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.

Little is known for certain about early Wisconsin (isotope stage 4) lakes and glaciers of the Great Basin. A moderate lake-level rise in the Bonneville basin is not well dated, but on the basis of amino-acid and radiocarbon ages, is thought to be early Wisconsin in age. A moderate rise of lakes in the basins of Lake Lahontan is dated as ca. 50 ka by U-series ages on tufa, but may have occurred earlier. In the southern Great Basin, Searles Lake fluctuated at levels below the threshold connecting it with Panamint Valley, and Panamint Valley apparently did not contain a large lake during the early Wisconsin. The glacial record is even less-well dated than the lacustrine record. The extent of glaciers in and around the Great Basin during the early Wisconsin is not known; ice extent was certainly greater than at present, but probably was less than the late Wisconsin maximum in most glaciated valleys. Further work is necessary to refine lacustrine and glacial chronologies, and to investigate the causes of lake vs. glacier expansion. Important clues to these questions will come from detailed studies of lacustrine and glacial sequences in different parts of the Great Basin.