Abstract Alaska contains a number of volcanogenic massive sulfide districts: Amblet district (Brooks Range); Delta, Bonnifield, and Kantishna districts (Alaska Range); Prince William Sound district; and numerous districts of variable age in southeastern Alaska. Because of a lack of infrastructure and transportation in much of the state production has occurred only at deposits in Prince William Sound and southeastern Alaska. This lack of access has resulted in relatively few published data being available on most deposits. With the exception of the Prince William Sound deposits, deposits by ophiolite and nearby sedimentary rocks, most Alaskan volcanogenic massive sulfide deposits are associated with bimodal volcanic suites; associated intermediatecomposition roeks are rare. The volcanic and sedimentary rocks hosting these deposits suggest that they formed In rift-related settings, rather than in the island-are settings characteristic of many Phanerozoic volcanogenic massive sulfide deposits. Alaskan deposits of this type outside of Prince William Sound are generally polymetallic and have elevated precious metal contents; although many contain barite, baritic caps above deposits are rare. The deposits characteristically display large envelopes of chloritic and sericitic alteration, and a few have well-developed footwall alteratiou zones. Definition of sulfide, gangue, and alteration mineralogy allows explanation of the variation in precious metal content among deposits as a function of hydrothermal fluid oxidation state. Intense deformation in many of the districts has resulted in deformed, flat sulfide sheet morphologies and irregular metal zoning. Sulfur and lead isotope data from Alaskan volcanogenic massive sulfide deposits are highly regular and conform to patterns found in other such districts worldwide. The lead data are particularly helpful in clarifying poorly constrained deposit ages. The large numher of Alaskan volcanogenie massive sullfitde deposits is ultimately related to the large number of accreted volcanic terranes that make up Alaska; it is unclear why there are so few deposits in normal submarine islandare settings.

Abstract The Tintina gold province comprises numerous (>15) individual gold belts and districts throughout interior Alaska and Yukon and has a recently defined lode gold resource of ~35 Moz and past placer-lode production of ~33 Moz Au. This recently defined province is underlain by a diverse geology comprising the following: (1) highly deformed and polymetamorphosed Neoproterozoic and Paleozoic schists and meta-igneous rocks of the pericratonic Yukon-Tanana terrane in eastern Alaska and western Yukon, (2) deformed continental margin Neoproterozoic and Paleozoic clastic rocks and limestones of the Selwyn basin in eastern Yukon, and (3) weakly deformed flysch of the Cretaceous Kuskokwim basin in western Alaska. Most gold lodes show a spatial and temporal association with mid-Cretaceous plutons in the former two areas and with Late Cretaceous plutons in the latter area. These calc-alkaline, reduced, radiogenic intrusions were emplaced within a wide variety of tectonic settings along the Cretaceous margin of North America. Significant lode gold resources, newly defined during the last decade, include those at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz) Mineralization styles throughout the Tintina gold province are extremely varied. Most deposits are well defined as zones surrounding a central causative pluton. Sheeted quartz-feldspar veins are a distinctive ore style where ore is localized in the apical parts of plutons or in immediately adjacent hornfels, and they are typically characterized by a Te-W-Bi-Au geochemical signature. Polymetallic replacement bodies, auriferous breccias, and disseminated ores occur farther out within thermal aureoles; gold-rich skarns develop where reactive carbonate units are part of the surrounding country rock sequence. Other ore styles, although spatially and temporally associated with intrusions, continue to require more data before a definitive genetic link to magmatism can be assured. These styles include the epizonal fracture networks within carbonaceous sedimentary rocks and/or intruded dikes and sills, which have a consistent As-Au-Hg-Sb signature (e.g., Brewery Creek, Donlin Creek, True North). Also of uncertain origin are the shear-zone—related veins (e.g., Ryan Lode, Longline, Pogo) that have more abundant sulfide minerals (2—3 vol %) than other styles, common crack-seal textures, ductile and brittle features, and little metal zoning. Recognition in 1992 that an old prospect, known since 1913 at the site of the presently producing Fort Knox deposit, could be a world-class gold deposit changed precious metal exploration strategies in the northern North American Cordillera. Bulk tonnage orebodies with economic grades <1 g/t Au were for the first time seen as conventional milling or heap leaching targets. The resulting exploration boom throughout the 1990s in interior Alaska and Yukon led to four years of recent gold production at Brewery Creek, the present mining at True North, and the large defined resource at Donlin Creek. In addition, the Pogo deposit was discovered in the mid-1990s; it has ore grades (~18 g/t Au) about one order of magnitude greater than those of the other new resources in the Tintina gold province. Regional stream geochemistry followed by soil geochemistry has been most effective in discovering new orebodies in this vast region (e.g., Brewery Creek, Pogo), although the application of the intrusion-related deposit model to areas with known mineral deposits has also been successful (e.g., Fort Knox, True North, Donlin Creek). The notable characteristics of the Tintina gold province deposits pertinent to exploration (such as vast, remote under explored areas, unallocated regions with variable oxidation depths, and discontinuous permafrost), in combination with an evolving geologic model, create exploration challenges that can be overcome with the pragmatic application of selected exploration techniques and strategies.

Abstract The Cordilleran orogen of western Canada and Alaska records tectonic processes than span over 1.8 billion years, from assembly of the Laurentian cratonic core of Ancestral North America in the Precambrian to sea-floor spreading, subduction, and geometrically linked transform faulting along the modern continental margin. The evolution of tectonic regimes, from Proterozoic intracratonic basin subsidence and Paleozoic rifting to construction of Mesozoic and younger intraoceanic and continent-margin arcs, has led to diverse metallogenetic styles. The northern Cordillera consists of four large-scale paleogeographic realms. The Ancestral North American (Laurentian) realm comprises 2.3 to 1.8 Ga cratonic basement, Paleoproterozoic through Triassic cover successions, and younger synorogenic clastic deposits. Terranes of the peri-Laurentian realm, although allochthonous, have a northwestern Laurentian heritage. They include continental fragments, arcs, accompanying accretionary complexes, and back-arc marginal ocean basins that developed off western (present coordinates) Ancestral North America, in a setting similar to the modern western Pacific basin. Terranes of the Arctic-northeastern Pacific realm include the following: pre-Devonian pericratonic and arc fragments that originated near the Baltican and Siberian margins of the Arctic basin and Late Devonian to early Jurassic arc, back-arc, and accretionary terranes that developed during transport into and within the northeastern paleo-Pacific basin. Some Arctic realm terranes may have impinged on the outer peri-Laurentian margin in the Devonian. However, main-stage accretion of the two realms to each other and to the Laurentian margin began in mid-Jurassic time and continued through the Cretaceous. Terranes of the Coastal realm occupy the western edge of the present continent; they include later Mesozoic to Cenozoic accretionary prisms and seamounts that were scraped off of Pacific oceanic plates during subduction beneath the margin of North America. Each realm carries its own metallogenetic signature. Proterozoic basins of Ancestral North America host polymetallic SEDEX, Cu-Au-U-Co-enriched breccias, MVT, and sedimentary copper deposits. Paleozoic syngenetic sulfides occur in continental rift and arc settings in Ancestral North America, the peri-Laurentian terranes, and in two of the older pericratonic Arctic terranes, Arctic Alaska, and Alexander. The early Mesozoic peri-Laurentian arcs of Stikinia and Quesnellia host prolific porphyry Cu-Au and Cu-Mo and related precious metal-enriched deposits. Superimposed postaccretionary magmatic arcs and compressional and extensional tectonic regimes have also given rise to important mineral deposit suites, particularly gold, but also porphyries. Very young (5 Ma) porphyry Cu deposits in northwestern Vancouver Island and sea-floor hotspring deposits along the modern Juan de Fuca Ridge off the southwest coast of British Columbia show that Cordilleran metallogeny continues.