Hard rock silver mining began in 1874, then rich copper veins were discovered in 1882. The district quickly switched from silver to copper, and by 1887, Butte was the leading copper-producing district in the United States. The Anaconda Copper Mine was the world's most productive copper mine from 1892 through 1903, and intermittently for years thereafter. Open-pit mining began at the Berkeley Pit in 1955; the Berkeley Pit has been inactive for years, and continues to fill with acidic water. Through 1964, Butte produced 7.3 million tonnes of copper, 2.2 million tonnes of zinc, 1.7 million tonnes of manganese, 380 thousand tonnes of lead, 645 million troy ounces of silver, and 2.5 million ounces of gold. The only remaining active copper mine at Butte is the Continental pit, operated by Montana Resources LLP. Some copper is also produced by Troy unit silver mine in the northwest corner of the state, and by two platinum mines in the Stillwater igneous complex: the Stillwater mine and the East Boulder project. The first commercial copper mining district in Nevada was at Yerington in Lyon County. The Ludwig Mine opened in 1865, but the district produced only modest amounts of copper until a railroad was built to the district in 1911, and a smelter built in 1912 at nearby Thompson. The Anaconda Copper Mine produced from an open pit from 1918 to 1978. The copper ore bodies are contact metamorphic replacement deposits in limestone. Production through 1921 was 39 thousand tonnes of copper. The largest copper producer in Nevada has been the Ely district (also called the Robinson district) in White Pine County. A Native American showed mineralization to prospectors in 1867, and the district started in a small way as a lode gold producer. A railroad link in 1906 made it economically possible to start large scale open pit mining of the large porphyry copper deposits, and the first copper was produced in 1908. Mining was halted in recent years due to low copper prices, but the open pit was reopened in 2004 by Quadra Mining Ltd. In 2007, the mine produced 121 million pounds (55 thousand tonnes) of copper, plus byproduct molybdenum. Newmont's Phoenix mine in Lander County produced 6.2 million pounds (2800 tonnes) of copper in 2007, as a byproduct of gold mining. New Jersey was the site of one of the first attempts to mine copper in what is now the United States. Copper ore was discovered about 1712 in what is now North Arlington, and the Schuyler Copper Mine extracted ore and shipped it in casks to the Netherlands.