Deposition Of Iron Formations Research Articles

The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA

A layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the Paleoproterozoic iron ranges of northern Michigan, in the Lake Superior region of the United States. Radiometric age constraints from events predating and postdating depo si tion of the breccia are ca. 1875 Ma and 1830 Ma. The major bolide impact that occurred at 1850 Ma at Sudbury, Ontario, 500–700 km east of these sites, is the likely causative event. The Michigan sites described here, along with previously described sites in Minnesota and Ontario, defi ne an extensive ejecta-bearing deposit throughout the Paleoproterozoic iron ranges of the Lake Superior region that we refer to as the Sudbury impact layer. The layer at the sites in Michigan exhibits a range of thicknesses, lithologic characters, and sedimentary settings. The diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented, but the detailed investigations needed to document them are incomplete. Many of the sites had been described and interpreted previously as products of common terrestrial processes, but the presence of relict shock-induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extra terrestrial impact. At most localities, this layer also contains relict fragments of altered devitrifi ed glass and/or accretionary lapilli. One immediate use of the impact layer is as an ultraprecise time line that ties together the well-known stratigraphic sequences of the various geographically separated iron ranges, the correlation of which has remained controversial for many decades. The Sudbury impact layer most commonly lies at a horizon that records a signifi cant change in the character of sediments across the region. The impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of ficlastic rocks, commonly black shales. The impact may have produced regional, if not global, changes in the environment that resulted in this widespread synchronous change in sedimentation style.

Brine pool deposition for the Zn–Pb–Cu massive sulphide deposits of the Bathurst mining camp, New Brunswick, Canada. I. Comparisons with the Iberian pyrite belt

The Ordovician Zn–Pb–Cu massive sulphide ore deposits of the Bathurst mining camp share many features with those of the Devonian/Carboniferous Iberian pyrite belt, particularly the tendency to large size (tonnage and metal content); shape, as far as can be determined after allowing for deformation; metal content, particularly Fe/Cu, Pb/Zn and Sn; mineral assemblages (pyrite + arsenopyrite ± pyrrhotite and lack or rarity of sulphates); sulphide textures (particularly framboidal pyrite); lack of chimney structures and rubble mounds; irregular metal or mineral zoning; and the low degree of zone refining compared to Hokuroku ores. The major differences between the provinces are the lack of vent complexes and the presence of Sn–Cu ores in the Iberian pyrite belt. There are also similarities in the geological setting of the two camps: both lie within continental terranes undergoing arc-continent and continent–continent collision, and in each case massive sulphide mineralisation followed ophiolite obduction; the ore deposits are associated with bimodal volcanic rocks derived from MORB and continental crust and marine shales; and mineralisation was locally accompanied or followed by deposition of iron formations. Fluid inclusion data from veins in stockworks from at least six of the Iberian massive sulphide deposits point to sulphide deposition having taken place in basins containing mostly spent saline, ore-forming fluids (brine pools), and it is suggested that most of the major features of the Bathurst deposits can be explained by similar processes. The proposed model is largely independent of ocean sulphate and O 2 content, whereas low values of each are requisites for the current, spreading-plume model of sulphide deposition in the Bathurst camp.

Evolution of the oceanic sulfur cycle at the end of the Paleoproterozoic

Here, we present new measurements of 32S, 33S, 34S, and 36S in sedimentary sulfides and couple these measurements with modeling treatments to study the sulfur cycle of a late Paleoproterozoic marine basin. We target the transition in ocean chemistry from the deposition of Paleoproterozoic iron formations (Gunflint Formation, Biwabik Formation, Trommald Formation, and Mahnomen iron formations) to the inferred sulfidic ocean conditions recorded by overlying shale (Rove Formation). The data suggest that certain features of the global sulfur cycle, such as a control by sulfate reducing prokaryotes, and low (mM) concentrations of oceanic sulfate, were maintained across this transition. This suggests that the transition was associated with changes in the structure of the basin-scale sulfur cycle during deposition of these sediments. Sulfide data from the iron formations are interpreted to reflect sedimentary sulfides formed from microbial reduction of pore-water sulfate that was supplied through steady-state exchange with an overlying oceanic sulfate reservoir. The sulfide data for the euxinic Rove Formation shales reflect the operation of a sulfur cycle that included the loss of sulfide by a Rayleigh-like process. We suggest that the prevalence of large and variable heavy isotope enrichments observed in Rove Formation sulfide minerals reflect a sustained and significant net loss of sulfide from the euxinic water column, either as a result of a shallow chemocline and degassing to the atmosphere or as a result of a water column pyrite sink. The inclusion of 36S measurements (in addition to 32S, 33S, and 34S) illustrates the mass-dependent character of these sedimentary environments, ruling out contributions from the weathering of Archean sulfides and pointing to at least modest levels of sustained atmospheric oxygen (>10 −5 present atmospheric levels of O 2).

Iron Formation in Neoarchean Deltaic Successions and the Microbially Mediated Deposition of Transgressive Systems Tracts

Abstract Neoarchean deltaic deposits present in Superior Province, Canada, contain an excellent record of shallow-water siliciclastic and iron formation (IF) deposition. The deltas were fed by braided fluvial systems, which commonly resulted in the development of small, active lobes that repeatedly cycled through progradation and abandonment, although a thick progradational deltaic assemblage was also investigated. Magnetite-rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore-proximal fluvial channels and distributary-mouth bars of the braid-delta deposits. Magnetite laminae are also interbedded with some distributary-mouth sediments draping reactivation surfaces in ripples to barforms. Thicker accumulations of IF (up to 73 m) form portions of transgressive systems tracts where chemical sedimentation dominated. Offshore equivalents of these successions do not contain chemical sediments. Sedimentologic data imply that IF deposition in this well-mixed, nearshore water mass was not compatible with slow rates of constant iron-compound precipitation in the world ocean combined with siliciclastic sediment starvation. The model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore, stimulating photosynthetically induced oxidation of Fe2+. Under these conditions, elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts.

Stratigraphic and geochemical framework of the Agouron drill cores, Transvaal Supergroup (Neoarchean-Paleoproterozoic, South Africa)

Two long drill cores through the Transvaal Supergroup (Neoarchean-Paleoproterozoic) record the vertical transition from a mixed siliciclastic-carbonate ramp (Schmidtsdrif Subgroup) through the Campbellrand-Malmani carbonate platform (Campbellrand Subgroup) to deeper-water banded iron formations (Asbestos Hills Subgroup). The cores were drilled in proximal (core GKF01) and distal (core GKP01) positions relative to the margin of the Campbellrand-Malmani carbonate platform. Correlation between cores is based on three types of marker horizons: sequence stratigraphic, volcanic/impact and lithologic markers. Several surfaces can be correlated to the platform succession. Gamma-ray data and bulk-rock XRF geochemistry of siliciclastic mudstones and tuff beds support the correlation. Following flooding of a shallow-water carbonate ramp (Boomplaas Formation of Schmidtsdrif Subgroup), deposition of slope carbonates with minor siliciclastic mudstones, chert and banded iron formation (BIF) occurred below storm wave base. Cycles of in situ microbialites and reworked slope carbonates are prominent in both cores. A final transgressive pulse near the top of the succession led to deposition of thick cherts and iron formations across the former platform, slope and basinal areas. Deposition on the slope was controlled by cementation potential in microbial facies and by sediment input, which in turn was linked to variations in platform margin geometry and changes in accommodation space. Results indicate preferred sediment accumulation in the proximal slope position, while chemical sediments were deposited preferentially beyond the reach of carbonate and siliciclastic detrital sediment. Similarly, sediment input and possibly early marine cementation influenced cycles of microbialites and reworked carbonates. Sequence stratigraphic analysis and siliciclastic mudstone geochemistry suggest a possible sea level fall and a change in provenance of the siliciclastic detritus at the transition from the Monteville Formation to the Nauga Formation (basal Campbellrand Subgroup). These changes were probably related to a major depositional hiatus on the platform.

Kajian Endapan Pasir Besi di Daerah Pantai Selatan Kabupaten Ende, Flores, Provinsi Nusa Tenggara Timur

Iron as One of primary raw material in steel and construction industry, it's presence and potencies in Indonesia have an important role. Genetically, known 3 formation of iron deposits (1) Primary, (2) Lateritic, (3) Secondary/reworking process.Based on the survey conducted in Ende in 2005, the typical of iron deposits in this area is classified into sedimentary iron deposits (due to leaching, transporting, accumulating and depositing in somewhere). The accumulation of magnetite grains suggested is due to by leaching, washing in the long term and occurs intensively, so that produce high magnetite concentration in certain area. The interesting area is spread in four sectors; Rapo Rindu, Bheramari, Ruku Ramba and Ondorea sectors.The result of physical laboratory analysis (in concentrate) show the magnetite degree (MD) is vary from 10% to 50%, whereas from chemical analysis known the highest grade reach 37.10% and lowest grade is around 4.4%. However, the everage grade, in general are from 10 to 25 %.The potential of the deposits is obtained from combination between field work laboratory analysis and for those areas mentioned above the total hypothetics resources is 57.134.358,4 ton (concntrate). It seem the deposits of this area is not economic in mining view because very small (under 100 millions tonnes).

The potential significance of microbial Fe(III) reduction during deposition of Precambrian banded iron formations

ABSTRACTDuring deposition of late Archean–early Palaeoproterozoic Precambrian banded iron formations (BIFs) the downward flux of ferric hydroxide (Fe(OH)3) and phytoplankton biomass should have facilitated microbial Fe(III) reduction. However, quantifying the significance of such a metabolic pathway in the Precambrian is extremely difficult, considering the post‐depositional alteration of the rocks and the lack of ideal modern analogues. Consequently, we have very few constraints on the Fe cycle at that time, namely (i) the concentration of dissolved Fe(II) in the ocean waters; (ii) by what mechanisms Fe(II) was oxidized (chemical, photochemical or biological, the latter using either O2 or light); (iii) where the ferric hydroxide was precipitated (over the shelf vs. open ocean); (iv) the amount of phytoplankton biomass, which relates to the nutrient status of the surface waters; (v) the relative importance of Fe(III) reduction vs. the other types of metabolic pathways utilized by sea floor microbial communities; and (vi) the proportion of primary vs. diagenetic Fe(II) in BIF. Furthermore, although estimates can be made regarding the quantity of reducing equivalents necessary to account for the diagenetic Fe(II) component in Fe‐rich BIF layers, those same estimates do not offer any insights into the magnitude of Fe(III) actually generated within the water column, and hence, the efficiency of Fe and C recycling prior to burial. Accordingly, in this study, we have attempted to model the ancient Fe cycle, based simply on conservative experimental rates of photosynthetic Fe(II) oxidation in the euphotic zone. We estimate here that under ideal growth conditions, as much as 70% of the biologically formed Fe(III) could have been recycled back into the water column via fermentation and organic carbon oxidation coupled to microbial Fe(III) reduction. By comparing the potential amount of biomass generated phototrophically with the reducing equivalents required for Fe(III) reduction and magnetite formation, we also hypothesize that another anaerobic metabolic pathway might have been utilized in the surface sediment to oxidize the fermentation by‐products. Based on the premise that the deep ocean waters were anoxic, this role could have been fulfilled by methanogens, and maybe even methanotrophs that employed Fe(III) reduction.

Patterns of sedimentation in the Precambrian

The principle of uniformitarianism may be applied to Precambrian basin evolution and to the sedimentary record as a whole. The major difference in the Precambrian Eon lay in variability of rates and intensities of processes controlling weathering, erosion, transport, deposition, lithification, and diagenesis. This paper examines Precambrian sedimentation patterns within the larger framework of Earth evolution. Pre-rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis, resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension, all reworked by marine current systems and localised turbidites. From c. 4 to 3.2 Ga, greenstone belts provided the predominant settings for the thin passive margin carbonates, BIF, stromatolitic evaporites, pelites and quartzites, and lesser synorogenic turbidites, conglomerates, and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings. Common palaeoenvironments were high gradient alluvial fans, low sinuosity braided rivers, and relatively shallow marine settings, subject to wave and tidal action, and turbidity currents. Although continental crustal growth continued largely through greenstone belts until c. 2.7 Ga, the Witwatersrand basin (c. 3.0–2.7 Ga; Kaapvaal craton, South Africa) reflects initial stabilisation of the oldest craton, with an epeiric sea accumulating largely fluvial detritus subject to tidal (inland) and storm-wave (craton-marginal) reworking within a retroarc foreland basin setting. Neoarchaean–Palaeoproterozoic sedimentation is discussed within a framework of two global “superevents”, at c. 2.7 Ga and 2.2–1.8 Ga, each encompassing major changes in Earth's evolution related to the supercontinent cycle, mantle superplumes, peaks in crustal growth rates, and significant biochemical changes within the atmosphere–hydrosphere system. Concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed, and deposition of iron-formation peaked globally at c. 2.5 Ga. The first global glaciation at c. 2.4–2.2 Ga provides little support for the “Snowball Earth” theory, but does suggest a negative feedback loop model whereby intraglacial CO 2-related warming and synglacial decreases in weathering alternated up to three times. Models for palaeo-atmospheric and -oceanic evolution are mutually exclusive, but by c.1.8 Ga, the existence of large landmasses and free oxygen in Earth's atmosphere enabled erg development and red bed sedimentation globally; the full spectrum of Phanerozoic–Modern sedimentary environments was thus present on Earth. A third postulated “superevent” at c. 0.8–0.6 Ga essentially recreated conditions experienced at c. 2.2–1.8 Ga, with, additionally, at least three global refrigeration events.

Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria

The mechanism of banded iron formation (BIF) deposition is controversial, but classically has been interpreted to reflect ferrous iron [Fe(II)] oxidation by molecular oxygen after cyanobacteria evolved on Earth. Anoxygenic photoautotrophic bacteria can also catalyze Fe(II) oxidation under anoxic conditions. Calculations based on experimentally determined Fe(II) oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that, even in the presence of cyanobacteria, anoxygenic phototrophs living beneath a wind-mixed surface layer provide the most likely explanation for BIF deposition in a stratified ancient ocean and the absence of Fe in Precambrian surface waters.

Hotspots and mantle plumes: global intraplate tectonics, magmatism and ore deposits

Intraplate tectono-magmatic phenomena, including the emplacement of layered intrusions, and the giant dyke swarms, anorogenic (hotspot) volcanism, oceanic plataeux, rifting processes, basin formation, and geomorphological features are discussed in the context of the mantle plume theory. A review of the relationships between mantle plumes and ore deposits focuses on direct links, proxied by the emplacement of mafic-ultramafic magmas (e.g. PGE and Ni–Cu sulphides associated with flood basalts) and indirectly in rift systems where high geothermal gradients are set up in the crust above the plume, induce large scale circulation of hydrothermal fluids, which result in the generation of a wide range of ore deposits. Peak periods in the deposition of iron formations coincide with plume events in the Archeaen and Proterozoic. Passive margins, which evolve from continental breakups and triple junctions, host abundant mineral and hydrocarbon resources.

Geological Setting of the Meadowbank Gold Deposits, Woodburn Lake Group, Nunavut

The Meadowbank gold deposits are hosted by a series of polydeformed and metamorphosed auriferous iron formations located in the Neoarchean Woodburn Lake group, western Churchill province, Nunavut. The supracrustal rocks in the Meadowbank area consist of a thick sequence of intermediate-composition volcanic rocks with intercalated iron formation, ultramafic volcanic rocks, and quartzite to quartz arenite. U-Pb geochronology gives an interpreted age of 2711 ± 3 Ma for the host strata at Meadowbank. The intermediate volcanic rocks are predominately volcaniclastic and have a geochemical composition that is consistent with an active continental margin setting. Iron formation deposition was coeval with the intermediate volcanism, as indicated by incorporation of volcanic detritus in the chemical precipitate. The geochemistry of the iron formation indicates that it precipitated from a fluid that at one time was at high temperature (>250°C), reducing, and likely acidic. Intercalated ultramafic rocks rarely show spinifex textures and have trace element ratios that are consistent with mantle plume-related undepleted mantle asthenosphere, likely recording episodic mantle upwelling associated with rifting in an active continental margin. The contact between the ultramafic volcanic rocks and the overlying quartz arenite is a disconformity marked by a quartz pebble conglomerate. However, similar geology on the structural hanging wall and footwall of the contact suggests that the conglomerate does not represent a significant hiatus in deposition, and is more likely a prograding terriginous siliciclastic unit. Three metamorphic events are recognized. The first, M1, is a cryptic greenschist facies event that is pre-D2. The second event, M2, is a mid-greenschist to amphibolite facies, syn-D2 event. M3 is the last event recognized, and is a post-tectonic greenschist facies event that is regional but inhomogeneous in extent, possibly reflecting thermal aureoles around post-tectonic 1.7 to 1.8 Ga Nueltin granites. The structural geology of the area is complex with four phases of deformation recognized, two of which had a significant effect on the geometry of the deposit. All of these regional events are interpreted as Paleoproterozoic in age. Relationships between deformation fabrics and mineralization, as well as the overall geometry of the mineralized envelopes, suggest that the deposit formed during syn- to late-D2. Superimposed on the mineralization are D4 structural elements that postdate gold mineralization. The main control on gold mineralization is replacement of magnetite by pyrrhotite and pyrite in high-strain corridors. The composition of amphiboles and chlorites associated with gold mineralization is remarkably consistent and shows no spatial or temporal variation, suggesting that it was buffered by the iron-rich nature of the host rocks. The bulk metasomatic effect on the intermediate volcanic rocks is characterized by the addition of K2O and the loss of CaO, Na2O, and MgO, with little variation in the total iron content. The elemental losses are likely the result of destruction of feldspars, stilpnomelane, and ferroactinolite, which are relatively common in the unmineralized rocks, and the formation of sericite, chlorite, and grunerite. Textural and timing relationships suggest that the mineralization is syn- to late-D2, and by inference M2. Fluid inclusions, chlorite, and arsenopyrite geothermometry all suggest that mineralization took place at about 325° to 375°C and 1.3 kbar pressure. These are lower P-T conditions than is indicated by M2 mineral assemblages, suggesting that the deposit formed during the waning stages of metamorphism. Strain partitioning due to the mechanical contrast of the iron formation preferentially localized dilational settings during D2 deformation. This allowed fluid influx, sulfidation of the iron formation, and resulting precipitation of gold to be concentrated in high-strain corridors. Geologic relations and geochronology collectively suggest that a Paleoproterozoic (ca. 1.8–1.9 Ga) deformation event was responsible for the introduction of gold into the Neoarchean supracrustal sequences.

Geophysical exploration for iron ore in the Middleback Ranges, South Australia

Iron has been mined in the Middleback Ranges-Katunga Hills in the northeast Eyre Peninsula since the turn of the last century. The iron ore mineralisation was formed primarily by supergene enrichment of Palaeoproterozoic iron formation and most deposits are located close to outcropping iron formation. Faults and fold axes probably acted as fluid conduits localising and focusing the fluids required for the ore-forming processes. Basic and ultrabasic intrusives probably acted as impervious barriers, further localising these fluids. The iron formations in the Middleback Ranges are both dense and highly magnetic when compared with other lithologies in the area and this has enabled gravity and magnetic data to be used successfully to map structure and stratigraphy. The iron ores are in general denser, but less magnetic, than the iron formation, although there is overlap in their range of physical properties. Most iron-ore deposits give rise to positive gravity anomalies, but the recognition of these anomalies is hindered by the presence of larger anomalies due to the iron formations. Also, similar anomalies may be caused by basic intrusives. There is no recognisable response from the ores in magnetic data. Trial electromagnetic surveys defined anomalies in some areas of mineralisation. However, petrophysical measurements show the ores are not conductive and these responses are probably caused by saline groundwater and clay minerals.

Using Ge/Si ratios to decouple iron and silica fluxes in Precambrian banded iron formations

Banded iron formations are prominent sedimentary deposits of the Precambrian, yet the source of their silica remains unresolved. Here we show that Ge/Si ratios preserved in banded iron formation chert layers are indicative of weathering of continental landmass. This conflicts with the accumulation of evidence suggesting that chemical components were sourced at mid-ocean-ridge hydrothermal systems. Instead, it implies that the sources of silica and iron were decoupled during banded iron formation deposition, silica being dominantly derived from weathering of continental landmass and iron having a hydrothermal origin. Thus, the chemistry within banded iron formation depositional basins underwent clear switching that varied on a periodic basis and is recorded in the alternation of iron- to silica-rich layers.

Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region

A rhyolite in the Hemlock Formation, a mostly bimodal submarine volcanic deposit that is laterally correlative with the Negaunee Iron-formation, yields a sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon age of 1874 ± 9 Ma, but also contains inherited Archean zircons as old as 3.8 Ga. This precise age determination for the classic Paleoproterozoic stratigraphic sequence of northern Michigan, the Marquette Range Supergroup (MRS), necessitates modification of previous depositional and tectonic models. Our new data indicate that the Menominee Group, previously ascribed to continental rifting during early, pre-collision phases of the Penokean orogenic cycle, is coeval with arc-related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit. We interpret these to be second-order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin. Along with a recently reported age for the Gunflint Formation in Ontario of 1878 ± 2 Ma, our data suggest that an extensive foredeep in the western Lake Superior region was the locus of iron-formation deposition during arc accretion from the south. Further, we interpret the lower MRS (Chocolay Group), a glaciogenic and shallow-marine succession that lies atop Archean basement, to be equivalent to the upper part of the Huronian Supergroup of Ontario and to represent the original continental rifting and passive-margin phase of the Penokean cycle. The upper MRS (Baraga Group) represents deeper marine basins, dominated by turbidites and lesser volcanic rocks, resulting from increased subsidence and continued collision. A stitching pluton, which cuts correlatives of the Hemlock Formation in a thrust sheet, yielded a U–Pb zircon age of 1833 ± 6 Ma, consistent with other post-tectonic plutons in Wisconsin and northern Michigan, indicating that Penokean convergence lasted no longer than ~40 million years.

Geochemistry of the Palæoproterozoic Mooidraai Formation: Fe-rich limestone as end member of iron formation deposition, Kalahari Manganese Field, Transvaal Supergroup, South Africa

The Mooidraai Formation forms the uppermost portion of the Palæoproterozoic Transvaal Supergroup in Griqualand West, Northern Cape Province, South Africa and is developed immediately above the Mn-bearing Hotazel Formation. Though previously described as a (cherty) dolomite, the portion of the Mooidraai succession investigated here contains microcrystalline calcite as the sole carbonate constituent. Variable amounts of magnetite and quartz are commonly present in the rock, giving it a mineralogical/geochemical character that approaches, at times, that of the underlying Hotazel iron formation. Bulk rock and rare earth element data compare well with those of the Hotazel iron formation and suggest that the Mooidraai Formation constitutes the end member of chemical precipitation that produced the cyclic Fe-Mn sequence of the Hotazel Formation. Calcite C and O isotope data indicate a diagenetic origin for the carbonate component with deviations from typical marine bicarbonate isotope values proportionally related to the amount of Fe present. The Mooidraai data thus support models of organic matter oxidation-ferric Fe reduction, as observed in modern O deficient diagenetic environments.

Phototrophs in high-iron-concentration microbial mats: physiological ecology of phototrophs in an iron-depositing hot spring.

At Chocolate Pots Hot Springs in Yellowstone National Park the source waters have a pH near neutral, contain high concentrations of reduced iron, and lack sulfide. An iron formation that is associated with cyanobacterial mats is actively deposited. The uptake of [(14)C]bicarbonate was used to assess the impact of ferrous iron on photosynthesis in this environment. Photoautotrophy in some of the mats was stimulated by ferrous iron (1.0 mM). Microelectrodes were used to determine the impact of photosynthetic activity on the oxygen content and the pH in the mat and sediment microenvironments. Photosynthesis increased the oxygen concentration to 200% of air saturation levels in the top millimeter of the mats. The oxygen concentration decreased with depth and in the dark. Light-dependent increases in pH were observed. The penetration of light in the mats and in the sediments was determined. Visible radiation was rapidly attenuated in the top 2 mm of the iron-rich mats. Near-infrared radiation penetrated deeper. Iron was totally oxidized in the top few millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration in the surface sediments, the cyanobacteria could potentially increase the rate of iron oxidation in situ. This high-iron-content hot spring provides a suitable model for studying the interactions of microbial photosynthesis and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help clarify the potential role of photosynthesis in the deposition of Precambrian banded iron formations.

Two Neoarchean supercontinents? Evidence from the Paleoproterozoic

An unresolved question in Precambrian geology is the relationship between Archean crustal fragments that are now separated by younger orogens: were they once contiguous? Williams et al. (1991)proposed the name `Kenorland' for a speculative Neoarchean supercontinent comprising the Archean provinces in North America. Recently, a large number of ca. 2.5–2.0 Ga magmatic, metamorphic, detrital and xenocrystic ages have been reported from North America. We interpret that the wide geographic distribution and temporal spread of these ages may signify long-lived, regional-scale mantle upwelling, and anorogenic magmatic and metamorphic processes related to the protracted breakup of Kenorland. Breakup may have extended from ca. 2.5 to 2.1 Ga, culminating with dispersion of continental fragments at ca. 2.1–2.0 Ga. In North America, ca. 2.5–2.1 Ga intracratonic basin successions (e.g. Hurwitz Group) formed in the interior of Kenorland before dispersion, and passive margin sequences flanking the Superior Province (e.g. Huronian Supergroup) and the Wyoming Province (e.g. Snowy Pass Supergroup) defined the edges of Kenorland. Earliest Paleoproterozoic magmatic and sedimentary rocks, which include voluminous quartz arenites and glacigenic deposits, are consistent with a high-standing supercontinent and a mantle superplume. The Paleoproterozoic record from the Baltic and Siberian shields is similar to that of North America, suggesting inclusion in Kenorland. A slightly different record from the southern continents suggests a second, coexisting supercontinent that included the Zimbabwe, Kaapvaal, and Pilbara cratons, (`Zimvaalbara' of I.G. Stanistreet), the São Francisco Craton, and possibly, cratonic blocks in India. Attenuation of this second supercontinent started earlier than in Kenorland (ca. 2.65 Ga) and was accompanied by high sea level and deposition of vast Lake Superior-type iron formations. Immediately thereafter, both supercontinents became emergent and were subject to global cooling and glaciation.

Carbonate precipitation and oxygen stratification in late Archean seawater as deduced from facies and stratigraphy of the Gamohaan and Frisco formations, Transvaal Supergroup, South Africa

The correlative 2521+ or -3 Ma Gamohaan and Frisco formations, Transvaal Supergroup, South Africa, consist of peritidal and subtidal carbonate lithofacies that pass conformably upward into the deep subtidal Kuruman and Penge iron-formations. The stratigraphic setting and lithofacies transitions demonstrate that the Gamohaan and Frisco formations were deposited in open marine environments during a transgression that resulted in drowning of the underlying Campbellrand-Malmani carbonate platform. The Gamohaan and Frisco formations contain complex microbial structures associated with abundant sea floor-encrusting and void-filling calcite. In a 40 m section of the Gamohaan Formation, more than 35 percent of the rock consists of marine calcite that precipitated as crystals directly on the sea floor or in primary voids. Individual beds of precipitated, sea floor-encrusting calcite are up to 30 cm thick and are laterally continuous for the entire 7000 km 2 of good stratigraphic control. The abundance of precipitated carbonate and the lateral continuity of individual beds demonstrate that deep subtidal seawater was supersaturated with respect to calcite, that carbonate precipitation was controlled by regional seawater chemistry, and that in situ calcite precipitation directly on the sea floor was an important rock-forming process in late Archean oceans. Transitions from the Gamohaan Formation to basinal equivalents laterally and to the Kuruman Iron Formation vertically show a progressive change from precipitated calcite to shale to siderite-facies iron-formation mixed with oxide-facies iron-formation deposition. This facies succession can be attributed to an increase in [Fe (super 2+) ] with depth: As [Fe (super 2+) ] increases, calcite precipitation slows, and siderite becomes supersaturated resulting in a change from limestone to iron-formation accumulation. This gradient probably ranges from several 100 mu mol/1 Fe (super 2+) in deep seawater to a few mu mol/1 Fe (super 2+) in the mixed zone of the oceans. The presence of any Fe (super 2+) in the mixed layer of late Archean oceans requires low atmospheric [O 2 ].

Lithological and geochemical evidence of Fe and Mn pathways during deposition of Lower Proterozoic banded iron formation in the Krivoy Rog Basin (Ukraine)

Abstract Data on stratigraphy, chemical and mineral composition of siliciclastic and ferrugionus rocks of greenschist to epidote-amphibolite metamorphic grade, multi-order cyclicity and banding, isotopic composition of sulphur and carbon and other features of the Krivoy Rog Supergroup metasedimentary rocks are summarized. They provide an insight into the primary sources and the nature of siliciclastic and chemogenic components of banded iron formation (BIF), as well as on pathways of Fe, Mn and Si in the Krivoy Rog palaeobasin at the time of BIF accumulation. Skelevat and Saxagan iron-ore Groups were deposited in a single cyclic transgressive-regressive sequence, probably in a trough-like landlocked basin with complex bottom topography defined by syn-sedimentary faults inherited from the earlier rift structures. Variations in sea level, humidity of climate, continental run-off, oceanic water inflows etc. many times provoked the rapid facies changes ranging from sedimentation of land-derived siliciclastics to deposition of nearly pure chemogenic sediments of the oxide facies BIF in the deeper parts of the basin. Up to seven pairs of schist ( s -) and BIF ( f -) horizons were deposited in the Saxagan Group. Lateral variations of their thicknesses, distribution of free carbon in the rocks and other data indicate that only parts of the basin at a given time were optimal for chemical sedimentation; major part of dissolved Fe, Si and Mn was supplied by constant or event inflow of suboxic ocean water. The Mn content in the Krivoy Rog Supergroup BIFs could serve as an indicator of the fraction of total Fe that precipitated chemically as carbonate. The main part of Fe precipitation in form of Fe(III) hydroxide particles, possibly coated with amorphous silica, is due to a number of oxidation mechanisms operating in the photic surface water layer. On the basis of physicochemical considerations, it is concluded that the Mn CO 3 content in siderite ( c. 2 mol %) contained in the BIF rocks reflects a uniform dissolved Mn concentration ( c. 1 ppm) in the ocean and basin waters. Sedimentation of the typical association of Mg-Fe carbonates and silicates of the carbonate-silicate facies BIF could occur in the near-bottom water only in the case of strong enrichment in dissolved Fe(II) (up to 100–200 ppm) relative to the ocean water (1–10 ppm), which required a stratified brackish-water basin with an estuarine-type circulation. No enrichment in dissolved Mn(II) was possible until p O 2 in the atmosphere raised high enough for precipitation of particulate Mn oxide in the photic zone and its dissolution in the suboxic near-bottom waters.

Was the Deposition of Large Precambrian Iron Formations Linked to Major Marine Transgressions?

Geochemical evidence suggests the iron- and silica-rich precipitates that compose large Neoarchean and Paleoproterozoic iron formations formed along a chemocline between iron-rich deep water and iron-poor surface water. Sedimento-logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions. We suggest this occurred because the chemocline could only impinge on continental shelves during times of sea-level rise or highstand. On the other hand, the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgressions in others. To explain this, we propose that iron concentrations in the Precambrian ocean were greatest at mid-water depths in a zone on the order of 100 m thick. Radiometric age dates from the Hamersley Basin of Western Australia suggest major iron-formation deposition had a periodicity of ca. 20 to 125 m.y., close to second-order cycles in Phanerozoic sea level driven by long-te...