The large-size Chagangnuoer iron deposit is hosted in andesite and andesitic volcaniclastic rocks in the middle-upper part of Carboniferous Dahalajunshan Formation, with widespread high hydrothermal alteration (represented by garnet) distributed extensively around the uppermost ore body FeI. The result indicates that magnetite intergrown with garnet was formed at the late stage of Early Carboniferous period. Therefore, the iron metallogeny and high hydrothermal alteration might have resulted from the metasomatism between the post-magmatic hydrothermal solution derived from eruption of Dahalajunshan Formation volcanic rocks and the underlying marble, rather than the skarnization caused by the reaction between magmatic hydrothermal solution from Permian intrusion and marble in the ore district.