Constraints on the Petrogenesis of a Proterozoic Talc Deposit in Southwestern Montana: A Petrological and Geochemical study

Talc, a magnesium phyllosilicate, is used in many products, including paints, rubber, ceramics, cosmetics, and plastics. Talc mineralization generally occurs in low-grade metamorphic conditions and requires a significant source of magnesium. Large amounts of Al, Ca, or K in the formational environment limit talc mineralization in favor of other minerals such as chlorite, tremolite, and biotite. Formation processes, such as metamorphism or hydrothermal events, of the talc bodies, control the inherent compositions and can dictate which impurities are present. This study focuses on a talc deposit near Alder, Montana, one of a series of high purity Precambrian deposits within this region. Petrographic results indicate that dolomitic marble was pseudomorphically replaced by talc. This implies that sufficient magnesium was supplied from the host rock and silica was supplied by the hydrothermal fluid. Relatively pure (>90% by XRD) talc samples have only trace amounts of Al, Ca, and K, and are very low in rare earth elements (REE), with generally flat chondrite-normalized REE patterns by XRF and ICP-MS. For these samples, the most common accessory mineral is clinochlore. Acetic acid leachates from carbonate-rich units yield light REE enriched patterns and higher REE concentrations than the pure talc samples. Rare earth element patterns and concentrations suggest that the talc inherited its REEs from the carbonate during recrystallization from carbonate to talc. Sm-Nd isotope data from the carbonate samples define a linear trend corresponding to an age of 1.42±0.07 Ga, which is consistent with the inferred age of the hydrothermal event responsible for the talc formation