PRIMARY CO2 FLUID INCLUSIONS FROM THE TRANSITION ZONE OF THE GORE MOUNTAIN GARNET DEPOSIT, CENTRAL ADIRONDACKS, NEW YORK, USA

The Gore Mtn. garnet deposit is characterized by megacrystic garnet hosted by amphibolite and hornblendite. A transition zone between garnet ore and metagabbro contains 0.5 to 1 cm garnets accompanied by coarse-grained plagioclase, hornblende and orthopyroxene. Several researchers have proposed that influx of H₂O fluid along the boundary between metagabbro and metasyenite led to garnet formation. However, no aqueous fluid inclusions have ever been described in Gore Mtn. garnet. Furthermore, recent rehomogenization of cristobalite + kumdykolite-bearing polyphase inclusions as well as the confirmation of primary glass (Ferrero et al., 2019, AGU abs) in Gore Mtn. megacrystic garnets favors an origin in the presence of tonalitic to trondhjemitic melt.

To date, only polyphase crystalline inclusions have been reported from Gore Mtn. megacrystic garnet. However, transition zone garnet from the number eight pit of the Barton Mine contains small (10 to 30 μm), isolated, fluid inclusions having a negative rhombohedral crystal shape, and are inferred to be of primary origin. These fluid inclusions contain one phase at 20°C, however, birefringent solids were observed in some. Upon cooling, all fluid inclusions froze between -70°C and -88°C. Melting of CO₂ solid occurred from -57.7°C to -56.6°C (n = 51), indicating CO₂ as a dominate phase but not the only volatile. The inclusions homogenized by CO₂ vapor bubble disappearance between -11.2°C and +5.0°C, but the average and standard deviation of 49 measurements were -3.0°C ± 3.7°C. Calculated densities (assuming pure CO₂) over the standard deviation range are 0.965 to 0.923 gms/cm³.

Preliminary laser Raman analysis confirms the presence of CO₂ but also reveals small amounts of CH₄. Birefringent solids show peaks for Mg-calcite and pyrophyllite. These phases are interpreted as step-daughter phases, formed after trapping by a reaction between the host garnet and the inclusion fluid. No liquid H₂O was detected but the presence of pyrophyllite confirms that H₂O was at one time a fluid component, at least in the transition zone at Gore Mtn. We infer that H₂O entering the metagabbro generated tonalitic to trondhjemitic melt that aided garnet growth. The primary CO₂-rich fluid inclusions in the transition zone may represent trapping of a distal fluid depleted of its H₂O component.