The possible role of thiosulfate in the precipitation of 34S-rich barite in some Mississippi Valley-type deposits

The precipitation of extremely ³⁴S-rich barite in the late stage of mineralization in the Mississippi Valleytype deposits of the Illinois-Kentucky district (U.S.A.) may be explained by reactions involving thiosulfate (S₂O₃⁼). Inorganic processes are known to concentrate ³⁴S in the sulfonate site of thiosulfate and ³²S in the sulfate site. In the mineralizing solution, these inorganic processes may have fractionated sulfur between the two sites by about 40 per mil. At the low temperatures of the late barite stage of mineralization, bacteria are known to metabolize thiosulfate by various reactions. In one of these, dissimilatory reduction, hydrogen sulfide and sulfite are produced. Isotopically light sulfite is preferentially reduced to sulfide by bacteria to leave a residual sulfite enriched in ³⁴S. Part of the residual sulfite may be oxidized to form isotopically heavy sulfate; part may recombine with hydrogen sulfide to form thiosulfate. The recombination also enriches the sulfonate site in ³⁴S and the sulfane site in ³²S. Recycling the newly formed thiosulfate through the above steps further enriches sulfite and sulfate from oxidation of sulfite in ³⁴S. During genesis of the ores, the aggregate effect of these reactions may have been the precipitation of extremely ³⁴S-rich barite. The sequence of reactions suggested above requires the presence of organic matter. Previously proposed reactions to account for the precipitation of sulfide minerals and fluorite and for the carbonate paragenesis also require the presence of organic matter. Thus, organic matter in the host rocks may cause the various ore-zone reactions and account for the localization of the ores.