The Geochemistry Characteristcs And The Occurrence Reason Of The U-rich Source Rocks In Chang 7 Member Of Yanchang Formation, Ordos Basin

At the bottom of the Chang7 member of Yan Chang Formation in Ordos basin,the source rocks, show obviours radioactive abnormal phenomenon which mainly caused by radioactive uranium element based on study. The U-rich source rocks developed in the deep lake that charactered by concentrating on collophane, frambodial pyrite, organic matter and so on in mineralogy.With the help of synthesis using many modern analysis and test technologies, including thin slice identification, scanning electron microscope, electron probe,energy spectrum analysis, elements geochemistry analysis, sequential chemical extraction,α-radiography photo and so on,we studied the geochemical characteristics of the source rocks at the bottom of Chang7 formation and the occurrence of uranium element detailedly.At the same time,we found the relationship between major and trace elements and uranium element.What is more,we always analyzed the occurrence condition of U-rich source rocks, the origin of uranium in the source rocks and the mechanism of its occurrence.We,firstly, found the independent uranium mineral in the U-rich source rocks in Chang7 member.Uranium can occurrent in collophane as independent mineral more than allomerism in collophane or absorbing in pyrite and organic matters.The independent mineral can divided into uranium stone,and titanium with their particle size between 2μm-14μm.The evidence indicates that it developed in the low to moderate temperature and reduced to strong reduced environment and the U-rich source rocks in Chang7 formation had influend by epithermal action. For the further study,the highest proportion of uranium in the collophane is 50%,the proportion of absorbing uranium in pyrite is nearly 20%.There is less than 10% uranium absorbing in organic matters and clay mineral by supposed.Integrating the recognition that there is the thin bed of ash tuff,evidences of activities of lots microbe and epithermal action in Chang7 formation,we hold that the origin and the occurrence of urament in Chang7 source rocks is the synthetical result of biogenicagency, volcanic action, hydrothermal fluid action at the bottom of the deep lake and so on.