Bedrock geologic map of the northern Fairbanks mining district, Circle Quadrangle, Alaska

Bedrock geologic map of the northern Fairbanks mining district, Circle Quadrangle, Alaska, Preliminary Interpretive Report 2022-2, presents a 1:50,000-scale bedrock geologic map of the northern Fairbanks mining district, covering parts of the Circle A-4, A-5, B-4, and B-5 quadrangles. The 188-square-mile study area is 55 direct miles northeast of Fairbanks, straddling the Steese Highway between miles 66 and 85. The field area is coincident with the northern tip of the Fairbanks mining district, Alaska's largest historic producer at just less than 17 million troy ounces of gold. This area is within the northeast-oriented trend between plutonic-related gold mineralization in the central Fairbanks mining district and placer gold deposits in the northwest Circle mining district. During June 2007, personnel from the Alaska Division of Geological & Geophysical Surveys (DGGS) and the University of Alaska Fairbanks spent approximately 189 person-days conducting fieldwork in the northeast Fairbanks area. Geologic units are defined by field observations and analysis of samples collected at more than 2,222 stations. Unit descriptions are also based on the petrographic examination and modal analysis of 301 thin sections (this study), 116 thin sections collected in the Circle Quadrangle by the U.S. Geological Survey from 1979-1981, and 26 thin sections from the 1986 Lime Peak-Mt. Prindle project. Metamorphic rocks and younger, unmetamorphosed igneous rocks were primarily defined by chemical composition, examination of hand samples, and petrography. Forty-four rock samples were analyzed for major- and minor-oxide by a commercial analytical laboratory and trace elements by the University of Alaska Fairbanks. These data are used to suggest possible protoliths of metamorphosed and altered rocks, identify types of alteration, and assign trace element-indicated tectonic settings to igneous and metamafic rocks. Non-carbonate carbon in 16 samples was measured by a commercial analytical laboratory. Garnet-biotite geothermometry and garnet-plagioclase-muscovite-biotite equilibria geobarometry were conducted on 16 samples to assess the metamorphic history of the units. Five 40Ar/39Ar ages were used to constrain the timing of igneous events, mineralization, and metamorphism in the northeast Fairbanks area. One U-Pb detrital zircon age provides a maximum age of the metamorphosed sedimentary rocks and for regional unit correlations. Total field magnetic and electromagnetic geophysical data aided our bedrock mapping, especially in areas covered by vegetation and unconsolidated Quaternary deposits. The complete report, geodatabase, and ESRI fonts and style files are available from the DGGS website: http://doi.org/10.14509/30891.

Tags: actinolite, airborne geophysical/geological mineral inventory, amphibolite, analyses, antimony, arsenopyrite, bear creek, bedrock, calcite, cambrian, charity creek, chatanika river, chlorite, circle mining district, conglomerate, cretaceous, crooked creek, deep creek, dggs, diorite, early cretaceous, epidote, fairbanks mining district, faith creek, feldspar, frozenfoot creek, garnet, gems db l3 compliant, gems db v1pt1, geologic, geologic map, gold, granite, granodiorite, graphite, harrington fork, hematite, hope creek, hornblende, horse creek, idaho creek, igneous, late proterozoic, lode, magnetite, marble, mcmanus creek, metamorphic, metamorphic minerals, metamorphic rocks, metamorphism, mica, mineral deposit, minerals, mining, montana creek, monzonite, mount ryan, mudstone, placer, placer gold, plagioclase, polar creek, pool creek, proterozoic, pyrite, pyrrhotite, quartz, quartzite, rocks, ruby creek, sand, schist, sedimentary, sericite, smith creek, sourdough creek, staurolite, steese highway, stibnite, swamp saddle, tertiary, tough luck creek, tourmaline, yukon-tanana upland, zephyr creek, zircon
Organization: State of Alaska