File:Berkeley Pit & Continental Mine (Butte, Montana, USA) 2.jpg

English: (photo on display at the Montana Resources office; north is to the left)

---

This aerial photo shows two major mines in Butte, Montana (= “Richest Hill on Earth”; = "The Mining City"). The Butte Mining District has produced gold, silver, copper, molybdenum, manganese, and other metals.

At center is the Berkeley Pit (Berkeley Mine). At the top is the Continental Pit (Continental Mine).

---

The area's bedrock consists of the Butte Quartz Monzonite (a.k.a. Butte Pluton), which is part of the Boulder Batholith. The Butte Quartz Monzonite ("BQM") formed 76.3 million years ago, during the mid-Campanian Stage in the Late Cretaceous. BQM rocks have been intruded and altered by hydrothermal veins containing valuable metallic minerals - principally sulfides. The copper mineralization has been dated to 62-66 million years ago, during the latest Maastrichtian Stage (latest Cretaceous) and Danian Stage (Early Paleocene). In the supergene enrichment zone of the area, the original sulfide mineralogy has been altered.

---

The Berkeley Pit is a huge open-pit copper mine that closed in 1982. I saw the site in 1991, 2010, and 2011. The water level has been slowly rising each year and is approaching the level of the local water table. If water levels reach the same elevation as the aquifer, polluted water from the pit will drain into surrounding areas. The Berkeley Pit's water is iced tea-colored. The water filling the mine comes from about ten thousand miles of flooded underground mine tunnels. Dewatering of the Butte area by water pumps allowed operation of these deep mines, until they were turned off by the Anaconda Copper Mining Company in 1982. Dissolved oxygen in the water in the flooded mine tunnels oxidized sulfide minerals in the rocks, resulting in acidic and heavy metal-rich water - this is why the Berkeley Pit lake is polluted. Oddly, the water is rich enough in copper that the water is mined! Water is pumped out, copper is precipitated out, and the used water is returned to the pit.

The environmental nightmare at Butte, Montana was made worst by politicians. A proposal to push mine waste rocks into the Berkeley Pit would have prevented the pit's water from becoming so acidic - the waste rock tailings are high pH (about 10). Politicians refused, creating a worse situation. The lesson here is to NEVER, EVER, EVER let politicians make scientific decisions. They're all bone-headed morons.

When I visited in 2010, I obtained the following Berkeley Pit water analysis from the Montana Bureau of Mines & Geology: 64 parts per million (ppm) copper, 500 ppm zinc, 0.1 ppm arsenic, 500 ppm iron, 200 ppm manganese, and a pH of 2.5 to 2.7 (= cranberry juice). The acidity value varies with season and sample depth in the water.

---

The Continental Mine was started in 1980 by the Anaconda Copper Mining Company - it is currently owned by Montana Resources. The mine targets a low-grade copper and molybdenum deposit on the eastern side of the Continental Fault, a major Basin & Range normal fault in the Butte area with about 3500 feet of offset. The mine's rocks consist of BQM with disseminated copper sulfides, plus copper- and molybdenum-bearing hydrothermal veins that intrude the BQM. Minerals at the site include chalcopyrite, molybdenite, malachite, azurite, tenorite, and cuprite. The latter four minerals are secondary copper minerals, produced by alteration of the primary copper sulfides.

In 2010, the Continental Mine was making 50,000 to 52,000 tons of ore each day. This mine can operate down to an ore grade of 0.1% copper. Most of the mineralization is disseminated copper, but veins are also present. Two stages of mineralization occurred in the Butte area - a porphyry copper system and a main stage system with large veins. The bottom of the porphyry copper system is ~ less than 12,800 feet below the surface. Veins peter out at 5600 to 5800 feet below the surface. At the Continental Mine, veins are small - they're veinlets less than 6 inches wide.

Mining is done 24 hours a day, 365 to 366 days per year. There's 1 to 2 days of down time at the mill. During those days, mining stops and waste material is moved. The ore:waste ratio is 8:10 (= strip ratio). The alluvial overburden consists of 7 paleosol horizons, including some caliches - the lime content results in an average pH of 8. The caliche material can be used to treat acidic materials.

This mine has 14 shovels and 15 trucks. A large Bucyrus shovel can load a 240-ton truck in three passes. The mine's benches are forty feet tall. Blasting is done with ANFO - ammonium nitrate and fuel oil. 0.65 pounds of explosives are used per ton of rock. The mine uses ~45 megawatts of power per day, which is about the same as the city of Butte itself.

Continental Mine ores are crushed in two stages. The crushed ores are then sent to the mill, where they are ground down to the fineness of talcum powder. Flotation and lime are used in procesing. Sulfides are collected. 1% of the mined material goes to the concentrator. 99% of mined material becomes tailings. The tailings powder is wet (33% solid and the rest is water) and piped uphill to a pond. The tailings pond water has a pH of 10. Water from the pond is recycled to make tailings slurry. 27 million gallons a day enters the pond. An earthen dam around the pond is designed to withstand a powerful earthquake.

Copper and molybdenum concentrates produced at the Continental Mine are not smelted locally - they are not even smelted in America. Concentrates are sold around the world, where material is smelted and the metals are produced. America shipping rocks overseas and buying back the finished product is the behavior of an underdeveloped country - America is not interested in smelting anymore - a sad reality.