40Ar/39Ar geochronology data from the Ladue River-Mount Fairbanks area, eastern Alaska, Raw Data File 2024-32, provides 40Ar/39Ar geochronology results from rock samples collected in the Tanacross Quadrangle of eastern Alaska within the Ladue River drainage and the upper Dennison Fork of the Fortymile River drainage around Mount Fairplay. During the 2019 field season, geologists from the Alaska Division of Geological & Geophysical Surveys (DGGS) conducted geologic mapping and sampling in the Tanacross Quadrangle of eastern Alaska within the Ladue River drainage and the upper Dennison Fork of the Fortymile River drainage around Mount Fairplay. The area lies northeast of the Alaska Highway between Tok, Alaska, and the Canadian border. A recent geologic map of the area lies within the Yukon-Tanana Uplands, which DGGS and U.S. Geological Survey (USGS) identified as having the potential to host deposits of multiple critical minerals, as well as gold, copper, molybdenum, lead, zinc, and silver. Most known mineralization in the region is related to Cretaceous-Paleogene magmatism. These igneous rocks intrude metamorphic rocks of the North American continental margin and the structurally overlying allochthonous Yukon-Tanana Terrane, both of which are multiply deformed and juxtaposed along low-angle faults. Samples were collected for 40Ar/39Ar geochronology to further understand the crystallization ages of igneous rocks and the exhumation history of metamorphic rocks. This report's 18 40Ar/39Ar samples include one volcanic crystallization age, two pluton-alteration ages, and 15 metamorphic cooling ages. Differences in metamorphic cooling ages have been used to distinguish the allochthonous Yukon-Tanana Terrane from parautochthonous North America. The 15 metamorphic samples with cooling ages reported here were collected to aid in DGGS field mapping near this major terrane boundary. Allochthonous samples include eight samples from the Ladue River assemblage with muscovite cooling ages between ca. 105 and 235 Ma; a sample from the Klondike assemblage with a muscovite cooling age of ca. 147 Ma; a sample from the Fortymile River assemblage with a muscovite cooling age of ca. 186 Ma. Parautochthonous North America samples include a sample from the Jarvis assemblage with a muscovite cooling age of ca. 149 Ma and four samples from the Lake George assemblage with muscovite cooling ages of ca. 100 and 105 Ma and hornblende cooling ages of ca. 127 and 255 Ma. These data and report are available from the DGGS website: http://doi.org/10.14509/31454.

Geologic map of the McGrath D-6 Quadrangle, Alaska, Professional Report 79, provides 1:63,360-scale geologic mapping of the McGrath D-6 Quadrangle, which lies on the eastern edge of the Kuskokwim Mountains, a maturely dissected upland of accordant rounded ridges and broad lowlands. Two highlands, the Candle Hills and Takotna Mountain, are surrounded by sediment-filled troughs of the Takotna, Tatalina, and Kuskokwim River valleys. Elevations range from 300 to 2,600 ft. This report is the result of 40 man-days of reconnaissance geologic mapping in 1977, 1978, and 1982. Bedrock exposures were mapped on foot and by boat. Four-fifths of the quadrangle is covered by Quaternary deposits, which have been subdivided on the basis of a photo-geologic reconnaissance and limited field checking. The report also includes detailed geologic, structural, stratigraphic, and geochronologic data. The complete report, geodatabase, and ESRI fonts and style files are available from the DGGS website: http://doi.org/10.14509/2253.

During the 2017 and 2018 field seasons, geologists from the Alaska Division of Geological & Geophysical Surveys (DGGS) conducted geologic mapping and sampling in the Richardson mining district southeast of Fairbanks, including parts of the Big Delta B-4, B-5, B-6, and C-6 quadrangles. The project area has produced approximately 122,000 ounces of gold, mostly from placer mines, and it includes the Uncle Sam, Montecristo, Richardson, Tower, and Hilltop lode gold exploration properties. The goal of DGGS's work in this area is to conduct a mineral-resource assessment and to build an improved understanding of the area's geology and controls on gold mineralization to facilitate industry exploration targeting. The four 40Ar/39Ar ages reported here are all from samples at or near mid-Cretaceous intrusion-related gold prospects. Skarn hornblende from the Banner Creek area (18RN069) yielded an age of 104.0 +/- 1.6 Ma, which places its formation relatively early in the magmatic history of the district. Fine-grained, holocrystalline granodiorite in the immediate vicinity of the Democrat prospect yielded a biotite cooling age of 96.9 +/- 2.0 Ma (18RN568), slightly older than the white mica age reported for the nearby mineralized rhyolite dike (89.4 +/- 0.7 M). White mica from a sericite-pyrite altered, quartz-feldspar porphyry dike in the Rosa Pass area yielded an age of 87.0 +/- 1.5 Ma (17MBW526). Sericite-altered intrusive rock from drill core at the Lone Tree prospect yielded an age of 93.2 +/- 1.2 Ma (18ET057), somewhat younger than the 40Ar/39Ar age determined for presumably magmatic white mica from a dike at the nearby Naosi prospect on the Montecristo property (100.0 +/- 0.7 Ma). The complete report and digital data are available through the DGGS website: http://doi.org/10.14509/30530.

Mineral Resources Section personnel from the Alaska Division of Geological & Geophysical Surveys (DGGS) carried out a geologic field survey, including mapping and sampling, in the eastern part of the Bonnifield mining district in the Fairbanks A-1 and A-2 and the Healy D-1 and D-2 quadrangles, Alaska, from June 16 through July 18, 2008. The fieldwork provides basic information critical to building an understanding of Alaska's geology and is part of an integrated program of airborne geophysical surveys followed by geologic mapping. Specifically, this work provides geologic context for geophysical surveys conducted in 2006. Interpretation and synthesis of this data has been presented in professional and trade meetings. This report and associated geologic map data are preliminary, have not undergone rigorous peer review, and will be superseded by a subsequent Report of Investigations map and report that will be issued later in 2016. The objective of the eastern Bonnifield project is to produce a 1:50,000-scale geologic map to foster a better understanding of the area's geology and mineral potential. Although DGGS concentrated on mapping the Paleozoic metamorphic rocks that host the volcanogenic massive sulfide deposits (VMS) and Mesozoic igneous rocks associated with Au-Ag-As-Sb veins, we also studied the Tertiary sedimentary section, which could contain coal resources. The geologic map data are available in digital format as ESRI shapefiles.

Geologic map of the Iditarod D-2 and eastern D-3 quadrangles, Alaska, Geologic Report 72, provides 1:63,360-scale geologic mapping of the Beaver Mountains and the surrounding area within the Kuskokwim Mountains uplands. This work was part of a series of reports on the geology, geochemistry, and mineral resources of the Iditarod quadrangle, Alaska. This publication also includes detailed geologic, structural, stratigraphic, and geochronologic data. The complete report, geodatabase, and ESRI fonts and style files are available from the DGGS website: http://doi.org/10.14509/432.

Mineral Resources Section personnel from the Alaska Division of Geological & Geophysical Surveys (DGGS) carried out a geologic field mapping survey in the eastern part of the Bonnifield mining district in the Fairbanks A-1 and A-2 and the Healy D-1 and D-2 quadrangles. The map area contains several significant polymetallic sulfide prospects and transects three metamorphic assemblages, which include the Healy Schist, Keevy Peak Formation, and the Totatlanika Schist. The fieldwork (June 16 through July 18, 2008) augments publically available information critical to building an understanding of Alaska's geology and is part of an integrated program of airborne geophysical surveys followed by geologic mapping. This raw data release provides petrographic thin section observations of 264 samples from the East Bonnifield Project rock collection. Project staff examined thin sections to better distinguish rock units, assist with evaluating the structural and stratigraphic relationships among the various lithologies, and to determine suitability for lithogeochemical and geochronological analysis. Observations in the data tables include lithologic classification, texture, mineral crystal structure, and mineral components. Users can access the complete report and digital data from the DGGS website: http://doi.org/10.14509/30171.

This Alaska Division of Geological & Geophysical Surveys (DGGS) Raw Data File presents 40Ar/39Ar age dating results for a volcanic whole-rock sample, selected minerals from various metamorphic rocks and igneous dikes, and white mica from both metamorphic- and hydrothermal-mineralization-related veins encountered on the southern Seward Peninsula, as well as select minerals from one plutonic rock from Cape Denbeigh, eastern Norton Sound, Alaska. Cooling ages on glaucophane, barroisite, amphibole, paragonite, and biotite from metamorphic rocks range from 84 Ma to 209 Ma. The cooling age of white mica in a possible fault zone is 144 Ma. Cooling ages on late-stage metamorphic minerals (winchite and white mica) from veins cutting foliation in metamorphic rocks range from 117 Ma to 139 Ma. Cooling ages on white mica and adularia from hydrothermal mineralization-related veins range from 105 to 131 Ma. The cooling age for one highly alkalic dike is 107 Ma. Cooling ages for younger mafic dikes range from 80 Ma to 84 Ma. The preferred age on a whole-rock sample of basalt is 0.780 +/- 0.011 Ma. Closure and cooling ages on the Cape Denbeigh granite (hornblende, K-feldspar) are 119 Ma and 113 Ma, respectively.

40Ar/39Ar analyses were performed on igneous rocks from the Ray Mountains area of west-central Alaska. The plutonic samples have ages from about 89 Ma to 109 Ma, while the volcanic samples show ranges from about 30 Ma to 64 Ma. The three volcanic samples fall into two age groups: the younger sample, a basalt, has an age of about 30 Ma, while the two older, rhyolitic samples fall between 58 Ma and 64 Ma. Analyses were performed by the University of Alaska Fairbanks Geochronology Laboratory, and results were reported by Paul Layer and Jeff Benowitz. Products included in this data release are: a summary of sample collection method; the laboratory report, analytical data tables and associated metadata; and plots of the 40Ar/39Ar age spectra, Ca/K ratios, and Cl/K ratios. All components of this data release are available for download on the DGGS website at no charge.