Engineering & Mining Journal

JUL 2018

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Page 27 of 75

ENVIRONMENTAL MANAGEMENT 26 E&MJ • JULY 2018 Metals loading adversely impacts the An- imas River in the area around Silverton, San Juan County, Colorado. The metals of concern include iron, aluminum, cad - mium, copper, lead, zinc, arsenic and nickel. The metals loading in the Animas River is due to acid rock drainage that is created when sulphide minerals are exposed to air and water to produce sul- phuric acid. The acidic water can dissolve area minerals and then deposit metals in rivers like the Animas. Metals loading in the Animas River has limited aquatic life, including the trout fishery downstream from Silverton. Sunnyside Gold Corp. (SGC) was formed and acquired the Sunnyside mine in 1985 and mined it from 1986 until 1991 under modern environmental reg- ulations and using modern mining tech- niques. Since 1985, SGC has engaged in more than 30 years of reclamation and remediation in the Silverton Caldera. This article analyzes the geologic setting and historic mining that have caused the met- als loading in the Animas River as well as the effect of SGC's mining and extensive reclamation activities. It is also details how the remediation actions that SGC has taken have substantially reduced acid rock drainage and metals loading in the Animas River. The Sunnyside mine is located ap- proximately 8 miles north of Silverton, in the Eureka mining district. The Eureka mining district is in the northern portion of the Silverton Caldera, which is part of the San Juan Volcanic complex, which was active during the mid-Tertiary (25 to 35 million years ago). The area is natu - rally mineralized, has been for millions of years, and forms part of the Upper Ani- mas River basin. A large volume of literature has been generated on the geology and mineral- ization of the Silverton area, but the ma- jor structural feature in the area is the northeast-trending Eureka Grabben. An intricate system of radial fractures and faults, including the Ross Basin and Bo- nita faults, formed around the San Juan Caldera and provided the pathway for mineralizing solutions and locations for deposition on the vein deposits. Widespread propylitic alteration in the area has affected many cubic miles of volcanic rocks throughout the Eureka district and beyond. Pyrite is ubiquitous in the propylitized rocks, which forms 0.1 to 2 weight percent of the rock mass. It is estimated that hundreds of million tons of pyrite is present in the rocks in the vicinity. Natural weathering of altered and mineralized rock can be an important source of metals and acidity to surface and ground water. Weathering of the al- tered rock in the Upper Animas basin over the last million years provided a natural source of metals and acidity to the basin waters long before mining began. Histori- cally, parts of Cement and Mineral Creeks were always acidic, contained high-met- als loads, and likely did not support any aquatic life other than species of algae that can tolerate a pH less than 4. Franklin Rhonda, a topographer with the 1874 Hayden Survey who worked in the Silverton area, described the water in both Cement and Mineral Creeks as iron sulfate waters that were undrink- able. Pre-mining geochemical conditions in Cement Creek were not very different than they are today. A viable macroinver- tebrate community probably did not exist in either Mineral Creek upstream from the confluence with South Fork Mineral Creek or in Cement Creek prior to mining. The area has been naturally discharg- ing heavy metals for literally thousands and thousands of years. A several-thou- sand-year history of acidic drainage is re- corded in many of the surficial deposits, where iron-rich ground water derived from pyrite weathering has infiltrated these de- Remediation Helps Rescue a River Long-term reclamation projects lower the historically high-metal loading in the Animas River By Steven Lange San Juan Caldera—The area includes the drainage basins of three main tributaries: Mineral Creek, Cement Creek and the Animas River. Elevations range from 9,305 feet in Silverton, Colorado, to more than 13,800 feet in the surrounding mountains.

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