Based on field survey and microscope observations, the authors selected representative samples of the ores from the Longmen kaolinite–pyrophyllite deposit in Shangrao of Jiangxi Province, and analyzed mineral compositions by such means as electron microprobe analysis, X-ray powder diffraction and hydrogen and oxygen isotope analysis. It is suggested that the ore deposit was formed by replacement of tuff by hydrothermal fluids, which mainly came from the meteoric water. The formation temperature of the ore deposit was 75 ℃～300 ℃ and the ore-forming pressure was less than 1 kb.