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The Sanjiu uranium ore field,located in the central of Zhuguangshan granitic batholith,is a newly discovered granite-related uranium ore field in South China.The main sulfide in the ore field is pyrite,which is closely related to uranium mineralization.The textures,major and trace elements,S-He-Ar isotopes compositions of pyrites in ores of different grade were observed and/or analyzed by optical microscope,scanning electron microscope,electron microprobe,laser ablation inductively coupled plasma mass spectrometry,and noble gas mass spectrometer (Helix-SFT).It is observed that these U-related pyrites are generally euhedral-subhedral with dissolution textures,anhedral variety with colloform texture,veinlet and fine particles,and the color of the associated minerals is mostly dark hue,such as purple-black fluorite,dark-red hematite,and dark-green chlorite,etc.The analytical results show that the average compositions of major elements in pyrite are FeS1.944.Pyrites are characterized by S-deficiency,low content of Co and Th,and Co/Ni ＞ 1,which indicate that these ores are of low-temperature hydrothermal origin.We found that the higher the grade of ore,the more deficient in S,the more obvious negative δ34S,and the higher REE content (close to U-rich granitic pluton) of pyrite.The S-He-Ar isotopic compositions of various varieties of pyrites indicate that the ore-forming fluids mainly come from crust-derived fluids and mixed with mantle-derived fluids.