DEPOSIT-SCALE GEOPHYSICS AT THE PEBBLE DEPOSIT, SW ALASKA

The Pebble Cu-Au-Mo porphyry deposit of southwest Alaska contains the largest gold reserve and second-largest copper resource in North America. The deposit, associated with the late Cretaceous Kaskanak batholith, lies near a litho-tectonic boundary where Mesozoic sedimentary rocks are intruded by Mesozoic to Cenozoic plutons. The deposit comprises vein stockworks and disseminated mineralization hosted by sills, intervening sedimentary rocks, and small plutons. The west zone, discovered in 1988 through surface geological and geochemical exploration, is covered by glacial deposits, while the east zone, discovered in 2004, is covered by Late Cretaceous to Tertiary volcanic and sedimentary rocks.

In cooperation with the Pebble Limited Partnership (PLP), the USGS has been carrying out geophysical surveys on site to better understand the geophysical signature of such concealed deposits and the structural controls on deposit emplacement and preservation. Magnetotelluric (MT), self-potential (SP), and gravity data have been collected by the USGS and analyzed together with airborne magnetic and MT data collected by PLP.

MT data detect variations in electrical resistivity with depth at scales of hundreds of meters to tens of kilometers. The three-dimensional MT resistivity models suggest zones of quartz-sericite-pyrite alteration peripheral to the deposit, and delineate the contact between the conductive Cretaceous rocks and the Tertiary volcanic cover. Self-potential data that measure changes in redox state highlight several prominent anomalies associated with geochemically reduced columns within the glacial overburden. An examination of correlations between SP, alteration, and surface geochemical datasets is ongoing. Airborne magnetic data map the variation in abundance of magnetic minerals (mainly magnetite). Decreased magnetic susceptibility values in drill core within the deposit suggest that hydrothermal fluids reduced the magnetite content in the host rocks. The gravity data show significant variation within the deposit area relating to structural trends as predicted by drill core density variations. The integrated interpretation of these data sets is underway in order to refine structure and alteration models of the deposit.