GEOCHEMISTRY OF LATE (APPROXIMATE-TO-1.1-GA) FLUID INCLUSIONS IN ROCKS OF THE KAPUSKASING ARCHEAN CRUSTAL SECTION

Three outcrops, well constrained by geochronological and structural st udies, and representing a traverse running from tonalite-dominated out crops in the eastern Wawa gneiss terrane to high-grade granulites of t he Kapuskasing structural zone, were mapped and sampled in detail in o rder to study the trapped fluids. All fluid inclusions in quartz are s econdary and consist mostly of CO2-dominated (type II) and saline aque ous (type ma) fluids usually occurring on separate healed fractures bu t also coexisting on some fractures. Healed fractures in quartz contai n fluid inclusions but are associated with carbonate- sericite alterat ion where they pass into adjacent mineral grains. Homogeneous H2O-CO2- salt fluid inclusions (type Ia) in carbonate-rich veins of probable Ke weenawan (similar to 1.1 Ga) age were trapped at 400-550 degrees C and ambient pressures of 1.5-2 kbar (1 kbar = 100 MPa). As these fluids c ooled on penetration into cool (similar to 200 degrees C) country rock s along fractures they underwent open-system H2O-CO2 phase separation from similar to 350 degrees C down to similar to 190 degrees C, produc ing a range of fluid compositions, including physically segregated CO2 -rich (type II) and H2O-salt-rich (type IIIa). Combined gas and ion ch romatographic bulk fluid inclusion analyses show that fluid types II a nd IIIa are not related to shield brines. Br-/Cl- ratios of samples co ntaining phase-separated fluids are similar to the Br-/Cl- ratio of fl uids in the carbonate-rich vein. The results of this study show that K eweenawan alkalic magmatism caused widespread carbonate alteration thr oughout the Kapuskasing structural zone and Wawa gneiss domain. The CO 2 component of the fluids is probably magmatic in origin, whereas the aqueous part could also be magmatic or, alternatively, formation water s activated by Keweenawan magmatism.