File:Continental Mine (Butte, Montana, USA) 3.jpg

English: The town of Butte, Montana (pronounced “byoot”) 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.

Seen here is the Continental Mine, also known as the Continental Pit. It is the only active mine in modern Butte. Mining here was started in 1980 by the Anaconda Copper Mining Company - it is currently owned by Montana Resources. The mine is situated on the eastern side of the Continental Fault, a major Basin & Range normal fault in the Butte area with about 3500 feet of offset. Over 100 different copper minerals are known from Butte to the west of the fault - many are minor minerals. East of the fault, lower grade rocks are present. The Continental Mine targets this low-grade deposit, which consists of disseminated copper sulfides plus copper- and molybdenum-bearing hydrothermal veins that intrude the BQM. Minerals include chalcopyrite, molybdenite, malachite, azurite, and cuprite. A secondary biotite mica halo is present around the deposit - the biotite is derived from hornblende amphibole. The photo shows copper-molybdenum ores piled up on the working floor of the mine.

Copper and molybdenum concentrates are produced at the Continental Mine, but they are not smelted locally and 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.

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.

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.

"An ore deposit is a mine if it can stand total mismanagement and still make money."