The Hemingway property, located 1.3 km due north of the Kidd Creek volcanogenic massive sulfide deposit in Kidd township, Ontario, is part of the Kidd Creek volcanic complex. Ore reserves and past production of the mine total 130 million metric tons of ore at 2.86 percent Cu, 6.14 percent Zn, 0.22 percent Pb, and 85 g/t Ag. Felsic volcanic rocks and ultramafic flows and sills intercalated with the felsic rocks have been overprinted by extensive hydrothermal-metasomatic alteration at the Hemingway property and at the Kidd Creek mine. The earliest significant alteration identified in the ultramafic rocks is serpentinization, and in the rhyolite, silicification. Pervasive CO 2 metasomatism of the serpentinized ultramafic rocks and the silicified rhyolite resulted in extensive carbonate alteration of both rock types. Late sericite-fuchsite, chlorite, and minor (second generation) carbonate alteration overprinted the first generation carbonates. It is suggested that silicification and carbonate alteration may represent an early synvolcanic hydrothermal episode associated with ore deposition, whereas the sericite-fuchsite, chlorite and late carbonate alteration was superimposed on the deposit during later metamorphism-metasomatism.Pervasive CO 2 metasomatism at the Hemingway property was a relatively low temperature event. Microthermometric measurements were obtained on primary fluid inclusions in carbonates and quartz from the ultramafic rocks. Since phase separation was observed, the mean homogenization temperature of fluid inclusions provides a formation temperature estimate of 248 degrees + or - 12 degrees C. The CO 2 content of the fluid ranges from <0.85 mole percent in the talc-carbonate-altered ultramafic rocks to 10 to 75 mole percent in the quartz-carbonate-altered ultramafic rocks. Fluid inclusions in the quartz-carbonate rocks contain liquid CO 2 and up to 1 mole percent CH 4 . The salinity of the fluid ranges from 1 to 6 wt percent NaCl equiv. The calculated oxygen isotope composition of the fluid in equilibrium with the carbonates ranges from 0 to 3.5 per mil; delta 13 C values of carbonates define a narrow range of 0 to 1 per mil. The oxygen isotope composition of the fluid is consistent with a modified seawater source, but the presence of liquid CO 2 in the inclusions precludes seawater as the source of the CO 2 . One possible explanation for the high CO 2 concentration may be that CO 2 had a volcanic source and was produced during magma degassing. Although the delta 13 C composition of magmatically derived CO 2 is -3 to -5 per mil, delta 13 C values could have increased as CH 4 was generated from the interaction of the fluid with carbonaceous sediments.The hydrothermal-metasomatic evolution of the rocks at the Hemingway property and the Kidd Creek massive sulfide deposit is comparable to some ultramafic rock-hosted gold deposits in the Abitibi belt. The similarity in previously reported ages of secondary accessory minerals such as monazite, titanite, rutile, fuchsite, and sericite at gold deposits and at Kidd Creek suggests that the hydrothermal and metasomatic evolution of these deposits occurred on a regional scale and should be evaluated in a regional context. Early, pervasive carbonate alteration may have been contemporaneous with the syngenetic deposition of massive sulfides at Kidd Creek, and pervasive carbonate alteration of ultramafic host rocks in gold deposits, such as Dome, Kerr Addison, and Aquarius, may have predated subsequent alteration events (quartz-carbonate veins, fuchsite, chlorite + or - rutile) associated with gold.