Not long ago I had a reader ask about rubies, the gemstone, and any possible collecting localities in Colorado. The reader had read the posting on sapphires (Post:5-1-2013) and had been whispered “rumors” about rubies down near Buena Vista on the Arkansas River. What about those rubies?
I was sorry to disappoint the reader but the “Colorado rubies” at Ruby Mountain are actually a variety of aluminum garnet called spessartine [MN3Al2(SiO4)3]. These stones occur in a Tertiary rhyolite exposed as a series of intrusive dikes at Ruby Mountain and adjacent Sugarloaf Mountain and Dorothy Hill. Rhyolite is a light-colored, silica-rich, volcanic rock that is, mineralogically speaking, the equivalent of granite (they have about the same mineral composition). However, the rapid cooling of rhyolite did not allow for large groundmass crystals to form—you need to examine the rocks with a loupe in order to see the crystals. Rhyolite also has the tendency to form gassy vugs and cavities, essentially hollow spaces, in the rapidly cooling lava. The garnet crystals at Ruby Mountain formed in such cavities from “late stages vapors…enriched in fluorine, sodium, and potassium” (Modreski and Murphy, 2002).
|Ruby Mountain, Chafee County, Colorado, as
seen from the
BLM Ruby Mountain Campground.
Topaz crystals, many are sherry in color when “fresh”, and obsidian nodules (Apache Tears: marekanite ) also occur at Ruby Mountain with the tears being exceedingly common and the topaz rare. The topaz is often found in association with the garnets in vugs or cavities while the Apache tears are found in a perlite near the north end. Perlite is a light (low specific gravity) rock formed when the rhyolitic magma was frothy and full of gas bubbles and cavities. An abandoned perlite mine may be observed along the BLM hiking trail at Ruby Mountain.
One of the more interesting aspects of Ruby Mountain is the chance to see the intrusive rock (rhyolite) in contact with the Precambrian granite. The rhyolite has been isotopically dated as 30.1 Ma (McIntosh and Chapin, 2004) while the Precambrian granite is approximately 1700 Ma years (Tweto, 1979).
The contact between the massive bedded Precambrian granite (left) with the Tertiary intrusive rhyolite dike (right).
To locate Ruby Mountain travel south from Buena Vista on U. S. 285 to Chaffee County Road (CCR) 301 (about a mile and a half north of Nathrop) and turn east for one-half mile. At the intersection with CCR 300, turn right toward Ruby Mountain Campground and travel about two and one-half miles. At the Campground (located along the Arkansas River) take the left fork of the road up a small valley to a dead end parking lot. Ruby Mountain is to the south.
|Spessartine garnet from Ruby Mountain. Photo courtesy www.wmnh.com|
Ruby Mountain, the public sector, is now part of the Bureau of Land Management (BLM) Brown’s Canyon Wilderness Study Area, and has been proposed to an upgrade of National Monument. Therefore, there are rules (explained on an information board at the trail head). However, rockhounding with hand tools is allowed. Please remember---no blasting to loosen up the rock!
Ruby Mountain is also a wonderful place to “look west” across the valley of the Arkansas River to the mighty Sawatch Range. Mt. Antero, with its aquamarine collecting, is easily visible as well as Mt. Princeton with the display of magnificent glacial valleys.
|Looking west across the valley of the Arkansas River at the mighty Sawatch Range.|
When visiting the somewhat isolated Nevada Rubies I am always reminded of a sentence by Sigurd Olson (one of my Minnesota heroes): “The smell of the morning is an adventure, and if you can start the day by going outdoors and sniffing the air, there is always a lift to the spirit.”
Cross, R. F., 1896, Topaz and Garnet in Rhyolite: American Journal of Science, ser.3, v.31.
Eckel, E. B. and others, 1997, Minerals of Colorado: Denver Museum of Nature and Science and Fulcrum Publishing.
McIntosh, W.C. and C.E. Chapin, 2004, Geochronology of the Central Colorado Volcanic Field: New Mexico Bureau of Geology and Mineral Resources, Bulletin 160.
Modreski, P. J. and J. A. Murphy, 2002, A Tour of Colorado Gemstone Localities: Rocks and Mineral 77.