GRAVITY, MAGNETOTELLURIC, AND MAGNETIC FIELD DATA DISTINGUISH MAGMATIC FEATURES IN THE ILIAMNA-LAKE CLARK REGION NEAR THE PEBBLE PORPHYRY DEPOSIT

In areas covered with glacial till, geophysical methods can provide significant information regarding geologic features at depth. The Iliamna-Lake Clark region, situated within the southern Kahiltna terrane of southwestern Alaska, is of particular interest because it hosts the world-class Pebble Cu-Au-Mo porphyry deposit. This deposit is associated with a granodiorite batholith intruded during the Late Cretaceous, but is concealed by younger volcanic rocks and glacial cover. We combine gravity, magnetotelluric (MT), and aeromagnetic data with drill core data collected within the deposit and surrounding region to model the likely types and depth extents of igneous rocks in the area. One goal is to distinguish magmatic episodes reflecting different compositions and time periods.

Drill cores provide samples of a suite of rock types including mostly Cretaceous granodiorites, monzonite and monzodiorites, Tertiary basalts, and a variety of sedimentary and meta-sedimentary rocks. The densities of the more felsic rocks are generally comparable to those of surrounding sediments, but lower than those of the mafic materials. In contrast, magnetic susceptibilities of the felsic and mafic samples lie within comparable ranges. A notable exception is within the Pebble deposit, where susceptibilities of ore-hosting granodiorites are much lower than those of the surrounding areas.

A ~40 x 50 km region of high magnetic anomalies representing both Cretaceous and Tertiary felsic and mafic rocks characterizes the region. Using MT data to distinguish more resistive igneous rocks from surrounding sedimentary rocks, and gravity data to distinguish felsic from mafic rocks, we model several 5-10 km wide felsic intrusions extending to ~2-4 km depth, with the greatest concentration of such intrusions occurring near the Pebble deposit. These models also suggest the presence of a few similarly scaled mafic intrusions, but most mafic bodies appear to be less than 1 km thick. The granodiorite hosting the Pebble deposit is characterized by lower resistivity and magnetic susceptibility, with the latter expressed in magnetic field derivative maps. We attribute this difference in geophysical character to hydrothermal alteration associated with formation of the deposit.