GEOCHEMISTRY OF SOIL SAMPLES FROM THE RED DOG AREA, WESTERN BROOKS RANGE: DISTINGUISHING NATURAL VERSUS ANTHROPOGENIC METAL SOURCES

Numerous massive sulfide deposits in the western Brooks Range (containing ~31Mt Zn) are distributed over an area of about 600 km2. Natural weathering of these deposits has dispersed metals in water, soil, and stream sediments. Local anthropogenic sources also contribute Pb and Zn to the environment; ore concentrates of galena and sphalerite are produced at the Red Dog mine and transported along a haul road from the mine to a port storage facility. Transportation and storage of the concentrate has resulted in dispersal of dust along the road and near the port facility.

Geochemical characteristics of soil samples and their mineralogic character distinguish naturally formed soils from soils containing concentrate dust. Vertical element distributions within a soil profile are particularly distinctive. In soils over mineral deposits, there is a consistent increase in Pb, Sb, or Ag concentration with depth/proximity to the metal source. The profile over the unmined Suds deposit provides an example: Pb ranges from 74 ppm in surface soils (within 10 cm of surface) to 1920 ppm (40-60 cm depths), Sb from 4.5 to 88 ppm, and Ag from 1.4 to 4.3 ppm. In contrast, soils with mining-produced concentrate dust show high concentrations of Pb and Zn within the upper 10 cm of soil profiles (one sample has 5,240 ppm and 50,000 ppm of Pb and Zn, respectively), with sharp decreases in concentration with depth (43 ppm and 130 ppm). Element associations are also distinctive. The acidity produced by weathering of sulfide deposits (e.g., abundant pyrite) creates an environment in which elements like Se and Mo are stable and Ca is not. Consequently, high Mo (30-170 ppm) and Se (6-17 ppm) and low Ca (<0.4%) contents characterize soils near deposits. These soils have high Pb/Zn ratios (10-100) because Zn is mobile and preferentially removed in an acidic environment. In contrast, soils contaminated with galena- and sphalerite-concentrate dust (pyrite absent) have low Mo (<6 ppm) and Se (<3 ppm), high Ca (0.4 to 3.6%) and Pb/Zn ratios of about 0.2. Mineralogy and morphology of grains differ greatly between soil types. Secondary jarosite, plubojarosite, and anglesite are developed in deposit soils, with rare rounded/etched galena grains and no sphalerite, whereas euhedral galena and sphalerite grains remain in dust-bearing soils, and secondary phases are absent.