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 (>100 µm) and easily analyzed by electron microprobe methods. However, the microperthite and microantiperthite commonly had exsolution lamellae <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 (>10 wt%) the total range of the average value from each of these sets varies by < ± 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

The iron oxide-apatite (IOA) deposits near Mineville in the Adirondack Mountains, New York, have been of interest for their rich magnetite ore since the mid-1700s but have attracted renewed attention due to their potential as rare earth element (REE) resources (McKeown and Klemic, 1956; Lupulescu and others, 2016; Taylor and others, 2018). Apatite is the main REE-host and is found in variable concentrations within ore seams of the regional magnetite deposits. Some apatite crystals are unaltered, relatively homogenous, and inclusion-free, whereas other deposits contain heterogenous apatite with zones of abundant secondary mineral inclusions that were formed through metasomatic reactions with the apatite after initial precipitation. The heterogeneous apatite crystals may have inclusion-free bright zones and intermediate zones in back-scattered electron imagery (BSE), and dark BSE zones that contain inclusions of monazite and thorite. Apatite crystals from twenty-seven samples, including twenty-four ore and three rock samples from a total of nineteen different ore deposits, were analyzed by electron microprobe to obtain their major and minor element geochemistry. Additionally, some magmatic apatite crystals from the ore-hosting Lyon Mountain Granite Gneiss were analyzed for comparison with the ore apatite. The electron microprobe data was collected by personnel of the Southwest Region Geology, Geophysics, and Geochemistry Science Center in Denver, Colorado, for the U.S. Geological Survey (USGS) Mineral Resources Program (MRP). A JEOL 8900 Electron Microprobe with five wavelength dispersive analyzers operated at 20keV accelerating voltage, a 50-nA current (measured on the Faraday cup), and an electron beam diameter of 10 micrometers was utilized. All analyzed crystals are considered fluorapatite, with fluorine contents ranging from approximately 3.5 to 6.6%. Some apatite crystals from ore contain greater than 15% total REE, whereas some others contain less than 1%. Commonly, Y, La, Ce, and Nd are the most abundant REE in the apatite crystals. The magmatic apatite crystals are notably purer with low contents of actinides, REE, and other common minor impurities. Analyses that contained total elemental weight percentages between 97% to 103% were accepted; those analyses with poor totals falling outside of this range were rejected. The different zones within heterogeneous apatite crystals contained lower concentrations of REE and other minor element components in the dark BSE zones than in the bright BSE zones, but both zones had nearly parallel REE profiles. The zones of differing BSE brightness are interpreted to be caused by metasomatic alteration. Although the REE profiles were consistent for a given sample, variations in total REE content and overall chemistry were noted between different deposits and even different ore seams within a given deposit.

The iron oxide-apatite (IOA) deposits near Mineville in the Adirondack Mountains, New York, have been of interest for their rich magnetite ore since the mid-1700s but have attracted renewed attention due to their potential as rare earth element (REE) resources (McKeown and Klemic, 1956; Lupulescu and others, 2016; Taylor and others, 2018). Apatite is the main REE-host and is found in variable concentrations within ore seams of the regional magnetite deposits. Some apatite crystals are unaltered, relatively homogenous, and inclusion-free, whereas other deposits contain heterogenous apatite with zones of abundant secondary mineral inclusions that were formed through metasomatic reactions with the apatite after initial precipitation. The heterogeneous apatite crystals may have inclusion-free bright zones and intermediate zones in back-scattered electron imagery (BSE), and dark BSE zones that contain inclusions of monazite and thorite. Apatite crystals from twenty-seven samples, including twenty-four ore and three rock samples from a total of nineteen different ore deposits, were analyzed by electron microprobe to obtain their major and minor element geochemistry. Additionally, some magmatic apatite crystals from the ore-hosting Lyon Mountain Granite Gneiss were analyzed for comparison with the ore apatite. The electron microprobe data was collected by personnel of the Southwest Region Geology, Geophysics, and Geochemistry Science Center in Denver, Colorado, for the U.S. Geological Survey (USGS) Mineral Resources Program (MRP). A JEOL 8900 Electron Microprobe with five wavelength dispersive analyzers operated at 20keV accelerating voltage, a 50-nA current (measured on the Faraday cup), and an electron beam diameter of 10 micrometers was utilized. All analyzed crystals are considered fluorapatite, with fluorine contents ranging from approximately 3.5 to 6.6%. Some apatite crystals from ore contain greater than 15% total REE, whereas some others contain less than 1%. Commonly, Y, La, Ce, and Nd are the most abundant REE in the apatite crystals. The magmatic apatite crystals are notably purer with low contents of actinides, REE, and other common minor impurities. Analyses that contained total elemental weight percentages between 97% to 103% were accepted; those analyses with poor totals falling outside of this range were rejected. The different zones within heterogeneous apatite crystals contained lower concentrations of REE and other minor element components in the dark BSE zones than in the bright BSE zones, but both zones had nearly parallel REE profiles. The zones of differing BSE brightness are interpreted to be caused by metasomatic alteration. Although the REE profiles were consistent for a given sample, variations in total REE content and overall chemistry were noted between different deposits and even different ore seams within a given deposit.

This dataset accompanies planned publication 'Unmixing multiple metamorphic muscovite age populations with powder X-ran diffraction and 40Ar/39Ar analysis'. The 40Ar/39Ar and electron microprobe data are from samples adjacent to a lower greenschist facies shear zone in western New Hampshire. The geochronology coupled with the electron microprobe data provide a petrochronologic framework for the rocks studied in the manuscript. Samples collection was supervised by Ryan McAleer, Gregory Walsh, and Peter Valley of the USGS.

This dataset accompanies planned publication 'Unmixing multiple metamorphic muscovite age populations with powder X-ran diffraction and 40Ar/39Ar analysis'. The 40Ar/39Ar and electron microprobe data are from samples adjacent to a lower greenschist facies shear zone in western New Hampshire. The geochronology coupled with the electron microprobe data provide a petrochronologic framework for the rocks studied in the manuscript. Samples collection was supervised by Ryan McAleer, Gregory Walsh, and Peter Valley of the USGS.

The exploration for porphyry deposits in some parts of Alaska may require unconventional exploration geochemical methods, depending on type of cover. The Taurus deposit and others in the region are mostly concealed by residual soils that in part include ash and loess, and therefore traditional stream sediment samples typically contain subdued geochemical signatures. Indicator mineral studies include collection of stream sediment samples and analysis using automated SEM mineralogical techniques. The presence of select minerals in the stream sediments may indicate mineralization. In addition, the chemistry of specific minerals may be used to distinguish a hydrothermal origin as opposed to others, and include apatite, rutile, and titanite. The electron probe data in this data release were collected for apatite, rutile, and titanite by personnel of the Geology, Geophysics, and Geochemistry Science Center in Denver, Colorado, for the U.S. Geological Survey (USGS) Mineral Resources Program (MRP). Appreciable differences in chemistry were noted for these minerals in mineralized rock and stream sediment samples draining these rocks compared to sediment samples away from mineralization.

The exploration for porphyry deposits in some parts of Alaska may require unconventional exploration geochemical methods, depending on type of cover. The Taurus deposit and others in the region are mostly concealed by residual soils that in part include ash and loess, and therefore traditional stream sediment samples typically contain subdued geochemical signatures. Indicator mineral studies include collection of stream sediment samples and analysis using automated SEM mineralogical techniques. The presence of select minerals in the stream sediments may indicate mineralization. In addition, the chemistry of specific minerals may be used to distinguish a hydrothermal origin as opposed to others, and include apatite, rutile, and titanite. The electron probe data in this data release were collected for apatite, rutile, and titanite by personnel of the Geology, Geophysics, and Geochemistry Science Center in Denver, Colorado, for the U.S. Geological Survey (USGS) Mineral Resources Program (MRP). Appreciable differences in chemistry were noted for these minerals in mineralized rock and stream sediment samples draining these rocks compared to sediment samples away from mineralization.

This data release provides geochemical results of in situ electron probe microanalyses of hydrothermal muscovite associated with gold-bearing quartz veins from the Klamath Mountains, California. Samples were collected from eight different mines in the summer of 2013 and electron probe microanalyses were carried out May 27, 2014 and November 12, 2019.

This data release provides geochemical results of in situ electron probe microanalyses of hydrothermal muscovite associated with gold-bearing quartz veins from the Klamath Mountains, California. Samples were collected from eight different mines in the summer of 2013 and electron probe microanalyses were carried out May 27, 2014 and November 12, 2019.