PETROGENESIS OF A SAPPHIRE-BEARING PEGMATITE, OLD PRESSLEY MINE, CANTON, NORTH CAROLINA

The Old Pressley Sapphire Mine is a muscovite-class pegmatite mine located in the Blue Ridge Physiographic Province near Canton, North Carolina. This pegmatite is intruded into the Earlies Gap Biotite Gneiss (EGBG), which consists of middle Proterozoic age, upper amphibolite to granulite facies metamorphic rock. Regional metamorphism during the Middle Ordovician associated with the Taconic Orogeny caused most of the visible metamorphic textures. This metamorphic event primarily caused folding, recrystallization, and partial melting of "wet" quartzofeldspathic sediments. The EGBG is comprised of micaceous and hornblendic gneiss, mica schist, and amphibolite characterized by wavy foliation, and is locally migmatitic in the Canton Quadrangle. At the Old Pressley Mine, the EBGB consists of granulite-facies gneiss: major phases include plagioclase, augite, hornblende, quartz, and garnet; minor phases include sphene and apatite, with retrograde scapolite, epidote, and chlorite. The Old Pressley pegmatite consists of approximately 77% orthoclase, 13% muscovite, 8% biotite, and 2% corundum var. sapphire. The pegmatite lacks quartz. The high concentration of Al₂O₃ (est. 22 wt%) and K₂O (est. 15 wt%) suggests an origin by partial melting of a pelite or metapelite, although the relatively low concentration of SiO₂ (est. 58%) and lack of quartz suggest otherwise. Phase-equilibria data suggest that orthoclase, muscovite, and corundum equilibrated between 730 to 770°C over a range of 5000 to 7000 bars of total pressure (assuming P-total = P-H₂O). We evaluate several competing hypotheses for the origin of this pegmatite in light of the unusual mineralogy of this pegmatite, the mineralogy and petrology of the host rock, and the results of phase-equilibria data.