|M E E T I N G N O T E S|
Crystal Peak Geologic Perspectives
by Andy Weinzapfel, geologist
Walk into any large geologic museum featuring rare and exceptional minerals, and you are likely to stumble across an attractive display of unusual, bright green or greenish-blue specimens labeled Crystal Peak, Teller County, Park County, or Pikes Peak region, Colorado. These unique, euhedral (smooth-faced) crystals are amazonite, a relatively rare variant of a common mineral, microcline feldspar. Feldspar, along with quartz, is a major constituent of granite, the most prevalent igneous rock found in continental mountain ranges.
The geology of the Pikes Peak region is dominated by the 1.07-1.09 billion-year-old Pikes Peak batholith, a large body of once-molten rock that was likely derived from the earth’s deep mantle and injected upward to a depth of 3 miles or less below the surface. The Pikes Peak Granite, extending over an area of 1200 square miles, is exposed at the surface today only because the rocks that once covered it have gradually eroded away.
Several well-known, scattered mineral collecting districts are hosted within the Pikes Peak Granite. Probably the most famous and productive is a roughly circular, 16 square mile area, centered 4 miles northwest of Florissant. The primary mineralized part is in the vicinity of Crystal Peak, on private property. A significant and poorly explored portion of the district, however, is located on public lands in Pike National Forest, north of Crystal Peak. Unfortunately, part of that area has been adversely impacted by the Hayman fire, and thus may not be available for public collecting in the near future.
A common but erroneous belief is that Crystal Peak is an old volcano. Its pyramidal shape is actually due to differential erosion, a process whereby fine-gained granite (aplite) on the peak weathers away more slowly than the surrounding coarser grained variant. However, the zoned intrusive exposed today are probably the deeply weathered roots of a volcano.
For many years, the agent responsible for the beautiful green or bluish-green color of amazonite was a mystery. In the 1970s, it was discovered that lead, bonded with water, was the mechanism. The deeper the color, the higher the lead content. A legacy of past Colorado mining activity is that lead, within the common sulfide mineral, galena, has and is being released into the environment by weathering of mine tailings. Can the lead in amazonite at Crystal Peak cause pollution of surface or ground water? Fortunately, no, because it is bonded very tightly and remains immobile upon exposure to air.
The Ute Indians were the first collectors of crystals from this area, used for spiritual purposes. In the 1870s, Dr. A. E. Foote of Philadelphia systematically explored the area, employing 19 men, and shipped many specimens back east. On February 20, 1878, Authur Lake, who accompanied Samuel Scutter of Harvard University on an early paleontological investigation of fossils of the area, sketched the first geologic map of the Florissant valley while sitting on Crystal Peak. In 1908, A. B. Whitmore established the Gem Mines north of Crystal Peak, a popular collecting site. Successful collecting in the area continues today.
Other interesting minerals occur in the Crystal Peak area, including smoky quartz, topaz, goethite, fluorite, and phenakite. What may be the largest smoky quartz crystal found in Colorado was recently discovered north of the peak. Black smoky quartz and green amazonite, when together on the same specimen, are highly prized. The best specimens are usually found in open spaces within fractures. A diorama of a mineralized cavity, with Crystal Peak as a background, can be found at the Denver Museum of Nature and Science.
What new theory suggests a relationship between Crystal Peak, the Florissant Fossil Beds, Cripple Creek gold district, and topaz-bearing Redskin Granite of the Tarryalls? How do you find amazonite and smoky quartz? Come join us at 2:00 P.M., November 16, 2003 at the Florissant Old Schoolhouse, for a Chautauqua presentation titled: The Geology of Crystal Peak.