosted uranium deposits in the SW Songliao Basin have different geological features compared to typical sandstone-hosted uranium deposits. Studies on the Upper Cretaceous Yaojia Formation, the uranium-bearing layer, were conducted using various analytical techniques to investigate the role of hydrothermal fluids in the formation of these deposits. The results showed the presence of kaolinite alteration and hydrothermal fluids with medium-low temperature and high salinity. Three types of hydrothermal fluids, including acid fluid, groundwater heated by mafic magma, and alkaline fluid rich in Ca2+ and CO32-, were identified. These fluids played successive roles in the formation of the uranium deposits, with kaolinite absorbing uranium, thermal energy changing the redox environment, and alkaline fluid facilitating the formation of soluble uranium complexes and the precipitation of uranium minerals.