At Aberfeldy the bulk of barite sulfur (delta 34 S = 27-36 per mil) was derived during periods of rapid surface and subsurface (?) mixing of hot ( approximately 200 degrees C?) brine with Vendian sea water, enriched in 34 S as a result of bacterial sulfate reduction within the enclosed Dalradian basin. During periods of slower brine-seawater mixing, greater proportion of isotopically light sulfate was derived from the slow oxidation of exhalative reduced sulfur (delta 34 S <28 per mil). The delta 34 S distribution is bimodal (31 and 35 per mil) and individual barite layers become isotopically light upward. Pyrite in the footwall and hanging-wall schists at Aberfeldy incorporates sulfur of bacteriogenic origin. Indications that no equilibrium exchange occurred between barite-pyrite and sulfide-sulfide pairs during greenschist-amphibolite facies metamorphism.--Modified journal abstract.