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Despite countless advances in recent years, exploration for volcanogenic massive sulfide (VMS) deposits remains challenging. This is particularly the case in the Yilgarn Craton of Western Australia, where outcrop is limited, weathering is deep and extensive, and metamorphism is variable. At Erayinia in the southern Kurnalpi terrane, intercepts of VMS-style mineralization occur along ~35 km strike length of stratigraphy, and a small Zn (-Cu) deposit has been defined at King (2.15 Mt at 3.47% Zn). An extensive aircore and reverse circulation drilling campaign on the regional stratigraphy identified additional VMS targets, including the King North prospect. Through a combination of detailed rock chip logging, petrography (inc. SEM imaging), and lithogeochemistry, we have reconstructed the volcanic stratigraphy and alteration halos associated with the King North prospect. Hydrothermal alteration assemblages and geochemical characteristics at King North (Mg-Si-K enrichment, Na depletion, and high Sb, Tl, Eu/Eu*, alteration index, CCPI, and normative corundum abundance values) are consistent with an overturned VMS system. The overturned footwall stratigraphy at King North is dominated by metamorphosed volcanic rocks, namely the following: garnet amphibolite (tholeiitic, basaltic), biotite amphibolite (andesitic, calc-alkaline), chlorite–quartz schist (dacitic), and narrow horizons of muscovite–quartz schist (dacitic to rhyolitic, HFSE-enriched). The hanging-wall to the Zn-bearing sequence is characterized by quartz–albite schists (metasedimentary rocks) and thick sequences of amphibolite (calc-alkaline, basaltic andesite). An iron-rich unit (>25% Fe2O3) of chlorite–actinolite–quartz schist, interpreted as a meta-exhalite, is associated with significant Cu-Au mineralization, adjacent to a likely syn-volcanic fault. Extensive Mg metasomatism of the immediate felsic footwall is represented by muscovite–chlorite schist. Diamond drilling into the deep hanging-wall stratigraphy at both King North and King has also revealed the potential for additional, stacked VMS prospective horizons in the greenstone belt stratigraphy. The discovery of HFSE-enriched rhyolites, zones of muscovite–chlorite schist, presence of abundant sulfide-rich argillaceous metasedimentary rocks, and a second upper meta-exhalite horizon further expand the exploration potential of the King–King North region. Our combined petrographic and lithogeochemical approach demonstrates that complex volcanic lithologies and VMS alteration signatures can be established across variably metamorphosed greenstone belts. This has wider implications for more cost-effective exploration across the Yilgarn Craton, utilizing RC drilling to reconstruct the local geology and identify proximal halos, and limiting more costly diamond drilling to key areas of complex geology and deeper EM targets. [ABSTRACT FROM AUTHOR]

The discovery and development of world-class Lithium–Caesium–Tantalum (LCT) spodumene-bearing pegmatites in Western Australia underpins growth of a significant new sector of its mining industry. Recently, several new spodumene discoveries have been delineated in the Yilgarn and Pilbara Cratons. Contrary to exploration narratives that new economic mineral discoveries will generally be made at increasingly greater depths, beneath barren cover rocks, or in remote geological environments, all new lithium discoveries have clear surface expressions in relatively 'mature' greenstone belts. The exploration implication is that the search space for pegmatite-hosted spodumene deposits in Western Australia remains immature. These recently discovered LCT pegmatites have geological features relevant to exploration including their age, amphibolite-facies metamorphic setting and syn-metamorphic timing, and 3D geometry, particularly their typically gentle dips, that match other such world-class pegmatites globally. Further spodumene discoveries within pegmatites at or near surface are likely in the Archean terranes of Western Australia based on these consistent exploration criteria and supportive capital market conditions. [ABSTRACT FROM AUTHOR]

A huge amount of legacy drilling data is available in geological survey but cannot be used directly as they are compiled and recorded in an unstructured textual form and using different formats depending on the database structure, company, logging geologist, investigation method, investigated materials and/or drilling campaign. They are subjective and plagued by uncertainty as they are likely to have been conducted by tens to hundreds of geologists, all of whom would have their own personal biases. dh2loop (https://github.com/Loop3D/dh2loop , last access: 30 September 2021​​​​​​​) is an open-source Python library for extracting and standardizing geologic drill hole data and exporting them into readily importable interval tables (collar, survey, lithology). In this contribution, we extract, process and classify lithological logs from the Geological Survey of Western Australia (GSWA) Mineral Exploration Reports (WAMEX) database in the Yalgoo–Singleton greenstone belt (YSGB) region. The contribution also addresses the subjective nature and variability of the nomenclature of lithological descriptions within and across different drilling campaigns by using thesauri and fuzzy string matching. For this study case, 86 % of the extracted lithology data is successfully matched to lithologies in the thesauri. Since this process can be tedious, we attempted to test the string matching with the comments, which resulted in a matching rate of 16 % (7870 successfully matched records out of 47 823 records). The standardized lithological data are then classified into multi-level groupings that can be used to systematically upscale and downscale drill hole data inputs for multiscale 3D geological modelling. dh2loop formats legacy data bridging the gap between utilization and maximization of legacy drill hole data and drill hole analysis functionalities available in existing Python libraries (lasio , welly , striplog). [ABSTRACT FROM AUTHOR]

• Zircon petrochronological (U/Pb-O-Hf) constraints for the far-east Yilgarn Craton. • Relatively old (2840 – 2775 Ma) crustal block at the eastern edge of the craton. • Identification of the newly defined Dorothy Hills Block and Yamarna Rift. • Re-assessment of the nomenclature in the Eastern Goldfields Terrane (EGT). • The EGT formed via sustained, autochthonous crustal growth. The Archean Yilgarn Craton in Western Australia represents one of the key areas for investigating the mechanisms of crustal growth during Earth's infancy. The far-eastern margin of the Yilgarn Craton has, however, received limited scientific scrutiny. In this study, in-situ zircon petrochronology (U-Pb, O, and Lu-Hf isotopes) is used to constrain the formation and evolution of the easternmost Yamarna and Burtville regions as well as the encompassing Eastern Goldfields (Super)Terrane (EGT) of the Yilgarn Craton. The new data highlight the unequivocal existence of relatively old (ca. 2840 – 2775 Ma), compared to the EGT, and isotopically juvenile (εHf: +2.1 to +3.5) and mantle-like (δ18O: 5.2 to 5.9‰) crust at the eastern edge of the exposed craton. We refer to this crust as the 'Dorothy Hills Block'. The Dorothy Hills Block shows geochronological and isotopic similarities with the adjacent 'Burtville Block' and the Youanmi Terrane (>400 km further west), which suggests a shared history up to ca. 2720 Ma. In addition, linear greenstone belts characterised by younger magmatism (ca. 2720 – 2660 Ma) with a constant, mildly supra-chondritic Hf isotope composition (εHf: 0.0 to +3.5) are identified between the Burtville and Dorothy Hills Blocks. We refer to this region as the 'Yamarna Rift', by analogy to the better-understood 'Kalgoorlie-Kurnalpi Rift'. It is inferred that the Burtville and Dorothy Hills Blocks formed via peri-cratonic magmatism (ca. <2970 Ma), associated with mantle upwelling focused along the eastern edge of the ca. >3300 Ma Yilgarn proto-craton. The Yamarna and Kalgoorlie-Kurnalpi Rifts (ca. 2720 – 2660 Ma) overlie this extended older substrate and formed via intra-cratonic rifting broadly coeval with a major mantle upwelling event. The revised Meso- to Neoarchean crustal evolution model for the EGT, including new nomenclature, suggests a sustained (>200 Myr) and autochthonous geological development of the eastern Yilgarn Craton resulting from mantle upwelling events, potentially within a back-arc tectonic setting. Comparable crustal evolutionary trends are recorded in other Archean cratons (e.g., Superior Craton), which suggests that similar geodynamic regimes operated on a global scale in the Archean. [ABSTRACT FROM AUTHOR]

The nature of Earth's earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ~4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemitegranodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism. [ABSTRACT FROM AUTHOR]

The early tectonic history of the Eastern Goldfields Terrane (EGT) is poorly understood, but in places ca 2800 Ma mafic–ultramafic sequences are conformably overlain by ca 2720–2670 Ma sequences (e.g. Leonora and Laverton districts), suggesting minimal early deformation. The first significant angular unconformities occur at the base of the ca 2670–2655 Ma late basins and indicate that deformation was contemporaneous with deposition of the late basins. These basins mark the end of the volcano-sedimentary record in the EGT and typically grade upwards from polymictic mafic-dominated conglomerates to more siliciclastic compositions with abundant granitic clasts. The clastic sequences record the uplift and exhumation of granite-cored domes. There is a distinct lack of clasts with internal deformation fabrics (e.g. schist or gneiss). The timing of late basin formation overlaps with the ca 2672–2660 Ma D1 event, which has been variously described as extensional or compressional. D1 structures comprise a bedding-parallel S1 that is axial planar to F1 folds. The late basins and early D1 fabrics are overprinted by upright north-trending F2 folds and a sub-vertical S2. A period of crustal thickening was achieved by autochthonous processes with deposition of greenstone sequences into local basins between ca 2720 and 2670 Ma. Partial convective overturn (or granite-up, greenstone-down tectonics) developed as a result of gravitational instability. As the large granite–gneiss bodies rose, solid-state extensional D1 shears developed around the granites with a radial pattern of L1 extension lineations. Areas of compression developed within the sinking greenstone sequences. A final phase of subsidence in the central parts of the greenstone belts produced depocentres (late basins). A shift from dominantly vertical tectonics to horizontal tectonics at ca 2655–2650 Ma is marked by the onset of east–west D2 compression and the change in granite compositions from high-Ca to low-Ca granites. Older ca 2800 Ma mafic–ultramafic sequences are conformably overlain by ca 2720–2670 Ma sequences suggesting minimal early deformation in the EGT. The first significant angular unconformities are at the base of the ca 2665 Ma late basin sequences. Late basin sequences record the uplift and exhumation of granite-cored domes and sedimentation is contemporaneous with the regional ca 2672–2660 Ma D1 event. The D1 event represents granite-up, greenstone-down tectonics (D1 event) that developed as a result of gravitational instability arising from autochthonous crustal thickening from ca 2720–2670 Ma. Granite-up, greenstone-down tectonics precludes an accretionary model for the development of the EGT. [ABSTRACT FROM AUTHOR]

The United States Geological Survey (USGS) Three-part Undiscovered Mineral Resource Assessment provides a framework for estimating undiscovered mineral endowment. Previous studies that applied the Three-part Assessment to estimate the undiscovered orogenic gold endowment of the Sandstone Greenstone Belt, Western Australia, have relied upon dated or expert-derived grade-tonnage models. Here, several assessments are conducted using local grade-tonnage models, comprising known orogenic gold deposits within the entire Yilgarn Block and several individual terranes with contrasting lithosphere- to terrane-scale characteristics. These models are generated through comprehensive review of historical exploration, resource and production data. Based on these models, the Sandstone Greenstone Belt is estimated to contain significant undiscovered gold mineralisation, with a median total endowment of between 166 and 298 t gold, and mean of 167–319 t gold. Although these updated grade-tonnage models provide an approximately 80 per cent variation in predicted gold endowment, it is still evident that the belt remains an under-explored region within the Yilgarn Block, Western Australia. [ABSTRACT FROM AUTHOR]

We present 40Ar/39Ar ages and boron isotopes of tourmalines from a quartz-tourmaline intrusion cross-cutting the Hadean detrital-zircon-bearing metasedimentary rocks from the Mt. Alfred and Brooking Hills localities of the Illaara Greenstone Belt, Western Australia. Concordant 40Ar/39Ar plateau ages on the tourmalines at Mt. Alfred give a weighted mean age of 2935 ± 9 Ma (2σ). These results are the first reported Archean 40Ar/39Ar ages directly obtained on tourmaline, and provide a minimum depositional age for this sequence, placing constraints on the depositional timing of detrital Hadean zircons to between 2935 ± 9 Ma and c.a. 3250 Ma (youngest detrital zircon age in sequence). This is the last known occurrence of abundant detrital Hadean zircons having been directly deposited in a sedimentary sequence, and thus suggests that most Hadean zircon sources have been exhausted by this time. The 2935 ± 9 Ma tourmalines occur within a quartz-tourmaline intrusion with associated stratiform layers and veins, inferred to have grown during a hydrothermal fluid circulation event. A younger generation of tourmalines from the Brooking Hills, 30 km south of Mt. Alfred yielded concordant 40Ar/39Ar plateau ages with a weighted mean age of 2624 ± 16 Ma. These tourmalines occur within and along the margins of post-kinematic quartz veins, parallel to the foliation fabric and marks the end stage of high temperature tectono-thermal events within the Illaara Greenstone Belt. The retention of 2935 ± 9 Ma tourmaline ages within the Illaara Greenstone Belt shows that the K/Ar system of tourmaline remained closed throughout repeated upper-greenschist metamorphic events (∼450 °C) between the ages of c.a. 2930 to 2630 Ma in contrast to muscovite recording plateau ages of about 2600 Ma. This is in agreement with an approximate Ar closure temperature between ∼534 and 628 °C experimentally determined in this study. [ABSTRACT FROM AUTHOR]

The study of climate‐driven effects on seed traits such as germination has gained momentum over the past decade as the impact of global warming becomes more apparent on the health and survival of plant diversity.Seed response to warming was evaluated in a suite of short‐range endemic species from the biodiverse Greenstone Belt of southern Western Australia. The temperature dimensions for germination in 20 woody perennials were identified using small unreplicated samples over 6 weeks on a temperature gradient plate (constant and fluctuating temperatures between 5 and 40 °C). These data were subsequently modelled against current and forecast (2070) mean monthly minimum and maximum temperatures to illustrate seasonal changes to germination timing and final percentage germination.All but one species attained full germination in at least one cell on the gradient plate. Modelling of the data suggested only minimal changes to percentage germination despite a forecast rise in diurnal temperatures over the next 50 years. Nine species were predicted to experience declines of between <1% and 7%, whilst 11 species were predicted to increase their germination by <1% to 3%. Overall, the speed of germination is predicted to increase but the timing of germination for most species shifts seasonally (both advances and delays) as a result of changing diurnal temperatures.The capacity of this suite of species to cope with warmer temperatures during a critical early life stage shows a degree of adaptation to heterogeneous environments. Predicting the effects of global change on terrestrial plant communities is crucial to managing and conserving plant diversity. [ABSTRACT FROM AUTHOR]

The 920 km2 Archean Sandstone greenstone belt lies in the central-northern part of the Southern Cross Domain of the Youanmi Terrane, Western Australia. The belt, forming an arrow-head geometry, is bounded by the Youanmi and Edale shear zones on its eastern and western margins, respectively. The belt has a total gold endowment, including historic production, resources and reserves, of approximately 62 t (2.0 Moz). Most known deposits are situated within a central-southern ultramafic domain, although this is likely a function of exploration focus. The distribution of historical exploration efforts and domaining of deposit types, based on litho-structural controls on ore formation, highlight the potential for significant undiscovered deposits, hosted within arrays of shear zones parallel and adjacent to the lithosphere-scale Youanmi and Edale shear zones. [ABSTRACT FROM AUTHOR]

Post-magmatic alteration of certain magmatic Ni sulfide ores in Western Australia, the Miitel deposit and the Sarah's Find prospect, produced Ni–As–PGE haloes around massive sulfides. A study of the composition of arsenide grains from these hydrothermal haloes, along with arsenides from various magmatic and hydrothermal mineralized environments in other localities, was conducted in order to compare their composition, and assess their potential use as indicator minerals for exploration vectoring, as well as to gain knowledge on their crystallization history. Concentrations in trace elements such as platinum-group elements (PGEs), Au and other metals was obtained by laser ablation inductively coupled plasma mass spectroscopy analyses. Results show that variations in PGEs and Au compositions can be related to the magmatic vs. hydrothermal origin of the grains; and to their provenance from deposits enriched in either Ni, Au or both. Magmatic NiCoFe sulfarsenides have strongly correlated, high IPGE (Os, Ir, Ru, Rh) contents up to 100 ppm Ir, compared with maximum values in hydrothermal sulfarsenides of ~1 ppm. Gold in hydrothermal sulfarsenides from Au-mineralized ultramafic rocks extends up to 500 ppm, with typical values of 3–30 ppm; similar values are also found in nickeline (also called niccolite). These results suggest that nickel arsenides could potentially be used as indicator minerals for nickel and gold exploration. Trace-element contents of arsenide grains in shear zones could be used to deduce the presence of Ni or Au mineralization upstream in the fluid pathway. [ABSTRACT FROM AUTHOR]

The Murchison domain forms the northwest part of the Youanmi Terrane, a tectonic unit within the Neoarchean Yilgarn Craton in Western Australia. In the Cue-Weld Range area the Murchison domain has experienced a complex magmatic and deformation history that resulted in a transposed array of greenstone belts that host significant iron, gold, and base metal deposits. In this study, we interpret the upper 2 s (about 6 km) of a deep crustal seismic profile 10GA–YU1, near the town of Cue, and correlate rock units and structures in outcrop with corresponding reflections. We performed 3D constant velocity ray-tracing and calculate the corresponding travel times for the reflections for time domain pre-stack and post-stack seismic data. This allows us to link shallow reflections with mafic volcanic rocks of the Glen Group and basaltic rocks of the Polelle Group in outcrop. Based on our interpretation and published geological maps and data, we propose a model in which the local stratigraphy represents a refolded thrust system. To test our hypothesis, we applied 2D acoustic finite difference forward modeling. The corresponding synthetic data were processed in the same way as the acquired data. Comparisons between the acquired and the synthetic data show that the model is consistent with observations. We propose a new model for the subsurface of the Cue-Weld Range area and argue that some of the lithologies in the area are repeated structurally at different levels. Our approach highlights the benefit of imaging and modeling of deep seismic transects to resolve local structural complexity in Archean granite-greenstone terrains. [ABSTRACT FROM AUTHOR]

The paper presents new geological and geochemical data on the Perseverance komatiite complex (PK) that hosts the Perseverance Ni-sulphide deposit and the volcanic and sedimentary rocks forming the footwall of the PK. Using the 3D reconstruction of the PK and its' footwall volcano-sedimentary succession the heterogeneity of the komatiite channel has been characterised on the basis of spatial distribution of the geochemical petrogenetic indicators. The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the 3D models as the segments with anomalously high MgO concentrations coupled with low contents of Al 2 O 3 , TiO 2 and CaO (Fig - abstract). The geochemical indicators of komatiite fertility, Ni/Cr and Ni/Ti ratios, are commonly in the range of 1 to 3 for Ni/Cr and 2 to 5 for Ni/Ti within the main pathway. The Ni/Cr exceeds 5 and Ni/Ti 20 in the mineralised parts of the channel. In the peripheral zone komatiites these values commonly drop to less than 1, corresponding to barren komatiites. Similarly large variations are exhibited by the ratios of mantle normalized Pt and Pd to Al and Ti over short distances from 1 to 25. Closely spaced high and low values of these ratios hinder their use for discriminating fertile from barren komatiites. However, within the komatiite main lava pathway they can be used as geochemical pathfinders for magmatic sulphides. Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. For these purposes a special reference in this study was made to the geochemical systematics of rocks that make up footwall succession of the PK complex. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastics (mafic-1) and intrusive gabbro (mafic-2). The mafic volcaniclastics are strongly enriched in LREE, possibly due to presence of sedimentary epiclastic material or reflecting magma contamination by the crustal material during ascent through the crust. Additionally, they have notably high-Th contents, allowing them to be classified as having high-Th geochemical affinities. The second group (mafic-2), interpreted as coherent in origin, are characterised by flat REE patterns and lower overall abundances of the rare earths and Zr corresponding to a tholeiitic composition. Contents of Th and TiO 2 of these rocks are clustered in the two narrow intervals of compositions, corresponding to low-Th and intermediate-Th basalts, which form different amphibolite bodies. Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic material derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. In conclusion, abundances and ratios of the trace elements in composition of the mafic rocks are matching to that of the different parts of the Agnew Wiluna greenstone belt and can be used for overall stratigraphic correlations. [Display omitted] • The paper presents new geological and geochemical data on the Perseverance komatiite complex (PK) that hosts the Perseverance Ni-sulphide deposit and the volcano-sedimentary succession forming the footwall of the PK. • The 3D reconstruction of the PK and its' footwall volcano-sedimentary succession has been undertaken that has allowed to enhance the heterogeneity of the komatiite channel on the basis of spatial distribution of the geochemical petrogenetic indicators. • The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the geochemical 3D models as the segments with anomalously high MgO concentrations coupled with the anomalous values of the geochemical indicators of komatiite fertility, including Ni/Cr, Ni/Ti ratios and the ratios of mantle normalized Pt and Pd to Al and Ti. • Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastics (mafic-1) and intrusive gabbro (mafic-2). • Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic material derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. The paper presents new geological and geochemical data on the Perseverance komatiite complex (PKC) that hosts the Perseverance Ni-sulphide deposit and the volcanic and sedimentary rocks forming the footwall of the PKC. Using the 3D reconstruction of the PKC and its' footwall volcano-sedimentary succession the heterogeneity of the komatiite channel has been characterised on the basis of spatial distribution of the geochemical petrogenetic indicators. The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the 3D models as the segments with anomalously high MgO concentrations coupled with low contents of Al 2 O 3 , TiO 2 and CaO. The geochemical indicators of komatiite fertility, Ni/Cr and Ni/Ti ratios, commonly are in the range of 1 to 3 for Ni/Cr and 2 to 5 for Ni/Ti within the main pathway, The Ni/Cr exceeds 5 and Ni/Ti 20 in the mineralised parts of the channel. In the peripheral zone komatiites these values commonly drop to less than 1, corresponding to barren komatiites. Similarly large variations are exhibited by the ratios of mantle normalized Pt and Pd to Al and Ti over short distances from 1 to 25. Closely spaced high and low values of these ratios hinder their use for discriminating fertile from barren komatiites. However, within the komatiite main lava pathway they can be used as geochemical pathfinders for magmatic sulphides. Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. For these purposes a special reference in this study was made to the geochemical systematics of rocks that make up footwall succession of the PKC complex. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastic rocks (mafic-1) and intrusive mafic rocks, apparently metagabbro sills (mafic-2). The mafic volcaniclastic rocks are strongly enriched in LREE, possibly due to presence of sedimentary epiclastic material or reflecting magma contamination by the crustal material during ascent through the crust. Additionally, they have notably high-Th contents, allowing them to be classified as having high-Th geochemical affinities. The second group (mafic-2), interpreted as coherent in origin, are characterised by flat REE patterns and lower overall abundances of the rare earths and Zr corresponding to a tholeiitic composition. Contents of Th and TiO 2 of these rocks are clustered in the two narrow intervals of compositions, corresponding to low-Th and intermediate-Th basalts, which form different amphibolite bodies. Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic materials derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. The different geochemical affinities of the mafic rocks distributed in the footwall succession of the PKC apparently suggests different magma sources and, possibly, can also reflect the position of the rocks within the mantle plume structure. In conclusion, abundances, and ratios of the trace elements in composition of the mafic rocks matches that of the different parts of the Agnew Wiluna greenstone belt and can be used for overall stratigraphic correlations. [ABSTRACT FROM AUTHOR]

The thick package of ∼2.7 Ga mafic and ultramafic lavas and intrusions preserved among the Neoarchean of the Kalgoorlie Terrene in Western Australia provides valuable insight into geological processes controlling the most prodigious episode of growth and preservation of juvenile continental crust in Earth's history. Limited exposure of these rocks results in uncertainty about their age, physical and chemical characteristics, and stratigraphic relationships. This in turn prevents confident correlation of regional occurrences of mafic and ultramafic successions (both intrusive and extrusive) and hinders the interpretation of tectonic setting and magmatic evolution. A recent stratigraphic drilling program of the Neoarchean stratigraphy of the Agnew Greenstone Belt in Western Australia has provided continuous exposures through a c. 7 km thick sequence of mafic and ultramafic units. In this study, we present a volcanological, lithogeochemical and chronological study of the Agnew Greenstone Belt, and provide the first pre-2690 Ma regional correlation across the Kalgoorlie Terrane. The Agnew Greenstone Belt records ∼30 m.y. of episodic ultramafic–mafic magmatism that includes two cycles, each defined by a komatiite that is overlain by units that become more evolved and contaminated with time. The sequence is divided into nine conformable packages, each consisting of stacked subaqueous lava flows and comagmatic intrusions, as well as two sills without associated extrusions. Lavas, with the exception of intercalations between two units, form a layer-cake stratigraphy and were likely erupted from a system of fissures tapping the same magma source. The komatiites are not contaminated by continental crust ([La/Sm] PM ∼ 0.7) and are of the Al-undepleted Munro-type. Crustal contamination is evident in many units (Songvang Basalt, Never Can Tell Basalt, Redeemer Basalt, and Turrett Dolerite), as judged by [La/Sm] > 1, negative Nb and Ti anomalies, and geochemical mixing trends towards felsic contaminants. Crystal fractionation was also significant, with early olivine and chromite (Mg# > 65) followed by plagioclase and clinopyroxene removal (Mg < 65), and in the most evolved case, titanomagnetite accumulation. Three new TIMS dates on granophyric zones of mafic sills and one ICP-MS date from an interflow felsic tuff are presented and used for regional stratigraphic correlation. Cycle I magmatism began at ∼2720 Ma and ended ∼2705 Ma, whereas cycle II began ∼2705 Ma and ended at 2690.7 ± 1.2 Ma. Regional correlations indicate the western Kalgoorlie Terrane consists of a remarkably similar stratigraphy that can be recognised at Agnew, Ora Banda and Coolgardie, whereas the eastern part of the terrane (e.g., Kambalda Domain) does not include cycle I, but correlates well with cycle II. This research supports an autochthonous model of greenstone formation, in which one large igneous province, represented by two complete cycles, is constructed on sialic crust. New stratigraphic correlations for the Kalgoorlie Terrane indicate that many units can be traced over distances >100 km, which has implications for exploration targeting for stratigraphically hosted ultramafic Ni and VMS deposits. [ABSTRACT FROM AUTHOR]

The geodynamic setting of the Neoarchaean Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton is the subject of debate. Some authors propose plume models, while others advocate variants on a subduction accretion model for the origin of mineralised greenstone belt sequences. Felsic volcanism in the Kalgoorlie Terrane, the westernmost terrane of the EGS, is considered to have a tonalite-trondhjemite-granodiorite/dacite (TTG/D) geochemical affinity. The Cosmos greenstone succession, which lies in the Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie Terrane, contains several komatiite-hosted nickel sulphide deposits, the volcanic footwall to which consists of an intercalated succession of fragmental and coherent rocks ranging in composition from basaltic andesite to rhyolite. Light rare earth elements (LREEs) and large ion-lithophile elements (LILEs) are strongly enriched relative to high field strength elements (HFSEs) across all volcanic units, and the rocks display strong positive Pb and negative Nb anomalies. These geochemical characteristics resemble closely those of modern high-K calc-alkaline to shoshonite continental arc successions. Contrasting REE, LILE and HFSE concentrations, coupled with assimilation-fractional crystallisation (AFC) modelling, shows that the intercalated dacitic and andesitic volcanic rocks within the footwall succession are not co-genetic. Xenocrystic zircons within the felsic volcanic lithologies indicate that some assimilation of older continental crust contributed to the generation of the footwall volcanic sequence. The geochemical characteristics of the Cosmos volcanic succession indicate that parental melts were derived via partial melting of enriched peridotite that had been contaminated by subducted crustal material within the mantle wedge of a subduction zone. In contrast, two younger felsic porphyry intrusions, which cross-cut the volcanic succession, have a distinct TTG/D affinity. Therefore, these intrusions are considered to be generated via partial melting of a subducting slab and are related to local high-Ca granitoid intrusions. The Cosmos volcanic succession represents the first extrusive high-K calc-alkaline to shoshonitic volcanic arc sequence described in the Kalgoorlie Terrane and, coupled with age dating of the stratigraphy, is indicative of formation in a long-lived volcanic arc setting active from 2736 Ma to later than 2724 Ma. The composition and geochemical affinity of the volcanic footwall succession to the Cosmos komatiite-hosted nickel-sulphide deposits contrasts with the majority of felsic volcanic rocks within the AWB and also the wider Kalgoorlie Terrane, suggesting that the overall architecture of this region is more complex than is currently thought. Our conclusions not only have consequences for recent models of the tectonic evolution of the EGS but also contribute to the debate on the operation of plate tectonics during the late Archaean in general. The arc affinity of the Cosmos volcanic succession, containing abundant high-K calc-alkaline andesite lavas, provides further support for the operation of plate tectonics in the Neoarchaean. [ABSTRACT FROM AUTHOR]

• New lithological, whole-rock geochemical and U-Pb geochronological constraints provided for the West Laverton Greenstone Belt (WLGB) in the Kurnalpi Terrane. • Greenstones erupted sub-aqueously onto rifted sedimentary basement in three volcanic events between 2854 ± 4 and 2717 ± 4 Ma, from a depleted mantle source. • Comparisons with better-characterised successions in the west Kalgoorlie Terrane indicate a common ca. 2.96–2.69 Ga stratigraphic framework, supporting an autochthonous setting. • The Yilgarn Craton reveals remarkable > 2.71 Ga lithostratigraphic similarities with the Zimbabwe and Slave Cratons, supporting NeoArchean ancestory. The time-interval between 2.8 and 2.7 Ga is associated with the worldwide appearance of thick tholeiite-komatiite successions. However, their origin and affiliation, if any, is poorly understood. The Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, is dominated by the 2.7 Ga Kalgoorlie and Kurnalpi terranes, both of which consist of well-preserved mafic-ultramafic volcano-stratigraphy. On a local scale, these supracrustal sequences permit reconstruction of volcano-sedimentary environments, which provides important constraints to the local conditions of magmatism and/or deposition. Regionally, these observations can be used to test correlations between greenstone belts, constrain basin architecture and the extent of volcanism. Studies of this nature provide important insights to global, Archean volcano-sedimentary processes. Here we present an integrated field, geochemical and U-Pb geochronological study from the well-preserved West Laverton Greenstone Belt in the Kurnalpi Terrane and compare it to the relatively well constrained Kalgoorlie Terrane. The sequence comprises three volcanic 'events' consisting dominantly of low-Th tholeiitic basalt and komatiite derived from depleted mantle sources. Magmatism commenced after 2854 ± 4 Ma and ceased by 2717 ± 4 Ma. Prominent banded iron formations separate volcanic cycles, indicating an overall quiescent subaqueous setting. The West Laverton Sequence is correlated with volcanic events in the neighbouring Kalgoorlie Terrane, where pre-existing basement with remarkably similar character is documented near Norseman, over 360 km away. Both terranes contain basal paragneiss (2.96–2.93 Ga) and epiclastic sediments (∼2.93–2.85 Ga) that are overlain by three variably preserved and widespread tholeiite-dominated volcanic events, here defined as the 2.81–2.71 Ga 'Goldfields Tholeiitic Super Event'. These correlations support an autochthonous setting for the Kalgoorlie and Kurnalpi terranes with the eruption of volcanic tholeiites onto submerged continental crust. Voluminous tholeiitic volcanism at 2.8–2.7 Ga was a global event. The Zimbabwe and Slave cratons preserve remarkably similar basement overlain by thick basaltic units, that are compositionally similar to the 'Goldfields Tholeiitic Super Event'. This correlation indicates that a globally significant rifting event at 2.8–2.7 Ga affected much of the continental crust extant. [ABSTRACT FROM AUTHOR]

The chemistry of the major reservoirs in the silicate Earth reflects a long history of differentiation into- and interaction between- incompatible element enriched crust and depleted mantle. Evidence from 142Nd variability in ancient mantle-derived rocks imply such differentiation began very early in Earth's history, while the short-lived 146Sm (half-life 103 Ma) was extant, but the size and composition of primitive reservoirs in the Hadean and early Archean is debated. Therefore, pivotal to the study of Earth evolution is the recognition of well-preserved ancient magmatic rocks that can provide robust constraints on source compositions to test geodynamic models. We present the first combined petrological, geochemical and isotope (146-147Sm-142-143Nd and 176Lu–176Hf) study of basaltic rocks from the East Pilbara Terrane, Pilbara Craton, Western Australia. The basaltic pillow lavas, from the ca. 3470 Ma Mount Ada Basalt of the Pilbara Supergroup, include quartz-phyric quartz-normative tholeiites, plagioclase-phyric olivine-normative tholeiites and clinopyroxene spinifex textured olivine-normative tholeiites and include samples that classify as siliceous high Mg basalts. Their major and trace-element compositions suggest that the basalts are high-degree partial melts (20–30%) produced at shallow mantle depths of 0.5–1 GPa, which is consistent with formation of both quartz-normative and olivine-normative parental magmas. Isotopic data from this sample suite defines Sm–Nd and Lu–Hf isochrons with dates of 3484 ± 113 Ma and 3463 ± 50 Ma, respectively. The samples yield coupled initial ε Nd = +1.1 and ε Hf = +2.1, and μ142Nd values indistinguishable from the modern mantle. The uniform initial ε Nd and ε Hf in samples from this study and other Mount Ada Basalt basaltic samples across four greenstone belts indicates that this portion of the Paleoarchean mantle was well mixed at the scale of melt generation and evolved with slightly suprachondritic Sm/Nd and Lu/Hf. Absence of 142Nd variations suggests the portion of the Paleoarchean Pilbara mantle sampled by these basalts retains no signature of Hadean Sm/Nd fractionation. • Mount Ada Basalt quartz- and olivine-normative basalts formed by high-degree partial melting at shallow depths • Sm-Nd and Lu-Hf isochron dates of 3484±113 Ma and 3463±50 Ma are indistinguishable from the 3470 Ma Mount Ada Basalt age • Initial ε Nd +1.1±0.6 and ε Hf +2.1±0.2 suggest the Paleoarchean Pilbara mantle was slightly incompatible-element depleted • Mount Ada Basalt samples show no resolvable 142Nd anomalies indicating no retained signature of Hadean Sm/Nd fractionation [ABSTRACT FROM AUTHOR]

[Display omitted] • The variolitic texture in mafic pillow lavas of the Mount Ada Basalt of the Pilbara Supergroup formed due to liquid immiscibility. • Clinopyroxene element mapping indicate that grains hosted in both variole and host regions preserve shared magmatic histories. • The liquid immiscibility reaction occurred due to H 2 O saturation in the precursor magmas. • Variole bearing lavas have εNd i −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites. • The fourth volcanic cycle in the Pilbara Supergroup, the Mount Ada Basalt and the Duffer Formation, preserves distinctly different volcanic types that indicate significant changes in magma genesis during a 10 Ma timeframe. Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe 2 O 3 (total) and higher SiO 2 and Al 2 O 3 concentrations. However, the variole and host materials have indistinguishable TiO 2 , Cr 2 O 3 , NiO 2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H 2 O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H 2 O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNd i −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation. [ABSTRACT FROM AUTHOR]

The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle. [ABSTRACT FROM AUTHOR]

• Clasts derived from Archean conglomerates support diapirism and sagduction models. • Felsic and mafic clasts indicate systematic erosion of supracrustal sequence. • Orogenesis (>2655 Ma) unsupported as clasts lack pre-emplacement tectonic fabrics. Archean greenstone belts commonly consist of a stratigraphy that records subaqueous deposition capped by subaerial sedimentation, yet it remains unclear if the controls on this change were driven by horizontal or vertical processes. Clasts in conglomerates can provide important constraints on this question because they inform on the changing nature of sources. The 2.7 Ga Agnew Greenstone Belt, Yilgarn Craton, Australia, provides the ideal opportunity for such a study because it contains abundant and diverse conglomerate facies and its basal stratigraphy is well understood. This study focusses on the texture and composition of lithic clasts derived from three stratigraphic conglomerate units and their host successions. The lowermost conglomerate-bearing unit, hosted in the Vivien Formation, unconformably overlies the mafic-ultramafic lavas and intrusions of the Kalgoorlie Group (ca. 2720–2690 Ma) and is dominated by aphyric to porphyritic mafic and felsic volcanic clasts. The conformably overlying Maria Mine Formation conglomerates contain a higher proportion of felsic clasts with medium- and plutonic groundmass textures. The Vivien and Maria Mine Formations (ca. 2690–2670 Ma) are dominated by cherts, turbidites and conglomerates, and represent subaqueous sediments formed in the ring-plain surrounding an emergent volcanic island. The unconformably overlying Scotty Creek Formation (ca. 2665–2655 Ma) contains conglomerates with many plutonic felsic clasts, and is dominated by bedded and cross-bedded sandstones formed in a subaerial environment. Whole-rock geochemistry of mafic and ultramafic clasts matches the local Kalgoorlie Group rocks and indicates progressive erosion of the underlying supracrustal sequence. Undeformed conglomerates lack clasts with metamorphic foliations, indicating uplift by horizontal compression could not have been significant before ca. 2655 Ma, which is considered as the time of the first major compressional event in many published structural frameworks for the region. The systematic erosion of a felsic stratovolcano, its associated plumbing system, and any resultant upwarping of underlying sequences through vertically-driven diapirism, better explains the progressive changes observed in clast texture and composition, as opposed to horizontally-driven uplift, such as compression associated with subduction. [ABSTRACT FROM AUTHOR]

The oldest known evidence of large impacts in Early Earth history are Archaean to Palaeoproterozoic impact spherule layers of the Barberton Greenstone Belt in South Africa and the Pilbara craton in Western Australia. Four drill core sections (BH5901, BH5907, BH5911 and BH5949) containing spherule layers were intersected in exploration drilling at Fairview Gold Mine in the northern Barberton Greenstone Belt. Here, we provide a first detailed mineralogical study of these new spherule layer intersections. The spherule layers are densely packed with sand-sized, spherical to ovoid, i.e., in part strongly deformed, "beads". They generally have morphological and textural features similar to other 3.4–3.2 Ga old spherule layers of the Barberton Greenstone Belt. The Fairview spherule layers are stratigraphically positioned at the contact between the silicified volcanic tuff of the uppermost unit of the Onverwacht Group and the lowermost siliciclastic rocks of the Fig Tree Group, like the S2 and S4 layers in the southern, and S3 layer in the northern, domain of the greenstone belt. Micro-XRF scanning allowed (i) to distinguish spherule layers from their host rocks by their major element distribution, and (ii) to characterize secondary alteration and micro-deformation features in the SL and host rocks. Ni–Cr spinel, the only primary mineral in evidence in these sections, is not distributed uniformly along a spherule layer intersection, and occurrences can not be stratigraphically correlated between the four layer intersections either. Spinel crystals vary with respect to Ni, Cr, Fe and Zn contents and respective zonation patterns for these elements. Zinc abundances are likely related to post-depositional hydrothermal overprint. The results of this investigation comprising geological, petrographic, semiquantitative micro-X-ray fluorescence spectrometry, and quantitative electron microprobe analysis on spinel support a complex and heterogeneous genetic process that must have taken place during spherule formation in an impact vapor plume and in post-depositional times. The Fairview spherule layers are thought to possibly represent the product of a single impact event. They represent fallout into deep-water conditions, as they lack evidence for reworking or sorting by current or wave action. The heterogeneous Ni–Cr spinel concentrations are similar to what has been reported for spinel occurrences in the S3 layer, but considering the complex tectonic overprint on this lithostratigraphic sequence at the changeover from the Onverwacht to the Fig Tree Group it is not possible to conclude whether or not these spherule layers represent the same impact event. • The oldest known evidence of large impacts in Early Earth history are Archaean to Palaeoproterozoic impact spherule layers of the Barberton Greenstone Belt in South Africa and the Pilbara craton in Western Australia. • A new set of spherule layer intersections is reported from Fairview Gold Mine. • Detailed petrographic characterisation of these new intersections is reported. • The Fairview spherule layers are thought to possibly represent the product of a single impact event. [ABSTRACT FROM AUTHOR]

Studies investigating the structure and diversity of Earth's record of life older than 3.2 Ga are restricted to two locations worldwide in which sedimentary rocks have escaped regional high-grade metamorphism and penetrative deformation: the Pilbara Craton of Western Australia and the Barberton Greenstone Belt in the Kaapvaal Craton of South Africa. This paper provides a South African perspective on the evidence of Paleoarchean life; a record that is often overlooked in the literature. It aims to summarize and critically review previously reported claims of early life in the BGB, gives an overview of the latest findings, and provides an outlook on potential future discoveries. The ~15 km thick, volcanic-sedimentary succession making up the Barberton Supergroup was deposited between 3.55 to ca. 3.20 Ga and can be subdivided in three stratigraphic units that provide a unique window into a diverse and widespread Paleoarchean microbial ecosystem landscape. Putative biosignatures occur almost throughout the entire BGB stratigraphy and range from carbonaceous cherts containing filamentous, spheroidal, and lenticular microstructures, traces of hydrothermal biofilms, photosynthetic microbial mats, remnants of pseudocolumnar stromatolites, and large, organic-walled spheroidal microfossils of currently unknown affinity. The BGB also contains one of the world's oldest known record of tufted microbial mats, which extensively colonized tidally-influenced, siliciclastic shorelines and were most likely formed by filamentous photosynthesizers. Other mat-associated biosignatures include silicified gas bubbles, domes and lenses that likely formed due to metabolic activity or the decay of buried organic matter. Some of these subsurface voids beneath the cohesive mats were inhabited by the earliest known forms of cavity-dwelling microbial communities that were probably dominated by chemotrophic or photosynthetic microbes. Recently discovered terrestrial microbial mats, once thriving in a fluvial-dominated setting, represent the oldest macroscopically-visible fossil traces of life on land, which is also supported by the occurrence of nearby paleosols that carry signals of biogenic sulfur fractionation. The wealth of preserved microbial biosignatures from marine, fluvial, hydrothermal, and possibly planktonic settings combined with the high spatial and temporal resolution of the Barberton Greenstone Belt deposits is truly exceptional, Consequently, the BGB deserves an equal level of attention and protection for future generations like its Australian counterpart. [ABSTRACT FROM AUTHOR]

Four decades have now passed since Schidlowski et al. [1] proposed that a light carbon isotope signature in metasedimentary rocks, as found in the 3.7 Ga Isua Greenstone Belt (IGB) of southwest Greenland, might be used to indicate the presence of life in some of the earliest preserved geology. Isotopic evidence was relied upon because the host rocks have been metamorphosed and deformed to a degree that precludes preservation of unambiguous biomorphic features, such as those seen in the ca. 3.4 Ga algal mats at Strelley Pool, Western Australia [2]. Since Schidlowski et al.'s seminal work, a number of claims have been made for low δ13C indicating life in the Eoarchean, including those from Isua [3] and Akilia [4] in west Greenland, and the Nuvvuagittuq Greenstone Belt [5] and Saglek region, Labrador [6] in northern Canada. Such cases, often hailed at the time of publication as evidence for 'the Earth's oldest life', attract great publicity and debate, and are remarkable not least because of the overlap with a time when the rate of meteorite bombardment was considerably greater [7], with a concomitantly increased "impact frustration" to the establishment of terrestrial life [8].A number of key criteria, required for the credibility of claims for early life, were proposed in a review paper by Whitehouse and Fedo (2007) [9] and have recently been reiterated by Whitehouse et al. [10]. Along with the veracity of the carbon isotopic evidence itself, upon which most Eoarchean life claims rely, these criteria include (1) the suitability of host rocks as possible environments in which life might have developed and (2) temporal constraints provided either directly by the isotopic dating of the host rock or through field relationships with well-dated lithologies. Using the case studies cited above, this presentation will stress the important of both unambiguous geological relationships and geochronology in the quest for finding evidence of Earth's earliest biogenicity, and will review the status of such claims. It is noteworthy that the preservation of fossil evidence in all claims prior to the Paleoarchean is severely limited by the effects of tectonism and metamorphism, and sole reliance on chemical or isotopic evidence cannot provide an unambiguous basis for biogenicity.References: [1] Schidlowski et al. (1979) Geochim. Cosmochim. Acta 43, 189. [2] Allwood et al. (2007) Precam. Res. 158, 198. [3] Rosing (1999) Science 283, 674. [4] Mojzsis et al. (1996) Nature 384, 55. [5] Dodd et al. (2017) Nature 543, 60. [6] Tashiro et al. (2017) Nature 549, 516. [7] Bottke and Norman (2017) Annu Rev Earth Pl Sci 45, 619. [8] Maher & Stevenson, (1988) Nature 331, 612. [9] Whitehouse & Fedo (2007) Developments in Precambrian Geology (Elsevier) 15, 841. [10] Whitehouse et al. (2019). Precam. Res, doi.org/10.1016/j.precamres.2019.01.001. [ABSTRACT FROM AUTHOR]