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.

Abstract Many strata-bound Zn-Pb ± Ag mineral occurrence in carbonate rocks of Alaska have characteristics of Mississippi Valley-type deposits as well as similarities to Irish Carboniferous Zn-Pb deposits. Common characteristics are a simple mineralogy low Fe sulfide content, no apparent igneous association, partial dolomitization or silicification of wall-rock limestone, variable barite and fluorite content, and structural preparation, often brecciation, of host rocks. Zinc-rich, smithsonite-dominant prospects in the Reef Ridge district, central Alaska, and the Charley River and Livengood areas of east-central Alaska contain solution, karst, and shatter breccias indicating structural preparation of host rocks; they are the closest Alaskan analogues to Mississippi Valley-type deposits. Epigenetic prospects of various forms, with variable amounts of Zn, Pb, Ag, fluorite, and barite, in southeastern, northern, and east-central Alaska and on the Seward Peninsula, may also have formed by Mississippi Valley-type mechanisms and are similar to areas such as the Illinois-Kentucky fluorspar district. Several prospects on the Seward Peninsula and in central and eastern interior Alaska with lensoid and disseminated Pb-Zn-Ag mineralization may have a diagenetic origin similar to that of Irish stratabound Zn deposits. Lead- and Ag-enriched occurrences on the Seward Peninsula and in southeastern Alaska may be higher temperalute carbonate-replacement deposits distal to, and genetically derived from, felsic intrusions. The age of formation of carbonate-hosted Zn-Pb ± Ag mineral occurrences in Alaska ranges from no earlier than latest Proterozoic (Three Castle Mountain) to Tettiaay(?) (Mo-Udall). The fluids that formed them were apparently derived from latest Proterozoic(?). Paleozoic, and Mesozoic mixed carbonate and siliciclastic basins. The sedimentologic, diagenetic, and tectonic history of these basins, and the chemistry and migration pathways of mineralizing fluids, are poorly known. Strata-bound Cu ± Co ± Zn sulfide stockworks in Alaska are unusual epigenetic deposits in Late Proterozoic to Devonian carbonate rocks. The larger Alaskan occurrences (Ruby Creek, Omar) are similar to Irish Cu deposits related to basin dewatering and to Australian Cu deposits interpreted as proximal vent sites for shalehosted Zn-Pb-Ag massive sulfide deposits ("sedex"). These Cu ± Co ± Zn stockworks have stratigraphic and tectonic settings similar to many of the carbonate-hosted Zn-Pb ± Ag deposits. Their different metal contents may result from fluids derived from different sources passing through aquifers of different composition and/or may indicate a mafic rock source for elements such as Co. Small noneconomic Cu occunences on the Seward Peninsula have been interpreted as a tectonogenic mineralization related to thrust fault formation. Extensive, locally thick Late Proterozoic and Paleozoic platform carbonate rocks, and some mixed carbonateclastic slope or basinal facies in Alaska, are prospective for additional Zn-Pb ± Ag strata-bound and stockwork Cu ± Co ± Zn deposits. Thinner, Pennil to Jurassic carbonate sequences outboard of the displaced North American continental margin are less likely hosts for these deposits.

Abstract Alaska is commonly regarded as one of the frontiers of North America for the discovery of metalliferous mineral deposits. A recurring theme in the history of the state has been “rushes” or “stampedes” to sites of newly discovered deposits. Since about 1965, mining companies have undertaken much exploration for lode and placer mineral deposits. During the same period, because of the considerable interest in federal lands in Alaska and the establishment of new national parks, wildlife refuges, and native corporations, extensive studies of mineral deposits and of the mineral resource potential of Alaska have been conducted by the U.S. Geological Survey, the U.S. Bureau of Mines, and the Alaska Division of Geological and Geophysical Surveys. These studies have resulted in abundant new information on Alaskan mineral deposits. In the same period, substantial new geologic mapping has also been completed with the help of new logistical and technical tools. One result of the geologic mapping and associated geologic studies is the recognition of numerous faultbounded assemblages of rocks designated as tectonostratigraphic (lithotectonic) terranes. This concept indicates that most of Alaska consists of a collage of such terranes (Silberling and others, this volume, Plate 3). The purpose of this report is to summarize the local geology, geologic setting, and metallogenesis of the major metalliferous lode deposits and placer districts of Alaska. The term “major mineral deposit” is defined as a mine, mineral deposit with known reserve, prospect, or occurrence that the authors judged significant for any given geographic region. This report is

Abstract Arctic Ridge (67°09'18''N, 156°23'18''W to 67°12'42''N, 156°21'12''W) is located in the Ambler mineral district on the southern flank of the Brooks Range, northwest Alaska (Fig. 1), in the Ambler River A-1 Quadrangle. The ridgeline is 1.5 mi (2.5 km) due east of VABM Riley, between the Kogoluktuk and Shungnak rivers (Fig. 2). Access to the area is easiest by helicopter. Arctic Ridge lies 270 mi (430 km) northwest of Fairbanks, 170 mi (270 km) east of Kotzebue, and 310 mi (500 km) northeast of Nome. Commercial helicopters are usually available for charter in Fairbanks and Nome. The traverse of the Ambler sequence begins on the highest peak (locally known as Arctic peak, elevation 3,560 ft; 1,085 m, Fig. 2) of the southern ridgeline, southeast of the buildings in Subarctic Creek valley (Fig. 3). Permission to cross private land and patented claims should be obtained prior to a visit from the NANA regional corporation (Northwest Alaska Native Association, based in Kotzebue) and Kennecott Mining Company (Salt Lake City, Utah), respectively. An alternative is to charter a small single-engine plane to Arctic airstrip (Fig. 2). This is a private airstrip, and permission for landing must be obtained from Kennecott Mining Company prior to any visit. From the airstrip, walk approximately 7.5 mi (12 km) northeast along a dirt road to Arctic camp, and from there up to Arctic peak. The ridge is often covered by patchy snow into early July, but most outcrops are accessible by early June. The area usually remains free of snow cover until mid-September; however flurries can occur in any month. The ridgeline is steep, craggy, and locally difficult to traverse. Allow 5 to 10 hours in addition to the time required to arrive in the area. Interesting sidehill outcrops are 500 to 1,500 ft (150 to 450 m) in elevation below the ridgeline and often on steep slopes; allow extra time or use a helicopter to reach these.

Abstract Arctic Ridge (67°09'18''N, 156°23'18''W to 67°12'42''N, 156°21'12''W) is located in the Ambler mineral district on the southern flank of the Brooks Range, northwest Alaska (Fig. 1), in the Ambler River A-1 Quadrangle. The ridgeline is 1.5 mi (2.5 km) due east of VABM Riley, between the Kogoluktuk and Shungnak rivers (Fig. 2). Access to the area is easiest by helicopter. Arctic Ridge lies 270 mi (430 km) northwest of Fairbanks, 170 mi (270 km) east of Kotzebue, and 310 mi (500 km) northeast of Nome. Commercial helicopters are usually available for charter in Fairbanks and Nome. The traverse of the Ambler sequence begins on the highest peak (locally known as Arctic peak, elevation 3,560 ft; 1,085 m, Fig. 2) of the southern ridgeline, southeast of the buildings in Subarctic Creek valley (Fig. 3). Permission to cross private land and patented claims should be obtained prior to a visit from the NANA regional corporation (Northwest Alaska Native Association, based in Kotzebue) and Kennecott Mining Company (Salt Lake City, Utah), respectively. An alternative is to charter a small single-engine plane to Arctic airstrip (Fig. 2). This is a private airstrip, and permission for landing must be obtained from Kennecott Mining Company prior to any visit. From the airstrip, walk approximately 7.5 mi (12 km) northeast along a dirt road to Arctic camp, and from there up to Arctic peak. The ridge is often covered by patchy snow into early July, but most outcrops are accessible by early June. The area usually remains free of snow cover until mid-September; however flurries can occur in any month. The ridgeline is steep, craggy, and locally difficult to traverse. Allow 5 to 10 hours in addition to the time required to arrive in the area. Interesting sidehill outcrops are 500 to 1,500 ft (150 to 450 m) in elevation below the ridgeline and often on steep slopes; allow extra time or use a helicopter to reach these.