TEXTURAL AND MODAL ANALYSIS OF GORE MOUNTAIN-TYPE GARNET RIMS, ADIRONDACKS, NEW YORK

The Gore Mountain megacrystic garnet deposits (GM types) in the Adirondack Mountains of northern New York are home to the largest garnets in the world (up to 1 m diameter). Most came from the Barton Mine, which opened in 1878 and was for a time the largest producer of garnet in the United States. The rocks that contain the garnets underwent granulite facies metamorphism about 1050 Ma, which is interpreted as the extensional collapse of the Ottowan Orogen. GM types contain hornblende, garnet, orthopyroxene, clinopyroxene, and plagioclase. Most GM-type deposits are hosted in megacrystic garnet amphibolites, but lithologies range from a hornblendite to an anorthositic gabbroic gneiss (what they call their “white ore”). We have collected samples from three main GM type deposits: Barton, Ruby, and Hooper, which together span the range of lithologies. We have begun analyzing these samples using petrography and image analysis. We find the modal abundances of mafic minerals in the samples to range from 52% to 13%, and the subset of garnets ranges from 36% to 9%. The more felsic the rocks are, the smaller the garnets typically are. The hornblende percentage seems to correlate positively with the size of the garnets. The garnets typically have reaction rims which extend to about 1-2 times the radii of their garnets. These rims are variable and may be either enriched or depleted in hornblende. Within our samples garnets range up to 3 cm; Barton Mountain has the most mafic minerals, and Ruby Mountain has the fewest, while Hooper falls between them. Past work has suggested that the hornblende is not forming at the expense of garnet and that the generalized reaction is plagioclase + mafic phases + H₂O = garnet + hornblende. This is difficult to reconcile with the observations of hornblende-depleted rims that we see in many of our samples, which suggest dehydration of hornblende may in part lead to garnet (and pyroxene) growth. We are using textural evidence and modal analysis of garnet rims across GM-type lithologies to assess these prior models and to support one or more pathways to garnet growth.