Geochemical and Petrographic Analysis of Gore Mountain Garnets, Adirondacks NY

The mine at Gore Mountain is famous for its giant garnets and long history of garnet abrasive production. This location, Warrensburg (Wall St.), and others were examined to attempt to better understand the petrogenesis of these remarkable rocks. Our studies have emphasized the geochemistry of bulk rocks and rock and mineral separates (122 analyses), thin section petrology, and thermodynamic modeling of mineral assemblages. At Gore Mtn. the ore has the assemblage hornblende-plagioclase-garnet-OPX-biotite, and formed over a ~2 m thick transition zone from a layered olivine corona gabbro, metamorphosed at granulite facies. Petrographically the delicate corona structures and fine-grained garnets were replaced by hornblende and progressively larger garnets across the transition zone, indicating that the small number of large garnets in the ore resulted from growth driven by surface energy reduction rather than low nucleation rate. That chemical components must have rapidly migrated through the rock volume via a fluid phase seems obvious. The transition zone is compositionally the same as the gabbro in most cases, except addition of water to form hornblende. However, in the ore there are highly variable trace element composition differences compared to the gabbro. There is also a subtle, variable, loss of LREE relative to HREE. This suggests strongly channelized fluid flow. The situation at Warrensburg is not so clear because gabbro-garnet amphibolite contacts are not exposed. The big garnet rock and nearby corona gabbro have more evolved magmatic compositions than the Gore Mtn. rocks, and the chemical changes are less extreme and different. This may indicate more homogeneous fluid flow and a somewhat different fluid composition. Regional granulite facies metamorphism has been reported at ~750°C and ~8 kbar. New thermodynamic modeling indicates that Gore Mtn. ore formation took place at ~750°C and 4.5-5 kbar, in the presence of silicate melt and hydrous fluid, indicating isothermal uplift followed by fluid infiltration