Hydrogeochemical Metallogenic Mechanism Of Sandstone-type Uranium Deposits In Yili Basin

The Yili Basin is one of the important uranium resource bases in China.A series of large sandstone-type uranium deposits were discovered in the southern of the basin,while only a few scattered small-scale uranium deposits or uranium mineralization points were found in the northern part of the basin.The different tectonic activity intensity and the different evolution characteristics of groundwater flow systems may be the important reasons for the differences in uranium metallogenic conditions in different structural parts of the basin.Therefore,the migration and transformation of Uranium-bearing fluids are investigated by comprehensively considering the coupling of hydrodynamic and hydrochemical processes from the perspective of basin groundwater system evolution,to reveal the hydrogeochemical mechanism of sandstone-type uranium ore-forming in Yili Basin.It is of great significance to deepen the understanding of uranium formation and to predict the spatial location of uranium deposits in Yili Basin.Based on the analysis of the regional geological data of the Yili Basin,this thesis systematically carried out outcrop geological surveys and rock,surface water and groundwater sampling work.Several methods such as rock radioactivity analysis,rock and hydrogeochemical analysis,and groundwater 3D numerical simulation,are used to analyze the uranium source conditions,hydrodynamic conditions and water chemical conditions of uranium mineralization in Yili Basin.Using the reactive flow and transport model,migration and transformation process of uranium in groundwater was simulated to reveale the hydro-geochemical mechanism of uranium mineralization.Then,uranium metallogenic models and favorable ore-forming targets are discussed by combining the characteristics of the tectonic and groundwater system evolution in the Yili Basin.The mainly results and understanding are listed as follows:1.The magmatic rocks,volcanic rocks and pyroclastic rocks in the source area provide a good source of uranium for the basin groundwater system.The analysis results of uranium content in rock and surface water from the source area indicate that both the southern and northern parts of the basin have good uranium source conditions.Ganite is an important source of uranium which has both high uranium content and high uranium precipitation capacity under the action of oxygenated water.Volcanic rocks and volcanic clastic rocks are also good sources of uranium which have general uranium content but stronger precipitation capacity.Sedimentary rocks and neutral magmatic rocks are poor uranium sources which have low uranium content.2.Multi-level groundwater system provide favorable hydrodynamic conditions for uranium mineralization.Several kilometers of sand-bearing coal-bearing rock series were continuously deposited during Mesozoic Era in the alternating evolution of river and lake sedimentary structures in the Yili basin,which provides the foundation for the formation of a multilayered confined water-bearing system.The Triassic-Jurassic confined aquifer system opened at the edge of the basin to accept the infiltration of uranium-containing oxygenated water in the piedmont.At the same time,the development of deep fault structures within the basin leads to the hydraulic connection between the deep confined aquifers and the unconfined aquifer system,and improves the groundwater circulation conditions.Moreover,it creates a premise for water circulation of the confined aquifer system,and forms a complete groundwater recharge-runoff-excretion system,which provides favorable hydrodynamic conditions for uranium mineralization.3.The complete oxidation-reduction environment system in the aquifer provides the necessary hydro-geochemical conditions for uranium mineralization.The characteristics of groundwater in the Yili Basin indicate that both sides of the Yili Basin develop complete oxidation-reduction environment systems and obtain the basic hydrochemical condition for uranium mineralization.From the recharge zone(oxidation zone)to the runoff zone(oxidation-reduction transition zone),the groundwater chemical characteristics are gradually changed from HCO3-Ca type water with low TDS and low or medium uranium content to HCO3·SO4-Na or SO4·Cl-Ca·Na type water with medium TDS and medium-high uranium content.As grounwater continuingly moves into the deep runoff-excretion zone(The reduction zone),it appears the SO4·Cl-Na·Ca or Cl ·SO4-Na·Ca type water with high TDS and low-medium uranium content.Under the control of oxidation-reduction environment,the groundwater circulation mode from “uranium goes with water” to “water leaving uranium depositing” determines the precipitation,migration and enrichment process of uranium in groundwater system.4.The circulation intensity of groundwater and the vertical heterogeneity of the aquifers control the scale of uranium mineralization.The results of groundwater flow simulations and uranium reactive and transport simulation show that the water exchange conditions in the southern part of the Yili basin are much stronger than those in the northern part of the basin,so the uranium metallogenic conditions in the south are better.Especially in the southwestern part of the basin,the hydrodynamic conditions of uranium mineralization are optimal.In vertical direction,the permeability of Badaowan Formation and Xishanyao Formation is strong,while the permeability of Sangonghe Formation and Xiaoquangou group is weak.It lays a foundation for the uneven distribution of ore-bearing fluids in the vertical direction.More oxygen-containing uranium-containing fluids mainly flow through the Badaowan Formation and the Xishanyao Formation,which is more conducive to the integration of minerals in these two horizons.5.Tectonic movement controls the characteristics of groundwater flow and uranium metallogenic scale in different parts of the basin.Since the opening of Triassic to Jurassic confined aquifers in the southern Yili Basin,the tectonic movement intensity is relatively weak.With stable groundwater flow and complete oxidation-reduction system,the uranium mineralization is stable and continuous.Uranium metallogenic conditions are good,especially in southwest basin with the most stable metallogenic conditions.The formation of folds in the southeast changes the groundwater flow characteristics and uranium metallogenic conditions,resulting in less metallogenic scale than the southwest.Under strong tectonic movement in the northern basin,the basin edge rises sharply and forms a fold belt,resulting in the uranium reservoirs and the early uranium deposits destroyed when uplifted to the surface.Only the southern slope of the fold belt developed the uranium ore-forming conditions,however,it is worse than those in the south basin due to dispersive and instable groundwater flow.