ABSTRACT

The Pohndorf amethyst mine is located in the northern Highland Mountains, approximately 15 mi (24 km) southeast of Butte. The mine has been worked principally for its gem quality amethyst crystals. Museum specimens of scepter amethyst on tourmalinated quartz have been recovered from cavities within a megacrystic microcline phase of the pegmatite. The exposed portion of the pegmatite is distinctly zoned and is hosted by sparsely porphyritic granodiorite of the Boulder batholith. Zoning in the pegmatite ranges from a nearly pure, coarse-grained quartz zone through a two-feldspar phase, to a massive microcline phase with abundant late-stage vugs. The geochemistry of the pegmatite indicates SiO2 content of 70% to 74%, and elevated copper and zinc. Rare Earth Element content of the pegmatite is generally low, as is the gold content. However, gold values as high as 0.47 opt are found in thin quartz veins approximately 1/2 mi (1 km) east of the Pohndorf Mine.

The Pohndorf pegmatite is one of many late-stage pegmatites associated with the Boulder batholith in southwestern Montana. This paper summarizes work recently conducted in the Pohndorf mine area, with particular focus on hand specimen investigation of principal rock types, zoning, and paragenesis in the pegmatite.