The Leadville mining district was the most productive silver-mining district in the US state of Colorado. It is located immediately east of the town of Leadville.
Cumulative production through 1963 was 240 million troy ounces of silver, 3 million troy ounces of gold, 987 million tonnes of lead, 712 million tonnes of zinc, and 48 million tonnes of copper. The district also produced byproduct bismuth, and iron-manganese ore.[1]
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Although silver was discovered in Colorado in 1859, the state's largest silver district, Leadville, was not discovered until 1874. Placer gold was mined at nearby Oro City starting in 1860, but it was not until 1874 that miners realized that the heavy brown sand that interfered with their gold cleanups was silver-bearing cerussite. Prospectors traced the brown sand to its lode sources, and started the Colorado Silver Boom.
The last active mine in the district, the Black Cloud mine, owned by ASARCO, closed in 1999.[2]
The district is a highly faulted area, intruded with Tertiary quartz monzonite porphyries, on the east side of the Arkansas River graben, part of the Rio Grande Rift system.
The silver occurs associated with manganese and lead in veins, stockworks, and manto-type deposits in the Mississippian Leadville Limestone (here a dolomite), the Devonian Dyer Dolomite, and the Ordovician Manitou Dolomite. Ore minerals are pyrite, sphalerite, and galena, in jasperoid and manganosiderite gangue. In upper levels, the ore minerals are oxidized to cerussite, anglesite, and smithsonite.
As in many mining districts, as the mines extended deeper, keeping the water pumped out of the workings became a major expense. To more economically drain the mines, two tunnels were driven to allow the water to drain by gravity. Water from both tunnels ultimately flows into the Arkansas River.
The Yak tunnel, 3.5 miles long and built between 1895 and 1923 to drain the southern part of the district, has its outlet in California Gulch east of the town of Leadville. The tunnel became part of the California Gulch Superfund site in 1983. In October 1985, a large surge of water from the Yak tunnel reached the Arkansas River, and elevated the dissolved metals content of the river for tens of miles downstream. Water flowing from the tunnel has been treated by its owner, ASARCO, since June 1991, to remove metals.
The Leadville tunnel was started in 1943 by the US Bureau of Mines to drain the mines of the northern part of the district, and so increase metal production. The tunnel has its outlet north of the town of Leadville, on the East Fork of the Arkansas River. In 1959 the US Bureau of Reclamation bought the tunnel for US$1, as a source for irrigation water.
Since March 1992, the Bureau of Reclamation has treated the water flowing out of the tunnel, to remove dissolved metals and bring the water quality into compliance with the Clean Water Act.
Collapses within the tunnel that began in 1995 partially blocked flow, and have created a large reservoir of an estimated one billion gallons (3.8 million cubic meters) of water within the tunnel behind the collapse. In February 2008, concerns became public that if the collapse dam should suddenly fail, as has happened in other mine drainage tunnels in Colorado (such as the Yak tunnel and the Argo Tunnel), a large slug of contaminated water would suddenly flow out of the tunnel, overwhelm the treatment facilities, and flow into the Arkansas River. The rise in water level inside the tunnel has caused water with high concentrations of dissolved metals to leak out to the ground surface through springs.[3]
Opinions as to the threat posed varied widely. County Commissioner Mike Hickman said "If it blows, it could be a national catastrophe, not only to Leadville and Lake County but to the entire Arkansas River." On the other hand, Leadville Mayor Bud Elliott stated "This is what happens when you create an emergency when there isn't one."[4] On June 30, 2008, the Bureau of Reclamation issued a report that concluded that a sudden burst of water from the tunnel was unlikely, and that the tunnel posed "no imminent public safety hazard."[5]
On 27 February 2008, The US EPA began pumping 150 gallons per minute (216 thousand gallons per day, 818 cubic meters per day) from the tunnel system, to relieve water pressure upstream from the blockage. The water, pumped from the Gaw mine shaft, was clean enough to discharge to the Arkansas River without treatment.[6] Meanwhile, the EPA drilled a new well into the tunnel system; a pump test was completed in early June 2008 to determine optimal pumping rate.[7]