Abstract The Pebble deposit is located ∼320 km southwest of Anchorage, Alaska. It is one of the largest porphyry deposits known, with a total resource of 10.78 billion metric tons (Bt) of mineralized rock divided between the contiguous West and East zones. The oldest rocks in the Pebble district are Jurassic-Cretaceous Kahiltna flysch, which contains interbedded basalt and associated gabbro intrusions. These were cut between 99 and 96 Ma by coeval granodiorite and diorite sills, followed by alkalic intrusions and related breccias. Subalkalic hornblende granodiorite porphyry plutons were emplaced at ∼90 Ma and include the Kaskanak batholith and smaller stocks related to Cu-Au-Mo mineralization. Porphyry mineralization has been dated between 89.5 and 90.4 Ma by Re-Os on molybdenite. Late Cretaceous volcanic and sedimentary rocks completely conceal the East zone. Eocene volcanic rocks and subvolcanic intrusions occur east and southeast of the Pebble deposit and glacial sediments are widespread. The East and West zones represent two coeval hydrothermal centers within a single system. The West zone extends from surface to ∼500-m depth and is centered on four small granodiorite plugs emplaced into flysch, diorite and granodiorite sills, and alkalic intrusions and breccias. The much higher grade East zone is hosted by the larger East zone granodiorite pluton and adjacent granodiorite sills and flysch and extends to at least 1,700-m depth below surface. The granodiorite intrusions merge with depth. Lower grade, less extensive mineralization occurs in the center of the deposit where the peripheries of the East and West zones converge. On the eastern side of the deposit, faulting dropped high-grade mineralization 600 to 900 m into the NE-trending East graben where the deposit remains open. Variations in hypogene grade and metal ratios reflect multiple stages of metal introduction and redistribution. Premineralization hornfels formed around the Kaskanak batholith. Early disseminated and vein-hosted Cu-Au-Mo mineralization formed with potassic alteration in the East zone and sodic-potassic alteration in the West zone. In the East zone, potassic alteration is underlain by weakly mineralized sodic-potassic ± calcic alteration. Slightly younger quartz veins introduced additional molybdenum. Illite ± kaolinite alteration overprinted the early alteration assemblages and variably redistributed copper and gold. Late-stage advanced argillic alteration is associated with high-grade Cu-Au mineralization in the East zone; it was controlled by a synhydrothermal brittle-ductile fault zone and comprises a core of pyrophyllite alteration associated with chalcopyrite bounded to the west by sericite alteration with hypogene bornite, digenite, covellite, and trace enargite and tennantite. Copper was removed by quartz-sericite-pyrite alteration that forms a halo to the deposit and yields outward to propylitic alteration. A weakly mineralized quartz-illite-pyrite cap is preserved in the upper central part of the deposit. Weak supergene mineralization is present only in the West zone where the Late Cretaceous cover sequence was eroded. The large size and high hypogene grades of the Pebble deposit may reflect a combination of multiple stages of metal introduction with vertically restricted, lateral fluid flow induced by hornfels aquitards in flysch. The Pebble deposit occurs in one of several large, deep-seated magnetic anomalies which occur at the intersection of crustal-scale structures both parallel and at high angles to an arc, which formed in southwest Alaska during the Cretaceous. This setting is similar to fertile porphyry environments in northern Chile and suggests that southwestern Alaska is highly prospective for porphyry exploration.

Abstract The Virginia zone is one of several copper-gold mineralized zones in the Copper Mountain camp, located 15 kilometres south of Princeton, British Columbia. Reserves consist of 4.5 million tonnes grading 0.39 per cent copper (Tim Carew, personal communication, 1992) and approximately 0.17 gram gold and 1.49 grams silver per tonne (calculated from median Cu/Au and Cu/Ag ratios in samples within the deposit). This zone has special interest because it contains higher gold grades than other previously or currently mined zones in the camp and because it is in the initial stage of production. As a result, there is currently an opportunity to document the geology of this zone more fully, and to use this information to understand the genetic controls on copper and gold deposition.

Abstract The Crescent deposit is one of several porphyry-style deposits located within the Iron Mask batholith. Other mined deposits in the district include the Afton, Ajax East and West, and Pothook deposits; the Big Onion, DM, and Python zones have published reserves but have had no production (Figure 1; Kwong, 1977). The Iron Mask is a composite intrusion of alkalic affinity which was emplaced at about 207±3 Ma (Ghosh, 1993) into coeval volcanic rocks of the Nicola Group which is part of the Quesnellia oceanic island-arc terrane (Souther, 1992). The copper-gold deposits within the batholiths have been classified within the silica-saturated group of alkalic porphyry deposits (Lang et al., 1992). The Crescent deposit is located 3 kilometres due east of the Afton deposit, the largest orebody in the district (Figure 1), and yielded 1.36 million tonnes of ore with an average grade of 0.46% copper and 0.2 gram per tonne gold during production in 1989 and 1990.

Abstract The Iron Mask batholith is an earliest Jurassic (207±3 Ma; Ghosh 1993), composite alkalic intrusion lo-cated approximately 10 kilometres southwest of Kam-loops, British Columbia (Figure 1). It lies in the southern part of the Quesnel Terrane, a volcanic arc that lay somewhere offshore of North America during the Late Triassic (Souther, 1992). The batholith is an elongate, northwest-trending body approximately 22 kilometres long and 5 kilometres wide, and intrudes volcanic and sedimentary rocks of the Upper Triassic Nicola Group (Preto, 1968). The batholith is exposed in the Iron Mask pluton to the southeast and in the smaller Cherry Creek pluton to the northwest which are separated by a graben of down-faulted Eocene Kamloops Group volcanic and sedimentary rocks (Kwong, 1987).