Uranium and Sm isotope studies of the supergiant Olympic Dam Cu–Au–U–Ag deposit, South Australia

Abstract

The Olympic Dam Cu–U–Au–Ag deposit in the Archean–Proterozoic Gawler Craton (South Australia) is a type example of the iron oxide–copper–gold (IOCG) spectrum of deposits and one of the largest Cu–U–Au resources known. Mineralization is hosted in a lithologically and texturally diverse, hematite-rich breccia complex developed within a granite of the 1.59Ga Gawler Silicic Province. Emerging evidence indicates that both the breccia complex and its metal content developed over ∼1000Ma, responding to major tectonic events, e.g., at 1300–1100, 825 and 500Ma. However, metal sources and exact mechanism/s of ore formation remain poorly known.New high-precision 238U/235U data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range (δ238UCRM112a=−0.56‰ to +0.04‰). At the scale of sampling, there is no correlation of δ238U with lithology, degree of alteration or U mineralogy, although ores with U>5wt.% have subtly higher δ238U values (−0.20‰ to 0.00) than the majority of samples (<0.7wt.% U, −0.56‰ to −0.23‰). The new U isotope data are consistent with published data for uraninites from Olympic Dam, and with published results from high-temperature U deposits. They overlap completely with the range of δ238U values in granitoids (including the host granite, −0.18‰ to −0.32‰) and with estimates of the upper continental crust in general. This similarity suggests that Olympic Dam δ238U values reflects the crustal sources of U, which probably include felsic volcanic rocks and granitoids. The isotopic homogeneity suggests depositional mechanisms that involve minimal isotopic fractionation of U; alternatively, primary fractionation signatures may have been erased during the long history of the U mineralization.High-grade U ores may record isotopic neutron-capture effects related to fissionogenic neutrons. High-precision Sm isotope data for five high-U (>5wt.%U, U/Sm≫500) Olympic Dam ores define a neutron capture line, with correlated depletions in 149Sm (up to ∼2εunits) and excesses in 150Sm (up to ∼ 4εunits), but fission fragment contributions to Sm are below detection. These observations provide evidence for small-scale neutron-capture effects, with calculated neutron fluences of 1015 to 1016ncm−2, similar to those observed in several Proterozoic and Phanerozoic U deposits. The apparent lack of fission fragment contributions in Olympic Dam high-grade ores can be explained with an age of U deposition, or re-deposition that is substantially younger than the initial 1.59Ga age of the oldest IOCG-style mineralization.The results presented here thus (i) suggest uranium sources in common (likely igneous) upper crustal lithologies, (ii) support geochronological evidence for gradual addition of U in several stages over 1000Ma at elevated temperatures of mineralization, and (iii) do not show the high δ238U signatures expected from low-temperature reworking of older low- δ238U ores.