Search

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 238 U/ 235 U data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range (δ 238 U CRM112a = −0.56‰ to +0.04‰). At the scale of sampling, there is no correlation of δ 238 U with lithology, degree of alteration or U mineralogy, although ores with U > 5 wt.% have subtly higher δ 238 U 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 δ 238 U 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 δ 238 U 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 149 Sm (up to ∼2 ε units) and excesses in 150 Sm (up to ∼ 4 ε 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 10 15 to 10 16 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 δ 238 U signatures expected from low-temperature reworking of older low- δ 238 U ores. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sm-Nd dating yields robust chronologies for several carbonate associations. • First Lu-Hf isotopic dates for hydrothermal carbonates in an ore deposit. • Three carbonate-forming events confirmed (~1.59–1.55 Ga, 0.8–1.0 Ga, 0.5 Ga). • Sr isotope ratios in carbonates support multi-stage, protracted evolution of deposit. Mineralization at the supergiant Cu-U-Au-Ag Olympic Dam deposit (South Australia), the 'uranium endmember' of the iron-oxide copper–gold (IOCG) spectrum of ore deposits, is hosted in a breccia complex developed entirely within granite of the 1.59 Ga Hiltaba Suite (Gawler Craton). Earlier studies suggested brecciation and mineralization occurred within a magmatically-driven hydrothermal system at 1.59 Ga, with a critical role for mafic–ultramafic intrusions. In contrast, recent radiometric dating of the breccia complex indicates a prolonged, multi-stage history of brecciation and mineralization from 1.59 to 0.5–0.4 Ga. Ca-Fe-Mg-Mn-carbonate gangue minerals are associated with ore minerals at virtually every stage of mineralization. In a companion study (Apukhtina et al., 2020), this mineralogically, texturally and compositionally diverse carbonate mineral suite was assigned to seven associations defined on the basis of host lithology and texture. Here we report Sm-Nd, Pb-Pb and Lu-Hf isotope ages for these carbonates, which are used to examine the chronology of carbonate deposition. Initial Sr-Nd isotopic compositions are used to place constraints on fluid sources. Sm-Nd and Pb-Pb isotope systematics of calcite veins in ~1.59 Ga IOCG ore indicate 1.59–1.55 Ga deposition ages. Likewise, locally abundant laminated siderites have Sm-Nd ages in this age interval. A world-first attempt to apply Lu-Hf dating to carbonate gangue in an ore deposit yields ages that are 70–100 Ma younger than corresponding Sm-Nd ages, presumably reflecting isotopic exchange of carbonate Lu-Hf isotope systems with host rocks. Sm-Nd ages for carbonates assigned to other carbonate associations (hosted in highly altered inferred 1.59 Ga basalt and picrite; diverse settings within granite-dominated breccia; locally abundant megaclasts of green and red bedded sandstone/mudstone sequences; ~0.82 Ga doleritic dykes) are more diverse and range from ~1.59 to 0.5 Ga. The structurally youngest carbonates (unbrecciated fluorite-barite veins; carbonate matrix in polymict conglomerate above the breccia complex) yield ~0.50 Ga Sm-Nd ages. Inferred carbonate ages are broadly consistent with radiometric dates for other hydrothermal minerals (e.g., hematite, uraninite, apatite, fluorite). They suggest that mineralization initiated at 1.59 Ga was reworked and possibly increased in size in response to large-scale tectonic, magmatic, sedimentary and hydrothermal events. Initial 87Sr/86Sr in the carbonates is higher and more variable (0.710–0.752, average ~0.721) than could be explained by ore and gangue mineral formation from magmatic-hydrothermal fluids during a single event at 1.59 Ga, a model favored in several earlier studies. By contrast, carbonate formation over a long period, as inferred from the Sm-Nd chronology presented here, would allow ingrowth of 87Sr in the granitic host rocks to develop the heterogeneous initial 87Sr/86Sr recorded in the carbonates. Carbonate-bearing fluids appear to have sourced Nd (and most likely also Sr) locally, within the host granite and breccia, with contributions from mafic rocks. The emerging evidence for protracted, multi-stage mineralization implies that single-stage models for Olympic Dam need to be revisited and that all studies of sulfide and gangue minerals in this deposit require careful radiometric dating. We speculate that the polymetallic nature and unusually large metal reserves of the Olympic Dam mineralization are related to its multi-stage formation history. [ABSTRACT FROM AUTHOR]

Abstract: The mineralisation potential of Palaeoproterozoic strata from the central Gawler Craton, South Australia, is poorly known. This study defines the timing of Zn-rich skarn formation within Palaeoproterozoic calcsilicate and highlights this as a new mineralisation style for the Gawler Craton. Sulphides within the garnet–diopside skarn in the No. 17 Bore Prospect are predominantly in the form of sphalerite, associated with galena, minor chalcopyrite, pyrrhotite and pyrite. Sulphide is present in disseminated form and as a coarse-grained sulphide within a sericite-rich cavity-fill. Mineralisation is inferred to have formed at 1710±16Ma through a Sm–Nd isochron from garnet and diopside aliquots. A weakly mineralised and altered granite immediately below the calcsilicate skarn crystallised at 1729±13Ma (LA-ICPMS U–Pb zircon), within error of the skarn mineralisation. The skarn is interpreted to have formed through the initiation of fluid circulation as a result of high-level granite emplacement within the Palaeoproterozoic strata. Exploration for skarn Zn–Pb deposits such as the No. 17 Bore Prospect is assisted by their geophysical properties. [Copyright &y& Elsevier]

The origins of many supergiant ore deposits remain unresolved because the factors responsible for such extreme metal enrichments are not understood. One factor of critical importance is the timing of mineralization. However, timing information is commonly confounded by the difficulty of dating ore minerals. The world's largest uranium resource at Olympic Dam, South Australia, is exceptional because the high abundance of U allows U-Pb dating of ore minerals. The Olympic Dam U(-Cu-Au-Ag) ore deposit is hosted in ca. 1.59 Ga rocks, and the consensus has been that the supergiant deposit formed at the same time. We argue that, in fact, two stages of mineralization were involved. Paired in situ U-Pb and trace element analyses of texturally distinct uraninite populations show that the supergiant size and highest-U-grade zones are the result of U addition at 0.7-0.5 Ga, at least one billion years after initial formation. This conclusion is supported by a remarkable clustering of thousands of radiogenic 207Pb/206Pb model ages of Cu sulfide grains at this time. Upgrading of the original ca. 1.59 Ga U deposit to its present size at 0.7-0.5 Ga may have resulted from perturbation of regional fluid flow triggered by global climatic (deglaciation) and tectonic (breakup of Rodinia) events. [ABSTRACT FROM AUTHOR]

• We suggest a near-continuous model of hydrothermal reworking in the Olympic Cu-Au Province resulting in recrystallisation of the Cu-(Fe)-sulfides, and Pb (re)mobilisation at nm- to deposit-scales. • Pb-isotope ratios in galena, clausthalite, altaite hosted within the Cu-(Fe)-sulfides, and the Cu-(Fe)-sulfides themselves, are highly radiogenic and are indistinguishable from one another. • Hydrothermal autoclave experiments showed that the Pb-chalcogenide inclusions likely formed within porous zones of the Cu-(Fe)-sulfides during coupled dissolution reprecipitation reactions. • The data presented here are consistent with other studies in the Olympic Cu-Au Province which record the impact of tectonothermal events younger than the 1600–1585 Ma 'main' mineralisation event. Lead-isotope data were obtained for a population of Pb-chalcogenides (galena, clausthalite and altaite) and Cu-(Fe)-sulfides in Cu-(Fe)-sulfide mineral separates from the Prominent Hill iron-oxide copper gold deposit, Mt Woods Inlier, South Australia. Each mineral displays wide variability in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, ranging from moderately radiogenic Pb signatures to strongly radiogenic. Any contribution from non-radiogenic (common) lead at the time of initial deposit formation (1600–1585 Ma) is minor. Lead is overwhelmingly derived from the decay of U, with a minor contribution from the decay of Th also present within the ores. The heterogenous nature of the recorded Pb isotope values suggests a mixing of isotope signatures during repeated cycles of fluid-assisted dissolution, remobilisation and reprecipitation of U- and Pb-bearing phases. Hydrothermal experiments showed that the incorporation of Pb-chalcogenide (galena) occurs rapidly (days) at relatively mild conditions (≤150 °C), via precipitation in reaction-induced porosity formed during coupled dissolution reprecipitation reactions of the host Cu-(Fe)-sulfides. Hence, even minor hydrothermal activity may result in some trapping of radiogenic Pb by pre-existing Cu-(Fe)-sulfides. Construction of Pb-Pb pseudo-isochrons from the data supports these assessments, returning a range of possible (theoretical) ages (0–0.535 Ga) for Pb-chalcogenide formation. An additional (two-stage) Pb evolution model is preserved by the data with separation of each reservoir from the bulk silicate earth at ∼ 830 Ma and mixing of the two reservoirs at ∼ 500 Ma. Although the Pb-isotope data can be interpreted as resulting from a number of discrete events that generated regional-scale fluid flow, near-continuous Pb (re)-mobilisation at the nm- to deposit-scales is an equally valid interpretation, and is also consistent with observations of modern radionuclide mobility in the ores. The observations made within this study should be considered when discussing the mobility of elements within and between mineral phases in ore-forming systems. They show that many mineral phases, particularly the sulphide mineral phases discussed here, should not be considered as closed systems, but potentially, as systems in which chemistry and mineral stoichiometry evolve continuously over geological time. [ABSTRACT FROM AUTHOR]

The felsic-dominant Gawler Range Volcanics and cogenetic Hiltaba Suite granitoids constitute the ca. 1590 Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. The province includes minor occurrences of olivine-phyric basalt at Kokatha and Mount Gunson. In this study, we describe additional olivine-phyric basalts and dykes intersected by drill holes at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and the Wirrda Well Cu–Au prospect. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb dating results (un-anchored) of apatite in the basalts and dykes at Mount Gunson (1576 ± 33 Ma), Wirrda Well (1596 ± 17 Ma), and Olympic Dam (1621 ± 20 Ma) confirm their effective temporal correlation with the ca. 1590 Ma Gawler silicic large igneous province. Compositions of Cr-spinel inclusions enclosed in former olivine phenocrysts (forsterite number >80) in basalts at Mount Gunson, Wirrda Well and Olympic Dam imply derivation from a heterogeneous mantle source that may have been modified by subduction. Least-altered olivine-phyric basalts at Mount Gunson and Kokatha are characterized by negative Nb and Ta anomalies, which are typical of arc basalts but are also common in back-arc basin basalts, in accordance with the result suggested by the Cr-spinel source indicator. The mafic components of the Gawler Range Volcanics generally have higher Zr contents and Zr/TiO 2 ratios than those of high-Mg basalts and picrites produced in variable tectonic settings worldwide, possibly reflecting continental crustal components involved in their mantle source. High abundances (typically ∼20 vol.%) of former olivine phenocrysts in the basalt at Olympic Dam imply high-Mg whole-rock compositions and a high temperature of the primary magma. This result is consistent with previous models in which the heat flux from mantle magmas caused large-scale partial melting of crustal rocks that subsequently gave rise to silicic magmas erupted and intruded to form the Gawler silicic large igneous province. [ABSTRACT FROM AUTHOR]

A suite of basaltic to doleritic dykes (named the Olympic Dam dolerite) has been intersected in drill holes at the Olympic Dam iron oxide Cu–U–Au–Ag deposit in the Gawler Craton, South Australia. The dykes intrude the ca. 1600 Ma Roxby Downs Granite as well as the Olympic Dam Breccia Complex, and range from several centimetres to over 100 m in width, strike NW and dip subvertically. Clinopyroxene and plagioclase are the major minerals, together with accessory apatite and Ti-magnetite. Magmatic apatite crystals in medium-grained doleritic dykes have been dated by LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry), and yielded a U–Pb age of 825 ± 18 Ma. The Olympic Dam dolerite has invariably been altered. Secondary apatite and titanite crystals from post-magmatic veinlets cutting the same dykes give similar results of 821 ± 15 Ma and 817 ± 11 Ma, respectively. All these results are, within error, coincident in age with the Gairdner Dyke Swarm, the longest dyke swarm (∼1000 km in length) in Australia. The Olympic Dam dolerite shows similar compositional trends and variations to the Gairdner Dyke Swarm, especially in terms of the relatively immobile High Field Strength Elements (HFSE) and Rare Earth Element (REE). We conclude that the Olympic Dam dolerite belongs to the Gairdner Dyke Swarm, expanding its known extent and compositional range. The Gairdner Dyke Swarm is a major component of the ca. 820 Ma Gairdner Large Igneous Province (LIP) generated during the break-up of the supercontinent Rodinia. The ca. 1070 Ma Warakurna and the ca. 820 Ma Gairdner LIP in Australia are compositionally distinct, whereas the Gairdner LIP and LIP and mafic suites associated with the break-up of Rodinia in South China and North America are broadly similar. This provides geochemical support for a Gairdner LIP origin for some mafic suites, in particular the Beda Volcanics in South Australia, and for a juxtaposition of South China, South Australia and North America in the tectonic reconstruction of Rodinia at ca. 820 Ma. [ABSTRACT FROM AUTHOR]

• Olympic Dam Cu-U-Au-Ag deposit is a significant carbon anomaly. • Carbonates occur in nearly all lithologies in the Olympic Dam Breccia Complex. • Recycling and new input of carbonate have occurred. The supergiant Olympic Dam Cu-U-Au-Ag deposit in South Australia is a type example of the iron-oxide copper–gold (IOCG) deposit family. Hosted entirely within heterogeneous breccia in 1.59 Ga granite, the deposit contains a volumetrically substantial and mineralogically diverse component of carbonate minerals. Carbonate minerals are always associated with ore minerals (sulfides, uraninite), implying a genetic relationship and providing an opportunity to use gangue carbonates to better understand ore formation. This study provides the first detailed and comprehensive petrographic and chemical/isotopic study of Olympic Dam carbonates, with a particular emphasis on petrography and texture, and an attempt is made to relate carbonate formation to local and regional events that have affected Olympic Dam. Based on a set of 196 carbonate-bearing samples, carbonate minerals are observed in all lithologies present at Olympic Dam. Carbonates occur as cement in breccia and conglomerates, as breccia clasts, in veins crosscutting ore-rich breccia and other rock types, in pores and ooids, and in the form of laminated carbonate. Siderite and siderite-rhodochrosite-magnesite solid solution are by far the most common carbonate types, whereas calcite, dolomite-ankerite solid solution and REE-fluorocarbonates are locally abundant. Single carbonate grains typically show compositional zones (simple or oscillatory) and replacement textures (including mutual replacement of carbonates with other carbonates and with hematite) are common. In the absence of consistent, deposit-wide paragenetic relationships, the carbonates were placed in seven associations based on host rock, mineralogy and texture: (1) coarse-grained calcite veins in weakly brecciated granite and rhyolite, (2) carbonates in strongly brecciated granite, (3) carbonate veins in bedded clastic facies, (4) carbonates in mafic and ultramafic igneous rocks, (5) massive barite-fluorite-dominated veins with minor carbonate, (6) laminated siderite, and (7) carbonate matrix in conglomerate-breccia-sandstone above the unconformity. Some of these associations can be related to regional tectonic events based on local context and relationships with dated assemblages. δ13C (−6.5‰ to −2‰) values for the carbonates show a relatively limited range whereas δ18O is more variable (+9.4‰ to + 20.9‰). C-O isotopic compositions for the various carbonate associations tend to overlap, suggestive of mixed fluid sources, recycling of older carbonate and perhaps other fractionation processes. The C-O isotope data overlap the compositional fields of several major carbon–oxygen reservoirs (magmatic, sedimentary) and carbon sources in local granite, felsic volcanics, older BIF and sedimentary rocks are all possible at different stages of carbonate deposition. [ABSTRACT FROM AUTHOR]