Cu isotope fractionation during bornite dissolution: An in situ X-ray diffraction analysis

Andrew J. Wall, Ryan Mathur, Jeffrey E. Post, Peter J. Heaney

Research output: Contribution to journalArticlepeer-review

38 Scopus citations

Abstract

Low-temperature ore deposits exhibit a large variation in δ65Cu (~12%), and this range has been attributed, in part, to isotope fractionation during weathering reactions of primary minerals such as chalcocite and chalcopyrite. Here, we examine the fractionation of Cu isotopes during dissolution of another important Cu ore mineral, bornite, using a novel approach that combines time-resolved X-ray diffraction (XRD) and isotope analysis of reaction products. During the initial stages of bornite oxidative dissolution by ferric sulfate (<5mol% of total Cu leached), dissolved Cu was enriched in isotopically heavy Cu (65Cu) relative to the solid, with an average apparent isotope fractionation (Δaq-min65Cuaq65Cumin0) of 2.20±0.25%. When >20mol% Cu was leached from the solid, the difference between the Cu isotope composition of the aqueous and mineral phases approached zero, with Δaq-min0 values ranging from -0.21±0.61% to 0.92±0.25%. XRD analysis allowed us to correlate changes in the atomic structure of bornite with the apparent isotope fractionation as the dissolution reaction progressed. These data revealed that the greatest degree of apparent fractionation is accompanied by a steep contraction in the unit-cell volume, which we identified as a transition from stoichiometric to non-stoichiometric bornite. We propose that the initially high Δaq-min values result from isotopically heavy Cu (65Cu) concentrating within Cu2+ during dissolution. The decrease in the apparent isotope fractionation as the reaction progresses occurs from the distillation of isotopically heavy Cu (65Cu) during dissolution or kinetic isotope effects associated with the depletion of Cu from the surfaces of bornite particles.

Original languageEnglish (US)
Pages (from-to)62-70
Number of pages9
JournalOre Geology Reviews
Volume42
Issue number1
DOIs
StatePublished - Nov 2011

All Science Journal Classification (ASJC) codes

  • Geology
  • Geochemistry and Petrology
  • Economic Geology

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