Study On The Redox Conditions Of Uranium Metallogenesis In Mianhuakeng Deposit In Zhuguangshan Area

The red microcrystalline quartz-type ores are widely developed in hydrothermal-type uranium deposits in the Zhuguangshan area,which are red due to the large amount of finely dispersed hematite,and the grade of this type of ore is usually positively related to the content of hematite.But the redox conditions between hematite and pitchblende are different In previous work,so it is necessary to have a study on the redox conditions of pitchblende.This dissertation takes the Mianhuakeng deposit,a typical uranium deposit in the area,as the research object,and used a lot of methods and methods such as chronology,mineralogy,geochemistry,fluid inclusion geochemistry to find out the metallogenic age,the characteristics of ore minerals and altered minerals,the nature and evolution of ore-forming fluids,and the uranium metallogenic redox conditions have been systematically discussed.The main understandings gained in this work are as follows:?1?The metallogenic period of the Mianhuakeng deposit was divided into two stages,whose the typical mineral combination in the early stage is red microcrystalline quartz-hematite-pitchblende,and gray microcrystalline quartz-pyrite-fluorite-pitchblende in the late stage.The in-situ LA-ICP-MS U-Pb dating technology of two stages ages are 84.7±1.2Ma and 65.3±1.6Ma.Uranium mineralization is controlled by the geodynamics of the extension of the lithosphere in the Cretaceous to Paleogene.?2?The results of the composition of pitchblende in the two-stage ores by micro-identification,electronic probes and LA-ICP-MS was that the pitchblende was poorly crystallized with iron oxide minerals,high Si,Pb,Al,?Ce positively anomalous,heavy rare earth element enrichment in the early mineralization stage while pitchblende was well crystallized and with lots of pyrite,high U,Fe,Ca,Na,Mn,P,?Eu negative anomaly,enrichment of light rare earth elements in the late one.The former was formed in an oxide environment,but the latter is formed in a reduce environment.?3?The pyrite in the Miankengkeng deposit were divided into 6 types?Type?-??according to the metallogenic stage and mineral morphological:pyrite from pre-ore is mainly cube and pentagonal dodecahedron?Type??and enriched Li,Co,Cu,low uranium content,formed in high temperature.Pyrite from the red microcrystalline quartz stage in syn-ore is self-shaped cube pyrite?type II?and deficient in Fe,S,Sr,enriched in U,Pb,Sm.,Nd,Cu,Co,Ti,and has a low content of rare earth elements.The pyrite from early stage mineralization of gray microcrystalline quartz has two crystal forms:automorphic pentagonal dodecahedron?Type III?and semi-automorphic cube shape?Type IV?,which have similar geochemical characteristics and are rich in K,P,As,Tl,Au,Sb and other elements,loss of Nb,Zr,Hf,Ba,Sr and other elements,rare earth element content is extremely low.There are two types of gray microcrystalline quartz in veins?type V?and colloidal pyrite?type VI?at the late gray microcrystalline quartz mineralization stage,which are strongly depleted of Fe and S and enriched in U,Pb,As,Th,Sm,Nd and Bi,high content of rare earth elements.Scanning electron microscopy results showed that the pitchblende and pyrite contact site has a reactive corona texture,and the composition gradually changes.The closer to the bituminous uranium,the pyrite has Fe,S content reduced while the content of U,Pb,REE,Co,Ni,As and Se increasing.Pyrite can reduce and Fe²⁺can also participate in the reduction process,but to a limited extent.?4?Hematite in Mianhuakeng deposit were divided into 4 types:porphyry texture in pre-ore,cloud texture and disseminated texture in syn-ore,and net vein in post-ore.The hematite crystal closely related to pitchblende is poor and its particle size is small,mainly in the shape of cloud.The porphyritic hematite before mineralization period was the product of altered feldspar and biotite,rich in Fe,Al,Mg,K,LREE,and deficient in Ba,Sr,Nb,Ta,Zr,Hf.The co-existing cloud haematite is rich in Fe,Si,Al,U,Pb,Li,Rb,and has obvious?Ce positive anomalies.?5?The test results of fluid inclusions by micro temperature measurement and laser Raman was that the pre-ore fluid was high temperature?360??400??,medium density?average 0.9g/cm³?,low salinity?6.0?9.0wt%Na Cl?fluid.The fluid in the red microcrystalline quartz ore is medium-high temperature?240-320??and medium density?average 0.826g/cm³?,low salinity?5.0?9.0wt%Na Cl?,fluid inclusions were rich in O₂ in gas.The fluid was medium-high temperature?240-300??,medium density?average 0.869g/cm³?,low salinity?5.0?8.0wt%Na Cl?fluids,rich in H₂,CH₄,CO₂ in the gas phase of the quartz and fluorite inclusions in the gray microcrystalline quartz ore while medium-low temperature?120?180??,medium density?average 0.918g/cm³?,low salinity?2.0?4.0wt%Na Cl?fluid in post-ore.As the metallogenic stage progresses,the metallogenic fluid has a tendency to decrease in temperature and salinity and increase in density.The early stage of the mineralization period was oxidized fluid,and the late stage was reduced fluid.?6?In the early stage in syn-ore from the Mianhuakeng deposit,the ore fluid was oxidative account for the deep source reducing fluid was mixed with more atmospheric precipitation,forming red microcrystalline quartz vein-type uranium mineralization;yet the late stage was reductive due to the deep source reducing fluid mixed less atmospheric precipitation,and forming gray microcrystalline quartz vein-type uranium mineralization.Pitchblende is not only formed in a strong reducing environment,but also in an oxidizing environment.The reducing environment is not a necessary condition for the precipitation and enrichment of pitchblende.The reducing environment is more significent to the enrichment and precipitation of pitchblende and easier to form rich ore.