Engineering & Mining Journal

JUL 2018

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

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ENVIRONMENTAL MANAGEMENT JULY 2018 • E&MJ 27 posits and cemented them with oxides of iron, forming what is referred to as ferricrete. These recent geologic events have exposed mineral deposits to surface weathering prior to mining. Before the first miner arrived, there was massive natural metals loading in the Animas River, which limited aquatic life, including trout populations downstream from Silverton. Historic Mining As early as the 1700s, Spanish explor- ers discovered placer gold in Arrastra Creek, a tributary of the Upper Animas River. Serious mineral exploration began in the early 1870s, following discovery of placer gold by the U.S. Army's Baker reconnaissance team. In the 1870s, the Upper Animas watershed became a prime location of mining activity for gold, silver, lead and copper. The area contains more than 1,500 mines. 1 In addition, the area was home to more than 50 separate mill sites, eight distinct smelters and 35 dif- ferent aerial trams. Mining and milling processes in the Animas River watershed were typical of those practiced throughout the West in the 19 th and 20 th centuries. Beginning in 1872, vein deposits were mined un- derground by small crews who selectively mined high-grade ore. Ore was hand-sort- ed and sent to smelters by mule pack train and later by wagon. Waste rock was discarded in open mine stopes or on mine-waste dumps outside the portal where the hand sorting was done. As more efficient methods of mining and milling were developed and rail trans- port became available, increasingly larger amounts of lower grade ore were mined and processed and additional metals re- covered. For example, before 1917, the available mineral processing methods could not recover zinc so it was discarded with the other tailings. After 1917, zinc was recovered from the ore. Waste rock was disposed of in waste dumps outside the mine portal, and mill wastes were de- posited into the nearest stream course. An estimated 8.6 million tons of mill waste, about 47.5% of the total ore pro- duced, was discharged directly into sur- face streams between 1872 and 1935. With the arrival of mining in the Up- per Animas 140 years ago, drainage from mine adits, discharges of tailings to water ways, and the weathering of waste rock added to the naturally high metal and acid loads in the Animas Riv- er and its tributaries. These discharges continued long after the mines were no longer active. The total quantity of tail- ings generated from 1873 until 1964 is enormous. "The total amount of tailings created in the watershed in 125 years of The discharge from the Gold King mine can be seen in this historic photo. The mine was discovered in 1887 and operated until 1907 when a fire destroyed the surface structures. SGC constructed a water treatment facility for Cement Creek at Gladstone. 1 One of these historic mines was the Gold King, where, in August 2015, an EPA contractor breached an earthen plug and caused the release of over 3 million gallons of wa- ter. The released waters eroded a historic waste rock pile, washing material into Cement Creek. According to EPA, the amount of metals loading to the Animas River from the Spill was small when compared with ongoing metals load- ing from natural and other historic sources in the area and the impacts of the Spill on wildlife were minimal. (US EPA, 2017, Fate and Transport Report Executive Summary).

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