Fact Sheet PDF File

The Natural Dispersal of Metals to the Environment in the Wulik River–Ikalukrok Creek Area, Western Brooks Range, Alaska Introduction layers were deposited. Metal-bearing fluids deposited minerals such as pyrite (iron sulfide), galena (lead sulfide), and sphalerite Reconnaissance geologic investigations by I.L. Tailleur in (zinc sulfide). These metal-bearing sedimentary rocks are now 1968 noted the presence of widespread iron-oxide staining in exposed at the surface and are being eroded and dispersed to the creek beds in the Wulik River and Ikalukrok Creek drainages environment along the slopes, creeks, and rivers of the region. of the western Brooks Range. A brief examination of one of Erosion and dispersion of metal-bearing rocks have been ongo- these creeks revealed the presence of weathered sulfide-bearing ing for at least hundreds of thousands of years and have resulted bedrock and stream sediments containing more than 10 percent in very large amounts of lead, zinc, and other metals that are lead (Tailleur, 1970). Tailleur named this creek Red Dog Creek, now found in the Wulik River and Ikalukrok Creek drainages. and subsequent exploration in this area resulted in the discovery Mining of the Red Dog lead-zinc-silver deposit began in of the huge Red Dog lead-zinc-silver deposit. As exploration 1989. Ore is mined in an open pit and transferred to a nearby continued in this region, many other metal-bearing deposits processing facility where it is crushed, ground, and concentrated were discovered (fig. 1). Today there are at least eight signifi- using a flotation process. The process separates metal-bear- cant areas of high metal concentrations in bedrock at or near the ing minerals (galena and sphalerite) from other minerals in the surface in the Wulik River and Ikalukrok Creek areas (fig. 1). metal-bearing host rock. This processing produces a concentrate Most of the iron-, lead-, and zinc-bearing rocks in the of fine-grained, clean, and angular galena and sphalerite grains. western Brooks Range originally formed on or immediately Trucks are used to transport concentrates from the mine to the below an ancient sea floor about 330 to 340 million years ago coastal port facility where they are temporarily stored dur- (Kelley and others, 2003). The sea floor was at the bottom of a ing winter. During the summer, concentrates are transferred to deep-water basin where thin black shale, chert, and carbonate barges and then to deep-water ships. Transportation and storage Barrow 0 100 KM Wulik Anarraaq Cub Area of Map Oak 02AH16 Su-Lik Brooks Suds Range 02AH24 02AH15 Arctic Ferric Creek 02AH20 Circle 02AH18 02AH29 Red Dog 68°00' Cr ver Ri 02AH25 Ikalukrok C River H Kivalina U Wulik K port road C H Kivalina I S E A 0 10 Km Sulfide prospect/deposit 02AH28 Sample site port facility Noatak 163°30' Figure 1. Location map of sulfide mineral deposits and sample sites. U.S. Department of the Interior Fact Sheet 107–03 U.S. Geological Survey �� October 2003 of the concentrate has inadvertently resulted in dispersal of dust There are two characteristic patterns of metal concentra- from the concentrates along roads, and around the mine and port tion in natural surface materials of the region. In residual facility. colluvium profiles over metal-bearing bedrock deposits, metal High concentrations of lead or zinc in surface materials in contents characteristically increase with depth and proximity northwestern Alaska are of concern to villagers in the region to the metal-bearing bedrock source. The Cub prospect collu- (i.e., Kivalina and Noatak) (fig. 1). It is difficult, however, to vium profile shown in figure 2 illustrates this type of variation. determine whether the source is from natural erosion or disper- The metal-bearing bedrock at the Cub prospect is on a small sion or related to fugitive dust, based solely on high concentra- tributary to the upper Ikalukrok River, about 9 km upstream tions of lead or zinc. This study shows how metal-rich samples from the confluence with Red Dog Creek (fig. 1). The second that are derived from fugitive concentrate dust can be distin- type of metal-concentration variation is developed in silt, sand, guished from those that are naturally dispersed to the environ- and gravel (alluvium) deposited by streams in the area. In these ment by weathering and erosion of metal-bearing deposits. sediments, metal concentrations remain nearly constant from the surface downward through sampled profiles. Data for the Natural Metal Dispersal Processes lower Ikalukrok Creek area (fig. 2) illustrate this type of varia- Natural metal dispersal in the Wulik River and Ikalukrok tion. The physical erosion and transport of alluvium by streams Creek area effectively started several hundred thousand years effectively mixes and homogenizes metal-bearing material in ago when glaciers covered the region. These glaciers physi- the alluvium. These two types of metal-concentration variation cally eroded and moved bedrock materials—including parts of distinctly contrast with that developed where mining-produced metal-bearing deposits in their path—to the limits of glaciation fugitive concentrate dust is the source of high metal concentra- that were at or beyond the present coastline (Coulter and others, tions. As one would expect, ore-concentrate dust is localized at 1965). The glaciers eventually melted and retreated to higher the surface, and metal concentrations sharply decrease down- parts of the nearby mountains, where remnants were still ward in soil profiles (fig. 2). present until about 11,500 years ago (Hamilton, 1994). The mineralogic character, including mineral species and the Since the glaciers retreated from the Wulik River and morphology of specific mineral grains, also distinguishes natu- Ikalukrok Creek areas, both physical and chemical weathering ral metal-rich surface materials. The effects of chemical weath- processes have continued to disperse metals from bedrock ering, especially oxidation, are evident in natural colluvium to the environment. Physical weathering breaks up near- developed over metal-bearing bedrock. Here, secondary miner- surface bedrock and creates a cover of soil and rock fragments als such as jarosite, plumbojarosite, and anglesite are developed; as much as several meters thick. Some of these materials, called sphalerite is lacking, and galena occurs in only minor amounts, colluvium, may slowly migrate downslope to valley lowlands if at all (samples 02AH16, 20, and 24; table 1). Surface materi- where they are then eroded and transported by streams to als at the port facility where fugitive concentrate dust is present, lower elevations—thus forming alluvium. Both processes—the contain both galena and sphalerite (02AH28; table 1). breaking up of near surface bedrock and deposition of materi- The morphology of galena grains produced by anthropo- als in streams—are called mechanical or physical weathering genic input (mine concentrate and port facility) is different processes. Chemical weathering involves the alteration and dis- than galena that occurs naturally (alluvium and colluvium over solution of minerals. Oxidation of sulfide minerals, especially and surrounding natural deposits) (fig. 3). Individual grains pyrite (iron sulfide), is a very important chemical-weathering of galena and sphalerite in soil at the port facility are angular, process that develops acidic surface materials and waters. The acidic conditions and the abundance of oxidized iron enable LEAD CONCENTRATION (PPM) further sulfide-mineral breakdown, precipitation of secondary 1,000 2,000 3,000 4,000 5,000 6,000 14,000 minerals, and dissolution of metals in surface and ground water. 0 Metal-bearing bedrock deposits in the Wulik River and Ikaluk- t facility (dust) rok Creek drainages have been affected by chemical weathering por 10 and physical weathering and erosion for at least the last 10,000 Middle Ik years. These are the processes that produced very high (greater Cub pr than 10 percent) lead concentrations in the sediments of Red 20 ospect Dog Creek before the Red Dog mine existed. alukr (colluvium ) 30 ok Cr Distinguishing Naturally Dispersed Metals from eek (alluvium) DEPTH (CM) Mining-Produced Fugitive Concentrate Dust 40 The complicated processes that lead to naturally dispersed metals in the Wulik River and Ikalukrok Creek areas produce 50 several characteristics of the resulting metal-bearing materials that distinguish them from those that are the product of mining- 60 produced fugitive concentrate dust. Naturally dispersed metals can be identified by their patterns of metal concentrations in Figure 2. Diagram showing variation in lead concentrations in profiles surface materials, their mineralogic character, and their associa- from the Cub prospect colluvium, middle Ikalukrok Creek alluvium, and tion with certain other elements. port facility surficial materials. ��

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