Gold Prospectors of the Rockies Gold-bearing Tellurides

Gold-bearing Tellurides
As appearing in The Gold Nugget, September 1999
by Harvey S. Eastman, August 29, 1999

Gold-bearing tellurides loom large in the history of gold mining in Colorado. These mineral species are found in the San Juan Mountains, the La Plata Mountains, and at Cripple Creek. Other major localities in the United States include the huge gold fields of the Sierra Nevada foothills in California.
Gold-bearing tellurides are silvery to pyrite-yellow minerals, commonly striated, unlike gold which is a deeper yellow and rarely crystalline; and many of the lodes carrying gold-bearing tellurides were not discovered until the late 1800's. Placer gold was often found below the Cripple Creek Mine, but the lodes of the area were not recognized until the 1890's. Similar histories are common in other areas where gold-bearing tellurides were the major ore mineralogy. The miners of the 1860's would find gold placers below these vein deposits. However, the veins themselves were not recognized for what they were until some miner, perhaps cooking a meal on a camp fire surrounded by rocks, noticed gold appearing where a silvery, or pyrite-like mineral had been before.
This discovery led to a second gold rush to find these veins with a silvery to pyrite-like mineral, testing them with fire to determine if that mineral contained gold. Out of this rush came the mining camps of Cripple Creek and Telluride along with other mining camps in the San Juan region of Colorado. Of these, the mining camp of Cripple Creek is the most famous, producing nearly 20 million ounces of gold over a 70 year period from 1891 to 1961 (Smith, Raines, and Feitz, 1985). Additional gold was produced in the 1980's from heap leaching of low-grade gold-bearing mine tailings and ‘waste’ dumps.
A tip on finding tellurides: The mineral commonly has a greenish halo around it when weathered, due to oxidation of the telluride.
Gold-Bearing Telluride Mineralogy
Gold-bearing tellurides are common in gold-bearing deposits throughout the world. The following mineral species are the more common varieties. Localities listed are restricted primarily to the United States except when the mineral is not known to occur in this country.
Petzite: Ag3AuTe2 (silver-gold telluride)
Steel gray to black, metallic.
Gold Hill and Sunshine Mines in Colorado and various mines in Calaveras and Tuolumne Counties in California.
Krennerite: (Au,Ag)Te2 (gold-silver telluride)
Silver-white to pale yellowish white, metallic, good cleavage.
Cripple Creek District, Teller County, Colorado.
Sylvanite: AgAuTe4 (silver-gold telluride)
Silver white, sometimes a yellow tint, metallic. Good cleavage, brittle.
Foothills gold belt, California; Cripple Creek District and other areas, Colorado
Calaverite: AuTe2 (gold telluride)
Brass yellow to silver white, metallic, brittle.
Looks like pyrite, but when roasted, gold appears.
Foothills gold belt, California; La Plata, Montezuma, Boulder, Hinsdale, and Teller Counties, Colorado, including the Cripple Creek District.
Montbrayite: (Au,Sb)2Te3 to Au2Te3 (gold + antimony telluride)
Tin-white to pale yellow, metallic. Brittle.
Robb-Montbray mine in Abitibi County, Quebec, Canada.
Nagyagite: Pb5Au(Te,Sb)4S5-8 (lead-gold tellurium sulfosalt)
Dark lead-gray, good cleavage with flexible laminae, metallic.
Gold Hill in Boulder County, Colorado; Trinity, Calaveras, and Shasta Counties, California; Kings Mountain Mine, Gaston County, North Carolina.
Kostovite: AuCuTe4 (gold-copper telluride)
Grayish white, metallic.
Chelopech, Bulgaria
Other telluride species are known to exist, including combinations with lead, iron, sulfur, and selenium. For more information refer to Wilson (1982) and other authors on tellurides.
References

Lindgren, W. and Ransome, F. L. 1906. Geology and Gold deposits of the Cripple Creek District, Colorado, USGS Professional Paper 54.
Ramdohr, P. 1969. The Ore Minerals and their Intergrowths, Pergammon Press, London.
Roberts, W. L., Rapp, G. R., and Weber, J. 1974. Encyclopedia of Minerals, Van Nostrand Reinhold Company, New York.
Smith, A. E. Jr., Raines, E., and Feitz, L. 1985. The Cresson Vug, Cripple Creek, The Mineralogical Record, vol. 16, no. 3, pp. 230 - 238.
Wilson, W. E. 1982. The Gold-Containing Minerals: a Review, The Mineralogical Record, vol. 13, no. 6, pp. 389 - 400.

Dr. Eastman, who has a PhD in geology from Stanford University, has fifteen years experience in mining copper, uranium, silver, and gold, and most recently, eight years experience in groundwater resources and environmental consulting. He currently is a Project Manager in hydrogeology with Jehn Water Consultants, Inc. and Jehn Environmental, Inc., Denver, Colorado.

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