We also present data for the vapour-liquid fractionation of the stable isotopes of copper (65Cu/63Cu) in the CuCl-NaCl-H2O system. During closed-system equilibrium partitioning, the isotopic compositions of the vapour and liquid sample pairs were generally equal within uncertainty (although the ¦¤65CuL-V or most pairs were nominally positive). However, from the compositions of the staring solutions to those of the final, lowest-pressure V and L sample pairs extracted from the autoclave, a shift to heavier values of ¦Ä65Cu was seen. Specifically, between the starting compositions and those of the lowest-pressure vapour samples, the differences in ¦Ä65Cu values were 0.16¡ë, 0.69¡ë and 0.10¡ë (all ¡À0.07¡ë) at 350, 400 and 450 ¡ãC, respectively. This compositional shift is roughly proportional to the volume of vapour extracted between sample sets (6-35 % of the total bulk fluid removed as vapour), indicating that the V-L fractionation of 65Cu/63Cu may be described by a Rayleigh distillation process.
These results indicate that in a boiling, porphyry-type ore-forming environment, elemental Cu will fractionate such that the concentration in the vapour is less than that of the liquid, but will more readily enter the vapour phase in sulphide-bearing fluid systems (although the Cu concentration in the vapour remains less than that in the coexisting liquid). In a closed system where the vapour and liquid remain in coexistence after boiling, Cu isotopes will exhibit conservative fractionation, thus the V and L will preserve the isotopic signature of the fluid source. However, in a structurally open system in which periodic vapour removal occurs, Rayleigh-type fractionation may give the evolving vapour a progressively lighter ¦Ä65Cu than the residual fluid near the source. It follows that vapour and liquid fluid inclusions derived from an evolved magmatic vapour and trapped at shallower depths (e.g., epithermal vein systems) will likely have lighter ¦Ä65Cu values than their deeper magmatic counterparts and the source of the ore fluids.