Electron microprobe analyses of feldspars and petrographic, geochemical, and geochronologic data from the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss in the Adirondacks of New York (ver. 2.0, May 2023)

Iron oxide-apatite (IOA) deposits of the Adirondack Mountains of New York locally contain elevated REE concentrations (e.g. Taylor and others, 2019). Critical to evaluating resource potential is understanding the genesis of the IOA deposits that host the REE-rich minerals. As part of this effort, the U.S. Geological Survey (USGS) is conducting bedrock geologic mapping, geochronology, geochemistry, and geophysics in the region. Published and ongoing research demonstrates the spatial association of IOA deposits with the Lyon Mountain Granite Gneiss (LMG), so understanding the relationship of the LMG to the IOA deposits is important for resource evaluation—however the age and origin of the LMG remain contentious. As a result, the USGS undertook a petrologic and geochronologic study of the LMG and Hawkeye Granite Gneiss to better understand the temporal relationship between ores and the LMG. Electron microprobe (EMP) analyses of the feldspars in the sampled rocks was conducted as part of this research.


Twelve samples, including four samples of Hawkeye Granite Gneiss, seven samples of Lyon Mountain Granite Gneiss, and one amphibolite were collected from the Adirondack massif in upstate NY (see Aleinikoff and others, 2021 Figure 1 and Table 1). Feldspar grains from these samples were analyzed by electron microprobe to determine their major and minor element geochemistry. The electron microprobe data was collected by personnel of the Florence Bascom Geoscience Center in Reston, Virginia, for the U.S. Geological Survey (USGS) National Cooperative Geological Mapping Program (NCGMP). A fully automated JEOL 8900 Electron Microproben with five wavelength dispersive analyzers operated at 15keV accelerating voltage, a 20-nA beam current, and an electron beam diameter of 3-10 micrometers was utilized. The microprobe was operated using Probe for EPMA software (Donovan, 2015).


Twelve samples, including four samples of Hawkeye Granite Gneiss, seven samples of Lyon Mountain Granite Gneiss, and one amphibolite were collected from the Adirondack massif in upstate NY (see Aleinikoff and others, 2021 Figure 1 and Table 1). Feldspar grains from these samples were analyzed by electron microprobe to determine their major and minor element geochemistry. The electron microprobe data was collected by personnel of the Florence Bascom Geoscience Center in Reston, Virginia, for the U.S. Geological Survey (USGS) National Cooperative Geological Mapping Program (NCGMP). A fully automated JEOL 8900 Electron Microproben with five wavelength dispersive analyzers operated at 15keV accelerating voltage, a 20-nA beam current, and an electron beam diameter of 3-10 micrometers was utilized. The microprobe was operated using Probe for EPMA software (Donovan, 2015).