Garnet megacrysts commonly 30 centimters (cm) ranging up to 1 meter (m) in diameter occur at the summit of Gore Mountain, Adirondacks, NY and were mined there for abrasives for more than a century. The mine, owned by Barton Mines Co., LLC, is roughly 2 km x 150 m and is located in a hornblende-rich garnet amphibolite at the southern boundary of a metamorphosed olivine gabbro body that is in fault contact with charnockite. Barton supplies garnet, a chemically homogeneous pyrope-almandine, to the waterjet cutting, lapping, and abrasive coatings industries. The garnet megacrysts are reliably dated at 1049 ± 5 Ma. The growth of the garnet megacrysts was facilitated by an influx of hydrothermal fluid emanating from the ore body’s southern boundary fault. The fluids were most probably associated with the intrusion of the Lyon Mountain Granite (1049.9 ± 10 Ma) and/or associated pegmatitic rocks late in the tectonic history of the Adirondacks.



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