Open Pit Mining - Its Impacts
Exploration adit at Pogo |
With the exception of the Nixon Fork gold mine, which was an underground operation, and Red Dog, which is a very high-grade base metal deposit, all of these mines share the common trait of being bulk-tonnage, open-pit leach gold operations. These means that because the gold is low grade and disseminated throughout bedrock, large quantities of rock have to be stripped, crushed, and soaked in cyanide solution to recover the gold. The resulting tailings, which contain residual cyanide and other process chemicals, as well as heavy metals from the ore, are stored in impoundments that are most often created by damning up a natural creek valley, diverting the stream, and filling the valley with tails.
"The mining industry produces more solid waste than all the municipal landfills in the United States." (Mineral Policy Center)
Mill and waste rock dumps,
Fort Knox mine |
Bulk-tonnage mines, unlike underground mines, also impact a much larger surface area. Because these mines are essentially strip mines, huge quantities of uneconomical or un-mineralized rock are considered waste by the company, and permanently placed in huge rock dumps on-site.
And because the ore is low-grade, vast quantities of ore-bearing rock must be processed, creating millions of tons of tailings that are also stored, permanently, at the mine site. For example, Fort Knox, at an average grade of 0.025 ounces of gold per ton of rock, produces roughly 400,000 ounces of gold a year.
Tailings slurry flows from
pressure release junction, Fort Knox |
This means that they also produce and store approximately 14 and ½ million tons of tails a year, and 14 and ½ million tons of waste rock. Since this mine had a projected mine life of about 16 years (although that has now shrunk to about half as a result of current gold prices), the mine property encompasses an area big enough to accommodate a 1,177 acre tailings impoundment and several huge waste rock dumps.
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