IRON OXIDE-APATITE WASTE PILES AS UNCONVENTIONAL RARE EARTH ELEMENT RESOURCES IN THE EASTERN ADIRONDACK HIGHLANDS, NEW YORK

The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands of New York represent high-grade magnetite ore that contains variable quantities of rare earth element (REE)-bearing apatite. These IOA deposits and their associated waste and/or tailings piles are renewed focus areas for potential domestic REE resources.

Most of the deposits are hosted within Na-altered Lyon Mountain Granite Gneiss (LMG); however, some Fe deposits occur in paragneiss, gabbro, anorthosite, or K-altered LMG. The mineralogy and chemistry of the Fe deposits appear to be partially dependent on the host rock type, with the highest REE and apatite concentrations found in ore hosted by Na-altered LMG.

The amount of P and associated apatite varies from negligible to nearly 10 wt% P₂O₅ within individual ore seams. The associated REE content within the apatite crystals also varies between individual deposits ranging from <1% to ~15% total REE. Initial ore formation resulted in the uneven distribution of apatite between deposits and subsequent Na-poor metasomatic events led to irregular dissolution-reprecipitation reactions of apatite that resulted in heterogeneous REE distribution within individual apatite crystals.

At four tailing and twenty-five tailing sites, composite sample were collected by combining 30 to >50 subsamples evenly distributed over each pile. The composite pile samples consist of a mixture of ore and host rock pieces. The REE content of 34 ore samples varies from ~15 to ~48,000 ppm, whereas 29 sampled waste/tailings piles range from ~11 to ~22,000 ppm. A positive correlation exists for the REE content of ore and waste pile for the deposits (R²=0.8159).

In the IOA deposits sampled from the eastern Adirondack Highlands, there is an enrichment in HREE compared to many REE mines (such as Bayan Obo and Mountain Pass); a median L/HREE value of 2.25 for ore and 2.14 for waste/tailings piles exists. The overall concentration of REE and relative HREE enrichments suggest that some of the deposits and waste piles are prospective as potential REE resources that would be easily exploitable. Reprocessing the piles has the advantage of partial “recycling” of the waste material to produce a critical set of metals and environmental remediation of these historic mines.