NPR recently interviewed journalist Matthew Hart who wrote Gold: The Race For The World's Most Seductive Metal. He spoke about the huge Mponeng Mine of South Africa which produces about one billion dollars of gold a year. The biggest mines are in the Carlin Trend of Nevada, U.S. but the Mponeng is huge, 2.5 miles deep, serviced by a project with tunnels and chutes the size of Manhattan and worked by 4,000 men a day.
As one goes deeper into the Earth (or any planet) there is an increase of temperature, called the geothermal gradient. This is the result of radioactive decay, the effect of ancient impacts and some other, debatable, causes. The gradient averages 1°F per 70 feet of depth in most of the world.
In Mponeng the geothermal gradient at the mine's deepest level is 140 degrees Fahrenheit. They cool the depths with an ice-salt slurry they pump to deep reservoir and use giant fans to cool the area down to about 85 degrees.
A big component of mining in South Africa is theft. From 10 to 20 percent of the ore in all of South Africa is stolen, probably 2 billion dollars a year. The thieves, called "ghosts" because of their sickly appearance, live in the mines and refine the gold there with dangerous chemicals (mercury). This requires a lot of collaboration, including the legitimate miners themselves who make a good side income with black market food. (One dollar bread on the surface brings twelve dollars underground.)
Abuse. Theft. Deprivation. Cruelty. Another system to make a living.
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