[Illustration: Cripple Creek, on the flanks of the Pikes Peak massif,
has produced more than $400,000,000 worth of gold. The Sangre de Cristo
Mountains are visible in the distance beyond the Arkansas River valley.
(Jack Rathbone photo)]
In 1890, two sheepherders stumbled on some richly mineralized rocks near
Cripple Creek. A boom developed immediately, for the rocks contained
both gold and silver. Since then, the area has produced more than
2,000,000 ounces of silver and nearly 19,000,000 ounces of gold.
Cripple Creek has produced almost half of all the state’s gold and
silver. The ores are located in or at the edge of a large mass of middle
Tertiary volcanic rocks which form an elliptical basin or _caldera_
several miles across. The caldera, surrounded by Precambrian gneiss and
granite of the Pikes Peak massif, was probably formed by collapse of a
volcanic center that had erupted through the older rock. The collapse
shattered the rocks around the basin margin, and subsequent volcanic
activity introduced mineral-rich solutions into the many faults and
fissures produced by the collapse. Tellurides of gold, silver, and
copper, as well as pyrite, sphalerite, galena, tetrahedrite, and other
minerals, are characteristic.