Geology of the Sleetmute A-5, A-6, B-5, and B-6 quadrangles, southwestern Alaska

Geology of the Sleetmute A-5, A-6, B-5, and B-6 quadrangles, southwestern Alaska, Professional Report 99, presents a compilation geologic map of the Chuilnuk Mountains and Kiokluk Mountains in the Kuskokwim Mountains area of southwestern Alaska. Geologic mapping of the Sleetmute quadrangles in southwestern Alaska utilized detailed topographic maps, aerial photographs, and field data from U.S. Geological Survey studies, with key contributions published in 1984 and supplemented by Alaska DGGS data files. The region's geology spans from the Late Proterozoic to the Holocene, with the oldest rocks informally grouped under the broadly defined Holitna Group. Most of the area is underlain by the Kuskokwim Group, a thick sequence of marine turbidites primarily dated to the Late Cretaceous. Overlying this is the Holokuk Basalt, a Late Cretaceous volcanic unit associated with granodiorite intrusions, followed by younger dikes and olivine basalt dated at 38 Ma. Quaternary deposits record four glacial advances, and the area exhibits complex folding and faulting tied to the Denali-Farewell fault system. The lower Kuskokwim region is rich in mineral resources, including placer deposits of gold and tungsten, and vein and greisen deposits containing mercury, antimony, tin, and other metals. Hydrothermal quartz veins near Fortyseven Creek host gold and scheelite, likely formed by a rhyolite intrusion. Mercury and antimony deposits, such as the Mountain Top site, occur in sheared and brecciated rocks, possibly linked to hot spring activity. Tin is found in quartz-tourmaline veins and stockworks near pluton contacts, with mineralized zones showing distinct downstream geochemical signatures.The complete report, geodatabase, and ESRI fonts and style files are available from the Alaska Division of Geological & Geophysical Surveys website: http://doi.org/10.14509/2280.

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Organization: State of Alaska