title: Pb-isotopic insights into the late Palaeoproterozoic crustal evolution and metallogenesis of the Gawler Craton, South Australia
Creators: name: Chapman, ND
Publisher: name: University of Tasmania
Year: 2020

description: Pb-isotopes present a powerful tool to investigate ancient crustal evolution and metallogenesis. When coupled with an analytical techniques such as laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) which yield high analytical throughput and high-spatial resolution at a level of precision which is fit-for-purpose, Pb-isotopes represents a cost-effective method through which to conduct large-scale, regional studies. This study examines the late Paleoproterozoic Pb-isotopic evolution of the Gawler Craton, South Australia in order to investigate igneous petrogenesis and ore deposit metallogenesis. The late Paleoproterozoic-Early Mesoproterozoic period (1690 – 1590 Ma) represents an important period for metallogenesis (i.e. Olympic Dam, Prominent Hill, Carapateena and Tarcoola) and igenous crustal evolution in the Gawler Craton. However, like many of Australia’s Paleoproterozoic terranes, extensive Neoproterozoic and younger cover sequences largely obscure the geological relationships of the underlying igneous basement, limiting straightforward geodynamic and petrogenetic interpretations. Isotopic investigations are ideal in such situations, providing a great deal of constraint on melt-sources and the tectonic regimes driving magmatism. New Pb-isotopic results presented here, are integrated with extant geochemical, geochronlogical and Sm-Nd isotopic data to provide new insights in the igneous crustal evolution and metallogensis of the Gawler Craton. The tectonic setting of 1690 – 1670 Ma Tunkillia Suite remains unclear, and has been variably attributed to both arc and continental-collisional/ post-collisional tectonic environments. In situ Pbisotopic results from alkali feldspars reveal a complex petrogenesis, instigated by mantle-driven magmatism, but with dramatic differences between the crustal-Pb components of the eastern and western Tunkillia Suite. The crustal Pb component in the western Tunkillia Suite is particularly noteworthy as it records a rare occurrence of retarded Pb-isotopic behaviour (i.e. less radiogenic than predicted by growth models), caused by significant, ancient, U-depletion related to granulite-facies metamorphism. Similarities between the geochemical and Pb-isotopic compositions of the eastern Tunkillia Suite and the nearby 1630-1600 Ma St Peters Suite which are considered to represent island arc-style magmatism, demonstrates some inconsistencies in the way igneous suites of the Gawler Craton have been previously classified. Comparison between the Pb-isotopic, Nd-isotopic and geochemical characteristics of the Tunkillia Suite shows that it has a greater similarity with modernday continental arc-style granitoids than either continental-collisional or post-tectonic granitoids. It is therefore suggested that the late Paleoproterozoic ((&lt;1700 Ma) crustal evolution of the Gawler Craton is dominated by subduction-related tectonics, beginning around ~1690 Ma with the Tunkillia Suite and terminating around ~1600 Ma with the St Peters Suite. Less than 13 Ma after the completion of the St Peters Suite subduction cycle, hightemperature (~870°C), volumetric (100 000 km\(^3\)), A-type (K-HFSE-REE-rich) felsic magmas of the 1595-1575 Ma Hiltaba Suite/ Gawler Range Volcanics (GRV) were emplaced across the Gawler Craton. Recent high-precision geochronological data on the GRV provides constraints on how the Hiltaba-GRV magmatic event evolved through time. The dominantly mantle-like Pb-isotopic signatures in the 1592 Ma Lower GRV contrast strongly with the high-µ signature in the 1589 Ma Upper GRV, suggesting that either the GRV may not have been co-magmatic sensu stricto. Alternatively, the Upper GRV may reflects a period of the Hiltaba-GRV magmatic event characterised by large-scale crustal melting (4200 km\(^3\)). Isotopic (Pb-Pb, Sm-Nd), geochemical, and physicochemical (T°C, αH\(_2\)O etc) similarities between the GRV and modern-day volcanic terranes which have generated large volumes of felsic volcanics (e.g. Basin and Range Province), suggest that the Hiltaba/ GRV event may have developed in an intracontinental, back-arc tectonic setting, consistent with the interpretation of the late Paleoproterozoic Gawler Craton tectonic regime presented here. Initial Pb-isotopic compositions from alkali feldspars belonging to the widespread Hiltaba Suite granitoids show a diverse range of melt-sources identical to the GRV, from enriched-mantle to U-Thenriched lower crust. Mantle-like signatures are limited to the Hiltaba Suite granites in proximity of the Nuyts Terrane, highlighting a possible geodynamic link between the subduction-related paradigm interpreted for the Tunkillia and St Peters Suites. To the north-northwest of the Nuyts Terrane, Hiltaba Suite granites have incorporated weakly-retarded, high-κ, Pb-isotopic signatures, demonstrating that U-depleted granulites are an extensive component of the western Gawler Craton lower crust. In contrast, Hiltaba Suite granites in the eastern Gawler Craton contain high-µ, high-κ Pb-isotopic signatures indicating that crust underlying the IOCG province contains U and Th concentrations which are anomalously enriched compared to average crust. A particular emphasis was placed on acquiring the initial Pb-isotopic signature of the Roxby Downs Granite (RDG), a member of the Hiltaba Suite which hosts the world-class Olympic Dam Cu-AuU-Ag deposit. Complex textural, paragenetic and crystallographic features of alkali feldspars from the RDG has inhibited previous initial Pb-isotopic determinations using conventional dissolution-based methods. A series of high-spatial resolution geochemical and crystallographic techniques (SC-XRD, SEM-WDS and synchrotron) were employed to determine pristine, orthomagmatic domains in leastaltered alkali feldspar phenocrysts, and were crucial in validating the authenticity of the initial Pbisotopic signatures determined by LA-MC-ICP-MS. The initial Pb-isotopic signature shows that the RDG was partially-derived from a crustal source (20:80 crust:mantle melt) that was ~14% more enriched in U and ~7% more enriched in Th than average crust. Growth curve modelling suggests that the U-Thenriched crustal reservoir was formed at 3.2-3.1 Ga, which is consistent with the age of the Cooyerdoo Granite – a known U-Th-rich granite. It is therefore likely that U-contribution from an enriched crustal melt represents a first-order explanation for the anomalously U-rich mineralisation of Olympic Dam. Pb-isotopic variations between high-temperature (830-870°C) alkali feldspars, and low-temperature (300-400°C), paragenetically-later K-feldspars indicates that some open-system Pb-exchange occurred between the RDG and surrounding country rock during the transition from orthomagmatic conditions, to magmatic-hydrothermal conditions, consistent with the fluid-mixing model invoked by previous studies for the formation of Olympic Dam. The Olympic Dam Cu-Au-U-Ag deposit is one of the largest polymetallic resource currently known, however, little is understood about how it formed. A growing body of evidence suggests that the deposit has a protracted history of reworking. 397 analyses of Pb-rich mineral species from across the deposit were analysed for their Pb-isotopic compositions, in order to constrain potential disturbances to the U-Th-Pb system of the deposit. Calcite veins in the deepest part of the deposit, and galena from the highest-Pb zone of the deposit contains the least radiogenic signatures encountered. The galena Pb-isotopic compositions form linear arrays which intersect the RDG initial Pb-isotopic signature demonstrating that most primary Pb (and possibly other metals) were contributed by the RDG itself. The high-µ trajectory of the array suggests that high U/Pb conditions were already established within the deposit from at least 1527 ± 147 Ma, and therefore there is no Pbisotopic requirement to introduce U after initial formation of the deposit. However, Pb-isotopic signatures from across the deposit show no relationship with U, Th or Pb concentrations suggesting that U and Pb have been redistributed to some degree. Strong, uranogenic departures in the Pbisotopic signatures from bornite and chalcocite zones of the deposit indicates that at least one major reworking event has dramatically affected U-mineralisation. Radiogenic Pb-isotopic signatures show a positive correlation with modal % bornite, yet reflect a negative relationship with modal % chalcopyrite, suggesting that the U-disturbance event recorded by Pb-isotopic compositions may have also upgraded chalcopyrite to bornite. Pb-Pb isochrons from bornite and chalcocite-rich areas of the deposit yield repeatable radiogenic Pb ages of av. 664 ± 66 Ma (MSWD = 2.3, n = 7) which is consistent with the depositional age of Ediacaran sediments (~676 Ma) above the unconformity which unroofs the Olympic Dam deposit. To explain these phenomena, it is suggested that Cryogenean-Ediacaran marine transgression following the Marinoan Glaciation, led to the ingress of oxidised marine fluids into the Olympic Dam breccia complex, remobilising some U and forcing the Cu-sulphide stability field from chalcopyrite-stable to bornite-stable conditions. It is therefore considered likely that reworking of Olympic Dam during the Neoproterozoic upgraded some Cu-U mineralisation, ultimately contributing to the economic tenor of its world-class resource.