Interior Alaska amphibolite mineral compositions: A key to distinguishing metamorphic assemblage

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Frequently anticipated questions:


What does this data set describe?

Title:
Interior Alaska amphibolite mineral compositions: A key to distinguishing metamorphic assemblage
Abstract:
Interior Alaska amphibolite mineral compositions: A key to distinguishing metamorphic assemblage, Preliminary Interpretive Report 2026-2, provides electron microprobe mineral compositional data used to distinguish metamorphic assemblages in eastern Interior Alaska. Metamorphic rocks in this region have historically been grouped into several packages and, more recently, assemblages that are often differentiated using 40Ar/39Ar age dating, which is a costly and time consuming method. Although some assemblages show clear differences in metamorphic grade, others do not, and three amphibolite facies units (the Fortymile River, Lake George, and Fairbanks Chena assemblages) contain similar lithologies with no obvious hand specimen distinctions. Localized contact and retrograde metamorphism have further complicated classification. This study analyzes amphibole and plagioclase compositions from 79 metamafic samples collected between 1981 and 2024, with most collected between 2021 and 2024. Most analyses were conducted using energy dispersive spectrometry (EDS), while samples collected in 2024 were analyzed using the more accurate wavelength dispersive spectrometry (WDS). Several samples were analyzed by both methods and show broadly comparable results. Amphibole compositions provide the strongest discrimination among assemblages because plagioclase is more susceptible to recrystallization during both higher temperature contact metamorphism and lower temperature retrograde alteration. Results generally support the assemblage assignments proposed by Dusel Bacon 2006 and indicate that the Fairbanks Chena assemblage is more extensive than previously recognized. In addition to the interpretive discussion, this report includes comprehensive data tables containing sample identifiers, geographic locations and descriptions, petrographic context, mineral phases, oxide weight percentages, and calculated cation proportions for amphibole and plagioclase populations. These data and accompanying discussion are released as a DGGS Preliminary Interpretive Report, and all publication components are available from the DGGS website. at: http://doi.org/10.14509/32081.
Supplemental_Information:
electron‑microprobe mineral‑chemistry data:    Sample identifiers, geographic locations and description, petrographic locations, mineral phases, oxide weight percentages, and calculated cation proportions for amphibole and plagioclase populations.
  1. How might this data set be cited?
    Newberry, R.J., and Moshrefzadeh, J.A., 2026, Interior Alaska amphibolite mineral compositions: A key to distinguishing metamorphic assemblage: Preliminary Interpretive Report PIR 2026-2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 33 p.
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -148.202609
    East_Bounding_Coordinate: -141.721891
    North_Bounding_Coordinate: 65.249229
    South_Bounding_Coordinate: 63.261399
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Calendar_Date: 2025
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: spreadsheet
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a point data set.
    2. What coordinate system is used to represent geographic features?
      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.0000001. Longitudes are given to the nearest 0.0000001. Latitude and longitude values are specified in decimal degrees. The horizontal datum used is NAD83.
      The ellipsoid used is GRS 80.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257222101.
  7. How does the data set describe geographic features?
    pir2026-2_plagioclase_data.csv, pir2026-2_plagioclase_data_dictionary.csv, pir2026-2_amphibole_data.csv, pir2026-2_amphibole_data_dictionary.csv
    Sample identifiers, geographic locations and description, petrographic locations, mineral phases, oxide weight percentages, and calculated cation proportions for amphibole and plagioclase populations. (Source: DGGS)

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
  2. Who also contributed to the data set?
    The authors thank J. Wes Buchanan for his review, which improved the quality of the manuscript, and the University of Alaska Fairbanks Advanced Instrumentation Laboratory (UAF-AIL) manager, Dr. Nathan Graham, who helped set up the 2024 WDS analyses. We also thank Evan Twelker, Travis Naibert, Mike Barrera, Dave Szumigala, Alicja Wypych, Michelle Gavel, Wes Buchanan, Alec Wildland, Sean Regan, David Harvey, Izzy Muller, Serena Fessenden, Noel Blackwell, Lily Norwood, and Marisa Acosta, who mapped with the authors at various times between 2021 and 2024, procured samples used in this study, and provided helpful observations and ideas. K. Clautice, D. Solie, T. Bundtzen, G. Laird, and R. Reifenstuhl mapped with Newberry in 1995 and provided samples and discussion. Dr. Ken Severin assisted with those WDS analyses. Much of this work was performed at the UAF-AIL, and the authors gratefully acknowledge use of the UAF-AIL electron microprobe acquired with support from the National Science Foundation, Major Research Instrumentation Program Award Number 1126898. We profusely thank past and present personnel of UAF-AIL. This project was jointly funded by the State of Alaska and the U.S. Geological Survey Earth Mapping Resources Initiative (Earth MRI) through cooperative agreements G20AC00156, G21AC00336, G22AC00288, G23AC00372, and G24AC00323. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions or policies of the U.S. Geological Survey. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey.
  3. To whom should users address questions about the data?
    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
    Contact_Instructions:
    Please view our website (https://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.

Why was the data set created?

The Yukon Tanana Upland of east central Alaska is a region with significant critical mineral potential and complex metamorphic geology. The purpose of this dataset is to contribute to ongoing efforts to modernize geologic mapping across east central Alaska by providing mineral chemistry information that improves the interpretation of rock units and regional geologic relationships.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: 2025 (process 1 of 1)
    Sample analysis - Geologists collected rock samples during field mapping from 2021 to 2024 and selected specimens suitable for polished thin section preparation. All thin sections were examined, and only metamafic samples with minimal weathering or retrograde alteration were chosen for analysis. Thin sections showing alteration of hornblende to actinolite or chlorite, or alteration of plagioclase to white mica, calcite, or chlorite were rejected. At least four sites containing amphibole and plagioclase were randomly selected on each thin section, and multiple grains of each mineral were analyzed using standard modified energy dispersive spectrometry (EDS) on the JEOL JXA 8530 electron probe microanalyzer at the University of Alaska Fairbanks Advanced Instrumentation Laboratory. Additional spot analyses were performed as needed to confirm mineral identification. Samples collected in 2024 were also analyzed using wavelength dispersive spectrometry (WDS) to obtain higher accuracy results and to verify EDS measurements. For each mineral grain, three or more analyses were averaged to reduce analytical variability. Previously unpublished hornblende and plagioclase analyses generated using the Cameca SX 50 microprobe at the same facility were reviewed and checked against new EDS data collected in 2025.
  3. What similar or related data should the user be aware of?
    Buchanan, J.W., Wypych, Alicja, Barrera, M.L., Biegel, J.M., Gavel, M.M., Harvey, D.A., Ketcham, R.A., Muller, I.P., Naibert, T.J., Newberry, R.J., Regan, S.P., Szumigala, D.J., Twelker, Evan, and Wildland, A.D., 2025, Geochemical data from samples collected in 2023 for the Chena and Mount Harper projects, Big Delta, Circle, Fairbanks, and Eagle quadrangles, Alaska: Raw Data File RDF 2024-2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 5 p
    Gavel, M.M., Wypych, Alicja, Naibert, T.J., Twelker, Evan, Newberry, R.J., Barrera, M.L., Szumigala, D.J., Truskowski, C.M., Muller, I.P., Fessenden, S.N., Blackwell, N.J., Harvey, D.A., and Wildland, A.D., 2025, Bedrock geologic map of the Mount Harper-Middle Fork area, Eagle Quadrangle, Alaska: Preliminary Interpretive Report PIR 2025-2D, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:100,000
    Joy, B.R., Keskinen, M.J., and Newberry, R.J., 1996, Preliminary thermobarometry and microprobe mineral compositions, Fairbanks area schists and amphibolites: Public Data File PDF 96-12, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 15 p
    Moshrefzadeh, J.A., and Newberry, R.J., 2025, Electron probe microanalyzer data collected on samples from the Chena and Steese projects, Yukon-Tanana Uplands, Alaska: Raw Data File RDF 2025-13, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 6 p
    Naibert, T.J., Wypych, Alicja, Newberry, R.J., Twelker, Evan, Gavel, M.M., Wildland, A.D., Barrera, M.L., Avirett, D.F., Muller, I.P., Blackwell, N.J., and Szumigala, D.J., 2024, Bedrock geologic map of the Taylor Mountain area, Eagle and Tanacross quadrangles, Alaska: Preliminary Interpretive Report PIR 2024-6B, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:100,000
    Naibert, T.J., Newberry, R.J., Twelker, Evan, Wypych, Alicja, Gavel, M.M., Barrera, M.L., Szumigala, D.J., Truskowski, C.M., Muller, I.P., Fessenden, S.N., Buchanan, J.W., Blackwell, J.M., Harvey, D.A., and Wildland, A.D., 2025, Bedrock geologic map of the Volkmar River-Healy River area, Big Delta and Mount Hayes quadrangles, Alaska: Preliminary Interpretive Report PIR 2025-2C, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:100,000
    Newberry, R.J., Bundtzen, T.K., Clautice, K.H., Combellick, R.A., Douglas, Tom, Laird, G.M., Liss, S.A., Pinney, D.S., Reifenstuhl, R.R., and Solie, D.N., 1996, Preliminary geologic map of the Fairbanks mining district, Alaska: Public Data File PDF 96-16, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 17 p., 2 sheets, scale 1:63,360
    Robinson, M.S., Smith, T.E., and Metz, P.A., 1990, Bedrock geology of the Fairbanks mining district: Professional Report PR 106, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 2 sheets, scale 1:63,360
    Twelker, Evan, Newberry, R.J., Wypych, Alicja, Naibert, T.J., Wildland, A.D., Sicard, K.R., Regan, S.P., Athey, J.E., Wyatt, W.C., and Lopez, J.A., 2021, Bedrock geologic map of the Ladue River-Mount Fairplay area, Tanacross and Nabesna quadrangles, Alaska: Report of Investigation RI 2021-5A, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: p. 1-32, 1 sheet, scale 1:100,000
    Twelker, Evan, Newberry, R.J., Naibert, T.J., and Wypych, Alicja, 2025, Bedrock geologic map of the Richardson Mining District, Big Delta Quadrangle, Alaska: Preliminary Interpretive Report PIR 2025-2A, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:100,000
    Twelker, Evan, Newberry, R.J., Naibert, T.J., Wypych, Alicja, Gavel, M.M., Barrera, M.L., Szumigala, D.J., Truskowski, C.M., Muller, I.P., Fessenden, S.N., Blackwell, N.J., Harvey, D.A., and Wildland, A.D., 2025, Bedrock geologic maps of the Mount Harper-Middle Fork area, Volkmar River-Healy River area, Goodpaster River-Shaw Creek area, and the Richardson Mining District, Alaska: Preliminary Interpretive Report PIR 2025-2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 38 p
    Wypych, Alicja, Naibert, T.J., Athey, J.E., Newberry, R.J., Sicard, K.R., Twelker, Evan, Werdon, M.B., Willingham, A.L., and Wyatt, W.C., 2018, Major-oxide and trace-element geochemical data from rocks collected in 2018 for the Northeast Tanacross project, Tanacross C-1, C-2, D-1, and D-2 quadrangles, Alaska: Raw Data File RDF 2018-4, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 4 p
    Wypych, Alicja, Gavel, M.M., Naibert, T.J., Avirett, D.F., Barrera, M.L., Hubbard, A.K., Newberry, R.J., Regan, S.P., Twelker, Evan, Wildland, A.D., and Wyatt, W.C., 2022, Geochemical data from samples collected in 2021 for the Taylor Mountain project, Tanacross and Eagle quadrangles, Alaska: Raw Data File RDF 2022-4, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 3 p
    Wypych, Alicja, Gavel, M.M., Naibert, T.J., Avirett, D.F., Barrera, M.L., Hubbard, A.K., Newberry, R.J., Regan, S.P., Twelker, Evan, Wildland, A.D., and Wyatt, W.C., 2022, Geochemical data from samples collected in 2020 and 2021 for the Western Tanacross project, Tanacross Quadrangle, Alaska: Raw Data File RDF 2022-5, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 3 p
    Wypych, Alicja, Twelker, Evan, Naibert, T.J., Gavel, M.M., Newberry, R.J., Szumigala, D.J., Wildland, A.D., Barrera, M.L., Harvey, D.A., Muller, I.P., Fessenden, S.N., and Blackwell, N.J., 2023, Geochemical data from samples collected in 2022 for the Mount Harper geologic mapping project, Big Delta, Mount Hayes, and Eagle quadrangles, Alaska: Raw Data File RDF 2023-24, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 3 p
    Wypych, Alicja, Naibert, T.J., Newberry, R.J., Twelker, Evan, Gavel, M.M., Wildland, A.D., Szumigala, D.J., Regan, S.P., Avirett, D.F., Barrera, M.L., Bernard, C.M., Blackwell, N.J., Fessenden, S.N., Harvey, D.A., Hubbard, A.K., Masterman, S.S., Muller, I.P., Turner, M.M., and Wyatt, W.C., 2024, Bedrock geologic map of the Western Tanacross area, Tanacross Quadrangle, Alaska: Preliminary Interpretive Report PIR 2024-6C, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:100,000

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
    These data are being released in the DGGS Preliminary Interpretive Report (PIR) series. PIRs present emergent interpretations of geologic mapping or analytical investigation. These publications are reviewed for clarity and consistency and receive limited peer review. The accompanying report provides a comprehensive discussion of processing methods, with errors reported in the tables where applicable.
  2. How accurate are the geographic locations?
    Location data for field stations were collected using GPS-enabled tablets or smartphones running the Esri Field Maps app with an estimated location error of 10.0 m.
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
    This data release is complete.
  5. How consistent are the relationships among the observations, including topology?
    Not applicable

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints:
This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Use_Constraints:
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  1. Who distributes the data set? (Distributor 1 of 1)
    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
    Contact_Instructions:
    Please view our website (https://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.
  2. What's the catalog number I need to order this data set? PIR 2026-2
  3. What legal disclaimers am I supposed to read?
    The State of Alaska makes no expressed or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 15-May-2026
Metadata author:
Alaska Division of Geological & Geophysical Surveys
Attn: Simone Montayne
Metadata Manager
3354 College Road
Fairbanks, AK
USA

(907)451-5020 (voice)
(907)451-5050 (FAX)
dggspubs@alaska.gov
Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:

Generated by mp version 2.9.50 on Tue May 19 14:09:49 2026