GEMOC ARC National Key CentreA. Botfield, S.E. Jackson, W.L. Griffin and N.J. Pearson
GEMOC Macquarie
The isotope ratios of heavy metals commonly found in sulphide ore deposits (e.g., Cu, Zn, Fe, Sb, Ag) may provide important information on the source, transport and depositional mechanisms of these metals. However, very little work has been done on these isotopic systems because of the analytical difficulties in making precise measurements. With the recent advent of laser ablation (LA)-multi-collector (MC)-ICP-MS technology, a technique has been born which is capable of rapid, in situ determination of the stable isotope ratios of these metals. This study reports on the development of an analytical protocol for the measurement of the 65Cu/63Cu ratio in chalcopyrite and its preliminary application to the analysis of samples from a wide range of copper-bearing mineral deposits.
The instrumentation used in this study was a Merchantek LUV266 laser sampler operated with either Ar or He as the sample carrier gas. The ablated material was transported into a Nu Plasma MC-ICP-MS for isotopic determination. All data were acquired using the time resolved analysis data acquisition feature, which reports signal intensities at 0.2 s intervals throughout the analysis, allowing recognition of inclusions and isotopic heterogeneity within the ablation volume, together with other analytical artifacts. Instrumental mass bias was corrected using the measured 66Zn/64Zn ratio of a Zn solution with a 66Zn/64Zn ratio previously established by analysis versus NIST SRM 976, Cu isotopic standard. The Zn solution was added continuously, as an aerosol, to the sample carrier gas flow via a T-junction. The Zn aerosol was generated using a desolvating micro-concentric nebulisation system (CETAC MCN6000),.
Initial Cu isotopic analyses showed the following features:
(1) The 65Cu/63Cu ratio invariably increased during the course of an
ablation analysis. The increase was unrelated to the signal intensity.
The magnitude of the increase was typically ca. 3 per mil over a 4-minute
ablation period.
(2) The 66Zn/64Zn ratio showed no measurable concomitant variation
during analyses.
(3) The 65Cu/63Cu ratio measured by LA-ICP-MS, corrected for mass bias
of the ICP-MS, was 1 to 4 per mil lower than the ratio determined by solution-MC-ICP-MS
analysis of digested aliquots of the same samples.
These observations indicate that Cu isotopes are fractionated by the laser sampling/transport processes similar to the manner in which elements are known to be fractionated. The mechanism clearly involves volatilisation and condensation processes, both of which favour depletion of 65Cu relative to 63Cu in the transported material. This is in keeping with the observed relationship between solution and LA data. Similar observations have been made with Sb and Ag isotopes but on a reduced scale.
Fractionation of Cu isotopes is affected by ablation conditions and may be reduced by using high pulse energies, low laser repetition rates and, most significantly, by ablation in He which dramatically reduces the condensation blanket of sulphidic material around the ablation site. Using these conditions, relatively stable Cu isotope ratios may be maintained over a significant ablation period (up to 4 minutes). However, absolute ratios are still significantly different to those measured by solution-MC-ICP-MS. To produce accurate ratios, therefore, all analyses were referenced to a chalcopyrite "standard" measured repeatedly during an analytical session under identical ablation conditions to the "samples". This approach has been verified by accurate reproduction, using LA sampling, of the solution-MC-ICP-MS *65Cu/63Cu value between two chalcopyrite samples, one from the Bougainville porphyry copper deposit, PNG, and one from Mount Isa.
In this study, the standard chosen was a chalcopyrite sample from the Bougainville porphyry copper deposit, which has a 65Cu/63Cu value of 0.44580 ± 0.000028 as determined by solution-MC-ICP-MS. All data are reported as delta65Cu/63Cu values relative to NIST SRM 976 (65Cu/63Cu = 0.44563). All samples were analysed 5 times, with typical external precision (2 S.E.) of ca. 0.5 per mil. Samples analysed from a wide range of deposits (magmatic, magmatic-hydrothermal, sedimentary) show the following isotopic features:
- The samples show a range in delta65Cu/63Cu values of ca. 8 per mil.
- All the chalcopyrite samples associated with igneous activity (magmatic
segregation, porphyry Cu, VMS, epithermal) have delta65Cu/63Cu976 values
between -1 and +1.
- Samples from sedimentary deposits, including the sandstone-hosted
Zambian Cu belt, and the shale-hosted White Pine (Montana) and HYC (Northern
Territory) deposits, have significantly lower delta65Cu/63Cu976 values
(by up to 7 per mil).
These data suggest that igneous Cu has a relatively limited range in 65Cu/63Cu, but that sedimentary processes may significantly fractionate Cu isotopes. Further investigations are now underway to:
1. determine the relationship between Cu isotopic signatures and biogenic
reduction, a process known to fractionate S isotopes.
2. study spatial trends in Cu isotope fractionation within individual
ore deposits.