Aspects Ore Genesis at the Lost Creek Uranium Deposit, Sweetwater County, Wyoming

Robert Gregory, Susan Swapp, B. Ronald Frost, Jonathan McLaughlin, and Carol Frost

Compositional and isotopic studies of core samples from barren altered and barren unaltered arkoses bracketing a well-defined uranium roll front deposit in the Lost Creek Uranium Project have been examined to address fundamental questions of ore deposit genesis includingdetrital sources, nature of ground water / rock interactions associated with ore genesis, and nature of reductant. The Lost Creek uranium ore deposit lies in arkoses of the Eocene Battle Spring Formation in the northeastern Great Divide Basin, Sweetwater County, Wyoming. The source for uranium in these deposits is inferred to have been either the Archean basement rocks exposed in Laramide uplifts or the Tertiary volcanic rocks postdating those uplifts. Uranium mineralization may be strongly associated with other groundwater/mineral reactions, especially the formation of clays. Finally, inferred reductants for the deposition of this uranium include buried organic matter, detrital sulfides and oxides, mixing with reduced fluids, and/or biogenic activity. Zircon was extracted from the heavy mineral suite (>3.2 s.g.) and remaining minerals were identified optically and by energy dispersive X-ray spectroscopy. Zircon from both regimes show a consistent age of 2.6-2.8 Ga. Older ages, ca. 3.2 ñ 3.4 Ga, which are common in the adjacent Granite Mountains, were not found in an isotopic study of over 500 zircon grains from Lost Creek drill cores. Framboidal pyrite, a texture that is diagnostic of a biogenic origin, occurs in some collected samples of barren altered rock but is absent from unaltered rock, where only sparse cubic pyrite has been observed. Because very little relict organic matter occurs in either altered or unaltered rock, we suggest that bacterial reduction of H2S may have played an important role in uranium deposition in this deposit.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012