IRON-OXIDE APATITE DEPOSITS OF THE EASTERN ADIRONDACKS, NY, ILLUMINATED USING AEROMAGNETIC, AERORADIOMETRIC, AND GEOCHEMICAL SAMPLE ANALYSIS DATA

The eastern Adirondacks contain numerous iron-oxide apatite deposits that were extensively mined for iron in the 1800-1900s but are currently prospective for rare earth element (REE) resources. We use geophysical, geochemical and geological approaches to evaluate the REE resource in deposits and tailings/waste piles and to better understand their distribution within the mineralizing system architecture. New data include high-resolution airborne magnetic and radiometric (for K, U and Th) surveys in Dec 2015 as well as outcrop geophysical measurements, ground gravity, and geochemical analyses of samples collected 2015-2017.

The magnetic data show a >1000 nT high centered near Ticonderoga with one NNW branch towards Mineville and another WSW branch towards Hammondville. Most deposits lie within the high, which is predominantly associated with leucogranite (~50x10-3 SI). High-pass filtering of the magnetic field delineates shallow or surficial magnetite-apatite deposits. To estimate the shape and rough depth extent of these deposits we employed 3D inversions for magnetic susceptibility. The models suggest several south- or west-dipping deposits, consistent with historical mining records where available, and other deposits that are more vertical. The models also suggest a larger complex for deposits near Mineville.

Regional 3D models of magnetic susceptibility approximate the extent of the leucogranite and in some places other magnetized rocks. The models suggest a thick (> 5 km) central magnetic source near Ticonderoga, and thinner (2-3 km) branches toward the NNW and WSW. Higher apatite concentrations are observed near the distal ends of these two branches, but with significant local variability, and in an area isolated from the broader magnetic high (the Cheever deposit). We speculate whether more rapid fluid cooling in these areas led to preferential precipitation of apatite.

The radiometric data mostly reflect variations in surface geology as verified on outcrops. However, K, U and Th are highly variable within the leucogranite, perhaps due to a history of multiple alteration episodes that mobilized these elements. The most prominent Th anomalies are associated with large tailings piles. Those that could be sampled show high concentrations of REE-bearing apatite (with 0.659% REE oxide).