Gawler Range Volcanics Research Articles

Insights into lithospheric architecture, fertilisation and fluid pathways from AusLAMP MT

The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) has the goal to map the electrical resistivity of the Australian lithosphere to constrain the geodynamic framework of the continent. Data acquisition in South Australia has covered two-thirds of the state to date. Three-dimensional resistivity models of subsets of the AusLAMP grid across the Gawler Craton show a generally electrically resistive crust and lithosphere, but an area of low resistivity between depths of 100 km and 200 km beneath the Gawler Range Volcanics exists. A possible explanation is a fertilised mantle signature as a result of metasomatic events in the Proterozoic. The continuous low resistivity connection along the margins of the Gawler Craton core to the surface coincides with the prospective IOCG belt along the eastern margin of the Gawler Craton. The results support the importance of the AusLAMP project to define the lithospheric architecture of the continent and the value of primary lithospheric architecture for mineral exploration.

Spectral characteristics of the Gawler Range Volcanics in drill core Myall Creek RC1

ABSTRACTTraditional core-logging methods in conjunction with spectral scanning techniques have been used to log volcanic successions of the lower Gawler Range Volcanics. The open-file drill core Myall Creek RC 1 was re-logged and scanned using HyLogger™ core scanning technologies as a part of the Geological Survey of South Australia's Southern Gawler Rangers mapping program and the Mineral System Drilling Program. Myall Creek RC 1 is one of the key stratigraphic drill cores for the region, owing to the intersection of a large section of the Gawler Range Volcanics. Spectral characterisation of the Eucarro Rhyolite revealed differential weathering of plagioclase phenocrysts, while high-resolution imagery and spectral results were used to log new basaltic flows and small flow features in the Roopena Basalt. The composition and distribution of feldspars in the unnamed lower Gawler Range Volcanic units were used to aid traditional logging of visually similar lithologies. A spectral scalar, the felsic–mafic index, was used to identify unusual features in the unnamed sequence and was found to identify iron oxides as either fracture coatings or finely disseminated in the matrix of the sample. Iron oxides were also used to identify features within lithological units, which were difficult to discern visually, especially the layers in the deepest layered ignimbrite at the end of the drill core.

A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic iron oxide copper-gold (IOCG) province of South Australia

The Prominent Hill deposit is a world-class iron oxide copper–gold (IOCG) deposit in South Australia, characterized by a high Cu/S ratio of the dominant Cu-(Fe) sulfides hosted by hematite breccias. It contains a total resource of 278Mt of ore at 0.98% Cu and 0.75g/t Au. Prominent Hill is one of several IOCG deposits and numerous prospects in the Olympic IOCG province that are temporally associated with the 1603–1575Ma Gawler Range Volcanics, a large igneous province including co-magmatic granitoid intrusions of the Hiltaba Suite. Globally, IOCG deposits share many similar features in terms of their geological environment and mineral association. However, it is not yet clear whether sulfur and copper originate from the same source rocks and which hydrothermal redox processes created the characteristic iron oxide enrichment. Highly variable sulfur isotope compositions of sulfides and sulfates in IOCG deposits have previously been interpreted in terms of diverse sulfur sources that may include contributions from magmatic, sedimentary, seawater or evaporitic sulfur. In order to test these alternatives, we performed a detailed sulfur isotope study of Cu-(Fe) sulfides from Prominent Hill and IOCG prospects nearby. The Prominent Hill deposit shows a wide range in δ34SV-CDT between −33.5‰ and 29.9‰ for Cu-(Fe) sulfides, and a narrower range of 4.3‰ to 15.8‰ for barite. Iron sulfides (pyrite, pyrrhotite) show a narrow range in sulfur isotope composition, whereas Cu-bearing sulfides show a much wider range, and more negative δ34SV-CDT values on average. We propose a two-stage sulfide mineralization model for the IOCG system in the Prominent Hill area, in which all hydrothermal sulfur is ultimately derived from a magmatic source that had a composition of 4.4±2‰. The diversity in sulfur isotope composition can be produced by different fluid evolution pathways along reducing or oxidizing trajectories. A reduced sulfur evolution pathway is responsible for stage I mineralization, when intrusion-derived magmatic-hydrothermal fluids produced early pyrite and minor chalcopyrite at Prominent Hill, and iron±copper sulfides in regional magnetite skarns and in some pervasively altered volcanic rocks of the Gawler Range Volcanics. Shallow-venting magmatic-hydrothermal fluids and subaerial volcanic gases that became completely oxidized by reaction with atmospheric oxygen produced sulfate and sulfuric acid with a sulfur isotope composition equal to their magmatic source. This highly oxidized ore fluid probably consisted dominantly of water from the hydrosphere, but contained magmatic solute components, notably sulfate, acidity and Cu. Sulfate reduction produced hydrothermal Cu sulfides with a wide range in sulfur isotope compositions from very negative to moderately positive values. Partial reaction of the Cu-rich stage II fluid with earlier stage I sulfides resulted in mixing of sulfur derived from sulfate reduction and from sulfides deposited during stage I. Modeling of the sulfur isotope fractionation processes in response to reducing and oxidizing pathways demonstrates that the entire spectrum of sulfur isotope data from stage I and stage II mineralization can be explained with a single, ultimately magmatic sulfur source. Such a magmatic sulfur source is also adequate to explain the complete spectrum of sulfur isotope data of other IOCG prospects and deposits in the Olympic province, including Olympic Dam. The results of our study challenge the conventional model that suggests the requirement of multiple and compositionally diverse sulfur sources in hematite-breccia hosted IOCG style mineralization.

Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: Examples at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and other localities

The felsic-dominant Gawler Range Volcanics and cogenetic Hiltaba Suite granitoids constitute the ca. 1590Ma 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±33Ma), Wirrda Well (1596±17Ma), and Olympic Dam (1621±20Ma) confirm their effective temporal correlation with the ca. 1590Ma 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/TiO2 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 ∼20vol.%) 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.

Lithology and Hydrothermal Alteration Control the Distribution of Copper Grade in the Prominent Hill Iron Oxide-Copper-Gold Deposit (Gawler Craton, South Australia)

The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca. 278 million metric tons (Mt) of ore at 0.98% Cu, 0.75 g/t Au, and 2.5 g/t Ag. In contrast to the predominantly granite-hosted Olympic Dam IOCG deposit, Prominent Hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate, including local massive dolomite. Essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate-composition lavas, inferred to be members of the lower Gawler Range Volcanics, host the economically mineralized hematite breccias. The volcanic-sedimentary package was downfaulted and tilted along a major east-west fault, north of which similar but regionally low-grade metamorphosed rocks were affected by subeconomic skarn mineralization, and (on a more regional scale of the Mount Woods domain) intruded by granitic and gabbroic bodies. Hydrothermal alteration and mineralization at Prominent Hill involved pervasive and texturally destructive replacement of formerly calcareous, dolomitic, and siliciclastic breccia components. Hydrothermal alteration minerals comprise hematite, magnetite, siderite, ankerite, quartz, sericite, chlorite, kaolinite, fluorapatite, fluorite, barite, REE-U minerals (including monazite), uraninite, and coffinite, together with Cu sulfides including chalcopyrite, bornite, and chalcocite in the highest grade ore. Brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies, such that sedimentary contacts became obscured. Mass-balance calculations identify Al, Ti, Si, and Zr as least mobile components during hematite-chlorite-sericite to weak hematite-quartz alteration. Because Zr was not regularly assayed in drill cores, we use concentration ratios of Ti, Al, and Si from the deposit-scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and Cu mineralization. The resulting lithochemical model, based on one horizontal and five vertical cross sections, is used as a basis for mapping alteration patterns calculated from molar (Fe + Si)/(Fe + Si + Al), K/Na, and K/Al ratios. These chemical patterns, in conjunction with mineral stoichiometry, indicate that the spatial distribution of hematite, chlorite, variably phengitic sericite (and/or illite) ± kaolinite ± quartz-bearing alteration is superimposed on the pattern of interpreted lithologic contacts. The alteration patterns confirm visual logging results, showing that hematite enrichment correlates only partially with the distribution of Cu grades of >0.25 wt %. A subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a Cu-barren core in the central and eastern parts of the deposit. Zones of increasing K/Al and K/Na ratios extend upward and westward from this Cu-barren core, transgressively overprinting lithologic contacts. The degree of hematite-quartz replacement can be measured by a hematite-quartz alteration index, here termed the HMSI value [(Fe + Si)/(Fe +Si + Al)], which inversely correlates with the normal probability for Cu grade. Areas of highest Cu grade (>1 wt %) spatially correlate with irregular zones having intermediate molar alteration indices: 0.34 < K/Al < 0.40, 20 < K/Na < 36, and HMSI < 0.98. Hematite breccias and Cu ore deposition developed after tilting of the host sequence into its present steep orientation, as indicated by geopetal structures within the breccia matrix. Thus, the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower Gawler Range Volcanics. The formation of the Prominent Hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic Cu in skarns north of the fault. Faulting as well as ore formation may be related to orogenic processes in the central and northern part of the Mount Woods domain. Iron oxide introduction was decoupled from, and at least partly preceded, hydrothermal deposition of high-grade Cu. Geochemical and petrographic data indicate that economic Cu mineralization occurred together with mildly acidic hematite-chlorite-sericite ± siderite alteration of originally carbonate-, illite-, and feldspar-bearing sedimentary rocks. The presence of copper enrichment with an intermediate degree of cation leaching from the host rocks indicates that pH neutralization of initially highly acidic metal-transporting fluids was an essential factor causing Cu sulfide deposition. Distinct ranges in Na-K-Al ratios and low HMSI values offer potential as exploration indicators pointing toward higher ore grades. These results from Prominent Hill are consistent with recently published mineralogical studies at the giant Olympic Dam deposit, indicating similar ore depositional controls despite lithologically different host rocks.

Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact events

This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism, from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of shock metamorphism and impact-induced recrystallization. ZrO2-bearing granular zircons indicate shock pressures in excess of ∼65–70GPa, which are considerably higher than previous shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly shocked crystalline grains yield ages on concordia at ∼1.59–1.60Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40Ar/39Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from shocked Yardea Dacite, resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500Ma and ∼450Ma that selectively affected the impactites exposed in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.

Mt Woods 2D Seismic Reflection Survey, Gawler Craton, South Australia: An Integrated Minerals Exploration Case Study

SUMMARY Five seismic reflection profiles for ca 130 line km were acquired over the southern Mt Woods Inlier, near the Prominent Hill IOCG deposit in South Australia. The aim was to provide high resolution images of the undercover region in the shallow to mid crust, to augment existing potential field and drilling data and, through a combined interpretation, to optimise exploration targeting. The application of seismic reflection using vibroseis sources in hard rock terranes presented significant challenges to processing and interpretation. Notwithstanding this, the resulting 2D images clarified a number of important fault and fold patterns that were also apparent in, but poorly constrained by, the potential field data. The major faults include: the Southern Overthrust imaged as a major north dipping feature that truncates a series of gently dipping reflectors within the Palaeoproterozoic and Archaean basement, and the Bulgunnia Fault as a complex set of steeply dipping divergent faults. The major folds are large amplitude refolded features that include the Kennedys Dam antiform and Larissa synform. The White Hills mafic complex, which has an impressive positive gravity and magnetic response, occupies a broadly synformal position. The Gawler Range Volcanics and their contact with Palaeoproterzoic metasediments is well imaged using the seismic method. A thrust duplex structural setting has been inferred around the Prominent Hill deposit.

Uranium-bearing hematite from the Olympic Dam Cu–U–Au deposit, South Australia: A geochemical tracer and reconnaissance Pb–Pb geochronometer

Uranium-bearing hematite (containing up to several wt.% U), also containing Al, Mo, W and radiogenic Pb, is described from Olympic Dam, South Australia. These elements are present in grains that display both oscillatory zonation and porous structures. Laser-ablation inductively coupled mass spectrometry (LA-ICP-MS) element mapping confirms oscillatory and sectorial zoned domains in hematite grains are enriched in 238U- and 206Pb, and are distinct from those of W and Mo. The crystal structure and absence of inclusions within zoned hematite was assessed by transmission electron microscopy on foils obtained by in situ slicing across zonation patterns using the scanning electron microscope-focused ion beam technique. Satellite reflections on the electron diffraction patterns obtained from banded zones in hematite are attributable to long-range superstructure ordering, that is inferred to assist metal incorporation via the substitution 2Fe3+↔Me6++vacancy, where Me=U, W, Mo.The suitability of U-bearing hematite for Pb–Pb geochronology as a first pass was tested on both zoned and porous hematite grains via LA-ICP-MS, using the GJ-1 zircon as the primary external standard. Only Pb–Pb ages were considered and resulted in 207Pb–206Pb ages of 1590±8Ma and 1577±5Ma for oscillatory and sector zoned hematite from two samples. Although reconnaissance in nature, these ages potentially support the supposition that mineralization is coeval with emplacement of the Gawler Range Volcanics and associated Hiltaba Intrusive Suite. The geochronological application utilizing an abundant refractory mineral represents a new tool for dating iron-bearing ores.

Electrical conductors in Archean mantle—Result of plume interaction?

From long‐period magnetotelluric data across the Gawler Craton, we obtain a three‐dimensional resistivity image of the lithosphere and provide constraints on tectonothermal events dating back to the Proterozoic. Contrary to common observations of Archean cratons displaying high electrical resistivity in the mantle lithosphere, the magnetotelluric data show low resistivity of around 10Ω m at 80km depth underneath the 1595Ma Gawler Range Volcanics, a silicic large igneous province. The resistivity distribution appears to be a signature of plume‐modified orogenesis with low‐degree partial melting at the base of the lithosphere in a back‐arc setting. The enhanced conductivity is explained through higher hydrogen and iron content in the crystal lattice and also along the grain boundaries of the mantle constituting minerals. Older, arc‐related magmatism to the southwest across 1620–1610Ma St. Peter Suite does not display enhanced conductivity and suggests a depleted mantle lithosphere. The data show that Yellowstone‐type mantle plume analogues are preserved through time and still display an elevated electrical signature in the lithosphere today.

The fluorine link between a supergiant ore deposit and a silicic large igneous province: COMMENT

I welcome the description by [McPhie et al. (2011)][1] of the major role played by fluorine in the genesis of mineral deposits at Olympic Dam (Australia), and more broadly in the Gawler Range Volcanics. It is important to recall that intrusive granitic rocks with 1%–2% fluorine (F) are a

Evolution and emplacement of high fluorine rhyolites in the Mesoproterozoic Gawler silicic large igneous province, South Australia

The Gawler Range Volcanics (GRV) and the Hiltaba Suite (HS) of South Australia form a silicic-dominated large igneous province (the Gawler SLIP) emplaced in an intracontinental setting during the Mesoproterozoic. Emplacement of the GRV lasted for a short period of time (∼2Ma), and can be separated into two main phases. The first phase (lower GRV) is composed of thick (≤3km) sequences erupted from distinct centres, and includes small to moderate volume (up to >150km3) felsic lavas, ignimbrites, and minor mafic and intermediate lavas. The upper GRV include extensive felsic lavas that are up to >1000 of km3 in volume and >200km across. Using well preserved, quartz-hosted melt inclusions, we investigated the composition of the lower GRV, including major, trace, and volatile elements. The results indicate high concentrations of K2O (≤7–8wt.%), rare earth and high field strength elements, and low concentrations of Ca, Mg, Ni, Cr, Sr and Ba in comparison with felsic continental crust. Overall, melt inclusion compositions match whole-rock geochemical characteristics. We demonstrate that the GRV magma was F-rich (≤1.3wt.%), and had high temperature for a silicic magma. High F concentrations and high temperature would have resulted in lower than usual polymerisation of the melt and relatively low viscosity. These characteristics help explain how very voluminous felsic magma was erupted effusively and emplaced as lavas. Other intracontinental SLIP contain extensive felsic lavas and ignimbrites which appear to share similar geochemical characteristics. We also show that selective alteration caused depletion of whole-rock compositions in some trace elements, namely Pb, U, and Sn.

Geochemistry and geochronology of the c. 1585 Ma Benagerie Volcanic Suite, southern Australia: Relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province

New ion microprobe (SHRIMP) U–Pb zircon data indicate that voluminous A-type volcanic rocks were extruded c. 1585Ma in the Benagerie Ridge region of the north-central Curnamona Province in southern Australia. Incompatible trace element ratios and whole-rock Sm–Nd isotope compositions (ɛNd (1585)=−4.3 to −2.2) suggest a dominant crustal source region for the felsic rocks. Incompatible trace element ratios in the basalts, enriched light REE/heavy REE ratios (La/Yb=6–7.5), and ɛNd (1585) values between −1.5 and 0.2 suggest that the basalts were derived from a HFSE-enriched magma from within the mantle. These new data show that the Benagerie Volcanic Suite share geochronological, geochemical, and isotopic affinities with the upper Gawler Range Volcanics of the Gawler Craton indicating these two volcanics suites can be considered part of a formerly contiguous Mesoproterozoic silicic large igneous province. We invoke aspects of the two-phase model for the generation of this A-type silicic large igneous province presented by previous workers and attempt to place the initial ‘developmental’ and subsequent ‘mature’ phases of this event into a tectonic framework for the early Mesoproterozoic of the Curnamona Province and the adjacent Gawler Craton. Initiation of the developmental phase likely resulted from lithospheric extension and was accompanied by localised basaltic magmatism. The mature phase, which involved eruption of voluminous felsic volcanic rocks, was a result of widespread crustal melting, potentially induced by the elevated geotherm caused by extension, by ponding of mafic melt in the lower crust, or a combination of these factors. The high geothermal gradient that resulted from extension and magmatism likely primed the crust for the early Mesoproterozoic regional deformation and metamorphism that occurred across the Curnamona Province and Gawler Craton. The far-field tectonic drivers for this silicic large igneous province remain uncertain, although the association between extensive bimodal A-type magmatism, high-temperature metamorphism, and localised compressional deformation, is suggestive of an intracontinental setting, possibly mechanically connected to a far-field subduction zone.

Mesoarchean to Mesoproterozoic evolution of the southern Gawler Craton, South Australia

The Gawler Craton preserves a complex and prolonged tectonic history spanning the interval c. 3200-1500 Ma. Reworking of Paleoarchean, c. 3400-3250 Ma crust led to the formation of c. 3150 Ma granites now exposed within a narrow belt in the eastern Gawler Craton. Following this, there is no known record of significant tectonic activity until the onset of bimodal magmatism during the Neoarchean to earliest Paleoproterozoic, c. 2560-2470 Ma. This magmatism was terminated by high temperature metamorphism and deformation during the 2465-2410 Ma Sleafordian Orogeny. Magmatic events associated with widespread sedimentation over the interval c. 2000-1740 Ma largely sources this older crust. The c. 1730-1690 Ma Kimban Orogeny reworked these Paleoproterozoic basins and the Neoarchean basement in a pre-dominantly transpressional orogenic system. Juvenile mantle input followed by widespread crustal melting occurred over the interval c. 1620-1570 Ma. This period of intense magmatism initiated with emplacement of the relatively juvenile c. 1620-1608 Ma St Peter Suite. This was followed by the economically significant c. 1600-1570 Ma Gawler Range Volcanics/Hiltaba Suite magmatic event, which resulted from widespread mid-crustal melting. Synchronous deformation and high temperature metamorphism accompanied the Gawler Range Volcanics/Hiltaba Suite magmatic event indicating it occurred in an orogenic environment. Far field stress was distributed around a central core zone of largely undisturbed Gawler Range Volcanics with deformation localised in the northern and southern Gawler Craton. The Gawler Range Volcanics/Hiltaba Suite magmatic event resulted in formation of a province of major economic significance that includes the giant Olympic Dam Cu-Au-U ore body.

Magma chamber dynamics in a silicic LIP revealed by quartz: The Mesoproterozoic Gawler Range Volcanics

Silicic-dominated large igneous provinces (SLIP) represent vast amounts of magma (≥ 10 5 km 3) erupted onto the Earth's surface or injected into the crust over short time spans, and are important components of the continental crust. The conditions of formation and evolution of these large magmatic provinces and their magma chambers are still poorly constrained. In this contribution, we examine cathodoluminescence textures and trace element (Al, Ti, Fe) zoning of quartz in a Mesoproterozoic SLIP, the Gawler Range Volcanics (GRV), South Australia. We describe intra-granular textures such as truncation of growth textures and reverse zoning (rimwards increase of Ti content). These characteristics of quartz, together with remelting of already crystallised portions of the magma chamber (felsic enclaves), suggest a complex history of crystallisation and resorption, and fluctuating magma temperature. Titanium-in-quartz geothermometry indicates that adjacent quartz zones record temperature variations (ΔT) up to 70 °C in volcanic units. We also report contrasting (non-correlatable) zoning patterns amongst quartz crystals, each indicating different crystallisation conditions. The juxtaposition of quartz crystals with contrasting zoning patterns is consistent with a dynamic regime (convection, stirring, overturning) of the GRV magma chamber. These results point to pulsating magmatic conditions, compatible with a non-linear evolution of the GRV magma chamber. Heat, necessary to explain both intra-granular and infra-granular textural variations, may have been provided in different pulses by underplating of mafic magma.

Magnetic characteristics of the Hiltaba Suite Granitoids and Volcanics: Late Devonian overprinting and related thermal history of the Gawler Craton

The ∼1590 Ma Gawler Range Volcanics (GRV) and 1595–1575 Ma Hiltaba Suite Granitoids (HSG), from the central Gawler Craton of South Australia, can be divided into an oxidised Roxby Downs-type containing hematite and magnetite, and a less oxidised Kokatha-type that commonly contains ilmenite and titanomagnetite. In both types the predominant magnetic phase is pure end-member magnetite, except for a minor amount of titanomagnetite present in the Kokatha-type Black Yardea Dacite. The characteristic remanence observed in the GRV is directed upward to the northeast. A fold test is strongly negative, indicating that this characteristic remanence is a post-folding overprint although the age of folding is uncertain. Rock magnetic properties identify magnetically unstable multidomain magnetite particles. The magnetisation has been blocked as the crust cooled from above multidomain (MD) unblocking temperatures probably during a mid-phase of the Alice Springs Orogeny (ASO). The in situ mean direction, not corrected for tilt, from 69 samples is declination = 22.6°, inclination = −44.9°, α 95 = 6.9° and the corresponding pole position is latitude λp = 69.5°S, longitude ϕp = 36.1°E, dp = 5.5°, dm = 8.7°. No directions of opposite polarity were found and no consistent remanence was found in either type of the HSG. The pole position from GRV is consistent with other Australian paleopoles from rocks of about 360–370 Ma. Apatite fission track ages (AFTA) for the Gawler Craton have yielded a mean age of 331 ± 30 Ma. It is likely that the remanence and fission tracks have been reset during exhumation/cooling after the ASO. The younger AFTA of ∼330 Ma are consistent with the track retention temperature (100 ± 20°C) being attained after the higher remanence blocking temperature of ∼>200°C. The apatite ages are consistent with thermal modelling of 40Ar/39Ar data from K-feldspars from the nearby Mount Painter Province.

A Mesoproterozoic continental flood rhyolite province, the Gawler Ranges, Australia: the end member example of the Large Igneous Province clan

Abstract. Rhyolite and dacite lavas of the Mesoproterozoic upper Gawler Range Volcanics (GRV) (&gt;30 000 km3 preserved), South Australia, represent the remnants of one of the most voluminous felsic magmatic events preserved on Earth. Geophysical interpretation suggests eruption from a central cluster of feeder vents which supplied large-scale lobate flows &gt;100 km in length. Pigeonite inversion thermometers indicate eruption temperatures of 950–1100 °C. The lavas are A-type in composition (e.g. high Ga/Al ratios) and characterised by elevated primary halogen concentrations (~1600 ppm fluorine, ~400 ppm chlorine). These depolymerised the magma such that temperature-composition-volatile non-Arrhenian melt viscosity modelling suggests they had viscosities of &lt;3.5 log η (Pa s). These physicochemical properties have led to the emplacement of a Large Rhyolite Province, which has affinities in emplacement style to Large Basaltic Provinces. The low viscosity of these felsic magmas has produced a unique igneous system on a scale which is either not present or poorly preserved elsewhere on the planet. The Gawler Range Volcanic Province represents the erupted portion of the felsic end member of the family of voluminous, rapidly emplaced terrestrial magmatic provinces.

Ediacaran ice-rafting and coeval asteroid impact, South Australia: insights into the terminal Proterozoic environment

Isolated quartzose pebbles, clusters of quartz granules, orthogonal aggregates of poorly sorted quartzose coarse sand, and ovoid pellets (≤2 mm long) of quartz silt occur in hemipelagic marine mudstone of the mid-Ediacaran Bunyeroo Formation exposed in the Adelaide Geosyncline (Adelaide Rift Complex), and ovoid pellets of quartz silt in cores of the correlative marine Dey Dey Mudstone from deep drillholes in the Officer Basin, South Australia. This detritus is interpreted respectively as dropstones, dumps, and frozen aggregates dispersed by sea ice possibly of seasonal origin, and till pellets transported by glacial ice. The ice-rafted material in the Bunyeroo Formation only has been found <10 m stratigraphically below and above a horizon of dacitic ejecta related to the 90 km diameter Acraman impact structure in the Mesoproterozoic Gawler Range Volcanics 300 km to the west. Furthermore, till pellets have been identified 4.4 to 68 m below distal Acraman ejecta in the Dey Dey Mudstone >500 km northwest of the impact site. The Acraman impact took place at a low paleolatitude (∼12.5°) and would have adversely affected the global environment. The stratigraphic observations imply, however, that the impact occurred during, but did not trigger, a cold interval marked by sea ice and glacial ice, although the temporal relationship with Ediacaran glaciations elsewhere in Australia and on other continents is unclear. Release from the combined environmental stresses of a frigid, glacial climate near sea-level and a major impact in low latitudes may have been a factor influencing subsequent Ediacaran biotic evolution.

U–Pb geochronology of the Western Channel Diabase, northwestern Laurentia: Implications for a large 1.59 Ga magmatic province, Laurentia's APWP and paleocontinental reconstructions of Laurentia, Baltica and Gawler craton of southern Australia

U–Pb baddeleyite ages of 1592 ± 3 and 1590 ± 4 Ma are reported for paleomagnetic sites in sheets and dykes of Western Channel Diabase (WCD) that intrude Proterozoic rocks of the flat-lying Hornby Bay Group in the Hornby Bay basin and the deformed volcanic-plutonic Great Bear Magmatic Zone of Wopmay orogen of northwestern Laurentia. A published WCD paleomagnetic pole at 9°N, 115°W ( A 95 = 6°) has been demonstrated primary. The new ages indicate that the WCD pole falls midway in time between poles for the 1.74 Ga Cleaver dykes and 1.48–1.42 Ga Elsonian-aged plutons, filling an important gap in the Proterozoic apparent polar wander path (APWP) for Laurentia. The WCD pole can be compared with poles reported from similar-aged magmatic units on other cratons in order to test paleocontinental reconstructions. A comparison of the Laurentian WCD pole with primary ca. 1.63 Ga and ca. 1.575 Ga poles for Baltica, along with an earlier comparison of precisely dated 1.27–1.255 Ga poles for Laurentia and Baltica, suggests that the two cratonic blocks drifted as a single entity with Baltica adjacent to eastern Greenland during the ca. 1.59–1.27 Ga interval. On the basis of less well constrained ca. 1.84–1.83 Ga poles from Laurentia and Baltica, it is possible that this reconstruction existed as early as ca. 1.83 Ga. The WCD is the same age as Wernecke breccias of the Wernecke and Ogilvie Mountains of northwestern Laurentia and bimodal Gawler Range Volcanics (GRV) and related Olympic Dam breccias of the Gawler craton. It has been proposed by others that the Gawler craton lay adjacent to northwestern Laurentia at 1.59 Ga, with the Olympic Dam and Wernecke breccias forming a large hydrothermal province. The primary WCD pole provides an opportunity to test Laurentia–Gawler craton reconstructions at 1.59 Ga. A paleopole has been reported for the GRV, although its primary or secondary nature is open to interpretation. If primary, or if acquired as an overprint during the later stages of 1.60–1.58 Ga Hiltaba-GRV magmatism, then a position for the Gawler craton adjacent to northwestern Laurentia is permitted. If the GRV pole is a later secondary overprint then a reliable comparison with Laurentian poles cannot be made.

Crustal imaging of a mobile belt using magnetotellurics: An example of the Fowler Domain in South Australia

A long‐period magnetotelluric data set was obtained in 2005 along a two‐dimensional profile across the western part of the late Archaean‐early Proterozoic Gawler Craton, South Australia. The study is aimed at delineating the electrical conductivity structure of the crust and upper mantle underneath an east‐west trending profile extending from the Gawler Range Volcanics in the east, crossing the Nuyts Domain and the highly prospective Meso‐Proterozoic Fowler Domain, and terminating in the Eucla Basin to the west. The resistivity model shows a very electrically resistive crust and upper mantle underneath the Nuyts and Fowler Domain, possibly representing the cratonic root of the Gawler Craton extending to depths of ∼160 km. The resistive cratonic root is closer to the surface underneath the Fowler Domain compared to the Nuyts Domain which supports findings from outcrops of metasediments of higher metamorphic grade in the Fowler Domain. A subvertical conductor marks the western terminus of the Fowler Domain and is imaged to upper mantle depths. On the eastern side of the Fowler Domain, another subvertical conductor extends to similar depths. These features spatially coincide with the Tallacootra and Coorabie shear zone at the surface, respectively, and their higher conductivity is likely due to a reduction in grain size of olivine associated with an increase in influence of grain boundary diffusion and thus enhanced conductivity. A comparison of the results with other surveys across mobile belts worldwide shows a more resistive response of the interpreted mobile belt of the Fowler Domain raising questions as to the nature of the domain.

The role of fluorine in the concentration and transport of lithophile trace elements in felsic magmas: Insights from the Gawler Range Volcanics, South Australia

Rhyolites of the Mesoproterozoic Gawler Range Volcanics (GRV) of South Australia are characterised by high concentrations of some trace elements (REE, Y. HFSE, Rb and F, in particular). Whole rock geochemical data suggest that these elements were incompatible during magma crystallisation. Accessory minerals (fluorite, zircon, REE-F-carbonate, Ti oxide, apatite, and titanite) can account for most of the trace element content of the rocks. These minerals occur in vesicles, micromiaroles, lithophysal vugs and in interstices between major mineral phases (quartz and feldspar as both phenocrysts and groundmass). Such textural evidence indicates that accessory minerals crystallised late in the history of the magma and that they were deposited from a volatile-rich (fluid) phase. These features are explained by the following sequence of events: 1) F dissolved in the magma lowered the crystallisation temperature of accessory minerals, causing trace elements (REE, Y. and HFSE) to behave as incompatible elements. 2) Protracted crystallisation of major mineral phases (quartz, feldspar, and oxides) formed a volatile- and trace element-enriched residual liquid. 3) A volatile element (H2O, F, and CO2)-rich phase (late-stage magmatic fluid) evolved from the magma. High concentration of fluorine and other complexing agents in this phase allowed trace elements to be transported in solution. 4) Accessory minerals crystallised from such a phase in vesicles, micromiaroles and interstices between the major mineral phases