PALEOGEOGRAPHIC AND METALLOGENIC IMPLICATIONS OF PHOSPHATIC ROCKS IN THE LISBURNE GROUP (PERMIAN-CARBONIFEROUS), NORTHERN ALASKA

DUMOULIN, Julie A., U.S. Geological Survey, 4200 University Dr, Anchorage, AK 99508, [email protected], WHALEN, Michael T., Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, AK 99775, HARRIS, Anita G., 1523 East Hillsboro Blvd #1031, Deerfield Beach, FL 33441, and SLACK, John F., U.S. Geol Survey, National Center, MS 954, Reston, VA 20192

Phosphatic rocks are a small but notable part of the Lisburne Group (Permian-Carboniferous), a chiefly carbonate platform succession found in outcrop and subsurface throughout northern Alaska. New sedimentologic, paleontologic, and geochemical data allow us to better define the geographic and stratigraphic extent of these strata and explore their implications for tectonic and paleogeographic reconstructions. Lisburne phosphorites are best developed in the Endicott Mountains allochthon in a discontinuous belt that extends for 265 km in the central Brooks Range (CBR). Similar phosphatic rocks occur in the Kelly River allochthon in the western Brooks Range (WBR), and in autochthonous strata in the Ikpikpuk well. Conodonts, foraminifers, and goniatite cephalopods indicate that Lisburne phosphorites are Late Mississippian (mainly early Chesterian). Phosphatic strata are interbedded with black shale and lime mudstone rich in radiolarians and sponge spicules, and formed largely in dysoxic outer ramp to slope settings.

Where best developed in the CBR, Lisburne phosphatic strata consist of 10- to 20-cm-thick beds of sandstone and rudstone made of phosphatic peloids, ooids, oncoids, and bioclasts cemented by carbonate or silica. These beds cap regressive parasequences of lime mudstone and shale and occur through an interval ≤12 m thick. High gamma ray response through this interval indicates strongly condensed facies, likely related to sediment starvation and development of phosphatic hardgrounds. Lisburne phosphorites contain up to 26 wt % P₂O₅, 5.1 wt % F, and 162 ppm U; interbedded black shales have up to 20 wt % TOC, are potential petroleum source rocks, and are locally metalliferous with up to 1690 ppm Cr, 1870 ppm V, 461 ppm Ni, 4670 ppm Zn, and 32 ppm Ag.

Paleogeographic reconstructions of northern Alaska imply that Lisburne phosphorites formed in the Ikpikpuk basin and along both sides of the mud-rich Kuna basin, which hosts giant massive sulfide and barite deposits of the Red Dog district. Lisburne phosphatic strata are coeval with these deposits and formed as part of a high-productivity upwelling regime. Resultant nutrification likely contributed to the demise of the Lisburne carbonate platform in the WBR and its penultimate drowning in the CBR, and was important in development of the Red Dog district deposits.