Syngenetic theories of sulfide ore formation often include microbial sulfate-reduction as the source of sulfide. The conditions required for large scale reduction of sulfate, quantitative aspects, the potential and the limitations of microbial sulfate-reduction are sometimes not fully appreciated or may even be misunderstood by those participating in the debate on syngenesis of sulfides. The biochemical factors involved are considered separately and this is followed by a general discussion. Both qualitative and quantitative aspects of microbial sulfate-reduction are adequate for syngenetic theory. The conditions required for sulfate-reduction cannot be specified simply but are satisfied by various combinations of circumstances that result in oxygen depletion in a wide variety of physical environments. Metals entering such environments, whether adsorbed or not, are converted to metal sulfides. Sulfate-reduction does not explain the origin of the metal moiety. Separation and concentration of adsorbed metals and the identification of biochemicals in ore-bearing strata need further research. Objections to syngenesis based upon toxicity of metals are not valid. Biochemical factors cannot conclusively establish the origin of sulfide deposits but are consistent with a sedimentary origin. Microfossils in Precambrian pyrite are not identifiable with modern sulfate-reducers but do contain organic matter. On biochemical grounds, it is probable that microbial sulfate-reduction had evolved by the early Precambrian.

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