The generation of the H₂S required to precipitate the ore minerals found in MVT deposits can be accomplished by sulfate reduction, or by the heating of sulfur-bearing organic compounds in shales or in petroleum. Bacterial sulfate reduction (BSR) requires temperatures well below those that occur during the formation of MVT deposits, and therefore must take place before ore deposition, and be held in a trap that is intersected by the metal-bearing fluid in order to be effective. The rates of both TSR and of the release of H₂S from organic material are very low at these temperatures, and it is suggested that MVT deposits having large, well crystallized sphalerites such as those in Central Tennessee are formed when H₂S is generated from sulfur-bearing organic material either carried in the ore solution itself or in the rock column through which the metal-bearing ore solution passes, and that ore deposition occurs during the evolution of the H₂S. The heating of the organic material may well be caused by the introduction of the hot aqueous metal-bearing fluids.