The Characteristics Of In-situ Stable Isotopes And Their Geological Significance About Sandstone Uranium Deposits In Northern Ordos Basin

Over the past decade,A series of large and super large sandstone uranium deposits have been found in the Dongsheng-Hang Jinqi banner area,Yimeng uplift,in northern Ordos Basin.Up to now,it has become the largest sandstone type uranium deposit in China.In the early stage,some scholars have done some work on the genesis of ore deposits from the perspective of geological characteristics.It is considered that the main feature of the deposit is that the ore body is controlled by the green alteration zone of the sandstone in the Zhi Luo formation.At the same time,the genesis of the ore deposits is studied from stable isotopes,with the metacarbonization of sandstone white,carbonated and pyrite mineralization associated with the associated ore layer and its adjacent areas.The shortcomings of this part of the work are that the testing of stable isotopes is a mass spectrometric analysis of all rock or mixed minerals,which can not distinguish the phases,types and properties of altered minerals,and the understanding of the results is limited to some extent.This work takes the North Hang Jin Qi Daying uranium mining area in the northern part of the basin as the object,taking uranium enrichment and pyrite mineralization,sandstone white mineralization,carbonation and other metallogenic phenomena as the link,abandoning the past whole rock or mixed single mineral isotopic analysis method,for the first time to apply the determination methods of the stable isotopes of the in-situ micro area?pyrite S,Pb,carbonate C,O,etc.?.The genesis characteristics and identification marks of major alteration of ore deposits.It is hopeful to identify the time and nature of altered minerals accurately,and it is a useful attempt to improve the testing accuracy and to understand the metallogenic regularity of sandstone uranium ore and the genesis of ore deposits more accurately,and the results are expected to provide new information,knowledge and scientific basis for the formation process and cause of formation of sandstone type uranium deposits in this area.The types,periods and causes of pyrite in sandstone uranium deposits are determined by the observation of mineral morphology under the pyrite electron microscope and the test of S and Pb isotopes by fs-LA-ICP-MS in situ microregion.In the metallogenic period,pyrite and uranium minerals are interspersed with each other,indicating that it is formed at the same time.Most of the gelatinous and strawberry pyrite in this area belong to this situation,and the delta 34 S is basically negative.The boundary between pyrite and uranium minerals is clear in diagenetic stage.Common uranium minerals grow around pyrite.Most massive and annular pyrites belong to this situation.The delta ³⁴S has a large negative value and a large positive value,and there is no definite rule.The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb of pyrite in the diagenetic stage are less affected by the downstream fluid,showing a smaller normal value.In the metallogenic period,pyrite is greatly influenced by the metallization fluid or mineralization,and its 206Pb/204 Pb and 207Pb/204 Pb can reach several times the normal value,which is obviously abnormal,and the distinguishing feature is very obvious.Therefore,combining morphological observation with S and Pb isotopic data can be more accurate in judging the type and stage of pyrite.The analysis of S and Pb isotopes in the situ microregion of pyrite fs-LA-ICP-MS shows that the metallogenic period and the late pyrite Delta ³⁴S have a larger negative value,indicating the characteristics of organic matter and microorganism.The comparison of the pyrite Delta ³⁴S in the metallogenic period with the organic sulfur isotopes in the basin is considered to be similar to the S isotopic characteristics of the upper Paleozoic source rocks,and the fluid mineralization is related to the dissipation of natural gas in the upper Paleozoic.The Pb isotopic characteristics of pyrite in diagenetic stage indicate that the source of the ore bearing layer is mainly from the Mount Daqing in the orogenic belt,which is closely related to the upper Paleozoic granites,and the metallogenic material is derived from the crust.The SIMS calcite C and O isotopic studies on epigenetic carbonation indicate that its formation mainly includes organic fluid and C source.O isotopes indicate that the fluid phase mainly comes from meteoric water.From the mineral type of uranium,we know that the fluid fluid has the characteristics of low temperature.After comparing the C isotopic values of the organic source areas in the basin,it is found that the C quality of the carbonation of the region is derived from the upper Paleozoic gas source rocks,not from the oil and gas source rocks of the Mesozoic petroleum,coal or lower Paleozoic,which are related to the dissipation of the upper Paleozoic sky gas.Using the work results of the stable isotopes of S,Pb,C and O in this micro area,combined with the study of H,O and Si isotopes of sandstone white and uranium mineralization,as well as the basic characteristics and geological background of the deposit,the formation and genetic characteristics of the sandstone uranium deposits in this area are synthetically analyzed.The uranium mineralized fluid is considered as the low-temperature hydrothermal solution of the atmospheric water as the main component,and contains the composition and function of the organic matter.The organic matter comes from the dissipation of natural gas in the upper Paleozoic of the basin on a large scale to the north.This mixed low temperature hydrothermal fluid containing natural gas and water is the key factor for the reduction and preservation of uranium mineralization and the accumulation and enrichment of uranium deposits in the area.This work provides evidence for the understanding of the basic genesis of sandstone uranium deposits in the northern part of the basin.