Metallogenic Characteristics And Genesis Of Qianjiadian Sandstone-type Uranium Deposit In The Songliao Basin

The Songliao Basin,where multiple energy sources such as oil,gas,coal,and uranium coexist,is a large Mesozoic and Cenozoic sedimentary basin in China.It is also a important basin for the exploration of the sandstone-type uranium deposits.The Qianjiadian uranium deposit is located in the southwest of the Songliao Basin and is the first super-large sandstone-type uranium deposit in eastern China.Many research work have done to the Qianjiadian uranium deposit in recent years,so there are many understandings and achievements in its metallogenic background,uranium mineralization characteristics,metallogenic conditions,and ore-controlling factors.However,there are still different opinions on the mineralization age,the occurrence of uranium,the source of ore-forming minerals and other aspects related to metallogenic regularity,which affects and restricts the practice of prospecting and exploration.Previous dating studies used the traditional whole-rock isochron method,and the results were greatly affected by many factors such as open conditions.So this article uses the LA-ICP-MS in-situ U-Pb dating method,a new dating method,to research the metallogenic age of uranium,and combined it with the geological background of the area to divide the metallogenic stage,which provided new information for the understanding of the mineralized age of the deposit.At the same time,in view of the sources of minerals,this study uses Pb isotopes to identify the mineralization-related and unrelated pyrite,and compare the in-situ S isotope geochemical characteristics of pyrite in the ore and the basic dyke.Combined with the occurrence of uranium in the ore and other characteristics,this article reveals provenance and the genesis of the Qianjiadian sandstone-type uranium deposit.These understandings may provide a new scientific theoretical basis for prospecting and exploration of sandstone-type uranium deposit.The occurrence of uranium in Qianjiadian uranium deposit is mainly adsorption state,including water soluble form,carbonate bound form and Fe-Mn oxides bound form,accounting for 73.5%,and the carbonate bound form contains the most.The content of independent uranium minerals is relatively small,mainly residual state and pyrite bound form,accounting for 26.5%.Independent uranium minerals mainly include pitchblende,brannerite,and Ti-containing uranium minerals.The morphology of pitchblende is different from that of brannerite and Ti-containing uranium minerals.Most pitchblende are mainly in close symbiosis with pyrite,occurring in the cracks or surface of pyrite;while brannerite and Ti-containing uranium minerals mostly occurred as mineral particles,it may be the result of metasomatism between uranium and Ti-containing minerals.Uranium has different distributions of valence states.When?(U⁴⁺)/?(U⁶⁺)<1,the content of U⁴⁺is less than that of U⁶⁺,indicating that there is relatively more uranium in the oxidation state.And the oxygen coefficient of uranium is relatively high,so uranium are easy to dissolve in crystal water,intergranular solution or liquid inclusions.When?(U⁴⁺)/?(U⁶⁺)>,the content of U⁴⁺is relatively high,and the oxygen coefficient is relatively low,so uranium is easy to precipitate and forms uranium minerals.LA-ICP-MS U-Pb isotopic dating mainly gets two ages of 0.179±0.025 Ma and1.408±0.060 Ma,which are the Pleistocene mineralization period.This result is quite different from the previous dating data of the whole rock dating,and the age is relatively young.The fission track ages of the basin show that there was a tectonic uplift activity in the Songliao Basin at the end of the Pliocene.This tectonic activation provided favorable metallogenic conditions for mineralization,which is corresponding to this metallogenic period,indicating that the results are reasonable.So there is a relatively concentrated mineralization period in Qianjiadian Uranium during the Pleistocene stage.Based on previous mineralization age studies,the metallogenic stages of the Qianjiadian uranium deposit can be divided into four phases:Late Cretaceous synsedimentary preconcentration period,Eocene mineralization period,Pliocene superimposed mineralization period,and Pleistocene superimposed mineralization and preservation period.The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb values of pyrite in the ore of the Qianjiadian uranium deposit are obviously high,which is caused by radioactive decay of high content U:U produces a large amount of ²⁰⁶Pb and ²⁰⁷Pb,so the pyrite in the ore is polluted.And this is the characteristic of the pyrite in the mineralization period.The S isotope data of pyrite during the mineralization period shows that its?³⁴S value is a large negative value,indicating that the S in the Qianjiadian uranium deposit is mainly derived from organic matter and bacterial sulfate reduction(BSR).There is just little coal dust in the Yaojia Formation,but it is rich in oil and gas in the research area,indicating that the formation of the Qianjiadian deposit is closely related to the role of oil and gas,and it is accompanied by microbial degradation to provide H₂S.Comparing the S isotope of pyrite in the mineralized sandstone and the diabase in the area,it is found that their data are quite different,so their source of S is obviously different.Therefore,it is believed that the diabase does not provide the uranium source for the Qianjiadian uranium deposit,but only plays a role of providing a heat source.The hydrothermal fluid causes a large amount of hydrothermal alteration in the Qianjiadian uranium deposit,which is beneficial to the superimposed mineralizatiion of uranium.Combined with the geological background of the Qianjiadian deposit and analyzing the research results of S,Pb,C,H,O isotopes of pyrite,carbonate and kaolinization in the area,this article comprehensively discusses the genesis of the Qianjiadian sandstone uranium deposits:The Qianjiadian uranium deposit is an interlayer oxidation zone sandstone-type uranium deposit of superimposed mineralization and multiple genesis.The formation of this deposit is the result of the superimposition of two ore-forming fluids and the superimposed mineralization of three sources of uranium.The two ore-forming fluids include“O&U-bearing supergene fluid”and“low-temperature hydrothermal oil and gas fluid”,and three sources of uranium include“synsedimentary preconcentration uranium”,“uanium reduced by O&U-bearing supergene fluid”and“uranium reduced by low-temperature hydrothermal oil and gas fluid”.