DISTRIBUTION, GEOLOGIC SETTING, AND RARE-EARTH ELEMENT (REE) POTENTIAL OF HISTORIC IRON DEPOSITS IN SOUTHERN ESSEX COUNTY, NEW YORK

Rare earth element (REE) potential exists in abandoned iron oxide apatite (IOA) deposits in the “Iron Ore Tract” of the southeastern Adirondacks. New 1:24,000-scale geologic mapping in the Paradox Lake, Eagle Lake, and Crown Point 7.5’ quadrangles provides modern context to the deposits. New LiDAR percent slope maps greatly enhance the ability to locate abandoned mines, railroads, tailings, and geologic structure in the densely wooded terrain. Digital mapping with GPS-enabled iPads running Fieldmove expedites data collection and allows for access to high-resolution imagery, LiDAR, and new geophysical data. New airborne magnetic and radiometric data locally delineates the host granitic rocks and IOA deposits and suggests that additional deposits remain undiscovered.

Apatite enriched in REEs occurs in mine tailings, magnetite seams, and adjacent country rock. Mapping shows that many, but not all, deposits occur within the Lyon Mountain Granite Gneiss (LMG). Small magnetite-pyrite deposits occur within biotite paragneiss that was intruded by apophyses of nearby syenite, leucogranite, and pegmatite, but these deposits show low eTh, eU, and K anomalies suggesting that they have limited REEs. Some deposits are associated with cross-cutting magnetite-biotite granitic pegmatite. Magnetic anomaly maps filtered to highlight shallow sources show highs that locally correspond to sill-like bodies of LMG, and many of the filtered highs represent IOA deposits, but in other places the highs cross all map units. The generally low magnetic susceptibility paragneiss and marble locally contain cross-cutting magnetite-biotite pegmatite. Some magnetic highs may represent small magnetite-ilmenite deposits in metagabbro, thinner bodies of LMG in less magnetic rocks, areas of abundant pegmatite, or buried LMG bodies. Thus, magnetic highs alone are not always a proxy for the occurrence of IOA deposits, and suggest that some magnetite deposits are late. Large magnetite deposits and REE-bearing apatite occur in intrusive light gray quartz-albite rock, thought to be an alteration of LMG. Away from large deposits, unaltered LMG is usually pink microperthite granite (ca. 1140-1130 Ma) supporting the conclusion that subsequent alteration produced and re-mobilized the deposits during multiple events (ca. 1115-1000 Ma).