Abstract The world-class Zn-Pb-Ag deposits of the Red Dog district, Alaska, occur in severely shortened, late Paleozoic sedimentary rocks of the western Brooks Range fold and thrust belt. Red Dog-style strata-bound mineralization occurs within a localized Mississippian black shale facies of the Lisburne Group informally termed the Ikalukrok unit. Prior work has developed paleogeographic models in which the Ikalukrok unit formed in a starved, second-order basin flanked by carbonate platforms. Collision with and northward (modern coordinates) obduction of the oceanic Angayucham terrane telescoped Devonian to Early Jurassic passive margin sediments along thrust faults with displacements ranging from meters to tens or hundreds of kilometers. The most significant thrusts bound allochthons that juxtapose coeval stratigraphy from previously widely separated parts of the former continental margin. Thrusts within the allochthons created structurally and stratigraphically defined thrust plates and subplates, which are in turn deformed internally by smaller faults and associated folds. Steeply dipping extensional faults cut all compressional structures in the district. The Red Dog mine area and Anarraaq deposits are highly enriched in Zn and Pb owing to the superposition of as many as four phases of sulfide mineralization through carbonate replacement, brecciation, silicification, and veining. Barite and secondary silica are pervasive and intimately associated with base-metal mineralization in the mine area deposits. A giant barite body occurs in the structural hanging wall of the Anarraaq deposit but is spatially separated from it. The Ikalukrok unit hosts other strata-bound Zn-Pb-Ag deposits and occurrences in the Red Dog district that are referred to herein as laminated deposits. Sulfides within laminated deposits such as Su and Lik are typically laminated and brecciated. These deposits lack evidence of associated barite or widespread, multiphase massive base-metal sulfides, which form the high-grade cores of the mine area and Anarraaq deposits. We suggest that differences in character of Zn-Pb-Ag deposits in the Red Dog district can be attributed to variability in the original composition of the Ikalukrok unit host and the location of the deposits within the original subbasin.

Abstract The Lisburne Group (Carboniferous-Permian) consists of a carbonate platform that extends for >1000 km across northern Alaska, and diverse margin, slope, and basin facies that contain world-class deposits of Zn and Ba, notable phosphorites, and petroleum source rocks. Lithologic, paleontologic, isotopic, geochemical, and seismic data gathered from outcrop and subsurface studies during the past 20 years allow us to delineate the distribution, composition, and age of the off-platform facies, and to better understand the physical and chemical conditions under which they formed. The southern edge of the Lisburne platform changed from a gently sloping, homoclinal ramp in the east to a tectonically complex, distally steepened margin in the west that was partly bisected by the extensional Kuna Basin (~200 by 600 km). Carbonate turbidites, black mudrocks, and radiolarian chert accumulated in this basin; turbidites were generated mainly during times of eustatic rise in the late Early and middle Late Mississippian. Interbedded black mudrocks (up to 20 wt% total organic carbon), granular and nodular phosphorite (up to 37 wt% P 2 O 5 ), and fine-grained limestone rich in radiolarians and sponge spicules formed along basin margins during the middle Late Mississippian in response to a nutrient-rich, upwelling regime. Detrital zircons from a turbidite sample in the western Kuna Basin have mainly Neoproterozoic through early Paleozoic U-Pb ages (~900-400 Ma), with subordinate populations of Mesoproterozoic and late Paleoproterozoic grains. This age distribution is similar to that found in slightly older rocks along the northern and western margins of the basin. It also resembles age distributions reported from Carboniferous and older strata elsewhere in northwestern Alaska and on Wrangel Island. Geochemical and isotopic data indicate that suboxic, denitrifying conditions prevailed in the Kuna Basin and along its margins. High V/Mo, Cr/Mo, and Re/Mo ratios (all marine fractions [MF]) and low MnO contents (<0.01 wt%) characterize Lisburne black mudrocks. Low Qmf/Vmf ratios (mostly 0.8-4.0) suggest moderately to strongly denitrifying conditions in suboxic bottom waters during siliciclastic and phosphorite sedimentation. Elevated to high Mo contents (31-135 ppm) in some samples are consistent with seasonal to intermittent sulfidic conditions in bottom waters, developed mainly along the basin margin. High d 15 N values (6-120) imply that the waters supplying nutrients to primary producers in the photic zone had a history of denitrification either in the water column or in underlying sediments. Demise of the Lisburne platform was diachronous and reflects tectonic, eustatic, and environmental drivers. Southwestern, south-central, and northwestern parts of the platform drowned during the Late Mississippian, coincident with Zn and Ba metallogenesis within the Kuna Basin and phosphogenesis along basin margins. This drowning was temporary (except in the southwest) and likely due to eutrophication associated with upwelling and sea-level rise enhanced by regional extension, which allowed suboxic, denitrifying waters to form on platform margins. Final drowning in the southcentral area occurred in the Early Pennsylvanian and also may have been linked to regional extension. In the northwest, platform sedimentation persisted into the Permian; its demise there appears to have been due to increased siliciclastic input. Climatic cooling may have produced additional stress on parts of the Lisburne platform biota during Pennsylvanian and Permian times.