File:Bucyrus - 822 shovel (Continental Mine, Butte, Montana, USA) 37.jpg

English: Seen here is Bucyrus # 822, a very large shovel engaged in bulk mining at the Continental Mine (= Continental Pit) in Butte, Montana. The town is known as the “Richest Hill on Earth” and "The Mining City". The Butte Mining District has produced gold, silver, copper, molybdenum, manganese, and other metals.

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 Continental 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 consists of 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.

When I visited 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.

This mine has 14 shovels and 15 trucks. The large Bucyrus # 822 shovel is operated by one person and 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.