|Fact Sheet 092–95|
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The U.S. Geological Survey (USGS) is investigating the environmental effects of undisturbed (unmined) silver-lead-zinc deposits in the northwestern Brooks Range (fig. 1). Metal concentrations and metal dispersion in stream waters draining the deposits have been determined. Using these data, scientists can:
assess the degree of metal contamination in local streams due to weathering of these deposits;
predict the environmental effects associated with other deposits of this type in similar physiographic and geologic settings and from possible future mining of such deposits; and
provide information about the natural background concentrations in mineralized areas. This information can be used to devise realistic remediation and monitoring programs. For example, in mineralized areas it may not be scientifically feasible to remediate the water to standards that are lower than the natural background concentrations.
Figure 1. Location of silver-lead-zinc deposits in the northwestern Brooks Range.
The Brooks Range zinc-lead-silver deposits consist of layers rich in sulfide minerals that are dispersed in black shale and chert. The dominant minerals are sphalerite (zinc sulfide), silver-rich galena (lead sulfide), pyrite (iron sulfide), and marcasite (iron sulfide).
Red Dog is one of the largest zinc deposits in the world. Mining of lead and zinc began in 1990 and continues today; the Lik and Drenchwater deposits have not been mined. All three deposits are exposed at the surface and were discovered by orange staining on surrounding hillsides or by significant heavy metal contamination in creeks draining the deposits.
Northwest Alaska is home to more than 6,000 people, living in 11 villages, who have traditionally obtained their food supply by hunting and fishing. Unmined mineral deposits in the area have the potential to generate acidic, metal-bearing waters when pyrite and other metal (silver, lead, and zinc) sulfides are exposed to air and water. These waters may become toxic, adversely affecting drinking water and disrupting growth and reproduction of aquatic organisms. When drainage from these deposits enters streams and rivers that are part of the local ecosystems, they constitute a potential hazard to residents and wildlife.
Water-quality data from streams draining the Red Dog deposit prior to mining (Dames and Moore, 1983) reveal that the waters were acidic and contained highly toxic levels of cadmium, lead, and zinc that exceeded the drinking water standards recommended by the State of Alaska and the instream concentrations that the U.S. Environmental Protection Agency (EPA) indicates may result in acute toxic effects to aquatic life. Other metals, such as aluminum, chromium, copper, iron, manganese, mercury, nickel, and silver, slightly exceeded EPA water quality criteria for aquatic life. These contaminated waters were toxic to most aquatic life, and streams immediately draining the deposit did not support any significant fish populations.
Stream waters draining the Drenchwater deposit have low pH values and high concentrations of dissolved solids (fig. 2). The most acidic water in the region (pH 2.8 to 3.1) is in False Wager Creek, which partly drains the deposit on the east side. These waters contain high concentrations of dissolved aluminum, arsenic, iron, cadmium, copper, lead, manganese, nickel, and zinc. Sulfate concentrations are also high. Locally, the iron- and sulfate-rich waters have precipitated a bright-orange material consisting dominantly of jarosite (fig. 3).
Figure 2. Plot demonstrating how the water from Discovery Creek, which drains mineralized areas, impacts the dissolved zinc concentrations in lower Drenchwater Creek. Aluminum, cadmium, copper, iron, lead, manganese, and nickel show similar variations in concentrations that reflect the input of naturally contaminated water. In addition, the pH of water downstream from the deposit dramatically decreases (acidity increases).
Although metal concentrations in waters draining the Drenchwater deposit are relatively high compared to those in unmineralized areas, many metal concentrations are still below the maximum levels allowed for safe drinking water according to standards recommended by the State of Alaska. Exceptions include aluminum, iron, cadmium, manganese, nickel, and sulfate, which occur in concentrations that are several times greater than the standards. Levels of cadmium, copper, and zinc exceed the EPA maximum instream concentrations determined to cause acute toxic effects to aquatic life.
Figure 3. False Wager Creek is locally precipitating jarosite (potassium and iron-rich sulfate mineral), seen as a bright orange "ferricrete" layer cementing stream pebbles and cobbles (stream width is about 1.5-2 m)
Waters collected from the Lik deposit differ from those near the Drenchwater deposit, particularly in pH and total dissolved metal concentrations. Stream waters draining the Lik deposit are about neutral and most water samples contain low concentrations of total dissolved metals. Zinc was the only metal that occurred in consistently high concentrations in water samples below the deposit. These data indicate that waters that are not acidic can still contain high concentrations of zinc. Differences in chemistry between the Drenchwater and Lik deposits are attributed to the presence of carbonate rocks at Lik. Carbonate rocks neutralize acid in the water and lower its ability to carry most metals in solution.
The results of this study show that natural metal contamination associated with silver-lead-zinc deposits in the Brooks Range is significant, particularly with deposits that have no associated carbonate rocks. The metal concentrations in some of these waters are toxic to fish and exceed the maximum levels allowed for safe drinking water. These waters are a potential hazard to wildlife and residents. Knowledge of the natural background concentrations in mineralized areas that have not been mined is crucial for predicting the environmental effects likely to result from natural weathering of deposits of this type in other areas and for predicting the effects of mining from such deposits.
Dames and Moore, 1983, Environmental baseline studies, Red Dog Project. Prepared for Cominco Alaska, Inc., Anchorage, Alaska.
Karen D. Kelley
U.S. Geological Survey
P.O. Box 25046, MS 973
Denver, CO 80225-0046
Fax (303) 236-3200
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