BIG GARNET ROCKS AT GORE MTN. AND WARRENSBURG, NY: GEOCHEMICAL EVIDENCE OF FLUID FLOW AND CONDITIONS OF GARNET GROWTH

The garnet deposit at Gore Mountain is famous for its giant garnets and long history of garnet abrasive production. This location, Warrensburg (Wall St. outcrop), 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), 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. Layering can be traced from the gabbro through the transition zone into the garnet ore. Petrographically the delicate corona structures and fine-grained garnets are 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 fluids 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 composition differences compared to the gabbro: increased Li (average, x5) and Sr (1.4) and decreased Rb (0.7), CO₂ (0.5), U (0.4), S (0.4), Cu (0.3), Th (0.2), Sb (0.2), Cs (0.12). 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 contacts are not well 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: increased Li (x1.4), S (1.6), Pb (1.8), Cs (7), decreased Th (0.7), CO₂ (0.6), and Sb (0.3), and similar subtle loss of LREE. 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.