An experimental study of the elemental and isotopic fractionation of copper between aqueous vapour and liquid to 450°C and 400bar in the CuCl–NaCl–H2O and CuCl–NaHS–NaCl–H2O systems

• Published in: Geochimica et cosmochimica acta Vol. 94; pp. 199 - 216
• Main Authors: Rempel, Kirsten U., Liebscher, Axel, Meixner, Anette, Romer, Rolf L., Heinrich, Wilhelm
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2012
• Subjects: autoclaves; boiling; copper; distillation; isotope fractionation; partition coefficients; sodium chloride; stable isotopes; sulfur; uncertainty; vapors
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2012.06.028

In the magmatic-hydrothermal ore-forming systems typical of porphyry Cu deposits, the separation of aqueous vapour and liquid by boiling promotes the vapour–liquid (V–L) fractionation of copper and other ore components. To better understand the processes controlling this behaviour, this study investigates the V–L equilibria in the CuCl–NaCl–H2O system at 350, 400 and 450°C and pressures from 140 to 400bar, as well as the effect of the addition of NaHS to the experimental system at 400°C. The V–L partitioning of NaCl in the sulphide-absent experiments corresponds very well to previous data for H2O–NaCl, whereas the addition of NaHS caused a narrowing of the vapour–liquid coexistence field. For copper, the partition coefficients (DCuV/L) measured in S-absent fluids were higher than those measured in earlier work, particularly at 350°C, but the DCuV/L for S-bearing fluids were typically lower than those from previous studies in which higher S concentrations were employed. As with previous work, however, our results are also in accordance with an increase in DCuV/L in the presence of reduced sulphur, from 0.007±0.006 – 0.07±0.1 in S-free samples at 248.3–274.6bar, to 0.07±0.01 – 0.22±0.11 in S-bearing samples at 242.4–272.7bar. The values of DCuV/L remained below unity at all conditions, in keeping with previous experimental studies of the V–L fractionation of Cu. 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 Δ65CuL–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 δ65Cu was seen. Specifically, between the starting compositions and those of the lowest-pressure vapour samples, the differences in δ65Cu values were 0.16‰, 0.69‰ and 0.10‰ (all ±0.07‰) at 350, 400 and 450°C, 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 δ65Cu 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 δ65Cu values than their deeper magmatic counterparts and the source of the ore fluids.