File:Berkeley Pit visitor center (Butte, Montana, USA) 1.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 entrance to a viewing platform at the Berkeley Pit (Berkeley Mine), a huge open-pit copper mine that closed in 1982. I saw this site in 1991, 2010, and 2011. The water level has been slowly rising each year and is approaching the level of the area's 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.

Locality: Berkeley Pit, Butte Mining District, northeastern Silver Bow County, southwestern Montana, USA

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See info. at:

en.wikipedia.org/wiki/Berkeley_Pit