Cenozoic Basins In Northwest China Sandstone-type Uranium Geology And Mineralization,

Through study emphasisly from epigenetic alteration and fluid geochemistry, the characteristic of organic geologic process in the formation of sandstone type uranium deposit, and combining with the mineralization of its dynamic evolution, dating in mineralization process , the evolvement of paleo-climate environment, the substantial source of ore-forming event, the ways of uranium migration and its mechanism of sedimentation, this paper studies and contrasts organic and inorganic geologic metallogeny and their evolvement of typical sandstone type uranium deposit (Ordos, Turpan-Hami , Yili basin) in the Northwest area in China Based on this, the author summarized the evolvement feature of metallogeny of sandstone type uranium deposit and their types, holded up six patterns of the evolvement of basins which are the main ones that produces sandstone type uranium deposit in the world, stated some of the author's own opinions and new apprehend on the Metallogenic theory of hydatogenic uranium deposit and the means and ideas of ore finding.For the fluid geologic process, from the qualitative and quantitative analysis, the author revealed the liquid ingredient and its physical and chemical property and the C, H, O, S isotope of basement rock, the time of diagenesis, oxidation, bleaching and green alteration, mineralizing, autogenetic rock (zone of reduction ) by using alteration mineralogy and inclusion geochemistry; and consider that the metallogenic fluid is not from deep-seated basement rock , but superficial and normal temperature atmospheric precipitation; but the gas organic matter from deep is of importance to bleaching alteration, green alteration and metallization of uranium in Dongsheng, and the main cause of metallogenic fluid having the property of thermal fluid of low temperature. The degree of mineralization and temperature of fluid in the diagenesis time are higher,and its acidity and alkalinity and Eh are in the state of modification;and the reduction process of reducting SO42" for the sulfate reduction bacterial is the important cause of the form of H₂S and the pyrite.The content of uranium and organic matter is lower in the oxidizing liquid,and the temperature and degree of mineralization are also lower.The ratio of Th/U and Ra/U of the oxidized rocks reach their big peak.Bleaching alteration fluid is reductive and highly acidic,and the ratio of Th/U and totalcontent of hydrocarbon and kaolinite in the bleachead rocks are the highest,but the content of its orgc and ï¿¡Fe is clearly lower. In metallization alteration phase, the content of 5Ds 518Ch U, Orgc> Mo, Re is the highest in fluid or in the alteration rocks, and the fluid property is (neutral^alkalescent) — (faintly reductive—reductive) . green alteration fluid is highly reductive and alkal, in this phase the values of Temperature> PH all reach their own big peak .There are no phases clearly of bleaching and green alteration in the deposits of Turpan-Hami and Yili deposits, it demonstrates that compared with these two areas , metallogenic fluid had reached highly reductive phase in Dongsheng. It is also the big difference between Dongsheng and the others (Turpan-Hami , Yili ore area).In the feature of organic geologic process, the influence that gas organic matter to uranium deposit is the the most important in Dongsheng deposit, solid organic matter take second place;In Yili deposits, the influence of biological bacteria is the most obviously,that of gas organic matter second; In Turpan-Hami, the three influences mentioned above jointly determine its feature.The solid organic matter in Turpan-Hami and Yili ore area is maimly vitrain;but it cantains about percent two to ten of asphaltene in the Dongsheng ore area.The type of organic-carbon is humus.but it tends to the type of sapropel in Yili ore area.The organic matter originates from higher plants of land facies.The formation enviroment of organic-carbon is in the process of diagenesis in Turpan-Hami and Yili ore area.but it is in the process of later phase of diagenesis(that is the phase of forming the gas-oil).The Fulvic acids and Humic acids that are decomposed into from organic matter are the main vehicle for the migration of uranium,and uranium is precipitated and forms rich ores by reductive little molecule chemical compounds.The wide carbonation is formed by decarbonation of the organic matter,but it is also affected by coal gas in Dongsheng ore area.The feature that the consistency of times indicated by the four geologic events that later stage reformation and uplift, dissipation injection of the gas organic matter , metallogeny of uranium deposit and dry-hot aero-climate in basin, show that migration and injection of oil and gas caused by geodynamical action formed highly reductive circumstance and offered needed reducer for precipitation reaction of uranium. So, the metallogeny of sandstone type uranium deposit doesn't occur isolatedly, it is the result of organic and inorganic interaction in the united dynamical setting. Therefore, the trap structure(neotectonic folds, faults and lithologic-lithofacies interfacies etc.) is favorable condition of metallogeny. The direction of dissipation of gas organic matter is advantageous for the formation of sandstone type uranium deposit.Alterting the dynamical mechanism of moving rate and direction of underground water,and forming dissipation injection of gas organic matter along the faults,neotectonic folds and faults(cover faults and reviving basement faults) are the main ore-holding tectonic in Turpan-Hami and Yili ore area.But in Dongsheng ore area,the reving basement faults are the main ore-holding tectonec.The migration of gas organec matter upward and in the direction of north made by later reformation and uplift in the eastern area in Ordos basin on time of J2-3, K2-E1 and Ni is the important factor forming bleaching alteration and the "orebody of plate shape". The migration of gas organic matter along the data in the direction of east made by later reformation and uplift in the western area on time of J3-Ki,K2-Eiand Ni is the main factor forming thegreen alteration and the "similar roll orebody" in Dongsheng uranium deposit.About the source of metallogenic substance. One side, the ore-holding strata itself have high value of uranium, in addition to the high Th/U ratio caused by oxidizing and bleaching action, all these show that the two alterations offer substance for metallogeny of uranium deposit; the other, volcanic rock and granite also have severe depletion of uranium in denuded area, show that they are one of source of metallogenic substance. In conclusion, for metallogeny of sandstone type uranium deposit, the ore-holding strata itself and volcanic rock and granite in denuded area are all the important source of metallogenic substance.In Dongsheng , Turpan-Hami and Yili area, the uranium migrates and occurs in U02 (CO3) /\U02 (C03) 2' ?. uranium organic complex and U02 (OH) 2H2O .When the Eh value fall, it react with reducer such as H2s H2S> C0> CHun gas organic matter ,and then uraninite and bilibinite come into being. In addition, the adsorption by organic matter .. clay and clastic debris to uranium have great significance to precipitation of uranium.According to the feature of organic and inorganic action of metallogeny of uranium deposit and dynamical evolvement of the basin, the basin models that uranium mine occur are devided . The main sandstone type uranium deposits in the world occur in the tectonic area of "faintly neotectonic district" and "second orogenic belf'.one of the types is the oxidation excedinly.that is," cy-c a p h c y ft — cup n a p b h h "type, its characteristic is of having a long time of interlayer oxidizing action, abundant provision of the groundwater% the great scale of interlayer oxidizing zone. Dongsheng is of deacidizing extreme model, there are rich resource in basin ,such as oil, gas and coal, and it has high "deacidizing atmosphere". This two types wholly are situated at" faint new tectonic movement district". From the scale of ore deposit,it can be see that both ought to belong the super large-scale deposit(when the oxidizing time is not long enough, the quantity of reducer that induce reaction which will give birth touranium mineral would have critical significance). So, in the light of this theory, there are still tremendous prospecting potential in Dongsheng field.