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