Joining the dots: Insights into the magmatic nickel sulphide potential of the western Gawler Craton

Reid, Anthony1,2, Pawley, Mark1

1Geological Survey of South Australia, Department for Energy and Mining, 11 Waymouth St, Adelaide, South Australia, 5001, Australia 2Department of Earth Sciences, University of Adelaide, South Australia, 5005, Australia

Nickel is back in high demand, with the result that the risk equation for greenfield exploration is significantly improved. Of Australian Proterozoic terranes, the Gawler Craton in South Australia stands out as a terrane with a perceived lack of nickel potential. But is this really the case, or is the current prospectivity telling us more about a lack of data, than a lack of metals?

Recent exploration drilling in the western Gawler Craton intersected over 200m (down-hole length) of nickel and copper bearing sulphides hosted by a pyroxenite intrusive. This greenfields discovery by Western Areas in JV with Iluka Resources is highly encouraging. While previous drilling has intersected mafic and ultramafic intrusions with minor sulphide, this has been the first drilling to demonstrate that significant sulphide mineralisation is present in the region.

A review of the geological and geophysical features of the western Gawler Craton demonstrates why this intersection is unlikely to be the last. Key features of the western Gawler Craton include:

  • Curvature of the lithosphere-asthenosphere boundary upwards towards the western edge of the craton.
  • Topography on the Moho, with up to 5-10 km offsets across major structures imaged in crustal reflection seismic.
  • Trans-lithospheric conductivity zones images in magnetotelluric data that correspond to major structures in regional aeromagnetic data.
  • A complex network of anastomosing shear zones, with evidence for poly-metamorphism and syn-deformation intrusion.
  • The localisation of different magmatic events into specific regions, with the boundaries between these magmatic domains corresponding to major structural boundaries in the region.
  • Variation in (admittedly sparse) radiogenic isotope composition of these magmatic rocks suggesting variation in composition of the deep crust across the region.
  • Geochemistry of mafic and ultramafic rocks suggests an enriched mantle source coupled with crustal interaction and assimilation.
  • A metasedimentary component to the region, including an Archean to earliest Paleoproterozoic succession (Mulgathing Complex) and sedimentary rocks deposited at c. 1700 Ma and c. 1660 Ma.
  • Variable depth of exhumation across the region, with upper crustal lithologies juxtaposed against lower crustal granulites across major structures.

Joining the dots along these lines of evidence suggests a broad geological setting that is comparable to other terranes with proven nickel-copper-PGE resources. In the Gawler Craton melting of previously fertilised lithospheric mantle produced mafic and ultramafic magmas that were able to ascend towards the crust. Assisted by trans-lithospheric structures and complex deformation these magmas were likely focused into the crust during one or more tectonothermal events, where they underwent crustal assimilation. Variation in exhumation depth across the region as a result of the complex anastomosing shear zone network means magma chambers, magma transfer zones and the root zones are likely present in different parts of the region providing opportunities for the formation and preservation of sulphide accumulation zones. Future work in this region including mapping of sparse outcrop, geophysical interpretation, systematic geochronology and ultimately further exploration drilling will assist to better understand this mineral system.


Biography

Anthony has an interest in tectonics, magmatism, orogenic processes and the relationship of these to the formation of mineral deposits. Anthony works at the Geological Survey of South Australia.

Crustal-scale controls on the evolution of the Yeneena Basin

Tyler, Ian1; Kohanpour, Fariba1; Gorczyk, Weronika1

1Centre for Exploration Targeting, School of Earth Sciences, University of Western Australia, Perth WA 6009, Australia

The Neoproterozoic (<911 Ma to >655 Ma) Yeneena Basin is exposed at the northeastern margin of the West Australian Craton, deposited on extended crust of the Archean Pilbara Craton and the Proterozoic Capricorn Orogen. It is deformed and metamorphosed within the Proterozoic Paterson Orogen and hosts the world-class Telfer Au–Cu mine. The recent Winu Cu–Au discovery is focussing exploration interest under Canning Basin cover.

An understanding of the tectonic setting and geodynamic history of the Yeneena Basin can be derived from published data, enhanced by new information extracted from key exploration and stratigraphic drillholes held in the Geological Survey of Western Australia Core Library, including rock properties, structural analysis and petrography, geochemistry, geochronology, isotope geology, sedimentology and sequence stratigraphy. When integrated with geophysical imagery, including gravity, magnetics and passive and active (reflection) seismic surveys, it can form the basis of a consistent crustal-scale 3D map, which can be used as a framework to understand the geodynamic evolution of the basin and it’s mineral systems. 

In the Yeneena Basin, the lower Throssell Range Group occurs between the Vines Fault, and the Southwest Thrust to the west and the Parallel Range Thrust to the east. The basal Coolbro Sandstone, which has a low magnetic signature, unconformably overlies the 1.8 to 1.3 Ga crystalline basement of the Rudall Province, and is overlain by the Broadhurst Formation, a sequence of carbonaceous and sulfidic shale, sandstone and dolomite that extends beneath shallow cover to the northwest. The overlying Isdell Formation is predominantly carbonate and is overlain in the Parallel Range Thrust zone by the Lamil Group comprising the Malu Formation (quartz sandstone), the overlying Puntapunta Formation (carbonate, sandstone, shale and siltstone) and the Wilki Formation (quartz sandstone and shale).

The Tarcunyah Group in the adjacent North West Officer Basin is part of the 850–700 Ma Supersequence 1 of the Centralian Superbasin. It includes a basal siliciclastic package, which unconformably overlies the Archean Fortescue Group west of the Vines Fault, and an overlying shallow-water shelf evaporate-carbonate package, which contains distinctive stromatolite assemblages. In magnetic images, a transition can be seen from the non-magnetic evaporate-carbonate package across the Southwest Thrust and into the non-stromatolitic and magnetic sedimentary rocks of the Broadhurst Formation. This is consistent with the Throssell Range Group comprising deeper water, basin floor deposits, laterally equivalent to Supersequence 1.

The sediment-hosted Nifty Cu deposit has been dated at 823–791 Ma, and can be linked to 830 Ma mafic intrusions emplaced during basin extension. Inversion of the basin by southwest verging thrusting and associated dextral strike-slip faulting took place during the Miles Orogeny at 655 Ma. Au–Cu and W mineralisation is associated with intrusions of the 650–610 Ma Mount Crofton Granite east of the Parallel Range Thrust. To the north and east of Telfer, the upper Lamil Group is buried by less than 300 m of Phanerozoic sedimentary rocks deposited in the overlying Canning Basin. Important elements for Au-Cu mineralisation targeting can be identified in geophysical images.


Biography

Fariba Kohanpour has completed her PhD in February 2019 in the Centre for Exploration Targeting. she applied geodynamic modelling, geophysical interpretation, and isotope analysis to understand tectonic evolution of the Halls Creek Orogen. Now she is doing her postdoc for investigating geodynamics of Paterson region.

On the destructive tendencies of cratons:  3D geodynamics modelling of cratons and subduction

1Farrington, Rebecca; 2Cooper, Katie; 3Miller, Meghan.

1School of Earth Sciences, The University of Melbourne, Melbourne, Victoria 3010, Australia 2School of the Environment, Washington State University, Pullman, Washington 99164, USA 3Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory 2600, Australia

Subduction of lithosphere at convergent plate boundaries drives large-scale mantle flow patterns. When in the vicinity of craton margins this mantle flow can provide the potential for craton destruction. Examples of this setting can be seen along the northern margin of South America and north-western Africa where active subduction zones occur adjacent to craton margins. We present a 3D numerical geodynamic study exploring the interaction between subducting lithosphere and craton margins. We propose that subducting slabs can direct flow along craton margins, a process that may shape and carve these margins and impact the overall stability of the craton.


Biography

Rebecca is a senior research fellow in the School of Earth Sciences, Faculty of Science and the Petascale Campus Initiative, Chancellery (Research). With research expertise in computational mathematics and geodynamics, she leads a team developing the solid Earth dynamics code Underworld. As a leader in the University of Melbourne wide Petascale Campus Initiative and Auscope, the geoscience National Research Infrastructure organisation, she champions the sustainable development of academic and community-led data-intensive research programs.

Evidence for a 3.2–3.1 Ga accretionary orogeny along the south-eastern edge of the Kaapvaal Craton: a regional setting for late-stage gold mineralisation in Barberton

Taylor, Jeanne1,2

1Department of Earth Sciences, Stellenbosch University, South Africa. 2Institute of Geoscience, Goethe University Frankfurt am Main, Germany.

The Barberton Granite-Greenstone Belt (BGGB) of South Africa is one of only a few exceptionally preserved Paleo- to Mesoarchaean terranes in the world able to advance our understanding of tectonic processes operating on the early Earth. Together with the Pilbara Craton of Western Australia, it has fuelled the debate of uniformitarian (modern-style subduction-accretion) versus non-uniformitarian (vertical) geodynamic models for Archaean crustal evolution. Barberton is also known for its world-class lode-gold deposits, which formed late in its tectonic evolution during at least two episodes, at 3080 Ma and at 3040–3015 Ma. However, the thermal-tectonic processes responsible for gold mineralisation remain somewhat cryptic.

The Ancient Gneiss Complex (AGC) of Swaziland constitutes a fragment of pristine 3.7–2.7 Ga continental crust in direct contact with the south-eastern margin of the BGGB, and represents a unique opportunity to interrogate contrasting tectonic models. A large body of recent data from 3.23–3.22 Ma granitic rocks, and high-grade, aluminous clastic meta-sediments deposited at 3.53–3.43 Ga and ~3.2 Ga, is largely inconsistent with a non-uniformitarian geodynamic model. Granulite-facies meta-sedimentary rocks display extraordinarily complex metamorphic histories within single hand-samples, which have been deciphered by high-resolution, in situ dating of accessory phases. These rocks seemingly experienced an early thermal episode at 3.43 ̶ 3.40 Ga, followed by two granulite-facies events at 3.23 Ga and 3.15 ̶ 3.05 Ga. Peak metamorphic conditions of 830–875 °C and 6.5–7.6 kbar were reached by 3.11–3.07 Ga, accompanied by extensive in situ partial melting. A final retrograde thermal overprint took place at 2.73 Ga involving rehydration and further decompression. Syn- to post-peak metamorphic (i.e., 3.11–3.07 Ga) deformation fabrics in the granulites (NW-SE directed compression and synchronous NE-SW extension) are similar in character, and coaxial with large-scale deformation features in Barberton and the surrounding 3.23–3.07 Ga AGC granites.

Detrital zircon U-Pb age spectra from clastic meta-sediments deposited at ~3.2 Ga display patterns comparable to those found in modern convergent margin settings (after Cawood et al. 2012), in particular, trench and fore-arc basin environments. Significantly, detrital zircon εHf isotope signatures indicate that these (meta)-sediments were largely derived from an isotopically distinct (i.e., younger, more juvenile) source terrane compared to the BGGB or the AGC, such as a ≤ 3.32 Ga primitive island arc. Detrital zircon age data, combined with in situ dating of metamorphic monazite inclusions in the cores of high-grade garnets (from the same samples), further testify to the rapid, deep burial of these meta-sediments soon after their deposition to ± 25–30 km crustal depths by ~3.10 Ga. In combination, the AGC data provide strong evidence for a long-lived ~3.2–3.1 Ga accretionary margin (involving north-westward subduction) along the south-eastern edge of the proto-Kaapvaal Craton, and support the idea that gold mineralisation in Barberton was linked to late orogenic, transtensional tectonics following terrane accretion. Further connections can be made with long-lived southward subduction and tectonic accretion in the Pietersburg Block (the northernmost terrane of the Kaapvaal Craton), during a comparable time interval between 3.15 and 2.97 Ga.


Biography

My PhD and Postdoctoral research has focused on geodynamic processes operating on the early Earth and the evolution of Archaean cratons; high-grade metamorphism, partial melting of the crust and the production and extraction of granitic magma from their source; radiogenic isotope dating and its behaviour in accessory phases during poly-metamorphism.

Composition and evolution of the southern African lithosphere from combined xenocryst and magnetotelluric data

Özaydin, Sinan1, Selway, Kate1

1Department of Earth and Environmental Sciences, Macquarie University, Australia

Cratons provide us sparse clues about their composition and evolution through xenoliths and the few geophysical methods that can penetrate the cratonic lithosphere. Alone, these methods do not provide enough information to fully understand cratons so it is vital that they be interpreted together so that the fullest possible understanding of cratons can be developed.

In this talk, we focus on the southern African lithosphere, which is arguably the most well-studied cratonic region on Earth. Comparatively voluminous kimberlite magmatism has produced large quantities of mantle xenoliths and xenocrysts that have been used to interpret the composition and evolution of the southern African cratons. However, questions remain about whether these exhumed mantle rocks are representative of the wider lithospheric mantle.

To address the question of cratonic composition, we compare the southern African mantle xenolith and xenocryst data with new, 3D magnetotelluric (MT) models produced from the SAMTEX dataset. Detailed comparisons between the geochemical and MT data around the Jagersfontein and Kimberley kimberlite pipes show that modal mineral compositions and water content measurements from the xenoliths generally agree with those we would interpret from the geophysical data, subject to uncertainties in the geotherm and experimental constraints. However, the water contents interpreted from the MT data and those from the xenoliths do not agree uniformly, suggesting that the region has experienced some localised metasomatism at the scale of the kimberlite pipes that has not affected the mantle more regionally.

In addition to the detailed comparisons around kimberlite pipes, we also consider the broader implications of the MT model for southern African lithosphere composition. As has been observed for most cratonic regions, the MT model over the southern African cratons shows considerable heterogeneity, showing that cratonic compositions must also be heterogeneous. These conductivity contrasts do not generally follow the surface expressions of tectonic boundaries and are more likely to reflect metasomatic enrichment and melting depletion events than lithospheric architecture related to continental assembly. Outcropping kimberlite pipes avoid the regions of highest mantle conductivity, suggesting an interplay between the processes of mantle metasomatism, which produce the strong mantle conductors, and kimberlite magmatism.


Biography

Sinan Özaydin is currently in the final stages of his PhD at Macquarie University. He has developed methods for quantitative interpretation of magnetotelluric data, including publishing the open-source software MATE, and has applied those methods to understanding the southern African lithosphere.

Geodynamic influences on volcanological, paleoenvironmental and tectonic evolution of the Archean Kalgoorlie Terrane LIP, Western Australi

Cas RAF1, Hayman PC2, Squire RJ3, IH Campbell IH4, Wyche S5, Sapkota J6, Smithies H7

1School of Earth, Atmosphere and Environment, Monash University, Vic, 3800, Australia 2Queensland University of Technology, Brisbane, QLD, 4074, Australia 3School of Earth, Atmosphere and Environment, Monash University, Vic, 3800, Australia 4Research School of Earth Sciences, Australian National University, ACT, 4074, Australia 5Geological Survey of Western Australia, Perth and Kalgoorlie, WA, Australia 6Geological Survey of Western Australia, Perth and Kalgoorlie, WA, Australia 7Geological Survey of Western Australia, Perth and Kalgoorlie, WA, Australia

The stratigraphy of the late Archean (>2.7 Ga to 2.658 Ga) Kalgoorlie Terrane Large Igneous Province, in the Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia, preserves an evolution of magmas, eruption processes paleo-environments, sediment provenance, and deformation that are inconsistent with a plate tectonic setting. An initial, deep submarine LIP komatiite and basaltic succession several kms thick (Stage 1 ~2.72-2.69 Ga) of lavas, hyaloclastite breccia and sills, with intercalated chert, black mudstones and minor felsic volcanics, extends > 600km along strike. It represents a widespread mantle plume event during regional extensional (D1) rift volcanism. Eruptions occurred in an open, anoxic, deep-water setting with no evidence of nearby emergent continents. Hydrostatic pressure suppressed all explosive activity.

From ~2.69 to 2.67 Ga (Stage 2) felsic TTG magmatism was dominant, represented by submarine lavas, hyaloclastite, monomictic tuffaceous megaturbidites, polymictic volcanogenic turbidite and conglomeratic mass flow deposits, and contemporaneous high-level granitoid intrusions. A major mafic event at ~ 2.80 Ga indicates mantle heat likely caused felsic crustal magmatism. The felsic volcanics and volcaniclastics represent marine intra-basinal felsic volcanoes (lava domes, stratovolcanoes?), some of which grew into shallow water and became emergent, were explosive under low ambient pressures, and produced the submarine tuffaceous megaturbidites. The Stage 2 volcanic conglomerates, including some granitoid clasts, were not generated by orogenic deformation and uplift, but were derived exclusively from the intra-basinal volcanic centres and sub-volcanic plutons, and from Stage 1 komatiite and mafic stratigraphy that was up-domed by diapiric granitoids. Ongoing plume buoyancy and contemporaneous local granitoid diapirism caused regional and local uplift (D2’) and shallowing of basins during ongoing regional extension (D1).

From ~2.67 Ga a transition into more widespread subaerial paleoenvironments is represented by felsic tuffaceous volcanic sediments deposited in fluvial braid-plain, alluvial fan, shallow marine, and local deep-water settings (Stage 3A). At < 2.658 Ga “late basin” polymictic, alluvial fan – fan delta conglomerates and sandstones represent major, regional crustal compression, uplift (D2/D4’, depending on scheme), emergence of large landmasses, and far-field sediment provenance (Stage 3B). There is no geological evidence of a plate tectonic regime prior to 2.658 Ga (i.e. no ophiolites, accretionary prisms, blueschist belts, uplifted orogenic belts, mountainous continents). A long-lived mantle plume caused partial melting of thick metasomatized mafic crust causing uprise of TTG plutons, leading to plutonic diapiric updoming, and changing paleoenvironments and eruption styles from Stages 1 to 3 (~ 50 Myr). Widespread TTG magmatism does not signify a subduction setting, but widespread anatexis of a lid-like crust/lithosphere.


Biography

Ray Cas has interesets in all aspects physical volcanology, including (paleo)environmental influences on eruption styles, geodynamic settings of volcanism, associated mineralisation and Archean paleovolcanology. He leads an annual professional shortcourse on these topics hosted by Monash U, UTasmania and Queensland University of Technology.

Multiple ages of rutile from a single sample of granulite

Durgalakshmi1, Ian S. Williams1, K. Sajeev2

1 Research School of Earth Sciences, Australian National University, Canberra, Australia, 2 Centre for Earth Sciences, Indian Institute of Science, Bengaluru, India

Rutile (TiO2) is a common accessory mineral in hydrothermal and metamorphic rocks that is stable across a wide range of P-T conditions. It can incorporate up to 200 ppm of U, and has a lower closure temperature than zircon, making it a reliable mineral with which to date retrograde metamorphism and low- to medium-grade metamorphic events by the U-Pb technique. Further, Pb and Th are incompatible in rutile, so corrections for its small initial Pb can be made accurately using its 208Pb content.

During prograde metamorphism, rutile forms following the breakdown of biotite or ilmenite as part of a continuous reaction. During retrogression, rutile can be replaced by ilmenite and titanite. Secondary rutile can be formed by hydrothermal alteration, oxidation or exsolution. In high-grade metamorphic rocks, rutile occurs as single crystals in the matrix and/or as inclusions in other minerals such as garnet, pyroxene and amphibole. In low- to medium-grade rocks it usually occurs as needles or polycrystalline aggregates. Using an ion microprobe (SIMS), rutile can be dated in its textural context in thin section, providing age information directly linked to metamorphic reactions. Dating and trace element analysis of separated rutile grains have a wide range of applications from sedimentary provenance studies to dating vein mineralisation and granitic pegmatites that host mineral deposits.

In the Neoarchæan Southern Granulite Terrane, India, rutile preserves a record of the late thermal history that is not provided by other datable minerals such as zircon or monazite. One studied sample of granulite grade felsic gneiss contains at least three distinct generations of rutile that preserve a range of Neoproterozoic to early Palæozoic U-Pb ages. The zircon from the same sample is Neoarchæan, with no evidence of a younger component. The rutile occurs as single ~ 0.5–0.8 mm crystals in the matrix, some of which are rimmed by titanite. Rutile-forming reactions, which can be linked to the metamorphic conditions, have been dated, contributing to unravelling the polymetamorphic and tectonic history of this complex terrane.


Biography

Miss Durgalakshmi is a PhD student at Research School of Earth Sciences, ANU. She works on the Archaean rocks of Southern Granulite Terrane, India.

Way out west – does the Arunta Orogen continue westward beneath the Canning Basin?

Kelsey, David E.1,3, Spaggiari, Catherine V.1, Wingate, Michael T.D.1, Lu, Yongjun1, Fielding, Imogen O.H.1, Finch, Emily G.1,2,3

1Geological Survey of Western Australia, 100 Plain St, East Perth, WA 6004, Australia, 2University of South Australia, 101 Currie St, Adelaide, SA 5001, Australia, 3MinEx CRC

Crystalline basement lies beneath the southeastern margin of the Canning Basin and immediately west of the exposed Arunta Orogen, although whether that basement is the continuation of the Arunta Orogen is unknown. The major bounding fault of the Canning Basin coincides with the inferred trace of the Lasseter Shear Zone, which truncates dominant east-trending structures of the orogen and potentially also terminates it. The Top Up Rise prospect is located above a distinct northeast-trending gravity anomaly bound by northeast-trending shear zones coincident with the Lasseter Shear Zone. Five co-funded Exploration Incentive Scheme diamond drillcores from Top Up Rise contain partially melted or melt-injected upper-amphibolite to low-granulite facies basement rocks. These are currently the only drillcores that intersect the Canning Basin basement in this region and provide a means to test its tectonic affinity, and the significance of the Lasseter Shear Zone.

Petrography of granitic gneiss and paragneiss indicates distinctly higher metamorphic grade than is observed in exposed rocks of the west Arunta Orogen, consistent with their separation by a significant shear zone. SHRIMP U–Pb zircon geochronology of several granitic gneiss samples has so far revealed a single magmatic protolith crystallization age of c. 1875 Ma, which is distinctly older than known magmatic rocks in the Arunta Orogen. Maximum deposition ages of 1877–1822 Ma for the metasedimentary rocks are similar to or younger than the magmatic protolith ages of the granitic gneiss, suggesting that emplacement of granitic rocks predated deposition of at least some of the sedimentary rocks. Zircon rims in both granitic gneiss and paragneiss samples record high-grade metamorphism at 1622–1604 Ma. Although metamorphism of this age is unknown in the west Arunta Orogen, thermal events of similar age may have occurred in the central Arunta Orogen (Alessio et al., 2020, Lithos, https://doi.org/10.1016/j.lithos.2019.105280), and granitic magmatism occurred at this time in the Haast Bluff Domain of the Warumpi Province (NTGS Special Publication 5, 2013).

The granitic protolith ages of c. 1875 Ma from Top Up Rise are different to those of granitic suites in the Lamboo Province and Granites–Tanami Orogen; however, felsic volcanic rocks of the lower Halls Creek Group have been dated at c. 1880 Ma (Phillips et al., 2016, GSWA Report 164) and c. 1880 Ma granitic rocks are known in the Arnhem Province (NTGS Record 2017-008). In contrast, detrital zircon age components at c. 1875 Ma are widespread and form significant age components in the Lander Rock Formation of the Arunta Orogen, the Marboo Formation and Tickalara Metamorphics of the Halls Creek Orogen, as well as in Top Up Rise paragneisses. Hence, the newly identified granitic basement in the Top Up Rise drillcores may be representative of a major source component that fed detritus into turbidite fan systems that included the Lander Rock Formation. This basement could represent part of the ‘missing’ Arunta basement, which would support interpretations that the Arunta does continue westwards, or it could be part of a previously unrecognized Proterozoic crustal element that underlies the Canning Basin.


Biography

The author team is made up of geologists and geochronologists from the GSWA. Emily Finch is a MinEx CRC Embedded Researcher at the GSWA.

Neodymium and oxygen isotope maps of Western Australia

Lu, Yongjun1, Smithies, RH1, Champion, DC2, Wingate, MTD1, Johnson, SP1, Martin, L3, Jeon, H4, Poujol, M5, Zhao, J6, Maas, R7, Creaser, RA8

1Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, 2Geoscience Australia, GPO Box 378, Canberra ACT 2601, 3Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, WA 6009, 4Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden, 5Univ Rennes, CNRS, Géosciences Rennes – UMR 6118, 35000 Rennes, France, 6Radiogenic Isotope Facility, School of Earth Sciences, The University of Queensland, Brisbane, QLD 4072, Australia, 7School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia, 8Dept. Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada

Multi-isotope maps can characterise lithospheric architecture through time, play an increasingly important role in predictive exploration targeting, and are consequently sought-after datasets by industry. We present the first zircon oxygen isotope map and an updated whole-rock Sm–Nd isotope map of Western Australia. These shed new light on crustal evolution and mineral system distributions. The isotope maps were generated from datasets that are subject to ongoing updates as new data are generated and compiled.

Median zircon δ18O values for about 125 igneous rocks have been spatially visualized so far, and coverage currently extends across the Pilbara and Yilgarn Cratons, the Capricorn, Paterson, and Albany-Fraser Orogens and the Eucla basement (Madura and Coompana Provinces). The Pilbara and Yilgarn Cratons are dominated by mantle-like δ18O values (4.7 – 5.9‰), consistent with reworking of igneous material that had not been exposed at the surface. A c. 3.47 Ga diorite, four c. 3.3 Ga hornblende-bearing granitic rocks, and a c. 2.95 Ga hornblende monzogranite in the Pilbara Craton exhibit weakly elevated zircon δ18O values (5.9 – 6.5‰), which together with trace element enrichment were attributed to hydrous sanukitoids or to derivation from a sanukitoid-enriched source. The Capricorn, Paterson, and Albany–Fraser Orogens and the Eucla basement also contain rocks with elevated δ18O values (6.6 – 9.9‰), suggesting significant reworking of upper crustal material during magma genesis. Zircons with sub-mantle δ18O values (<4.7‰) were found for granitic rocks of c. 3.55 Ga in the Sylvania Inlier, of c. 3.44 Ga in the northern Pilbara Craton, and of c. 3.0 and 2.67 Ga in the South West Terrane, suggesting recycling of crustal material subjected to high-temperature hydrothermal alteration, such as observed in post-Archean rift systems or calderas.

Sm–Nd isotopes for about 1120 felsic igneous rocks provide regionally extensive images of crustal architecture. The map of two-stage depleted mantle model ages (TDM2) highlights the distinction between Archean cratons (TDM2 >2.6 Ga) and Proterozoic orogens (TDM2 <2.2 Ga), and isotopic boundaries correlate well with most existing proposed terrane boundaries. However, the isotopic boundary between the South West Terrane and the Youanmi Terrane appears to be about 100 km west of the previously proposed boundary, but correlates well with magnetic and gravity anomaly zones and the distribution of gold mineralization. The crustal residence map highlights predominantly short residence times (<0.5 Ga) for the Pilbara and Yilgarn Cratons, and much longer crustal residence times (>0.8 Ga) in the Paterson, Albany–Fraser, Pinjarra and Capricorn Orogens, suggesting decreased juvenile crust generation in these orogens. 

These maps are directly applicable to metallogeny. For example, most giant gold deposits in WA are located on or near significant isotopic boundaries and tectonic structures. Interestingly, Telfer, Plutonic and giant gold deposits in the Murchison are aligned along a northeast-trending isotopic boundary. Similar boundaries occur between the eastern and western parts of the Pilbara Craton and between the Yilgarn Craton and the Albany–Fraser Orogen. These isotopically defined discontinuities may be important clues to the earliest architectural elements in Western Australia.


Biography

Dr. Yongjun Lu is Senior Geochronologist Isotope Geologist at GSWA. Yongjun has over 15 years’ geological experience, extensive collaboration with industry, government and academia. He has made important contributions to mineral systems science, highlighted by being the 52nd recipient of the Waldemar Lindgren Award of Society of Economic Geologists (SEG).

Records of the Earth’s early crust from apatite inclusions in zircon – development and applications of in situ 87Sr/86Sr analysis by SIMS

Gillespie, Jack1, Kinny, Pete1, Martin, Laure2, Kirkland, Christopher1, Nemchin, Alexander1, Cavosie, Aaron1

1The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, WA 6845, Australia, 2Center for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia

Rb-Sr isotopes in geological materials provide a system for tracing crustal differentiation processes and insights into the evolution of planetary bodies. The ingrowth of radiogenic 87Sr from the decay of 87Rb leads to increased 87Sr/86Sr over time, and due to the strong relationship between the Rb/Sr and SiO2 contents of igneous rocks, this provides a time-integrated window into the evolution of geochemical reservoirs. However, the high geological mobility of both Rb and Sr in whole rocks during metamorphism and fluid alteration means that this record becomes progressively less reliable in older rocks that have experienced post-crystallization geological events.

Strontium is easily substituted into the crystal lattice of apatite, occurring as a trace element in concentrations ranging from less than a hundred parts per million to several weight percent. In contrast, apatite nearly entirely excludes Rb (<1 ppm) resulting in negligible radiogenic ingrowth of 87Sr, and consequently the initial 87Sr/86Sr ratio of the melt from which an apatite crystallizes is faithfully recorded by the mineral. Inclusions of apatite within magmatic zircons are particularly valuable as they are armoured by the more robust host mineral, allowing them to survive subsequent events that might otherwise cause isotopic reset or recrystallization. However, the typically very small size of apatite inclusions in zircon and the complex isobaric interferences on the isotopes of Sr during in-situ mass spectrometry have previously limited the information that can be obtained from this archive.

Our recent development of a method to measure the 87Sr/86Sr ratio in apatite by SIMS with a spot size appropriate for accessing typical mineral inclusions in zircon (<15 µm) makes it possible to routinely analyse the commonly occurring inclusions of apatite in zircon. We have applied this method to determine the initial 87Sr/86Sr ratios of various Eo-Meso Archean igneous rocks by analysing the Sr isotope composition of apatite inclusions. High resolution SEM imaging and EPMA analysis illustrate the primary nature of these inclusions. Combining the measured 87Sr/86Sr of apatite inclusions with the U-Pb age and Hf isotopic composition of the co-genetic zircon host allows for the ‘triangulation’ of the Rb/Sr necessary for the ingrowth of radiogenic strontium over the crustal residence interval calculated from the crystallization and Hf model ages. Examples from SW Greenland and the Narryer Gneiss Terrane of Western Australia suggest that these rocks were derived from the melting of ancient crustal material that was on average of intermediate-felsic rather than mafic composition.


Biography

Jack Gillespie is a post-doctoral researcher at Curtin University working on developing new methods for understanding the evolution of the early earth

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