Recent developments in understanding the Archaean granitic basement of the Barberton Mountain Land and adjacent witwatersrand basin hinterland

Contemporaneous ultramafic and felsic intrusive and extrusive magmatism in the Archaean Boorara Domain, Eastern Goldfields Superterrane, Western Australia, and its implications

The Abitibi Greenstone Belt (AGB) of the Superior Province in Canada, and the extensive greenstone sequences of the Eastern Goldfields Superterrane (EGS) of the east Yilgarn Craton in Western Australia, both contain widespread and well-studied komatiite sequences. Both of these 2˙7 Ga belts contain broadly similar assemblages of volcanic rocks ranging from komatiites to rhyolites, and both contain abundant lode gold mineralisation. Despite this similarity, the two belts have widely differing metal endowments. The EGS is heavily endowed with magmatic Ni–Cu–PGE deposits, but contains only minor volcanogenic massive Cu–Zn sulphide occurrences, whereas the situation in the AGB is the reverse. The komatiite assemblages of the two belts are compared using a locality-weighted analysis of large whole-rock geochemical datasets. The EGS contains a higher relative proportion of highly olivine-rich cumulate rocks relative to the AGB, and shows more extensive evidence for crustal contamination of komatiite magmas. The most highly mineralised region of the AGB, the Shaw and Bartlett Dome area, is the most similar to the EGS in the distribution of komatiite rock types and also the presence of a crustal contamination signature. The bulk of samples in both data sets show no evidence for extensive PGE depletion attributable to sulphide liquid segregation, indicating that the komatiite suites in both belts were undersaturated in sulphide during partial melting, ascent and eruption. The combination of abundant thick cumulate-rich komatiite units and widespread crustal contamination in the EGS can be attributed to a predominance of rapid, high volume eruptions, relative to generally lower eruption rates in the AGB. Differences in lithospheric structure are likely to be the cause.

Two main deformational phases are recognised in the Archaean Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia, primarily involving south-over-north thrust faulting that repeated and thickened the stratigraphy, followed by east-northeast – west-southwest shortening that resulted in macroscale folding of the greenstone lithologies. The domain preserves mid-greenschist facies metamorphic grade, with an increase to lower amphibolite metamorphic grade towards the north of the region. As a result of the deformation and metamorphism, individual stratigraphic horizons are difficult to trace continuously throughout the entire domain. Volcanological and sedimentological textures and structures, primary lithological contacts, petrography and geochemistry have been used to correlate lithofacies between fault-bounded structural blocks. The correlated stratigraphic sequence for the Boorara Domain comprises quartzo-feldspathic turbidite packages, overlain by high-Mg tholeiitic basalt (lower basalt), coherent and clastic dacite facies, intrusive and extrusive komatiite units, an overlying komatiitic basalt unit (upper basalt), and at the stratigraphic top of the sequence, volcaniclastic quartz-rich turbidites. Reconstruction of the stratigraphy and consideration of emplacement dynamics has allowed reconstruction of the emplacement history and setting of the preserved sequence. This involves a felsic, mafic and ultramafic magmatic system emplaced as high-level intrusions, with localised emergent volcanic centres, into a submarine basin in which active sedimentation was occurring.

Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped geology. New Lu–Hf isotopic results, combined with existing Lu–Hf and Sm–Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane.

In this study, we present summary statistics for multi-element soil geochemistry across Western Australia based on over 74 000 soil samples using the UltraFine+ ® method that extracts and analyses the clay (<2 μ m) fraction of a soil sample. This method is a critical advancement for the detection of mobile element signatures for soil geochemical mineral exploration surveys in cover. However, existing estimates of background metal abundances acquired with other methods and on different sample media do not readily provide context for these analyses, as recovery from the fine fraction differs to that of whole-sample analysis. We therefore present herein the geochemical results for 52 elements, including precious, base and critical metals, as well as commonly associated pathfinder elements for Western Australian samples analysed during several research projects by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). This dataset is separated by tectonic unit into the Eastern Goldfields Superterrane and the Youanmi Terrane in the Yilgarn Craton, the Pilbara Craton and Sylvania Inlier, the Gascoyne, Lamboo and Aileron Provinces, and the Bryah and Earaheedy Basins to provide exploration-relevant context in these areas. We discuss some of the general trends observed for 12 of these elements, as well as some considerations for the use of these data in comparison to other geochemical soil surveys and with regards to mineral exploration settings. The samples presented in this study are not evenly distributed across Western Australia and limited information is available to correlate whether lithology at depth is mineralized or barren. However, in the absence of other systematic datasets using the <2 μ m-size fraction, these data present a suitable first-pass resource of element abundance ranges in areas of mineral exploration interest using the UltraFine+ ® method in some of the mineral-endowed areas of Western Australia. Supplementary material: Supplementary data A, UltraFine+ ® Soil Sampling Guide; B, UltraFine+ ® analyses results for available samples in Western Australia; C, Summary statistics of UltraFine+ ® analyses by simplified tectonic units; Supplementary Figure 1, Comparison of elemental mean values and uncertainties for ultrafine reference materials QC 320 and QC 422; Supplementary Figure 2, Violin plots of Ni, Cd, Mg and Au by simplified tectonic unit; Supplementary Figure 3, Location of soil samples in the Eastern Goldfields Superterrane analysed via UltraFine+ ® , and those collected as part of the NGSA and GSWA regolith programme; Supplementary Figure 4, Comparison of W, Al, Mg and Ca results from UltraFine+ ® , NGSA and GSWA analyses in the Eastern Goldfields Superterrane are available at https://doi.org/10.6084/m9.figshare.c.6919933

An Archaean submarine volcanic debris avalanche deposit, Yilgarn Craton, western Australia, with komatiite, basalt and dacite megablocks: The product of dome collapse

Age, origin and significance of nodular sulfides in 2680 Ma carbonaceous black shale of the Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia

Geochemie und Geochronologie des Erongo-Komplexes, Namibia

Detrital-zircon age-spectra for Late Archaean synorogenic basins of the Eastern Goldfields Superterrane, Western Australia

New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geological understanding of this region. The Yilgarn Craton had previously been divided into a series of terranes, with the easternmost Eastern Goldfields Superterrane separated from the Youanmi Terrane, which forms the core of the protocraton, by the Ida Fault zone. The Eastern Goldfields Superterrane was subdivided into the western Kalgoorlie, central Kurnalpi, and eastern Burtville terranes, with the latter, easternmost terrane the focus of the new field mapping and geochronology. Four main episodes of greenstone crustal growth have been recognised in the northeast Yilgarn Craton: ca 2970–2910 Ma, ca 2815–2800 Ma, 2775–2735 Ma, and ca 2715–2630 Ma. Rather than a single Burtville Terrane, as previously proposed, the distribution of greenstone magmatism reveals a previously unrecognised young (<2720 Ma) Yamarna Terrane in the northeast corner of the craton. The Yamarna Terrane is separated from the older (>2735 Ma) redefined Burtville Terrane by the Yamarna Shear Zone, which is now regarded as a terrane boundary. The correlation of lithologies and ages of magmatism in the northeast Yilgarn Craton with the rest of the craton indicates that the Burtville Terrane has affinities with the Youanmi Terrane that forms the nucleus of the craton, whereas the Yamarna Terrane has affinities with the Kalgoorlie Terrane in the west of the Eastern Goldfields Superterrane. The Burtville and Youanmi terranes shared a common history from ca 2970 Ma until ca 2720 Ma, when regional extension accommodated deposition of the Kambalda Sequence in the Kalgoorlie Terrane. It appears that extension also occurred along the Yamarna Shear Zone after ca 2720 Ma, accommodating the deposition of greenstones in the Yamarna Terrane. Like the Kalgoorlie and Kurnalpi Terranes, the Yamarna Terrane contains inherited zircon and local older rocks. This suggests that the ca 2720 Ma extension did not result in widespread rifting and the formation of extensive oceanic crust. Rather, there was thinning of older crust that extended right across the current Yilgarn Craton.

Magnetic grid resolution enhancement using machine learning: A case study from the Eastern Goldfields Superterrane

Apatite was separated from four samples of syenite porphyry, taken from the Karari gold deposit, in the Kurnalpi Terrane of the Archean Kalgoorlie-Kurnalpi Rift, Eastern Goldfields Superterrane (EGST). The alkalic composition of the syenitic magmas inhibited zircon crystallisation, so apatite provided the best mineral for geochronological investigations. LA-ICP-MS analysis of U, Th and Pb isotopes in the apatite gave a relatively wide range of lower intercept ages, with large errors, ranging from 1 to 3%, using OD-306 apatite as the primary standard. Cathodoluminescent (CL)-darker cores that comprise the major volume of apatite grains are relatively homogeneous in two samples, with one having clear oscillatory zoning. These samples yielded intercept ages of 2701 ± 34 Ma and 2699 ± 25 Ma, respectively. These ages are interpreted to approximate the magmatic crystallisation age of the apatite. Younger intercept ages were generated by apatite from two other samples, which display more complex and heterogeneous patterns of CL brightness. The apatite ages from these two samples are interpreted to have been produced by integrated analysis of apatite that has been heterogeneously modified by younger events. However, the magnitude of the temporal gap between magma emplacement and closure of the U–Pb system in apatite from these two samples remains unknown. Our best estimate of the age of the magmatic apatite from at least two of the syenitic intrusions at Karari is ca 2.70 Ga, which identifies these as the oldest intrusions of the Syenitic Group of magmas yet identified in the EGST. However, if ages are corrected to offset observed in the 401 apatite secondary standard, the two oldest syenitic intrusions are dated at ca 2660 Ma.

Pre-1.6 Ga rocks comprise around 45% of the onshore area of Western Australia (WA), constituting the West Australian Craton (WAC) (including the Archean Yilgarn and Pilbara Cratons) and the western part of the North Australian Craton (NAC). These areas provide the conditions suitable for diamond formation at depth, and numerous diamondiferous lamproite and kimberlite fields are known. As emplacement ages span close to 2500 Ma, there are significant opportunities for diamond-affinity rocks being present near-surface in much of the State, including amongst Phanerozoic rocks. WA’s size, terrain, infrastructure and climate, mean that many areas remain underexplored. However, continuous diamond exploration since the 1970s has resulted in abundant data. In order to advance future exploration, a comprehensive database of results of diamond exploration sampling (Geological Survey of Western Australia 2018) has been assessed. The Yilgarn and Pilbara Cratons have spinel indicators almost exclusively dominated by chromite (>90% of grains), whereas (Mg,Fe,Ti)-bearing Al-chromites account for more of the indicator spinels in the NAC, up to 50% of grains at the Northern Territory (NT) border. Increasing dominance of Al in chromites is interpreted as a sign of weathering or a shallower source than Al-depleted Mg-chromites. Garnet compositions across the State also correlate with geological subdivisions, with lherzolitic garnets showing more prospective compositions (Ca-depleted) in WAC samples compared to the NAC. WAC samples also show a much broader scatter into strongly diamond-prospective G10 and G10D compositions. Ilmenites from the NAC show Mg-enriched compositions (consistent with kimberlites), over and above those present in NT data. However, ilmenites from the WAC again show the most diamond-prospective trends. Numerous indicator mineral concentrations throughout the State have unknown sources. Due in part to the presence of diamondiferous lamproites, it is cautioned that some accepted indicator mineral criteria do not apply in parts of WA. For example Ca-depleted garnets, Mg-depleted ilmenites and Cr-depleted and Al-absent clinopyroxenes are all sometimes associated with strongly diamondiferous localities. Quantitative prospectivity analysis has also been carried out based on the extent and results of sampling, age of surface rocks relative to ages of diamond-prospective rocks, and the underlying mantle structure. Results show that locations within the NAC and with proximity to WA’s diamond mines score well. However, results point to parts of the WAC being more prospective, consistent with mineral chemical data. Most notable are the Hamersley Basin, Eastern Goldfields Superterrane and the Goodin Inlier of the Yilgarn Craton. Despite prolific diamond exploration, WA is considerably underexplored and the ageing Argyle mine and recent closure of operations at Ellendale warrant a re-evaluation of diamond potential. Results of mineral chemistry and prospectivity analysis make a compelling case for renewed exploration.

Paleoarchean variole-bearing metabasalts from the East Pilbara Terrane formed by hydrous fluid phase exsolution and implications for Archean greenstone belt magmatic processes

2.7 Ga plume associated VHMS mineralization in the Eastern Goldfields Superterrane, Yilgarn Craton: Insights from the low temperature and shallow water, Ag-Zn-(Au) Nimbus deposit