40 The Adirondack Mountains and the Grenville Orogenic Cycle: New Results and Syntheses Regarding the Timing and Nature of Deformation, Metamorphism, Intrusion, and Formation of Ore Deposits II: A Session Honoring the Work of Bruce Selleck

Monday, 19 March 2018
1:30 PM - 3:30 PM
Emerald Ballroom II

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

Paper number: 40-4
Program: Theme Sessions
Time: 2:30 PM – 2:50 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 2

doi: 10.1130/abs/2018NE-311114

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.