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

Newberry, R.J.Moshrefzadeh, J.A.2026Interior Alaska amphibolite mineral compositions: A key to distinguishing metamorphic assemblagespreadsheetPreliminary Interpretive ReportPIR 2026-2Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys33 p.https://doi.org/10.14509/32081Interior 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.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.>electronâmicroprobe mineralâchemistry data: Sample identifiers, geographic locations and description, petrographic locations, mineral phases, oxide weight percentages, and calculated cation proportions for amphibole and plagioclase populations.2025ground conditionCompleteNone planned-148.202609-141.72189165.24922963.261399ISO 19115 Topic CategorygeoscientificInformationAlaska Division of Geological & Geophysical SurveysActinoliteAlbiteAmphiboleAmphiboliteAnalysesBatholithBedrockChemistryClinopyroxeneClinozoisiteDGGSDiopsideEpidoteGarnetGeochemical AnalysisGeochemical CompositionGeochemical DataGeochemistryGeologicGeologyGreenschistGreenstoneHornblendeIlmeniteMetamorphicMetamorphismMethodsMicroprobe AnalysisMineral AssessmentMineral CompositionMineral ExplorationMineral ResourcesMineralogyMineralsPetrologyPlagioclasePlutonicResourcesRocksSchistAlaska Division of Geological & Geophysical SurveysAlaskaAlder CreekBedrock CreekBoulder CreekBuckskin CreekCalifornia CreekCement CreekChatham CreekChena RiverCircle Mining DistrictCleary CreekCleary Hill MineCoffee DomeCottonwood CreekCripple CreekDeep CreekDemar CreekEngineer CreekEster DomeEva CreekFairbanks Mining DistrictFirst Chance CreekFortymile Mining DistrictFrenchman CreekGeorge CreekGoldstream CreekGoodpaster Mining DistrictGulch CreekHappy CreekJohnson CreekJoseph CreekKokomo CreekLast Chance CreekLawson CreekLiwa LakeMiller CreekMolly CreekMontana CreekMoose CreekMount NeubergerMunson CreekNewsboy MineOhio CreekOlympia CreekOttertail CreekPaldo CreekPorcupine CreekReady Bullion CreekRichardson Mining DistrictRock CreekSand CreekSerpentine CreekSheep CreekSmallwood CreekSorrels CreekSteele CreekStibnite CreekStuart CreekTok Mining DistrictVictoria CreekVolkmar LakeWalnut CreekWigwam CreekWilliams CreekWillow CreekWolf CreekYukon-Tanana UplandAlaska Division of Geological & Geophysical SurveysFairbanks-Chena AssemblageFortymile River AssemblageKlondike AssemblageLake George AssemblageWalker, J.D., Geissman, J.W., Bowring, S.A, and Babcock, L.E., comp., 2012, Geologic Time Scale v. 4.0: Geological Society of AmericaMesozoicPaleozoicPermianProterozoicThis 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).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.Alaska Division of Geological & Geophysical SurveysMetadata Managermailing and physical

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FairbanksAK99709-3707USA(907)451-5020(907)451-5050dggspubs@alaska.gov8 am to 4:30 pm, Monday through Friday, except State holidaysPlease 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.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.Buchanan, J.W.Wypych, AlicjaBarrera, 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, EvanWildland, A.D.2025Geochemical data from samples collected in 2023 for the Chena and Mount Harper projects, Big Delta, Circle, Fairbanks, and Eagle quadrangles, AlaskaRaw Data FileRDF 2024-2Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys5 phttps://doi.org/10.14509/31123Gavel, M.M.Wypych, AlicjaNaibert, T.J.Twelker, EvanNewberry, R.J.Barrera, M.L.Szumigala, D.J.Truskowski, C.M.Muller, I.P.Fessenden, S.N.Blackwell, N.J.Harvey, D.A.Wildland, A.D.2025Bedrock geologic map of the Mount Harper-Middle Fork area, Eagle Quadrangle, AlaskaPreliminary Interpretive ReportPIR 2025-2DFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys1 sheet, scale 1:100,000https://doi.org/10.14509/31652Joy, B.R.Keskinen, M.J.Newberry, R.J.1996Preliminary thermobarometry and microprobe mineral compositions, Fairbanks area schists and amphibolitesPublic Data FilePDF 96-12Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys15 phttps://doi.org/10.14509/1736Moshrefzadeh, J.A.Newberry, R.J.2025Electron probe microanalyzer data collected on samples from the Chena and Steese projects, Yukon-Tanana Uplands, AlaskaRaw Data FileRDF 2025-13Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys6 phttps://doi.org/10.14509/31537Naibert, T.J.Wypych, AlicjaNewberry, R.J.Twelker, EvanGavel, M.M.Wildland, A.D.Barrera, M.L.Avirett, D.F.Muller, I.P.Blackwell, N.J.Szumigala, D.J.2024Bedrock geologic map of the Taylor Mountain area, Eagle and Tanacross quadrangles, AlaskaPreliminary Interpretive ReportPIR 2024-6BFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys1 sheet, scale 1:100,000https://doi.org/10.14509/31168Naibert, T.J.Newberry, R.J.Twelker, EvanWypych, AlicjaGavel, 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.Wildland, A.D.2025Bedrock geologic map of the Volkmar River-Healy River area, Big Delta and Mount Hayes quadrangles, AlaskaPreliminary Interpretive ReportPIR 2025-2CFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys1 sheet, scale 1:100,000https://doi.org/10.14509/31651Newberry, R.J.Bundtzen, T.K.Clautice, K.H.Combellick, R.A.Douglas, TomLaird, G.M.Liss, S.A.Pinney, D.S.Reifenstuhl, R.R.Solie, D.N.1996Preliminary geologic map of the Fairbanks mining district, AlaskaPublic Data FilePDF 96-16Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys17 p., 2 sheets, scale 1:63,360https://doi.org/10.14509/1740Robinson, M.S.Smith, T.E.Metz, P.A.1990Bedrock geology of the Fairbanks mining districtProfessional ReportPR 106Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys2 sheets, scale 1:63,360https://doi.org/10.14509/2287Twelker, EvanNewberry, R.J.Wypych, AlicjaNaibert, T.J.Wildland, A.D.Sicard, K.R.Regan, S.P.Athey, J.E.Wyatt, W.C.Lopez, J.A.2021Bedrock geologic map of the Ladue River-Mount Fairplay area, Tanacross and Nabesna quadrangles, AlaskaReport of InvestigationRI 2021-5AFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveysp. 1-32, 1 sheet, scale 1:100,000https://doi.org/10.14509/30735Twelker, EvanNewberry, R.J.Naibert, T.J.Wypych, Alicja2025Bedrock geologic map of the Richardson Mining District, Big Delta Quadrangle, AlaskaPreliminary Interpretive ReportPIR 2025-2AFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys1 sheet, scale 1:100,000https://doi.org/10.14509/31649Twelker, EvanNewberry, R.J.Naibert, T.J.Wypych, AlicjaGavel, M.M.Barrera, M.L.Szumigala, D.J.Truskowski, C.M.Muller, I.P.Fessenden, S.N.Blackwell, N.J.Harvey, D.A.Wildland, A.D.2025Bedrock geologic maps of the Mount Harper-Middle Fork area, Volkmar River-Healy River area, Goodpaster River-Shaw Creek area, and the Richardson Mining District, AlaskaPreliminary Interpretive ReportPIR 2025-2Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys38 phttps://doi.org/10.14509/31648Wypych, AlicjaNaibert, T.J.Athey, J.E.Newberry, R.J.Sicard, K.R.Twelker, EvanWerdon, M.B.Willingham, A.L.Wyatt, W.C.2018Major-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, AlaskaRaw Data FileRDF 2018-4Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys4 phttps://doi.org/10.14509/30113Wypych, AlicjaGavel, M.M.Naibert, T.J.Avirett, D.F.Barrera, M.L.Hubbard, A.K.Newberry, R.J.Regan, S.P.Twelker, EvanWildland, A.D.Wyatt, W.C.2022Geochemical data from samples collected in 2021 for the Taylor Mountain project, Tanacross and Eagle quadrangles, AlaskaRaw Data FileRDF 2022-4Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys3 phttps://doi.org/10.14509/30843Wypych, AlicjaGavel, M.M.Naibert, T.J.Avirett, D.F.Barrera, M.L.Hubbard, A.K.Newberry, R.J.Regan, S.P.Twelker, EvanWildland, A.D.Wyatt, W.C.2022Geochemical data from samples collected in 2020 and 2021 for the Western Tanacross project, Tanacross Quadrangle, AlaskaRaw Data FileRDF 2022-5Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys3 phttps://doi.org/10.14509/30844Wypych, AlicjaTwelker, EvanNaibert, 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.Blackwell, N.J.2023Geochemical data from samples collected in 2022 for the Mount Harper geologic mapping project, Big Delta, Mount Hayes, and Eagle quadrangles, AlaskaRaw Data FileRDF 2023-24Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys3 phttps://doi.org/10.14509/31089Wypych, AlicjaNaibert, T.J.Newberry, R.J.Twelker, EvanGavel, 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.Wyatt, W.C.2024Bedrock geologic map of the Western Tanacross area, Tanacross Quadrangle, AlaskaPreliminary Interpretive ReportPIR 2024-6CFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys1 sheet, scale 1:100,000https://doi.org/10.14509/31169These 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.Not applicableThis data release is complete.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.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 2025.2025point0.00000010.0000001decimal degreesNAD83GRS 806378137298.257222101pir2026-2_plagioclase_data.csv, pir2026-2_plagioclase_data_dictionary.csv, pir2026-2_amphibole_data.csv, pir2026-2_amphibole_data_dictionary.csvSample identifiers, geographic locations and description, petrographic locations, mineral phases, oxide weight percentages, and calculated cation proportions for amphibole and plagioclase populations.DGGSelectronâmicroprobe mineralâchemistry dataAlaska Division of Geological & Geophysical SurveysMetadata Managermailing and physical

3354 College Road

FairbanksAK99709-3707USA(907)451-5020(907)451-5050dggspubs@alaska.gov8 am to 4:30 pm, Monday through Friday, except State holidaysFGDC Content Standard for Digital Geospatial MetadataFGDC-STD-001-1998If the user has modified the data in any way they are obligated to describe the types of modifications they have performed in the supporting metadata file. User specifically agrees not to imply that changes they made were approved by the Alaska Department of Natural Resources or Division of Geological & Geophysical Surveys.https://dggs.alaska.gov/metadata/dggs.extdggs metadata extensions