INITIAL RESULTS OF GEOPHYSICAL MAPPING IN THE WESTERN FORTYMILE MINING DISTRICT, EAST-CENTRAL ALASKA

Geologic mapping at scales of 1:63,360 and larger is essential to the accurate assessment of mineral potential, particularly for localized mineral deposits, such as base- and precious-metal occurrences. However, complicated logistics, coupled with cover materials such as vegetation, young surficial deposits, or rubble that conceal geologic exposures, adds to the cost and difficulty of producing outcrop-based geologic maps. Hence, airborne geophysical measurements are critical to accurate geologic mapping. Effective integration of geophysical information into geologic mapping requires interpretation of geophysical features in terms of geologically significant units and structures. In large part this integration depends on measurement or estimation of physical properties associated with geologic map units.

As part of a larger effort to improve mineral assessment for concealed deposits, the USGS is studying the genesis and setting of base- and precious-metal occurrences in the western part of the Fortymile mining district of eastern Alaska, located mostly in the Eagle quadrangle. Recent airborne geophysical data (including magnetic and electro-magnetic measurements), with a flightline spacing of 400 m, were collected in two separate surveys by the State of Alaska, Division of Geological and Geophysical Surveys (ADGGS), in 1998 and 2008. The surveys covered the region around Mt. Veta in the southwestern Eagle quadrangle. In 2008, we began field characterization of magnetic susceptibilities for the major lithologies exposed in the Mt. Veta area. These measurements, combined with earlier magnetic susceptibility characterization by ADGGS geologists, provide an initial physical property framework for the area. Initial analysis from geophysical mapping allows the identification of structural trends, including dip of structural contacts, as well as mapping of some igneous rock units. More work is needed in order to characterize the complex magnetic susceptibility signatures of the multi-stage magmatic rock units in the area.