Disc- and spherical-shaped sphalerite concretions, containing variable amounts of carbonate, occur in thin-bedded Silurian dolomite of southern Ontario. Seven of these range from 27 to 92 weight percent ZnS and trace element analyses show that the sphalerite is relatively pure. Sulfur isotope compositions of 15 specimens ranged from --1.48 to--5.41 delta 34 Sppm with an average of --3.05ppm. The presence of possible gypsum molds in the concretions, the isotopic composition of sulfur, the high zinc and hydrocarbon content of the host dolomite, and the broader geological environment of their occurrence, together suggest a biogenic-diagenetic process for the formation of the concretions.

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