GEOTHERMOBAROMETRY AND OXIDE CHEMISTRY OF THE SANFORD LAKE DEPOSIT AND OTHER FE-TI-V-P ORES IN THE ADIRONDACK HIGHLANDS, NEW YORK

Fe-Ti-V-P oxide ores, like those at the historic Sanford Lake (Tahawus) deposit, New York, are typically associated with Proterozoic massif-type anorthosite and related gabbroic rocks, although other ores in Adirondack region are related to the Lyon Mountain granite, and others have unknown parental or conjugate components (Port Leyden nelsonite). The textures that form as a result of crystallization and subsolidus processes vary among the different localities. Fe-Ti oxide ores from Sanford Lake, examined by reflected light and scanning electron microscopy, are characterized by cumulate magnetite and ilmenite with extensive exsolution of hercynitic spinel [(Fe,Mg)Al₂O₃]. Ilmenite formed as a result of oxy-exsolution was rarely observed in our samples, suggesting most Ti was partitioned into early-formed cumulate ilmenite, leaving the primary magnetite with a low initial ulvöspinel content. Oxide mineral compositions were determined by EPMA with additional trace element analyses by LA-ICP-MS. Determination of the primary oxide composition by integration of exsolved hercynitic spinel with the host magnetite reveals an aluminous magnetite with 3.15-9.28 wt.% Al₂O₃. The high MgAl₂O₄ component is consistent with crystallization at pressures >0.5 GPa. Magnetite-ilmenite geothermobarometry applied to Sanford Lake ore gives an estimate of re-equilibration temperatures (485-665°C) and oxygen fugacity (log fO₂ = -16.57 to -21.20). In addition to Ti, Al, and Cr, magnetite of Sanford Lake ore contains higher V (1243-5250 ppm) than the six other Fe-Ti oxide occurrences of the Adirondack Highlands included in this study. The partitioning behavior of V between magnetite and melt of ferrogabbroic composition provide an independent estimate of oxygen fugacity, with V incorporation in magnetite favored under more reducing conditions. Oxygen fugacity estimated between FMQ +2.12 and FMQ +3.55 using this method, is within the range calculated using the two-oxide geothermobarometer at 0.5 GPa. Aluminum and V concentrations in Sanford Lake magnetite suggest separation and crystallization of the oxide liquid probably occurred at deeper crustal levels greater than 0.5 GPa, and at high fO₂ relative to similar anorthosite-hosted oxide deposits worldwide.