Fortescue Group Research Articles

Phreatomagmatic boulder conglomerates at the tip of the ca 2772 Ma Black Range dolerite dyke, Pilbara Craton, Western Australia

Unusual volcanic conglomerates with a mixture of well-rounded granitic boulders (to 1.2 m diameter) derived from adjacent basement rocks, and smaller (1 – 10 cm) subspherical basaltic droplets with chilled margins occupy a linear zone along strike of the northern end of the Late Archaean Black Range dolerite dyke in the Pilbara Craton, Western Australia. The matrix of the volcanic conglomerates becomes more angular with decreasing grainsize and grades to rock flour, a trend opposite to that in sedimentary conglomerates. In other places, the matrix consists of chlorite that cuts through, and resorbs, granitic clasts, indicating an origin as volcanic melt. The volcanic conglomerates have peperitic contacts with immediately adjacent flows of the Mt Roe Basalt of the Fortescue Group. A welded volcanic tuff at the peperitic contact is dated at 2767 ± 3 Ma, within error of the 2772 ± 2 Ma Black Range dolerite dyke and the Mt Roe Basalt (2775 ± 10 Ma), confirming the contemporaneity of formation of these geological elements. Subsequent normal faulting has juxtaposed the higher level conglomerates down into their present exposure level along strike of the Black Range dolerite dyke. The linear zone of volcanic conglomerates is interpreted to represent a phreatomagmatic pebble dyke that formed immediately above, and as a result of intrusion of, the Black Range dolerite dyke. Interaction of magma with groundwater caused phreatomagmatic brecciation of the country rock, in situ milling of granitic boulders, incorporation of basaltic melt droplets, and the formation of a mixed matrix of devitrified volcanic glass and granitic material. This process was accompanied by along-strike epithermal Cu – Hg – Au mineralisation.

SHRIMP U – Pb depositional age for the lower Hardey Formation: evidence for diachronous deposition of the lower Fortescue Group in the southern Pilbara region, Western Australia

A SHRIMP zircon U – Pb age is reported for the Hardey Formation, lower Fortescue Group in the southern Pilbara region of Western Australia. Zircons extracted from an accretionary lapilli-bearing siltstone of the Hardey Formation give a 207Pb/206Pb age of 2750 ± 5 Ma (n = 36), which is regarded as an age of felsic volcanism contemporaneous with deposition of the Hardey Formation. Published zircon ages, primarily on felsic intrusives within the Hardey Formation in the northern Pilbara are statistically significantly different, and older than the age of ca 2750 Ma presented here, suggesting that a diachronous surface existed across the Pilbara Craton during deposition of the Hardey Formation.

SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia

The Mt Bruce Supergroup of Western Australia was laid down between ca 2.8 Ga and ca 2.2 Ga in the Hamersley Basin, unconformably over a basement of the older, granite‐greenstone, component of the Pilbara Craton. The Mt Bruce Supergroup consists of three groups: the Fortescue Group, Hamersley Group and Turee Creek Group in upward sequence. The Hamersley Group, which is divided into eight formations, has a general thickness of ∼2.5 km, and is characterised by major banded iron‐formation (BIF) units. Reported here are SHRIMP U–Pb zircon results (406 grain analyses) from 13 samples taken from the Hamersley Group and near the top of the underlying Fortescue Group. In combination with SHRIMP results previously published from 12 Hamersley Group samples, the present results provide significant new constraints on the depositional chronology of the group, and suggest that the average (compacted) depositional rates of each of the main depositional lithologies (BIF, carbonate, shale) were ∼180 m per million years, 12 m per million years and 5 m per million years, respectively. Some recently published SHRIMP ages from the Joffre Member differ slightly from those that are interpreted from the present data, and it is suggested that the two datasets may be reconciled if non‐zero‐age Pb loss is taken into account. The total body of zircon U–Pb age data from the Fortescue and Hamersley Groups is consistent with a model involving continuous accumulation of basin‐fill for at least 330 million years, from ca 2780 Ma to the top of the Hamersley Group at ca 2449 Ma. The word ‘continuous’ in this context means that there may have been no breaks in deposition longer than 1 million years. However, this model is not proven, and a major challenge for future work is to measure the length of any proposed non‐depositional intervals.

Middle Archaean gold deposits in the North Shaw–Coongan Greenstone Belt, Australia: the relative timing of events responsible for gold mineralisation

Gold deposits of the Mid-Archaean East Pilbara Granitoid–Greenstone Terrane, Australia, are unique because they characterise metallogenic evolution of the Early Earth. In the North Shaw–Coongan Greenstone Belt, quartz–sulphide lodes within shear and fault zones are host to gold mineralisation. Detailed field mapping of crosscutting structures at the Sharks Well Mining Centre and Blue Bar Mine in the Coongan Greenstone Belt, and at the North Shaw Mining Centre in the North Shaw Greenstone Belt, have established a relative chronology for gold deposits. The results suggest that the accepted classification of regional gold mineralisation, in terms of two events, may need modification. The Sharks Well Mining Centre is hosted by meta-basalt to andesite and carbonatised quartz–chlorite schist of the Mt. Ada Basalt (Warrawoona Group, ca. 3.49–3.31 Ga). The gold-bearing quartz–sulphide lodes are principally situated in the Shark Gully Shear Zone. The lodes and veins formed early in D1 (local) and prior to development of cleavage and folds. Gold occurs in NW-trending fractures that cut across the quartz lodes and veins, suggesting syn- to post-D1 gold mineralisation. The maximum relative age of gold mineralisation at the mining centre is constrained by the age of the Duffer Formation, which is stratigraphically younger than the Mt. Ada Basalt and dated at 3471–3463 Ma. The Blue Bar Mine is hosted by quartz–fuchsite schist of the Central Coongan Shear Zone, which trends northerly and dips steeply eastward. The mine consists of a single, brecciated, gold-bearing quartz lode that is unconformably overlain by scree breccia, and in turn, by the Mount Roe Basalt of the Fortescue Group (ca. 2.7 Ga). The maximum relative age of gold mineralisation at the mine is constrained by the age of the Euro Basalt at 3395–3325 Ma, while the minimum relative age is constrained by the Mount Roe Basalt at ca. 2.7 Ga. The North Shaw Mining Centre consists of several gold-bearing quartz lodes (including the Auroria and Eldorado lodes), as well as a number of historic gold workings (alluvial and shaft mining) on the plains adjacent to the Shaw River. The mining centre is located on the contact margin of the North Shaw Suite of the Shaw Granitoid Complex. The host rocks consist of contact metamorphosed basalt–andesite and hornblende amphibolites. Five stages of (local) deformation are recognised in the rocks of the mining centre: D1—macroscopic folding of the North Shaw Greenstone Belt about a northerly trending fold axis; D2—NNE-trending foliation and folds; D3—development of a NE-trending disjunctive cleavage, folds and conjugate kinks; D4—development of faults and master joints during emplacement of dykes of the Black Range Suite; D5—formation of the WNW-trending Callina Fault Zone. The fault zone crosscuts mafic dykes of the Black Range Suite, which places constraints on the minimum relative age of gold mineralisation in the fault zone. Based on crosscutting structural relationships, the relative and absolute chronology of the gold deposits in the North Shaw–Coongan Greenstone Belt indicates they are not all of the same age. The crosscutting structural relationships justify a review of the currently accepted ‘Event 1 or 2’ classification of Huston et al. [Econ. Geol. 97 (2002) 691]. Two new events, one at 3325–2774 Ma, and one younger than 2.7 Ga, also affected the mafic and ultramafic rocks of the Warrawoona Group.

Geochronology of a Late Archaean flood basalt province in the Pilbara Craton, Australia: constraints on basin evolution, volcanic and sedimentary accumulation, and continental drift rates

Eleven high precision (±2–5 million years) SHRIMP zircon U–Pb ages have been obtained from felsic rocks within a single stratigraphic section of late Archaean volcanic and sedimentary rocks in the east Pilbara of Western Australia. The stratigraphic succession (Nullagine and Mount Jope Supersequences in sequence-stratigraphic terminology, Fortescue Group in lithostratigraphic terminology) is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional, rift-related environment between about 2772 and 2715 Ma. The geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy. Field mapping, geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs. While flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession, felsic volcanism is now known to have been active periodically through each geotectonic cycle. The succession covers a time period of about 57 million years. The lower ∼1400 m of a thick (∼1700 m) clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years. In contrast, a younger ∼150 m thick cogenetic tuff-basalt unit accumulated in less than 3 million years, and others probably accumulated at similar rates, comparable to those of Phanerozoic flood basalts. Unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time-gap of more than 10 million years. The unconformity-based stratigraphic framework, the new geochronology and palaeomagnetic studies [J. Geophys. Res. 108 (2003) B12, 2551, EMP 2-1 to 2-21] have been combined to determine a possible late Archaean continental drift rate for one part of the succession, implying a period of motion as fast as or up to five times faster than any known from the Phanerozoic.

The North Pole Dome: a non-diapiric dome in the Archaean Pilbara Craton, Western Australia

The northeastern part of the Archaean Pilbara Craton is characterised by an ovoid dome and basin pattern of domal granites separated by synformal greenstones (supracrustal rocks). This ovoid pattern is variably preserved at various structural and stratigraphic levels, with the most deeply eroded domes being granite dominated, and the least eroded domes being greenstone dominated. The North Pole Dome (NPD) is an example of a relatively high-level dome that has a flanking syncline preserving some of the youngest rocks of the craton (Fortescue Group) to be involved in the doming process. At the apex of the NPD is a small intrusive granite that was considered by some workers to be the top of a large underlying domal granite batholith. The NPD comprises mostly dense basaltic greenstones that range in age from approximately 3500 to 2700 Ma. The approximately 3460 Ma North Pole Monzogranite, a volumetrically insignificant intrusive granite body, intrudes the greenstones in the apex of the dome. No marginal shear zones occur around the intrusion. A new model, based on a detailed gravity traverse across the monzogranite, suggests that the granite intrusion is plug-like, up to 1.5 km thick and does not represent the exposed top of a larger underlying domal batholith. Results from potential field modelling show that the dome is relatively flat bottomed, with a base around 5.5–6.5 km deep. The NPD has no significant granitic material within the dome, but like all greenstones, is underlain by felsic crust (granite) below its base. The development of the NPD (and flanking syncline) was a multistage process. The first stage of doming involved relatively minor doming/tilting, possibly associated with the emplacement of the monzogranite, because palaeocurrents of synchronous volcanic rocks flowed radially outward from the dome. It is likely that this doming was minor as there are no recorded unconformities in the Warrawoona Group (in the NPD) above these volcanic rocks. A major dome-forming event (tilting >20°) occurred in the period between 3240 and 2772 Ma, and was unrelated to the emplacement of the small granite plug (diapirism). Regional folding and refolding from horizontal compression deformed the area into a domal shape (Type I fold interference pattern). Uplift and erosion of the dome was superseded by extension and deposition of flood basalts (Fortescue Group) that flowed towards the dome. Three further stages of shortening folded the regional unconformity and the underlying and overlying units, further amplified the underlying dome, developed the flanking Marble Bar Syncline, as well as fold interference patterns in the Fortescue Group. The NPD was developed over a 800 Ma time frame, ostensibly by a process of fold interference due to multiple stages of horizontal compression. This work shows that diapirism was not the cause of the development of the domal geometry of the NPD, and its flanking syncline, rather folding and refolding due to horizontal compression was the principal controlling factor.

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

Cyclic continental mafic tuff and flood basalt volcanism in the Late Archaean Nullagine and Mount Jope Supersequences in the eastern Pilbara, Western Australia

A late Archaean (ca. 2775–2715 Ma) continental flood basalt and extensional basin succession in the eastern Pilbara of Western Australia has been divided into 11 unconformity-bounded packages of rock, and four mafic dyke suites have been correlated chemically with subaerial flood basalts in the succession. The stratigraphic divisions follow unconformity-based mapping, whole-rock geochemical, palaeomagnetic, volcanological, sedimentological, petrographic and airborne geophysical studies. The 6 km thick lower Chichester Range Megasequence (Fortescue Group minus the Jeerinah Formation) in the Nullagine Synclinorium comprises lower and upper subaerial flood basalt units separated by a thick succession of terrestrial clastic sedimentary and felsic volcanic rocks. A repeated pattern of mafic volcanism is apparent in the upper part of the succession comprising six tuff to basalt cycles, each representing a flood basalt event. Subaerial flood basalt volcanism in each cycle was preceded by mafic hydrovolcanism resulting in a mafic tuff horizon beneath each subaerial flood basalt pile. Mafic tuff horizons were deposited and resedimented in mostly terrestrial settings. Initial pyroclastic volcanism is attributed to interaction between rising mafic magma batches and shallow ground water or surface water. Overlying flood basalt piles represent relatively rapid outpourings of mafic lava that complete the volcanic cycle. Most geological time in this part of the succession is represented either by boundaries between tuff/basalt cycles or by the resedimented tuff horizons. This study supports a rift-related geotectonic model for the succession as a whole, though it is shown to be more complex than previously proposed.

Discovery of a layer of probable impact melt spherules in the Late Archaean Jeerinah Formation, Fortescue Group, Western Australia

Individual layers rich in sand‐sized spherules interpreted as distal ejecta from Late Archaean to Early Palaeoproterozoic impacts have already been reported from three stratigraphic units in the Hamersley Group of Western Australia: the Wittenoom Formation, the Carawine Dolomite, and the Dales Gorge Member of the Brockman Iron Formation. Here we report the occurrence of a similar layer near the top of the Jeerinah Formation, the uppermost unit in the underlying Fortescue Group. This layer, which we informally name the Jeerinah Impact Layer, was deposited ca 2.63 Ga below wave‐base in a marine deep shelf to upper slope environment. As in all other spherule layers, the spherules in the Jeerinah Impact Layer are sand‐sized (up to 0.83 mm across), dominantly spheroidal, and show a mix of crystallisation and devitrification textures internally, indicating they are droplets of former low‐silica silicate melt. The morphologies of some of the crystallites indicate they were originally plagioclase, yet they now consist of K‐feldspar; this, too, is common in all of the other spherule layers. Despite the fact that it is ≤⃒6 mm thick, the Jeerinah Impact Layer pinches and swells laterally and consists of several different subunits internally. This indicates that more than the passive settling of particles in still water was involved in the formation of the Jeerinah Impact Layer, which probably involved traction of coarse sand‐sized grains. Despite its thinness, the Jeerinah Impact Layer still represents a thicker accumulation of spherules than all but the most proximal parts of spherule layers formed by some of the largest Phanerozoic impacts, suggesting the Jeerinah Impact Layer represents a major impact. If the Jeerinah Impact Layer and the other spherule layers in the Hamersley Basin did form by impacts, this also means that ejecta from a minimum of three distinct impacts are preserved with age dates suggesting a recurrence interval similar to the inferred rate of impacts in the Phanerozoic. The discovery of the Jeerinah Impact Layer also reconfirms that basinal successions such as black shale are among the most favourable environments for the preservation of distal layers of impact ejecta. It also raises intriguing questions about stratigraphic correlations in the Hamersley Basin of Western Australia and to another spherule layer discovered recently in the Transvaal Supergroup of South Africa.

Sulfur and carbon isotope analyses of the 2.7Ga Jeerinah Formation, Fortescue Group, Australia.

Sulfur and carbon isotope ratios have been determined on carbonaceous shales of the 2.7 Ga Jeerinah Formation, Hamersley Group, Australia. The analyses were performed using the Nd-YAG laser microprobe method. The δ13C(PDB) values of organic matter range from -38.3‰ to -35.1‰ with an average of -37.0‰ (44 analyses). These carbon isotope compositions support the previously proposed hypothesis that methanogens and methanotrophs were involved in the carbon cycles in the 2.7 Ga Hamersley ocean. Sulfur isotope compositions are ranging from +0.4‰ to +10.2‰ with an average of +4.6‰ (90 analyses). A notable feature is variable δ34S values within a micro-scale area; approximately 6 to 7‰ variations of δ34S values were detected within 5 × 5 mm areas. Such δ34S variations clearly indicate that pyrites in the Jeerinah shales were formed as a result of the sulfate reduction. This suggests that the 2.7 Ga Hamersley ocean contained appreciable amount of dissolved sulfate, opposing to the previously popular H2S-rich ocean model. Because of no evidence for the hydrothermal alterration on the examined samples, pyrites in the 2.7 Ga Jeerinah shales were most likely formed by the biological sulfate reduction in sediments.

Chronological correlations between the Pilbara and Kaapvaal cratons

The early geological development of the Pilbara and Kaapvaal cratons has many features in common. Attempts have been made to correlate geologically similar features of the two cratons, and it has been postulated that they originated as contiguous components of a single continent, ‘Vaalbara’, during this time. The early geological histories of the Pilbara and Kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed. These comparisons indicate significant differences in the chronologies of magmatic events within the granite–greenstone crusts of the Pilbara and Kaapvaal cratons. In addition, igneous correlatives emplaced during ca 2985 and 2782 Ma magmatic events on the Kaapvaal Craton have not been identified on the Pilbara Craton, and a well-defined 2760 Ma magmatic event, manifest as widespread emplacement of granitic rocks into the Pilbara granite–greenstone basement and eruption of flood basalts of the lower part of the Fortescue Group, is absent from the Kaapvaal Craton. Furthermore, similarities in first- and second-order transgression–regression cycles within the sedimentary supracrustal sequences may be attributable to global sea-level fluctuations, and thus may be irrevelant to the question of former contiguity. However, similarities in some aspects of the geological development of the Pilbara and Kaapvaal cratons imply that there were periods, extending for between 60 and 200 Ma, of the Archaean era during which the style of crust formation, intensity of volcanism and subaerial erosion, and magnitude of sea-level fluctuations may have varied on a global scale. Such similarities include the overall duration of formation of the granite–greenstone crusts from ca 3650 to 3100 Ma, the onset of craton-wide erosion in the interval ca 3125 to 3000 Ma, the major episodes of flood basaltic volcanism between 2760 and 2680 Ma, the predominance of chemical (carbonate and banded iron-formation) sedimentation between ca 2630 and 2440 Ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 Ma.

Ion microprobe baddeleyite and zircon ages for Late Archaean mafic dykes of the Pilbara Craton, Western Australia

Ion microprobe U–Th–Pb analyses of baddeleyite and zircon yield precise ages for several mafic intrusions in the Pilbara Craton of Western Australia. Baddeleyite was dated from four dolerite dykes of the north‐northeast‐trending Black Range swarm intruded into granitoid‐greenstone basement in the northern part of the craton. The mean 207Pb*/206Pb* age of 2772 ± 2 Ma, interpreted as an unambiguous age of emplacement for the dykes, is within error of previous ion microprobe U–Pb zircon ages for the Mt Roe flood basalts and confirms that the dykes acted as feeders to the volcanic rocks. The Sylvania Inlier, in the southeastern Pilbara Craton, also contains north‐northeast‐trending dykes that were correlated previously with the Black Range swarm. Based on concordant and discordant zircon analyses from samples of two dykes, the best estimate of the age of the Sylvania dykes is 2747 ± 4 Ma. The Sylvania dykes thus appear to be significantly younger than, and hence unrelated to, the Black Range swarm, but may have acted as feeders to younger volcanic units in the Fortescue Group such as the Kylena Formation.

Precise zircon U‐Pb ages from the Marra Mamba Iron Formation and Wittenoom Formation, Hamersley Group, Western Australia

The Mt Bruce Supergroup of the Pilbara Craton of Western Australia was laid down in the Hamersley Basin, unconformably over a basement of granite‐greenstone terrane, and consists of the Fortescue, Hamersley and Turee Creek Groups. The base of the ∼230 m‐thick Marra Mamba Iron Formation, the lowest formation of the Hamersley Group, marks the onset of the major banded iron‐formation deposition that characterises the group. Ion microprobe U‐Pb isotope analyses of zircons from a tuff band (NS3) within a high‐grade iron orebody in the uppermost (Mt Newman) member show two distinct, and non‐overlapping, age populations. Fifteen grains from the younger group of 16 have a pooled age of 2597±5Ma (95% confidence), interpreted as the age of syndepositional volcanism; individual grains of the older group of 8, with ages between ca 2950 Ma and ca 2820 Ma, are interpreted as xenocrysts derived from basement rocks transected by the rising magma. The conformably overlying Wittenoom Formation, ∼300–600 m thick, consists largely of dolomitic rocks. Zircons from a submarine fallout tuff within the uppermost (Bee Gorge) member also have two non‐overlapping zircon age populations. Most of the 23 zircons analysed belong to an older population with near‐concordant ages between ca 2750 and ca 2650 Ma, but 12 analysed spots from 7 grains have a pooled age of 2561 ±8 Ma. This is interpreted to be the age of volcanism coeval with deposition. The older zircons were probably derived from tuffs of Fortescue Group age, entrained with the Crystal‐rich Tuff ejectamenta during its explosive, probably phreatoplinian, eruption.

Archean Oil: Evidence for Extensive Hydrocarbon Generation and Migration 2.5-3.5 Ga

Archean sedimentary rocks from the Pilbara Craton, Australia, contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons around detrital uraninite, thorite, and monazite grains. The nodules are preserved in sandstones at several stratigraphic levels in the Fortescue Group (~2.75 Ga) and Lalla Rookh Formation (~3.0 Ga), both nonmarine successions, and in deltaic sediments of the Mosquito Creek Formation (~3.25 Ga). The most ancient evidence comes from the Warrawoona Group (>3.46 Ga), where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification. Bituminous relics of petroleum are also commonly preserved in shallow-marine sandstones of the Black Reef Formation (~2.59 Ga) and the Witwatersrand Supergroup (~2.85 Ga) from the Kaapvaal Craton, South Africa, along with subeconomic methane accumulations. In all cases, the petroleum was apparently sourced from Archean shales, generated during the Archean, and migrated before the late Archean or early Early Proterozoic metamorphism occluded fluid pathways. Given this widespread and abundant evidence for hydrocarbon generation and migration in Archean depositional basins, it seems that primordial bacterial biomass, producing labile type I kerogen, was often buried in sufficient quantities to successfully generate and expel petroleum. Depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration, and probably to the accumulation, of petroleum. Thus, the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in Archean basins are the subsequent destructive effects of deformation and metamorphism, which causes trap breaching, imperfect sealing, or thermal obliteration. However, there are ancient stable cratons where such disruption may not have occurred, and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in Archean rocks.

Palaeomagnetism and magnetic anisotropy of Proterozoic banded-iron formations and iron ores of the Hamersley Basin, Western Australia

Rock magnetic properties and palaeomagnetism of weakly metamorphosed banded-iron formations (BIFs) of the Palaeoproterozoic Hamersley Group, Western Australia, and Proterozoic BIF-derived iron ores have been investigated. The BIF units sampled here are slightly younger than 2500 Ma. At Paraburdoo, Mount Tom Price and Mount Newman iron ore formation was completed before 1850 Ma. Sampling was mainly from the Mount Tom Price and Paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh (unweathered and unaltered) BIF samples has allowed the nature of the remanence of the BIFs to be defined. The remanence of the BIFs is carried by late diagenetic/low-grade metamorphic magnetite after primary haematite. This remanence is pre-folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low. Determination of palaeofield directions from measured remanence directions is complicated by self-demagnetization effects in strongly magnetic, highly anisotropic BIF specimens. We present a method for correcting measured directions for the effects of self-demagnetization and anisotropy. For typical BIFs, the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes, but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes. Anisotropy also causes cones of confidence to be underestimated, due to compression of the range of inclinations. In principle, deflection of post-folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn-folding signature, with best agreement between directions from different fold limbs after partial unfolding. Thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors, but can also mislead interpretation of the time of remanence acquisition. The anisotropy of anhysteretic remanent magnetization (ARM) probably yields an upper limit to the anisotropy of the chemical remanent magnetization (CRM) carried by the BIFs. Therefore, from the anisotropy of ARM, a maximum inclination deflection of 9° is suggested for the sampled BIFs. This corresponds to less than 5° change of palaeolatitude. The palaeomagnetic pole position calculated for BIFs at Paraburdoo is 40.9°S, 225.0°E (dp=2.9°, dm=5.8°) after tilt correction, but without correction for anisotropy. Other pole positions reported include that from flat-lying BIFs from Wittenoom at 36.4°S, 218.9°E (dp=4.6°, dm=9.1°), from Mount Tom Price iron ore at 37.4°S, 220.3°E (dp=5.7°, dm=11.3°) and from Paraburdoo ore at 36.4°S, 209.9°E (dp=4.7°, dm=8.8°). The poles from the BIFs, the Paraburdoo ore and the part of the Tom Price deposit that was sampled in this study are indistinguishable from each other and from the Mount Jope Volcanics overprint pole. The magnetization of the BIFs was probably acquired during burial metamorphism of the Hamersley Group, soon before the main folding and uplift event in the southern part of the Hamersley Province. This tectonic event exposed magnetite-rich BIFs to near-surface oxidizing conditions, producing extensive martite-goethite orebodies and also appears to have produced the syn-folding overprint magnetization recorded by the Mount Jope Volcanics of the underlying Fortescue Group. Ages of magnetization are tentatively interpreted as ∼ 2200±100 Ma for the BIFs, ∼ 2000±100 Ma for the supergene enrichment of BIF to martite-goethite ore, recorded by the Parabudoo and Mount Tom Price orebodies, and ∼ 1950±100 Ma for the metamorphic martite-microplaty haematite ore, recorded as an overprint by the Tom Price orebody and as the only surviving magnetization of the Mount Newman orebody.

REE chemistry and Sm-Nd systematics of late Archean weathering profiles in the Fortescue Group, Western Australia.

Two weathering profiles, each consisting of an upper, sericite-rich zone and a lower, chlorite-rich zone, are preserved between flows of the Mt. Roe Basalt in the Fortescue Group, Hamersley Basin, Western Australia. REE concentrations in samples from these two profiles, which originally developed ca 2,760 Ma, show large variations depending on stratigraphic position. LREE abundances and (La/Yb)N are greatest at depths of 3-6 m below the paleosurface of the Mt. Roe #1 profile and are somewhat lower in samples above this level. The LREEs reach concentrations 6-9 times greater than in the underlying basalt, and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part. LREE concentrations in the #2 profile show a similar distribution but with a sharp increase in all REE concentrations within 50 cm of the paleosurface. The distinction between the REE profiles in the two paleosols may be related to the difference in the overlying material. The #1 paleosol is overlain by a few meters of sediments and then by basalt, whereas the #2 paleosol is directly overlain by basalt. The LREEs appear to have been mobilized both during chemical weathering of the parental basalt and during later lower-greenschist-facies metamorphism and metasomatism of the paleosols. Remobilization of the REEs during the regional metamorphism of the Fortescue Group is confirmed by a whole-rock Sm-Nd reference isochron of Mt. Roe #1 samples with an age of 2,151 +/- 360 Ma. Variable initial 143Nd/144Nd values of unweathered basalt samples which may represent the paleosol protolith prevents a confident determination of the magnitude of LREE mobility. Both the initial mobilization of the REEs during weathering and the metasomatic remobilization appear to have taken place under redox conditions where Ce was present dominantly as Ce3+, because Ce anomalies are not developed within the sericite zone samples regardless of concentration. Europium anomalies in the paleoweathering profile are somewhat variable and were probably modified by mobilization of Eu2+ at metamorphic conditions. In all samples, the HREEs appear to have been relatively immobile and correlate with Al, Ti, Cr, V, Zr, and Nb. Sm-Nd systematics and REE patterns of four unweathered basalt samples indicate derivation of the Mt. Roe Basalts from a heterogeneous and enriched source having epsilon Nd between -4.0 and -7.4. Initial 143Nd/144Nd values of these basalts are even lower than those reported by NELSON et al. (1992) for Fortescue Group basalts and indicate a substantial crustal component in the generation of Mt. Roe Basalts.

Geology and major and trace element chemistry of late Archean weathering profiles in the Fortescue Group, Western Australia: implications for atmospheric PO2

Abstract We have identified two thick weathering profiles between flows of the Mt. Roe Basalt unit of the Fortescue Group, in the Pilbara area of Western Australia. The age of weathering is constrained by UPb zircon dating to lie between 2765 and 2715 Ma. The profiles, herein referred to as the Mt. Roe #1 and #2 paleosols, were mapped in detail for 1500 m and 760 m along strike, respectively. Samples were collected along traverses across both profiles where they were well-developed and well-exposed. The profiles, which may be structurally offset parts of the same weathering horizon, have well-developed upper sericite zones (1–15 m thick); these grade downward into chlorite-rich zones which are chemically and physically heterogeneous. The chlorite zones grade downward into unweathered basalts. The Mt. Roe #1 profile is overlain by 2–6 m of bedded, probably lacustrine sediments; these in turn are covered by succeeding lava flows. Evidence of a surficial origin for the paleosols includes mud cracks, ripple marks, and rip-ups of the paleosol material in the post-paleosol sediments. Soft-sediment deformation and probable dewatering structures are present along the paleosol-sediment contact. The Mt. Roe #2 profile is overlain directly by the succeeding lava flow with little or no intervening sediments. Some brecciation and pillowing is observed along the base of the overlying basalt flow in both cases, probably due to emplacement of lava into a shallow lake or onto wet soil. In the sericite zones of both profiles nearly all of the Fe, Mn, Mg and Zn and a substantial fraction of the Ca, Pb, Na, Ni and Si have been leached. The chlorite zones have elevated Fe, Pb, Ni and Zn contents and appear to be repositories of elements lost from the sericite zone. Al, Ti, Zr and Th were immobile in the Mt. Roe # 1 paleosol, and are concentrated by factors of 2.5–2.6 over the unweathered basalt. V, U and Cr were also nearly immobile. Major element and trace element data indicate that weathering took place under conditions of low atmospheric oxygen. K is strongly enriched toward the upper contact of the sericite zone, and was introduced during post-weathering metasomatism together with Rb and Sr. Na and Ca are also enriched near the top of each profile, and small amounts of each may have been introduced during regional, low-grade metamorphism of the Hamersley Basin at 2168 ± 10 Ma.

Rare earth element and neodymium isotopic compositions of the banded iron-formations and associated shales from Hamersley, western Australia

Major, trace element, and Nd isotopic compositions are reported for the early Proterozoic iron-formations from the Hamersley Basin, western Australia. These data enable new constraints to be placed on the relative roles of continental weathering and submarine hydrothermal systems in the REE budget of the Hamersley banded-iron formations (IF). The lower part of the Hamersley Group (Jeerinah shales/cherts, Marra Mamba IF, Wittenoom Dolomite and McRae Shale units) have an average ϵ Nd = −0.6 ± 0.9 at 2.6 Ga. This uniformity suggests that Nd was mostly supplied by erosion products coming from the nearby upper Fortescue Group and also that these sediments were deposited in a restricted basin. The overlying Dales Gorge and Joffre banded iron-formations at Wittenoom record a major metamorphic event at 2.14 ± 0.03 Ga. Initial ϵ Nd values are estimated to be +1.0 ± 0.5 at the time of deposition (2.5 Ga), after correction for metamorphic effects. This more positive ϵ Nd value is consistent with the increased influence of submarine hydrothermal sources, probably in response to exchange with the open ocean. The Hamersley IF are therefore viewed as shelf deposits in a newly formed ocean, possibly of an evolving rift. The shaley bands of the Dales Gorge-Joffre formations containing around 30–60% detrital material give ϵ Nd = −0.9 ± 0.6. An ϵ Nd ≈ −2 is therefore inferred for river waters/aeolian sources in the Hamersley basin. The Nd mass balance obtained from the Dales Gorge-Joffre IF indicates a proportion of Nd derived from hydrothermal sources associated with mid-ocean ridge volcanism of 50 ± 10%.

A comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons

Geochemical and SmNd isotopic data are reported for igneous rocks of the 2.76 to 2.69 Ga Fortescue Group of the Pilbara Craton, Western Australia, and the ∼ 2.70 Ga Ventersdorp Supergroup of the Kaapvaal Craton, southern Africa. The metamorphic history of the Fortescue Group has also been investigated using whole-rock RbSr and PbPb isochron techniques. Igneous of both sequences have mixed tholeiitic and calc-alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous, LREE-enriched sources. Both sequences have similar negative ϵ Nd values (between −1.5 and −4.4 for the Fortescue Group and 0 and −3.4 for the Ventersdorp Supergroup) which are not correlated with Mg/(Mg + Fe 2+). The negative ϵ Nd values of the Fortescue Group rocks probably do not result from crustal assimilation within crustal-level magma chambers during or following differentiation as the Fortescue Group felsic differentiates have ϵ Nd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the Fortescue Group. Instead, these results are consistent with either extensive crustal contamination of primitive (komatiitic or picritic) Fortescue Group parent magmas prior to their differentiation or derivation from negative ϵ Nd (or enriched) mantle sources. The presence of zircon xenocrysts in some Ventersdorp samples provides compelling evidence of some crustal contamination, although correlations between immobile element ratios (such as La/Yb, Ti/Zr, Ti/Sc and V/Zr) and ϵ Nd indicate derivation from chemically heterogeneous, enriched mantle sources. A whole-rock SmNd isochron date of 3308 ± 138 Ma, ∼ 600 Ma older than the time of eruption indicated by UPb zircon data and within, ∼ 300 Ma of the formation of the Kaapvaal granite-greenstone terrane, may date an SmNd fractionation event within the subcontinental lithospheric mantle sources of the Ventersdorp rocks. RbSr and PbPb isochrons obtained for several flows from the Fortescue Group register hydrothermal events occurring between ∼ 2.45 and 2.0 Ga and probably associated with burial metamorphism. Similarities in the field geology, geochemistry and isotopic characteristics of the Fortescue and Ventersdorp sequences and Phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin, possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes.

The age of the Fortescue Group, Hamersley Basin, Western Australia, from ion microprobe zircon U‐Pb results

U‐Pb zircon ages are reported for nine samples from the volcano‐sedimentary Fortescue Group, the lowest of the three groups (Fortescue, Hamersley, Turee Creek) in the Hamersley Basin, and for a 10th sample from the Munni Munni Complex, which immediately underlies the basal unconformity of the Fortescue Group. Three widely separated felsic lava or tuff samples close to the base of the group give ages of 2775 ± 10, 2764 ± 8 and 2763 ± 13 Ma (all errors 2σ). Two other stratigraphically higher samples give ages of 2760 ± 10 and 2756 ± 8 Ma. The latter, from the Bamboo Creek Porphyry, is close to Pidgeon's (1984) published result of 2768 ± 16 Ma for the related Spinaway Porphyry. The mean of all six results, 2765 Ma, is the best estimate of the age of an abrupt initiating volcanic event in the Fortescue Group; the (short) duration of this bimodal volcanic event is not precisely defined by these results, although an age of 2715 ± 6 Ma for the youngest zircon population of the volcaniclastic Pillingini Tuff sets an upper stratigraphic limit. The age of 2925 ±16 Ma obtained from magnetite ferrogabbro pegmatite of the Munni Munni Complex is consistent with a major non‐depositional interval immediately preceding Fortescue Group deposition at ∼ 2765 Ma in the western part of the Pilbara Craton. Two widely separated samples of contemporaneous volcaniclastic rocks from the Jeerinah Formation, the uppermost stratigraphic unit of the Fortescue Group, have ages of 2690 ± 16 and 2684 ± 6 Ma; the mean of these, 2687 Ma, provides a good estimate of the age of the top of the group. From these results, the integrated depositional rate for the whole of the Fortescue Group lies in the range 40–120 m/Ma; during the initial 2765 Ma volcanic event local depositional rates may have been as high as 1000 m/Ma. By contrast, Compston et al.’s (1981) zircon age of 2490 ± 20 Ma from the Dales Gorge Member of the Hamersley Group, some 650 m stratigraphically higher than the Jeerinah Formation, suggests that the depositional rate of the mainly chemical sediments of the Hamersley Group was between 3 and 4 m/Ma. These results collectively confirm that much of the Fortescue Group is older than the greenstones of the southern part of the Norseman‐Wiluna Belt of the Yilgarn Craton, and demonstrate that there is no unique time within the Australian Precambrian marking the transition from granite‐greenstone terrane to overlying platform cover.