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The Gas Hills area is near the geographic center of Wyoming, in eastcentral Fremont County and west-central Natrona County. Structurally it is situated at the south-central margin of the Wind River Basin and on the north flank of the Sweetwater Arch. The northwest-trending Gas Hills anticline (Dutton Basin anticline) and associated Laramide folds were deeply dissected in pre-Wind River (early Eocene) time, and about 900 feet of arkosic sediments of the Wind River formation were deposited with angular discordance on rocks ranging in age from Cambrian to Late Cretaceous. Resting with apparent conformity on the Wind River formation are about 800 feet of tuffaceous sediments of middle and late Eocene, Oligocene, and Miocene age. During a period of general regional uplift in post-Miocene time, the Sweetwater Arch subsided along normal faults of large displacement and the Gas Hills area was tilted gently toward the south. The cycle of erosion which accompanied this uplift has continued to the present. Uranium deposits occur in the upper coarse-grained facies of the Wind River formation with the exception of two minor occurrences in subjacent Mesozoic rocks. Meta-autunite and an unnamed yellow uranium phosphate are common ore minerals, and they generally occur in irregular deposits in OFFICIAL USE ONLY OFFICIAL USE ONLY 6 reddish-brown to gray arkosic sandstones of fluviatile origin. Uraniniteliebigite-pyrite ore has been mined in one locality. Water from seeps, springs and wells in the Oligocene and Miocene rocks generally contains more than normal amounts of uranium (7 to 33 parts per billion), and water from areas near uranium deposits generally c-ntains even greater amounts of uranium (20 to 1300 parts per billion). Possible controls for ore deposition include (1) changes in permeability at lithologic or fault contacts of the coarse-grained Wind River sediments with fine-grained rocks; (2) character and relief of the pre-Wind River erosion surface; (3) presence of potential precipitating agents such as carbonaceous material, iron oxide, calcium carbonate, or natural gas; (h) presence of channel deposits in the White River formation which may have provided direct access of mineralizing solutions into the underlying host rocks of the Wind River formation.

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