Metallogenic Mechanism Of Paleozoic Jiling Sodium-metasomatic Type Uranium Deposit In Longshoushan Uranium Metallogenic Belt,Gansu Province,China

The Longshoushan Metallogenic Belt(northwestern China)is known for its word class Jinchuan Ni-Cu sulfide(Pt)deposit and is also an important uranium metallogenic belt.There are some different types of uranium deposits occurred in this belt,the Hongshiquan pegmatitic uranium deposit occurs in the western part of the belt;the sodium metasomatic type uranium deposit,Xinshuijing and Jiling uranium deposit,and Ge Minggou perigranitic uranium deposit occurs in the middle part of the belt;Jin Biansi leaching type uranium deposit occurs in the eastern part of the belt.The Xinshuijing and Jiling uranium deposits are the typical sodium-metasomatic uranium deposit among them.A series of systematic studies of Jiling Na-metasomatic type uranium deposit is significance for clarify the characteristics of Na-metasomatic type uranium deposit in China.In the thesis,the studies on petrology,mineralogy,whole rock geochemistry.Isotope geochemistry,fluid inclusion and U-Th-Pb chemical in situ dating on uraninite and pitchblende in the Jiling Na-metasomatic uranium deposit have been introduced.The aim is to unravel the genesis of Jiling pluton,regional magmatic activity,alteration characteristics of deposit,source of ore-forming fluid and uranium,and the geochronology for mineralization.Finally,to establish the metallogenic model of the Jiling Na-metasomatic uranium deposit.The Jiling granitoids display relatively high contents of Zr+Nb+Ce+Y(282.20 ppm?929.00 ppm),?REE(161.97 ppm?443.85 ppm),and isotope compositions.And most of the Jiling granitoids have high Al contents and low 1000×Ga/Al ratios,also display low A/CNK rations(<1).Indicate that the Jiling granitoids belong to KCG and ACG I-type granite.Because of the Jiling granitoids and microgranular mafic enclave show large variation range of major elements and isotope compositions(?_Nd(t)value range from-9.10to 0.21),combined the feature of petrography and minerals geochemistry of Jiling granitoids and microgranular mafic enclave.It is concluded that the Jiling granitoids were formed by magmatic mixing in an extensional condition.Through the studied of the geochemical characteristics of trace and rare earth elements among the Jiling granitoids,mineralization granites and uranium ore rock,indicate the mineralization granites and uranium ore rocks have inheritable to the Jiling granitoids.Combined that the Jiling granitoids show a higher Th/U ratio(6.4)than average upper crust(3.9),suggests that U loss from the Jiling granitoids.So the Jiling granitoids may provide the U source for the Jiling uranium deposit.And the similar REE patterns of apatites from the medium-coarse grained fleshy granodiorite(granite),the fine-grained granite and the mineralization granites,indicates that the uranium mineralization mainly occurred in the medium-coarse grained fleshy granodiorite(granite)and the fine-grained granite.The uranium minerals in the Jiling uranium deposit include uraninite,pitchblende,and coffinite,and the pitchblende is the mainly uranium mineral among them.The uraninite in the Jiling uranium deposit show high contents of Th+REEs+Y,have similar REE patterns with the uraninite in the typical magmatic uranium deposit,and there is no significant timespan between the uraninite and the Jiling granitoids.Indicates that the uraninite in the Jiling uranium deposit were formed under a high temperature with the magmatic-hydrothermal fluid.It is the first time that identified of this type uraninite in the Jiling uranium deposit.Comparing the chemical composition variation of uraninite and pitchblende which underwent different degrees of alteration,recognized that both uraninite and pitchblende underwent at least two hydrothermal alteration stages.The altered uraninite A enriched in Ca,the altered uraninite B enriched in Si,and compared to the altered pitchblende A,the altered pitchblende B enriched in Si,but with similar content of Ca.The high temperature crystallized uraninite and fresh pitchblende in the Jiling uranium deposit are 435.9±3.3 Ma and 361 Ma,respectively,obtained by EMPA U-Th-Pb chemical in situ dating.According to that there is no significant timespan between Jiling granitoids and the high temperature crystallized uraninite,suggests the the high temperature crystallized uraninite were formed under an extensional background.Through the studied of the chlorite and the pink calcite which have closely related to pitchblende show that,the formation temperature of chlorite range from 112??264?,mainly concentrated in 150??250?,with a mean value of 190?;the homogenization temperature of fluid inclusion in the pink calcite range from 164.0??224.8?,mainly concentrated in 170??200?,with a mean value of 188.4?;the salinities of the fluid inclusion the pink calcite range from 0.4 wt.%?6.0 wt.%,mainly concentrated in 1.0wt.%?3.0 wt.%,with a mean value of 2.2 wt.%.All these features show that the ore-forming hydrothermal fluid during the pitchblende ore stage is medium to low temperature and low salinity hydrothermal fluid.The value of?¹³C_V-PDB and ¹⁸O_V-SMOW in the pink calcite are range from?2.24‰??2.07‰,with a mean value of?2.13‰;10.03‰?10.50‰,with a mean value of 10.29‰,respectively.The value of?¹⁸O_H2Oin the carbonate equilibrium fluid during the pitchblende ore stage range from 0.33‰?0.80‰.These indicate that the hydrothermal fluid during the pitchblende ore stage is the meteoric water-dominated fluid source,also have the mixing characteristics of meteoric water and magmatic hydrothermal fluid.Based on the studies in this thesis and previous research,the metallogenic model of the Jiling uranium deposit was established.The formation of the Jiling granitoids and the extensional background during the early Silurian provided uranium and heat source for the uranium mineralization of Jiling uranium deposit.Then the extensive developed Na-metasomatic alteration among the Jiling granitoids resulted in a large amount dissolution of quartz and provides a favorable space for uranium precipitation.The high temperature magmatic-hydrothermal fluid which enriched in uranium actived among the sodium metasomatic altered granite and crystallized into uraninite in it.Also as the uranium source of pitchblende ore stage,the primary uraninite was altered and the uranium was extracted during the later hydrothermal fluid activities.Then the uranyl carbonate complex in the ore-forming fluid was precipitated by deadration of CO₂ and reduction of Fe²⁺to formed pitchblende.