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The 1.88 Ga Gunflint Formation in Ontario, Canada, has played a key role in the development of current models for the deposition of iron formations. The presence of hematite-rich iron formation intercalated with chert stromatolites containing purported cyanobacterial microfossils sparked the idea that biology was the principal driver of Fe2+ oxidation and iron deposition. However, despite the abundance of hematite in the Gunflint Formation, a primary depositional origin has not been established. Here we present evidence for the replacement of Fe-silicate granules by hematite in drill core intersecting the Gunflint Formation. Iron-oxide replacement proceeded inwards from granule boundaries and along intergranular fractures, producing iron oxide-rich rims around Fe-silicate cores. The abundance of organic matter in shaly iron formation implies that the iron-rich mudstones experienced anoxic diagenesis and that coexisting hematite was not depositional but formed after burial. Widespread distribution of the alteration textures indicates that this was a large-scale process and that much of the hematite is not primary. Lifting the veil of oxidative overprinting reveals an iron-rich sediment that was originally more reduced and dominated by Fe(II)-rich minerals. Our results imply that a major assumption underpinning the original model for biological iron oxidation as the driver of iron formation deposition may be flawed, raising broader questions about the origin of hematite in other iron formations and the role of biology in iron deposition in the early oceans. [ABSTRACT FROM AUTHOR]

The Great Oxidation Event (GOE) is marked by the loss of readily oxidizable detrital minerals and the onset of oxidative weathering. One of the oldest post-GOE weathering surfaces, which extends for almost 350 km along strike, occurs in Griqualand West, South Africa. It is best preserved east of the Blackridge thrust, where oxidized paleoweathering profiles are developed below the unconformity at the base of Mapedi-Gamagara red beds. In the Maremane Dome, the red beds preserve pisolitic hematite laterites, which indicate a highly oxygenated atmosphere and suggest hot and humid climatic conditions. The Mapedi and Gamagara Formations are undated east of the Blackridge thrust but were thought to be lithological correlatives of the ≥1.91 Ga Mapedi red bed sequence to the west. Here, we report a U-Pb zircon age of 1.25 Ga for a felsic tuff in red beds of the Mapedi Formation in the Kalahari manganese field. The new tuff age shows that the Mapedi red beds east of the thrust were deposited >650 m.y. after the Mapedi Formation to the west, and therefore they are part of a distinct Mesoproterozoic sequence. Based on lithologic and sedimentological similarities, the Mapedi-east and Gamagara formations are likely to be correlatives that were deposited on an ancient weathering surface at ca. 1.25 Ga. Our findings suggest that key evidence for a highly oxygenated atmosphere during the early Paleoproterozoic actually formed at ca. 1.25 Ga during a major episode of Mesoproterozoic oxidative weathering. [ABSTRACT FROM AUTHOR]

The abundance of Precambrian organic-rich shales, heated beyond the oil and gas window, requires that enormous volumes of hydrocarbons were generated and transported through the ancient crust. However, the former passage of fluid hydrocarbons rarely leaves a trace, so evidence for this process in the early Precambrian rock record is sparse. Here, we report the widespread presence of solidified oil (pyrobitumen) in the iron formation of the 1.88 Ga Gunflint Formation, Ontario, Canada. Petrographic textures indicate at least two phases of oil migration, an early phase marked by pyrobitumen in granules and intergranular pores, synchronous with synsedimentary silica cementation, and a later phase restricted to crosscutting fractures. The paragenetic relationships between the pyrobitumen and iron oxides indicate that oil migration commenced before hematite and some magnetite growth. Our evidence for early oil migration can be explained by petroleum generation during the 1.86–1.80 Ga Penokean orogeny, expelling hydrocarbons generated in the core of the fold-and-thrust belt outwards and updip through the sediments of the Gunflint Formation. [ABSTRACT FROM AUTHOR]

Highlights • Hematite in 1.8 Ga marine "red beds" formed >600 m.y. after deposition. • Hematite laths aligned in spaced cleavage overgrown by 1.2 Ga metamorphic monazite. • Pervasive oxidation of metasedimentary rocks linked to syn-orogenic uplift. • The Stirling biota organisms are not terrestrial but inhabited marine environments. Abstract The timing of hematite growth in ancient red-colored sedimentary rocks ("red beds") is controversial. While most of the hematite is considered to have formed at or near the land surface shortly after deposition, paleomagnetic evidence suggests that some hematite in ancient red beds formed long after deposition during orogenic fluid flow. Metamorphosed red sandstones and shales in the Stirling Range Formation, Western Australia, were deposited in a shallow marine environment between 2.0 and 1.8 billion years ago. The sandstones, which preserve an assemblage of discoidal and trace-like fossils, are stained red by abundant fine-grained hematite crystals. The hematite is aligned in a spaced cleavage defined by elongate laths of muscovite and chlorite, and its occurrence as inclusions in ∼1.2 Ga metamorphic monazite indicates that the oxide formed during the 1.2–1.15 Ga Albany-Fraser orogeny, >600 million years after deposition. Thermometry of two distinct populations of chlorite, an earlier Fe-chlorite (Chl1) and a later Mg-chlorite (Chl2), indicates a cooling trend from ∼370 °C (Chl1) to ∼290 °C (Chl2), consistent with evidence for rapid late-stage uplift elsewhere in the orogen. The results indicate that most of the hematite in the Stirling "red beds" formed during syn- to late-orogenic oxidation of previously more reduced marine sediments. The migration of oxidizing fluids during orogenesis, possibly related to rapid uplift and gravity-driven influx of surface water, could explain the origin of other post-orogenic "red beds". [ABSTRACT FROM AUTHOR]

Banded iron formations (BIFs) were deposited as deep-water facies distal to the late Archean Campbellrand carbonate platform, Transvaal Supergroup, South Africa. They are traditionally interpreted to have formed from iron oxides/hydroxides and silica that precipitated when upwelling hydrothermal water enriched in Fe 2+ and silica mixed with cooler, shallow seawater enriched in dissolved oxygen. The ferric oxides/hydroxides were then converted to hematite or, in the presence of organic matter, reduced and incorporated into siderite. New high-resolution microscopy of BIFs from distal facies of the Campbellrand platform questions that interpretation, revealing the presence of abundant greenalite nanoparticles, interpreted to represent primary precipitates from ferruginous seawater, consistent with recent results from equivalent-aged BIFs of the Hamersley Group, Western Australia. The particles locally define primary sedimentary lamination, and are engulfed in diagenetic chert that preserves shrinkage structures, considered to have formed during dehydration and recrystallization of amorphous silica cement. The replacement of greenalite nanoparticles by coarser grained oxide, silicate and carbonate minerals, with destruction of primary textures and structures, indicates that iron-silicate muds were originally much more widespread. Based on our findings we propose a new model for the deposition of the BIFs, involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor. We suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids. Upon mixing with cooler, more alkaline seawater, the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron-silicate nanoparticles over vast areas of the seafloor. The close association between iron and silica during the deposition of BIFs may be explained by the precipitation of iron-silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles. Furthermore, on the seafloor, iron-silicate nanoparticles presumably acted as nucleation sites for dissolved silica, promoting early diagenetic silica precipitation and cementation of the iron-rich muds. Our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of BIFs. [ABSTRACT FROM AUTHOR]

Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A fundamental tenet of this model is that fine-grained hematite (so-called dusty hematite) in least-altered BIFs represents the dehydration product of original oxide/hydroxide precipitates. However, this premise has never been proven. We have investigated the origin of the earliest-formed iron oxides in chert in well-preserved BIFs of the 2.63–2.45 billion-year-old Hamersley Group, Australia. We find that laminated chert in BIFs show progressive stages of in situ alteration from gray–green chert, containing iron-silicate nanoparticles, to red chert with abundant hematite dust. Analysis of textures by transmission electron microscopy of samples from the transition zone between gray–green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles by the precipitation of iron oxides in resulting cavities. These observations suggest that hematite dust is not a relict of an original seawater precipitate but the end-product of post-depositional oxidation. Our observations are consistent with paleomagnetic results from the Hamersley Group, which record two major phases of magnetic remanence carried by hematite that post-date deposition by more than 200 million years. Our results may provide an alternative explanation for the origin of jasper in BIFs deposited before the start of the Great Oxidation Event about 2.4 billion years ago. If correct, it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early Precambrian seawater. Furthermore, our results suggest that the primary iron precipitate in BIFs was iron-silicate mud that was silicified at or just below the sediment–water interface, a hypothesis that requires neither dissolved oxygen nor photosynthetic life, but was an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica. [ABSTRACT FROM AUTHOR]

In their recent paper entitled "Early Archean biogeochemical iron cycling and nutrient availability: New insights from a 3.5 Ga land-sea transition", Johnson et al. (2022) use the Fe isotope and trace-element compositions of ~3.5 Ga jaspilites from the North Pole Dome area of the Pilbara Craton, northwestern Australia, to investigate links between the ancient Fe biogeochemical cycle, continental exposure and nutrient (P) availability. The geochemical arguments are based on the underlying assumption that the jaspilites contain primary hematite that was initially deposited as Si-ferrihydrite precipitates following Fe(II)(aq) oxidation by photoferrotrophs. In support of a primary origin for the iron oxide, the authors present observations from standard light petrography showing the presence of fine-grained hematite dispersed amongst the chert. Here, we question whether the fine-grained hematite is the sole iron precipitate, and indeed, whether it is primary. These concerns arise from work on nearby and slightly younger jaspilites (3.46 Ga Marble Bar Chert Member), in which SEM and TEM studies revealed the presence of fine-grained particles (typically <1 µm) of greenalite and siderite, as well as evidence for pervasive secondary oxidation. Although fine-grained hematite is readily identified using light microscopy, tiny Fe(II)-rich particles can easily be overlooked. For this reason, we believe that the Dresser jaspilites, which closely resemble the nearby Marble Bar jaspilites, may also contain Fe(II)-rich precipitates. In addition, given the widespread evidence for secondary oxidation in the Marble Bar jaspilites, there remains a distinct possibility that the fine-grained hematite in the Dresser jaspilites is not primary. The precipitation of Fe(II)-rich phases, rather than ferrihydrite, would undermine interpretations for Fe biogeochemical cycling centred on photoferrotrophy. We suggest that the conclusions of Johnson et al. (2022) should be treated with caution until the jaspilites have been examined by high-spatial resolution electron imaging and analysis. [ABSTRACT FROM AUTHOR]

The Boulder Lefroy-Golden Mile fault system in the Archean Yigarn Craton is the most productive gold-mineralized structure in Australia (>2300 t Au). The New Celebration deposit (51 t Au) is part of a group of hematite- and anhydrite-bearing mesothermal deposits and Fe-Cu-Au skarns associated with monzodiorite-tonalite intrusions in the strike-slip fault system. Ore-grade biotite-carbonate and late sericite-carbonate-alkali feldspar replacement is bound to the contacts of a felsic (low Cr, Ni, V) quartz-plagioclase porphyry dyke dated at 2676 ± 7 Ma. The sodic-potassic alteration of the felsic boudinaged dyke contrasts with the albite-actinolite alteration in the adjacent mafic (high Cr, Ni, V) plagioclase porphyry dated at 2662 ± 4 Ma, although both share the same sulfide-oxide assemblage: pyrite ± chalcopyrite, magnetite ± hematite. The younger porphyry locally crosscuts foliation and is bordered by post-kinematic actinolite-pyrite selvages overprinting talc-chlorite-phlogopite-dolomite schist. It contains auriferous pyrite (70 ppb Au; 610 ppb Ag) where sampled for zircon U-Pb chronology at +224 m elevation. Above the sample site, the dyke was mined as gold ore (1–6 g/t Au) at +300–350 m. Temperature estimates based on actinolite-albite pairs (300–350 °C) agree with the fluid inclusion trapping temperature of main-stage auriferous veins (330 ± 20 °C). These relationships are interpreted to indicate syn-mineralization emplacement. Gold-related albite-altered porphyry dykes (albitites) also occur in the world-class Hollinger-McIntyre (986 t Au) and Kerr Addison-Chesterville deposits (336 t Au), Abitibi greenstone belt, Canada. [ABSTRACT FROM AUTHOR]

The history of atmospheric oxygen prior to the Great Oxidation Event (2.45-2.2 Ga) is not well understood. Hematite in the Marble Bar Chert from a NASA-funded drill hole (ABDP1) in the Pilbara craton, Australia, has been cited as evidence for an oxygenated ocean 3.46 b.y. ago. However, isotopic data from the same drill hole have been used to argue for an anoxic ocean. It is generally agreed that the hematite is primary, representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis. Here we present new petrographic evidence from the Marble Bar Chert (in drill hole ABDP1) that shows that hematite in jasper bands formed via mineral replacement reactions. The hematite mostly occurs as sub-micron-sized inclusions within chert (so-called "dusty" hematite) that are typically arranged into polygonal clusters surrounded by a rim of clear quartz, resembling shrinkage structures. The lateral transition from laminated chert enclosing minute inclusions of greenalite, siderite, and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron-bearing minerals. The presence of hematite-rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite. This interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement, from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries. The occurrence of dusty hematite in fractures, sedimentary laminae, and the outer margins of polygonal clusters containing greenalite is consistent with fluid-mediated oxidation of iron-rich precursor minerals. The presence of syn-sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition. The abundance of "dusty" greenalite inclusions, which are texturally the earliest components of the laminated chert, suggests that the precursor sediment contained an iron-rich clay mineral. Our results show that hematite has replaced ferrous-rich minerals after deposition and provide a mechanism to explain the origin of hematite in the Marble Bar Chert, which is consistent with the origin of hematite in adjacent basalts. A secondary origin for hematite invalidates arguments for an oxygen-bearing ocean ∼3.46 b.y. ago and provides a viable explanation for the formation of Archean jasper bands. Our findings show that misinterpretations about the origin of hematite in early Precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere. [ABSTRACT FROM AUTHOR]

High-grade hematite ore (so-called "hard ore") was first mined in the Lake Superior region of North America in A.D. 1848. The development of ore deposit models from the region has strongly influenced research on the origin of this deposit type worldwide. The inability to date high-grade hematite ores has hindered the testing of competing genetic models. In the Marquette Range, northern Michigan (USA), high-grade hematite ore is hosted by the 1.9-1.85 Ga Negaunee Iron Formation and unconformably overlying Goodrich Quartzite. Field relations and new in situ U-Pb geochronology of monazite and xenotime intergrown with hematite from six mines across the Marquette Range, Michigan, indicate that the formation of high-grade hematite ore bodies involved up to three events. The presence of pebbles of hematite ore in basal conglomerate of the Goodrich Quartzite indicates that the first episode of iron-oxide mineralization occurred after deposition and deformation of the Negaunee Iron Formation, but before the ca. 1.85 Ga Sudbury impact. Monazite in iron-ore deposits within the hinges of Penokean-aged folds yields an age of ca. 1.8 Ga, which is interpreted to record a second phase of hematite enrichment. Monazite and xenotime in iron-ore deposits along the limbs of regional folds give ages of ca. 1.77 Ga and are interpreted to record a later episode of hematite upgrading. The ca. 1.8 Ga and ca. 1.77 Ga phosphate ages coincide with the timing of regional tectonic events, suggesting hematite mineralization was coeval with the Penokean orogeny and Yavapai-interval accretion. Our results support the proposal that the formation of high-grade hematite ore bodies is a multistage process linked to discrete orogenic events. [ABSTRACT FROM AUTHOR]