GEOPHYSICAL, GEOCHEMICAL, AND GEOLOGICAL APPROACHES TO EVALUATING RARE EARTH RESOURCES IN THE EASTERN ADIRONDACKS, UPSTATE NEW YORK

The eastern Adirondacks in upstate New York contain recognized resources of iron and rare earth element (REE)-bearing minerals in several iron-oxide-apatite (IOA) deposits and occurrences. Near Mineville and Skiff Mountain, historical iron mines and tailings show significant REE enrichments. REEs are reported largely in apatite (as well as other minerals) within and at the margins of magnetite ore. This association suggests that geophysical methods may provide a tool for evaluating the REE resources.

High-resolution airborne geophysical (magnetics and gamma spectrometry for K, U, and Th) surveys were flown in December 2015 over a ~70-km x 22-km region west of Lake Champlain between Essex and Hague. The surveys were flown at 250-m line spacing with a nominal height of 125 m. Ongoing complementary efforts include geologic mapping; ground gravity, magnetic susceptibility (MS), and gamma spectrometry measurements; and sample collection for geophysical, geochemical, mineralogical, and geochronological analyses.

Initial observations show a broad magnetic high over an area comprising mostly leucogranite gneiss which intruded paragneiss and marble. The adjacent Marcy anorthosite massif shows relative magnetic lows with local highs over metagabbros. High-pass filtered anomalies show linear, 200-400-m wide, N-S highs that are aligned with historical mines. In some areas offsets of these highs suggest tectonic modification after emplacement. Field MS values range 100-1000 x 10^-3 SI in ore and 10-50 x 10^-3 SI in both host rock and more distal country rock. The latter includes paragneiss, suggesting possible fluid alteration, magnetite remobilization and emplacement. Gamma spectrometry data exhibit elevated K within charnockite and increased eTh and eU (equivalent Th and U) within the leucogranite. Tailings piles that are large enough to be visible in satellite imagery show very high eTh and eU. Elevated eTh and eU are also observed over roads and developed areas, perhaps because of the use of tailings as a gravel resource. The Marcy massif shows low K, eTh and eU. Continuing ground truth via field efforts and sample analyses will help determine the consistency and distribution of these relations.