According to the theory of hydrothermal uranium deposits, the formation mechanism of in-situ leachable sandstone-type uranium deposits can be summarized as follows: long-term stable uranium-bearing oxygen-bearing water forms good infiltration and migration in the aquifer, and through the redox action of reducing components in the osmotic layer, on the one hand, the oxidation zone continues to advance, on the other hand, it promotes the continuous enrichment of uranium, and finally mineralization (Dancev,1984; Luo et al.,1990; Wu, 2006). Sandstone type uranium deposits can be divided into interlayer oxidation zone type and phreatic oxidation zone type according to their genesis. Sandstone-type uranium deposits in interlayer oxidation zone refer to sandstone-type uranium deposits controlled by the leading edge of interlayer oxidation zone (transition zone) and related to interlayer oxidation of groundwater between two water-resisting layers along the direction of decreasing hydrodynamic gradient, such as Ili 5 1 1 and 5 12 deposits, Tuha Shihongtan deposit and Ordos Dongsheng deposit. Sandstone uranium deposits in phreatic oxidation zone are mostly ancient channel sandstone type. The genesis of this type of uranium deposit is mainly controlled by ancient channel sand body, phreatic oxidation or phreatic interlayer oxidation zone, and the genesis is complex. At present, such deposits are mainly found in Russia's Outer Urals, Western Siberia and Outer Baikal, as well as Mongolia and Ukraine. According to the types of mineralization, the sandstone-type uranium deposits in the ancient river channel can be divided into two types (Chen, 2002): basement type-uranium deposits cut into ancient valley sediments or the bottom of weathering crust on the folded basement or crystalline basement under erosion; Inter-structural type-ancient river channel is located on the sedimentary discontinuity between sedimentary caprocks in the basin, which is often found at the bottom of sedimentary subcycle, so it can be seen that it washes and cuts the underlying mudstone.

For exogenous sandstone-type uranium deposits, whether it is interlayer oxidation zone type or phreatic oxidation (ancient river channel) type, the uranium mineralization process begins with weathering (providing uranium source), and then goes through three stages: migration-precipitation-mineralization; They are all formed in ore-bearing strata by water infiltration and oxidation under the condition of hydrodynamic action, with uranium activation, migration and precipitation enrichment as the main line. Uranium mineralization is characterized by oxidation zone and uranium-bearing zoning, and the formation time of surrounding rock and mineralization is obviously different. The ore bodies are mostly rolled, layered and lenticular.

Uranium in uranium-rich rocks in basement and erosion area is transformed from tetravalent uranium (U4+) to hexavalent uranium (U6+) by weathering and leaching. U6+ combines with oxygen-containing water to form uranyl ion UO2+, which usually migrates in groundwater in the form of uranyl complex. The water quality types are HCO3- type and HCO 3-SO24- type, mainly uranyl carbonate [under the hydrodynamic circulation mechanism,

In the oxygen-deficient part of the oxygen-containing uranium-containing fluid, that is, the front of the oxidation zone, it is the interface between the oxidized rock and the reduced rock, which is the geochemical barrier and the enriched part of precipitation of uranium. There are organic substances such as hydrogen sulfide (H2S), dispersed pyrite (FeS2) and carbon chips in uranium-bearing water, which can destroy and precipitate uranyl complex in uranium-bearing water, and combine iron and sulfur to form pyrite, which will coexist with uranium. In addition, uranium can also be filled in the cell cavity of organic matter. Uranium precipitates and concentrates in this area.

Paleochannel sandstone-type uranium deposits in Erlian basin

In this process, it is restricted by structure-sedimentation, lithology and lithofacies, thus forming uranium deposits of different genesis. For PCSTU deposition, it is the recharge water flowing into the ancient river channel laterally (valley slope) or along the river channel direction or vertically ("skylight") to the discharge water flowing out of the ancient river channel. The process is that under the hydrodynamic conditions of the ancient river, the infiltration water containing uranium and oxygen interacts with the ore-bearing sandstone to form structural-sedimentary evolution and lithology-rock.