SULFUR ISOTOPE FRACTIONATION IN BACTERIALLY AND CHEMICALLY CONTROLLED ROLL-FRONT DEPOSITS

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.