In Earth's crust, thermochemical sulfate reduction (TSR) is a common organic-inorganic interaction, which is in close association with the carbon cycle in sedimentary basins and metal sulfide precipitation in Mississippi Valley-type deposits. However, the reaction pathway and mechanism of TSR need further investigation, mainly due to the complex sulfur species involved in this reaction. Here we applied in situ Raman spectroscopy to disclose the stepwise transformation from sulfate to sulfide and the reaction kinetics in the CH₄-Na₂SO₄/MgSO₄/H₂SO₄-H₂O systems at elevated temperatures and pressures. Our results indicate that sulfate is transformed to H₂S through a series of reduction and disproportionation reactions. Once dissolved H₂S reaches a certain concentration, it reacts with sulfate to form intermediate valence sulfur species (S-0 and S-3(-)), which quickly oxidize methane to generate CO₂ and H₂S. The formation of S-3(-) and S-0 is responsible for the autocatalytic effect in TSR. The sulfur speciation significantly affects the reaction kinetics, which could be divided into three stages based on the in situ observations. Extrapolated kinetic data show that TSR could proceed fast under the catalysis of H₂S, and the half-life of methane is similar to 7.10 million years at 200 degrees C. Subsequent numerical modeling strongly supports that in situ sulfate reduction could provide sufficient reduced sulfur for the formation of giant Mississippi Valley-type deposits, thus serving as an efficient mineralizing mechanism.