New Mexico Mineral Symposium — Abstracts

Montana hosts a fairly large number of mineral localities where hobbyists can collect nice specimens. Today's presentation will highlight four localities that are not world class, not well known to hobbyists outside of Montana, but are localities where hobbyists can find some very nice specimens.

The Crazy Sphinx mine is near Helena, Montana. The name "Crazy Sphinx" was coined by Bart Cannon who briefly had a claim on the deposit in the 1980s. Mineral collecting at the deposit dates back to at least 1930 and probably much earlier. The mineralogy of the deposit was first described by Knopf in his article describing clintonite from this deposit.

Knopf describes this deposit as a xenolith of limestone enclosed in granodiorite of the Boulder batholith. The skarn is composed predominantly of massive vesuvianite, grossular, and calcite. The most striking feature of this deposit is the blue spinel that is found here. The spinel occurs as octahedrons up to 1.5 inches and are almost always embedded in grossularite or vesuvianite. Careful preparation (or luck) is required to expose the crystals. Other collectible minerals are grossular, vesuvianite, stilbite, and clintonite. The grossular occurs as white to slightly yellowish crystals up to 9 cm. Vesuvianite crystals occur up to 9 cm, but are not common. Stilbite occurs as spherules up to 8 mm and as crusts. Clintonite occurs as snow white fans with a pearly luster.

The Sourdough mine is on the west side of Elkhorn, Montana. Elkhorn Goldfields LLC is presently developing the property. At this property the gold is hosted in skarn related to the Boulder batholith. In 2009, the Montana Crystal Collectors had a field trip to the mine site to collect vesuvianite from a roadcut near the mine. The rock at this roadcut is basically massive vesuvianite with small zones that have well-crystallized vesuvianite. Crystals can reach 2.5 inches in size. During the field trip, Lanny Ream found a number of small red grossular crystals in one small area. A feeding frenzy ensued by the 15 collectors in attendance, and many nice specimens of grossular and anorthite were found in the zone, which measured only 5 x 6 x 5 ft deep. There are vast areas of skarn exposed in the Elkhorn mining district. Surely there should be additional sites that afford excellent specimen collecting.

Bald Mountain is a skarn deposit located in Jefferson County, Montana, and about 25 mi southeast of Butte. The mineralogy of Bald Mountain has been described by Reuss. Minerals of interest are diopside, grossular, vesuvianite, apatite, spinel (black), and blue calcite. Massive fine-grained diopside forms a zone perhaps 100 ft thick and several thousand feet in length. A thick unit of massive white calcite underlies the diopside bed. Within the massive diopside are areas where the diopside is coarsely crystalline and cavities have formed. These cavities are filled by blue calcite that must be dissolved to expose the diopside crystals. The diopside forms excellent crystals up to 1 inch. Much of the diopside is twinned. A bright blue calcite fills the cavities. Grossular is sparingly found in the cavities. The grossular forms excellent crystals up to 1.5 inches. The color ranges from a brownish red to almost an orange color. The garnets occur in combination with diopside and can be very showy. Vesuvianite is fairly rare. It occurs as crystals up to 1/2 inch. Light-yellow apatite (calcium) is also found in the diopside zone, but is rare. Crystals are opaque and light yellow in color and are up to 1 inch. The spinel occurs as shiny black octahedrons up to 5 mm and is very rare.

The PC mine is a specimen mine located about 7 mi from Basin, Montana, and was mined by the author from 1989 to 2005. The deposit is an epithermal deposit hosted in Elkhorn volcanics. The deposit is highly brecciated and has (had) countless "pockets" or "vugs" that were filled with quartz crystals. The site produced a large number of Japan law twins and many tons of quartz crystals, singles and groups. Other minerals produced include pyrite, galena, and sphalerite. Sulfides were very rare in the pockets, but the crystal size was impressive. The sphalerite occurred in crystals up to 9 inches and the galena up to 5 inches. The sphalerite was green to brown, and flawless gems up to 99 carats have been cut.

References:

1. Knopf, A., 1953, Clintonite as a contact???metasomatic product of the Boulder batholith, Montana: American Mineralogist, v. 38, pp. 1113-1117.
2. Reuss, R., 1976, Pipestone mine, Bald Mountain, Boulder batholith, Jefferson County, Montana USA: Mineralogical Record, Jan./Feb.