Pegmatites of Montana

Abstract

Montana pegmatites are of three main age groups: pre-Beltian, both metamorphosed and unmetamorphosed types; Laramide, associated with the Boulder batholith; and Tertiary, related to scattered alkalic stocks and small intrusives. Deposits of sillimanite, kyanite, corundum, mica, fergusonite, and chromite occur with pre-Beltian pegmatites. Laramide pegmatites contain graphite, mica, feldspar, gem amethyst, and rose quartz. Syenitic Tertiary pegmatites are characterized by numerous unusual minerals and by associated vermiculite deposits.