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Isotopic evidence for microbial activity during supergene oxidation of a high-sulfidation epithermal Au-Ag deposit

¹ Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada

The light stable isotope chemistry of barite and acanthite in the Pierina Au-Ag deposit, Peru, provides strong support for microbial involvement at the economically critical supergene oxidation stage. Early hypogene barite yields ³⁴S values of 23.6 to 28.5 and ¹⁸O values of 5.8 to 10.9, and was precipitated with the original precious metal-enriched sulfide assemblage. In contrast, late barite, deposited during formation of supergene goethite and hematite, the main ore-hosts, yields ³⁴S and ¹⁸O values of 1.4 to 14.2 and –2.8 to 4.7, correlated mutually and with the ³⁴S values of coprecipitated acanthite (³⁴S = 0.4 to 3.9). Low ³⁴S values of barite and acanthite record sulfide mineral oxidation by meteoric waters. Increasing ³⁴S and ¹⁸O values of barite and ³⁴S values of acanthite indicate enrichment of the sulfate reservoir in ³⁴S and ¹⁸O, reflecting the preferential utilization of light isotopes by microbes during aqueous sulfate reduction. Continued sulfate reduction locally reduced supergene fluids, remobilizing iron and locally destroying the goethite-hematite assemblage. Precious metals were released but reacted with sulfide in the reduced waters to form Au-rich acanthite.

Key Words: geomicrobiology • supergene processes • barite • high sulfidation • epithermal processes • stable isotopes

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