As U.S. Highway 24 approaches Wilkerson Pass, Colorado, the 1.7 billion-year-old metamorphic rocks of the Puma Hills replace the younger Pikes Peak Granite. The Puma Hills were formed by the metamorphism of sedimentary rocks that were once oceanic sediments—sand, mud, and clay.
Before the highway reaches the summit of Wilkerson Pass, it goes past a dirt road to the M Lazy C ranch. The ranch road heads north into the hills where forest road 247 soon intersects the winding ranch road, and at this crossroad forest road 247 bears east, into the deep forest, past the old Pulver Gulch prospect.
The geology at Pulver Gulch is unlike the surrounding area. The sediments at Pulver Gulch contain more calcium carbonate, from impure and muddy limestones, than the surrounding ocean sediments that formed the Puma Hills. These calcareous sediments were heated, compressed, and transformed into calcium silicate rocks that host a group of interesting metamorphic minerals that include scheelite, vesuvianite, wollastonite, grossular garnet, and diopside. The Pulver Gulch prospect’s exploratory dump is an excellent place to search for these metamorphic minerals.
Prospectors worked the Pulver Gulch prospect over sixty years ago looking for scheelite (figure 1), a mineral that formed in the metamorphic rocks at the site. Scheelite is an important source of tungsten. Tungsten has many industrial applications, including filaments in light bulbs. Since scheelite is strongly fluorescent, prospectors searched the area with battery powered black lights at night.
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| Figure 1. Scheelite crystals and muscovite mica showing fluorescence under ultraviolet radiation. Image courtesy of Wikipedia. |
Brilliant brown granular crystals of vesuvianite (figure 2), a basic calcium magnesium silicate mineral, are common here. Short prismatic crystals can also be found. This mineral was named for Mt. Vesuvius, where it was discovered on the slopes of the Italian volcano.
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| Figure 2. Vesuvianite crystal. Image courtesy of the CSMS blog by Mike Nelson. |
Wollastonite (figure 3), a calcium silicate, occurs as milky-green masses of needle-like crystals at this site. Some of the massive specimens are larger than a football. This mineral is faintly fluorescent. Wollastonite is used as a component in refractory or heat resistant ceramics and as a filler for paint.
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| Figure 3. Wollastonite with diopside (green), garnet (red) and vesuvianite (dark brown) from the Stanislaw mine near Szklarska Poreba, Izerskie Mountains, Lower Silesia, Poland. Image courtesy of Wikipedia. |
Thick, banded layers of brown grossular (figure 4), a member of the garnet group, are associated with wollastonite at the Pulver Gulch prospect. These garnets also formed from the impure limestones and occur here in a massive and granular form.
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| Figure 4. Garnet crystals from the Jeffery Mine, Quebec. Image courtesy of the Canada. Bureau of Mines, specimens C-01687. |
By breaking open the host rocks on the Pulver Gulch dump, well-developed microcrystals of dark green diopside (figure 5) are exposed. The diopside crystals, a calcium magnesium silicate, are embedded in sparkling white calcium silicate rocks. These specimens of diopside can be interesting to micromount collectors.
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| Figure 5. Diopside crystal from De Kalb, New York. Image courtesy of Wikipedia. |
Today the Pulver Gulch dump is largely undisturbed. Occasionally a small group of geology students from Colorado College will stop by the dig site to study the local geology in this peaceful part of the Puma Hills.
Chesterman, Charles, 1978, The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York.
Chronic, Halka, 1980, Roadside Geology of Colorado, Mountain Press Publishing Co, Missoula.
Mineral Galleries, World Wide Web homepage URL: http://mineral.galleries.com/minerals/silicate/vesuvian/vesuvian.htm
Wobus, R.A., 1997 (Williams College) personal communication.