Pyramid Prospect, Aleutians East Borough, Alaska, USAi

Regional Level Types
Pyramid Prospect	Prospect
Aleutians East Borough	Borough
Alaska	State
USA	Country

This prospect is on land selected or patented by, or interim-conveyed to, the Aleut Corporation.
Location: This prospect is located on the Alaska Peninsula on the southeast flank of Pyramid Mountain approximately 4 miles northwest of Balboa Bay (Christie, 1974, color anomaly 90; MacKevett and Holloway, 1977, locality 6; Nokleberg and others, 1987, locality AP6; Wilson and others, 1988, locality 6; Young and others, 1997, locality 36). The map site is at an elevation of about 1,200 feet, in the S1/2 of sec. 35, T. 52 S., R. 74 W., of the Seward Meridian. The location is accurate.
Geology: The Pyramid copper-molybdenum deposit is associated with a quartz diorite pluton that cuts Paleocene to Eocene sedimentary rock of the Tolstoy Formation (Wilson and others, 1995). The stock has been dated at 6 million years (Wilson and others, 1996). The prospect is a classic copper-molybdenum deposit having a potassic core surrounded by a phyllic zone and an outer propylitic zone. The central potassic core contains secondary biotite after mafic minerals and 2 to 10 percent magnetite as fracture fillings and disseminated clots and grains. The core, entirely in the intrusive, is roughly 800 by 1,700 feet. It is essentially barren, with total sulfide content less than 0.25 percent and copper and molybdenum values in the 0.00X percent range (Christie, 1975). The phyllic zone is characterized by sericite, quartz, and andalusite in a zone surrounding the potassic core. It occurs mostly in the intrusive rocks and measures 700 to 2,500 feet in width. The propylitic zone is characterized by chlorite, magnetite, epidote, and calcite. The highest total sulfide content (5 to 10 percent) occurs in the inner part of the propylitic zone and outer part of the quartz-sericite zone. Pyrite:chalcopyrite ratios are 50:1 or greater and are associated with copper grades of 0.15 percent or less (Christie, 1975). Toward the inner part of the phyllic zone pyrite:chalcopyrite ratios decrease and copper grades increase to 0.3 to 0.4 percent; molybdenum grades are 0.03 to 0.04 percent. In this copper-rich part of the system the sulfides occur as disseminations and thin fracture fillings. The deposit has been oxidized to depths of 0 to 450 feet and exhibits a blanket of secondary copper enrichment as much as 300 feet thick. The enriched zone contains chalcocite and covellite, as well as some chalcopyrite. The best grades of copper, as much as 0.8 percent, occur in the upper 100 feet of the blanket. The thickest enrichment zones are not necessarily associated with the thickest zones of oxidation. Lead, antimony, and zinc are also reported in some assays. The Quintana-Duval-Aleut Joint Venture mapped, sampled, and drilled this deposit in 1974 and 1975. They diamond-drilled 19 holes for a total of 5,565 feet. As a result of this work they outlined an estimated resource of 126,000,000 tons of ore grading 0.403 percent copper and 0.025 percent molybdenum. Potential exists for another 49 million tons of chalcocite-enriched ore (Christie, 1975).
Workings: Quintana-Duval mapped, sampled, and diamond-drilled 19 holes totalling 5,565 feet in 1974-1975. The U.S. Geological Survey sampled the deposit in the 1980s.
Age: Six million years or younger.
Alteration: The alteration consists of a barren, potassically-altered core zone, an intermediate zone characterized by pervasive quartz and sericite, and a propylitic outer zone.
Reserves: The resource is estimated at 126 million tons of ore averaging 0.403 percent copper and 0.025 percent molybdenum. There is potential for an additional 49 million tons of chalcocite-enriched ore.

Commodities (Major) - Cu, Mo; (Minor) - Ag, Au, Pb, Sb, Zn
Development Status: None
Deposit Model: Porphyry Cu, Porphyry Cu-Mo (Cox and Singer, 1986; models 17, 21a)

Other Databases

Link to USGS - Alaska:	PM023

Other Regions, Features and Areas containing this locality

References