North Pole Dome Research Articles

Snapshot of a Paleoarchean seafloor: Evidence from 3.43-3.35 Ga Strelley Pool chert-pebble conglomerate for deposition, silicification, and erosion of hydrothermal greenalite-apatite precipitates

Some of the oldest, well-preserved exhalative sedimentary rocks (ferruginous cherts and jaspilites) occur in volcanic-dominated Paleoarchean sequences in the northern Pilbara Craton, Australia. Jaspilites contain fine-grained hematite particles that have been interpreted to have formed following seawater oxidation of vent-derived Fe2+(aq) by photoautotrophs. However, recent studies suggest that most of the iron in the jaspilites was originally deposited as Fe(II)-rich silicates such as greenalite, sparking renewed debate about how iron was precipitated in the early oceans. Here we show that rounded clasts of dusty ferruginous chert in basal conglomerates of the 3.43–3.35 Ga Strelley Pool Formation, North Pole Dome, Pilbara Craton, Australia, contain abundant nanoparticles of greenalite and apatite that are texturally, mineralogically and chemically near-identical to occurrences in Neoarchean banded iron formations in the Hamersley Group, Australia. Hematite where present occurs in trace amounts and its distribution along the edges of clasts implies that at least some of the hematite formed during weathering. The greenalite-rich clasts display a pronounced positive shale-normalized Eu anomaly, small positive Y anomaly and lack a positive La anomaly, features typical of Paleoarchean exhalites. The co-occurrence of greenalite- and apatite-rich chert-pebbles with clasts of hydrothermal black chert and silicified felsic volcaniclastic sedimentary rocks points to a volcanically active provenance with vigorous hydrothermal activity, consistent with derivation from the underlying 3.43 Ga Panorama Formation. We argue that the greenalite and apatite nanoparticles precipitated during mixing between hydrothermal vent fluids and anoxic seawater and were silicified on the seafloor. The round to oval shape of the chert clasts indicates that the silica cement had recrystallized prior to erosion, consistent with the presence of polygonal shrinkage structures in the dusty chert clasts. Our findings imply that greenalite, contrary to recent suggestions, not only formed in Paleoarchean seawater, but was also stable during deposition, diagenesis, and resedimentation. The precipitation and non-dissolution of apatite nanoparticles in Paleoarchean seawater points to elevated phosphate concentrations, whereas the absence of primary hematite is consistent with models advocating a role for abiotic iron deposition during the Archean.

Subsurface distribution of granites and greenstones in the Paleoarchean East Pilbara Terrane from 3-D gravity inversion

The Paleoarchean East Pilbara Terrane contains some of the oldest rocks on Earth, and has a distinctive surface architecture of granite domes surrounded by arcuate greenstone belts. Explanations for creation of the domes include fold interference, core complex formation and the subsidence of a dense, mafic crust between relatively buoyant granitic complexes. In the latter “sagduction” process, greenstones are often viewed as descending to the base of the crust. Here we present a 3-D inversion of Bouguer gravity data for density contrast constrained by shear wave velocities and a seismic reflection profile. We show that the East Pilbara Terrane is predominantly characterized by a two-layer crust with high-density greenstone rocks largely confined to the upper crust, which is inconsistent with vertical tectonic models requiring large-scale greenstone preservation in the lower crust. The eastern margin of the terrane is marked by a west-to-east transition from low-density ovoid bodies to linear northwest-trending sources associated with the Proterozoic Paterson Orogen. We identify the previously unknown 10–15 km wide Tumbinna Pool Dome, between the Mount Edgar and Yilgalong domes. Higher density sources related to the surface distribution of greenstone rocks largely occur above 10–15 km depth, though they may locally reach >15 km depth on the north side of the Mount Edgar Dome. Some granitic domes have higher density cores related to distinct granite suites, while the Split Rock Supersuite appears to exhibit systematically lower density at depths of 5–10 km. Low-density sources due to small intrusions such as the North Pole Dome only extend to depths up to 10 km, while the large Mount Edgar Dome can be identified at depths as great as 15–20 km. Some low-density bodies in the lower crust, one of which underlies the Yilgalong Dome, correlate with regions of reduced seismic reflectivity, which we interpret to be possible relict migmatisation zones or melt pathways. A weakly constrained ESE-trending high-density region immediately below the Moho correlates with isolated sub-Moho reflectors, and may be a relict zone of rifting and delamination.

A Reconstructed Subaerial Hot Spring Field in the ∼3.5 Billion-Year-Old Dresser Formation, North Pole Dome, Pilbara Craton, Western Australia.

Recent discoveries of geyserite and siliceous sinter with textural biosignatures in the ∼3.5 Ga Dresser Formation of the Pilbara Craton, Western Australia, extended the record of inhabited subaerial hot springs on Earth by ∼3 billion years, back to the time when siliceous sinter deposits are known to have formed on Mars (e.g., at Columbia Hills, Gusev Crater). Here, we present more detailed lithostratigraphic, petrographic and geochemical data collected from 100 measured sections across a ∼14 km strike length in the Dresser Formation. The data indicate deposition of a wide range of hot spring and associated deposits in a restricted interval that directly overlies a hydrothermally influenced volcanic caldera lake facies, with shoreline stromatolites. Hot spring deposits show abrupt lateral facies changes and include associated channelized clastic deposits that support fluvial, subaerial hot spring deposition. All Dresser hot spring and associated lithofacies have direct analogs with proximal, middle, and distal apron hot spring facies that are characteristic of those from New Zealand, Yellowstone National Park, USA, and Argentina. Rare earth element and yttrium geochemistry shows that the Dresser geyserite shares identical patterns with Phanerozoic hot spring sinters. This geochemical data further supports textural and contextual evidence that indicate the Dresser geyserite formed as a subaerial hot spring sinter. Further, the Dresser hot spring deposits are temporally associated with a diverse suite of textural biosignatures that indicate a thriving microbial community existed within in a Paleoarchean hot spring field. The results presented here underscore the importance of continued study of the early geological record for astrobiological research. In particular these findings reinforce the long-standing hypothesis that hydrothermal systems are optimal places to search for past life on Mars.

U–Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton

AbstractSupracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (204Pb/206Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (204Pb/206Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a 207Pb/206Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.

Multiple sulfur isotope constraints on microbial sulfate reduction below an Archean seafloor hydrothermal system.

Microbial sulfate reduction is among the most ubiquitous metabolic processes on earth. The oldest evidence of microbial sulfate reduction appears in the ca. 3.5Ga Dresser Formation in the North Pole area of Pilbara Craton in Western Australia. That evidence was found through analysis of quadruple sulfur isotopes of sulfate and sulfide minerals deposited on the seafloor. However, the activity of microbial sulfate reduction below the Archean seafloor remains poorly understood. Here, we report the quadruple sulfur isotopic compositions of sulfide minerals within hydrothermally altered seafloor basalt and less altered basaltic komatiite collected from the North Pole Dome area. The Δ33 S values of the sulfide minerals were nonzero negative, suggesting that sulfate reduction occurred below the Archean seafloor. To constrain the substrate sulfate sources and sulfate reduction processes, we constructed a numerical model. Comparing the modeled and observed sulfur isotopes, we show that the substrate sulfate comprises seawater sulfate with a negative Δ33 S anomaly and 34 S-enriched sulfate with no anomalous Δ33 S. The latter component probably represents sulfate produced by local hydrothermal processes. The maximum sulfur isotopic fractionation between the putative substrate sulfate and the observed sulfide minerals within the altered basalt and basaltic komatiite is 35‰, which is consistent with a microbial origin. Alternatively, thermochemical sulfate reduction may also produce sulfide. However, considering the hydrothermal temperature inferred from the metamorphic grade of the altered basalt, the sulfur isotopic fractionation produced by inorganic sulfate reduction is probably below 20‰. Collectively, larger fractionations imply the involvement of biological sulfate reduction processes, both in the hydrothermal system below the seafloor and in less altered subsurface settings.

Microstructure-specific carbon isotopic signatures of organic matter from ∼3.5 Ga cherts of the Pilbara Craton support a biologic origin

The ∼3.5 Ga Dresser Formation from the North Pole Dome of the Pilbara Craton (Western Australia) contains some of the oldest evidence for life on Earth. Here, we present a detailed study of microstructure-specific carbon isotopic composition of organic matter (OM) preserved in Dresser Formation bedded cherts and hydrothermal chert vein using in situ Secondary-Ion Mass Spectrometry (SIMS). The OM in these rocks occurs mainly as clots that, together with minor fine OM layers and laminae, are considered primary textures formed prior to host rock lithification. Other than rare OM-rich stylolites, no evidence was found for later OM migration beyond the micrometer scale. Average δ13C(OM) values in specific microstructural types range between −33.6‰ and −25.7‰. No correlation is seen between measured δ13C values and H/C ratios in the studied OM microstructures. This lack of correlation and the low metamorphic grade of the rocks studied argue against significant modification of OM isotopic composition by later metamorphic alteration. It is thus concluded that the range of δ13C values found in the samples represents primary OM isotopic variability. Within some individual samples variable δ13C(OM) values are correlated with specific microstructural types. This observation is not consistent with solely abiotic OM formation via Fisher-Tropsch type reactions. When compared with associated δ13C(ankerite) values, average δ13C(OM) values indicate C isotopic fractionation [Δ13C(Ank–OM)] of 25–33‰, which translates to dissolved CO2–OM isotopic fractionation [Δ13C(CO2–OM)] of 20–30‰. This range of Δ13C(CO2–OM) is consistent with enzymatic C fixation via the Calvin cycle utilized by photoautotrophs and the reductive acetyl-CoA pathway utilized by chemolithoautotrophs. Photosynthetic OM formation is supported by the relatively shallow water depth inferred for the Dresser environment and the restricted occurrence of stromatolites to shallow water deposits in this unit, whereas chemolithosynthesis is supported by the abundance of OM in sub-seafloor hydrothermal chert veins. The range of δ13C(OM) values observed in the samples may therefore represent the remains of different organisms utilizing different C-fixation pathways. Other biologic effects, such as the growth rate and density of microbial communities, and further heterotrophic overprinting of the autotrophic biomass may have also contributed to the observed range of δ13C(OM) values.

Relative retention of trace element and oxygen isotope ratios in zircon from Archean rhyolite, Panorama Formation, North Pole Dome, Pilbara Craton, Western Australia

In-situ analyses for oxygen isotope ratio and trace element concentration by ion microprobe of 3.3–3.7Ga zircons from rhyolite of Panorama Formation in the North Pole Dome, Pilbara Craton document intracrystalline patterns of variability, show compositional correlation, and provide metrics to assess alteration.Values of δ18O in the North Pole Dome (NPD) zircons average 5.78 ‰ with a tight range from 5.3 to 6.4 ‰ VSMOW. These values are similar to magmatic zircons throughout the Archean worldwide and there is no correlation of δ18O, UPb age, or trace elements. SEM/CL examination shows no evidence of mineral inclusions in the SIMS analysis pits. In-situ analysis of Ca concentration is useful to assess the degree of alteration in zircon and retention of trace element compositions. Most zircon analyses (74 %) have elevated [Ca]≥120ppm, with values up to 1614ppm suggesting alteration, probably facilitated by radiation damage. Many high-Ca zircons show enrichment in middle–heavy REE (M–HREE), a peak near Dy, and concave-down trends in chondrite-normalized diagrams. The calculated Dy anomalies (Dy/Dy*) correlate with [Ca]. High Ca also correlates with P, Ti, Fe, Y and REE, and with cumulative (unannealed) α-doses from radiactive decay of Th and U (3.1 to 20×1015events/mg). If not annealed, this α-dose could have caused radiation damage that exceeded the first percolation point. Radiation damage can also facilitate alteration of δ18O in zircon, but examples from other terranes show that alteration is heterogeneous and correlates to greater variability of δ18O within and among zircons. The tight range of oxygen isotope ratios in the NPD rhyolite zircons and their similarity to other Archean zircons, world-wide, suggests that the analyzed NPD zircons have retained original, magmatic oxygen isotope ratios.

Early Archean Hot Springs above Epithermal Veins, North Pole, Western Australia: New Insights from Fluid Inclusion Microanalysis

The world’s most ancient biogenic structures are found in the North Pole Dome of Western Australia, where 3.47-Gyr-old algal mats and stromatolites are closely associated with bedding-conformable and discordant laminar quartz, chalcedony, and barite. Barite-rich quartz hydrothermal veins with similar mineralogy occur throughout the stratigraphy below the conformable biogenic structures. With the exception of the large volume of barite, these bedding-conformable and discordant laminar quartz veins exhibit textures and associated hydrothermal alteration (quartz-chalcedony-chlorite-illite ± calcite-adularia-pyrite) typical of epithermal deposits formed from near-neutral pH fluids. We characterize the physical and chemical conditions of the ancient water responsible for depositing both the discordant and conformable quartz-chalcedony-barite as it passed through the upper parts of the Archean crust. Field relationships, combined with new fluid inclusion data, suggest that the best documented stromatolites in the North Pole Dome occur adjacent to quartz-chalcedony bands formed from cool (120°C), low-salinity (<3 wt % NaCl equiv) waters. Higher temperature (up to 300°C), more saline (up to 10 wt % NaCl equiv) and CO2-H2S-rich (±CH4) aqueous fluids occur in deeper level veins. Rare inclusions that are unusually rich in CO2 (containing liquid and gaseous CO2 and liquid H2O) support the existence of multiple batches of hydrothermal fluids (with variable densities and gas contents). Oxygen isotope data (8.7-3.7) suggest that the causative fluids comprised admixtures of deeply circulated surface water with variable input of magmatic components. Our findings reveal that the earliest life known on Earth lived in and around a hydrothermal system with temperatures from ~300°C at depth to 120°C near the paleosurface, in an environment closely analogous to modern hot springs, developed above epithermal veins. Evidence exists for the introduction of different batches of hydrothermal fluids (with variable densities and gas contents) during the development of veins. These findings support previous studies that demonstrate that the processes that form epithermal deposits have been active throughout geologic time, and the present-day distribution of epithermal deposits is dominantly a result of preservation, not process

Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia: Origin of 15N-depleted nitrogen

Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for 15N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu * anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ 13C vs. δ 15N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in 15N and 13C from a pristine source having δ 13C ⩽ −36‰ and δ 15N ⩽ −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH 4 + (rock) and N 2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large 13C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ 15N value (+11‰), while C (up to 120 ppm and δ 13C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The 15N- 13C-depleted pristine source has δ 15N values from −7‰ to −4‰ and 40Ar/ 36Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and 40Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated.

U-Pb Zircon Geochronology Using LA-ICP-MS in the North Pole Dome, Pilbara Craton, Western Australia: A New Tectonic Growth Model for the Archean Chert/Greenstone Succession

In situ, spot U-Pb ages of zircons from rhyolite, tuffaceous chert, spherule-bearing chert, and granite in the North Pole Dome obtained by laser ablation ICP-MS (LA-ICP-MS) combined with cathode luminescence imaging (CLI) reveal zircon growth patterns that reflect their igneous origins. Felsic lava forming the structural top of the succession, previously dated as 3458 + 9.1/-4.2 Ma by TIMS, has been dated as 3660 ± 52 Ma. These 3.6 Ga zircons from felsic lava, initially considered to be xenocrysts, are interpreted as euhedral igneous crystals as their CL images show no overgrowth rims and/or evidence of melting. Progressive downward in the section, zircons from tuffaceous cherts yield ages of 3463 ± 34 Ma and 3454 ± 46 Ma. This Archean volcaniclastic sequence was intruded by the North Pole Monzogranite at 3391 ± 33 Ma. Our reconstructed stratigraphy shows that the chert-greenstone secession, previously thought to be a single intact stratigraphic unit, actually consists of five units; most are typified by a base of mafic-ultramafic lavas and a capping chert sequence. The zircon ages demonstrate that these five units decrease in age progressively downward; thus the North Pole Dome chert/greenstone succession was formed by layer-parallel thrusting and horizontal shortening, and grew downward by imbricate stacking.

Hydrothermal alteration at the Panorama Formation, North Pole Dome, Pilbara Craton, Western Australia

An airborne hyperspectral remote sensing dataset was obtained of the North Pole Dome region of the Pilbara Craton in October 2002. It has been analyzed for indications of hydrothermal minerals. Here we report on the identification and mapping of hydrothermal minerals in the 3.459 Ga Panorama Formation and surrounding strata. The spatial distribution of a pattern of subvertical pyrophyllite rich veins connected to a pyrophyllite rich palaeohorizontal layer is interpreted to represent the base of an acid-sulfate epithermal system that is unconformably overlain by the stromatolitic 3.42 Ga Strelley Pool Chert.

Volcanic degassing, hydrothermal circulation and the flourishing of early life on Earth: A review of the evidence from c. 3490-3240 Ma rocks of the Pilbara Supergroup, Pilbara Craton, Western Australia

New data gathered during mapping of c. 3490–3240 Ma rocks of the Pilbara Supergroup in the Pilbara Craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism. For most cherts, fossil hydrothermal fluid pathways are preserved as silica ± barite ± Fe-bearing veins that cut through the footwall and up to the level of individual bedded chert units, but not above, indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events. Silica ± barite ± Fe-bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units, and occurred under alternating high-sulphidation and low-sulphidation conditions. These veins provided pathways to the surface for elements leached from the footwall (e.g., Si, Ba, Fe) and volcanogenic emissions from underlying felsic magma chambers (e.g., CO 2, H 2S/HS −, SO 2). Stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of Warrawoona Group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and/or degassing of these magmas. Heat from these intrusions drove episodes of hydrothermal circulation. Listric normal faulting during caldera collapse produced basins with restricted circulation of seawater. Eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components, thereby providing habitats suitable for early life forms. Fossil stromatolites from two distinct stratigraphic units in the North Pole Dome grew in shallow water conditions, but in two very different geological settings with different morphologies. Stratiform and domical stromatolites in the stratigraphically lower, c. 3490 Ma, Dresser Formation of the Warrawoona Group are intimately associated with barite and chert precipitates from hydrothermal vents, suggesting that component microbes may have been chemoautotrophic hyperthermophiles. Evidence of shallow water to periodically exposed conditions, active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water, tectonically active caldera and concentrated therein to produce an extreme habitat for early life. Widespread conical and pseudocolumnar stromatolites in the c. 3400 Ma, Strelley Pool Chert at the base of the unconformably overlying Kelly Group occur in shallow marine platform carbonates. Silicification was the result of later hydrothermal circulation driven by heat from the overlying, newly erupted Euro Basalt. The markedly different morphology and geological setting of these only slightly younger stromatolites, compared with the Dresser Formation, suggests a diversity of microbial life on early Earth. The biogenicity of putative microfossils from this and younger hydrothermal silica veins in the Warrawoona Group remains controversial and requires further detailed study.

Goldschmidt Conference Abstracts 2005

The c. 3.49 Ga Dresser Formation, a lower member of the Warrawoona Group in the North Pole Dome region, Western Australia, is a barite/black chert dominated, mafic volcanic hosted, shallow water (l100m), seafloor exhalative hydrothermal deposit. Carbonaceous matter (CM) aggregates, varying in size from 1mm to g20mm, have been isolated from black cherts in drill core from the Dresser Mine area. With total organic carbon (TOC) contents from 0.19% to 0.31%, organic petrology and SEM-EDS studies of whole rock samples reveals the CM as aggregated forms around siliceous nuclei in the majority of samples. Transmission electron microscopy (TEM) of carbonaceous concentrates show this aggregate material to be composed of sub-micron sized granular-textured particulates. The observed morphology is suggestive of the remains of microbial clusters or colonies. TEM investigation of one sample, however, showed it to be dominated by soot particles such as have been reported from the Sudbury impact and K-T boundary. The presence of soot particles supports previous evidence of meteoritic impact in the Pilbara during the early Archaean. Reflectance measurements (%Ro) of the CM have yielded three dominant groupings: 1) 0.5-1.5; 2) 2.4-2.8; and 3) 3.2-3.8. Different Ro populations in a single sample record several hydrothermal events, as well as relative proximity to a venting source. These results, combined with the bulk C-isotope values of the CM (d13C -32.1 to -38.2p PDB, n=11), are suggestive of a biogenic source, and as such, may represent remnants of some of the earliest primitive life forms on Earth.

Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia

The concentration of halogens (Cl, Br) and sulfate in seawater during the Archaean eon have important implications for the evolution of Earth's hydrosphere and atmosphere and the development of early life. Insights into the composition of Archaean seawater and hydrothermal fluids can be obtained by direct analysis of fluid inclusions preserved in Archaean sediments and hydrothermal systems. Here, we investigated a suite of well-preserved intrapillow quartz–carbonate pods that formed during oceanic hydrothermal alteration of the 3.49 Ga Dresser Formation, North Pole Dome, Western Australia. Texturally, the pods seems to contain a unique population of primary fluid inclusions which were analyzed individually using microthermometry and synchrotron radiation X-ray microfluorescence (μ-SR-XRF) techniques. Bulk chemical analyses were also performed using crush-leach method. Microthermometric data combined with crush-leach and μ-SR-XRF analyses yielded a model composition of 1100 mM Na, 2250 mM Cl., and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt.% salt equivalent. This high Cl concentration (ca. four-times present-day value) reflects a typical modern-day seawater evaporation trend in a shallow marine, closed basin environment. Individual fluid inclusion analysis using μ-SR-XRF revealed the presence of three main fluid populations: a metal-depleted fluid, a Ba-rich and S-depleted fluid, and a Fe–S-rich end-member. The Cl/Br ratio of metal-depleted fluid inclusions (630) is similar to the modern seawater value (649). By contrast, Ba- and Fe-rich brines have Cl/Br ratios (350 and 390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. The metal-depleted fluid displays low sulfate concentration (0–8 mM compared to 28 mM in present-day ocean). Sulfur content of the Fe-rich fluids ranges between 41 and 82 mM. Fluid–rock interaction processes occurring in pillow basalts located on the seafloor are unlikely to be the cause of Cl/Br fractionation between hydrothermal fluids and the “North Pole seawater” end-member. We hypothesize that high biological activities associated with the specific geological settings of North Pole (small isolated basin exposed to relatively intense terrigenous sedimentation) are responsible for the buffering of North Pole seawater Cl/Br value. Although the exact effect of water–rock interaction, barite precipitation, and biological activity on the sulfate concentration of analyzed fluids is difficult to asses, the low sulfate concentration recorded for North Pole seawater argues in favor of a poorly oxygenated Archaean environment.

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.

Archean ocean-floor metamorphism in the North Pole area, Pilbara Craton, Western Australia

Abstract Archean greenstones (ca. 3.49–3.46 Ga) in the North Pole Dome, Pilbara Craton, Western Australia, have undergone prograde metamorphism from prehnite-pumpellyite facies to greenschist-amphibolite facies. Observations of ∼3000 thin sections shows that a metamorphic aureole was restricted to within a few kilometers of the contact with the 3459 Ma North Pole Monzogranite. Three types of alteration were observed; carbonate, silicic and chlorite types. The distribution of the three types of alteration suggests that secondary chemical migration of elements occurred in the whole area, but that some samples were unaffected by this alteration. The southern North Pole area is outside of the contact metamorphic aureole, and is composed of five units from north (unit I) to south (unit V), dipping south about 30–70°. Metamorphism of the North Pole greenstones corresponds to the low-pressure type, which is broadly comparable to that inferred for ocean-floor metamorphism. Prehnite-pumpellyite (PP), transitional (TR), and greenschist (GS) facies prograde metamorphic zones occur from structural top to bottom within each of units. Metamorphic grade increases downward through the units and metamorphic isograds never cut unit boundaries.

Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia

This paper describes the results of detailed mapping and combined structural and sedimentologic investigation of the ∼3.5 Ga old North Pole Chert along the eastern and southern flanks of the North Pole granitoid dome of the Pilbara craton. The following relationships have been found. (1) Stepwise and repeated thickening over north block-down listric normal growth faults, now tilted to 120°E. Apart from the thickness distribution, the growth fault character has been inferred from progressive upward decrease of throw towards gentle flexuring, and the presence of roll-over anticlines, in a restored pre-tilt cross-section. The faults are unrelated to the actual geometry of the dome, and overprinted by pre-doming compressive deformation. (2) A reconsideration of the sedimentary model of the chert-barite unit with emphasis on vertical grainsize distribution and facies architecture shows direct synsedimentary fault control. (3) Black chert veins, clustered in the hangingwall blocks of the growth faults follow a conjugate fracture pattern in and immediately beneath the North Pole Chert, symmetrical with respect to the synsedimentary fault geometry. Upwards, the veins terminate in the lowermost chert-barite unit of the North Pole Chert; downwards, the majority of the veins converge towards centres at the growth faults in the underlying basalt. (4) Barite occurs: in veins, in association with black chert; as large synsedimentary mounds (15 m high, 50 m wide), formed on the original shallow subaqueous basin floor; as commonly silicified sinters and sedimentary or early diagenetic evaporite associated with littoral and/or stromatoloidal facies. The barite mounds are directly connected with chert-barite veins, and are uplapped by banded sedimentary chert and diamictite with barite clasts. Field relationships suggest precipitation of barite, particularly in the mounds, may have been primary and synsedimentary. Vein barite at or near the original depositional surface of the chert is associated with base metal sulphide at depth. The tectono-sedimentary relationships between normal faults, sedimentation patterns and thickness distribution, primary and secondary chert, synsedimentary barite mounds, stratiform barite and chert-barite veins, support a tensional fault-controlled, hydrothermal model with Ba, Si and sulphide emanation in ca 50 m depth of water. Such a model envisages boiling and/or degassing above vents to locally mix a normally stratified water body, causing instantaneous oxidization into sulphate. The sedimentary model of the North Pole Chert indicates repeated uplift and subsidence, with development of faults and fractures, the intensity of which diminished from the lowermost chert-barite unit of the North Pole Chert to the uppermost, fifth unit.

Timing and tectonic significance of Late Archaean, sinistral strike-slip deformation in the Central Pilbara Structural Corridor, Pilbara Craton, Western Australia

A north-south striking zone of sinistral strike-slip shear (20 × 100 km) and associated folding and thrusting within the central part of the Pilbara Craton is described. This zone, referred to herein as the Central Pilbara Structural Corridor (CPSC), was formed during regional D3 deformation and overprints two earlier sets of structures associated with granitoid doming. It is bound to the west by a continuous, north-south striking, curviplanar fault at greenschist-facies. In the east it is bound by a series of NE-SW striking fault segments located along sheared fold limbs, and by a kilometre-wide zone of amphibolite-facies mylonite within the western margin of the Shaw Batholith. Faults vary from narrow, high-level, brittle structures and retrograde breccia zones in the north, to broader, more ductile structures with S C mylonitic foliations in the south, suggesting a southerly increase in exhumation. A set of kilometre-scale, en-échelon folds with NE-SW striking, upright axial planes and NE-plunging axes characterize the interior of the CPSC and are wholly contained within the bounding faults, thus indicating coeval development with faulting during sinistral wrenching ( σ 1 = NW-SE). Strike-slip deformation in the CPSC is interpreted to have resulted from northwesterly indentation of Shaw Batholith (and contiguous rocks to the east). Westerly-directed tectonic escape of the Strelley Granite laccolith and associated supracrustal rocks from in between Shaw Batholith and North Pole Dome, was followed by ramping up the concave eastern margin of the Yule Batholith and eastward tilting during the transition from westerly to northerly displacement. Continued northward translation of the Strelley Granite at the leading edge of the system was terminated by constriction between the Carlindi Batholith and the North Pole Dome. Trailing greenstone successions were crumpled into a series of NE-trending, en-échelon folds and related faults. Kinematic evidence and structural geometry indicate that sediments of the Lalla Rookh Basin (De Grey Group) were deposited in advance of the north-moving Strelley Granite within a basin bounded by faults of opposite displacement, rather than in a pull-apart basin as previously suggested. The age of the D3 deformation is indicated by resetting of the ArAr and RbSr isotopic systems to ca. 2950 Ma in the Western Shaw region, and by UPb ages of syn-kinematic granitoid rocks.

Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia

This paper describes the results of detailed mapping and combined structural and sedimentologic investigation of the ∼ 3.5 Ga old North Pole Chert along the eastern and southern flanks of the North Pole granitoid dome of the Pilbara craton. The following relationships have been found. (1) Stepwise and repeated thickening over north block-down listric normal growth faults, now tilted to 120 E. Apart from the thickness distribution, the growth fault character has been inferred from progressive upward decrease of throw towards gentle flexuring, and the presence of roll-over anticlines, in a restored pre-tilt cross-section. The faults are unrelated to the actual geometry of the dome, and overprinted by pre-doming compressive deformation. (2) A reconsideration of the sedimentary model of the chert-barite unit with emphasis on vertical grainsize distribution and facies architecture shows direct synsedimentary fault control. (3) Black chert veins, clustered in the hangingwall blocks of the growth faults follow a conjugate fracture pattern in and immediately beneath the North Pole Chert, symmetrical with respect to the synsedimentary fault geometry. Upwards, the veins terminate in the lowermost chert-barite unit of the North Pole Chert; downwards, the majority of the veins converge towards centres at the growth faults in the underlying basalt. (5) Barite occurs: in veins, in association with black chert; as large synsedimentary mounds (15 m high, 50 m wide), formed on the original shallow subaqueous basin floor; as commonly silicified sinters and sedimentary or early diagenetic evaporite associated with littoral and/or stromatoloidal facies. The barite mounds are directly connected with chert-barite veins, and are uplapped by banded sedimentary chert and diamictite with barite clasts. Field relationships suggest precipitation of barite, particularly in the mounds, may have been primary and synsedimentary. Vein barite at or near the original depositional surface of the chert is associated with base metal sulphide at depth. The tectono-sedimentary relationships between normal faults, sedimentation patterns and thickness distribution, primary and secondary chert, synsedimentary barite mounds, stratiform barite and chert-barite veins, support a tensional fault-controlled, hydrothermal model with Ba, Si and sulphide emanation in ca 50 m depth of water. Such a model envisages boiling and/or degassing above vents to locally mix a normally stratified water body, causing instantaneous oxidization into sulphate. The sedimentary model of the North Pole Chert indicates repeated uplift and subsidence, with development of faults and fractures, the intensity of which diminished from the lowermost chert-barite unit of the North Pole Chert to the uppermost, fifth unit.

UPb zircon geochronology of Archaean felsic units in the Marble Bar region, Pilbara Craton, Western Australia

Archaean supracrustal rocks that are well exposed in the Marble Bar region, Pilbara Craton, have been assigned to the Warrawoona Group and younger sequences. The predominantly volcanic Warrawoona Group, previously dated at 3300 to 3500 Ma, is largely basaltic with locally intercalated thick felsic volcanic units. The supracrustal rocks have been folded and are distributed around the margins of large, roughly circular to ovoid, “granitic-gneissic” batholiths. A UPb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area, especially the Big Stubby, North Pole (barite), and Miralga Creek (ZnPbCuAu) deposits, in support of efforts to improve Archaean lead isotope models. The results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area. The new conventional zircon UPb data indicate that most of the Warrawoona Group (from the Duffer Formation upwards) was deposited over a period of ∼20 Ma, from ∼3470 Ma to 3450 Ma. The age of the Duffer Formation is closely constrained by results of 3471±5 Ma and 3465±3 Ma, and the age of the upper Salgash Subgroup appears to be established at 3454±1 Ma. The Panorama Formation at the southern margin of the North Pole Dome is 3458±1.9 Ma; unless the Duffer Formation was deposited over a period of at least 10 Ma, this casts doubt on a previously suggested Panorama-Duffer correlation. The age of the lower part of the North Pole succession is greater than ∼3458 Ma, consistent with the correlation of the basal greenstones at North Pole with the Talga Talga Subgroup. The North Pole chert-barite unit, which is well known for its preservation of Early Archean stromatolites, microfossils, and evaporite sediments, is clearly older than 3458 Ma. The 3325 Ma age now established for the Wyman Formation shows that this unit must be excluded from the Warrawoona Group. The age of the Talga Talga Subgroup remains uncertain; a felsic schist in the Mount Ada Basalt, which has been dated at 3449±3 Ma, is interpreted to be a sill, probably related to the 3450 Ma granitoid bodies. An age > 3724 Ma for a zircon xenocryst in the Panorama Formation is the oldest obtained so far for zircon from the Pilbara granite-greenstone terrane, and indicates the existence of pre-Pilbara Supergroup sialic basement. Ages of 3465±3 Ma and 3449±2 Ma applied to previous Pb isotope data for the Big Stubby and Miralga Creek deposits, respectively, in combination with comparable data for other syngenetic Archaean sulphide deposits, yield a general single-stage Pb evolution model with the parameters T 0=4560 Ma, A 0=9.0818, B 0=9.9002 and C 0=29.343. This gives model ages in accord with known constraints for many Archean deposits. A model lead age of 3490 Ma for the North Pole deposit, which is older than 3457 Ma, may be too great, but attempts to directly date this deposit have not yet succeeded. There are striking parallels between the Warrawoona Group ages reported here and those recently obtained for the Barberton region, Kaapvaal Craton, South Africa. The Talga Talga Subgroup and Lower Onverwacht (Tjakastad Subgroup) sequences have approximately equivalent constraints, and an age for the upper Salgash Subgroup agrees closely with those for the Hoogenoeg Formation of the upper Onverwacht Group (Geluk Subgroup).