Oxide-facies Banded Iron Formations Research Articles

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt‐hosted gold mineralization at the Tau deposit of the Mupane gold mine, Botswana

AbstractThe Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite‐rich zones associated with biotite‐chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold‐thiocomplexes in the ore‐forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two‐stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post‐depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold‐bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi‐processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.

Abiotic anoxic iron oxidation, formation of Archean banded iron formations, and the oxidation of early Earth

Iron in the early anoxic oceans of Archean age (4000-2500 million years ago) is believed to have been oxidized to form banded iron formations (BIF). Previously, it has been proposed that iron was oxidized either by free oxygen, H2O2, microbial oxidation, or photo-oxidation. However, these mechanisms are difficult to reconcile with evidence for the oceans at that time having been largely devoid of dissolved oxygen and oxidants, together with the rarity of microbial remains in BIF and restrictively slow rates of photo-oxidation. Experiments reported here show that ferrous iron readily oxidizes in analogs of Archean anoxic seawater following the precipitation of ferrous hydroxide. Once precipitated, ferrous hydroxide undergoes decomposition to elemental iron that reacts with water at room temperature to form ferric iron and release hydrogen gas. The ferric iron may then be incorporated into green rust, a mixed ferrous-ferric phase that ages into iron minerals commonly found in BIF. Our finding suggests that anoxic iron oxidation may have contributed to the formation of oxide-facies BIF, especially Algoma-type BIF that likely formed in semi-restricted basins where ferrous hydroxide saturation was more easily achieved. Additionally, ferrous hydroxide decomposition would have contributed to early Earth's oxidation, as a result of hydrogen escape to space, thus providing new insights into environmental and biological conditions on early Earth.

Banded iron formation from Antarctica: The 2.5 Ga old Mt. Ruker BIF and the antiquity of lanthanide tetrad effect and super-chondritic Y/Ho ratio in seawater

Here we report the first detailed geochemical data for Banded Iron-Formation (BIF) from Antarctica. Micro drill cores were taken from adjacent magnetite/haematite and metachert bands from the 2.5 Ga old Mt. Ruker BIF, Prince Charles Mountains, East Antarctica. Low concentrations of Al, Zr, Hf, and Sc reveal the purity of the samples. The chemical composition matches closely that of other early Precambrian BIF, except for an enrichment of Ni, and high Ba/Sr ratios in the metachert. Shale-normalized Rare Earths and Yttrium (REYSN) patterns show all the features of modern seawater such as depletion of light vs heavy REYSN, positive anomalies of LaSN, GdSN, and YSN (i.e. super-chondritic Y/Ho ratios) and the W-type lanthanide tetrad effect (LTE), but positive EuSN and no CeSN anomalies (and no positive Eu anomalies in chondrite-normalized REY patterns). This distribution is fully compatible with that of other pure Early Precambrian BIFs, suggesting high-temperature hydrothermal input of REY into seawater and a redox level of the atmosphere-hydrosphere system too reducing for Ce oxidation. The uniform, super-chondritic Y/Ho ratios in adjacent iron-oxide and metachert BIF bands most strongly suggest that the conspicuous banding does not result from post-depositional separation of an initially homogenous Fe-silicate precipitate, but argues for a primary origin of the banding. The super-chondritic Y/Ho ratios fit well within the range reported for other BIF worldwide, corroborating that positive Y anomalies, although possibly of smaller size than today, have always been a characteristic feature of seawater. The Mt. Ruker BIF further complements the set of Early Precambrian marine chemical sediments in which the W-type LTE has been observed, and demonstrates the antiquity of the LTE in seawater. These subtle features reveal that the distribution of the redox-insensitive REY in seawater did not change significantly over the almost 4 billion years geological record of seawater. The super-chondritic Y/Ho ratios and W-type LTE in this and other oxide-facies BIF are in marked contrast to the REY distribution in Modern and Cenozoic oxidic hydrogenetic ferromanganese crusts, pointing towards fundamentally different removal mechanisms of REY from Modern vs Early Precambrian seawater.

Decoupling of Neoarchean sulfur sources recorded in Algoma-type banded iron formation

Neoarchean Algoma-type banded iron formations (BIFs) are widely viewed as direct chemical precipitates from proximal volcanic–hydrothermal vents. However, a systematic multiple sulfur isotope study of oxide-facies BIF from a type locality in the ca. 2.74 Ga Temagami greenstone belt reveals mainly bacterial turnover of atmospheric elemental sulfur in the host basin rather than deposition of hydrothermally cycled seawater sulfate or sulfur from direct volcanic input. Trace amounts of chromium reducible sulfur that were extracted for quadruple sulfur isotope (32S–33S–34S–36S) analysis record the previously known mass-independent fractionation of volcanic SO2 in the Archean atmosphere (S-MIF) and biological sulfur cycling but only minor contributions from juvenile sulfur, despite the proximity of volcanic sources. We show that the dominant bacterial metabolisms were iron reduction and sulfur disproportionation, and not sulfate reduction, consistent with limited availability of organic matter and the abundant ferric iron deposited as Fe(OH)3. That sulfur contained in the BIF was not a direct volcanic–hydrothermal input, as expected, changes the view of an important archive of the Neoarchean sulfur cycle in which the available sulfur pools were strongly decoupled and only species produced photochemically under anoxic atmospheric conditions were deposited in the BIF-forming environment.

Silicon isotope fractionation during microbial reduction of Fe(III)–Si gels under Archean seawater conditions and implications for iron formation genesis

Microbial dissimilatory iron reduction (DIR) is a deeply rooted metabolism in the Bacteria and Archaea. In the Archean and Proterozoic, the most likely electron acceptor for DIR in marine environments was Fe(III)–Si gels. It has been recently suggested that the Fe and Si cycles were coupled through sorption of aqueous Si to iron oxides/hydroxides, and through release of Si during DIR. Evidence for the close association of the Fe and Si cycles comes from banded iron formations (BIFs), which consist of alternating bands of Fe-bearing minerals and quartz (chert). Although there has been extensive study of the stable Fe isotope fractionations produced by DIR of Fe(III)–Si gels, as well as studies of stable Fe isotope fractionations in analogous abiologic systems, no studies to date have investigated stable Si isotope fractionations produced by DIR.In this study, the stable Si isotope fractionations produced by microbial reduction of Fe(III)–Si gels were investigated in simulated artificial Archean seawater (AAS), using the marine iron-reducing bacterium Desulfuromonas acetoxidans. Microbial reduction produced very large 30Si/28Si isotope fractionations between the solid and aqueous phase at ∼23°C, where Δ30Sisolid–aqueous isotope fractionations of −3.35±0.16‰ and −3.46±0.09‰ were produced in two replicate experiments at 32% Fe(III) reduction (solid-phase Fe(II)/FeTotal=0.32). This isotopic fractionation was substantially greater than that observed in two abiologic controls that had solid-phase Fe(II)/FeTotal=0.02–0.03, which produced Δ30Sisolid–aqueous isotope fractionations of −2.83±0.24‰ and −2.65±0.28‰. In a companion study, the equilibrium Δ30Sisolid–aqueous isotope fractionation was determined to be −2.3‰ for solid-phase Fe(II)/FeTotal=0. Collectively, these results highlight the importance of Fe(II) in Fe–Si gels in producing large changes in Si isotope fractionations. These results suggest that DIR should produce highly negative δ30Si values in quartz that is the product of diagenetic reactions associated with Fe–Si gels. Such Si isotope compositions would be expected to be associated with Fe-bearing minerals that contain Fe(II), indicative of reduction, such as magnetite. Support for this model comes from recent in situ Si isotope studies of oxide-facies BIFs, where quartz in magnetite-rich samples have significantly more negative δ30Si values than quartz in hematite-rich samples.

Ge/Si in Hamersley BIF as tracer of hydrothermal Si and Ge inputs to the Paleoproterozoic ocean

Ge/Si and Ge/Fe ratios have been measured by laser ablation ICP-MS in selected sections of the Hamersley banded iron-formation (BIF), including ‘chert-siderite’ bands (i.e. carbonate- and silicate-facies BIF). The major Ge carriers are magnetite and hematite in the oxide-facies, and the Fe-rich silicates greenalite, minnesotaite and stilpnomelane in chert-siderite bands, while quartz has the lowest Ge/Si ratios (0.15–0.7μmol/mol), in accord with discrimination against Ge in quartz. The homogeneous Si/Fe ratio of 1.25 for Dales Gorge oxide-facies BIF is consistent with Si-ferrihydrite as precursor phase. This Si concentration is sufficiently high for formation of iron-free silica clusters at the surface of ferrihydrite, so that only a minor fractionation of the Ge/Si ratio should result from preferential Ge adsorption relative to Si. Hematite-chert bands that have been minimally altered by metamorphic reactions and/or fluid metasomatism, are the preferred material to determine the sources of silica in Hamersley BIF. Variations of Fe/Si and Ge/Si ratios in chert mesobands are interpreted as the result of diagenetic mineral segregation. A consistent Ge/Si ≈3μmol/mol is then derived by extrapolation of measured Ge/Si ratios to the average Fe/Si composition of Dales Gorge BIF. Simple mass balance calculations indicate hydrothermal inputs to the ocean of the order of 25% for Si and 90% for Ge, assuming Ge/Si=10μmol/mol for high-temperature hydrothermal fluids. The high Si concentration in siderite-chert bands (Si/Fe≈4) demands only a small amount of Si-ferrihydrite precursor and some excess silica likely precipitated in pore-water oversaturated with regard to amorphous silica. The average Ge/Si=0.80±0.1μmol/mol for bulk chert-siderite samples, including some stilpnomelane-rich cherts, is within the compositional range of modern riverine and aeolian inputs, pointing to a continental Si source during their deposition.

Depositional Environment of the Paleoproterozoic Yuanjiacun Banded Iron Formation in Shanxi Province, China

The Paleoproterozoic (~2.38–2.21 Ga) Yuanjiacun banded iron formation (BIF), located in Shanxi Province, is a Superior-type BIF in the North China craton. This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation, in the lower Luliang Group, which has undergone lower greenschist-facies metamor phism. Iron oxide (magnetite and hematite), carbonate, and silicate facies are all present within the iron-rich layers. The eastward transition from carbonate- into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east. These distinct lateral facies are also observed vertically within the BIF, i.e., the iron mineral assemblage changes upsection from sider ite through magnetite into hematite-rich iron formation. The oxide-facies BIF formed near shore, whereas carbonate (siderite)- and silicate-facies assemblages formed in deeper waters. Based on detailed analyses of these variations on a basinal scale, the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm wave base to relatively shallow waters. The BIF samples display distinctively seawater-like REEs + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams. Consistently positive Eu anomalies are also observed, which are typical of reduced, high-temperature hydrothermal fluids. In addition, slightly negative to positive Ce anomalies, and a large range in ratios of light to heavy REEs, are present in the oxide-facies BIF. These characteristics, in combination with consistently positive δ 56Fe values, suggest that deposition of the BIF took place along the chemocline where upwelling of deep, anoxic, iron- and silicarich hydrothermal fluids mixed with shallower and slightly oxygenated seawater. The ankerite displays highly depleted δ13C values and the carbonate-rich BIF has a high content of organic carbon, suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column; that same biomass was likely tied to the original oxidation of dissolved Fe(II). The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal, thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction. By contrast, in deeper waters more proximal to the hydrothermal vents, nutrients were abundant, and high biomass productivity was coupled to increased carbon burial, leading to the deposition of iron-rich carbonates. The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event (GOE; ca. 2.4–2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous, but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.

Dating the Oldest Greenstone in India: A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group, Singhbhum Craton

Abstract This article reports a precise 3506.8 ± 2.3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India. This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton, Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics,...

Trace element and isotopic characterization of Neoarchean and Paleoproterozoic iron formations in the Black Hills (South Dakota, USA): Assessment of chemical change during 2.9–1.9 Ga deposition bracketing the 2.4–2.2 Ga first rise of atmospheric oxygen

Banded iron formations (BIFs) were deposited in the Black Hills, South Dakota, prior to and after the first main oxygenation of the atmosphere at ∼2.4–2.2 Ga. The oldest, Neoarchean horizon (Nemo Iron Formation) is an oxide-facies, detritus-free BIF deposited at ∼2.9–2.6 Ga, in association with mafic phyllite. These BIFs show rare earth and yttrium elemental (REY) signatures typical of ambient seawater, including Pr/Yb < 1 and positive La, Eu, and Y anomalies in Post-Archean-Australian-Shale (PAAS)-normalized REE diagrams, but they lack clear Ce anomalies. Deposition of an Early Paleoproterozoic horizon (Benchmark Iron Formation) is constrained to the interval between 2.56 and 2.48 Ga. These BIFs also show the REE distribution patterns typical of ambient seawater. Benchmark BIFs also exhibit slightly negative Ce anomalies, but BIFs included within the younger, 2.10–2.02 Ga Estes metaconglomerate exhibit mostly positive Ce anomalies, which are probably the result of oxidative supergene remobilization of Ce during transport. Mixed, silicate–carbonate–sulfide–oxide–facies BIFs with slightly elevated detrital inputs were deposited in Middle Paleoproterozoic time, between 2.02 and 1.97 Ga (Homestake Iron Formation and equivalents) and between 1.89 and 1.87 Ga (Rochford Iron Formation). These BIFs are characterized by variably fractionated, PAAS-normalized REY patterns, which include: Pr/Yb ratios of 0.2–4.1; absence of La anomalies; and presence of positive Eu, positive and negative Ce, and mostly negative Y anomalies. Sm–Nd isotopes from individual microbands of the Neoarchean BIF define Eo- to Mesoarchean, Nd T DM ages of 3.6–2.9 Ga, which are generally compatible with crustal residence ages inferred from elsewhere in the Wyoming craton (i.e., in the 2.72–2.67 Ga Atlantic City BIF, Wind River Range). Model ages of microbands from the Early and Middle Paleoproterozoic BIFs are highly variable, ranging from 3.8 to 2.15 Ga. The youngest components in the Neoarchean and Middle Paleoproterozoic BIFs, inferred from the model ages, correspond to associated, mafic metatuffs and metabasalts, which yield ∼2.9 and ∼2.15 Ga model ages, respectively. Further, the oldest (i.e., ≤3.9–3.6 Ga) components indicate that Eo- to Mesoarchean crust was exposed during the entire BIF depositional time frame, between ∼2.9 and 1.88 Ga. Pb isotopes distinguish the Neoarchean from Early and Middle Paleoproterozoic BIFs. Whereas all BIFs reflect the generally high- μ ( 238U/ 204Pb) values typical of the Wyoming craton, thorogenic-versus-uranogenic Pb-isotope data arrays constrain sources of Pb in the various iron formations, particularly those deposited in Early and Middle Paleoproterozoic time. Early Paleoproterozoic BIFs exhibit low 208Pb/ 204Pb relative to 206Pb/ 204Pb ratios, implying a highly U/Th-fractionated source. This source is compatible with the peraluminous, ∼2.60 Ga Bear Mountain granite exposed in the W-central Black Hills, which itself was derived from even older crust. In contrast, the Middle Paleoproterozoic BIFs define a Pb-isotopic array that is compatible with a moderately fractionated source resembling the metaluminous, ∼2.56 Ga Little Elk granite exposed in the NE Black Hills. The Neoarchean BIFs are generally less radiogenic than their younger counterparts, and plot near the intersection of the two Paleoproterozoic arrays on the thorogenic diagram. Although the ∼2.9–1.9 Ga Black Hills BIFs bracket the worldwide, 2.4–2.0 Ga BIF depositional gap, which curiously coincides with the coupled, first atmospheric–biospheric oxygenation and subsequent, positive δ 13C excursion, we are unable to delineate major changes in BIF geochemical compositions that might reflect concomitant changes in atmospheric oxygenation. However, slightly negative Ce anomalies recorded in the 2.56–2.48 Ga, oxide–facies BIFs, and an overall trend of decreasing, positive Eu anomalies between ∼2.9 and 1.9 Ga among Black Hills BIFs generally (in agreement with the trend observed worldwide), might argue for the onset of bottom-water oxidation at the Archean-Proterozoic boundary and a decrease of high-temperature hydrothermal activity over the worldwide, BIF depositional gap. In the Black Hills, the 2.4–2.0 Ga depositional gap reflects either a ∼400 Myr absence of seawater between incipient, intracratonic rifting of supercraton Superia (or supercontinent Kenorland) at ∼2.5–2.4 Ga and its final breakup at ∼2.1 Ga, a ∼400 Myr missing rock record across a mapped unconformity, or both.

Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition

Oxidized Fe minerals in Archean–Paleoproterozoic banded iron formations (BIFs) are commonly taken to indicate the presence of biogenic O 2 or photosynthetic Fe(II)-oxidizing bacteria in the oceans' photic zone. However, at least one viable abiogenic oxidation mechanism has been proposed. Prior to the rise of atmospheric oxygen and the development of a protective ozone layer, the Earth's surface was subjected to high levels of ultraviolet radiation. Bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved Fe(II). Under such conditions, dissolved ferrous iron species, such as Fe 2+ and Fe(OH) +, would have absorbed radiation in the 200–400 nm range, leading to the formation of dissolved ferric iron [Fe(III)], which in turn, would have hydrolyzed to form ferric hydroxide [Fe(OH) 3] at circumneutral pH [Cairns-Smith, A.G., 1978, Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 76, 807–808; Braterman, P.S., Cairns-Smith, A.G., and Sloper, R.W., 1983, Photo-oxidation of hydrated Fe 2-Significance for banded iron formations. Nature 303, 163–164]. This process has been invoked to account for BIF deposition without need for biology [François, L.M., 1986, Extensive deposition of banded iron formations was possible without photosynthesis. Nature 320, 352–354]. Here, we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models. The experiments simulate the chemistry of ambient Precambrian seawater mixing with Fe(II)-rich hydrothermal fluids with, and without, UV irradiation. We find that if Fe(II) was effused from relatively shallow seamount-type vent systems directly into an anoxic photic zone, the photochemical contribution to solid-phase precipitation would have been negligible. Instead, most of the Fe(II) would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite ((Fe) 3Si 2O 5(OH) 4), and/or the ferrous carbonate, siderite (FeCO 3), depending on different simulated atmospheric pCO 2 levels. Conversely, in experiments where Fe(II) was exposed either to phototrophic Fe(II)-oxidizing bacteria or to O 2, ferric hydroxide formed rapidly, and the precipitation of ferrous iron phases was not observed. If, as suggested on mass balance grounds, BIF deposition requires that Fe be sourced from shallow seamount-type systems, then we are driven to conclude that oxide-facies BIF are the product of a rapid, non-photochemical oxidative process, the most likely candidates being direct or indirect biological oxidation, and that a significant fraction of BIF could have initially been deposited as ferrous minerals.

A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia

Bitumens extracted from 2.7 to 2.5 billion-year-old (Ga) shales of the Fortescue and Hamersley Groups in the Pilbara Craton, Western Australia, contain traces of molecular fossils. Based on a combination of molecular characteristics typical of many Precambrian bitumens, their consistently and unusually high thermal maturities, and their widespread distribution throughout the Hamersley Basin, the bitumens can be characterized as ‘probably of Archean age’. Accepting this interpretation, the biomarkers open a new window on Archean biodiversity. The presence of hopanes in the Archean rocks confirms the antiquity of the domain Bacteria, and high relative concentrations of 2α-methylhopanes indicate that cyanobacteria were important primary producers. Oxygenic photosynthesis therefore evolved > 2.7 Ga ago, and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere. Moreover, the abundance of cyanobacterial biomarkers in shales interbedded with oxide-facies banded iron formations (BIF) indicates that although some Archean BIF might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria, those in the Hamersley Group formed as a direct consequence of biological oxygen production. Biomarkers of the 3β-methylhopane series suggest that microaerophilic heterotrophic bacteria, probably methanotrophs or methylotrophs, were active in late Archean environments. The presence of steranes in a wide range of structures with relative abundances like those from late Paleoproterozoic to Phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late Archean, 900 Ma before visible fossil evidence indicates that the lineage arose. Sterol biosynthesis in extant eukaryotes requires molecular oxygen. The presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least ∼1% of the present atmospheric level (PAL) and may have been sufficient to support aerobic respiration.

The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia

The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different rock types are interpreted in terms of variations in eruption rate, which presumably reflect proximity to the vent or to a major lava river. Thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment, where lava flow was continuous and eruptive velocities were high. Sheeted bodies of olivine cumulate formed from voluminous, turbulent flow of lava over relatively refractory substrates, such as basalt and oxide-facies banded iron-formation. Ribbon-shaped olivine-cumulate bodies formed in an equally dynamic environment within major lava channels, where the substrate was less refractory and consisted of quartz-rich sedimentary rocks which were susceptible to thermal erosion. Distributary lava channels in more distal settings are represented by thinner ribbon-like accumulations of olivine mesocumulate to orthocumulate. Unusual poikilitic pyroxene-bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks. Pyroxene- and plagioclase-bearing cumulates are present at the top of other channel sequences and reflect late-stage ponding and in situ differentiation of komatiitic lava within the channel. The channel-facies rocks are flanked by sequences comprising multiple, thin, spinifex-textured flow-units, which formed from the episodic overflow of lava from the distributary channels. Similar rocks also occur stratigraphically above many of the thick olivine-cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle.

The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia

The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different rock types are interpreted in terms of variations in eruption rate, which presumably reflect proximity to the vent or to a major lava river. Thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment, where lava flow was continuous and eruptive velocities were high. Sheeted bodies of olivine cumulate formed from voluminous, turbulent flow of lava over relatively refractory substrates, such as basalt and oxide-facies banded iron-formation. Ribbon-shaped olivine-cumulate bodies formed in an equally dynamic environment within major lava channels, where the substrate was less refractory and consisted of quartz-rich sedimentary rocks which were susceptible to thermal erosion. Distributary lava channels in more distal settings are represented by thinner ribbon-like accumulations of olivine mesocumulate to orthocumulate. Unusual poikilitic pyroxene-bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks. Pyroxene- and plagioclase-bearing cumulates are present at the top of other channel sequences and reflect late-stage ponding and in situ differentiation of komatiitic lava within the channel. The channel-facies rocks are flanked by sequences comprising multiple, thin, spinifex-textured flow-units, which formed from the episodic overflow of lava from the distributary channels. Similar rocks also occur stratigraphically above many of the thick olivine-cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle.

Accretionary lapilli associated with Archaean banded iron formations of the Kraaipan Group, Amalia greenstone belt, South Africa

Abstract The Kraaipan Group in the Amalia greenstone belt, South Africa, consists of an Archaean volcano-sedimentary assemblage of mafic metavolcanic rocks and banded iron formations, together with subordinate greywacke, slate and carbonate rocks. The sequence is deformed and intruded by a variety of granites and gneisses. Interlayered with the volcano-sedimentary rocks, in close proximity to a small gold prospect, are pyroclastic rocks in which are described well-developed “coated” or “rim-type” accretionary lapilli of a variety hitherto rarely, if ever, recorded in association with oxide-facies banded iron formations. The accretionary lapilli are discussed in terms of their significance and as palaeoenvironmental indicators. It is concluded that the accretionary lapilli probably developed in a proximal setting with respect to an ancient eruptive centre and that the lapilli themselves do not constitute exceptional indicators of sedimentary environments but are useful mainly as time or stratigraphic markers. Their association with banded iron formations suggests the accretionary lapilli here described were deposited from volcanic centres close to some shallow-marine or barred lagoonal basin, the latter possibly analogous to a playa-lake complex developed in a primitive inter-arc or back-arc basin setting in existence during the evolution of the Kraaipan volcano-sedimentary assemblage found in the Amalia greenstone belt.