PETROGENESIS AND TIMING OF TALC FORMATION IN THE RUBY RANGE, SOUTHWESTERN MONTANA

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 ± tremolite, indicating that metamorphism occurred in the upper amphibolite to granulite facies (M1); crystallization of serpentine and chlorite, and dolomitization of the marbles in the greenschist facies overprinted the high-grade calc-silicate minerals in the latest stage of recrystallization which resulted in the formation of the main talc bodies (M3). Structural control of the talc formation is suggested by discontinuous occurrences of talc bodies along layer-parallel fractures. -from Authors