CHEMICAL VARIATIONS IN THE WATERS DRAINING THE HI-YU MINE, FAIRBANKS MINING DISTRICT, ALASKA

The Hi-Yu Mine is located on Moose Creek, approximately 56 km northeast of Fairbanks Alaska. Mineralization consists of northwest-trending gold-bearing pollymetallic veins. These veins consist mostly of quartz with sphalerite, pyrite, arsenopyrite, galena, stibnite, and antimony sulfosalts. The mining and milling of this gold-bearing ore results in the release of metals into the environment when the sulfide minerals contained in the ore, and mine tailings are exposed to water and oxygen. There are two sources of contamination at the Hi-Yu mine: the mill spring and the mine tailings. The mill spring has an acid pH (4.8) and contains ~21 ppb Cd and ~2400 ppb Zn. Water draining the mine tailings contains ~284 ppb As and 27 ppb Sb. These concentrations are above the EPA and State of Alaska Department of Environmental Conservation Maximum Contaminant Levels (MCL). Once the mill spring mixes with Moose Creek, the pH increases leading to the precipitation of iron and manganese hydroxides with associated As, Cd, Sb, and Zn. The iron precipitates contain > 10,000 ppm As, 19 ppm Cd, 240 ppm Sb, and 2600 ppm Zn. Precipitation of metals, along with interaction of organic material and dilution, lowers metal contaminant levels below MCL levels within 0.8 km from the mixing point. Moose Creek flows 1.2 km from the Hi-Yu mine into Fairbanks Creek. Although technically a point source of pollution, the spring and tailings cause no significant increase in the metal content of Fairbanks Creek at this time. The mixing and dilution of waters, adsorption of metals onto Fe and Mn oxy-hydroxides, and the settling of particles make the discharge benign, despite the iron hydroxide precipitate locally present.