Abstract Subeconomic molybdenum-bearing prophyry deposits and occurrences are scattered widely throughout mainland and southeastern Alaska. Most deposits in mainland Alaska are copper-molybdenum porphyries of probable calc-alkaline arc-related origins. These include Devonian examples in the Brooks Range, middle to Late Cretaceous examples in the northern Alaska Peninsula and northern Wrangell Mountains, and Miocene or younger examples in the Alaska peninsula and Aleutian Islands. Deposits in eastern Alaska and the northeastern Brooks Range are early Tertiary in age and may be related to a period or back-arc, extensional intraplate tectonics. Porphyry deposits in southeastern Alaska are primarily molybdenum bearing and can be roughly separated into two groups based upon their position relative to the Coast batholith. An outboard group of deposits is distributed between Prince of Wales Island and Glacier Bay and is mostly either Jurassic to Cretaceous or middle Eocene to early Oligocene in age. A secoud group of deposits lies within and along the western margin of the Coast batholith from north of Skagway to the latitude of Ketchikan. Isotopic age determinations suggest two periods of molybdenum mineralization during the Eocene-Paleocene and the Oligocene-Miocene. Alaska’s molybdenum-bearing porphyries, especially those of southeastern Alaska, span the classification spectrum from differentiated monzogranite-hosted, subduction-related, low-grade, low fluorine types to high silica rhyolite-alkalic-hosted, high-grade, high fluorine deposits that form in extensional tectonic settings. Endmember types of various ages are present in both southeastern Alaska groups, suggesting that multiple periods of botb lifting- and subduction-related porphyry molybdenum mineralization occurred. The giant Quartz Hill deposit is the only known potentially economic porphyry molybdenum deposit in Alaska. It is located 70 km east of Ketchikan and is hosted by an Oligocene felsic stock that has been emplaced into paragneiss and orthogneiss of the Coast batholith. Quartz Hill has characteristics of both rift-related and subduction-related types. Available data suggest that these unusual characteristics may be the product of the emplacement of a rift-generated porphyry into a subduction-related calc-alkaline arc. The Quartz Hill composite stock, the Travis stock to the southwest, and a smaller stock to the northeast define a 5- by 12-km area of igneous activity, localized along an N 30° E trend. The Quartz Hill composite stock has a surface exposure of 3 by 5 km and is located at the intersection of the southwestern extension of an N 30° E, major right lateral strike-slip fault and the N 55° W Stephens fault. The stock is composed predominantly of porphyritic granites and rhyolites. The intrusive sequence includes initial emplacement of an epizonal felsic magma with early-stage crystallization of biotite granite, followed by rapid crystallization of the Quartz Hill porphyry. Porphyritic rhyolite plugs and dikes intrude the Quartz Hill porphyry and are associated with the formation of semicontemporaneous breccia bodies. Associated breccia pipes suggest possible venting. Late emplacement of rhyolite dikes and small stocks represents the final stage of intrusive activity related to the Quartz Hill composite stock. Regional rhyolite and intermediate to mafic dike swarms cut all other rock types. The Quartz Hill molybdenum deposit occurs almost entirely within the Quartz Hill composite stock. The deposit is tabular to slightly convex upward and is exposed at the surface. Molybdenite is the only mineral of economic importance and occurs with or without quartz in randomly oriented veinlets forming a well-developed stockwork. Mineral inventory calculations reported by the U.S. Borax and Chemical Company in 1982 confirm approxmately 1,760 million short tons with an average grade of 0.127 percent MoS 2 using a 0.08 percent MoS 2 cutoff. The deposit is approximately 2,800 m long and 1,500 m wide and extends to depths of 370 to 500 m. Higher grade mineralization occurs in two zones that contain approximately 200 million short tons each with grades slightly greater than 0.20 percent MoS 2 . Pyrite concentrations average about 1 percent and occur with molybdenite in quartz veinlets and as disseminations in the stock. Concentrations of copper, lead, zinc, and tin are below average crustal abundances, and tungsten is about twice its average crustal abundance. No base metal halo has been recognized surrounding the deposit. The most important product of hydrothermal activity is the development of an intense quartz-molybdenite stocwork. Silicification and development of secondaty potassium feldspar occurs as vein selvages and irregular replacements. Chlorite is widespread and is best developed in more mafic rocks where it is observed replacing biotite. Sericite locally replaces plagioclase adjacent to quartz-pyrite veinlets. Moderate to intense argillic alteration is generally restricted to fault and shear zones. Weak argillic alteration is widespread, forming a clay dusting on feldspars. Calcite occurs as partial replacement after plagioclase, in veins with quartz, and as grains in the matrix. Although classic porphyry molybdenum alteration assemblages are recognnized at Quartz Hill, alteration zonation has not been clearly defined.