English: Fluorite veins in pyritic phonolite from the Oligocene of Colorado, USA.

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 dating to the Early Oligocene (32 Ma). 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: for example, calaverite - AuTe2, sylvanite - (Au,Ag)2Te4, 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 argentian tetrahedrite - (Cu,Fe,Ag,Zn)12Sb4S13.

Seen here are purple fluorite veins in phonolite from the Rosetta Mine. Precious metal tellurides are often intimately mixed with fluorite in these thin veins - they were the targets of historic subsurface mine tunnels.

Geologic unit: Cripple Creek Diatreme, Early Oligocene, 32 Ma

Locality: Rosetta Mine (just to the south of the Mollie Kathleen Mine), just east-northeast of the town of Cripple Creek, northwestern Cripple Creek Mining District, southern Teller County, central Colorado, USA