Characterization Of Ore-forming Fluids And Uranium Mineralization Events In The Tamusu Sandstone Type Uranium Deposit,Bayingobi Basin,China

The genesis of sandstone-hosted uranium deposits is generally considered to be related to the interaction of oxidizing fluids,however,there are studies showing that hydrothermal fluids activity was recorded in some sandstone type uranium deposits.The late hydrothermal reformation superimposed on the early oxidation mineralization,and the uranium mineralization characteristics are still similar to those of typical interlayer oxidation zone type uranium deposits.So,the role of hydrothermal activity in the mineralization of sandstone type uranium deposits is still in hot debate,mainly due to the lacking of detailed study of the process of interaction.The hydrothermal activity is obvious in the Tamusu sandstone-type uranium deposit,and the ore-bearing sandstone is generally consolidated under hydrothermal transformation,which resulted in the process of superimposed transformation of hydrothermal fluids on the early interlayer oxidation zone mineralization being fixed.This paper will study the mechanism of uranium enrichment under the action of different fluids from the fluids activity characteristics and diagenetic and metallogenic events,in order to find out the relationship between fluid activity and uranium enrichment the deposit.In this paper,field geological survey and laboratory analysis and testing are combined.The nature,source and time limit of different types of fluids are determined by the assemblage of minerals,fluid inclusions,C,O,S isotope analysis.Diagenesis and occur sequence of altered minerals are identified,by microscopic identification,EMPA,scanning electron microscopy and XRD analysis,and then determine the time series of diagenetic events.The spatial and temporal changes of metallogenic environment will be reconstructed by studying the zoning features of altered mineral assemblages,trace elements analysis,and in-situ LA-ICP-MS of pyrite,and the determine the relationship between different types of fluids and uranium enrichment from material sources.Finally,the role of different types of fluids in uranium metallogenesis is comprehensively discussed on the basis of the above research from spatial,temporal and metallogenic relationships,to reveal the process of ore-forming fluids and explore the genesis of the deposit.The main stduies and results are as follows:1)The weak compaction and strong cementation are the main reasons for the compactness of sandstone in the Tamusu deposit.The cements are hematite,limonite,carbonate,gypsum and chlorite,and the main types of cementation are porous cementation.The recrystallization of pyrite and calcite is widespread,and a large number of dolomite and metal sulfide occur,indicating that thermal fluid modification is the main factor affecting reservoir physical properties.The diagenetic stage can be divided into four stages according to the production sequence of altered minerals: synsedimentary/early diagenetic stage,oxidation stage,hydrothermal alteration stage and post-ore weak oxidation transformation stage.2)The mineral assemblages,temperatures,trace elements and C-O-S isotopes indicate that the Tamusu deposit was affected by two distinct types of fluids: supergene diagenetic alkaline oxidizing fluids and acidic reducing hydrothermal fluids.The C-O-S stable isotopes indicate that the hydrothermal fluids may originate mainly from the formation water in the deep strata,which is related to the basalt eruption of the Suhongtu Formation in the late Early Cretaceous.3 The etched track and EMPA show that the main ore mineral in the Tamusu deposit are pitchblende,brannerite and coffinite.Pitchblende coexists with subhedral pyrite,whereas brannerite and coffinite coexist with cubic pyrite and calcite.And metallic sulfides are commonly found around the uranium minerals.Mainwhile,trace element analysis shows that the high-grade uranium ores are further enriched in Zn,Co,Ni and Cu.The REE patterns in the high-grade U-bearing sandstone are similar to those in the veins,suggesting that the high-grade U-bearing sandstone is related to hydrothermal activity.4)The temporal and spatial variation law of the metallogenic environment is restored by the analysis of altered minerals assemblage characteristics and mineral production sequence.It is considered that the oxidation-reduction transition site during the transition from weak alkaline to acidic environment is the key to uranium precipitation.And the roles of oxidizing fluids and hydrothermal fluids in uranium metallogenesis are evaluated from the orebodies spatial occurrence location,uranium minerals and metallogenic age.The oxidizing fluids provided the necessary conditions for uranium migration and were the basis for the formation of the large-scale sandstone-type uranium deposit.The involvement of hydrothermal fluids changed the pH and Eh conditions of the mineralization,superimposed and reformed the low-grade uranium ores formed in the early stage,and formed high-grade uranium ores.5)Based on the study of uranium source,paleoclimate,reductant and oxidation fluid,the types of uranium mineralization in Taliu deposit are analyzed.It is considered that large-scale oxidation fluid mineralization exists in the early stage of the deposit and superimposition and transformation of thermal fluid mineralization occurs in the late stage.Therefore,the Tamusu deposit experienced four main stages:(i)sedimentary and early diagenetic uranium concentration,(ii)oxidation and ore-forming stage,(iii)hydrothermal alteration and superimposed mineralization,and(iv)post-ore stage.The relationship between different types of fluids and uranium mineralization and their position in the process of uranium mineralization are the key factors that perplex the genetic interpretation of the Tarim sandstone-type uranium deposit,and also directly affect the establishment of ore-forming model and the selection of prospecting direction of the deposit.This paper reveals the characteristics of different types of fluids and their relationship with uranium enrichment from the microscopic,which provides abundant evidence for the interpretation of deposit genesis and the establishment of prospecting model.Moreover,the results also show that the pH and the Eh play an important role in controlling the precipitation and enrichment of uranium.So,in the future exploration,attention should be paid to the influence of the change of pH.The results are of great help and guidance to the exploration of the Talignin uranium deposit,and are also of great help to the study of the relationship between the late thermal fluid superimposition and transformation and sandstone-type uranium mineralization in other basins.