COPPER ISOTOPE VARIATIONS IN MAGMATIC AND HYDROTHERMAL ORE DEPOSITS

Cu isotope ratios can be used to constrain the sources of Cu and the environments of Cu deposition in moderate to high temperature ore forming systems. The laboratory at Washington State University uses MC-ICPMS in conjunction with standard-sample bracketing to measure Cu isotope ratios relative to NBS976. Analytical solutions are prepared by direct mineral dissolution, and are diluted to about 100ppb for analyses. Measurement of chromatographically purified solutions yield Cu isotope ratios that are identical to those for un-purified solutions. Interference effects of major constituents in many Cu ore minerals (e.g., Fe and S) have been evaluated by doping solutions prepared by dissolution of pure Cu. Both the pure and doped solutions yield identical Cu isotope ratios at the dilutions of interest (about 100ppb). Potential heavy-ion matrix effects have also been evaluated by doping pure Cu solutions (100ppb) with Pb (less than 1ppm). In all cases, Pb doped solutions yield Cu isotope ratios within analytical uncertainty of the Cu isotope ratio of the pure solutions. Cu samples prepared by direct dissolution of sulfide, oxide, and native Cu minerals, therefore, provide precise Cu isotope ratio measurements using a standard-sample bracketing procedure. Natural Cu isotope variations range over 9 per mil. Chalcopyrite samples from mafic intrusions lie within a range of about 1.5 per mil, and most cluster tightly between -0.10 and -0.20 per mil, perhaps representing a bulk mantle value. Ratios for chalcopyrite and bornite from moderate to high temperature deposits as a group and within individual deposits exhibit a broad range of values, where variations of nearly 1 per mil are observed over distances on the order of one meter. Fractionations between chalcopyrite and bornite cluster near 0.40 per mil, suggesting an equilibrium Cu isotope fractionation. Weathering of hydrothermal copper minerals produces a wide range of values in secondary copper phases. In this supergene environment, cuprite typically has higher values than associated native copper, and redox states appear to exert a significant control over fractionation at low temperatures.