Fluids responsible for regional 18O-depletion of Neoproterozoic igneous rocks in Avalonia are investigated here through a petrographic, microthermometric, and stable isotopic examination of fluid inclusions and minerals from the abundant vein networks of the Mira terrane, Cape Breton Island. Six categories of vein assemblages — from oldest to youngest — are present: (i) quartz–albite, (ii) quartz–epidote, (iii) quartz, (iv) quartz–chlorite–calcite, (v) quartz–calcite, and (vi) calcite. Vein system temperatures were initially as high as ∼300 °C and gradually decreased to ∼200 °C. Moderate salinities (<8 equivalent wt.% NaCl) characterize entrapped fluids in the early quartz–albite veins but decrease in later quartz–epidote and quartz–calcite veins to <1 equiv. wt.% NaCl. The limited range of fluid δ18O values (–1.9‰ to +1.4‰) calculated for most of the vein assemblages is suggestive of a seawater-dominated system, as are the δDH2O values (–12‰ to –3‰) obtained for epidote. Decreasing fluid salinities, however, suggest that meteoric water became dominant during later stages of vein formation. The carbon isotopic compositions of trace CO2 and CH4 from the fluid inclusions (δ13CCO2 = –22‰ to –4‰; δ13CCH4 = –52‰ to –37‰) are indicative of externally derived (i.e., non-magmatic) fluids of organic origin.