Copper isotope variability in acid ferric sulfate dissolution of natural djurleite (Cu_1.94S)

The resurgence of interest in transition metal isotope chemistry is the result of improved instrument precision and sensitivity. This has been achieved using multiple collector inductively-coupled mass spectrometry (MC-ICPMS) which allows simultaneous measurement of several isotope ratios in low concentrations. Copper isotope investigations have benefited from this improved technology. This study has demonstrated that appreciable Cu isotope ratio variability accompanies progressive acid ferric sulfate leaching of natural djurleite, Cu1.94S. Copper-bearing solutions from early stages of reaction (less than 10% cumulative copper dissolution) are enriched in ⁶⁵Cu (δ⁶⁵Cu = 1.2 ± 0.04%) compared to the initial djurleite experimental feed material (δ⁶⁵Cu = -0.05 ± 0.04%). With continued reaction, ⁶⁵Cu of the copper solutions steadily decreases, approaching the isotopic value of the original djurleite. As Cu:S ratio of the remaining solid material decreases, mineralogy of the residuum progresses through a series of non-stoichiometric copper sulfide phases until reaching a kinetic barrier at about 44% copper dissolution. XRD analyses has verified that the mineralogy at this boundary is dominated by the mineral yarrowite, Cu _1.12S. From djurleite to yarrowite, the average Cu-S bond length increases, resulting in an increased proportion of 4-coordinated Cu-S polyhedra over 3-coordinated geometries. This trend suggests that Cu isotope fractionation may be linked to the preference of the heavier ⁶⁵Cu isotope for the lower coordinated configuration.