Talc deposits in the Ruby Range, southwestern Montana, have formed in Archean dolomitic marbles that were mineralized during retrograde talcification in the Proterozoic. Calcite-rich marbles contain the assemblage calcite-olivine-phlogopite + or - tremolite, indicating that metamorphism occurred in the upper amphibolite to granulite facies (M 1 ); crystallization of serpentine and chlorite, and dolomitization of the marbles in the greenschist facies overprinted the high-grade calc-silicate assemblages (M 2 ); and talc has replaced dolomite and the other greenschist-grade silicate minerals in the latest stage of recrystallization which resulted in the formation of the main talc bodies (M 3 ). Structural control of the talc formation is suggested by discontinuous occurrences of talc bodies along layer-parallel fractures. Preservation of relict textures indicates that the talc formed during constant volume replacement. Geochemical analyses and mass balance calculations demonstrate that introduction of large quantities of Si and Mg and removal of Ca is required to maintain constant volume. Model reactions indicate that infiltration of a minimum of 4.8 x 10 5 1 of water is needed to convert a cubic meter of the average high-grade marble to talc (volumetric water/rock ratio > 600). The infiltration mechanism is consistent with the observation that no isobaric univariant mineral assemblages were present to buffer the composition of the metamorphic fluids. Estimates of the maximum conditions of talc formation of 2 kbars and 400 degrees C are based on field observations and heterogeneous phase equilibria; it is probable that the talc actually formed at much lower temperatures and pressures, possibly in a near-surface hot spring deposit. Regional geologic relations indicate that the talc mineralization occurred during the middle Proterozoic.