Analysis of copper isotope ratios by multi-collector inductively coupled plasma mass spectrometry and interpretation of copper isotope ratios from copper mineralization

Copper isotope analysis by multi-collector inductively coupled plasma mass spectrometry can routinely determine copper ratios with 2sigma precision of +/-0.08‰ delta65CuNIST SRM 976, correcting machine mass fractionation by the standard-sample bracketing method. The zinc-doping method for machine mass fractionation correction provides similar reproducibility, depending on the zinc isotope pair utilized. Detailed evaluation of possible matrix effects on copper isotope measurements indicates that sample purification through chromatography is unnecessary for reliable analysis of Cu-Fe and Cu sulfide minerals.; Copper isotope ratios measured from high-temperature (>250°C) mineralization from several hydrothermal ore deposits span a larger range than recognized by recent investigations (Zhu et al., 2000). The large range suggests significant isotopic fractionation mechanisms operate at high temperatures and include fractionation during copper remobilization, possible fractionation between predominant metal-transporting complexes in solution, as well as significant equilibrium fluid-mineral fractionation. Mineralization due to high-salinity fluids is expected to have larger ranges in delta65Cu values than mineralization produced from lower-salinity fluids. Supergene mineralization shows large isotopic fractionations resulting from varied and repetitive isotopic fractionation processes including copper leaching, Cu+ to Cu 2+ oxidation-reduction reactions, and fluid-mineral fractionations. These processes are controlled by changes in the hydrologic system of supergene zones with time.; Hydrothermal chalcopyrite synthesis experiments have successfully precipitated chalcopyrite from elemental nutrient at 225°C and 300°C from fluids of different salinities. Resulting chalcopyrite delta65Cu values indicate that the fluid-mineral fractionation is probably negative, which means that chalcopyrite successively precipitated from the same hydrothermal fluid will be isotopically heavier than the residual fluid. Isotopic data from mineralization at Coroccohuayco, Peru, modeled using this equilibrium fluid-mineral fractionation, indicate that precipitation of chalcopyrite commences distally, likely due to thermal constraints, and advances toward the source of the mineralizing fluid. Zoned isotopic data from chalcopyrite can be successfully modeled as a series of small precipitation steps at fluid-chalcopyrite isotopic equilibrium with large changes in copper isotope ratios, punctuated by larger fractions of copper being precipitated with corresponding minor changes in copper isotope ratios of chalcopyrite.