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* Selleck: bselleck@mail.colgate.edu. Current address, Hamilton: Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada

Magmatic-hydrothermal processes produced significant metasomatic alteration of country rock, with resultant metal transport and deposition of low-Ti magnetite bodies, during the intrusion of late granitic magmas in the Adirondack Highlands of the Grenville Province. Manifestations of these ore-forming systems occur as sillimanite-bearing quartz-rich rocks within ca. 1040 Ma Lyon Mountain leucogranites in the southwestern Adirondack Highlands. Field and petrographic relationships demonstrate that emplacement of quartz-sillimanite and bull quartz veins into granite was accompanied by leaching of feldspar and other labile minerals from the granite to produce aluminum- and silica-rich residua. These relationships, coupled with fluid inclusion and stable isotope data from quartz in the host granite, in quartz-sillimanite veins, and in massive quartz veins, suggest that crystallization of granite released acidic magmatic fluids that resulted in high-temperature hydrolytic leaching of feldspars. These fluids also transported and redeposited silica and, locally, alumina to form a quartz-rich sillimanite-bearing carapace near the margins of the granite pluton. Later low-temperature hydrothermal processes mobilized silica and altered sillimanite to illite and diaspore. Zircons exhibiting U-Pb ages overlapping those of the host granite are abundant in the leached quartz-rich rock and are interpreted as residual grains incorporated from the granite. The spatial relationship of both high-temperature and low-temperature hydrothermal features relative to the intrusive Lyon Mountain leucogranite strongly suggests that emplacement of granitic magmas drove the hydrothermal system during progressive unroofing and cooling of the Adirondack orogen.

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