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Acid Mine Drainage, Coolidge, Montana

Not every geology story is positive and aesthetically pleasing. Some are simply necessary, like this short video on acid mine drainage from an abandoned mine in the Elkhorn mining district near Coolidge, Montana in the Pioneer Mountains. The old mining town of Coolidge was a silver-rich area discovered in 1872. The ore occurs in veins in the Pioneer batholith and includes quartz, pyrite, galena, sphalerite, chalcopyrite, and molybdenite. The crash of silver markets in 1893 closed the mines temporarily, but in 1917 construction of a narrow-gauge rail line brought the mines back to life. Coolidge boasted a population of 350 people at its peak, with electricity, telephone service, a post office, a school district, a restaurant, and a company store, but no saloon or church. In 1922, a mill was built to service the mines, but it never really prospered due to the low price of silver. By the mid- 1920s, more than 24,000 feet of underground tunnels had been excavated and 50,000 tons of gold, silver, copper, lead, and zinc ore had been processed by the mill. By the early 1930s, the town was largely abandoned. Today the abandoned tunnels produce a toxic reddish acid-mine drainage, which forms when the sulfide-rich ores are oxidized by contact with air and water to form sulfuric acid. The discharge contains measurable concentrations of iron, aluminum, cadmium, copper, nickel, lead, zinc, and arsenic, which reach Elkhorn Creek, a tributary to Wise River and ultimately the Big Hole River.