| Uranium is an important mineral resource for a country's economy and national defense.According to the International Atomic Energy Agency?IAEA?,the sandstone-type uranium deposit resources rank the second among all the discovered uranium resource types in the world.Recently,a series of sandstone-type uranium deposits have been discovered in Ordos Basin,which has greatly solved the problem of shortage of uranium resources in China.However,there are still hot debates about enrichment,transportation and ore-forming mechanisms of sandstone-type uranium deposits in the Ordos Basin.In this study,we performed detail field observation,mineral alteration and geochemical analysis on the Shuanglong sandstone-type uranium deposit,combined with changes in global climate and environment to explain its ore forming mechanism in the Ordos Basin.The following conclusions are drawn:1.Complete analytical methods were established for tracking the sources of sedimentary debris and ore-forming materials,which can be extrapolated for all sandstone-type uranium deposits worldwide.This study indicates that the source of sandstone-type uranium deposits in the Shuanglong area may not be from the Qinling area in the south,but from the Alxa block in the northwest,which has strong uranium supplying capacity.2.By analyzing trace elements and S isotopes of different types of pyrite,pyrite grains show close relationship to uranium enrichment,which imply that the ore deposit is associated with hydrothermal process.The main components of fluid inclusions include:H2O,CO32-,HCO3-,and CH4.Carbonate hydrothermal fluid seems to be the migration agent of uranium.Compiling previous research data,H and O isotopes of fluid inclusions indicate that the source of hydrothermal fluid is the mixture of atmospheric water and late magma hydrothermal fluid.Microscopic temperature measurement indicates that the hydrothermal fluid is range from 80 to 200°C.Therefore,low temperature and high oxygen fugacity calcite was formed by high oxygen fugacity atmospheric water mixed with late magma hydrothermal fluid.The low temperature hydrothermal fluid oxidizes UO22+to[(UO2?CO3?3]4-and promote these U ore-forming materials migration.3.The C and O isotopic characteristics of calcite?-12.4-8.3‰and 13.0-15.1‰?and the evidence of hydrocarbon inclusions?e.g.,CH4?indicate that the uranium-riched carbonate hydrothermal fluid is reduced by organic matter.?1?The redox reaction of organic materials produce a large amount of CO2,which is beneficial to the activation of uranium;?2?The organic material and reaction products,such as CH4,NH3,H2,H2S and other gases all can reduce uranium to UO2;?3?Organic material has a great adsorption capacity to uranium,which can provide a source of material for slow reactions;?4?After the hydrothermal fluid stops,the organic matter is still discharged continuously,which can provide a reducing environment for the mineralization of uranium even under the conditions of strong surface oxidation,forming a natural barrier for uranium mineralization.4.During the formation of uranium deposits,due to the effects of multiple fluids such as atmospheric water,hydrothermal fluids,organic matter,and groundwater,the migration and differentiation of elements in the oxidized zone,transition zone,and reduction zone result in new mineral formation.These changes have directly or indirectly played a role in the formation of uranium deposit.5.Global statistics found that mineralization times of sandstone-type uranium deposits were closely related to Global Oxidation and mass extinction events.Before the 2.3 Ga,uranium could not be migrated due to low oxygen concentration.After 2.3Ga,the O2 content in the atmosphere can activate the uranium.At this time,the main factors controlling the sandstone-type uranium mine are changed into reducing conditions.Before 500 Ma,due to the weak life activities on the earth,it is difficult to cause regional reduction on the earth,so there are few sandstone-type uranium deposits at this stage.After 500 Ma,life activities on the earth entered in a prosperous period,and the organic matter content increased greatly,especially several biological extinction events,leading to large-scale settlement of the Earth's biological remains,and the biological extinction caused the oxygen content in the water to decrease which provides great environmental conditions for the precipitation of uranium,leading to large-scale mineralization of uranium during this period.Since the middle Mesozoic,Alxa rocks with high uranium background could provided the uranium source for the Ordos Basin.During this period,the high oxygen level and arid environment are responsible for the activation of uranium.Due to late Mesozoic Yanshan movement,deep carbonate hydrothermal fluid intruded the stratum along the faults,mixing with atmospheric precipitation to form a high oxygen fugitive low-temperature hydrothermal fluid,carrying uranium migration;In the late Triassic mass extinction event,a large number of biological remains were deposited in the stratum,forming extensive sedimentary organic materials.These organic materials migrated upwards along the structural fractures during the Cretaceous,which leads to uranium enrichmen.Hydrodynamics of surface water led to continuous mixing of uranium hydrothermal solution and organic materials,resulting in uranium precipitation,enrichment and mineralization in the strata of the Zhiluo Formation. |
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