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

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• 作者：Maria Kirchenbaur^a ; ^b ; ^{mkirchen@uni-koeln.de" class="auth_mail" title="E-mail the corresponding author} ; Roland Maas^c ; Kathy Ehrig^d ; Vadim S. Kamenetsky^e ; Erik Strub^f ; Chris Ballhaus^b ; Carsten Mü ; nker^a
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 May 2016
• 年：2016
• 卷：180
• 期：Complete
• 页码：15-32
• 全文大小：663 K

文摘

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 1.59 Ga Gawler Silicic Province. Emerging evidence indicates that both the breccia complex and its metal content developed over ∼1000 Ma, responding to major tectonic events, e.g., at 1300–1100, 825 and 500 Ma. 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 (δ²³⁸U_CRM112a = −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 > 5 wt.% have subtly higher δ²³⁸U values (−0.20‰ to 0.00) than the majority of samples (<0.7 wt.% 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 (>5 wt.% U, U/Sm ≫ 500) Olympic Dam ores define a neutron capture line, with correlated depletions in ¹⁴⁹Sm (up to ∼2ε units) and excesses in ¹⁵⁰Sm (up to ∼ 4ε units), 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 10¹⁶ n cm⁻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 1.59 Ga 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 1000 Ma at elevated temperatures of mineralization, and (iii) do not show the high δ²³⁸U signatures expected from low-temperature reworking of older low- δ²³⁸U ores.