Open-Pit Mining and Block-Caving
Butte’s first small boom in the 1860s was based on miners finding gold using placer-mining methods (washing sands and gravels to recover particles of gold).
Butte’s sustained boom began in the 1870s, when miners found ways to exploit rich veins of ore from underground workings. That boom lasted seven decades, until the 1940s, when depletion of the rich veins and competition from copper mines elsewhere threatened to make mining in Butte unprofitable. Anaconda Copper Mining Company (ACM) responded by implementing newer methods, block-caving and open-pit mining, aimed at exploiting the remaining lower-grade ore beneath the Butte Hill.
Underground mining is traditionally a labor-intensive undertaking. In the nineteenth century, many aspects of underground mining had been mechanized. Machine drilling accelerated the drilling of holes in the rock for explosives; ore cars on wheels, which ran along tracks and were often pulled by horses or mules, greatly enhanced miners’ ability to move ore horizontally through mine workings to the shaft; and steam-powered hoisting allowed companies to lift large tonnages of ore vertically out of a mine. One task remained stubbornly labor-intensive: once the ore was broken by explosives, it still had to be laboriously loaded by muckers into ore cars using hand shovels. The need to use hand shovels for excavations changed broadly in the early nineteenth century with the invention of the steam shovel, but they were too bulky for use underground.
Surface mining using steam or electric shovels brought several low-grade copper mines in the American Southwest into production in the first quarter of the twentieth century, but underground mining remained the preferred method in the Butte mines because of the depth and richness of the ore. It remained profitable at Butte to have skilled miners follow and extract the rich veins of ore. Approaching mid-century, however, as the mines got deeper, it grew more difficult for ACM to remain profitable using conventional underground mining techniques in a copper market dominated by open-pit mines. The company’s first response after World War II was to develop the “Greater Butte Project,” implementing an underground-mining method called block-caving. This entails developing a series of underground chutes to convey ore to large haulage cars at deeper levels, and then using explosives to drop large blocks of ore over those chutes, thus eliminating the need for muckers. The company developed a new, larger shaft in upper Dublin Gulch, called the Kelley. The new hoist house had an engine capable of hoisting much greater volumes of ore than the conventional Butte shaft and hoisting engine. The Kelley mine began producing in 1952 with a capacity to hoist 15,000 tons of ore daily.
As the workings of the Kelley mine extended eastward, however, the grade of the ore became too low to be economically mined by block-caving, so ACM decided to develop an open-pit mine. Excavation began in 1955 at the Berkeley mine, for which the pit was named. The first haul trucks used in the Berkeley pit had capacities of 18 tons. The ore was less than 1% copper. As the pit went deeper, ACM introduced ever-larger trucks, getting as large as 200 tons, but 150-ton trucks proved most efficient. As the pit got deeper, its perimeter also expanded, causing the pit to swallow several storied neighborhoods, including Meaderville, East Butte, and McQueen. In 1980, the Berkeley Pit was producing 50,000 tons of ore per day.
Atlantic Richfield (ARCO), the oil company, purchased The Anaconda Company in 1977, and in 1979 ARCO decided to start excavating a new pit, east of the Berkeley, to exploit a different ore body. As the new pit went into production, ARCO closed the Berkeley Pit in 1982. A year later, ARCO suspended all mining in Butte and gave the order to turn off the pumps in the Kelley mine, which also served to keep the bottom of the Berkeley Pit dry. Soon thereafter, highly contaminated water from the flooding underground workings beneath the Berkeley Pit started rising. Had the water in the pit been allowed to continue rising, it would have contaminated shallow aquifers beneath “The Flat” south of the Butte Hill. Therefore, ARCO and the current mining company, Montana Resources (MR), began operating a water-treatment plant in 2019. The plant pumps water from the pit, neutralizes it, and removes the heavy metals so that the water is fit to discharge into Silver Bow Creek. The metals are collected as a sludge dumped back into the pit.
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