Ore assemblages in uranium roll-front deposits are highly variable and heavily dependent on Eh/pH conditions. Sulfur isotopes in pyrite traditionally have been employed to distinguish between biogenic or abiogenic redox pathways as drivers of roll-front propagation. However, the extent of and constraints on bacterial productivity have never been quantified, nor have the chemical conditions imposed by either primary formation mechanism. Moreover, this approach implicitly assumes that deposits form via one process or the other and disregards the possibility that both processes participate simultaneously in generating some orebodies. In this study, we analyzed sulfur isotopes from pyrite coprecipitated with uranium in two Wyoming roll-front deposits: Lost Creek and Willow Creek Mine Unit 10. The results document contrasting isotopic fractionation that correlates with pyrite morphology. Both deposits evolved with both abiogenic and biogenic redox mechanisms as active contributors to ore formation. In the past, bimodal fractionation behavior with pyrite morphology has been attributed to distinct temporal episodes of pyrite formation, driven by either a change in redox mechanism or multiple independent fluid events with unique isotopic signatures. However, neither explanation is appropriate for the isotopic trends identified in this study, where the two pyrite morphologies appear coeval in both deposits. Moreover, the contemporaneous formation of both pyrite morphologies cannot occur under the same conditions by the same precipitation mechanism because of the difference in their free energies of formation. The data suggest a third alternative in which pyrite morphology correlates to its biogenic or abiogenic mode of formation. Given the isotopic composition of pre-ore pyrite, sulfur isotope fractionation trends within the ore zone can be applied to establish prolificacy of bacteria and chemical conditions of the ore-forming solution.
In both study sites, framboidal pyrite occurred as the primary by-product of sulfur-reducing bacteria, and the corresponding fractionation pattern constrains the sulfur availability and bacterial productivity. Euhedral to anhedral pyrite precipitated from abiogenic redox, the sulfur fractionation recording Eh/pH gradients during ore evolution. At Lost Creek, framboidal pyrite produced δ34S values from –50.8 ± 0.5‰ to +142.8 ± 0.3‰, while subhedral pyrite ranged from –68.1 ± 0.4‰ to +33.8 ± 0.3‰ with δ34S values increasing toward the barren, unaltered contact. Pre-ore pyrite at Lost Creek ranged from –0.8 ± 0.5‰ to +70.6 ± 0.3‰. δ34S values from biogenically derived pyrite at Lost Creek indicate a closed system with limited sulfate availability and a slow rate of bacterial reduction, implying restricted bacterial activity. Abiogenic fractionation behavior indicates a system driven by an Eh drop under neutral or basic pH conditions, and pyrite distribution across the roll identifies abiogenic pyrite recycling as the dominant redox mechanism at Lost Creek. At Willow Creek Mine Unit 10, framboidal pyrite ranged from –32.5 ± 0.4‰ to +68.2 ± 0.4‰, and subhedral pyrite ranged from –45.1 ± 0.4‰ to +5.4 ± 0.4‰. The subhedral pyrite δ34S values initially increased into the center of the roll and subsequently decreased again approaching the barren, unaltered contact. Pre-ore pyrite ranged from –48.1 ± 0.4‰ to +15.6 ± 0.5‰. Willow Creek Mine Unit 10 biogenically produced δ34S values show minimal fractionation from pre-ore pyrite, indicating an open system with abundant sulfate and rapid reduction from prolific bacterial activity. The abiogenic trends indicate an Eh drop and low pH at the barren, altered contact progressively neutralized across the orebody. This correlates to the anticipated Eh/pH gradients in a system dominated by biogenic redox.