Some years back I visited the large CC&V open pit gold mine by Cripple Creek, Colorado. Standing at the bottom of the pit we could see haul trucks busily transferring ore to a staging site. Suspecting that it might be overburden, I asked what they were doing. Our guide, a geologist, said that this ore would be staged as unrefined until the price of gold rose to a certain higher value. The whole ore body had been mapped 3-dimensionally so at any given location and level where they blast, they have a rough idea of how much gold is present. At the time, ~10 years back, the geologist said that each large haul truck was typically carrying about $10,000 worth of gold. I don’t know how accurate that is, but there you have it.

The Cripple Creek gold load was discovered about 1893 and occurs in the throat of an extinct volcano. The ore contains gold and calaverite, AuTe2, a gold telluride mineral. The gold and AuTe2 is so finely dispersed that most people who work at the mine have never actually seen the gold. The recovery method they use is cyanide extraction. Unfortunately, tellurium interferes with this extraction process and unavoidably some of the gold as the telluride is left in the tailings. The ore is said to contain about 1 gram of recoverable gold per ton.

What prompted this essay was a moment of clarity I had reading a notice from the Energy Information Administration, EIA. It is common to hear about oil reserves. One might suppose that this refers to the total proven reserves in the ground. But this article referred to “economically recoverable oil resources”. When oil reserves are expressed in this way, the recoverable oil then becomes a function of the current oil prices. If oil prices are low, then the reserves are considered smaller than when oil prices are high. It seems so obvious but I never gave it a thought before. As with gold, the lesson is to pay attention to the type of reserves being discussed.