Sulfur Isotopes in Biogenically and Abiogenically Derived Uranium Roll-Front Deposits

摘要

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.