Abstract

Mississippi Valley-type ore samples from two mines in the Viburnum trend of southeast Missouri have been examined with the SHRIMP ion microprobe to determine the extent of their microscopic sulfur isotope variations. Approximately 140 SHRIMP delta 34 S values were obtained. Microscopic delta 34 S variations within and among the intergrown Fe-Cu-Pb-Zn sulfides are very large (-10 to +25%) and indicate an extremely complex history of mineral deposition. Textures record the general paragenesis: Fe sulfide, galena and sphalerite, chalcopyrite and bornite, and covellite. Early FeS 2 (delta 34 S = -10 to + 10%) is generally much more depleted in 34 S than later galena or chalcopyrite (delta 34 S = 0-25%). Both early FeS 2 and late chalcopyrite growth sequences exhibit large zonations in delta 34 S, typically from light to heavy values over a 15 per mil range. Isotopic zoning in galena crystals is comparatively negligible, but variations among different galena crystals encompass a large total range in delta 34 S values (0-25%) that is similar in magnitude to the total ranges observed in early FeS 2 and late chalcopyrite. Where later sulifides crosscut and replace earlier sulfides, the SHRIMP data indicate that there has been negligible local recycling of S from earlier sulfides in the ore-forming zone; each generation of metal appears to have brought in its own isotopically distinct and evolving batch of sulfur. During successive deposition of each metal generation, the observed delta 34 S variations require mixing among two or more isotopically distinct distal source fluids feeding already reduced sulfur to the ore-forming zone, and/or the existence of in situ sulfur isotope fractionation mechanisms, producing reduced sulfur whose isotopic composition varied progressively with time in the ore-forming zone.

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