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.

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