It is widely accepted that porphyry type deposits form from some oxidized ore fluids. According to related research results, the CH₄ is proposed to be derived from the contamination of S-type granite into I-type one, but it is also possible that CH₄ comes from the degassing process of the Earth. A chemical model is set in order to interpreting the role of reduced fluids in porphyry metallogenesis. According to the model, H₂S and CO₂ are derived from the reaction between CH₄ and SO₂ both of which come from one evolved igneous system, which will cause the precipitation of the molybdenite and cogenetic calcite.