Talc occurs as massive, economic deposits in upper amphibolite facies marbles of Archean age in southwestern Montana. Previous workers have demonstrated that the talc is a replacement of the marble that resulted from interaction with a large volume of fluid. δ18O (SMOW) values for dolomite and calcite range from 20–25‰ for the unaltered Archean marbles to as little as 8–10‰ in the talc deposits, suggesting that the metasomatic fluids had low δ18O values. In contrast, δ13C values for calcite and dolomite are similar for all samples (–2 to +2‰ PDB). Therefore, it is likely that the metasomatic fluids were oxygen-rich and carbon-poor, namely water-rich and CO2-poor. A CO2-poor fluid is also indicated by Δ13C (calcite-graphite) values (3.6–5.3‰), which appear little altered from values expected for upper amphibolite facies marbles, and by the occurrence of the mineral assemblage talc+calcite. 40Ar/39Ar age spectra for hornblende, phlogopite, and biotite record cooling at 1.72 Ga from a regional thermal event. 40Ar/39Ar age spectra of fine-grained muscovite associated with the talc date talc formation at 1.36 Ga. The Ar data limit the temperature of talc crystallization to below ~350 DEGC, the biotite closure temperature for Ar diffusion. If the metasomatic fluid was seawater (0‰), then the carbonate oxygen data require a minimum temperature of 270 DEGC for talc formation. Oxygen (δ18O = 4.7 to 8.8‰) and hydrogen (D/H = −49.9 to −57.6 SMOW) isotope data for the talc are consistent with a 200–300 DEGC metasomatic fluid derived from seawater, based on theoretical models of the fractionation of oxygen and hydrogen between talc and water. Regional, northwest-trending faults associated with the extension that formed the Belt Basin in the Middle Proterozoic may have provided channels for seawater to circulate in continental crust and to react with marble, forming talc at depths of 5–10 km.