Abstract We summarize the geologic settings, generalized geology, and inferred conditions of talc formation for two major deposits in southwestern Montana. Imerys Talc operates the Yellowstone Mine in the Gravelly Range. Barretts Minerals Inc., a subsidiary of Minerals Technologies Incorporated, mines talc from two large deposits—the Regal and the Treasure—in the southern Ruby Range. Talc mineralization in southwestern Montana is associated with hydrothermal alteration of Archean dolomitic marbles along faults in the southern margin of the middle Proterozoic Belt Seaway. Conditions of talc formation appear to have varied across the region and probably range from shallow hot spring systems to connate brine circulation pathways in Belt basin sediments. A road log description of the geology along a loop from Bozeman to Dillon, Montana, to visit both the Yellowstone and Regal talc mines accompanies this paper.

Standard petrographic (e.g., QFL) and chemical (e.g., bulk chemistry) methods for provenance determination successfully classify approximately 55 to 85% of the empirical data on which the methods are based. New approaches in provenance analysis include: (1) age dating of single zircon grains or rock fragments; (2) calculating the P-T-t paths of metamorphic clasts; (3) interpreting rare earth element distribution and Sm-Nd isotope systematics to infer mantle precursors; and (4) analyzing single groups of minerals; Fe-Ti oxide minerals appear to be an especially promising group of minerals because, despite extensive chemical alteration, intergrowth patterns assist in identifying ilmenite and magnetite precursors. The goals of quantitative provenance analysis are to estimate both the proportions of source rocks represented in a body of detrital sediments and the rates at which the detritus was derived. We have quantitatively compared the distribution and the chemistry of selected heavy mineral species in Holocene sands and the sandstones of the Oligocene Renova Formation in southwestern Montana; field evidence suggests that the source rocks of these sands and sandstones were the same. Our previous statistical analysis of the chemical and textural properties of detrital opaque oxide minerals in the sands and sandstones suggested that the Renova sandstones consist of 77% igneous (mostly granodiorites and dacites) and 23% metamorphic (mostly amphibolite-facies schists and gneisses) detritus. We have developed a first approximation of an algebraic mass balance model, using characteristic index minerals for specific parent rocks, which would calculate the proportions of parent rocks that were eroded to produce a body of detrital sediments. The largest uncertainty in the model is in the estimation of the proportions of parent rocks that must be eroded to release equal amounts of the respective index minerals, and in the relative preservation potentials of the index minerals during weathering and diagenesis. We have applied this model using garnets and zircons as two index minerals to compare the proportions of parent rocks of Holocene sands and Renova sandstones in southwestern Montana. Microprobe analyses of 574 single grains of garnet and zircon show that the compositions of detrital garnets and zircons in the Holocene sands and Renova sandstones are similar, suggesting that diagenetic alteration of these two minerals in the Renova has not been extensive. Our model calculations, based on modal analyses for zircons and garnets in 143,607 heavy mineral grains, suggest that ratios of igneous and metamorphic source rocks that were eroded to produce the Holocene sands and Renova sandstones in the study area are 81:19 and 82:18, respectively. The close correspondence of these two ratios possibly suggests that relative contributions of igneous and metamorphic sources in this locality have not changed much since the Oligocene time.