The Natural Dispersal of Metals to the Environment in the Wulik River–Ikalukrok Creek Area, Western Brooks Range, Alaska

Reconnaissance geologic investigations by I.L. Tailleur in 1968 noted the presence of widespread iron-oxide staining in creek beds in the Wulik River and Ikalukrok Creek drainages of the western Brooks Range. A brief examination of one of these creeks revealed the presence of weathered sulfide-bearing bedrock and stream sediments containing more than 10 percent lead (Tailleur, 1970). Tailleur named this creek Red Dog Creek, and subsequent exploration in this area resulted in the discovery of the huge Red Dog lead-zinc-silver deposit. As exploration continued in this region, many other metal-bearing deposits were discovered (fig. 1). Today there are at least eight significant areas of high metal concentrations in bedrock at or near the surface in the Wulik River and Ikalukrok Creek areas (fig. 1). Most of the iron-, lead-, and zinc-bearing rocks in the western Brooks Range originally formed on or immediately below an ancient sea floor about 330 to 340 million years ago (Kelley and others, 2003). The sea floor was at the bottom of a deep-water basin where thin black shale, chert, and carbonate layers were deposited. Metal-bearing fluids deposited minerals such as pyrite (iron sulfide), galena (lead sulfide), and sphalerite (zinc sulfide). These metal-bearing sedimentary rocks are now exposed at the surface and are being eroded and dispersed to the environment along the slopes, creeks, and rivers of the region. Erosion and dispersion of metal-bearing rocks have been ongoing for at least hundreds of thousands of years and have resulted in very large amounts of lead, zinc, and other metals that are now found in the Wulik River and Ikalukrok Creek drainages. Mining of the Red Dog lead-zinc-silver deposit began in 1989. Ore is mined in an open pit and transferred to a nearby processing facility where it is crushed, ground, and concentrated using a flotation process. The process separates metal-bearing minerals (galena and sphalerite) from other minerals in the metal-bearing host rock. This processing produces a concentrate of fine-grained, clean, and angular galena and sphalerite grains. Trucks are used to transport concentrates from the mine to the coastal port facility where they are temporarily stored during winter. During the summer, concentrates are transferred to barges and then to deep-water ships. Transportation and storage