Abstract: The Xiwang uranium ore deposit is one large-scale granite-hosted U deposit in the Xiazhuang U ore？eld in SouthChina. Uranium orebodies occur as thin veins along the fracture zones within the Maofeng granitic pluton. Pyriteis the main sul？de mineral in the ores. Textural observation suggested that formation of hydrothermal pyrite canbe divided into three stages: pre-mineralization stage pyrite (Py I), mineralization stage pyrite (Py II) accom-panying with U-minerals, post-mineralization stage pyrite (Py III). In this study, in-situ sulfur isotopic compo-sitions and trace element contents of pyrite from di？erent stages were analyzed by laser ablation (MC-) ICP-MSmethods. The δ34S values of Py I vary from ？28.5‰ to ？12.0‰ with an average value of ？18.4‰, Py II from？15.9‰ to ？10.2‰ with an average value of ？12.5‰ and Py III from ？22.3‰ to ？14.8‰ with an averagevalue of ？19.4‰. The variations of sulfur isotopic compositions of pyrites were caused by the changes ofoxygen fugacity of hydrothermal ？uids. The hydrothermal ？uids at the pre-mineralization stage had relativelyhigher oxygen fugacity (？O2 > 10？34). The decreasing of oxygen fugacity (to 10？36–10？35) at the miner-alization stage might promote the precipitation of uraninite. The mineralization was terminated as the increasingof oxygen fugacity of hydrothermal ？uids back to 10？35. Pyrite from the Xiwang deposit generally contains high U contents (up to ten thousand ppm) and very low Th contents (mostly < 0.1 ppm). U and W contents in pyriteshow good positive linear relationship, indicating that both U and W came from the hosting granite. Py II showshigher Mo and V contents than Py I, also suggesting more reduced environment at the U mineralization stage. Such high oxygen fugacity for initial hydrothermal ？uids supports that the ore-forming ？uids should be origi-nated from meteoric water. The circulating meteoric water can leach U and other metals from the hosting graniteto form U-rich hydrothermal ？uids. In fracture zones and under reduced conditions, U6+ in the ？uids can bereduced to precipitate uraninite (UO2). Thus, U-rich granites, circuiting meteoric water and reduced agents arepre-requisites for the formation of the granite-hosted U deposits in South China.