The migration pattern of oxidation fluids and its constraints on uranium mineralization in the Tamusu sandstone type uranium deposit of the Bayingobi Basin, China

Exploring the developmental stages and migration patterns of oxidation zones has always been a key task in the exploration of sandstone uranium deposits, but the involvement of deep fluid activity often makes it difficult to determine the characteristics of oxidizing fluids. The Tamusu deposit is a large sandstone-host uranium deposit, characterized of large scale oxidation zone, multiple layers of ore bodies, dense sandstone, and obvious hydrothermal alteration. However, the stage and mechanism of the occurrence of oxidative fluids are still unclear. Identifying the stages and migration patterns of oxidizing fluids and determining their relationship with uranium enrichment are of great significance for revealing the genesis of mineral deposits and exploring uranium deposits in other similar basins. In this study, core observation, microscopy, electron probe, scanning electron microscopy, cathodoluminescence, and α track etching were used to study the distribution of oxidation zones, the formation sequence of altered minerals, and uranium mineralogical characteristics. The oxidation zone occurs on a large scale in the upper member of the fan delta sand body, with almost all disappeared at the interface between sandstone and mudstone, and the oxidation fluid migration also occurs in many fractures of the target layer. The altered minerals in the Tamusu deposit mainly include hematite, limonite, dolomite, ankerite, gypsum, pyrite, and other metal sulfides. The uranium minerals in the ore are mainly pitchblende, some coffinite and UTi mixture，with a small amount of secondary uranium minerals. Uranium minerals are mainly distributed on the surface of debris particles, around pyrite or carbon debris, as well as in dissolution pores or growth zones of ankerite. The sedimentation and alteration of Tamusu sandstone can be divided into 4 stages: sedimentation to early diagenesis stage, early large-scale oxidation stage, thermal fluid alteration stage, and late weak oxidation stage. The sandstone with excellent pores in the early diagenetic stage and the unobstructed groundwater recharge, runoff, and drainage system are the main reasons for the formation of large-scale oxidation. At the end of the Early Cretaceous, hydrothermal activity formed a large amount of dolomite, ankerite, gypsum, etc., which solidified the sandstone and terminated the migration of oxidizing fluids in the pores of the sandstone. After the consolidation of sandstone, the oxidized fluid mainly migrated along the fractures in the formation, or also transformed some early consolidated sand bodies near the edge of the basin, and the oxidation scale significantly weakened. Therefore, the Tamusu deposit experienced three main stages: Large scale oxidation and low-grade U formation stage，hydrothermal alteration and U mineralization and late oxidation weak uranium mineralization stage.