The "sulfate-reduction plus mixing" hypotheses for the formation of Mississippi Valley-type deposits have suffered from an inability to show a plausible mass balance between sulfate, possible reductants, and sulfide ores, and a mechanism for mixing the reduced gas (H 2 S) with the metal-bearing fluid. Both these problems are resolved by focusing on the gases (mostly methane) generated from the organic matter in the host carbonate rocks during thermal maturation caused by heat from the hydrothermal solutions. In southeast Missouri, it appears that the volume of the Bonneterre Formation required to produce all the methane needed to reduce enough sulfate to precipitate all the ores is approximately the same as the volume of the brown-rock facies, which hosts most of the ore. Therefore it appears that metal- and sulfate-bearing solutions entered the Bonneterre at Lamotte Sandstone pinchouts, then followed permeability trends, which coincided mainly with the oolitic grainstones associated with the stromatolite reef, most of which are brown. Evolution of methane and other reducing compounds from the brown rock resulted in sulfate reduction and precipitation of sulfides. Several other regional and local factors in the geology of the area are found to be compatible with this idea.