The Geochemical Characteristics Of Ore-forming Fluid And Ore Genesis From The Xiangshan Uranium Orefield

Ore-forming fluid plays an important role in studying the basic problem of the geochemistry ofmineral deposits. The Xiangshan uranium ore field is a typical volcanic-related uranium ore field inChina, and ore-forming fluid plays an important role in it On the basis of previous work andachievement, we select typical uranium deposits as the research object, and by the field geologicalobservations, thin section identification in the microscope, electron microprobe analysis, the test ofmain and trace elements analysis, fluid geochemical characteristics and the study of isotopegeochemistry,we get several conclusions as follows:1. There are many different kinds of wall-rock alteration in Xiangshan ore fleid and overlap eachother. But it still can show its zonality which can be divided in five zones from ore to wallrock: themineralization center zone(fluoritization+hydromicazation+carbonatization+silicification)、Beside minealteration zone(hematitization+chloritization+fluoritization+silicification)、 nearly mineral alterationzone（chloritization+hydromicazation）、far mineral alteration zone（albitization+hydromicazation）andweak alteration zone（weak hydromicazation）. Alteration intensity weakens and new mineral speciesreduces from mineralization center zone to weak alteration zone. The acidity alteration minerals whichare near to mineralization center zone mainly include fluorite, illite, calcite, and those which are farfrom mineral alteration zone are alkaline alteration minerals such as albitite, chlorite and so on. Thisphenomenon shows the characteristic of after alkali acid in the time, and indicates that there are at leasttwo hydrothermal alteration processes in Xiangshan uranium ore field.2. The main elements such as K, Ca, Mn significantly increased, and the content of SiO2, Na2Ogradually reduced in the alteration process，what’s more, the content of Al, Fe, Ca, Ti and P in the oreare higher. In the albitization, chloritization, hydromica alteration, Na represent very great concentration;fluid is alkaline; the main alteration is alkaline alteration which includes albitization. Ti and P have ahigh value in the fluoritization, and indicate that fluid turns into the acidic solution, so we can confirmthat there are two heterogeneous nature of ore-forming fluids in Xiangshang ore field.3. The race element distribution pattern shows that the enrichment of Th、Rb is significant, and Ta、Nb、Ce、Zr、Hf、Sm、Y and Yb’s is slight; Sr, Ba, P, Ti significantly deficit; Mo, W, Pb, zinc can be usedas indicator elements in researching uranium mineralization; incompatible elements such as Sr、Th、P、Y、Yb are rich in the Ore-forming hydrothermal. The EER distribution pattern shows that the amount ofthe total rare earth elements (Σ REE), especially heavy rare earth elements is gradually increased, andindicates that uranium mineralization is closely related to REE especially HREE, and reflectsore-forming materials come from deep source. Ore-forming fluid enriches alkaline elements(Li Cs)、 transition elements(Sc Ti Y Mo)、LILE(Rb Sr)、HFSE(Nb Zr Hf Th)and indicates that theore-forming fluid comes from the mantle.4. The study of the Fluid inclusion shows that the range of homogeneous temperature is118.0~417.0℃,and the main metallogenic temperature is220~327℃;The characteristics of theore-forming fluid is midum-high temperature, low salinity（2.4~21.3％）, low density（0.53～1.08g/cm3）. The temperature and salinity bivariate chart shows that there is a mixing effect in theore-forming fluid and indicates that the ore-forming fluid includes not only mantle-derived fluids, butalso atmospheric precipitation. The study by the calculating shows that theoretical mineralizationpressure is131.25×105～525.19×105Pa; the theoretical mineralization depth is0.43~1.75Km; theaverage mineralization depth is about0.91Km. All of these show that there should be a goodprospecting in the deep of this deposit.5.By the study of the carbon and oxygen isotopic, we can draw the conclusion that C in thehydrothermal ore deposits mainly comes from CO2which forms in the deep place, and may be relatedto the mantle degassing。The study of sulfur isotopic shows that S mainly comes from the extraction andleaching of the surrounding rock in the ore-forming fluid circulation process.6. By the analysis the fluid characteristics and source in Xiangshan uranium ore field, combiningprevious study of the metallogenic material sources and chronology, and making an analysis of thetectonic location and significant geological events in Xiangshan uranium ore field, we discuss and set upthe possible uranium orefield metallogenic model which is related to mantle-derived fluids.