MAGNETITE-APATITE DEPOSITS IN THE ADIRONDACK MOUNTAINS, NEW YORK: A SOURCE FOR RARE EARTH ELEMENTS?

The Lyon Mountain granite gneiss (LMGG) in the northeastern Adirondack Mountains of New York is the host to numerous Kiruna-type, magnetite-apatite deposits. A combination of magmatic and hydrothermal processes produced the deposits and apatite that is extremely enriched in rare earth elements (REE) and some high field strength elements (HFSE). The LMG intruded the Adirondack Highlands between 1060-1050 Ma near the end of the Ottawan orogeny. Initial ore mineralization is related to granitic magmatism and forms deposits consisting of clinopyroxene, magnetite and apatite. The early deposits were altered first by K and then Na- and F-rich fluids 20 to 60 million years later, which remobilized Fe, Zr, HFSE and REE. Second generation deposits consist of quartz, magnetite, zircon and apatite that contain up to 20 weight percent of La, Ce, Nd, Th and Y. Historically, magnetite was mined for over 150 years as a source for iron, and apatite was ignored and left behind in the tailings piles, of which some are currently being used for road and construction sand. Mining for iron in the Adirondacks ended when more accessible and cheaper deposits were discovered elsewhere, but the deposits were never exhausted. Given the high demand for REE in all types of modern technology, reclaiming REE-rich apatite from old tailings piles and revisiting existing mines in areas outside the protected Adirondack Park may provide a viable REE source.