The Geological Characteristics And Metallogenic Mechanism Of The Zhangjiawu Uranium Deposit In Northern Zhejiang Area

The Northern Zhejiang area,the southeastern margin of the Yangtze plate,is located in the Tianmu Mountain uranium metallogenic prospective belt of the coastal Pacific metallogenic domain and the uranium metallogenic province of the Yangtze continental block.The Mesozoic volcanic activity in Northern Zhejiang area is relatively active,suggesting that that place should be a good potential area for uranium mineralization.Years of exploration results show that the zhangjiawu（605）uranium deposit is one of the largest volcanic rock type uranium deposits in Northern Zhejiang area.The previous work of the deposit only limited to the uranium prospecting and exploration,and the lack of the research on the geological characteristics and metallogenic mechanism of the deposit restricts the understanding of the genesis of zhangjiawu uranium deposit,the establishment of metallogenic model and further the prospecting and exploration of volcanic rock type uranium deposits in Northern Zhejiang area.This paper mainly focuses on the occurrence state of uranium and ore-bodies characteristics by using optical microscope,scanning electron microscope,energy dispersive spectrometer and electron probe based on the detailed field observations.The methods,such as zircon U-Pb chronology dating,petrology,rock geochemistry and isotope analyses,are used to determine the fomation age,and petrogenesis of the ore-hosting rhyolites.The method of means of mass balance calculation is also adopted to study the characteristics of hydrothermal alteration and the law of material migration.The main purpose of the paper is to add our understanding the genesis and the metallogenic mechanism of the deposit,and finally to establish the metallogenic model.The main achievements of the paper include flowing:The X-ray energy spectrum scanning shows that the distribution range of uranium elements in the image is consistent with that of uranium minerals,indicating that the uranium mainly exists in the form of independent uranium minerals.The SEM and EPMA quantitative analyses reveal that pitchblende and uranite are two of main uranium minerals in the deposit.The pitchblende is mainly in the form of colloidal small particles,small veinlets and network veins,and the uranite is wrapped in pitchblende in the form of thin shell.From the core to the edge,the uranium minerals are pitchblende,changing to"uranium petrochemical"with high silica uranite and normal uranite.Uranium minerals are composed of simple oxide and silicate minerals,containing only trace Th and REE elements.The alteration type of surrounding rock is medium to low temperature alteration and the mineralization mainly diffuses outward along the fault,indicating that zhangjiawu uranium deposit probably should be a medium to low temperature type of hydrothermal deposit.The mineral characteristics and metallogenic tectonic environment during the metallogenic period indicate that there is an obvious cooling and decompression boiling as a result during the uranium precipitation,precipitation and mineralization.LA-ICP-MS U-Pb dating of zircon reveals that the ore-hosting rhyolite in the zhangjiawu deposit formed at 130.8±2.0Ma,as the product of the Early Cretaceous volcanic activity,with high silicon,rich potassium,poor calcium and poor magnesium,suggesting a peraluminous high potassium calc alkaline A2 rhyolite.This rhyolite was probably partially melted in the mafic lower crust under the post orogenic setting of an active continental margin,and the magma stayed in the magma chamber in the shallow crust and formed through strong crystallization differentiation.The characteristics of zhangjiawu host rhyolite are similar to those of volcanic rocks related to uranium mineralization in eastern China,and tthe higher uranium content indicates better uranium metallogenic potential.The migration and of HREE and P₂O₅ in the altered rocks in each zoning with high oxidation index（OI）indicate that the ore-forming fluid is a mixed fluid,probably composed of a small part of deep source fluid and most of deep circulation atmospheric precipitation.The migration into of Ca²⁺and Mg²⁺plasma to a large extent in the mineralization center,reducing the activity of uranyl carbonate complex as a result,indicates that the migration of Ca²⁺and Mg²⁺plasma has a certain contribution to the precipitation of uranium mineralization.Fe₂O₃ and Fe O migrated to a small extent in the mineralization center and pyrite was generated during the mineralization period,suggesting that there should be a trend of change from oxidation environment to reduction environment during the mineralization.The migration of Si O₂ and the formation of chalcedony and microcrystalline quartz during mineralization show that the alkaline relatively high-temperature fluid has a trend of decreasing of the temperature and the value of p H in the process of mineralization.Combined with regional data of the geological-tectonic evolution,the mineralization age,it was proposed that the rhyolite in the Zhangjiawu deposit formed under the extensional setting in the early Cretaceous,and the continuous NW-SE extensional environment in the South China led to the mixing of the hydrothermal solution of deep source magma（similar to diagenetic magma）with the deep circulating atmospheric precipitation,and providing a channel for the migration of metallogenic hydrothermal solution and a favourable metallogenic space at the same time.The uranium probably mainly sourced from the deep fluid mixed with the uranium extracted continuously by water rock reaction when the deep circulating atmospheric precipitation flowed through uranium-rich-hosting rocks.Uranium probably transported in hydrothermal solution in the form of uranyl carbonate complex,and enriched,precipitated and mineralized after the action of decompression boiling and hydrothermal alteration in open space,companying the reducing the activity of U with the reduction-oxidation and neutralization to form a neutral weak alkaline environment,a favorable environment for conducive to uranium precipitation.