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 Zinc-lead-silver massivle sulfide and barite deposits hosted in dark, fine-grained clastic sedimentary rocks are economically important resources in Alaska and major producers worldwide. Alaska contains several identified belts of such deposits, the best known and largest of which is the Noatak district in the western and central Brooks Range. Most Alaskan occurrences contain either barite (Gargaryah, Cutaway basin) or base metal sulfides (Lik-Su, Drenchwater); Red Dog contains both. Knowu Zn-Pb-Ag deposits in Alaska occur mainly in Carboniferous strata, although the host rocks for other prospects are Late(?) Proterozoic to Triassic(?) in age. Many of the barite-only deposits are also hosted in Carboniferous rock, including some in facies that are not known to host sulfides. Other barite-only deposits are Cambrian(?), Silurian(?), Devomian(?). Permian, and Triassic in age. Deposits in the Cutaway basin of the north-central Brooks Range and in the Lime Hills quadrangle, southwestern Alaska, contain significant reserves of high-grade barite. The Red Dog mine is one of the few deposits where both barite and sulfides are abundant. The deposit formed under in a long-lived hydrothermal system, probably controlled by extensional faulting. The dominantly stratiform Zn-Pb-Ag massive sulfide deposits as well as some barite deposits formed by syngenetic to early diagenetic proeesses at and near the sea floor in highly restricted basins of at least modemte depth, with condensed, often highly siliceous sedimentation. Several deposits replaced primary siliceous or calcareous sedimentary layers, including fossils; others apparently formed from fluids vented directly to the sea floor. Dewatering of mixed carbonate and clastic basins of Proterozoic(?) to Paleozoic age along syntectonic faults is the most commonly proposted mechanism for formation of shale-hosted Zn-Pb-Ag and barite deposits, but the genesis of most Alaskan occurrences is still poorly known. Some barite lenses may have formed during oxygenation of a previously anoxic water column, without requiring a discrete hydrothermal system. The sedimentologic, diagenetic, and tectonic history of possible host basins and/or fluid-source basins and the chemistry and migration pathways of mineralizing fluids have not been defined for Alaskan deposits. Restricted basins of Carboniferous age in northwestern Alaska are interpreted to reflect widespread small-seale extensional faulting along the continental margin in Early to Middle Mississippian time. Extensional basin formation could reflect an incipient continental rift, a transtensional wrench environment, or a rift behind a continental collision zone. Sedimentation of chert and shale at very low depositional rates was subsequently restored, and diagenetic barite was precipitated in regioually extensive post-Mississippian condensed sequences. Dark, fine-grained clastic rocks are common in Late Proterozoic, Palezoic, and some Mesozoic Sedimentary-rock sequences in Alaska and formed in a variety of continental and marine tectonic and sedimentary setting. Most are prospective for additional Zn-Pb-Ag and barite deposits.