The Xiangshan uranium orefield is one of the largest volcanogenic hydrothermal ur anium orefields in China. A number of electronprobe tests indicates that francol i te extensively coexists with uranium minerals. Chemical analyses also show that the uranium content is in approximately positive correlation with P2O5 and C aO. Fu rther studies reveal that uranium and phosphorus have similar geochemical behavi ors in hydrothermal condition, and may be transported and precipi tated together. The authors propose that the formation mechanism of rich uraniu m deposits is originated from the colloidal coprecipitation of uranium and phos phoru s minerals. Studies of mantle xenoliths reveal that mantle fluids are rich in re du cing gases and K+, Na+ ions etc., so do the hydrothermal fluids. He and Ar i sotope a nalyses prove that the oreforming hydrothermal fluids have been m ixed with mantle fluids. On the basis of metallogenic features of this orefield, an ore formi ng model of extremelyrich uranium ores has been put forward.