Fe and Cu isotope fractionation between chalcopyrite and dissolved metal species during hydrothermal recrystallization: An experimental study at 350°C and 500 bars

Abstract:

The equilibrium Fe and Cu isotope fractionation factor between chalcopyrite and dissolved metal species was determined under hydrothermal conditions at 350°C and 500 bars. The experiments took advantage of gold-cell reaction technology, allowing time-series sampling of solution during the hydrothermal recrystallization of chalcopyrite over 3000 hours. One of the recrystallization experiments utilized an anomalous ⁵⁷Fe spike in solution to quantify the degree and rate of isotopic exchange towards equilibrium between mineral and fluid reservoirs. The time-series ⁵⁷Fe spike data suggests that chalcopyrite exchanges rapidly with dissolved Fe and Cu in solution and the isotopic fractionation between each metal-bearing reservoir throughout reaction progress, upon dissolution and recrystallization, represents close to equilibrium conditions. The isotope data indicate that the equilibrium fractionation between chalcopyrite and dissolved Fe and Cu at 350°C, Δ⁵⁶Fe_Cpy-Fe(aq), is 0.129±0.171‰ and Δ⁶⁵Cu_Cpy-Fe(aq), is -0.201±0.341‰ (2σ), and are in good agreement with recent theoretical equilibrium predictions. Comparison of the experimental data from this study with conjugate chalcopyrite and dissolved Fe and Cu pairs from a variety of hydrothermal systems along the mid-ocean ridge system indicates that chalcopyrite precipitates and recrystallizes at isotopic equilibrium with the fluid during cooling upon ascent to the seafloor. The rapid exchange between the mineral and fluid metal-reservoirs suggests that chalcopyrite effectively records the isotopic composition of the coexisting hydrothermal fluid during the evolution of hydrothermal systems. In addition, the pyrite-chalcopyrite equilibrium Fe isotope fractionation, Δ⁵⁶Fe_Pyr-Cpy, at 350°C is quantified by combination of pyrite-Fe²⁺(aq) equilibrium fractionation data from Syverson et al., [2013] with chalcopyrite-Fe²⁺(aq) from this study, resulting in a fractionation of 0.861±0.337‰ (2σ). The empirical sulfide mineral-mineral fractionation determined empirically is consistent with mineral-mineral theoretical predictions and can be coupled with mineral-fluid isotope fractionation systematics to differentiate between different ore depositional processes, of which, chalcopyrite is intimately is involved.