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

Underground Air Locomotive

In Part 1 of my post on the Mollie Kathleen Mine in Cripple Creek, Colorado, I described the ride down to the 1000 ft level.  Having been in mines considerably less developed, I was impressed with the quality of the skip lift equipment and the general state of the mine workings above and below ground. The mine make heavy use of pneumatic equipment to minimize ignition sources.  The air locomotive above features a pressure tank which energizes an air motor to drive the contraption. It works quite well.

Mechanized Mucking with Pneumatic Equipment

Once at the bottom of the shaft, the mine appears to be little more than a hallway with steel tracks on the floor. In fact, it is a series of hallways, or drifts, and shafts. The goldbearing formation that the Mollie Kathleen mine has penetrated is a volcanic formation called a diatreme and it is composed of highly disturbed rock from ancient volcanic activity. The district contains gold in varying abundances. Certain features of the formation are more enriched than others.

In general, one does underground hardrock mining to exploit a network of veins enriched in value, in this case, gold.  By definition, an ore is a body of rock or mineral that contains commercially exploitable value such as gold. 

Blasting pattern prior to a shot

Solid rock is fragmented with explosives and loaded into ore carts. The rubble accumulated from blasting activity is called “muck”. Muckers were very important workers in a mine and the mines productivity hinged on their ability to load the ore carts as fast as possible. Until carbide lamps arrived, miners toiled in very low light levels in candle or kerosene lamplight.

With the advent of better technology came more effective and safer blasting agents, fuse cord capable of adjusting the timing of a blast sequence, and more efficient ejection of fragmented rock.  Near the center of the photo above is a pattern of holes filled with blasting agent. Well, except for one hole in the center of the pattern. This empty hole is placed specifically to provide for space for expansion relief.  A shot is timed to trigger the charges around the empty hole first, followed by concentric detonation of the blast pattern. Finally, a set of charges low in the pattern lift the muck out of the hole and onto the floor.

Pneumatic hammer for pounding a drilling steel into the rock wall.

According to the tour guide, the Mollie Kathleen mine is fairly rich in gold but lacks access to a milling facility. Without milling and refinement, there is no point in pulling the ore out of the ground. So, until a scheme for beneficiation of the ore comes along, the gold will have to sit in the formation and make money for its owners as a tourist attraction.

As is common in mine tours, the staff is well versed in the history and mechanics of getting ore out of the formation. What seems to be glossed over or wholly ignored is the process of getting purified gold out of the ore. Being a chemist, I was naturally interested in the isolation process. The refining process I was able to “extract” from the mine tour operators was a simple but inefficient method.

Gold ore was pulverized and heated to high temperature in a way that resembles calcination. Diffuse wisps and pieces of elemental gold in the ore would melt and agglomerate so as to produce larger pieces of gold. The roasted ore could then be exposed to a mechanical/slurry agitation process that would dislodge the now larger pieces of gold and classify them by density much like the gold panning process.

The roasting process apparently oxidized the tellurium in the ore, resulting in a purification. The question is, did the roasting process just oxidize the free tellurium or did it free the gold from the gold telluride (Calaverite)?

Another process can be used to extract gold from the ore. It grossly resembles the mercury amalgamation method. Metallic lead is combined with gold ore and heated to some high temperature in a special container. A lead-gold alloy is formed which can be poured away from the gangue. The molten alloy is then exposed to air oxidation, forming litharge (PbO) and metallic gold which phase separate and can be separated mechanically. Assayers use a process called fire assay or cuppelation to extract refined gold for an assay.

Chlorine extraction was used to oxidize metallic gold to the soluble NaAuCl4 salt which could reduced by contact with carbon or by electrolysis. Chlorine water was used prior to the cyanide extraction methodology now in common practice.

Old Headworks by the Mollie Kathleen Mine