Wyloo Group Research Articles

A Paleoproterozoic Aeolianite (the Nummana Member) from the Lower Wyloo Group, Pilbara Craton, Western Australia, and Its Implication

AbstractIn the absence of vegetation, wind systems would have been very efficient in reworking terrestrial sediments. As a consequence, the Precambrian sedimentary succession should theoretically b...

U-Pb detrital zircon geochronology of the Turee Creek Group, Hamersley Basin, Western Australia: Timing and correlation of the Paleoproterozoic glaciations

The early Paleoproterozoic Turee Creek Group (TCG), Hamersley Basin, Western Australia, records a continuous siliciclastic-dominated sedimentary sequence across the Great Oxidation Event (GOE) and contains at least two glacial horizons showing many similarities with other Paleoproterozoic glaciogenic deposits from North America (Huronian Supergroup) and South Africa (Transvaal Supergroup). The correlation between these glacial events is uncertain, however, because of a lack of geochronological constraints, contradictory stratigraphic correlations among different cratons, and the occurrence of numerous sedimentary discontinuities in the Transvaal and Huronian successions. In this study, we performed in situ U-Pb dating of detrital zircons from drill core and surface samples of the TCG, the underlying Boolgeeda Iron Formation of the Hamersley Group, and the overlying Beasley River Quartzite of the Lower Wyloo Group. Over 1500 zircons analyzed by Laser Ablation coupled to ICP-MS yielded major age peaks spanning the Archean Eon at ca. 2.45 Ga, 2.54 Ga, 2.68 Ga and minor ones at 2.81 Ga, 2.96 Ga and 3.14 Ga. A younger age of 2340 ± 22 Ma obtained at the base of the main glacial deposit of the Meteorite Bore Member, represents a maximum age for this glacial event. A meter-scale level of diamictite has previously been described in the Boolgeeda Formation at the Boundary Ridge Locality (located about 50 km to the east of the Meteorite Bore Member succession) and a similar horizon was newly identified in drill core samples from the Hardey Syncline area. The youngest detrital zircons have an age of ca. 2.45 Ga in these diamictites supporting the hypothesis that these horizons represent a different and older glacial event than the Meteorite Bore Member. Considering that the TCG and underlying Boolgeeda Iron Formation are in sedimentary continuity, these results allow establishing a correlation with the other glaciogenic deposits recorded in the North America and southern Africa basins, and reconstructing the sequence of redox and climate changes attending the early Paleoproterozoic.

Palaeoproterozoic terrestrial sedimentation in the Beasley River Quartzite, lower Wyloo Group, Western Australia

Pre-2.2Ga aeolianite deposits are relatively rare in the geological record, due to reworking of aeolianites either by fluvial systems, transgression or non-recognition. Here, we present high resolution sedimentary facies analysis of a section through the 2.2Ga Beasley River Quartzite, lower Wyloo Group, Western Australia. The unambiguous presence of terrestrial (fluvial-aeolian) deposition is documented in the form of fluvial architectural elements (channel, bar, lateral accretion and overbank deposits) and aeolian features (dune, pin-stripe lamination, wind streaks, and adhesion features). These observations contrast strongly with a previous interpretation of marine deposition, which is discounted. Our data is consistent with the dominantly terrestrial depositional mode of the rest of the lower Wyloo Group, including the basal Three Corners Conglomerate Member and the subaerial Cheela Springs Basalt. We conclude that the lower Wyloo succession formed in a terrestrial regime during continental rifting.

Early Earth evolution: evidence from the 3.5–1.8 Ga geological history of the Pilbara region of Western Australia

The Pilbara region of Western Australia is one of only two areas on Earth - the other being the Kaapvaal Craton of southern Africa - that contain well preserved, near-continuous geological records of crustal evolution from the Paleoarchean into the late Paleoproterozoic. The Pilbara is famous for hosting fossil evidence of early life (stromatolites and microfossils), and for containing a record of the early Archean atmosphere. The geological record extends from granite-greenstone terranes and overlying clastic basins of the 3.53-2.83 Ga Pilbara Craton, across a major unconformity, to a series of 2.78-1.79 Ga volcanic and sedimentary successions. Between 3.53-3.23 Ga, a succession of mantle plume events formed a thick volcanic plateau on older continental crust, remnants of which include enclaves of c. 3.6 Ga granitic gneiss and abundant 3.8-3.6 Ga inherited and detrital zircons. During each of the plume events, the volcanic plateau was intruded by crustally-derived granitic rocks, leading to vertical deformation by partial convective overturn. By 3.23 Ga, these processes had established thick continental crust that was then rifted into three microplates separated by c. 3.2 Ga basins of oceanic crust. Subsequent plate tectonic processes to 2.90 Ga included subduction, terrane accretion, and orogeny. From 2.78-2.63 Ga the northern Pilbara Craton was affected by minor rifting, followed by deposition of thick basaltic formations separated by felsic volcanic and sedimentary rocks (Fortescue Basin). Rifting in the southern Pilbara resulted in progressively deepening marginal basin sedimentation, including thick units of banded iron formation (Hamersley Basin: 2.63-2.45 Ga). At c. 2.45 Ga, sedimentation in the southern Pilbara changed to a mixed assemblage of clastic and carbonate sedimentary rocks of the Turee Creek Basin, including one unit of glacial diamictites. Deposition of the unconformably overlying 2.21-1.79 Ga Wyloo Group in the Ashburton Basin followed the Ophthalmian Orogeny, and all of these rocks were deformed by the Panhandle (c. 2 Ga) and Capricorn (c. 1.78 Ga) orogenies.

Magnetic petrophysical results from the Hamersley Basin and their implications for interpretation of magnetic surveys

An extensive magnetic petrophysical investigation was carried out in the Hamersley Basin in Western Australia; which hosts large high-grade iron-ore deposits derived from banded iron formations (BIFs). Based on our new results, as well as those from previous studies, we demonstrate that the bulk susceptibility of BIF units has a bimodal distribution caused by the presence of chert-rich and magnetite-rich bands. The mean bulk susceptibility (MBS) of magnetite-rich bands is more than 10 times higher than that of chert-rich bands. Measurements of anisotropy of magnetic susceptibility (AMS) in BIF units show this to vary between 1.06 and 2.76. The natural remanent magnetisation (NRM) carried by iron ores varies with different deposits, and even within individual deposits. NRM measurements on non-ore units defined a northwesterly upward regional magnetic overprint (310°/–27°) present in the Fortescue and Hamersley Groups, and also in the lower Wyloo Group. Modelling of aeromagnetic data over the Hamersley Range demonstrates that the effects of AMS and NRM are significant and must be accounted for. The effects of AMS and NRM on magnetic anomalies due to BIFs depend on the volume fraction of magnetite-rich members in a BIF unit. Forward modelling of magnetic and gravity responses, using petrophysical results summarised in this study, over a mined hematite ore deposit in Tom Price indicates that TMI variations are expected to be significantly reduced in areas of mineralisation. However, magnetic data alone are not a reliable indicator of the presence of mineralisation because of other causes of reduced magnetism in BIFs.

Tectonic setting and regional implications of ca 2.2 Ga mafic magmatism in the southern Hamersley Province, Western Australia

Although the presence of post-Hamersley Group mafic intrusive and extrusive rocks in the southern Hamersley province of Western Australia has been known since the area was first mapped in the early 1960s, details of the age, tectonic setting and significance of these rocks have only recently been determined, and are still controversial. These rocks are most commonly interpreted as the products of two temporally distinct periods of continental extension, separated by a hiatus of ∼370 m.y. represented by the unconformity at the base of the Wyloo Group. However, the integration of published geochronological and geochemical data with detailed field observations documented in this study shows that ca 2.2 Ga dolerite sills pre- and post-date this unconformity, and were intruded during Ophthalmian orogenesis in a retroarc foreland basin. Furthermore, ca 2.2 Ga mafic magmatism is interpreted to include the Cheela Springs Basalt and is related to subduction beneath the southern Hamersley province, most likely resulting in accretion of part of the Gascoyne Province to the Pilbara Craton.

Structural controls of bedded iron ore in the Hamersley Province, Western Australia – an example from the Paraburdoo Ranges

Structure is the most important control on the location of hematite deposits in the Hamersley Province in the Pilbara region of Western Australia. Unravelling the structural history of mineralised areas, however, is complex because the Province underwent three orogenic events, and three regional extension events followed the main episodes of compression. Steeply E to ENE dipping normal faults immediately predated deposition of the Beasley Quartzite of the lower Wyloo Group after the Ophthalmian Orogeny. Steeply NE and SW–S dipping normal faults predated deposition of the McGrath formation of the middle Wyloo Group, following the Panhandle Orogeny. The resulting interpreted rift was subsequently reactivated and tilted during the Capricorn Orogeny. Late NE trending normal faults are confined to the eastern half of the Province. High-grade hematite deposits occur in rocks that range from intensely folded (Mt Whaleback, Giles Mini) to rocks with less, or only little deformation (Paraburdoo, Channar), suggesting that folding had at best limited influence on ore genesis. In contrast, extensional normal faults are always associated with high-grade hematite deposits, and in particular faults that link the underlying dolomites of the Wittenoom Formation with the overlying iron formation, thus providing a pathway for mineralising alkaline fluids.

Intracontinental reworking in the Capricorn Orogen, Western Australia: the 1680 – 1620 Ma Mangaroon Orogeny*

Structures and metamorphic mineral assemblages throughout much of the Proterozoic Capricorn Orogen have been attributed to the 1830 – 1780 Ma Capricorn Orogeny. However, in the northern part of the Gascoyne Complex at the exposed western end of the orogen, precursor sediments to pelitic gneiss and metamorphosed feldspathic sandstone were deposited after 1680 ± 13 Ma. The sediments were first deformed and intruded by voluminous granites during intracontinental reworking (the Mangaroon Orogeny) between 1680 Ma and 1620 Ma. The Mangaroon Orogeny comprises a subhorizontal gneissic layering and peak metamorphic conditions of upper amphibolite facies (D1m/M1m) followed by upright, macroscopic southeast-trending folds and an associated subvertical foliation formed at greenschist facies (D2m/M2m). Schlieric biotite – muscovite granodiorite dated at 1677 ± 5 Ma represents an anatectic melt that intruded during D2m. Cross-cutting biotite and biotite – muscovite monzogranites mainly crystallised at ca 1680 – 1660 Ma, but granite plutons were emplaced across the Gascoyne Complex until 1620 Ma. Our findings invalidate correlation of these metasedimentary rocks with lower grade rocks of the upper Wyloo Group along the northern margin of the orogen, and necessitate a reassessment of tectonic activity in the orogen attributed to the Capricorn Orogeny.

Giant iron-ore deposits of the Hamersley province related to the breakup of Paleoproterozoic Australia: New insights from in situ SHRIMP dating of baddeleyite from mafic intrusions

Banded iron formations of the ca. 2770–2405 Ma Hamersley province of Western Australia were locally upgraded to high-grade hematite ores during the Early Paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is ca. 2008 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes pre s ent a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050–2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma.

Stratigraphic architecture and evolution of the early Paleoproterozoic McGrath Trough, Western Australia

Palaeocurrents, provenance and stratigraphic analysis of the upper part of the Mount Bruce Supergroup and the Wyloo Group in the southwestern part of the Hamersley Province of the Pilbara region of Western Australia have been used to define an early Paleoproterozoic foreland basin, the McGrath Trough, in front of the northwards-advancing Ophthalmian fold belt. The foreland basin succession commences with the Boolgeeda Iron Formation and includes the overlying Turee Creek and lower Wyloo Groups. The basin geometry is asymmetric, thinning northwards away from the Ophthalmian fold belt, and the trough axis trends east-southeast parallel to the Ophthalmian folds. Multiple intraformational unconformities in the upper Turee Creek Group record the advance of the fold belt into the axis of the McGrath Trough in the Hardey syncline area. East-southeast-trending folds and steep southward-dipping cleavage post-date deposition of the Beasley River Quartzite, a shallow-marine to fluvial sandstone that records an interval of tectonic quiescence during basin evolution. Deposition of the Beasley River Quartzite was followed by renewed uplift of the southern hinterland, resulting in continued deposition of fluvial siliciclastics into the foreland basin, which was eventually filled with the eruption of the Cheela Springs Basalt. The age of the McGrath Trough is constrained broadly between ∼2.45 Ga, the age of the underlying Woongarra Rhyolite, and ∼2.2 Ga, the age of the Cheela Springs Basalt at the top of the succession. The sedimentary fill of the McGrath Trough was derived mainly from the south, and becomes texturally and compositionally more mature upward, reflecting its recycled orogen provenance. Two main sources are identified: a dominant cratonic source interpreted to be similar to the granitoid-greenstone basement of the Pilbara Craton, and a volumetrically lesser source from banded iron formation (BIF), and volcanic and sedimentary rocks of the Mount Bruce Supergroup. Felsic volcanic detritus, likely to have been derived from the underlying Woongarra Rhyolite, is most common in the lower Turee Creek Group and becomes increasingly less common upsection, although it is reworked from underlying formations. Chert and BIF clasts appear first in the upper Turee Creek Group, and are important components of the most proximal alluvial fan to braided stream deposits in the upper Turee Creek and lower Wyloo Groups. Grains of microplaty haematite, identical in appearance to microplaty haematite in the giant Hamersley iron-ore deposits, appear in conglomeratic units from the base of the Beasley River Quartzite upwards. These fluvial sediments were derived from the south and record erosion of ore-grade deposits from the uplifted Ophthalmia fold belt during lower Wyloo Group times.

A pre-2.2 Ga age for giant hematite ores of the Hamersley Province, Australia?

One current model for the origin of giant hematite ores of the Hamersley province requires a two-step process: formation of martite and goethite by supergene enrichment, followed by burial metamorphism to convert goethite to microplaty hematite. Our new structural, stratigraphic, and geoehronological data suggest that at least some microplaty hematite ores were present before metamorphism owing to burial under the lower Wyloo Group. A SHRIMP U-Pb zircon age from volcaniclastic breccia within the lower Wyloo Group suggests that hematite detritus in that group was derived from hematite ores older than 2209 + or - 15 Ma. The action of pre-2.2 Ga oxidizing fluids is implied, although their origin (supergene versus hypogene) remains controversial.

Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia

The Transvaal succession in the northeastern part of the Kaapvaal Province of southern Africa is the erosional remnant of four, 2.7 to 2.1 Ga unconformity-bounded stratigraphic units, or sequences, each of which is 10 2 to 10 3 m thick and spans 10 7 to 10 8 years. Three of these Phanerozoic-like cratonic sequences occur in the western part of the Kaapvaal Province and in the Pilbara Province of Western Australia; thus, the two provinces once were part of the same continent, Vaalbara. The stratigraphically lowest sequence historically assigned to the Transvaal succession is part of the Ventersdorp Supergroup in Kaapvaal and the upper part of the Fortesque Group in Pilbara. It consists predominantly of arkosic sandstone and basaltic andesite; it probably is ⩽ 2687 ± 2 Ma. The second sequence contains the quartz arenites, dolomites, and banded-iron formations of the Transvaal and Griqualand West successions in the Kaapvaal and the Hamersley Group in the Pilbara. The span of ages (2684 ± 6 to 2432 ± 31 Ma), lithologies, and geographic extent of this sequence imply that the Limpopo orogeny, which welded the Zimbabwe Province to Vaalbara, is ⩽ 2.47 Ga (not 2.67 ± 0.05 Ga). The third sequence (the Pretoria and Postmasburg groups in the Kaapvaal and the lower part of the Wyloo Group in the Pilbara probably is the clastic wedge generated by the Limpopo orogeny. The fourth sequence (dominated by the felsites of the Rooiberg Group) is restricted to northeastern Kaapvaal. Sequence stratigraphy, lithostratigraphy, and lithofacies indicate that the Pilbara rifted from the southern edge of western Kaapvaal. The Penge Iron Formation in northeastern Kaapvaal, the Asbesheuwels Iron Formation in southwestern Kaapvaal and the Brockman Iron Formation in the Pilbara are the same formation, in which cyclothemic units 0.6 to 15.0 m thick extended ⩾ 1200 km.

Geochemistry of precambrian carbonates: IV. Early paleoproterozoic (2.25 ± 0.25 ga) seawater

In an effort to narrow down the first- and second-order variations in isotopic composition of the Early Paleoproterozoic (2.25 ± 0.25 Ga) seawater, we report a comprehensive study of mineralogy, chemistry and isotopic composition of the Malmani Dolomite (Transvaal Supergroup, South Africa), Duck Creek Dolomite (Wyloo Group, Australia) and Bruce “Limestone” Member of the Espanola Formation (Huronian Supergroup, Canada). Based on trace element data, their diagenetic rank increases in the order Duck Creek < Bruce < Malmani. The interpolation of alteration trends to “best” values yields an estimate of 0.70550 for 87Sr 86Sr , comparable to our previously published value of 0.70496. For δ 13 C, the measured range of 0 ± 1.5%. PDB is similar to that observed for Phanerozoic marine carbonates. In contrast, the “best” δ 18 O value for dolostones is −5%. PDB, depleted in 18O relative to Phanerozoic counterparts, but comparable to the estimates obtained by similar approach for Archean facies. The isotope geochemistry and mineralogy of Bruce “Limestone” Member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment, possibly related to the recession of the Brace glaciers. If such interpretation proves unacceptable from a sedimentological standpoint, the origin of the unusual isotopic signatures must be sought, despite the above trace element interpretation, in as yet unspecified post-depositional alteration phenomena. In contrast to Phanerozoic carbonates, the Early Paleoproterozoic—as well as the previously described Archean—counterparts are enriched in Mn and Fe.

The origin of olistoliths in proterozoic rocks of the Ashburton trough, Western Australia, using zircon U‐Pb isotopic characteristics

Zircon U‐Pb systems have been used to investigate the origin of olistoliths (large rafted fragments) in the Ashburton Formation of the Proterozoic Upper Wyloo Group of Western Australia. Zircons from felsic volcanics from two olistoliths were analysed. 207Pb/206Pb ages ∼ 1825 Ma, based on the — 105 μm non‐magnetic zircon fractions, are reported as the best estimate of the age of volcanic rocks from the two olistoliths. This result rules out the possibility that the olistoliths were derived from the Woongarra Volcanics (dated at 2439 ± 10 Ma) which occur at the top of the Hamersley Group. However, the zircon U‐Pb systems are essentially identical to those of the June Hill Volcanics (207Pb/206Pb age of 1843 ± 2 Ma), which occur low in the Upper Wyloo Group, and this unit is considered as the most likely parent for the olistoliths.

Inliers of banded iron‐formation in the Proterozoic Wyloo Group sediments near Paraburdoo, Western Australia

Abstract Two inliers with a total outcrop length of 3000 m and a maximum width of 200 m, consisting of a sedimentary klippe (olistolith) and an olistostrome (both composed of banded iron‐formation and shale belonging to the Hamersley Group) occur within the Mininer Turbidite Member of the Wyloo Group, south of Paraburdoo, W.A., 2500 m from the top of the Hamersley Group proper. The olistostrome is a typical debris slide produced by slumping of unconsolidated material. The klippe was rafted into position as a solid block by a turbidity current. The pattern of mineralisation within the banded iron‐formation part of the klippe, which is identified as being from the Brockman Iron Formation, together with evidence from the basal conglomerate of the Wyloo Group, shows that the formation of the Hamersley iron ore deposits commenced prior to the deposition of the Wyloo Group sediments.

Geological history of the early Proterozoic Paraburdoo Hinge Zone, Western Australia

Palinspastic profiles across the Paraburdoo Hinge Zone, which forms the southwestern boundary to the Hamersley Shelf, indicate the existence of a fore-trough, adjacent to the Ashburton Trough, on the edge of the Hamersley Shelf. The fore-trough existed during Turee Creek Group and Lower Wyloo Group times of deposition and is characterized by unconformities or shallow-water facies on its margins. The distribution of mature iron-ores in the Hamersley Group suggests a relationship of this type of ore-body to the fore-trough margins. Continual uplift, or a gradual migration northeastwards of a geanticline, is suggested for the Ashburton Trough region from post-Hamersley Group times to pre-Ashburton Formation times of deposition.

Radiometric ages of some precambrian rocks from the Northwest division of Western Australia

Abstract Rb‐Sr age measurements are reported for acid volcanic rocks, a tuffaceous siltstone, and two granites, all of Precambrian age, from the northwest division of Western Australia. The lava samples are from four localities within the Woongarra Volcanics, a formation situated near the top of the “Proterozoic” Hamersley Group, formerly a part of the “Nullagine Beds”. On field evidence these samples are of the same age, so that the Rb‐Sr data have been interpreted as giving approximately 2,100 m.y. for this age, assuming a variation in initial Sr87Sr86 of 0.700 to 0.720. The tuffaceous siltstone, from the overlying Wyloo Group, has a maximum indicated age of 1,850 m.y. This value is subject to uncertainty since only one sample was analysed, but nevertheless it is significantly younger than the Woongarra Volcanics, in accordance with its strati‐graphic position, and it is likewise consistent with concordant total‐rock and mineral ages of 1,720 m.y. measured in the Boolaloo Granite, which is intrusive into the Wyloo Group. Owing to suspected later metamorphism only broad limits of 2,300 to 3,000 m.y. can be set for the age of an “Archaean” granite which is older than the oldest “Proterozoic” sediments. Two unpublished U‐Pb and Rb‐Sr ages from the same area communicated to the authors are also reported.