The San Antonio mine is a mesothermal gold deposit hosted in the leucogabbroic portion of the San Antonio sill, a differentiated mafic intrusion in the western part of the Superior province of the Precambrian Canadian Shield. The gold mineralization is associated with a system of quartz-carbonate veins within a moderately plunging cylinder of hydrothermally altered rocks.Alteration zones delimited by mapping alteration isograds encompass individual veins and merge to form the mine-scale alteration. The alteration zones formed through metasomatism of CO 2 , K, and lesser Na as determined by mass balance calculations on whole-rock chemical data. Alteration minerals are dependent on the original rock composition, with actinolite and epidote characteristic of unaltered host rock. Boundaries between individual zones can be mapped as alteration isograds. In order of increasing alteration or decreasing distance from a vein or shear zone, these isograds in the leucogabbro are (1) chlorite and calcite, (2) paragonite and ankerite, and (3) ankerite and muscovite, and in the melagabbro, (1) chlorite and calcite and (2) ankerite and chlorite.Zone 1 contains evidence of the transition from titanite to rutile and calcite. Paragonite, abundant at the San Antonio mine, is much less common in other areas of gold alteration, thus making the San Antonio alteration unique. The formation of paragonite is due to CO 2 metasomatism in an aluminum-rich host rock; its coexistence with muscovite and hydrothermal albite is significant as it defines the K (super +) /Na (super +) ratio of the hydrothermal fluid associated with gold mineralization.Using K (super +) /H (super +) , Na (super +) /H (super +) , and CO 2 , the path of alteration was modeled corresponding to observations of texture and paragenesis. Thermodynamic calculations also suggest that the isograds represent the degree of carbonate, potassic, and sodic metasomatism by reaction of the gold-bearing hydrothermal fluid with the unaltered metamorphic rocks and display the importance of original rock composition on the mineralogy of alteration zones. The conditions of alteration and gold mineralization can be directly determined by understanding the alteration mineralogy and changes at zone boundaries. This ore fluid contained subequal amounts of potassium and sodium, about 5 percent CO 2 , and was within the temperature range of 230 degrees to 375 degrees C.