Abstract: "carbonaceous sand slate" in Deerni area is a rock name used for more than 30 years, because this kind of rock is irrelevant to mineralization. Through detailed research work，the authors discovered that this kind of rock can be divided into two kinds: one is terrigenous clastic rock, which is a kind of normal sedimentary rock, i.e., carbonaceous sandy slate, having no direct relationship with mineralization, whereas the other is exhalative rock formed by submarine exhalation—ferro-siliceous rock. It is thus inferred that the Deerni large-size Co-Cu deposit was mainly formed by submarine exhalative-sedimentation and was later enriched by superimposition of multi-stage mineralization. This discovery has important theoretical and guiding significance in search for such ore deposits in Deerni area.