ABIOTIC CONTROLS ON COPPER ISOTOPE FRACTIONATION DURING THE DISSOLUTION OF COPPER SULFIDE MINERALS

Open Access
Author:
Wall, Andrew James
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 30, 2011
Committee Members:
  • Peter J Heaney, Dissertation Advisor
  • Peter J Heaney, Committee Chair
  • Susan Louise Brantley, Committee Member
  • Matthew Scott Fantle, Committee Member
  • William D Burgos, Committee Member
Keywords:
  • Cu isotopes
  • chalcocite
  • bornite
  • copper sulfides
  • isotope fractionation
  • synchrotron X-ray diffraction
Abstract:
Stable isotope measurements have long been used as a geochemical tool in the Earth sciences, and recent advances in analytical techniques have added intermediate mass stable isotopes (e.g. Cu, Zn, Fe, Ni) to this suite of interpretive methods. The Cu isotope system offers particularly high potential to solve geologic problems due to its large natural isotopic variation (~12 &permil;). However, the factors that control the fractionation of Cu isotopes, especially during the dissolution of Cu-sulfide minerals, remain incompletely resolved. <br><br> In this dissertation, I explore abiotic controls on Cu isotope fractionation during the dissolution of Cu- sulfide minerals by combining in situ time-resolved X-ray diffraction (TR-XRD) coupled with stable isotope analysis. As a foundational part of this study, I modified a pre-existing design for a TR-XRD flow-through cell in order to remove any metal content and to allow for automated sampling of the eluate fluid. The resulting device (described in Chapter 1) allowed us to correlate Cu isotope fractionation with changes in crystal structure during Cu-sulfide dissolution. <br><br> TR-XRD analyses of oxidative dissolution of chalcocite (Cu<sub>2</sub>S) and bornite (Cu<sub>5</sub>FeS<sub>4</sub>) enabled the development of rate equations that describe these reactions and the identification of the reaction sequences as Cu leached from the solid phase (Chapter 2). During chalcocite dissolution, XRD analysis revealed mineral transformations involving the following phases: djurleite (Cu<sub>1.94</sub>S), roxbyite (Cu<sub>1.75</sub>S), yarrowite (Cu<sub>1.13</sub>S), and covellite (CuS). Similarly, the dissolution of bornite by ferric sulfate solutions also produced changes to the mineral structure: a contraction of the bornite unit-cell volume as “non-stoichiometric bornite” formed. XRD results demonstrated that the structure of non-stoichiometric bornite is similar to mooihoekite (Cu<sub>2.25</sub>Fe<sub>2.25</sub>S<sub>4</sub>). These results clarified the reaction sequences that occur when ferric sulfate solutions dissolve chalcocite and bornite. <br><br> By combining time-resolved diffraction data of these Cu sulfide dissolution reactions with real-time sampling and isotopic analysis of the eluates, we were able to discern structural controls on Cu isotope fractionation during dissolution. As described in Chapter 3, during the initial stages of bornite oxidative dissolution by ferric sulfate (<5 mol% of total Cu leached), dissolved Cu was enriched in isotopically heavy Cu (<sup>65</sup>Cu) relative to the solid, with an average apparent isotope fractionation (&Delta;<sub>aq- min</sub> = &delta;<sup>65</sup>Cu<sub>aq</sub> - &delta;<sup>65</sup>Cu</sup><sub>min</sub>o, where &delta;<sup>65</sup>Cu<sub>aq</sub> is the isotopic composition of the leached Cu and &delta;<sup>65</sup>Cu<sub>min</sub>o is the isotopic composition of the beginning mineral powder) of 2.20 ± 0.25‰ (Chapter 3). When >20 mol% Cu was leached from the solid, the difference between the Cu isotope composition of the aqueous and mineral phases approached zero, with &Delta;<sub>aq-min</sub>o values ranging from -0.21 ± 0.61‰ to 0.92 ± 0.25‰. We propose that the decrease in the apparent isotope fractionation as the reaction progressed resulted from distillation of isotopically heavy Cu (<sup>65</sup>Cu) during dissolution or isotope effects associated with the formation of a leached layer on the surfaces of bornite particles. <br><br> Similarly, during the initial stages of chalcocite oxidative dissolution (Chapter 4), leached fluids were enriched in heavy Cu (<sup>65</sup>Cu) with &delta;<sup>65</sup>Cu values of ~3‰. As the dissolution reaction progressed and chalcocite transformed to covellite, the leached Cu became isotopically lighter and &delta;<sup>65</sup>Cu values of the leachate decreased to as low as -3.01‰. Isotope box models are consistent with two isotope effects that influence the degree of fractionation observed during the reaction: one due to oxidation (&alpha; ~ 1.003) and another due to changes in bonding during mineral transformations (&alpha; ~ 1.001). These results may be useful in interpreting the extent of weathering in Cu ore bodies and the potential for Cu release from acid mine drainage environments.