MICROSCOPIC SULFUR ISOTOPE VARIATIONS IN ORE MINERALS FROM THE VIBURNUM TREND, SOUTHEAST MISSOURI - A SHRIMP STUDY

Mississippi Valley-type ore samples from two mines in the Viburnum tre nd of southeast Missouri have been examined with the SHRIMP ion microp robe to determine the extent of their microscopic sulfur isotope varia tions. Approximately 140 SHRIMP delta(34)S values were obtained. Micro scopic delta(34)S variations within and among the intergrown Fe-Cu-Pb- Zn sulfides are very large (-10 to +25 parts per thousand) and indicat e an extremely complex history of mineral deposition. Textures record the general paragenesis: Fe sulfide, galena and sphalerite, chalcopyri te and bornite, and covellite. Early FeS2 (delta(34)S = -10 to 10 part s per thousand) is generally much more depleted in S-34 than later gal ena or chalcopyrite (delta(34)S = 0-25 parts per thousand). Both early FeS2 and late chalcopyrite growth sequences exhibit large zonations i n delta(34)S, typically from light to heavy values over a 15 per mil r ange. 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 parts per thousand) that is similar in magnitude to the total ranges observed in early FeS2 and late chalc opyrite. Where later sulfides crosscut and replace earlier sulfides. t he SHRIMP data indicate that there has been negligible local recycling of S from earlier sulfides in tile 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 g eneration, the observed delta(34)S variations require mixing among two or more isotopically distinct distal source fluids feeding already re duced sulfur to tile ore-forming zone, and/or the existence of in situ sulfur isotope fractionation mechanisms, producing reduced sulfur who se isotopic composition varied progressively with time in the ore-form ing zone.