Abstract

Sulfur isotopic compositions of coprecipitated anhydrite, chalcopyrite, and pyrite from late stage porphyry-type ore mineralization at Mines Gaspe, Murdochville, Quebec, suggest isotopic disequilibria between sulfate and H 2 S in the ore-forming fluid. The delta 34 S sulfate-sulfide values range between 6.6 and 12.0 per mil, yielding apparent isotopic temperatures between 800 degrees and 550 degrees C. However, microthermometric measurements of primary fluid inclusions in quartz, anhydrite, and fluorite indicate depositional temperatures between 425 degrees and 200 degrees C. The delta 34 S pyrite-chalcopyrite values from nonanhydrite-bearing assemblages also yield a wide range of isotopic temperatures not in agreement with the fluid inclusion data. The delta 34 S pyrite-chalcopyrite values from anhydrite-bearing assemblages yield isotopic temperatures in general agreement with the fluid inclusion data. In addition, these delta 34 S pyrite-chalcopyrite values increase with increasing delta 34 S sulfate-sulfide values. Thus, it appears that the anhydrites and sulfides are in systematic isotopic disequilibria.Pyrite and chalcopyrite approach sulfur isotopic equilibrium with an H 2 S-bearing fluid more quickly than aqueous sulfate and H 2 S equilibrate isotopically. We therefore suggest that the sulfur isotopic compositions of chalcopyrite and pyrite in anhydrite-bearing assemblages were controlled by their relatively rapid approach to sulfur isotopic equilibrium, whereas the delta 34 S sulfate-sulfide values were controlled by the relatively slower rate of isotopic equilibration between aqueous sulfate and H 2 S. The sulfur isotopic compositions of the isotopically lightest anhydrite and isotopically heaviest chalcopyrite (smallest delta 34 S anhydrite-chalcopyrite value) are assumed to reflect the original disequilibrium isotopic compositions of sulfate and H 2 S in the ore-forming fluid, respectively. The changing isotopic compositions of pyrite and chalcopyrite precipitated from the solution, with time, were calculated from the inferred original delta 34 S values of sulfate and H 2 S. These calculated model isotopic values for pyrite and chalcopyrite are in agreement with the measured values.The consistent isotopic disequilibria between sulfates and sulfides places severe constraints on the temporal evolution of the ore-forming fluids at Mines Gaspe and has important implications for: redox disequilibrium in the fluid, mixing of sulfate- and H 2 S-rich fluids; hydrothermal dissolution, transport, and redeposition of earlier sulfides; and the preservation of earlier, higher temperature isotopic equilibria.Reexamination of the sulfur isotopic systematics of other porphyry-type ore deposits, most notably the El Salvador, Chile, deposit, suggests that sulfur isotopic disequilibria between aqueous sulfate and H 2 S may be an important feature of porphyry copper ore formation and may reveal remobilization of earlier precipitated ore constituents as well as record changes in the chemistry of the ore-forming fluid.If the rate constant for sulfur isotopic exchange equilibria between aqueous sulfate and H 2 S is known, sulfur isotopic disequilibria in porphyry-type systems may place limits on the residence times of sulfur species in solution. If the location of the source of the initial sulfur isotopic disequilibrium is known, these residence times may yield estimates of fluid flow rates in porphyry-type vein systems.

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