The hydrothermal sediment samples from the 49.2°E-50.5°E hydrothermal fields of the Southwest Indian Ocean ultra-slow spreading ridge were analyzed for the mineral compositions and elemental geochemical characteristics. Moreover, the results show that the mineral compositions of the hydrothermal sediments in the study area are mainly pyrite and chalcopyrite, also possessed slightly sphalerite. Meanwhile, the silicon chimneys and residual oxides are composed predominantly of amorphous silicon, calcium carbonate, and iron hydroxides (such as goethite). The transition trend of the mineral assembles from the hydrothermal sediments to the residual oxides indicates that the changing process of its ore-forming temperature decreasing gradually. Furthermore, the main and trace element geochemical characteristics of the hydrothermal sediments were analyzed by XRF and ICP-AES. Geochemical characteristics of the main and trace elements show that the contribution of seawater increases gradually and reduces the influence of hydrothermal fluid. The formation environment also changes gradually from the hydrothermal plume flow condition to the low temperature conditions of the marine aquatic. In the analysis of rare earth elements (REE), the hydrothermal sediment samples contained the highest contents of rare earth elements ［∑REE: (26.37～32.86)×10-6］, residual oxides samples contained the second highest contents of rare earth elements ［∑REE: (5.58～30.43)×10-6］, and silicon chimneys samples contained the lowest contents of rare earth elements ［∑REE: (0.92～6.96)×10-6］. Besides, all of the samples enrich the light rare earth elements (LREE) and deplete the heavy rare earth elements, with Ce negative anomaly (δCe: 0.34～1.00) and Eu positive anomaly (δEu: 0.87～4.24). Geochemical characteristics of rare earth elements suggest that the samples not only inherited part of the geochemical characteristics of hydrothermal fluid but were also affected obviously by seawater, and the source of REE in the hydrothermal sediments has diversity characteristics. Comprehensive analysis shows that the metallogenic environment of the study area is predominantly in the low temperature due to limited weak hydrothermal activity, and the ore-forming fluid mainly originated in seawater. Therefore, the metallogenic process is greatly influenced by the ambient seawaters mixing action.