Research On The Metallogenic Mechanism Of The Miaoershan Granite-type Uranium Deposit In Guangxi

The Miao’ershan uranium ore field is a large granite-hosted hydrothermal uranium orefield,including at least the Shazijiang,Shuanghuajiang,Menggongjie,Zhangjia uranium deposits,et al.In this thesis,the geotectonic background,lithological sequences,structures,granitic plutons,veins,and deposit geologic characteristics in the Miao’ershan ore field have been firstly introduced.The heat source,fluid source as well as the uranium’s source then have been traced,while the migration,preservation and change of the uranium mineralization systems have also been discussed.The metallogenic model has been finally established.The major results are as follows:（1）The uranium contents in the Sinian Doushantuo formation and Cambrian Qingxi formation are both very high,which have been regarded as the possible uranium source.（2）The stress field of the study area evolves from North-Southcompression in Indosinian period to Northwest-Southeast compression in the Yanshanian period.The most important ore-controlling structures are NNE-and NE-trending faults.The Xinzi fault is a regional deep crust fault,which not only controlled the formation of the down-faulted red basin,but also the formation of the uranium deposits herein.（3）The geochronologic and geochemical studies of the granitic rocks in the ore field have been carried out,and the possible relationship between the granitic magmatism and uranium mineralization has then been discussed.The Caledonian base granitic rocks yield a SIMS zircon U-Pb age of 417.9±2.3Ma,while the LA-ICP-MS zircon U-Pb ages of the Xiangcaoping,Douzhashan and Zhangjia plutons are 223.9±1.3Ma,216.0±1.8Ma and 222.4±3.6Ma,respectively.They all belong to high-K calc-alkaline,low Ba-Sr series and S-type granitoids,and probably derived from partial melting of the Proterozoic sedimentary-metamorphic rocks.Different U contents of different plutons may result from the uranium heterogeneity of the source region.Two important factors favorable for the mobilization of uranium are auto-metamorphism and tectonic activities.（4）The age and rock geochemistry characteristics of a fine-grained biotite granite vein have been determined.Furthermore,the relationship between this vein and uranium mineralization has been analyzed.The LA-ICP-MS zircon U-Pb age of the fine-grained biotite granite trending to the NE direction is 199.4±1.5Ma.The age of this vein is at least 50 Ma older than the uranium mineralization age,implying that it might only provide favorable physical structural condition,rather than act as the heat,fluid or metal sources.（5）All orebodies are vein-type in shape.The most representative ores are the highly fractured granite with hematitization and silicification.The uranium minerals are mainly pitchblende,uramphite,coffinite,and the gangue minerals are mainly quartz,calcite and fluorite.During uranium mineralization,the heavy rare earth elements（HREEs）are relatively enriched than light rare earth elements（LREEs）.Minor Se was found enclosed within uranophane,suggesting the weakly oxidazed condition or oxidazed condition.（6）Geochemistry features and element migration regularity in one borehole in the Periphery of the Shazijiang uranium deposit have been studied.The uranium contents and SiO₂、CaO、Fe₂O₃、LOI、HREEs contents have a positive correlation relationship,and Na is almost depletion at the mineralization parts within this drillhole.In trace elements spider diagram and REEs distribution pattern diagram,the shapes of different mineralized and altered samples are very similar.Different from the fresh surrounding rock,cataclastic granites（ores）together with the altered surrounding rocks have much higher Sb,U and Sb,Cd,U contents.Sb,volatile components（LOI）,CaO,Ni,Cu,In,Sr are enriched,while Na₂O,FeO,Be,Cs,Th,Nb are depleted during mineralization.It is suggested that the U-enriched granite is one of the most important uranium sources,and the ore-forming fluid is alkaline with high oxygen fugacity.（7）The fluid inclusion characteristics of the Zhangjia deposit has been studied in detail,and comparation has been made with the Shazijiang deposit.The ore-forming fluid of the Zhangjia deposit is of moderate to low temperature,with mineralization temperatures ranging from 109°C to 229°C（concentrating within 125°C²36°C and averaging at 175°C）.The salinities of the fluid range from 0.35%to 1.40%NaCl eq,with the average of 0.87%NaCl,showing low salinity features.H₂ was detected as the main composition in the volatile,with a little N₂ and CH₄.The ore-forming fluids of both deposits are of the H₂O-NaCl system.（8）The mineralization ages of the deposits have been constrained by with the combination of traditional pitchblende TIMS and modern pitchblende LA-ICP-MS methods.Six uranium metallogenic stages have been classified,namely:⁷0Ma（71±1Ma,70.6±6.1Ma,main mineralization stage）,146.4±5.5Ma,124.4±4.6Ma,¹00 Ma（95.6±6.3Ma,94.1±38Ma）,⁶0 Ma（60±3Ma;61.7±2.8Ma）and 53.0±6.4Ma.（9）The comprehensive study on the source（fluid,uranium）,transportation,precipitation and supergene overprint have been discussed in the thesis.The heat source and the power source of uranium mineralization come from the extensional structure movement of the lithosphere.The ore-forming fluids are mainly from meteoric water,but CO₂ mainly derived from the degassing of the mantle.The uranium is mainly from the U-enriched strata and granitic plutons.The uranium is transported as the complex of UO₂（CO3）₂^2-and UO₂F₄^2-under the oxidized condition.The most key point for uranium precipitation is“the pressure decrease and degassing”.The ore-forming fluids may be of a weak oxidized condition during U precipitation.The secondary uranium minerals are the result of oxidation from the pitchblende.（10）The metallogenic model was finally established.In the phase of the formation of the Caledonian（440³81 Ma）granite and the phase of the formation of the Indosinian（250²00 Ma）granite,the uranium was preliminarily enriched,and the favorable physical structural condition was formed.In late Jurassic to Paleogene,6phases of uranium mineralization is a kind of response to 6 extensional structure movements of the lithosphere.