A TECHNIQUE FOR MEASURING COPPER ISOTOPES IN LOW CONCENTRATION (PARTS PER TRILLION) SALTS DERIVED FROM DRIED MOUNT ISA INLIER GROUNDWATERS

The weathering process of copper sulfide minerals results in the production of distinctively heavier copper isotope signatures in solutions (with a higher proportion of 65Cu to 63Cu). This reaction has the potential to be used as a mineral exploration tool. In this study, we employ this technique to analyze saline groundwater samples taken from the Mount Isa Block. These samples exhibit low copper concentrations (<1 ppb, with an average of 250 parts per trillion). Due to the high salt content and low copper concentration of the groundwater, various chemical separation methods were tested and a novel technique that removes unwanted ions from the measured solution was developed. Spatial analysis of 42 samples revealed that copper isotope compositions in groundwater near mineralization displayed a range of values (d65Cu= 1.2 to 4.4 per mil) resembling known waters resulting from the weathering of chalcopyrite and/or chalcocite from six deposits, which ranged from -1 to +3 per mil. The geographical overlap of known mineralization with predicted copper isotopic fractionation induced by sulfide weathering suggests that sulfide oxidation generates these copper isotope values. The data from this study demonstrates the effectiveness of copper isotope measurements in groundwater as a technique for identifying sulfide weathering associated with mineralization concealed beneath the surface.