0361-0128/01/3470/1555-13 $6.00 1555 Introduction THE RED DOG zinc-lead-silver deposit in northern Alaska (Fig. 1) is presently the largest zinc producer in the world (Jennings and King, 2002). The four primary deposits at Red Dog have combined reserves of 143 million tons at grades of 15.9 percent Zn, 4.3 percent Pb, and 82 g/t Ag. All four de- posits are hosted by Carboniferous shale on the western end of the east-west trending Brooks Range fold and thrust belt. The ore contains textures indicative of both syngenetic or syn- diagenetic and later epigenetic replacement processes. In ad- dition, silicification overprinted earlier sulfides and replaced barite (Young, 1989; Leach et al., 2004), and some remobi- lization of zinc likely occurred with zinc reprecipitation in late breccias that cut main stage ore (Kelley et al., 2004a, b). De- termining the age of the main stage mineralization, the pale- ogeographic position of northern Alaska at the time of miner- alization, and the effects of later events is critical to understanding the genesis and postore history of the deposits. The research presented here was conducted in concert with a paleomagnetic study of the Mesozoic Asik Mountain ultra- mafic complex (Lewchuk et al., 2004) and with two with ra- diometric dating studies of minerals related to the ores (Morelli et al., 2004; Rombach and Layer, 2004) to try to an- swer two questions: When did the ore form? What was the paleogeographic location of northern Alaska at that time? The paleomagnetic data from Asik Mountain provided a Mesozoic reference pole for northern Alaska, and the radiometric dat- ing research provided age estimates for the paleomagnetic di- rections isolated here. Stable Remanent Magnetizations Associated with Ore Deposits and Diagenetic Events Direct dating of either shale-hosted synsedimentary or epi- genentic carbonate-hosted Mississippi Valley type deposits is perhaps the most significant obstacle to understanding the origin of these deposits (Sangster, 1986). This difficulty largely results because their ore mineralogy commonly lacks minerals suitable for dating. Therefore, dating by less com- mon techniques such as paleomagnetism has been attempted. Recent paleomagnetic studies on Mississippi Valley-type ore deposits have demonstrated that remagnetization is a conse- quence of the formation of the ore deposits, whereby massive fluid-rock interaction generates authigenic magnetizations in the ores and adjacent host rocks (Symons et al., 1996; Leach et al., 2001). The general agreement of the paleomagnetic data with the most reliable radiometric ages provides evi- dence of the usefulness of the paleomagnetic method (Leach et al., 2001). Paleomagnetism of the Red Dog Zn-Pb Massive Sulfide Deposit in Northern Alaska MICHAEL T. LEWCHUK, †, * School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019 DAVID L. LEACH, KAREN D. KELLEY, U.S. Geological Survey, Box 25046, MS 973, Denver, Colorado 80225 AND DAVID T. A. SYMONS Department of Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada Abstract Paleomagnetic methods have isolated two ancient magnetizations in and around the Paleozoic shale-hosted Red Dog ore deposit in northern Alaska. A high-latitude, westerly magnetization carried by magnetite, termed characteristic remanent magnetization A, was found in rocks that have barite and/or substantial quartz re- placement of barite. An intermediate- to low-latitude, southerly magnetization (characteristic remanent mag- netization B) is carried by pyrrhotite and was found in rocks dominated by galena and sphalerite. The ages the two components are constrained by their relationship with geochemistry, radiometric age dating, and hy- potheses for the Mesozoic tectonic history of the Brooks Range. Characteristic remanent magnetization A fails the fold test so it must postdate the end of Brookian orogenesis (~150 Ma). It is always found with replacement quartz that has a radiometric date (white mica from a vug, 39 Ar/ 40 Ar) of 126 Ma. The paleolatitude for charac- teristic remanent magnetization B is too shallow to be Mesozoic or younger, regardless of the model for the tectonic origin of northern Alaska, and must predate Brookian orogenesis. Geologic mapping suggests that most of the ore is syngenetic, formed at 330 to 340 Ma, and a radiometric date (Re-Os on pyrite) yields an age of 338 Ma. Since characteristic remanent magnetization B predates deformation, is found in mineralized rocks and is carried by pyrrhotite, it was probably acquired during the mineralizing process as well. The combined radiometric ages and paleomagnetic data sets can be best interpreted by assuming that northern Alaska was part of an accreted terrane that was translated northward by about 30° into its current location relative to the rest of North America and then rotated counterclockwise by 50° to 70°. This tectonic interpretation yields plau- sible magnetization ages for both characteristic remanent magnetization A and B. Geologic evidence, isotopic ages, and paleomagnetic data indicate formation of the deposit at a paleolatitude that is much lower than today. ©2004 by Economic Geology Vol. 99, pp. 1555–1567 † Corresponding author: e-mail, lewchukm@casady.org *Present address: Casady School, 9500 North Pennsylvania Avenue, Okla- homa City, Oklahoma 73120.