Robert W. Gregory, Senior Geologist at the Wyoming State Geological Survey, and Vice-Chair (Government) for the Uranium (Nuclear and REE) Committee of the AAPG's Energy Minerals Division, reports that the Gas Hills uranium district is the most productive in Wyoming and one of the most important in the United States. The district, discovered in 1953, produced more than 111 million pounds of uranium concentrate (U3O8) between 1954 and 1988 when it ceased production due to declining prices. Production derived from five different mills, three in the Gas Hills, and one each in Riverton and Jeffrey City.
Uranium in the Gas Hills district was mined from roll-front deposits in the Puddle Springs Arkose Member of the Eocene Wind River Formation. The Puddle Springs Arkose Member comprises a vast system of arkosic conglomerates and sandstones derived from erosion of Precambrian rocks in the Granite Mountains, which were uplifted and exposed during the Laramide orogeny. Post deposition, oxygenated, uranium-bearing groundwater flowed northward into the Wind River Basin, promoting mineralization in this complex of very porous and permeable alluvial fans.
The resultant uranium roll-fronts form sub-vertical, en echelon stacks concentrated along interfaces in elongate frontal systems or ore trends between alluvial fans. The highest grade uranium ore is generally concentrated in three main ore trends in the west, central, and eastern parts of the district. Estimates indicate a remaining resource of more than 50 million pounds of equivalent uranium oxide concentrate (eU3O8).
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