Hydrothermal veins bearing calcite, quartz and chlorite (+/- fluorite, hematite, pyrite and REE silicates) occupy brittle fractures in granitic gneisses of the Adirondack Highlands. SEM- EDS, XRD and fluid inclusion analysis of vein minerals from two localities indicate that Na-Ca-Cl brines of high salinity (>25 wt. % NaCl equivalent) and temperatures of 140-180°C best characterize the hydrothermal system. At Lyon Mountain Mine in the extreme northern Adirondack Highlands, hydrothermal calcite is associated with hematite, REE silicate, and Mg-rich type Ib chlorite. In the north-central Adirondacks near Long Lake, calcite is associated with fluorite, pyrite, REE silicate, and Fe-rich type Ib chlorite. Fluid inclusions from the Long Lake fluorite have been previously studied by Bird and Darling (1996).
The presence of high-salinity hydrothermal fluids in Adirondack veins and in nearby Paleozoic rocks has been reported by a number of workers and is corroborated by our study. While downward infiltration of Paleozoic brines may have been the source of fluids involved, saline waters derived from late magmatic activity in the waning stages of the Ottawan Orogeny are also probable. Surface-derived saline brines could also have penetrated Adirondack crust during late Proterozoic unroofing of the orogen. Any of these sources could provide fluids with the characteristics observed, resembling the so-called 'Shield brines' described by Frape, et al (1984).
Analysis of other vein systems within the Adirondacks coupled with stable isotope and E-SEM analyses of inclusion fluids may provide information that will constrain the origin of the fluids.
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