The Mineralization Mechanism Related To Hydrothermal Fluid In The Xiangshan Uranium Orefield

The vast majority of volcanic-type uranium deposits in China are deposited in the late Yanshan intermediate-acid volcanic or subvolcanic rocks in South China.Among them,the Xiangshan uranium orefield is the largest volcanic-type uranium orefield in China.At present,controversy still exists about the source of ore-forming fluids,the uranium migration and precipitation mechanism in the Xiangshan uranium orefield.This article focuses on the U-Th type uranium deposit in the west（Zoujiashan deposit）and the U-only type uranium deposit in the north（Shazhou deposit）of the Xiangshan uranium orefield,using methods of element geochemistry and Sr isotope of both magmatic and hydrothermal apatite,microthermometry and LA-ICP-MS analysis for metals in single fluid inclusion,combining elemental and Sr isotope gechemistry of whole rocks,to better understand the precipitation mechanism of U mineralization,to constrain the origin of U and ore-forming fluids of the Xiangshan orefield,and to further disucuss the key factors controling the volcanic-type uranium mineralization in South China.The Xiangshan volcanic-intrusive complex includes rhyolite,porphyry lava and subvolcanic rocks（granite porphyry and quartz monzonite porphyry）.The mafic quenched enclaves are common in granite porphyry,while rare mafic enclaves were found in the quartz monzonite porphyry.Whole rock geochemical data shows that these types of volcano-subvolcanic magmatic rocks all have A-type granitoids afiinity.Except for quartz monzonite porphyry,the magmatic apatites in the other three types of rocks have high and stable F content and low Cl content（molar fraction of F:XFAp>0.833;XCl Ap:0.003-0.025）,showing one of the A-type granitoids features which is the enrichment of F in the source rocks.In addition,unlike apatite in the other three types of rock,apatite of quartz monzonite porphyry has high Cl and low F（XFAp:0.617-0.634,XCl Ap:0.188-0.234）,and the halogen content is close to that of the mafic enclaves in granite porphyry（XFAp:0.361-0.617,XCl Ap:0.192-0.364）.The apatite Sr isotopic composition of rhyolite,porphyry lava,and granite porphyry is relatively uniform（0.710137-0.711874）,while apatite in quartz monzonite porphyry has a very low 87Sr/86Sr ratio（0.706424-0.707141）,which is close to the initial87Sr/86Sr ratio of mafic enclaves core（0.70806）.These features indicate that the magma forming quartz monzonite porphyry has undergone complete crust-mantle magma mixing during its evolution.The halogen content and Sr isotope of apatite in typical uranium-producing and non-uranium-producing volcanic rocks on the Gan-Hang belt show that from west to east,as the late Yanshanian volcanic-intrusive complexes get younger,more mantle-derived magma was involved in the formation process of these volcanic rocks.However,there is no linear correlation between the Sr isotope of apatite and the U content of the whole rocks,indicating that there is no obvious correlation between the crust-mantle magmatic mixing and U-producing potential of the volcanic-intrusive complex.Based on petrographic observation and chemical composition analysis,the hydrothermal apatite related to uranium mineralization in the Zoujiashan uranium deposit and the Shazhou uranium deposit can be divided into three types:the first type is euhedral-subhedral apatite with no eveident chemical zoning（Ap1）,which distributes in the hematitization zone adjacent to uranium veins;the second type is sub-euhedral apatite（Ap2）with obvious chemical zoning along its rims which occurs in uranium veins with pitchblend scattered along its exterior edges;the third type of apatite（Ap3）occurs as anhedral aggregates closely associated with U-minerals,implying concurrent precipitation of Ap3 and U ores.Ap1 is homogeneous in composition and has a very similar chemical composition to the core of Ap2,including higher Cl（2.24-2.91%）content,lower Mn（842-4040 ppm）and F（0.81-1.29%）content.The Ap2 rims and Ap3 have relatively higher F（1.87-3.19%）,Mn（2661-13697 ppm）and lower Cl（0.39-1.53%）content.In addition,Ap3 has the highest Th content,and the Th content of Ap2 rims is the lowest,while the Th content of Ap2 core is in between.The U content of the three types of apatite is similar.Since Mn can only enter apatite in the form of Mn2+,the Mn content of apatite can be used as indicator of the redox state of the fluid/melt.The chemical characteristics of the three types of hydrothermal apatite show that Ap1 core and Ap2 crystallize in a Cl-rich and Th-poor oxidizing fluids,while Ap3 crystallizes in an F and Th-rich reducing fluids,and the rims of Ap2are altered by the F-rich fluid.Furthermore,uranium enters into apatite crystals mainly in the form of U⁴⁺.Therefore,although the U content in the three types of apatite is similar,the more oxidized Cl-rich fluid that forms Ap2 core and Ap1may have more U content than he reducing F-rich fluids which precipitates Ap3and alters rims of Ap2.The content of metal elements in fluid inclusions of hydrothermal minerals（fluorite,calcite）shows that Zoujiashan uranium deposit and Shazhou uranium deposit each have two end-member fluids in the uranium mineralization stage.One end-member is characterized by high U（26-51 ppm）,high salinity,and low content of Th（1.5-3.7 ppm）.The other end-element fluid has low salinity and U content（2.0-15.17 ppm）,yet high content of Th（7.8-24ppm）.Ap2 core,Ap1,and the U-rich,Th-poor end-member fluid all have low Th and Sr contents,yet high U content,while Ap3 and the U-poor,Th-rich end-member fluid have high Th and Sr contents,yet low U content,indicating that the U-rich end-member fluid correspond to the Cl-and U-rich oxidizing fluids precipitating Ap1 and Ap2.The F-and Th-rich reducing fluid indicated by Ap3compositions correspond to the mixed fluid after the involvement of U-poor,Th-rich end-member fluid.Mixing of the two end-member fluids causes the precipitation of U,Mo,Th,Sr,etc.,which is also consistent with the non-linear correlations of Mn,Sr with the halogen content in hydrothermal apatite.Apatite has very low Rb content and high Sr content,thus its ⁸⁷Sr/⁸⁶Sr ratio can directly reflect the Sr isotopic composition of the fluid/melt it crystalized from.The U-poor,Th-rich end-member fluid has low Rb/Cs（average 3.7）,and its Sr content and Sr isotope compositions are close to that of the enriched mantle fluid,indicating that the U-poor fluid was derived from the mantle wedge experienced addition of Cs by fluids generated from the devolatilization of the subducted paleo-Pacific slab.The Rb/Cs ratio（average 19.7）of the U-rich,Th-poor end-member fluids is higher,and its Sr content and ⁸⁷Sr/⁸⁶Sr are plotted on the evolution line to the composition of the Xiangshan volcanic complex.Combining its high U,Cl content and high oxygen fugacity,we suggest that it was originated from the oxidized basin brines and had leached U from the surrounding volcano-intrusive complex.The two stages of uranium mineralization in the Xiangshan orefield represented by the Shazhou uranium deposit and the Zoujiashan uranium deposit respectively is consistent in the aspect of hydrothermal fluid evolution and uranium precipitation mechanism.However,compared with the Shazhou uranium deposit,the U ores and Ap3 in the Zoujiashan uranium deposit have higher Th content,indicating that the F content of the mantle-derived end-member fluid has higher F,since Th can only be transported by F in form of Th F₂²⁺.This is consistent with the observations that more fluorites are found in the gangue minerals of the Zoujiashan deposit than that of the Shazhou deposit.When the fluid is rich in CO₂,the F content of apatite crystallized from this fluid will increase.Therefore,although Ap3 in the Shazhou uranium deposit is rich in F,it might be the result of elevated CO₂ in the mantle-derived end-member fluid than that of the Zoujiashan deposit.The Sr content and Sr isotopic composition of the mantle-derived end-member fluid in two uranium deposits show that the mantle-derived end-member fluid in the Shazhou uranium deposit experienced water-rock reaction with the basement metamorphic rock,while that in the Zoujiashan uranium deposit experienced water-rock reaction with the F-rich source rocks of the Xiangshan volcanic-intrusive complex.The different ascending pathways of the mantle-derived end-member fluids of the two deposits indicate that the tectonic environment controlling the two stages of uranium mineralization in the Xiangshan orefield is not the same.Combining ore-forming mechanism of the Xiangshan uranium orefield and the overall metallogenic characteristics of the volcanic rock-type uranium deposits in South China,this paper summarizes three important factors that control the volcanic rock-type uranium mineralization in South China:the Late Mesozoic U-rich volcanic rock,the Cretaceous red basin associated with U-rich volcanic rock,and the activity of mantle-derived fluids or magmas after the formation of the volcanic rocks.