New Mexico Mineral Symposium — Abstracts

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The Quartz Creek pegmatite field is located 17 miles almost due east of Gunnison, in Gunnison County, north and south of the road connecting Parlin and Ohio City. The 1,803 mapped pegmatites are centrally located around the Black Wonder pegmatitic granite and cover an area of 26 square miles (Hanley, Heinrich, and Page 1950; Staatz and Trites 1955). The exposed pegmatites form an example of a zoned field where the pegmatites closest to its source pluton are geochemically less evolved with the most distal pegmatites being the most geochemically evolved. The minerals found in these pegmatites are among the most rare in Colorado.

The Proterozoic country rock of the Quartz Creek pegmatite field formed during the Routt orogenic event that spanned the age range of 1,770 to 1,670 million years including post-orogenic granitic plutons. Extensive age dating, however, shows that the Quartz Creek pegmatite field was intruded during the Berthoud anorogenic event at 1.39 + 40 million years. Most of the pegmatites crystallized within fractures that crosscut the foliation of a hornblende gneiss and less frequently within the older granitic and volcanic rocks.

Cerny (1982, 422) was the first to suggest that the Black Wonder pegmatitic granite was the parental source to the pegmatite field, based solely on a rough mineralogical zoning of the surrounding pegmatites. Most of the pegmatites are simple, and frequently unzoned (comprising 78% of the total). They are located northeast of the Black Wonder and contain biotite and magnetite. The beryl-bearing pegmatites are found close to the Black Wonder as well as more distally to the south and west. The lithium-rich pegmatites, the Brown Derby, Bazooka, White Spar and Opportunity pegmatite groups, are the most distal, being found only to the south and southwest. The zoned pegmatites tend to be zoned in layers (as opposed to concentrically zoned), which is interpreted to reflect the influence of gravity during crystallization on moderately dipping pegmatite dikes.

At least twenty-seven minerals have been identified from this field (Elder 1998). Specimen quality lepidolite, non-gemstone elbaite, beryl, monazite, and columbite-tantalite have been found. Rarer minerals of microlite, rynersonite, gahnite, zircon, allanite, spodumene, amblygonite and stibiotantalite can also be found but are often unattractive. The Brown Derby pegmatite remains the best source of elbaite specimens in this district. Lepidolite is known from several other pegmatites in flat and curved crystals as well as the more common fine-grained masses that are ideal for polishing. Although beryl occurs at the Brown Derby, other pegmatites contain better crystallized specimens.

The Brown Derby pegmatite is the most highly geochemically evolved pegmatite in Colorado is. The premier indication of chemical evolution is the presence of pollucite, a cesium-bearing zeolite which has been found only in the Brown Derby pegmatite. This lithium-cesium-tantalum (LCT) geochemical class of complex pegmatite is formed at approximately 2.5 kilobars pressure (approximately 10 km below the surface) and 550º C. The Brown Derby was mined since its discovery in 1932 for beryl, microlite, lepidolite, and mineral specimens.

Most of the pegmatites are on National Forest land and can be accessed for viewing; although collecting may be restricted because of active claims. Private land is continuous adjacent to the Parlin to Ohio City road which follows Quartz Creek, thus access to the pegmatite area south of the Parlin to Ohio City road must be done via forest service dirt roads that enter the area from the south. Access to the areas north of the Parlin to Ohio City road must be done via the Willow Creek and the Big Gulch roads.

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References:

1. Elder, R. A. 1998. Geochemistry and Mineralogy of the Brown Derby No. 1 Pegmatite, Gunnison County, Colorado. Universi-
   ty of New Orleans, M. S. Thesis. 212 pp.
   
2. Hanley, J. B., E. W. Heinrich, and L. R. Page. 1950. Pegmatite investigations in Colorado, Wyoming, and Utah, 1942-1944:
   U.S. Geological Survey Professional Paper 227, 125 pp.