The Rose Mountain area northwest of Faro is underlain by a succession of phyllites, cherts, sandstones, chert-pebble conglomerates and basalt belonging to the Slide Mountain Terrane. At least two, laterally extensive, massive barite horizons are interbedded with phyllite, bedded chert, sandstone and chert-pebble conglomerate of the Early Carboniferous Mount Aho formation, the lowermost unit in this succession. Samples from two barite horizons produced 34S values of +14.3 ± 0.2 (2) and +13.8 ± 0.2 (2), which are consistent with 34S values for coeval dissolved sulphate in seawater. Ambient seawater is therefore the most probable source of the sulphate in the barite. The results suggest barite precipitated from mixing of sulphate-poor hydrothermal fluids with ambient seawater rather than inversion of anoxic, stagnant, stratified basin waters.