The Central City-Idaho Springs area was the principal metal mining
region in the state until the late 1880s. In 1858, rich placer deposits
were discovered in gravels and river terraces along both forks of Clear
Creek. Exploration upstream led to discoveries of rich oxidized quartz
veins at Central City, Black Hawk, and Idaho Springs. These veins, which
generally trend northeast-southwest, extend through the mountains in a
zone about six miles long and three miles wide between the two forks of
Clear Creek.
The ores filled a multitude of cracks and fissures in the Precambrian
bedrock. The veins are usually less than five feet thick, and are almost
vertical and often clustered in zones up to thirty feet wide. The
position of one of the vein systems may be seen clearly between Black
Hawk and Central City—the ore-bearing rock has been mined out, but a
series of collapsed tunnels marks the line where the veins crossed the
valley. A monument here commemorates the discovery of Gregory Gulch, one
of the richest localities in the state.
Several rich veins were mined in both directions—southwest from Central
City and northeast from Idaho Springs—until the mines met. The Argo
tunnel, marked by dilapidated buildings and extensive dumps on the north
side of Idaho Springs, connected the two districts; it was completed in
1904.
The “Patch,” a deep crater-like hole on Quartz Hill, about one mile
southwest of Central City, is an intriguing feature in this area. It was
produced by glory-holing, a mining technique in which a deep tunnel is
deliberately caved by blasting, so that ores above the tunnel can be
removed. This glory hole was dynamited below an irregular mass of highly
broken rock where many ore-rich veins converged. After the caving, ores
were taken out through the remaining part of the tunnel.
The principal ore minerals of Central City and Idaho Springs are native
gold, pyrite, sphalerite, galena, chalcopyrite, and tennantite.
Prospecting for uranium was carried out during the 1950s but no uranium
was ever mined here.
The area has produced almost $200,000,000 worth of gold, silver, lead,
zinc, and copper. A few mines still operate seasonally or on a small
scale, but tourists, many of them riding Jeeps across the mountainous
terrain to visit mines and ghost towns, are often more visibly active
than the mines.