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.

This DGGS report presents 40Ar/39Ar age dating results from selected mafic to intermediate intrusions encountered during a geological and geochemical resource assessment project in the Gulkana, Healy, Mount Hayes, and Talkeetna Mountains quadrangles. Three intrusions returned mid-Cretaceous crystallization ages, while one olivine gabbro returned a Late Triassic crystallization age. Two altered or metamorphosed samples yielded Late Jurassic and Early Cretaceous ages. 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 on the DGGS website doi:10.14509/29119 at no charge.

This report presents 40Ar/39Ar step-heating geochronology results for igneous and metamorphic rocks from the eastern Moran area. Field samples were collected by the DGGS Mineral Resources section during detailed geologic mapping campaigns in 2011. The data provided in this report add significant detail to the thermal history of the Moran area. These new data indicate that the minimum age of prograde metamorphism of Ruby terrane rocks ranges from 148.5 +/- 1.7 to 140.4 +/- 1.7 Ma, and retrograde greenschist metamorphism is 122.6 +/- 2.3 Ma. The retrograde metamorphism is roughly coeval with the age of fabric development parallel to the Kaltag fault (128.3 +/- 1.7) and Tozitna thrust/detachment fault (123.2 +/- 1.5 Ma). The new data also indicate that the Melozitna pluton is composite, with a biotite cooling age of 116.5 +/- 1.3 from coarse-grained granite, while cooling ages for dikes cutting the granite range from 110.1 +/- 1.3 to 102.8 +/- 1.2 Ma. The age of mineralized veins in the area are variable and include 119.0 +/- 1.3 Ma galena veins in the Tozimoran drainage and an interpreted age of 66.5 +/- 2.6 for an auriferous vein from the Monday Creek area, which is synchronous with ages of biotite samples from granite and schist from the Ruby Mining district. The complete report and digital data are available through the DGGS website: http://doi.org/10.14509/30117.

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.

This report presents 40Ar/39Ar step-heating geochronology results for igneous and metamorphic rocks from the Alaska Division of Geological & Geophysical Surveys' (DGGS) geologic mapping project in the Tanacross A-5 and A-6 quadrangles and adjoining areas. Our results indicate the Hona granodiorite pluton intruded the area in the Late Cretaceous between 76 and 71 Ma. We determined Cretaceous metamorphic ages from 126 to 121 Ma for hornblende, biotite, sericite and muscovite from multiple samples of upper greenschist to amphibolite grade rocks. This is consistent with regional argon results that imply this area is in the upper plate of the Yukon Tanana Terrane (Hansen and Dusel-Bacon, 1998). Basaltic andesite dikes returned Late Cretaceous (99.0 +/- 0.5 Ma) and Tertiary (58.4 +/- 0.3 Ma) ages from biotite; these constrain late brittle faulting in the area. Finally, we obtained a Late Cretaceous age (71.5 +/- 0.5) for a basaltic andesite flow; these ages constrain the timing of displacement on some of the faults in the map area. Analyses were performed by the University of Alaska Fairbanks (UAF) Geochronology Laboratory, and results were reported by Jeff Benowitz and Paul Layer. Products included in this data release are a summary of sample collection methods, the laboratory report, analytical data tables and associated metadata, and plots of the 40Ar/39Ar age spectra and Ca/K and Cl/K ratios.

40Ar/39Ar analyses of igneous rocks from the Alaska Highway corridor between Delta Junction and the Canada border show a range of Cretaceous ages from about 68 Ma to about 112 Ma. The 25 samples fall into two broad age groups. The younger group ranges from about 68 Ma to 73 Ma; the older group ranges from a minimum age of about 84 Ma to about 103 Ma. One sample, a mafic dike, yielded an older age of about 112 Ma.

This report presents 40Ar/39Ar step-heating geochronology results from a single outcrop of a mineralized near-trench intrusion exposed in a road cut near Whittier, Alaska. The outcrop is the best road-accessible occurrence of a Tertiary near-trench pluton in the region and is commonly visited by field trips, yet no prior geochronological data from this outcrop have been published. Two chronologically distinct intrusive bodies, the Paleogene Sanak-Baranof belt and the Eshamy-suite, are documented in the upper Cook Inlet area. Geochronological analysis of the two samples described in this report confirms that the Whittier road-cut intrusion and veins are coeval (53.0 +/- 0.3 Ma and 50.9 +/- 0.2 Ma, respectively) with the Sanak-Baranof suite and not the younger, Eshamy plutonic suite. Analyses were performed by the University of Alaska Fairbanks (UAF) Geochronology Laboratory, and results were reported by Jeff Benowitz and Paul Layer. Products included in this data release are a summary of sample collection methods, the laboratory report, analytical data tables and associated metadata, and plots of the 40Ar/39Ar age spectra and Ca/K and Cl/K ratios.

This dataset contains 40Ar/39Ar isotopic data and ages for rocks from the Yukon-Tanana upland of eastern Alaska and the northern Aleutian Range of south-central Alaska. The entire sample suite was collected as part of geological mapping and supporting geochemical and geochronological analysis were conducted from 2013 to 2019 by the Mineral Resources Program of the U.S. Geological Survey (USGS). Legacy bedrock samples archived from earlier USGS field investigations were also analyzed in some cases. Hornblende, muscovite, and (or) biotite were extracted from bedrock samples of igneous and metamorphic rocks and analyzed at the University of Alaska Fairbanks Geochronology Laboratory in order to determine the 40Ar/39Ar age of each mineral phase. The data tables accompanying this data release report the resulting 40Ar/39Ar analytical data and interpreted ages for each mineral separated from a bulk rock sample.

This data set contains 40Ar/39Ar isotopic data and ages for rocks from the Yukon-Tanana upland of eastern Alaska and the northern Aleutian Range of south-central Alaska. The entire sample suite was collected as part of geological mapping and supporting geochemical and geochronological analysis were conducted from 2013 to 2019 by the Mineral Resources Program of the U.S. Geological Survey (USGS). Legacy bedrock samples archived from earlier USGS field investigations were also analyzed in some cases. Hornblende, muscovite, and (or) biotite were extracted from bedrock samples of igneous and metamorphic rocks and analyzed at the University of Alaska Fairbanks Geochronology Laboratory in order to determine the 40Ar/39Ar age of each mineral phase. The data tables accompanying this data release report the resulting 40Ar/39Ar analytical data and interpreted ages for each mineral separated from a bulk rock sample.

40Ar/39Ar geochronology data from the Tanacross and Eagle quadrangles, Alaska, Raw Data File 2023-23, provides 40Ar/39Ar geochronology data from igneous and metamorphic rocks in or adjacent to the DGGS Western Tanacross and Taylor Mountain geologic mapping project areas. Western Tanacross and Taylor Mountain geologic mapping projects aim to produce more accurate and integrated modern geologic maps and supporting datasets to promote mineral resource exploration in eastern Interior Alaska. The Tanacross and southern Eagle quadrangles are within the Yukon Tanana Uplands, which DGGS and USGS identified as having the potential to host deposits of multiple critical minerals, as well as gold, copper, molybdenum, lead, zinc, and silver. Most of the known mineralization in the region is related to Mesozoic-Paleogene magmatism. Igneous rocks intrude a composite metamorphic province that includes parautochthonous North America and the allochthonous Yukon Tanana Terrane, which are multiply deformed and apparently juxtaposed along low-angle faults. Samples were collected (2009, 2018, 2019, 2021) for 40Ar/39Ar geochronology to understand the crystallization ages of igneous rocks, the exhumation history of metamorphic rocks, and the timing of mineralization. The 22 40Ar/39Ar ages reported here include six mineralization/alteration ages, one volcanic crystallization age, seven metamorphic cooling ages from the Yukon-Tanana terrane (YTT), and eight metamorphic cooling ages from parautochthonous North American rocks (pNA). The analytical data tables associated with this report are available in digital format as comma-separated value (CSV) files. These data are available from the DGGS website: http://doi.org/10.14509/31085.