Sulfur Isotope Fractionation in Bacterially and Chemically Controlled Roll-Front Deposits
Gretchen Ann Hough1, Susan Swapp
2 and Carol D Frost
1, (1)University of Wyoming, Laramie, WY, United States, (2)University of Wyoming, Laramie, United States
Contact First Author: Gretchen Ann Hough; ghough@uwyo.edu
English Abstract:
Redox mechanisms driving uranium precipitation in roll-front deposits have been debated for decades with minimal evidence confirming either bacterial or chemical models. In order to resolve the issue, naturally occurring deposits that represent each endmember need to be identified and compared. Two deposits in Wyoming have been selected as likely redox endmembers. One deposit contains abundant organic material; the other has almost none. Organic material is essential to host a sulfur reducing bacteria colony, and its presence would favor bacterial redox. However, without organic material present, the environment would be inhospitable to bacteria and chemical redox would be favored. Several other distinctions between the two deposits are also striking: associated with abundant organic material is abundant pyrite precipitation and locally uranium vanadates; in deposits lacking appreciable organic material only minor pyrite is present and the predominant uranium mineralogy is uraninite and coffinite. Distinguishing between redox mechanisms may therefore be key in understanding the wide variations in uranium mineralogy.
Sulfur isotope fractionation in ore-zone pyrite should vary with redox mechanism. Chemical redox should produce lower δ34S than biogenic redox processes. Because of the constant addition of sulfate to the system, bacterial redox may produce high δ34S values. In contrast, oxidation of pyrite as it is cycled across the roll-front continuously produces 34S-enriched sulfate that is removed from the system, leaving ore zone pyrite with low δ34S. Further, fractionation in a chemical deposit would show a trend of decreasing δ34S across the ore zone from unaltered to altered sandstone. Cores from the Wyoming deposits have been collected that span the ore horizon. Sulfur isotope values will be analyzed across the roll-front for each deposit using SIMS to confirm redox drivers and expected trends to positively differentiate between bacterial and chemical redox.
