UNRAVELING THE PETROGENESIS OF ORE BODY TALC DEPOSITS: A GEOCHEMICAL AND PETROLOGICAL STUDY

Talc, a magnesium phyllosilicate, is used in many products, including paints, rubber, ceramics, cosmetics and plastics. Talc mineralization generally occurs under metasomatic, 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 [1]. Formation processes of the talc bodies control the inherent compositions of the rock and can dictate which impurities are present. High purity talc deposits are found throughout the southwestern region of Montana, where recent litigation has highlighted the importance of understanding the petrogenesis of these deposits and the possible impurities present within the talc. We report petrographic, mineralogical, geochemical and rare earth element (REE) data from talc and surrounding metasedimentary rocks in a deposit near Alder, Montana, to determine the conditions present during the hydrothermal alteration of the carbonate host rock to talc. Preliminary results indicate that dolomitic marble was pseduomorphically replaced by talc. This implies that sufficient magnesium supplied from the host rock and silica 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 REEs, with generally flat patterns relative to chondrites. The most common accessory mineral within these samples is clinochlore. Most marble host units have similar REE patterns but higher REE concentrations than the pure talc samples, suggesting that the talc REE was inherited primarily from the carbonate. Reference Cited: [1] Greenwood, W, 1998, MS Thesis, Univ. Maryland, College Park, MD.