REGIONAL GEOPHYSICAL SETTING OF THE PEBBLE DEPOSIT, SOUTHWEST ALASKA

Recent commercial resource estimates for the Pebble porphyry copper-gold-molybdenum deposit in southwest Alaska indicate that the Pebble deposit contains the largest gold resource and second-largest contained copper resource in North America. The USGS is studying the Pebble deposit and surrounding region as part of a larger effort to improve mineral resource and mineral environmental appraisals and develop new assessment techniques.

Mineral exploration and reconnaissance geologic studies in southwest Alaska include the collection of regional as well as local-scale aeromagnetic data. Several large-scale geophysical domains, defined from regional magnetic compilations, trend through the Pebble region in southwest Alaska. The South Alaska Magnetic High is a profound zone of significant crustal magnetization. Just north of this magnetic high, the South Alaska Magnetic Trough is a profound magnetic low and represents a largely non-magnetic crust. The boundary between these two domains is roughly coincident with a strand of the Lake Clark fault near the Pebble deposit. This geologic boundary separates Jurassic and older magmatic-metamorphic rocks to the southeast from Jura-Cretaceous volcaniclastic sedimentary and Triassic volcanic and carbonate assemblages overlain by Tertiary volcanic rocks to the northwest. This region has also been characterized as the boundary between the Kahiltna and Penninsular teconostratigraphic terranes. In detail, there is significant complexity to the magnetic anomalies along this geologic and geophysical domain boundary zone. The Pebble deposit is situated on the flanking gradient of an ovoid, ~25-km wide magnetic high that forms an irregularity along the edge of the South Alaska Magnetic High. There are a number of other similar-scale ovoid magnetic highs in this part of southwest Alaska. These highs may represent intrusive bodies with similar magnetic properties. The magnetic gradients on the flanks of these magnetic highs may help map areas of increased likelihood for mineral potential.