The Acton Vale limestone units of probable Early Ordovician age in the external nappe zone of the Appalachian foldbelt in southeastern Quebec host numerous occurrences of vein- and breccia-type copper deposits. Associated with these are two significant occurrences of barite: one at Upton consisting of a stratiform mass of probable economic potential; and the other at Lord Aylmer consisting of thin, tabular, karstic infillings of barite. Studies of the mineral occurrences at Acton Vale, Upton, and Lord Aylmer show that the mineralization is epigenetic and formed largely by open-space filling at shallow depths in parts of the limestones that had undergone ground preparation by brecciation and (or) karsting. Fluid-inclusion, sulphur-isotope, and strontium-isotope analyses show that mineral deposition occurred at temperatures ranging from 110° to 135 °C, that sea water formed the bulk of the mineralizing fluids, and that altered basaltic volcanic rocks associated with the Acton Vale limestones are. the most important source of metals and Ba.The mineral deposits seem to have formed during the evolution of the passive margin of Laurentia. The preferred model for their genesis is one involving deep circulation of water, which, on coming into contact with the volcanic rocks, leached metals and Ba and subsequently deposited them in favourable parts of Acton Vale limestones. Recurrent movements of faults of the Ottawa Graben, which is the failed arm of the Sutton Mountains triple junction, may have been a factor that facilitated deep circulation of fluids. In their present setting, the Acton Vale limestones and the associated volcanic rocks and shales probably represent slivers of the passive margin incorporated into the Cambrian shale – feldspathic sandstone assemblages of the Granby Nappe.