GEOPHYSICAL CHARACTERIZATION AND RELATED MINERALOGY OF THE BOKAN MOUNTAIN RARE EARTH DEPOSITS, SOUTHEAST ALASKA

A high-resolution helicopter magnetic and gamma-ray survey over the Bokan Mountain peralkaline granite complex provides insight into the geologic setting and associated rare earth element (REE) mineralization. Between 2009 and 2010, airborne geophysical data were collected along flight lines spaced every 100 m at altitudes from 35 to 60 m above terrain. The survey covers approximately 41 km² of the Jurassic Bokan Mountain peralkaline granite complex, which is concentrically zoned, comprised of an arfvedsonite-aegerine granite core surrounded by an aegerine granite ring. The peralkaline granite core and outer granite ring are the least magnetic units in the study, but they contain high radioelement concentrations (eU, eTh, and %K). The Jurassic granites intrude mostly nonmagnetic and nonradioactive Silurian-Ordovician quartz monzonite, argillite and shale. The most magnetic rock unit is Ordovician diorite, located east of the pluton, which is associated with large amplitude positive magnetic anomalies. All geologic units are cut by pegmatite, andesite, dacite, and aplite dikes, which are associated with the U, Th, and REE deposits. The Dotson vein-dike system, host to REE deposits, is marked by intense metasomatism that produced albitization, resulting in an increase in sodium and a decrease in potassium. The mineralogy of the Dotson REE-bearing veins is extremely complex and includes thorite, xenotime, monazite, aeschynite, pyrochlore, and other more minor phases, which contain variable amounts of Th and U. The elevated level of radioactivity of the ore assemblage relative to the host rocks makes gamma-ray surveying particularly useful to identify high concentrations of REE-bearing minerals. Although the REE-bearing veins in the Dotson zone have distinct airborne gamma-ray signatures, they do not display an airborne magnetic signature likely due to the low volume of mineralized material relative to enclosing country rock. Other potential geologic structures are identified from filtered products of the airborne data, which map sets of west-northwest and east-west trends that strike across the pluton and are co-parallel with the trend of the REE-bearing veins.