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 Microprobe 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). 
The feldspars of the analyzed samples included plagioclase, k-feldspar, microperthite, and microantiperthite. The latter two are fine-scale exsolution intergrowths of plagioclase and K-feldspar. The grain size of plagioclase was typically coarse (&gt;100 µm) and easily analyzed by electron microprobe methods. However, the microperthite and microantiperthite commonly had exsolution lamellae &lt;10 µm in width. As a result, some EMP analyses overlapped lamellae of different composition and are mixed analyses. All analyses with total elemental weight percentages between 98 and 102% are reported.  
These data were collected in two analytical sessions on 4/26/2019 and 4/30/2019. The spectrometer configuration including analyzing crystal, X-ray line, and on peak count times were as follows: Spectrometer 1 - TAP crystal, NaKα, 20s, MgKα 30s, Spectrometer 2 - LIFH crystal, FeKα 20s, BaLα 20s, Spectrometer 3 - TAP crystal, SiKα 25s, AlKα 25s, Spectrometer 4 - PETJ crystal, CaKα 25s, TiKα 25s, Spectrometer 5 - PETJ crystal, KKα 20s, SrLα 20s.  All background corrections were done by linear interpolation of off-peak backgrounds, and off-peak background count times were half the on peak time for each background position. The matrix correction algorithm of Armstrong/Love Scott phi-rho-z (Armstrong, 1988) was used along with the mass absorption coefficients of Henke (1982) for all analyses.  Detection limits (3σ) based on counting statistics were ~0.02 wt% for SiO2, CaO, MgO, Al2O3, K2O, 0.03 wt% for Na2O, 0.04wt% for TiO2, 0.05 wt% for FeO, 0.06 wt% for SrO, and 0.07 wt% for Ba. Machine stability throughout the analyses and between sessions was documented by the analysis of secondary standard FSLC (Lake County, plagioclase) in sets of 7 analyses roughly every 12 hours. For major oxides (&gt;10 wt%) the total range of the average value from each of these sets varies by &lt; ± 3% relative, within QC bounds of ± 3%, and for the minor oxide Na2O varies by 9% relative within the QC bound of ±10%. The published value for FLSC (Huebner and Woodruff, 1985) and the value and uncertainty (1σ) measured during this study are as follows SiO2, 51.42, 51.05 ± 0.47; Al2O3, 30.76, 30.67 ± 0.18; CaO, 13.42, 13.33 ± 0.14; Na2O, 3.52, 3.81 ± 0.09; FeO 0.39, 0.41 ± 0.02; MgO, 0.14, 0.14 ± 0.01; K2O, 0.12, 0.12 ± 0.01; SrO 0.07, 0.09 ± 0.01; TiO2, 0.04, 0.04 ± 0.01; BaO, 0.007, 0.01 ± 0.02.
Plagioclase grains from the Hawkeye Granite Gneiss have typical anorthite component values of An15-An20. In contrast, 5 of 7 Lyon Mountain Granite Gneiss samples have plagioclase grains with values &lt;An10. Plagioclase in a sample from Moose River, near Lyonsdate, NY (MR-15-001) has an anomalous composition (~An45-50). Plagioclase from an amphibolite has a composition of An25.  
The composition of K-feldspar from the two units overlaps. However, the Hawkeye Granite Gneiss in general has a higher albite component. BSE and CL imaging indicates that in some cases there are relatively late generations of feldspars that can form veins (flame perthite) and/or patchy replacement of earlier feldspar generations. In some cases, the late feldspars are accompanied by microporosity and micron-sized inclusions of fluorite. These zones were generally avoided for the above analyses, but may explain part of the reason for overlap in the composition of K-feldspars from both units. 
One of the appendix tables provided in this data release, Table A2, contains whole rock geochemical data collected by the contracted lab, AGAT Laboratories. 
Abbreviations:
BSE – Backscattered Electron
CL – Cathodoluminescence
LiFH – lithium fluoride 
PETJ - pentaerythritol
TAP - thallium acid pthalate
Note: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
References: 
Aleinikoff, J.N., Walsh, G.J., and McAleer, R.J., 2021, New interpretations of the ages and origins of the Hawkeye Granite Gneiss
and Lyon Mountain Granite Gneiss, Adirondack Mountains, NY: Implications for the nature and timing of Mesoproterozoic plutonism, metamorphism, and deformation, Precambrian Research 358, 106112.
Armstrong, J.T., 1988, Quantitative analysis of silicates and oxide minerals: Comparison of Monte-Carlo, ZAF and Phi-Rho-Z procedures, in Newbury, D.E., ed., Microbeam analysis: San Francisco, California, San Francisco Press, p. 239–246 
Donovan, J.J., 2015, Probe for EPMA User Guide and Reference, online @ http://www.probesoftware.com/Technical.html 
Henke, B.L. , Lee, P., Tanaka, T.J., Shimabukuro, R.L. and Fijikawa B.K., 1982, Low-energy x-ray interaction coefficients: Photoabsorption, scattering, and reflection: E = 100-2000 eV Z = 1-94, Atomic Data Nucl. Data Tables 27(1), p. 1-144
Huebner, J.S. and Woodruff, M.E., 1985. Chemical compositions and critical evaluation of microprobe standards available from the Reston microprobe facility. US Geological Survey Open File Report, 85(718), p.45. 
Stacey, J., and Kramers, J., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model, Earth and Planetary Science Letters, 26, p. 207-221
Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology, Earth and Planetary Science Letters, 36, p. 359-362
Taylor, R.D., Shah, A.K., Walsh, G.J. and Taylor, C.D., 2019. Geochemistry and Geophysics of Iron Oxide-Apatite Deposits and Associated Waste Piles with Implications for Potential Rare Earth Element Resources from Ore and Historical Mine Waste in the Eastern Adirondack Highlands, New York, USA. Economic Geology, 114(8), p.1569-1598.

Tags: adirondack-mountains, chemical-analysis, dannemora, electron-probe-microanalysis, geochemistry, geoscientificinformation, hawkeye, keene, lyon-mountain, lyonsdale-new-york, mineral-resources, mineralogy, usgs-5eb04d5082cefae35a27a567
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