Category Archives: Mining

Cresson Gold Mine, Part 1.

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Early saturday morning 50 intrepid geotouristas packed into vans and drove to the CC&V mine in the Cripple Creek district of Colorado. Most of the group were professional geologists- professors, teachers, and geological survey folks. There were only a few interlopers like myself who were interested but untrained in geology. Not surprisingly, a background in chemistry is nearly useless when the discussion turns to stratigraphy and rock morphology.

The Cripple Creek gold district consists of an extinct volcanic structure called a diatreme. A diatreme is characterized by the presence of a volcanic pipe structure filled with brecciated rock. It is thought that the combination of shallow hot rock and ground water lead to violent explosions that resulted in fractured rock. Cripple Creek breccia has rounded clasts, indicating the rock fragments were exposed to rough, erosive treatment leading to attrition and rounding of the clasts prior to consolidation of the breccia.

In the past, gold mining at Cripple Creek was a underground activity. The district contains an extensive network of remnant subsurface works of drifts and shafts. Today, CC&V’s mining activity is limited to high throughput open pit excavation supplying pulverized rock to a cyanide heap leach field. A constant flow of ca 100 ppm aqueous sodium cyanide solution is leached from the top down through as much as 800 vertical ft of gold bearing rubble.

Abandoned Drift and Blue Bird Dike

Abandoned Drift and Blue Bird Dike

Columnar formations can be seen in certain locations of the mine (See photo: some features are enhanced with lines to show the margins). As the pit expands, drifts and shafts are exposed as seen in the photo above. The Blue Bird dike is an igneous intrusion into the surrounding formation. It is no coincidence that the drift in the photo is near the dike. It is common to see disturbed zones along the intrusion where gold can be found in higher abundance. The goal of the drift miner was to follow the enriched rock for more efficient reclamation of value.
Exposed Drifts During Pit Operations

Exposed Drifts During Pit Operations

A feature seen in the pit is a Lamprophyre, or igneous dike comprised of ultramafic, silica-poor, magnesium-rich rock. Biotite phenocrysts can be seen in samples. This is regarded as an unusual feature.

Lamprophyre formation in Cresson Mine

Lamprophyre formation in Cresson Mine

 The big haul trucks carry 300 tons of rock from the mine to the crusher. The crusher is actually a series of crushers that reduce the ore to pieces roughly 3/4 inch in diameter.

Haul Truck Carrying Rock from Blasting Site to Crusher

Haul truck carrying rock from blasting site to crusher The crushed rock is blended with lime and then driven to the leach pile for extraction. Another load for the leach heap.

 

A Massive Au/AuTe Deposit

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Th’ Gaussling attended a geology seminar thursday evening at the Colorado School of Mines. It was given by the chief geologist at the Cripple Creek & Victor gold mine (now AngloGold Ashanti) and was concerned with 3-D modeling of the volcanic formation that forms the center of the deposit.

What is unusual about the CC&V mine is the extent to which tellurium is present. There are a dozen or more tellurium minerals and many of them are present in the ore body. The CC&V load was discovered relatively late- about 1891. Due to the extensive fraction of AuTe and AuAgTe minerals, the presence of the ore body was not detected by placer prospecting. 

Prospectors panning for gold in local streams had no way of knowing that extensive gold was present because AuTe(Ag) minerals do not have a gold-like appearance.  Legend has it that it was discovered by a drunken cowboy who noticed some native gold in an outcropping in the area and took a sample down for assay. As I have mentioned before, the Cripple Creek district has produced about half of all the g0ld to come out of Colorado.

What is key to the formation is the fact that it has zones of extensively altered volcanic rock disturbed both mechanically in the form of fractures and faulting, and chemically in the form of its potassic-alkali nature. The formation has strongly brecciated zones and is desribed as “vuggy”, meaning that there are extensive voids. Native gold and gold telluride mineralization can be found on the surfaces of the vugs. The mineralization was deposited by hydrothermal streams extracting Au and Te from unknown source rock.

Presently the operation is surface mining which feeds to a cyanide leach field for gold extraction. The surface pit mine is working downward, digging through the extensive network of mineshafts. In the early days at Cripple Creek the mining was limited to underground activity. Miners would follow the extensive subsurface network of gold-rich veins in whatever direction they might go. The result is a very complex and extensive matrix of tunnels and shafts that extend downward to as low as 3000 ft. In the early days, the economics of subsurface vein mining were attractive enough to sustain the operation. Today, the economy of scale dominates and pit mining with heap leaching of the lower grade ore is what sustains the operation.

The gold is recovered by a cyanide leach field that is 800 ft thick in places. This method produces ca 300,000 toz/yr. The process does not recover Au from AuTe. It is left untouched in the leach heap and constitutes ca 1/3 of the total gold present.

Curiously, during the many eruption cycles in the distant past (~32 ma), debris from the surface has washed back deep into the formation. Bits of woody debris have been recovered within cementitious rock hundreds of feet below the surface. The CC&V geologist showed a core sample with a wood fragment imbedded within it. For a time reference, the current episode of Rocky Mountains (the Laramide Orogeny) began ~65 ma.

Chemist Gaussling will blend in with a group of geologists tomorrow morning and take an extensive geology tour of the mine site. Hopefully, there will be pictures to share. We’ll be going up to ~10,000 ft, so it will be chilly.

A Nevada Cinder Cone

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Whilst doing a survey of Lithium mining in North America, I blundered into a small cinder cone. It is found in the Clayton Valley of western Nevada south and west of Tonopah.

A link from the University of Nevada, Reno, gives some details on this cone as well as some interesting photographs.

Just to the south of this cinder cone is the Chemetall Foote Corporation Silver Peak brine facility. Lithium rich brines are pumped into evaporation ponds for concentration of the lithium salts. US 7390466 says that the Silver Peak brines contain 0.02 wt % Li.  The richest Li brine can be found in the Salar de Atacama brines in Chile. The Atacama brines contain from 0.15 to 0.193 wt % Li.

Gold Refining with Borax

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According to the GEUS, the Geological Survey Office of Denmark and Greenland, it is possible to concentrate and isolate gold from the ore using borax and charcoal. This method has the immediate benefit of making mercury “redundant” in gold isolation.

Extraction of gold by amalgamation with mercury is a simple means of producing metallic gold in the field.  After contact with gold enriched ore, mercury is evaporated into the air by direct application of a torch flame to the puddle of metal leaving purified gold metal.

It is thought that there are millions of miners who scratch out a subsistance living working a small patch of ground for gold. It’s called small scale mining. In the course of this activity, environmental contamination can accrue to the immediate area as well as the watershed at large. Sadly, the toxicological insult to the miners from exposure to mercury vapor can be severe.

This method is an inexpensive and simple alternative to the mercury process. Perhaps the chemistry community has something to contribute by way of education or improved methods of extraction.

8/25/10  Update.  I have revisited this post and am compelled to comment further.  While I am unable to offer a good chemical explanation for the effect of borax on gold ore, I can say that the use of borax as a flux  for smelting goes back to the 19th century during the American gold rush period.  The process described in the link appears to be a smelting process for enriched ore containing elemental gold, as opposed to sulfide, or sulphuretted ore. The function of a flux is to modify the flow and phase separation properties of host rock so as to partition away from the gold phase or layer.  In other words, a flux modifieds the slag to help the gold to separate cleanly from the rock.