The Sen-nin hematite deposits have been generally considered to be of contact metasomatic type related to nearby Cretaceous granitic rocks. Another opinion is that the mineralization is connected with the acidic volcanism of the Middle Miocene. The hematite ores have reversed remanent magnetization. Of five igneous rock bodies in the area, ranging in age from Cretaceous to Neogene, only the Miocne plagiorhyolite body has similar direction and polarity. The other bodies are normal. It is concluded that the Sen-nin hematite deposits were genetically related to the plagiorhyolite. It is also suggested that the mineralization is middle Middle Miocene in age.