| Xiangshan uranium ore-field is the largest volcanic type uranium ore-firld andholds an important place in uranium geology of China. The implement of scientificdrilling have promoted the depth and breadth of research in Xiangshan. This paperstudies the porphyry rhyolite of Ehuling formation and altering zones in it. Based onthe study of detailed geological survey, petrology, petrochemical, zircon U-Pbisotopic dating, altered rocks, element mass migration, microscopic thermodynamic offluid inclusion, fluid inclusion composition, we have lucubrated in petrology feature,microscopic characteristics of mineralization-alteration, mass change quantificationand property of mineralizing fluid. The main advance achievements from this studyare as followings:The porphyry rhyolite of Ehuling formation belongs to high-K calc-alkalineseries, with possessing relatively high SiO2, Al2O3, K2O and comparatively lowNa2O，and is enrichment of LILE, depletion of HFSE. The lanthanon distributionpattern is characterized by rightinclining type. LA-ICP-MS U-Pb zircon dating showsthat the formation age of porphyry rhyolite is132Ma. According to our analysis andthe previous studies, we hold that Xishang volcano-instrusive complex is formed inEarly Cretaceous. Whole-rock isotopic dating shows that porphyry rhyolite is themixing of mantle materials and crustal materials.The comparison study of mineralized sector and intensely altered sector showsthat the hydrothermal alteration consists of three stages and three stages are isospatialsuperimposed. We consider that acid illitization is the dominant factor ofmineralization. Three stage alterations and tectonism are not isospatial superimposed.The intensity, method of alteration and forms of tectonism take place in a certainpattern from the centre of mineralization to lateral alteration zonatins. Red alterationis characterized by multiple stages and diversity. Red alteration is not isospatialsuperimposed with albitization and can be purificated by late alteration Based on the study of elements geochemistry, we found that the elements presented asemigration, the lack of ore-forming element U and Ca, P and Mn is the main reason caused thenon-ore in the intensely altered sector. Based on the study of elements geochemistry inminealization sector, we found that LOI, CaO, P2O5, MnO, Na2O, FeO, Fe2O3are closely relatedto ore-forming. Chalcophile elements have excellent indicative function in uranium mineralization,such as Pb, Zn, Cd, Sb, In, As and S. The enrichment of HREE is closely related to uraniummineralization and this indicate that the hydrothermal solution is not evoluted from magma. Theore-forming hydrothermal solution was located in acid geochemical environment for REEfractionation and it has anatectic character.The analysis of elements migration and ore-fluid dyamics shows that elementsexist an conbective equilibrium progress during metasomatic alteration. In addition,the conbective equilibrium is affected by many factors. The alteration sections can notbe represented by similar alteration model with different structure openness.The study of fluid inclusion in uranium mineralization section shows thatore-forming fluid exist an evolution relationship from early stage to late stage and itbelong to low temperature, low salinity fluid. By comparing the formationtemperature of chlorite, we conclude that the fluid inclusion microthermometry canreflect the real ore-forming temperature.The laser Raman analysis shows that the liquid phase is mainly made up by H2Oand gaseous phase is mainly made up by CH4, CO2and N2. The group analysis offluid inclusion shows that gaseous phase is mainly made up by N2, H2O, CO2, O2, asmall amount of CO and the smallest amount of gaseous hydrocarbons. The liquidphase is mainly made up by Na+, K+, Mg2+and Ca2+. The ore-forming fluid hasreducibility and anatectic character. By comparing the ionic concentration in alterationgeochemistry progress, the component of liquid phase represent the property of residual solutionand can not really reflect the ionic composition of ore-forming fluid.The estimation to physicochemical conditions shows that ore-forming fluid haslow oxygen fugacity. The estimation by thermodynamic calculation is similar toevolution of alteration with alkali-acid-alkali.This paper summarizes the relationship between alteration and uranium mineralization. The alkali alteration enlarged the porosity of wall rock and providesadequate space for ore-forming fluid. The acid illite or illite smevtive mixed layerrepresent the geochemical environment of uranium mineralization and the reductionof pH value may cause the sedment of ore-forming element. |
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