A magmatic copper and fluid source for the sediment-hosted Mount Isa deposit

Abstract

The world class Mount Isa deposit is a unique, sediment-hosted, copper deposit with no known equivalent around the world and a controversial origin. We report δ 65 Cu values (n = 90) from chalcopyrite grains collected systematically across the entire deposit. The δ 65 Cu shows a unimodal distribution with limited variability (min = −0.87 ‰; max = 0.88 ‰) and an average value (þ0.13 ‰) comparable to average igneous rocks. In general, the δ 65 Cu values in chalcopyrite are lower near major structures and heavier further away, consistent with equilibrium fractionation with distance from the fluid source. The range in δ 65 Cu of chalcopyrite from the Mount Isa deposit is less variable compared to sedimentary copper, VMS and porphyry/ epithermal deposits, but similar to Michigan deposits; meanwhile, average δ 65 Cu at Mt. Isa is distinctly higher than sedimentary copper deposits, but similar to VMS, porphyry/epithermal and Michigan deposits. These data suggest that, from a copper isotope perspective, the Mount Isa deposit is clearly different from sedimentary copper deposits and more like VMS, porphyry copper/ epithermal and Michigan style deposits. The average δ 65 Cu (þ0.13 ‰) is almost identical to the average δ 65 Cu (þ0.14 ‰) from Proterozoic basalts and suggests that copper was sourced from the underlying mafic rocks; the limited fractionation and the normal distribution of the δ 65 Cu indicate a very effective leaching mechanism and transport by a hot fluid from which chalcopyrite precipitated without significant fractionation of copper isotopes.