The Early Cretaceous hydrothermal sedimentation and its influence on sandstone-type uranium mineralization in the Xinniwusu Sag, Bayingobi Basin, NW China

Abstract

The presence of Early Cretaceous hydrothermal sedimentation was initially observed in the Xinniwusu Sag, Eastern Bayingobi Basin, NW China. To gain deeper insights into the features of hydrothermal sedimentation in this region and its relationship with uranium mineralization, we conducted an analysis encompassing petrography, geochemistry, the electron microprobe, and low-temperature thermochronology of the hydrothermal sedimentary rocks. Petrographic analysis revealed that the predominant hydrothermal sedimentary rocks in the upper member of the Bayingobi Formation within the Xinniwusu Sag encompass dolomitic limestone and sinter, followed by clastic rocks mixing with hydrothermal sedimentation. Major and trace element analyses emphasized the enrichment of elements including Ca, Mg, P, U, REEs, Sr, Y, Mo, Sb, Cd, and Pb in the hydrothermal sedimentary rocks. Notably, a prominent positive correlation was identified between U and P, REEs, Sr, and Y. Electron microprobe analyses illuminated that uranium primarily exists within fluorapatite in the form of adsorption or isomorphism. Isotopic analyses, δ¹³C_V-PDB and δ¹⁸O_V-PDB, of carbonate cements in clastic rocks and limestone yielded values ranging from −3 to 2.1 ‰ and − 17 to −6.9 ‰, respectively. These findings suggest that the carbon in the hydrothermal fluids predominantly originates from marine strata, with some potential contribution from the mantle. The paleotemperatures based on the O isotope data range from 67 °C to 78 °C, with an average of 71 °C, implying that the hydrothermal sedimentation corresponds to the continental low temperature white smoker type. Furthermore, apatite fission-tracks of grayish-white gritstone within the upper member of the Bayingobi Formation were completely annealed at 116 ± 7 Ma, partially annealed at 112 ± 4 Ma, also indicating that there was a hydrothermal sedimentary environment for a long time. The hydrothermal sedimentation predominantly influenced the uranium preconcentration in the upper member of the Bayingobi Formation. The uranium source in the hydrothermal fluids is likely linked to the Mesoproterozoic Zhaertai Group within the southern basement of the basin. This revelation offers valuable insights into the distribution of uranium deposits and mineralization points in the Bayingobi Basin, primarily concentrated in the southern sector. Consequently, these areas characterized by well-developed faults connecting the basement and the Lower Cretaceous Formation in the south of Bayingobi Basin are favorable areas for uranium mineralization. This study bears immense significance in enhancing our understanding of the uranium metallogenic mechanisms in the Bayingobi Basin and hydrothermal sedimentation mineralization widely distributed in the world.