Technology & Modern
Mining in Idaho

Mining in Idaho was the state's first industry. Even today, more than 140 years after the discovery of gold, mining remains a keystone of the state's economy. Idaho still holds vast untapped mineral deposits. But the techniques used to find these deposits have come a long way from the days of Noah Kellogg's donkey. Modern prospecting begins with field studies conducted by the Idaho Geological Survey. Data from these studies give us clues as to how and where mineral deposits are formed.

Using their knowledge of the Earth's formation and state-of-the-art Idaho Geological Survey Maps, modern-day prospectors can concentrate their exploratory efforts at precise locations. The rock captured in the center of a drill, called a core sample, is examined by geologists for signs of mineralization. In Idaho, these exploratory projects are often restricted by weather and cost; an average of a quarter-million dollars apiece. Land disturbed by exploration activity is reclaimed, whether or not minerals are found. After hundreds of bore holes are drilled and thousands of core samples are examined and tested, a computer model of the ore body is developed.

The mining engineers first determine the size, shape and quality of the ore body and then select the best way to mine it. They must also estimate the quantity of the mineral within the ore body and determine how many years it will take to mine.

Before a single shovel of dirt is moved, a variety of environmental and engineering studies must be conducted. It may take two to five years and millions of dollars to complete this second phase. Mining is heavily regulated by both the state and federal governments. An Environmental Impact Statement is required along with state and federal permits and a period of time set aside for public comment. Today, mining companies must develop a long-term plan which includes not only the mining process, but a program to rehabilitate the land once the mining stops.

Once the permitting process is complete and the mine plan approved, the mine construction phase begins. Roads must be built to the site. Mills for processing ore are constructed. In a surface mining operation, topsoil and overburden, or non-mineral rock covering the ore, is removed and stockpiled for future reclamation.

Surface mines often resemble giant outdoor stadiums. The size of the pit, or cut, depends on the size of the ore body and how much overburden must be removed. Benches or steps are cut on the sides of the pit to stabilize surrounding rock and minimize erosion. Using a computer model of the mine, excavation is carefully designed by the mining engineer.

When constructing an underground mine, vertical or have inclined shafts must be sunk or horizontal tunnels or adits bored to reach the mineral vein. Mine construction can take several years and employ several hundred people. Underground mining is virtually a world unto itself. Productive silver mines in Idaho are over a mile deep and require very specialized people and equipment. Safety is paramount in the underground world.

Once the rock is loosened up by blasting, shovel operators selectively load ore into large trucks bound for the concentrating mill. The most common form of concentrator is the flotation mill. After being crushed, the ore passes through a series of large grinding mills. These giant drums reduce the rock to particles smaller than flour. These tiny particles are mixed with water and chemicals and sent through a series of flotation circuits. The mineral particles stick to the bubbles, float to the top and are skimmed off. Ore entering the first flotation circuit can contain as little as one-tenth of one percent mineral. By the time the concentrate emerges from the last flotation circuit, it can be more than 95 percent pure. Low-grade gold and silver ores are concentrated by a process called leaching. A chemical solution is mixed with the ore to dissolve the precious metals. In a heap leach operation, ore is crushed and stacked on a thick clay pad covered with a leak-proof liner. The chemical solution slowly seeps through the ore. The solution, now rich in dissolved gold and silver, is collected and processed.

Leaching can also take place in large tanks or vats. As the ore moves through a series of vats, gold and silver are dissolved by the chemical solution. The metals are recovered from the solution and the resulting "filer cake" is melted to produce a gold/silver mixture called "dore" (pronounced "door-ray"). These 50-pound dore buttons are shipped to a refinery where pure gold and silver are produced. In both vat leaching and heap leaching, the chemical solution is continually recycled and reused in a closed system.