| Xiangshan uranium ore field is the largest and richest volcanic-type uranium ore field in China.Predecessors conducted in-depth studies on the metallogenic mechanism of the rich and large ore bodies in the Xiangshan uranium ore field through petrographic observation,electron probe analysis,major and trace element analysis,fluid inclusions and other methods.But in the study of fluid inclusions,Most of them chose fluid inclusions in gangue minerals with good crystallization and high transparency for research.These gangue minerals are generally the products of crystallization in the late or post mineralization period,and the results are difficult to accurately reflect the information of ore-forming fluids.In view of this,apatite and its fine crystal transparent minerals,which are closely associated with uranium minerals,are selected from the ultra-rich ore(U>3%,the same below)of Zoujiashan uranium deposit.On the basis of petrographic observation and major elements analysis,electron microprope analysis,SEM-EDS analysis,fluid inclusion temperature measurement,Laser Raman Spectroscopy technologies and stable isotope researches are carried out to discuss the characteristics of ore-forming fluids and to make further research the discovery of the ore-forming mechanism of rich and large uranium deposits in volcanic hydrothermal uranium deposits.The main achievements are as follows:(1)Brannerite,uranium and coffinite are the main uranium minerals in the ultra-rich uranium ore,and the associated minerals are pyrite,potash feldspar,mica and apatite.Geochemical analysis shows that the U content of ultra-rich uranium ore is 3.14%?11.45%,P2O5 is 1.06%?7.066%,and U is positively correlated with P2O5.The U content of lean Uranium ore is 0.024%?0.047%,and the P2O5 content is 1.49%?2.11%.The contents of U and P2O5 in ultra-rich uranium ore are obviously higher than those in lean uranium ore.(2)There are two types of apatites in ultra-rich uranium ore,and their microscopic characteristics are obviously different.One is microcrystalline apatite(Ap1)with inclusions of black minerals,which has poor crystal shape,irregular shape,small particle size,light yellow or dark gray,poor transparency,particle like,wormlike and so on.It is obviously associated with brannerite,so it should be formed in main ore-forming stage.Microcrystalline quartz and purple black fine crystal fluorite are associated with microcrystalline apatite,and both are formed in main ore-forming stage;The another one is the medium coarse crystalline apatite(Ap2),which has high degree of automorphism,regular shape,hexagonal column shape and large particle size.It grows around brannerite or uranium and it should be formed in late ore-forming stage.(3)Characteristics of ore-forming fluid in Zoujiashan uranium deposit:(1)In main ore-forming stage,the homogenization temperature of fluid inclusions is concentrated in 270??330?,and the salinity range is 5.11%?8.95%Na Cleqv.The homogenization temperature of fluid inclusions is concentrated in 180??220?,the salinity range is 4.96%?9.47%Na Cleqv in the late ore-forming stage,and the lowest mineralization temperature is above 180?.(2)The average density of main ore-forming stage fluid is 0.79g/cm3,Forming pressure is271.06×105Pa,mineralization depth is 0.9km;The average density of late ore-forming stage fluid is 0.91g/cm3,Forming pressure is 188.25×105Pa,mineralization depth is 0.63km.In the main ore-forming stage,the metallogenic pressure is higher and the metallogenic depth is deeper,while in the late ore-forming stage,the metallogenic pressure is lower and the metallogenic depth is shallower,which indicates that the Zoujiashan deposit may be eroded or uplifted in the later stage.(3)Laser Raman measurement shows that the gas composition of inclusions in purple black fine crystal fluorite in Zoujiashan uranium deposit is mainly H2,and that of inclusions in microcrystalline quartz is mainly O2 and CO2,which indicates that the gas of the ore-forming fluids is mainly H2 and O2,and a small amount of CO2.The ore-forming fluid in the early stage of mineralization may be H2 rich reducing fluid,while in the late ore-forming stage,the oxygen fugacity increases,which may be more O2 rich oxidizing fluid.(4)By the study of the hydrogen and oxygen isotopic,we can draw the conclusion that Ore-forming fluids in the main ore-forming stage of Zoujiashan uranium deposit is dominated by magmatic water,while meteoric water is mixed in the late ore-forming stage.From the main ore-forming stage to the late ore-forming stage,the ore-forming fluid evolved from magmatic water to meteoric water.The study of sulfur isotopic shows that the?11B value of Zoujiashan uranium deposit varies from-38.29‰?-17.12‰,which indicates that the minerals of Zoujiashan uranium deposit are mainly derived from the earth's crust and may involve a small amount of mantle material.(4)Based on the study of fluid inclusions and previous studies,it is considered that the rich large uranium deposit is formed by the superposition of two ore-forming stages.In main ore-forming stage,the ore-forming fluid is H2 rich reducing fluid with medium(high)temperature,medium and low salinity.In the reducing environment,UO22+is reduced and precipitated to form uranium minerals.In the late ore-forming stage,the ore-forming fluid is oxidized fluid with medium low temperature and low salinity and rich in O2.The ore-forming fluid in the late ore-forming stage met the reducing ore-forming materials in the main ore-forming stage,which made the uranium activated and reprecipitated to form the rich ore-body. |
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