PALEOMAGNETIC DATING OF MINERALIZATION AT RED DOG AND THE MESOZOIC TECTONIC HISTORY OF THE BROOKS RANGE, ALASKA

Paleomagnetic methods have isolated two ancient magnetizations in and around the Paleozoic shale hosted Red Dog ore deposit in northern Alaska. A shallow southerly magnetization was found in rocks dominated by galena and sphalerite while a much steeper, westerly magnetization was found in rocks that have substantial quartz replacement. Based on inclination only, the shallow magnetization formed at equatorial latitudes and the steeper magnetization at high latitudes. The steep magnetization is similar to secondary magnetizations of Mesozoic age found previously throughout the Brooks Range and geochemical considerations require that the quartz formed during deep burial in the Mesozoic. Thus the quartz-rich rocks are recording regional remagnetization associated with quartz replacement during the Mesozoic. The older magnetization, in Zn/Pb ores lacking quartz replacement, is presumably the recording the age of the mineralization at an equatorial latitude. Further interpretations are dependent upon hypotheses for the Mesozoic tectonic history of the Brooks Range. If the Brooks Range was a part ancestral North America but rotated counter-clockwise by 50-70° coincident with the opening of the Canada Basin, then the shallow magnetization is contemporaneous with the age of the host rocks. Thus the main Zn/Pb mineralization is syngenetic and formed during the Carboniferous. Hypotheses that preclude counter-clockwise rotation of the Brooks Range and instead consider it an accreted terrane could allow for a younger magnetization age and an epigenetic origin for the ore. However this would require several thousand kilometers of northward translation. Thus we favor the hypothesis of a syngenetic Zn/Pb ore followed by counter-clockwise rotation of the Brooks Range during the Mesozoic.