0361-0128/13/4099/405-15 405 Introduction THE PEBBLE porphyry deposit (Fig. 1) was discovered during an exploration drilling program in 1988 in southwestern Alaska by Cominco Alaska geologists (Bouley et al., 1995). The drilling followed up geochemical anomalies in an area of Alaska with low relief and poor bedrock exposure, and no known signifi- cant mineral occurrences. Further exploration by Northern Dynasty Minerals during the early part of this century de- fined the present measured, indicated, and inferred cumula- tive resource of 3,337 metric tons (t) Au, 40 Mt Cu, and 2.8 Mt Mo (Northern Dynasty Minerals, 2011). Thus, Pebble contains the largest gold resource and fifth largest copper re- source of any known porphyry deposit and ranks third in the world for gold of all ore deposits, after the Witwatersrand pa- leoplacers and the Muruntau orogenic deposit. The most detailed description of the geology of the Pebble deposit is provided by Lang et al. (2013). A suite of mafic to intermediate intrusions were emplaced into flysch of the Kahiltna basin between ca. 99 and 89 Ma (Hart et al., 2010; Kremenetskii et al., 2012; Lang et al., 2013), with porphyry- style Cu-Au-Mo mineralization associated with ca. 90 to 89 Ma granodioritic stocks and sills, and their adjacent country rocks. The mineralized body is divided into a relatively shal- low Pebble West zone and a deeper Pebble East zone that are interpreted to be two coeval hydrothermal centers within a single system. The focus of this paper is to place the Pebble porphyry de- posit into a regional geologic context. Specifically, how was the formation of the Pebble deposit related to the tectonic history of southern Alaska and what factors controlled where it formed? We will try to define the most critical geologic factors that are responsible for the localization of the giant Tectonic Setting of the Pebble and Other Copper-Gold-Molybdenum Porphyry Deposits within the Evolving Middle Cretaceous Continental Margin of Northwestern North America RICHARD J. GOLDFARB, 1,2,3,† ERIC D. ANDERSON, 1 AND CRAIG J.R. HART 4 1 U.S. Geological Survey, Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225-0046 2 Centre for Exploration Targeting, School of Earth and Geographical Sciences, University of Western Australia, Crawley, Western Australia, Australia 60094 3 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China 4 Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4 Abstract The Pebble Cu-Au-Mo deposit in southwestern Alaska, containing the largest gold resource of any known porphyry in the world, developed in a tectonic setting significantly different from that of the present-day. It is one of a series of metalliferous middle Cretaceous porphyritic granodiorite, quartz monzonite, and diorite bodies, evolved from lower crust and metasomatized lithospheric mantle melts, which formed along much of the length of the North American craton suture with the Peninsular-Alexander-Wrangellia arc. The porphyry deposits were emplaced within the northernmost two of a series of ca. 130 to 80 Ma flysch basins that define the suture, as well as into arc rocks immediately seaward of the two basins. Deposits include the ca. 100 to 90 Ma Pebble, Neacola, and other porphyry prospects along the Kahiltna basin-Peninsula terrane boundary, and the ca. 115 to 105 Ma Baultoff, Carl Creek, Horsfeld, Orange Hill, Bond Creek, and Chisna porphyries along the Nutzotin basin-Wrangellia terrane boundary. The porphyry deposits probably formed along the craton margin more than 1,000 km to the south of their present latitude. Palinspastic reconstructions of plate kinematics from this period are particularly difficult because magmatism overlaps the 119 to 83 Ma Cretaceous Normal Superchron, a period when sea-floor mag- netic data are lacking. Our favored scenario is that ore formation broadly overlaps the cessation of sedimenta- tion and contraction and the transition to a transpressional continental margin regime, such that the remnant ocean basins were converted to strike-slip basins. The basins and outboard Peninsular-Alexander-Wrangellia composite superterrane, which are all located seaward of the deep crustal Denali-Farewell fault system, were subjected to northerly dextral transpression for as long as perhaps 50 m.y., beginning at ca. 95 ± 10 Ma. The onset of this transpression was marked by development of the mineralized bodies along fault segments on the seaward side of the basins. Geochemical and radiogenic isotopic data for igneous rocks associated with the Pebble porphyry deposit sug- gest continuous melt derivation from enriched lithosphere of a recently metasomatized mantle. These geo- chemical characteristics, coupled with the arc-continent-related collisional setting, suggest that lithospheric thickening and postcollisional lithospheric melting are the most likely cause of the ore-related magmatism. Subsequent to translation of the Alaskan margin terranes and early Tertiary oroclinal bending of Alaska, the northernmost Kahiltna basin and the Pebble deposit, as well as the other porphyry systems, reached their present-day locations along southern Alaska. † Corresponding author: e-mail, Goldfarb@usgs.gov ©2013 Society of Economic Geologists, Inc. Economic Geology, v. 108, pp. 405–419