Three outcrops, well constrained by geochronological and structural studies, and representing a traverse running from tonalite-dominated outcrops in the eastern Wawa gneiss terrane to high-grade granulites of the Kapuskasing structural zone, were mapped and sampled in detail in order to study the trapped fluids. All fluid inclusions in quartz are secondary and consist mostly of CO2-dominated (type II) and saline aqueous (type IIIa) fluids usually occurring on separate healed fractures but also coexisting on some fractures. Healed fractures in quartz contain fluid inclusions but are associated with carbonate–sericite alteration where they pass into adjacent mineral grains. Homogeneous H2O–CO2–salt fluid inclusions (type Ia) in carbonate-rich veins of probable Keweenawan (~ 1.1 Ga) age were trapped at 400–550 °C and ambient pressures of 1.5–2 kbar (1 kbar = 100 MPa). As these fluids cooled on penetration into cool (~ 200 °C) country rocks along fractures they underwent open-system H2O-CO2 phase separation from ~ 350 °C down to ~ 190 °C, producing a range of fluid compositions, including physically segregated CO2-rich (type II) and H2O–salt–rich (type IIIa). Combined gas and ion chromatographic bulk fluid inclusion analyses show that fluid types II and IIIa are not related to shield brines. Br−/Cl− ratios of samples containing phase-separated fluids are similar to the Br−/Cl− ratio of fluids in the carbonate-rich vein. The results of this study show that Keweenawan alkalic magmatism caused widespread carbonate alteration throughout the Kapuskasing structural zone and Wawa gneiss domain. The CO2 component of the fluids is probably magmatic in origin, whereas the aqueous part could also be magmatic or, alternatively, formation waters activated by Keweenawan magmatism.