English: Roasted gold ore from Cripple Creek, Colorado, USA. (field of view ~4.1 centimeters across)

The Cripple Creek Gold District of central Colorado 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 of Early Oligocene age (32 million years old).

The dominant lithology at Cripple Creek is phonolite, a scarce, 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 such as 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 seen here are surficial blisters and crusts of gold.