Study On Secondary Oxidation Transformation Paleo-sandstone Type Uranium Deposit In Shenshangou-Sunjialiang Area,Dongsheng

Ordos Basin is the most important sandstone type uranium exploration and development base in North China.Sunjialiang paleo-sandstone type uranium deposit is located in the north east of Ordos Basin.There are different oxidation types,such as paleo-oxidation and secondary oxidation.There are different oxidation fluids and multi-stage uranium formation.It is an ideal and rare area to study fluid periods,metallogenic environment changes and uranium ore body transformation and location.The red paleo-oxidation zone of the uranium deposit was almost completely reduced and transformed in the later stage,and mainly manifested as green paleo-oxidation zone.The original red paleo-oxidation zone only existed in the form of residue.After the Cenozoic tectonic uplift to the surface and denudation,the secondary oxidized oxygen-containing uranium bearing water entered the paleo-sandstone type uranium deposit after the mineralization period,resulting in the re activation and migration of the original ore body,and the reprecipitation and enrichment of mineralization near the new oxidation-reduction transition surface.Therefore,the Sunjialiang paleo-sandstone type uranium deposit has the unique characteristics of secondary oxidation in China.Taking the paleo-sandstone uranium deposit in Shenshangou-Sunjialiang area as the research object,this paper inverses the evolution process of ore-forming fluid in different stages through petromineralogical and geochemical analysis,recovers the spatiotemporal evolution law of metallogenic environment change,discusses the transformation of paleo-sandstone type uranium deposit by secondary oxidation,establishes the identification mark of secondary oxidation comprehensively,and reveals the uranium metallogenic-recrystallization process in the four stages of transformation,the oxidation-reduction transformation plane uranium metallogenic model is innovatively constructed,and the prediction criteria for paleo-sandstone type uranium deposits after secondary oxidation transformation are established.The main achievements and understandings are as follows:1)The paleo-sandstone type uranium deposits can be further divided into primary paleo-sandstone type uranium deposits and secondary paleo-sandstone type uranium deposits according to whether the surface exposed at the later stage is subjected to post mineralization secondary oxidation transformation.The primary paleo-sandstone type uranium deposit refers to the paleo-sandstone type uranium deposit which has been completely concealed in the geochemical environment of reducing rocks since the formation of the uranium deposit and has never been exposed to new secondary oxidation.The secondary paleo-sandstone type uranium deposit refers to the paleo sandstone type uranium deposit in which the surface is exposed to new secondary oxidation due to tectonic uplift.The types of oxidation alteration can be divided into two types according to time:paleo-oxidation and secondary oxidation.The spatial distribution characteristics of different secondary oxidation alteration zones and their relationship with uranium mineralization are summarized.The secondary oxidation zone refers to the secondary oxidation zone since Neogene.2)The formation and evolution process of the Sunjialiang paleo-sandstone type uranium deposit are reconstructed,and it is pointed out that it is exposed to the surface under the influence of Neotectonic uplift in the east of Pliocene.Based on the analysis of tectonic evolution and paleo-hydrogeological characteristics of the study area,it is considered that the evolution of paleo-sandstone type uranium deposits in the study area can be further subdivided into the uplift sedimentary metallogenic stage from Middle Jurassic to early Cretaceous,the NW-SE extension stage from Late Cretaceous to Oligocene,and the uplift metallogenic transformation stage from Miocene to the East Paragraph.After Pliocene,the study area rapidly tilted westward and Dongshengliang took shape,resulting in Sunjialiang paleo-sandstone type uranium deposit outcropping.At this time,the paleocurrent direction of the uranium reservoir is nearly perpendicular to the groundwater flow direction of the main mineralization period,and the paleoclimatic conditions of continuous drought caused the new secondary oxidation transformation of the Sunjialiang paleo-sandstone type uranium deposit,resulting in the formation of a large amount of uranium mineralization spots on the surface of Shenshangou-zhungeerzhao area.3)A comprehensive indicator of secondary oxidation transformation was established.The paleo-oxidation zone is dominated by hematite,goethite and anatase during the metallogenic period,while limonite is the main secondary oxidation zone,and pyrite is relatively developed in the reduction zone.The content of clay minerals in the paleo-oxidation zone is characterized by high kaolinite and low smectite,and chlorite exists at the same time.The secondary oxidation zone is mainly characterized by high smectite and low kaolinite.There are not only spatial zoning,but also temporal transformation and evolution.From oxidation zone to reduction zone,the contents of organic carbon,total sulfur,low sulfur and redox potential increased gradually,while Fe³⁺/Fe²⁺decreased gradually.In particular,the content of organic carbon in the mineralized sandstone in the reduction zone is more than twice that in the secondary oxidized sandstone.The post mineralization secondary oxidized sandstone is the reoxidation product of the post mineralization secondary oxidized sandstone,which is characterized by high content of Si O₂,low content of Mn O and Ca O,and low content of Na₂O and Mg O.The metallogenic age of paleo-oxidized uranium is consistent with that of primary paleosandstone type uranium deposit.The metallogenic age of secondary oxidized uranium after metallogenic period is mainly Neogene metallogenic age and modern surface oxidized metallogenic age.The uranium rich ore in the curl head of the Sunjialiang paleo-sandstone type uranium deposit has obvious characteristics of partial uranium,and has the phenomenon of repeated development of partial uranium and partial radium in the vertical direction,reflecting the multi-stage transformation and enrichment of the original ore body by secondary oxidation.In plane,the development direction of equilibrium coefficient is from NE to SW,that is to say,the more southwestward the uranium is,the more uranium is,which indicates that the secondary oxidation transformation after mineralization is obvious.4)There are unique uranium enrichment in the study area,mainly including chemical reduction and mechanical adsorption.Reduction enrichment is mainly manifested in two aspects:one is related to solid reduction medium in uranium reservoir;the other is related to infiltration of reduction gas in later stage.Adsorption includes adsorption of organic matter and adsorption of uranium by clay minerals.A new uranium metallogenic model,oxidation-reduction transition surface uranium metallogenic model,has been established.The uranium mineralization can be divided into four stages:sedimentary diagenetic preconcentration stage,paleo-oxidation-reduction transition surface,ore preservation stage of reduction and secondary oxidation-reduction transition surface after mineralization.5)The prediction criteria for paleo-sandstone type uranium deposits after secondary oxidation are proposed,which will be of great significance for uranium exploration in Cenozoic strata with similar ore controlling genesis.The results show that the oxidation-reduction anomaly is the first metallogenic type,and the spatial distribution of the secondary oxidation-reduction anomaly is the first metallogenic type.There are several aspects that should be paid attention to when using the surface uranium anomaly information to accurately predict the deep prospecting:first,to judge whether it is the cause of enrichment of paleo-sandstone type uranium ore body after secondary oxidation transformation;second,to judge the ore bearing horizon;third,to verify the extension of deep uranium mineralization;fourth,to delineate the scale of surface uranium anomaly;fifth,to judge the source of uranium anomaly.This paper summarizes the methods of spatial location of oxidation-reduction transition surface:color location,petro mineralogy location,geochemical location and sedimentary genetic facies location.Of course,in order to improve the accuracy of spatial distribution of oxidation-reduction conversion surface,the above methods are best used in combination.The above achievements and understandings reveal the transformation mechanism of secondary oxidation on paleo-sandstone type uranium deposits,creatively establish the metallogenic model of secondary oxidation,and establish the prediction criteria for paleo-sandstone type uranium deposits after secondary oxidation.This study will provide a reference for uranium prospecting in Cenozoic strata with similar secondary oxidation-reduction conversion surface.The innovative results of this paper are as follows:1)The mineralogical and geochemical indicators of secondary oxidation transformation are comprehensively established,the enrichment mechanism of secondary oxidation transformation of Sunjialiang paleo-sandstone type uranium deposit is revealed,the uranium metallogenic model of oxidation-reduction conversion surface in the study area is established,and the metallogenic model of Dongsheng sandstone type uranium deposit is improved.2)The prediction criteria for paleo-sandstone type uranium deposits after secondary oxidation are established.