The Central City mining district had its origin with the Pikes Peak gold rush. Placer gold deposits found in Denver area streams were quickly played out. Miners followed streams like Clear Creek up the canyon to their source, staking claims on the placer deposits in the streams along the way.  Eventually, the placer deposits played out causing miners to search for the lode deposits along the hillsides. Underground hardrock mining was the inevitable outcome of placer depletion.

The Pikes Peak gold rush began in 1859 and is named after the high  mountain peak that is visible from 100 miles into the eastern plains. While the immediate area of Pikes Peak produced little or no gold, it was a useful point of reference for arriving miners and settlers.

In many ways, the various gold rushes in American history are simply examples of economic bubbles 19th century style. The discovery of a resource that can generate substantial streams of cash will attract large numbers of wealth seekers. Not surprisingly, chance favored the early arrivals in the bubble. Many a weary participant gave up, hoping only to break even. Others realize that there is more stability in providing supplies and services to the miners.

Abandoned Mill Workings Along Russell Gulch Road (Copyright 2009, all rights reserved)

While mineshafts, adits, and tailings piles still mark the landscape in most mining districts, what has been lost for the most part are the mills. The photo above shows the remains of a mill operation between Central City and Russell Gulch. Mills were a crucial link in the generation of wealth from mining activity.

Mill Along Russell Gulch Road

Mills were constructed near the richer lodes and were configured in various ways. Some processing buildings were built along the hillside and early mills had chutes with which to convey material  within the facility. Later mills had conveyors to transport materials.

Remnant of Surface Workings, Central City, CO (Copyright 2009, all rights reserved)

Cominution was a key operation of the mill. Large rocks had to be reduced in size to expose greater surface area for value extraction. Stamp mills were very common and consisted of a powered camshaft that lifted and dropped a train of heavy cylindrical hammers on the ore. The output of the stamp mill was treated in various ways depending on the nature of the gold bearing ore.

Gold ore near the surface might be of a more highly oxidized nature from exposure to oxygen entrained in meteoric water. In that case, sulfur would have already leached from the formation leaving a higher level of metallic gold. Such ore was more amenable to extraction by amalgamation. Stamp mills could be constructed with mercury covered copper plates on the output side of the stamps. The mercury would amalgamate the gold particles from the mill feed, selectively trapping them on the plate. The amalgam would then be scraped off the plates and the mercury removed by the application of heat to evaporate the mercury. 

Gold ore from deeper deposits resisted direct amalgamation, however, and roasting was used to free the sulfur (and tellurium) from the value. Highly sulfurized ores would be roasted to liberate the gold from the sulfur in the matrix. The resulting calcined ore might be milled or just subject to mechanical agitation to dislodge the gold particles in a sluice or by amalgamation (source- discussions with mining museum people. I would like to find better documentation, however).

The extent to which amalgamation was used in gold recovery is largely forgotten or left unmentioned. The USGS has excellent documentation on this topic.  Chlorination by the application of Cl2 and cyanide extraction were introduced by the end of the 19th century as well, but that will be the topic of another post.

The folded rock in the photo below is shown only because it is visually interesting. Note how the various layers show signs of differential erosion. Nothing astonishing, just a reminder that seemingly ordinary things can be very interesting if one stops to have a look.

I’m going to get hit by a car one of these days from stopping at roadcuts. I can only hope it is over fast.

Folded Rock Formation, Roadcut on CO 119 near Blackhawk