By Steven Wade Veatch
Distinctive specimens of iridescent fireplace agate have lately been present in Park County within the USA, near Tarryall Creek and close to the Tarryall Reservoir. Hearth agate is a wide range of chalcedony (pronounced kal SED’ uh nee), a form of microcrystalline or cryptocrystalline (crystals too small to be seen without high magnification) quartz (SiO2). It contains inclusions of iron oxide (limonite) that produce an iridescent effect or ‘fire’.
Chalcedony is generally formed near the surface of the Earth, where temperatures and pressures are low. The Tarryall fire agate has a botryoidal (grape-like) growth form. The agate is also layered: it contains thin layers of plate-like crystals of iron oxide in various planes. When light travels through these thin layers, the planes produce the iridescent colour play of red, gold and green.
The fire agate specimens were found as seams in granite near the Tarryall Creek. This is a tributary of the South Platte River, approximately 25 miles (40km) long, in Park County, central Colorado. It drains a portion of north and central South Park, an intermontane grassland south-west of Denver. Tarryall Creek runs in several forks along the continental divide in the Pike National Forest and then flows to the south-west through a canyon where it enters South Park near the small town of Como, and then crosses US Highway 285 north-east of Fairplay, Colorado. The Creek then meanders roughly south-east, later joining the South Platte River in the south-eastern corner of South Park.
Dreams of gold and silver strikes lured early prospectors into the area. The Tarryall Creek, north of the present town of Como, was an active location for gold prospecting in 1859. The ‘Tarryall Diggings’ and other nearby sites brought in thousands of prospectors over Kenosha Pass, and the town of Tarryall (now gone) was soon founded near the creek (McConnell, 1966). Failing to get rich quick, many of the gold seekers moved on to other goldfields and mining camps, while a few stayed on to ranch and log. The valley was rich in water and lush meadows.
These early homesteaders cleared land, built cabins, strung up fences, broke ground for planting and dug irrigation ditches in order to raise hay, barley, oats, and rye. Potatoes, turnips, carrots and cabbage also did well but cattle ranching was the main economic activity. Several ranchers set up sawmills to provide lumber to other homesteaders along Tarryall Creek.
Prospecting still continues in the area today, chiefly for topaz crystals. I recently went on a weekend trip to explore parts of the Tarryall Valley, which resulted in this new discovery of gem-quality fire agate in the seams in granite rocks referred to above. The gem is thought to be formed when hot water, saturated with colloidal silica and iron oxide, invades cavities in country rock and then cools. As the solution begins to precipitate and grow layers of silica, iron oxide is deposited. These layers of silica and iron oxide cause the brilliant fire in the gem. As iron oxide is depleted in the solution, colorless chalcedony forms. This depletion of iron oxide can be seen along the edges of the specimen.
Fire agate is also found in the Sonora Desert region of northern Mexico and southern Arizona (for exmaple, on Saddle Mountain, near Tonopah, Maricopa County and near Safford, Graham County), as well as in the Central Basin of Mexico.
References
National Audubon Society, 1979. National Audubon Society Field Guide to North American Rocks and Minerals (Audubon Society Field Guide). Knopf, New York, 856p.
McConnell, Virginia, 1966. Bayou Salado: The Story of South Park. Sage Books, Chicago. 275 p.
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