Northwestern of Guizhou Province (it mainly includes Shuicheng area and Hezhang area), where plenty of middle—large scale magnesian siderite deposits and siderite deposits distributed, is located at the southwest margin of the Yangtze block. Hezhang area and Shuicheng area are the two largest iron ore clusters in Guizhou. The iron formation occurs in the carbonate of the Devonian and Carboniferous. The petrological, mineralogical, and geochemical characteristics of the Devonian magnesian siderite deposits in Shuitang, Xiongfei and Tiekuangshan at Hezhang county, Northwest Guizhou were reported in this paper. The ore bodies are mainly layered, stratoid or lode shaped in form and are hosted in the Devonian carbonate rocks. The major two types of ore, beige and gray magnesian siderite, usually have massive, banded or granular textures, and the paragenetic gangue minerals including dolomite, feldspar, pyrite and quartz. The feldspar that is associated closely with quartz shows the typical texture of saddle, which probably implies that hydrothermal superimposed transformation evolved in the process of iron mineralization. In addition, the higher Th (0.4×10-6~2.3×10-6, mean 1.1×10-6 and 0.2×10-6~3.2×10-6, mean 1.3×10-6, respectively), Zr (2.7×10-6~20.6×10-6, mean 8.1×10-6 and 1.3×10-6~17.8×10-6, mean 7.5×10-6, respectively), Sc (0.5×10-6~3.9×10-6, mean 1.3×10-6 and 0.1×10-6~3.6×10-6, mean 1.4×10-6, respectively) and Al2O3 (0.24%~1.78%, mean 0.85% and 0.06%~2.77%, mean 1.18%, respectively) content of Shuitang and Xiongfei magnesian siderite ores than Tiekuangshan magnesian siderite ores (Th: 0.2×10-6~0.8×10-6, mean 0.4×10-6, Zr: 0.6×10-6~9×10-6, mean 4×10-6, Sc: 0.1×10-6~1.7×10-6, mean 0.5×10-6 and Al2O3: 0.01%~1%, mean 0.38%) indicates that the weathering material from continental source evolved in the process of mineralization of former, while the high Eu/Eu* (6.73~8.94, mean 7.68) but low Y/Ho (26.67~33.33, mean 30.01) demonstrate that the hydrothermal activities played an more important role in the process of mineralization of later. Combing with the study of isotope sulfur and TOC, the sedimentation—transformation genesis for this deposit has been concluded.