KINETICS OF SULFATE REDUCTION, METASTABLE SULFUR, AND THE GENESIS OF HYDROTHERMAL URANINITE DEPOSITS, MISSISSIPPI VALLEY LEAD-ZINC DEPOSITS, BARITE DEPOSITS AND EPITHERMAL BASE AND PRECIOUS METAL DEPOSITS

At temperatures greater than approximately 250(DEGREES)C, the oxidation state of sulfur controls the redox potential of most hydrothermal solutions. As a sulfate-bearing hydrothermal solution cools to less than about 250(DEGREES)C, the rate of oxidation by sulfate diminishes. Once the solution has cooled to around 200(DEGREES)C, the rate of oxidation by sulfate is so slow that sulfate is essentially removed from the redox chemistry. This loss of an important oxidizing agent without a comparable loss of a reducing agent makes the solution more effective at reduction. Reduction by this process is a likely precipitation mechanism for uraninite, native gold and native silver in epithermal ore deposits.; As a consequence of kinetics, natural redox reactions involving H(,2)S at less than 250(DEGREES)C produce partly oxidized sulfur species. Available kinetic data indicate that these species can exist as long-lived metastable components of epithermal solutions. The partly oxidized sulfur species may act as complexing agents and, more importantly, they allow the solution to carry sulfur in a form compatible with the simultaneous transport of lead, zinc, and barium. In the case of Mississippi Valley-type lead-zinc deposits, reduction of partly oxidized sulfur species by organic carbon in the host rocks produces sulfide sulfur and triggers the precipitation of sulfide ores. Later in the paragenesis, mixing of the same mineralizing solution with oxygenated ground water oxidizes metastable sulfur to sulfate and precipitates barite. In convecting meteoric-hydroghermal systems around plutons, heating causes partly oxidized sulfur species to disproportionate to H(,2)S and SO(,4)('=), which initiates the simultaneous precipitation of sulfide and sulfate minerals.