GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 181-11
Presentation Time: 10:15 AM


HOUGH, Gretchen, Department of Geology and Geophysics, University of Wyoming, 1000 E University Ave, Dept 3006, Laramie, WY 82071, SWAPP, Susan M., Department of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 University Avenue, Laramie, WY 82071 and FROST, Carol D., Department of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 E. University Ave, Laramie, WY 82071,

Uranium mineralogy in roll-front deposits is highly variable and heavily dependent on Eh/pH conditions during ore formation. In this study we utilized pyrite as a redox proxy to constrain chemical evolutions of ore formation in three Wyoming roll-front deposits: LC, MU10, and MU7. Both bacterial and chemical processes may contribute to ore formation. Bacterial δ34S trends reflect bacterial productivity and sulfur availability; chemical δ34S behaviors record Eh/pH gradients during ore evolution. We distinguish the biogenic or abiogenic mode of formation through pyrite morphology. Differences in δ34S values between morphologies are evident in all three deposits.

1) The LC deposit propagated through chemical pyrite recycling in a buffered solution at near-neutral pH. Bacterial δ34S displayed an extreme range, exhibiting Rayleigh fractionation and indicating limited sulfate availability and restricted bacterial activity. Chemical δ34S identified ore precipitation driven by an Eh drop across the roll in a buffered solution at near-neutral pH as supported by the pervasive appearance of calcite cement. Ti4+ was more readily available than V5+ resulting in brannerite as the primary U-phase.

2) MU10 was controlled by bacterial redox evident from bacterial δ34S indicating prolific bacterial activity. Chemical δ34S trends identified decreasing Eh and increasing pH across the roll matching chemical conditions predicted for bacterial redox. The acid neutralization experienced by the semi-oxidized ore forming fluid resulted in tyuyamunite precipitation.

3) MU7 underwent a shift from bacterial to chemical redox during deposit evolution. Bacterial δ34S reflected prolific bacterial activity. Later oxidation of excess pyrite exhibited pH control within the deposit and drove late-stage ore precipitation under ultra-low pH, producing high δ34S values in late-stage chemical pyrite through a renewed influx of 34S from chemical sulfate reduction. Tyuyamunite formed early in deposit history under the influence of bacterial redox; coffinite and uraninite precipitated with the final acid-front.

Our results show that bacterial and chemical redox processes can be identified from δ34S analysis of different pyrite morphologies and strongly influence the resultant uranium mineralogies.