GEOPHYSICAL DATA USED TO CHARACTERIZE THE REGIONAL SETTING OF THE PEBBLE PORPHYRY DEPOSIT IN SOUTHWESTERN ALASKA

The Lake Clark-Iliamna region of southwestern Alaska hosts the world-class Pebble Cu-Au-Mo porphyry deposit. We use gravity and magnetotelluric (MT) data collected in 2008-2009 as well as previous magnetic field data and core sample properties to better constrain our understanding of the regional tectonic and magmatic setting associated with this deposit. The Pebble deposit falls within a ~50- by ~40-km-wide cluster of Jurassic to Tertiary intrusions and volcanics within the southernmost portion of Alaska’s Kahiltna terrane, close to the boundary with Peninsular terrane and south of the main trace of the Lake Clark Fault. The deposit is associated with a Late Cretaceous batholith and is partly covered with Tertiary volcanics and younger glacial materials. Other intrusive rocks of similar age may be present in the region, but volcanic and glacial cover make them difficult to discern from surface geologic observations alone.

The magnetic field of the southern Kahiltna terrane is broadly characterized by a smooth low, suggesting the presence of a thick sedimentary sequence. Amidst this background, Cretaceous and Tertiary plutons and volcanics near the Pebble deposit generate shorter-wavelength magnetic field highs resembling those associated with volcanic rocks of the neighboring Peninsular terrane. These magnetic highs indicate the presence of igneous materials beneath glacial deposits, but they do not provide easy distinction between older intrusives and later Tertiary volcanics. The gravity Bouguer anomaly, however, exhibits several highs of ~5-8 km width that may represent the roots of larger intrusive bodies. MT models further differentiate between more deeply emplaced intrusives and shallower Tertiary volcanics. Over a larger region, several magnetic lows form lineations both parallel and at moderate angles (25-35º) to the NE-SW-trending Lake Clark Fault. A set of these lineations lies ~30 km northwest and ~10 km southeast of the deposit. A 3D MT model shows that the Pebble deposit lies within a similarly oriented, ~10-km-wide conductive corridor flanked by resistive crust to >10 km depth. These findings suggest the presence of fault splays and/or a larger distributed shear zone that may have played a role in the formation of the deposit.