Earaheedy Basin Research Articles

Petrogenesis of Proterozoic alkaline ultramafic rocks in the Yilgarn Craton, Western Australia

The Yilgarn Craton and its northern margin contain a variety of petrogenetically poorly defined small-volume alkaline ultramafic rocks of Proterozoic age. This study documents the petrography, mineral and bulk-rock geochemistry and Nd-Hf-Sr-Pb isotope compositions of a selected suite of these rocks. They comprise ~2.03–2.06 Ga ultramafic lamprophyres (UML) and carbonatites from the Eastern Goldfields Superterrane (EGS), ~0.86 Ga UML from Norseman, and orangeites from the Earaheedy Basin, including samples from Jewill (~1.3 Ga), Bulljah (~1.4 Ga) and Nabberu (~1.8–1.9 Ga). The Proterozoic UML and carbonatites from the EGS and Norseman display very consistent chondritic to superchondritic Nd-Hf isotope compositions and trace-element ratios similar to modern OIBs, which are indicative of a common mantle source across this wide alkaline province. These Nd-Hf isotope compositions overlap with the evolution trends of global kimberlites through time, thus suggesting that this mantle source could be deep and ancient as that proposed for kimberlites. Conversely, the orangeites located in the Earaheedy Basin along the northern margin of the Yilgarn Craton display trace element signatures similar to subduction-related calc-alkaline magmas. Taken together with their highly enriched Sr-Nd-Hf isotope compositions, these characteristics indicate an ancient lithospheric mantle source, which was probably metasomatised by subduction-related fluids. As the ages of the Bulljah and Jewill orangeites overlap with the breakup of the Columbia supercontinent, it is proposed that orangeite magmatism was triggered by changes in plate stress conditions associated with this event. This study provides a comprehensive picture of the genesis of Proterozoic alkaline magmatism in the Yilgarn Craton, highlighting the complex tectono-magmatic evolution of this lithospheric block after its assembly in the Archean.

Imaging sedimentary basins from high-resolution aeromagnetics and texture analysis

Image texture is an important property of gridded aeromagnetic data that together with the availability of closer survey line-spacings and more sensitive instrumentation, has become more relevant for qualitative interpretations. Problems arise though when it is used as a basis for geological interpretation, since visual textures are difficult to define. This can introduce bias when interpretations are not validated against repeatable and testable quantitative measures. We propose the use of well-established unsupervised texture segmentation techniques that use multi-scale, orientation-selective Gabor filters to automatically domain contrasting textures in aeromagnetic maps. These methods are applied to tilt-derivative reduced-to-pole magnetics over the Palaeoproterozoic Earaheedy Basin, that was deposited over the northeastern Yilgarn Craton in a marginal setting prior to the ca. 1800 Ma Capricorn Orogeny Western Australia. These results are then validated against a geophysically constrained interpretation of the area and a review of the magnetic characteristics of the region. Our results demonstrate how automatic texture domaining from gridded magnetic-field data can produce geologically feasible results that can be used to constrain the architecture of basins that contain magnetic sedimentary rocks.

Archean controls on basin development and mineralisation in the Southern Capricorn Orogen

Basins along the northern margin of the Yilgarn Craton developed in response to extensional and compressional processes during the Paleoproterozoic. Early extension resulted in the formation of the Yerrida Basin as a large single basin over the northern Yilgarn Craton. Subsequent rifting led to voluminous volcanism in the northern part of the basin, within two depositional centres - the c. 2.03 to 1.96 Ga Bryah and Mooloogool Sub-basins. Cu-Au VMS deposits formed in the Bryah Sub-basin. Yilgarn Craton crust can be mapped using gravity and magnetic data beneath the Yerrida Basin, and Mooloogool Sub-basin. However, the Yerrida Basin can’t be mapped below much of the Bryah Basin, implying the formation of an ocean in this region. The degree of rifting of the Yilgarn Craton, and resulting architecture influenced subsequent basin development, and deformation in the region. For example, in areas where Yilgarn Craton crust can’t be mapped beneath basin sediments deformation is pronounced with the formation of disharmonic folds, refolded folds, and anastomosing shear zones. The southern part of the Yerrida Basin and the Earaheedy Basin formed shallow depositional centres over the Yilgarn Craton, and subsequent deformation in these regions is less intense. Base metal mineralisation in the region can, in part be related to the presence of deep crustal scale structures that initially developed in the Archean, and were re-activated during the Proterozoic. However, the location of c. 1800 Ma orogenic Au mineralization in the Bryah Sub-basin may not have been influenced by localisation of fluid flow around deep crustal-scale faults that formed as 200 Ma earlier, or even as early as the Archean.

U-Pb geochronology of monazite in Precambrian tuffs reveals depositional and metamorphic histories

Uranium-lead geochronology of zircon in felsic tuffs plays a key role in the development of chronostratigraphic frameworks for sedimentary successions. This is particularly true for Precambrian basins, where biostratigraphy is of limited use for age determination or stratigraphic correlation. Although zircon is widely used to date tuffs, it can yield null results due to the presence of xenocrystic populations or isotopic resetting. Other potential U-Pb chronometers, such as monazite, which also crystallize from felsic magmas, are largely unreported from felsic tuff layers. Consequently, there has been a lack of studies to evaluate the potential of monazite for U-Pb isotope geochronology of tuffs. We present results of in situ U-Pb ion microprobe dating of monazite that co-exists with zircon in felsic tuffs from two Precambrian basins: the Earaheedy Basin (Frere Formation) in Western Australia and the Bushveld Basin (Lephala Formation) in southern Africa. We show that 207Pb/206Pb ages from monazite in felsic tuffs from the Frere Formation (1891 ± 9 Ma) and the Lephala Formation (2307 ± 9 Ma) are identical to zircon ages from the respective tuffs. Also, monazite in the Bushveld tuff preserves additional information, including a chronological fingerprint of the volcanic source (2.4–2.5 Ga xenocrysts) and the timing of low-grade metamorphism (2.16 Ga overgrowths). The ca. 2.16 Ga low-grade metamorphism identified in the Bushveld Basin is reminiscent of that widely recorded in the Pilbara Craton, Western Australian, lending support to arguments for a connection between the Kaapvaal and Pilbara cratons. Igneous and low-grade metamorphic monazites are readily distinguished based on texture and chemistry. Our study demonstrates that monazite can be used to accurately date felsic tuffs, offering a powerful tool for constraining depositional and metamorphic histories of Precambrian sedimentary basins.

Newly identified 1.89 Ga mafic dyke swarm in the Archean Yilgarn Craton, Western Australia suggests a connection with India

The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408 Ma Widgiemooltha and the 1210 Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075 Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888 ± 9 Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858 ± 54 Ma, 1881 ± 37 and 1911 ± 42 Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300 million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890 Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.

A new Paleoproterozoic tectonic history of the eastern Capricorn Orogen, Western Australia, revealed by U–Pb zircon dating of micro-tuffs

The Earaheedy Basin is one of several Paleoproterozoic siliciclastic basins in the eastern part of the Capricorn Orogen in Western Australia. In the absence of an adequate chronostratigraphic framework, multiple conflicting models have been proposed for the tectonic history of the basin. We present in situ SHRIMP U–Pb zircon dates of 1971±11Ma, 1954±5Ma and 1949±10Ma from three thin tuff horizons (“micro-tuffs”) in the Yelma Formation at the base of the basin succession. These micro-tuffs are only slightly younger than the 1990±6Ma Imbin porphyry, which has been interpreted as a basement inlier. Our new dates, combined with field mapping, suggest that the Imbin porphyry is a volcanic–subvolcanic complex within the basin. Micro-tuffs in the Yelma Formation are more than 50m.y. older than c. 1890Ma micro-tuffs within the overlying Frere Formation. Deposition of the Yelma Formation was coeval with continent–continent collision farther west in the Orogen marked by the 2005–1950Ma Glenburgh Orogeny, and the formation is possibly correlative with peripheral foreland deposits of the Padbury Group. The Frere Formation was deposited in shallow marine conditions within an intracratonic basin about 60m.y. after the Glenburgh Orogeny. The Frere Formation is unconformably or disconformably overlain by the Miningarra Group, the youngest unit of which is the Mulgarra Sandstone. This sandstone was deposited after c. 1865Ma, and derived either from exposed Frere Formation or from a now-buried source within the Capricorn Orogen that was uplifted by inception of the 1820–1770Ma Capricorn Orogeny. Therefore, the Earaheedy Basin comprises at least three distinct, unconformity-bound successions deposited in contrasting tectonic settings.

The Paroo Station Mine supergene lead deposits, Western Australia: Geological and geochemical constraints

The Paroo Station Mine lead project is situated near Wiluna in the central part of Western Australia. It is located in an outlier of the Paleoproterozoic Earaheedy Group overlying the southeast Yerrida Basin. The total Measured and Indicated Mineral Resource is currently (as at 31st of December 2014) estimated at 31.8Mt of ore at 4.4% Pb. The deposits are hosted by deeply weathered carbonate and clastic sediments of the Paleoproterozoic Yelma Formation. The lead-only mineralisation contains only traces of sulfides and is composed of few supergene minerals, predominantly cerussite and anglesite with minor pyromorphite and plumbogummite. The Paroo Station Mine demonstrates a high degree of supergene Pb mobility which has resulted in almost complete destruction of primary Pb distribution pattern and development of characteristic for the oxidation zones coupled depletion zone and subhorizontal enrichment blankets below.The deposits are interpreted to have formed from Mississippi Valley-type (MVT), stratabound relatively low-grade sulfide PbZn mineralisation located within dolomites of the Sweetwaters Well Member of the Yelma Formation. The primary mineralisation based on modelling the Pb distribution was controlled by a network of northeast, northwest and meridional oriented fractures and small faults. Lead isotope study yielded from 1.80 to1.88Ga ages of the mineralisation based on the Stacey and Kramers (1975) two-stage model; the mineralising event is related to the Capricorn Orogeny. This data is well in line with previous UPb dating of monazites interpreted to be related to the mineralisation (1.82Ga; Muhling et al., 2012). High μ (238U/204Pb) values (10.26–10.58) are indicative of the Archean basement contribution and/or more likely with sediments originally derived from that basement. The Paroo Station Mine mineralisation is thought to be similar in style and formed synchronously with the MVT prospects in the Teague area in the Earaheedy Basin approximately 120km to the northeast.The mineralisation at the Paroo Station Mine was significantly upgraded through a multi-stage supergene remobilisation and reprecipitation of lead in the course of long weathering history of the region. Lead was almost completely depleted from the upper zone and accumulated below as subhorizontal enrichment blankets at three levels (from top to bottom): (1) the base of the collapse quartz-clay breccia unit marking the dolomite weathering front; (2) the contact of siltstone and sandstone packages coincident with the feldspar/kaolinite weathering front and; (3) the mid- to lower parts of the sandstone unit close to the base of weathering. The lowest enrichment zone extends as a continuous blanket at a significant, up to 500m distance (at a 1% Pb cut-off) beyond the upper mineralisation projection. This enrichment is interpreted to be formed by lateral Pb transport in waters close to the groundwater table through the most permeable layers of weathered sandstones. This active lateral lead dispersion is of high exploration significance for the region providing a much larger exploration target and vector to the primary mineralisation.

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fluvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fluvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution.

Oxygenation of shallow marine environments and chemical sedimentation in Palaeoproterozoic peritidal settings: Frere Formation, Western Australia

AbstractThe Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western Australia, is a ca 600 m thick succession of iron formation and fine‐grained, clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling. Lithofacies stacking patterns suggest that deposition occurred during a marine transgression punctuated by higher frequency relative sea‐level fluctuations that produced five parasequences. Decametre‐scale parasequences are defined by flooding surfaces overlain by either laminated magnetite or magnetite‐bearing, hummocky cross‐stratified sandstone that grades upward into interbedded hematite‐rich mudstone and trough cross‐stratified granular iron formation. Each aggradational cycle is interpreted to record progradation of intertidal and tidal channel sediments over shallow subtidal and storm‐generated deposits of the middle shelf. The presence of aeolian deposits, mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind‐blown sediment input. Iron formation in the Frere Formation, in contrast to most other Palaeoproterozoic examples, was deposited almost exclusively in peritidal environments. These other continental margin iron formations also reflect upwelling of anoxic, Fe‐rich sea water, but accumulated in the full spectrum of shelf environments. Dilution by fine‐grained, windblown terrigenous clastic sediment probably prevented the Frere iron formation from forming in deeper settings. Lithofacies associations and interpreted paragenetic pathways of Fe‐rich lithofacies further suggest precipitation in sea water with a prominent oxygen chemocline. Although essentially unmetamorphosed, the complex diagenetic history of the Frere Formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean‐atmosphere evolution. Unlike studies that focus exclusively on their chemistry, an approach that also considers palaeoenvironment and oceanography, as well the effects of post‐depositional fluid flow and alteration, mitigates the potential for incorrectly interpreting geochemical data.

Dating fluid flow and Mississippi Valley type base-metal mineralization in the Paleoproterozoic Earaheedy Basin, Western Australia

The ages of deposition and metamorphism of low-grade Precambrian metasedimentary sequences can be difficult to define in the absence of interlayered volcanogenic rocks. Monazite and xenotime can grow at temperatures below 400°C and can give vital evidence for the timing of diagenesis, hydrothermal fluid flow, and low-grade deformation and metamorphism in Precambrian basins. The histories of sedimentary basins in the eastern Capricorn Orogen of the West Australian Craton are generally poorly constrained. However, sandstones and siltstones of the basal Yelma Formation in the Earaheedy Basin contain authigenic monazite and xenotime associated with sulphide minerals related to Mississippi Valley Type base-metal mineralization hosted within carbonate of the overlying Sweetwaters Well Member. In the Teague area, within the Earaheedy Basin, siliciclastic rocks overlying granites of the Yilgarn Craton contain authigenic monazite that gives an in situ SHRIMP 207Pb/206Pb age of 1811±13Ma. Monazite in weathered sandstones and siltstones of the Cano secondary Pb deposit on Magellan Hill, overlying the Yerrida Basin, gives an indistinguishable age of 1815±13Ma. The Cano monazite is interpreted to have been intergrown with primary sulphide minerals and therefore to record the age of mineralization. Xenotime outgrowths on detrital zircons from Cano have high common Pb but define an isochron with an age of 1832±36Ma. The isochron is co-linear with Pb isotope ratios from Pb ore at Magellan, suggesting that xenotime growth is also related to mineralization. The timing of fluid flow is synchronous with the 1820–1770Ma Capricorn Orogeny, and is the first evidence for activity of this age in the eastern part of the Capricorn Orogen. The age of mineralization provides a firm minimum age for the Yelma Formation, and constrains its deposition to between ∼2.0Ga, the youngest age of detrital zircons, and the mineralization age.

Palaeomagnetism of iron-formation from the late Palaeoproterozoic Frere Formation, Earaheedy Basin, Western Australia: palaeogeographic and tectonic implications

Abstract Oriented cores of unweathered Frere Formation (1.9–1.8 Ga), including shaly and peloidal iron-formation, were obtained from two holes drilled to 116 m depth on opposite limbs of an anticline in the southern Earaheedy Basin at the northeastern margin of the Yilgarn Craton. Anisotropy of magnetic susceptibility measurements indicated that the Frere Formation in this area was unaffected by regional metamorphism. The predominant magnetic mineral in the iron-formation is hematite but a small amount of magnetite also is present in some samples. Stepwise thermal demagnetisation was conducted for 263 specimens. Many specimens acquired spurious components of magnetisation after treatment to 300–500 °C, ascribable to the oxidation of siderite to magnetite on heating. However, other specimens displayed a stable, high-temperature (680 °C), dual-polarity component A carried by hematite that gave a bedding dip-corrected direction of D=314.3°, I=−24.8° (K=38.5, α95=2.6°, n=78). A fold test was positive at >99% level of confidence, indicating acquisition of remanence prior to Stanley folding in the Earaheedy Basin that occurred at ∼1.78 Ga near the end of Capricorn orogenesis. The presence of polarity reversals in the stratigraphic section from one drill hole and within some specimens argues for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the Frere Formation, located at 45.2°S, 220.0°E (dp=1.3°, dm=2.4°). Component A is ascribed to a chemical remanent magnetisation (CRM) acquired soon after deposition, and indicates a palaeolatitude of 13.0±1.5° for the Frere Formation. The Frere pole plots adjacent to, but is still significantly different from, late to post-Capricorn Orogeny overprint poles from the Hamersley Province of the southern Pilbara. However, vertical-axis relative anticlockwise rotation of 10–15° for the Frere pole brings it close to these Pilbara overprint poles. The data imply that the Earaheedy Basin and possibly the Yilgarn Craton underwent clockwise rotation of 10–15° relative to the Pilbara Craton during Stanley folding. This finding accords with evidence for tectonic settings marked by sinistral strike-slip deformation adjacent to the Yilgarn–Pilbara cratonic boundary during deposition and subsequent folding in the Earaheedy Basin. The data also imply that the West Australian and North Australian cratons were in close proximity by ∼1.8 Ga.

Metallogeny in the Capricorn Orogen, Western Australia, the result of multiple ore-forming processes

The Capricorn Orogen includes metamorphosed tectonic units, intrusive igneous rocks, volcano-sedimentary and sedimentary basins situated between the Pilbara and Yilgarn cratons. These include the Gascoyne Complex, Ashburton, Blair, Bresnahan, Yerrida, Bryah, Padbury, and Earaheedy basins, and Edmund and Collier (former Bangemall) basins. Mineral deposits and occurrences in these units comprise stratabound base metals, including the giant Abra Pb–Cu–Ba (Edmund Basin) and Magellan Pb (Earaheedy Basin), and numerous hydrothermal veins and precious and/or base metal lodes. Some of the more important orogenic precious metal lodes are those of the Peak Hill group in the Bryah-Padbury basins and the Egerton field in the Gascoyne Complex. The metallogeny of the Capricorn Orogen reflects syn- to post-Capricorn orogenic events, linked to either protracted convergence tectonics between the Pilbara and Yilgarn cratons, or to a two-stage convergence history, one during the Capricorn Orogeny (ca. 1.83 Ga; Pilbara–Yilgarn convergence), the other during the Yapungku orogeny (ca. 1.79; North Australian–West Australian convergence). The metallogeny of the Capricorn Orogen was further sustained by continental mafic magmatism at about 1.45 and 1.07 Ga, during which heat advection may have caused widespread hydrothermal circulation, particularly in the Edmund, Collier, Earaheedy basins and adjacent tectonic units. An interesting, but as yet unexplained, aspect of the Capricorn metallogeny is the regional Pb–Ba enrichment. A conceptual model that attempts to explain the billion-year-long history of the metallogeny in the Capricorn Orogen is proposed. Mineral exploration in the Capricorn Orogen is somewhat limited and not as intensive as is in the brown-fields of the Archaean terranes of Western Australia.

Geology and tectonic evolution of Palaeoproterozoic basins of the eastern Capricorn Orogen, Western Australia

Abstract The Palaeoproterozoic Yerrida, Bryah, Padbury, and Earaheedy Basins extend for about 700 km along the northern margin of the Yilgarn Craton and are part of the Capricorn Orogen. The basins developed between ca. 2.2 and 1.8 Ga, recording periods of sedimentation, volcanism, rifting, accretion and passive-margin tectonism. The oldest is the 2.17 Ga Yerrida Basin, which began as an intracontinental sag within which siliciclastics and evaporites accumulated. At about 2.0 Ga convergence of the Yilgarn Craton and the Glenburgh terrane closed an oceanic arm, resulting in the accretion of oceanic crust and plateaux onto the Yilgarn margin and the western side of the Yerrida sag basin. This accretion produced the Narracoota Formation, which together with the clastic and chemical (banded iron-formation) sedimentation that followed, forms the Bryah Basin. Continuing collision resulted in the establishment, around 1.96 Ga, of the Padbury foreland basin, developed on top of the Bryah Basin rocks. The ca. 1.83 Ga convergence and collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny), resulted in deformation of the Bryah–Padbury Basins and orogenic uplift with coarse clastic sedimentation accompanied by the eruption of flood basalts into the Yerrida Basin. To the east of the collision zone, and possibly postdating collision, a passive margin was present. Sedimentation along it consisted of clastic and chemical sediments (granular iron-formation), which form the Earaheedy Basin. During the ca. 1.79 Ga Yapungku Orogeny, due to the convergence of the North Australian and Western Australian cratons, the northern margin of the Earaheedy Basin was deformed (Stanley Fold Belt). This last deformation event concludes the Palaeoproterozoic history of the eastern Capricorn Orogen basins.

Provenance of the Earaheedy Basin: implications for assembly of the Western Australian Craton

Abstract The Palaeoproterozoic Earaheedy Basin in Western Australia lies at the southern end of the Capricorn Orogen. The Earaheedy Group, the sole lithostratigraphic unit comprising the basin fill, is a 5 km thick succession of shallow-marine siliciclastic and carbonate sedimentary rocks. It consists from oldest to youngest of the Yelma, Frere, Windidda, Chiall, Wongawol, Kulele and Mulgarra formations. U–Pb age data for 312 detrital zircon grains from seven siliciclastic samples from the Yelma Formation, the Wandiwarra and Princess Ranges members of the Chiall Formation and the Mulgarra Sandstone reveal the presence of both Archaean (3.4–2.5 Ga) and Palaeoproterozoic (2.4–1.8 Ga) detritus. Zircon grains of Archaean age are dominantly in the range 2.7–2.6 Ga, similar to those of the Yilgarn Craton, which unconformably underlies the southern margin of the Earaheedy Basin. Age peaks in the range 2.3–2.2, 2.0 and 1.8 Ga, characterise the Palaeoproterozoic detritus. The two younger ages overlap with those of the adjoining Gascoyne Complex of the Capricorn Orogen to the west of the basin. Source components with ages in the range 2.3–2.2 Ga are extremely rare in Western Australia. Undeformed granite in the Mullingarra Complex within the Pinjarra Orogen to the west of the Darling Fault yielded an age of around 2.2 Ga. In addition, rare rims on detrital zircon grains from gneisses within basement of the Glenburgh Terrane of the Gascoyne Complex as well as a few detrital grains in metamorphosed quartz arenites of the terrane have yielded ages of around 2.4–2.2 Ga, suggesting early Palaeoproterozoic activity in this region. Detrital grains as young as 1800 Ma, combined with deposition of the unconformably underlying strata in the Yerrida Basin at c. 1840 Ma and deformation of the Earaheedy Group at around 1750 Ma, provide a relatively tight constraint on the depositional age of the Earaheedy Group.

Shoemaker impact structure, Western Australia

The Shoemaker impact structure, on the southern margin of the Palaeoproterozoic Earaheedy Basin, with an outer diameter of ∼⃒30 km, consists of two well‐defined concentric ring structures surrounding a granitoid basement uplift. The concentric structures, including a ring syncline and a ring anticline, formed in sedimentary rocks of the Earaheedy Group. In addition, aeromagnetic and geological field observations suggest that Shoemaker is a deeply eroded structure. The central 12 km‐diameter uplift consists of fractured Archaean basement granitoids of syenitic composition (Teague Granite). Shock‐metamorphic features include shatter cones in sedimentary rocks and planar deformation features in quartz crystals of the Teague Granite. Universal‐stage analysis of 51 sets of planar deformation features in 18 quartz grains indicate dominance of sets parallel to ω {1013}, but absence of sets parallel to π {1012}, implying peak shock pressures in the range of 10–20 GPa for the analysed sample. Geophysical characteristics of the structure include a —100 μs ‐2 gravity anomaly coincident with the central uplift and positive circular trends in both magnetic and gravity correlating with the inner ring syncline and outer ring anticline. The Teague Granite is dominated by albite‐quartz‐K‐feldspar with subordinate amounts of alkali pyroxene. The alkali‐rich syenitic composition suggests it could either represent a member of the Late Archaean plutonic suite or the product of alkali metasomatism related to impact‐generated hydrothermal activity. In places, the Teague Granite exhibits partial to pervasive silicification and contains hydrothermal minerals, including amphibole, garnet, sericite and prehnite. Recent isotopic age studies of the Teague Granite suggest an older age limit of ca 1300 Ma (Ar–Ar on K‐feldspar) and a younger age limit of ca 568 Ma (K–Ar on illite‐smectite).The significance of the K–Ar age of 568 Ma is not clear, and it might represent either hydrothermal activity triggered by impact‐related energy or a possible resetting by tectonothermal events in the region.

Evidence of hydrocarbon and metalliferous fluid migration in the Palaeoproterozoic Earaheedy Basin of Western Australia

Dolomitized limestones of the Earaheedy Basin contain solid bitumen and metal‐sulphides, sulpharsenides and arsenides within a network of fractures which acted as pathways for migrating fluids. The solid bitumen formed from residual oil that was thermally altered within those fractures. Immobilization of oil was postdated by precipitation of fracture‐filling calcite and later replacive dolomite and quartz cements, as well as of magnesian chlorite. Solid bitumen has high concentrations of Fe‐, Zn‐, Pb‐, Ni‐ and FeCu‐bearing sulphides, sulpharsenides and arsenides in minute inclusions that are randomly distributed throughout its groundmass. Galena and pyrite also fill cracks in the bitumen, whereas pyrite, arsenopyrite, galena, gersdorffite, enargite? and chalcopyrite fill contraction vugs. Small crystals of cassiterite are associated with cavity‐filling magnesian chlorite. Multiple phases of base‐metal precipitation are apparent. The first, indicated by the inclusions, probably formed during mixing of hydrocarbon and metal‐rich fluids. Later phases resulted from the filling of bitumen contraction structures that formed during and after thermal alteration and degassing processes, and which imply the migration of metal‐rich fluids after oil immobilization. The association of metal‐rich minerals and fracture‐filling bitumen, in this non‐ore setting, is a guide to the possible existence of ore‐grade mineralization elsewhere in the basin.

Stromatolites from the Palaeoproterozoic Earaheedy Group, Earaheedy Basin, Western Australia

Two new stromatolites, Ephyaltes edingunnensis and Eucapsiphora leakensis, are described from the Palaeoproterozic Earaheedy Group, in the Earaheedy Basin (formerly the eastern part of the Nabberu Basin), Western Australia. In addition, new localities are reported for Asperia digitata comb. nov. (= Yelma digitata Grey 1984), Pilbaria deverella Grey 1984, and Yandilla meekatharrensis Grey 1984 from problematic successions in the adjacent Glengarry Basin (formerly the western part of the Nabberu Basin), confirming correlation of these rocks with the base of the Earaheedy Group. Four of the five groups recorded, Asperia, Eucapsiphora, Ephyaltes and Pilbaria, are known from outside the Earaheedy Basin and their stratigraphic age is consistent with the probable minimum 1.65 Ga age of the Earaheedy Group. However, none of the forms has yet been recorded from outside the Earaheedy Group.