Abstract: The Caotaobei uranium deposit is a large-scale volcanic-hosted uranium deposit in the Hecaokeng ore field in Huichang area, southern Jiangxi Province. Granites and volcanic rocks occur in the deposit, and the orebodies are mainly hosted by the cryptoexplosive breccia and shattered granites. Chlorite, a widely developed alteration mineral in the granites, volcanic rocks and ores, is taken as the research object in this paper, and its chemical compositions are analyzed by EPMA and LA-ICPMS. The results show that the chlorite in the granites and the ores is mainly iron-rich chamosite and prochlorite, while the chlorite in the volcanic rocks is mainly magnesium-rich pycnochlorite. Ionic substitution mechanisms of Fe ⇔ Mg, Tschermak (TK) and di-trioctahedral (DT) are likely important for chemical variations of the chlorite at the Caotaobei uranium deposit. Chlorite in the ores has relatively high contents of Ti, Li, Be, B, Zn, Ga, Ge, Sn, Cs, U, Rb and Ba, and shows low Th/U ratios. Crystallization temperatures of the chlorite in the ores are calculated to be 201-269℃ with an average value of 242℃, indicating the mediumlow temperatures for ore-forming fluids. The oxygen fugacity (log f O₂) of chlorite in the ores varies from -48.4 to -41.2 with an average value of -44.5; the sulfur fugacity (log f S₂) varies from -7.5 to +2.8 with an average value of -1.8. The uranium mineralization in the Caotaobei deposit was formed in an environment of low oxygen fugacity and high sulfur fugacity. 

Key words: chlorite, geochemical characteristics, formation environment, caotaobei uranium deposit, southern Jiangxi