Environment

Printer friendly

Following the cessation of mining activities AngloGold Ashanti (Nevada) Corporation (AGANC) continues with environmental closure activities at Big Springs in north-eastern Nevada approximately 60 miles north of the town of Elko and 30 miles south of the Idaho border. The mine area is situated within the Independence Mountains on public lands inside the Humboldt-Toiyabee National Forest, which is administered by the US Forest Service (USFS). Elevations range from 7,200 to 9,200 feet above mean sea level (amsl) with average annual rainfall of 25 to 30 inches. Heavy snowfalls and severe winter conditions resulted in the mine operating on a seasonal basis, typically from March/April to October/November. Big Springs had a relatively short operating life from mid-1987 to mid-1993. The roaster system was shut down in 1994.

During the mine’s life, gold ore was extracted from 12 relatively small surface mining areas scattered around the site using conventional loader-truck methods. Ground water encountered during mining required dewatering in the northern portion of the mine area. The headwaters of the North Fork of Humboldt River were crossed to access one of the rock storage areas. Ore was crushed at the mine and then transported on highway-legal trucks on a gravel road to the mill site located four miles east of the mine, out of the mountainous terrain, and at a considerably lower elevation, which allowed for year-round milling and leaching operations.

Environmental mitigation measures at Big Springs started early, during the construction phase, when the dirt road along the North Fork of Humboldt River was replaced with a gravel road higher up the hill side outside the riparian corridor to reduce the impact on the resident threatened fish species, the Lahontan cutthroat trout. Other mitigation measures to protect the trout were: the construction of a five-mile fence to protect the riparian corridor (the narrow strips of land along the banks of the river) along the North Fork of the Humboldt River from livestock; structural improvements to the channel; riparian plantings, and numerous sediment control measures. By 1992, these collective mitigation measures contributed to the ‘good’ classification of the north fork of the Humboldt River and were recognised in the US Fish and Wildlife Service (USFWS) 1995 Recovery Plan for the cutthroat trout. This classification remains today, as chronicled in a report published by the Nevada Department of Wildlife in December 2004.

In 1992, certain constituent concentrations appeared to be increasing in the North Fork of the Humboldt River and its tributaries within and near the mine areas. This change in water quality was attributed to above normal rainfall after a prolonged drought. As a precautionary measure, a third party expert consulting firm was retained to determine if the changes were exclusively attributable to the flushing of dissolved salts that had accumulated during the drought years. Rigorous water quality sampling and geochemical testing was conducted to better understand the natural processes that might be contributing to the change. Considerable time, effort, and expense were involved, but it was determined that cover systems for the rock storage areas and a series of diversion channels to capture surface run-off were warranted.

Cover system and diversion installation started in 1995 as part of mine closure and reclamation and continued on a seasonal basis to the end of 1997. Improvements and repairs to the diversions continued through 2000. Since that time, little work has been required to operate and maintain these systems.

Simultaneous with this reclamation work, seven of the 12 surface mines were partially backfilled. Three of the surface mines had been backfilled during active mining operations. The partially backfilled surface mines were covered with soil and revegetated. The two remaining surface mines contain water, one of which supports a fish population.

In August and September 2005, the company conducted field reviews of the mine areas with government agency representatives. During these site visits, a good cover of vegetation was observed on all of the reclaimed areas. Similarly, the three large sedimentation ponds and fill across the north fork of the river were no longer apparent after being removed and creatively contoured to blend with the surrounding terrain. Likewise, the haul road on the steepest terrain was determined to have been successfully recontoured and stabilised with vegetation.

Water quality improvements have also been observed at selected locations since the covers and diversions were completed and the planted vegetation started to develop. Finally, ongoing aquatic studies involving fish and aquatic insects have demonstrated that no long-term adverse effects have resulted from the mining activities.

AngloGold Ashanti will continue to work with the agencies to monitor water quality and evaluate the effectiveness of the mine area reclamation and closure strategies. The company is implementing a new permit that calls for continued monitoring, reporting, and evaluation of water quality within and near the mine area for the next five years. The valuable experience in reclamation gained at Big Springs will stand the AngloGold Ashanti team in good stead for future planned closures.