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

Research Article| March 01, 2019 Sulfur Isotopes in Biogenically and Abiogenically Derived Uranium Roll-Front Deposits Gretchen Hough; Gretchen Hough 1University of Wyoming, Department of Geology and Geophysics, 1000 E. University Ave., Laramie, Wyoming 82071 Search for other works by this author on: GSW Google Scholar Susan Swapp; Susan Swapp 1University of Wyoming, Department of Geology and Geophysics, 1000 E. University Ave., Laramie, Wyoming 82071 †Corresponding author: email, swapp@uwyo.edu Search for other works by this author on: GSW Google Scholar Carol Frost; Carol Frost 1University of Wyoming, Department of Geology and Geophysics, 1000 E. University Ave., Laramie, Wyoming 82071 Search for other works by this author on: GSW Google Scholar Mostafa Fayek Mostafa Fayek 2University of Manitoba, Department of Geological Sciences, 240 Wallace Building, 125 Dysart Rd., Winnipeg, Manitoba R3T 2N2, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Gretchen Hough 1University of Wyoming, Department of Geology and Geophysics, 1000 E. University Ave., Laramie, Wyoming 82071 Susan Swapp 1University of Wyoming, Department of Geology and Geophysics, 1000 E. University Ave., Laramie, Wyoming 82071 Carol Frost 1University of Wyoming, Department of Geology and Geophysics, 1000 E. University Ave., Laramie, Wyoming 82071 Mostafa Fayek 2University of Manitoba, Department of Geological Sciences, 240 Wallace Building, 125 Dysart Rd., Winnipeg, Manitoba R3T 2N2, Canada †Corresponding author: email, swapp@uwyo.edu Publisher: Society of Economic Geologists Accepted: 07 Jan 2019 First Online: 27 Mar 2019 Online Issn: 1554-0774 Print Issn: 0361-0128 © 2019 Economic GeologyEconomic Geology Economic Geology (2019) 114 (2): 353–373. https://doi.org/10.5382/econgeo.2019.4634 Article history Accepted: 07 Jan 2019 First Online: 27 Mar 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gretchen Hough, Susan Swapp, Carol Frost, Mostafa Fayek; Sulfur Isotopes in Biogenically and Abiogenically Derived Uranium Roll-Front Deposits. Economic Geology 2019;; 114 (2): 353–373. doi: https://doi.org/10.5382/econgeo.2019.4634 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.