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Roasted Cripple Creek gold ore (Cripple Creek Diatreme, Early Oligocene, 32 Ma; Cripple Creek, Colorado, USA) 4 | by James St. John
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Roasted Cripple Creek gold ore (Cripple Creek Diatreme, Early Oligocene, 32 Ma; Cripple Creek, Colorado, USA) 4

Roasted gold ore from Cripple Creek, Colorado, USA. (public display, Montana Bureau of Mines and Geology Mineral Museum, Butte, Montana, USA) Artificial heating has driven tellurium (Te) away from the original telluride minerals (for example, calaverite), leaving behind vesicular blebs of gold.

 

The Cripple Creek Gold District of central Colorado, USA is famous for its unusual gold and silver mineralization. Precious metal mineralization occurs in the Cripple Creek Diatreme, the root zone of a deeply eroded volcano dating to the Early Oligocene (32 Ma).

 

The dominant lithology at Cripple Creek is the scarce igneous rock phonolite, an alkaline, intermediate, extrusive igneous rock. Cripple Creek gold can be found in its native state (Au), but it typically occurs in the form of gold telluride minerals: sylvanite ((Au,Ag)2Te4), calaverite (AuTe2), petzite (Ag3AuTe2), krennerite ((Au,Ag)Te2), and nagyagite (Pb5Au(Sb,Bi)Te2S6). Silver also occurs in some Cripple Creek minerals, including sylvanite, petzite, krennerite, hessite (Ag2Te), tennantite ((Cu,Ag,Fe,Zn)12As4S13), acanthite (Ag2S), and tetrahedrite ((Cu,Fe,Ag,Zn)12Sb4S13).

 

The gold telluride minerals common in the Cripple Creek Diatreme lack the wonderful, deep rich yellow color of native gold. Some Cripple Creek rock samples have been artificially “roasted” to drive off the tellurium. With heat, the Te readily volatilizes, leaving behind relatively pure gold. The gold patches on the rock below are surficial blisters and crusts of gold having a fine-scale vesicular texture (lots of tiny holes, like a pumice or scoria).

 

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Taken on August 11, 2010