Devil's Hole Mine, Fremont County, Colorado, USAi

Regional Level Types
Devil's Hole Mine	Quarry
Fremont County	County
Colorado	State
USA	Country

The Devils Hole Mine, Fremont County, Colorado
by Bob Carnein

Information for this article came from Eckel, et al., 1997; Hanley, et al., 1950; Heinrich and Vian, 1965; and mindat.org.

The Devils Hole (aka Zingheim; Wild Rose) mine is a famous source of K-feldspar, scrap mica, beryl, rose quartz, and columbite. Located in Fremont County, about 6 miles (9.6km) north of Texas Creek, the mine was discovered in the early 1900s and worked mainly into the early 1940s. It now belongs to Mike and Ed Tezak. Mike, with his wife Denise, operates the Gold Mine Rock Shop, on US Route 50 near the Royal Gorge. Currently, it is used mainly as a source of rose quartz for decorative purposes.

Historical records show that, besides an unknown amount of rose quartz, the mine has produced at least 300 tons of beryl, 17,000 tons of K-feldspar, 1600 tons of scrap mica (muscovite), and 200 pounds of columbite. The minerals occur in a large mass of pegmatite that is surrounded by high grade metamorphic rocks of varied composition. The pegmatite body trends north-south and has two major branches. Most mineralization occurs in the eastern part of the deposit, known as the "main body". That is the subject of this description.

The main body trends north, dips west at a variable angle, and is 35 to 200 feet (10 to 60m) wide. Its bulbous southern end has 2 minor branches: a west branch extending northwest, then south, then southeast (making a sort of circle); and an east branch trending northeast for 300 feet (91m). The whole body cuts across bedding and foliation, though contacts with surrounding metamorphic rocks are generally gradational. The main body, unlike the rest of the pegmatite, can be subdivided into 3 zones, each characterized by a distinct mineral composition.

The wall zone is the part in direct contact with metamorphic rocks. Ten to 30 feet (10m) thick, it consists of quartz-microcline-muscovite-albite pegmatite in which grain size averages 2 inches (5cm). Some large masses of quartz and microcline are present, and biotite, schorl, garnet, and magnetite occur near contacts with metamorphic rocks. Well formed garnet and schorl crystals are locally common.

An intermediate zone occurs discontinuously along the main body's strike and dip. It contains muscovite-albite-quartz pegmatite and is thickest adjacent to microcline pegmatite in the core zone (described below). In it, radial masses of gray-green muscovite to 5 feet (1.5m) across are made up of wedge shaped "books" of muscovite with pale pink to white quartz. Between the muscovite masses and the wall zone, pale pink albite may contain small black columbite crystals. Beryl occurs throughout but is especially common in the footwall area. The largest crystals mined, which were up to 4 feet (1.2m) across and 20 feet (6m) long, occurred in muscovite; those in albite were generally small.

The core zone consists of two distinct units. A microcline pegmatite, which consisted of 3 large masses, was nearly pure microcline. Cleavages up to 75 by 40 feet (23 x 12m) constitute some of the largest crystals ever described in Colorado. Because of its high aluminum content and scarcity of quartz blebs and stringers, this material fetched a premium price and was the main source of income during active mining in the first half of the 20th century. The microcline pegmatite contains occasional vugs near contacts with the intermediate zone. These may contain quartz crystals to 2 inches (5cm) and rare blebs of apatite and beryl with bright yellow fine grained muscovite.

The second core unit is a quartz pegmatite, consisting of almost pure pale to deep rose quartz. Accessory minerals are concentrated near the north end of the main body; minor bismuth minerals and free gold were identified in underground operations.

Currently, the mineral list for the Devils Hole mine includes the following:

Albite Biotite Gold Quartz
Almandine Calcite Magnetite Schorl
Apatite Columbite Microcline Tantalite
Beryl Columbite-(Fe) Muscovite

Beryl occurs as greenish blue, pale blue, bluish white, and brown crystals from 0.25 inches to 4 feet (0.6cm to 1.5m) across (average 4 inches). Crystals up to 20 feet (6m) long were found with muscovite concentrations in the intermediate zone. Although it occurs scattered throughout the main body, beryl production was concentrated in the intermediate zone and totaled about 300 tons.

Columbite was most abundant in the outer, albitic part of the intermediate zone. Crystals are generally blades that average less than an inch long, though they rarely reached over 4 x 2 inches (10x5cm). Locally, where muscovite concentrations occur along the footwall of the intermediate zone, columbite made up as much as 1 percent of the rock. Production totaled about 200 pounds; most columbite contains very little tantalum.

Muscovite occurred in masses up to 30 x 20 x 4 feet (9x6x1.2m). Crystals tend to be wedge shaped perpendicular to cleavage. Muscovite in this deposit was not suitable for higher value "sheet mica" production, but about 1600 tons of scrap mica were shipped.

Rose quartz owes its color to the presence of microscopic oriented inclusions of a pink borosilicate mineral related to dumortierite. At the Devils Hole mine, masses the size of cars are common in the quartz core, but larger masses often exhibit fractures coated by iron oxide. Careful selection yields small, translucent, somewhat milky, gemmy masses that are suitable for cabochons and other jewelry applications. As is typical or nearly all pegmatites, crystals do not occur here.

Schorl (black tourmaline) occurs as lustrous, euhedral, striated, prismatic crystals up to an inch (2.5cm) across and a few inches long. They are generally enclosed in milky quartz and are extremely brittle, making it a challenge to collect good examples.

Garnet (almandine?) occurs as dark red to black, fractured, euhedral trapezohedral crystals on and in pegmatite. Local concentrations consist of "pebbly" stringers of quarter- to half-inch crystals, many of which exhibit faces.

In addition to the above, the writer has found small (up to 2 inches; 5 cm) milky quartz crystals with calcite and euhedral apatite in vugs. The apatite fluoresces bright yellow, and calcite fluoresces and phosphoresces blue-white in SWUV. Also in the vugs is an unidentified brown mineral, intergrown with calcite, that fluoresces bright red. Other minerals found by the writer include azurite and malachite.

-References-
Eckel, E.B., et al., 1997, Minerals of Colorado, Updated and Revised: Golden, Colorado, Fulcrum Publishing

Hanley, J.B., et al., 1950, Pegmatite investigations in Colorado, Wyoming, and Utah, 1942-1944: U.S. Geological Survey Professional Paper 227.

Heinrich, E.W., and R.W. Vian, 1965, The Chief lithium pegmatite, Devils Hole, Fremont County, Colorado: The American Mineralogist, vol. 50, p. 96-104.