oolitic ironstones and contrasting ordovician and jurassic paleogeography distribution of abundant ordovician and jurassic oolitic ironstones provides a test for current ideas about factors controlling their origin these include warm climate and deep weathering dispersed continents and highstand of global sea level most of the jurassic ironstones developed in middle northern latitudes on the unstable european part of assembled laurasia at a time of low global sea level mild moist climate and abundant vegetation most of the ordovician ironstones accumulated around the northwestern margin of assembled gondwana and armorica during highstand of sea level and an absence of land plants and in high southern latitudes that supported an ice cap by the end of the period these contrasts demonstrate that neither dispersed continents nor major highstand of sea level was a necessary factor moreover requisite weathering in ordovician time may have been induced by adequate soil air co 2 pressure maintained by elevated atmospheric p co 2 in tne absence of land plants

facies analysis and palaeoclimatic significance of ironstones formed during the eocene greenhouse lower and middle eocene ironstone sequences of the naqb and qazzun formations from the north east bahariya depression western desert egypt represent a proxy for early palaeogene climate and sea level changes these sequences represent the only palaeogene economic ooidal ironstone record of the southern tethys these ironstone sequences rest unconformably on three structurally controlled cenomanian palaeohighs for example the gedida harra and ghorabi mines and formed on the inner ramp of a carbonate platform these palaeohighs were exposed and subjected to subaerial lateritic weathering from the cenomanian to early eocene the lower and middle eocene ironstone sequences consist of quiet water ironstone facies overlain by higher energy ironstone facies the distribution of low energy ironstone facies is controlled by depositional relief these deposits consist of lagoonal burrow mottled mud ironstone and laterally equivalent tidal flat stromatolitic ironstones the agitated water ironstone facies consist of shallow subtidal intertidal nummulitic ooidal oncoidal and back barrier storm generated fossiliferous ironstones the formation of these marginal marine sequences was associated with major marine transgressive regressive megacycles that separated by subaerial exposure and lateritic weathering the formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features such as colloform texture and alveolar voids implies periods of humid and warm climate followed major marine regressions the formation of the lower to middle eocene ironstone succession and the associated lateritic palaeosols can be linked to the early palaeogene global warming and eustatic sea level changes the reworking of the middle eocene palaeosol and the deposition of the upper eocene phosphate rich glauconitic sandstones of the overlying hamra formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions

origin of ooidal ironstones in relation to warming events cretaceous eocene bakchar deposit south east western siberia abstract an integrated study involving petrographical mineralogical and geochemical investigations proposes the origin of the bakchar ironstone deposit by upward migration of a mixture of fe rich brine and hydrothermal fluid through marine sediments trace element concentrations of the ironstone deposits rule out continental derivation of fe while high ree concentrations indicate mixed hydrogeneous and hydrothermal source three main types of ironstone within the bakchar deposit are characterized by distinct mineralogical assemblage of authigenic minerals corresponding to oxic methanic and weakly sulfidic dysoxic conditions changing redox conditions of the depositional environment methane seepage in warm seawater and availability of iron influx possibly determined the final mineralogy of three varieties of ironstones the association of siderite with ferrimagnetic sulfides like greigite and pyrrhotite in type 1 ore indicate anaerobic oxidation of methane with limited bacterial sulfate reduction the ironstone deposits of narym kolpashevo and bakchar are contemporaneous with geological events like ocean anoxic event 3 oae 3 cretaceous paleogene k pg boundary and palaeocene eocene thermal maximum petm respectively all corresponding to warm seawater conditions

discriminating between different genetic types of marine ferro manganese crusts and nodules based on rare earth elements and yttrium marine ferro manganese fe mn crusts and nodules are metal rich chemical sediments that are archives of paleoceanographic proxies and potential metal resources and targets of deep sea mining traditionally crusts and nodules are subdivided into hydrogenetic diagenetic and hydrothermal types because these are characterized by different compositions and hence economic resource potential a useable genetic classification is not only a tool for geochemical studies but may also help to characterize potential exploration targets we propose two easy to use yet robust discrimination diagrams based on geochemical relationships controlling the rare earths and yttrium rey inventory of marine fe mn oxyhydr oxide deposits the rey are a set of trace elements that show coherent behavior are routinely determined in geochemical studies and for which the analytical quality can easily be evaluated graphs of ce anomaly vs nd concentration and ce anomaly vs y anomaly effectively discriminate between the different types of precipitates regardless of their diverse mineralogical composition both hydrogenetic crusts and nodules show positive ce anomalies negative y anomalies and high nd concentrations of 100 mg kg 1 although nodules tend to have slightly lower nd concentrations than crusts in marked contrast hydrothermal deposits generally yield negative ce anomalies positive y anomalies and low nd concentrations of 10 mg kg 1 in spite of their large geochemical mineralogical and sedimentological diversity diagenetic nodules show negative ce anomalies negative y anomalies and intermediate nd concentrations between 10 and 100 mg kg 1 we discuss the geochemical background of these diagrams the respective sources of rey in the different precipitates and address the processes that control their rey inventory besides the three end members we also use rey systematics to define mixed type diagenetic hydrogenetic nodules and classify well studied international reference standards

the remarkable longevity of submarine plumes implications for the hydrothermal input of iron to the deep ocean the longevity of submarine plumes generated at sea floor hydrothermal systems constrains the hydrothermal input of chemical species into the deep ocean decades of observations of episodic event plumes suggest that a key process governing the dynamics of an hydrothermal cloud spreading out laterally from a buoyant rising plume is the production of internal layering here we use analog experiments on turbulent hot particle laden plumes and clouds to show that this layering occurs where particle diffusive convection driven by the differential diffusion of heat and small mineral precipitates gives rise to a large scale double diffusive instability where hydrothermal clouds are enriched in fine minerals this particle diffusive convection can extend the longevity of an event plume to 2 yr after its emplacement the very long residence time imposed by diffusive convective effects enables complete dissolution of fine sulfide and sulfate minerals we develop a new theoretical model that includes both sedimentation and dissolution processes to quantify the potential amount of iron produced by the dissolution of iron sulfide minerals settling through the cloud by diffusive convection a key prediction is that the concentration of dissolved iron in hydrothermal clouds can reach up to 19 3 nm19 3 nm which represents about 5 of the global hydrothermal discharge if these results are representative of all hydrothermal vent fields hydrothermal systems could provide 75 of the global budget of dissolved iron in the deep ocean regionally this flux is expected to scale in magnitude with mid ocean ridge heat flow consistent with observations and global ocean models

magnetic properties of oolitic iron ore on bell island newfoundland abstract magnetic properties of samples from bell island sedimentary rocks have been studied x ray analysis indicates that the main magnetic mineral is hematite in all samples the other iron bearing minerals identified are siderite and chamosite microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite the intensity of natural remanent magnetization nrm varies in the range of 0 03 0 4 a m 1 depending on the percentage of hematite the thermal demagnetization curves of nrm show in some cases a sharp increase in magnetization at temperatures in the range 500 600 c the peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating x ray analysis confirmed that the newly formed material is magnetite since the original nrm has been masked by the new intergrown material this would result in a serious error in the determination of paleomagnetic pole positions the samples showing this behaviour were not considered for paleomagnetic study the samples containing oolitic hematite in a calcite matrix exhibit very high stability of nrm including directional stability until almost 670 c for these samples a virtual pole position based on n 6 samples 32 specimens demagnetized to 665 c is 34 n 114 e not far from published ordovician poles for the north american craton

ironstone deposits hosted in eocene carbonates from bahariya egypt new perspective on cherty ironstone occurrences abstract this paper gives new insight into the genesis of cherty ironstone deposits the research was centered on well exposed unique cherty ironstone mineralization associated with eocene carbonates from the northern part of the bahariya depression egypt the economically important ironstones occur in the naqb formation early eocene which is mainly formed of shallow marine carbonate deposits periods of lowstand sea level caused extensive early dissolution karstification of the depositional carbonates and dolomitization associated with mixing zones of fresh and marine pore water in faulted areas the eocene carbonate deposits were transformed into cherty ironstone with preservation of the precursor carbonate sedimentary features i e skeletal and non skeletal grain types thickness bedding lateral and vertical sequential arrangement and karst profiles the ore deposits are composed of iron oxyhydroxides mainly hematite and goethite chert in the form of micro to macro quartz and chalcedony various manganese minerals barite and a number of subordinate sulfate and clay minerals detailed petrographic analysis shows that quartz and iron oxides were coetaneous and selectively replaced carbonates the coarse dolomite crystals having been preferentially transformed into quartz whereas the micro crystalline carbonates were replaced by the iron oxyhydroxides a number of petrographic sedimentological and structural features including the presence of hydrothermal mediated minerals e g jacobsite the geochemistry of the ore minerals as well as the structure controlled location of the mineralization suggest a hydrothermal source for the ore bearing fluids circulating through major faults and reflect their proximity to centers of magmatism the proposed formation model can contribute to better understanding of the genetic mechanisms of formation of banded iron formations bifs that were abundant during the precambrian

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

geochemistry and depositional environment of mn oxide deposits in the tokoro belt northeastern hokkaido japan numerous early cretaceous mn oxide and mn fe deposits are distributed in the tokoro belt northeastern hokkaido japan the mn oxide deposits usually occur in the southwestern part of the belt and within bedded cherts whereas the mn fe deposits are restricted to the northeastern part of the belt and occur between bedded cherts and underlying basaltic pillow lava the mn oxide ores are characterized by low fe mn ratios and the trace element concentrations and rare earth element ree patterns are similar to those of the submarine hydrothermal mn deposits distribution patterns of trace elements co ni cu and zn and ree in the mn oxide ores determine two types of deposits hydrogenous deposits which have relatively high concentrations and submarine hydrothermal deposits which have low concentrations these compositional trends and geologic evidences of the tokoro mn deposits suggest that the fe phases precipitated first closer to their source whereas the mn phases deposited later at a distance from their source some hydrogenous effects on the mn oxide deposits are due to the adsorption of trace elements from seawater trace element concentrations of tokoro mn oxide deposits show that they formed within 40 km from their source

mineralogical and geochemical investigations of the middle eocene ironstones el bahariya depression western desert egypt abstract the middle eocene ironstone succession is located in the northeastern part of el bahariya depression western desert egypt this succession is subdivided into lower and upper sequences and consists of two main shallow marine ironstone facies associations the first is a lagoonal manganiferous mud and fossiliferous ironstone facies association and consists mainly of goethite and hematite detrital minerals quartz rutile and feldspars manganese minerals todorokite psilomelane pyrolusite birnessite aurorite and manjiroite and authigenic clay minerals kaolinite and illite the second is a peritidal microbially mediated stromatolitic and nummulitic ooidal oncoidal ironstone facies association consists of goethite apatite and secondary minerals that include quartz jarosite psilomelane and pyrolusite organic materials such as proteinaceous compounds lipids cellulose and carotenoids were detected in the cortices of the ferruginous ooids and oncoids the marine ironstone facies were exposed to subaerial weathering and subsurface alteration processes the weathering resulted in the formation of lateritic iron ores and paleosols during humid climatic periods the lateritic iron ores consist mainly of colloform goethite hematite and psilomelane the identification of cavity filling sulfate nitrate carbonate and silicate minerals in the marine ironstones and the lateritic iron ore may indicate more recent alteration under arid climatic conditions the subsurface alteration is attributed to the oxidation of sulfides primarily pyrite and weathering of glauconitic clastic rocks in the underlying cenomanian bahariya formation during the interaction with acidic heated groundwater the formation of ferrous and ferric sulfate and silicate minerals and mobilization of trace metals are the products of the alteration process enrichments in ba co k pb and sr are correlated with manganese oxides whereas anomalous p v cr ni zn as mo and u are correlated with iron oxyhydroxides

review of cenozoic ooidal ironstones abstract cenozoic tertiary ooidal ironstones coi in 20 districts 39 deposits developed between the equatorial zone and 60 n except for one eocene district in mid southern latitude stratigraphic distribution paleocene oi occur in northern pakistan western siberia southern germany northwestern venezuela and northeastern colombia eocene oi in western siberia southern germany northwestern romania central north africa central west saudi arabia southwestern central africa northwestern venezuela northeastern colombia south central usa and northwestern australia oligocene oi in northwest and west central kazakhstan central denmark and north central iran miocene oi in northwestern venezuela northeastern colombia and southeastern malaysia pliocene oi in southeastern ukraine geotectonic framework ten districts developed in an interior or fractured craton five along the south trending uralian seaway and one along the east trending northern european seaway the south trending trans saharan seaway in or near an early red sea embayment in southeastern malaysia and in northwestern australia ten districts lay near a eratonic margin one along divergent margin and nine along the broad east trending caribbean and tethyan seaways almost all coi accumulated during the paleogene relatively high stand of sea level especially in early and middle eocene time as sea level fell gradually in neogene time coi developed in only three marginal districts including the giant pliocene kerch taman deposit in southeastern ukraine sedimentary environment almost all of the coi developed in deltaic to shallow marine facies these are commonly associated with shoaling upward siliciclastic sequences a few are in mixed siliciclastic carbonate sequences a few coi apparently occurred in fluvial and lacustrine facies some of these may have been reworked from laterite or from marine ironstones sedimentary petrology many coi are less than a few tens of centimeters thick and sporadic the ooids are entirely or predominantly ferric oxide a few contain a minor amount of berthierine 7a only the oldest one northern pakistan contains chamosite 14a more than half of the coi are associated directly or more remotely with glauconite nearly all the coi need much more rigorous examination for both their individual composition and their comparison with older ooidal ironstones

genesis of the precambrian copper rich caraiba hypersthenite norite complex brazil caraiba the largest brazilian copper deposit under exploitation consists mostly of disseminated and remobilised bornite and chalcopyrite hosted in early proterozoic norite and hypersthenite the mafic igneous complex comprises multiple intrusions of dykes veins and breccias of norites and hypersthenites with minor proportions of amphibolised gabbronorite and peridotite xenoliths transported by the magma from deeper levels in the lithosphere the country rocks are high grade gneisses granulites and metasediments compositions of plagioclase an60 40 and orthopyroxene en70 60 fall in a narrow range similar to the koperberg suite from the okiep copper district south africa and to that in many massif type anorthosites whole rock major and trace element geochemistry indicate a parental magma enriched in fe lree p k and cu negative nb anomalies on multi element plots and fractionated ree patterns along with sulphide sulphur isotopes in the range 34s 1 495 to 0 643 suggest a primary mantle lithosphere source although a lower crustal source for the gabbronorite and peridotite xenoliths cannot be excluded geochronological and field evidence indicate that both norite and hypersthenite are likely to have been emplaced during a major sinistral transcurrent partly transpressional shearing event associated with the waning stage of evolution of the early proterozoic salvador curaca orogen

a depositional model for the liassic minette ironstones luxemburg and france in comparison with other phanerozoic oolitic ironstones

a transient rise in tropical sea surface temperature during the paleocene eocene thermal maximum the paleocene eocene thermal maximum petm has been attributed to a rapid rise in greenhouse gas levels if so warming should have occurred at all latitudes although amplified toward the poles existing records reveal an increase in high latitude sea surface temperatures ssts 8 to 10 c and in bottom water temperatures 4 to 5 c to date however the character of the tropical sst response during this event remains unconstrained here we address this deficiency by using paired oxygen isotope and minor element magnesium calcium ratios of planktonic foraminifera from a tropical pacific core to estimate changes in sst using mixed layer foraminifera we found that the combined proxies imply a 4 to 5 c rise in pacific sst during the petm these results would necessitate a rise in atmospheric pco 2 to levels three to four times as high as those estimated for the late paleocene

pyrite framboids in the permian triassic boundary section at meishan china evidence for dysoxic deposition pyrite framboid evidence from the permian triassic global stratotype at meishan china indicates the late permian mass extinction is closely associated with a dramatic decline of benthic oxygen levels sizes and abundances of the framboids were determined from acid residues and in situ on polished sections of the samples the framboids are abundant in the thin pyrite lamina on the top of bed 24e the upper layer of bed 24 and beds 25 26 and 29 rare in bed 24e and bed 28 but absent in bed 27 they have narrow size distributions with average diameters from 4 6 mu m to 8 7 mu m which are similar to those formed under anoxic dysoxic conditions the abundances and mean diameters of the pyrite framboids indicate redox conditions of deposition changing from upper dysoxia bed 24e and lower dysoxia pyrite lamina beds 25 and 26 to oxygenation bed 27 and again to upper dysoxia bed 28 and lower dysoxia bed 29 our discovery confirms a worldwide dysoxic marine event during the permian triassic transition suggestive of a direct relationship with the permian triassic marine mass extinction c 2007 elsevier b v all rights reserved

debate about ironstone has solute supply been surficial weathering hydrothermal convection or exhalation of deep fluids ironstone is any chemical sedimentary rock with 15 fe an iron formation is a stratigraphic unit which is composed largely of ironstone the solutes which have precipitated to become ironstone have dissolved from the earth s surface from the upper crust e g the basaltic layer of oceanic crust or from deeper within the earth genetic modellers generally choose between surficial weathering e g soil formation and hydrothermal fluids which have convected through the upper kilometre of oceanic crust most genetic modellers attribute cherty laminated iron formations to hydrothermal convection and noncherty oolitic iron formations to surficial weathering however both types of iron formations are attributable to the exhalation of fluids from a source region too deep for convection of seawater evidence for a deep source of ferriferous fluids comes from a comparison of ancient ironstone with modern ferriferous sediment in coastal venezuela a deep source origin for ironstone has wide ranging implications for the origins of other chemical sedimentary ores e g phosphorite manganostone bedded magnesite sedimentary uranium ore various karst filling ores and even petroleum preliminary study of a modern oolitic iron deposit described herein suggests that the source of iron and silica to iron formations may have been even deeper than envisioned within most hydrothermal convection models

chamosite siderite and the environmental conditions of their formation in chamosite type phanerozoic ooidal ironstones abstract two characteristic ironstone minerals occurring in chamosite type ironstone deposits were investigated these are the minerals of the chlorite and the carbonate series the samples are from 31 localities and 8 countries czech republic france germany united kingdom egypt nigeria and usa these deposits are identical to those presented by mucke and farshad mucke a farshad f in press whole rock and mineralogical composition of phanerozoic ooidal ironstones comparison and differentiation of subtypes ore geology reviews with ordovician to late cretaceous ages electron microprobe analyses of 197 carbonates and 64 chlorites were carried out the analytical points of the carbonates in a magnesite calcite siderite rhodochrosite 100 mol diagram are distributed within a limited area close to the siderite end member the area can be subdivided into two fields 1 the carbonates of the ordovician ironstones of the welsh prague and thuringian basins have high siderite concentrations 87 1 to 90 6 mol magnesite contents from 6 7 to 8 3 mol and calcite concentrations from 1 6 to 2 4 mol in the ordovician ironstone of krusna hora prague basin the siderite concentrations are lower 78 3 mol and those of magnesite 15 6 mol and calcite 5 7 mol are higher 2 the carbonates of the other ironstones jurassic to late cretaceous have lower siderite 72 5 to 79 mol and higher magnesite 12 6 to 15 7 mol and calcite 6 9 to 12 2 mol contents the average values of the rhodochrosite end member lie in the range between 0 13 and 2 31 mol and the siderite rhodochrosite ratio 158 on average the analytical points of the chlorite analyses are presented in the fetot mg alvi 100 atom diagram one field containing exclusively chlorites with green internal reflections can be subdivided into two areas one area contains the chlorites of the ordovician ironstones consisting of 62 8 to 77 7 atom fetot 17 5 to 30 9 atom alvi and 4 1 to 7 4 atom mg and the other those of the jurassic to late cretaceous ironstones consisting of 55 7 to 62 5 atom fetot 26 to 33 8 atom alvi and 9 to 13 atom mg the second field contains altered chlorites of ferruginized ironstones aswan and red mountain formation these chlorites present brown internal reflections and have compositions 34 9 to 46 7 atom fetot 24 8 to 42 2 atom alvi and 16 0 to 28 5 atom mg that do not reflect the original diagenetic environment within the al al fe mg mg mg fe atom diagram the chlorite analyses are concentrated in a limited field which is distinctly separated from chlorites formed in other environments the occurrence of carbonate and chlorite both with slightly varying compositions dominated by their iron end members which occur characteristically in association with framboidal pyrite and organic matter are indications that three prerequisites for the formation of the ironstones are comparable in narrow limits these are 1 the composition of the protoliths 2 the depositional environment of the protoliths and their rates of deposition and 3 the environmental conditions that occurred during the lithification of the protoliths due to submarine diagenesis the protoliths consist of mixtures of kaolinite fe3 oxide hydroxide and organic matter the depositional environments are marine basins in which fully marine conditions occur and thus the availability of mg and ca not contained in the protoliths exists the rates of deposition control the amount of mg in the chloritess and that of mg and ca in the carbonatess and the environmental conditions during submarine diagenesis depend on the availability of organic matter creating reducing conditions due to bacterial oxidation the resulting conditions are reducing and depending on the amount of organic matter pass successively from post oxic formation of chamosite and authigenic rice grain shaped siderite via sulphidic formation of framboidal pyrite into methanic formation of abundant xenomorphic siderite that replaces chamosite

modern and palaeozoic iron ooids a similar volcanic origin abstract the discovery of an unconsolidated deposit of iron ooids of a kind not previously described and in the vicinity of a volcanic island in indonesia has shed new light on the genesis of many ancient iron oolites the discovery shows that iron ooid formation is geologically a rapid process the ooids form by chemical precipitation of cryptocrystalline iron oxyhydroxides on available grains on the seafloor from seawater enriched with fe al and si the enrichment can be a result of hydrothermal fluids volcanic ash falls into shallow basins or rapid weathering of fresh volcanic rocks ordovician iron ooids from northern europe are strikingly similar to the modern ooids in chemical composition ree distributions and internal structures the differences in mineralogy can be explained as diagenetic transformation reactions under different environments a model for the formation of the ordovician iron ooids in northern europe from volcaniclastic material is presented which could be applicable to other ancient oolites in the world

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

phanerozoic ironstones an introduction and review s u m m a r y ironstones particularly ooidal ironstones have long fascinated sedimentary geologists and have generated an enormous variety of interpretations but have remained poorly understood a recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences the first part of this introduction examines recent advances and attempts to produce a synthesis of them with particular reference to the genesis of marine ooidal ironstones current models for the formation of marine oolitic ironstone formations are discussed and a working model based on various lines of recent research is proposed this model invokes the intrasedimentary formation of berthierine ooids in marine environments during postoxic diagenesis the ooids may be mechanically modified and mineralogically transformed by reworking as well as by later diagenesis goethite ooids may be oxidized berthierine ooids reworked lateritic ooids or primary grains the formation of oolitic ironstoneformations is favoured by a break in clastic sediment supply continued supply of iron and physical reworking many sedimentary environments may supply these parameters but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply in the second part of this introduction the terminology and classification are discussed and conventions proposed the term berthieroid is introduced as a non specific term for material of undetermined berthierine or chamosite composition the term ironstone is proposed as a petrological term and as an informal lithostratigraphic term a deposit composed of ironstones may be termed an ironstone formation it is recommended that a dunham style of classification is employed petrographically with ironstone replacing the stone of the original carbonate classification the nomenclature of allochems in ironstones is also discussed a glossary of ironstone terminology is provided

microbial structures in oolitic iron formations ferriferous ooids and microoncoids occur in many sedimentary iron formations these ferriferous coated grains are found in fine grained carbonate rich groundmasses iron mineral encrusted microbiota are observed in both the coated grains and the groundmass the branching nature and other morphological features of the microorganisms suggest fungal origins for the oolitic iron ores particularly of the lower jurassic lorraine minette the similarity of the microbial structures in coated grains and their groundmasses suggests that both had developed within microbial mats growing under calm environmental conditions the contribution of stromatolitic marine fungal mats to the fast extraction and immobilization of iron and thus to the genesis of iron ores is demonstrated observations on well preserved freshwater derived fungal stromatolites of the tertiary and laboratory experiments with organotrophic fungal mats confirm the findings on the jurassic and imply a general role of microorganisms in the formation of such deposits

modern iron ooids from a shallow marine volcanic setting mahengetang indonesia an unconsolidated submarine deposit of iron ooids and pisoids located offshore the volcanic island mahengetang indonesia is described herein the deposit occurs in a shallow marine setting in an area characterized by venting of hydrothermal fluids and expulsion of gas across the sediment water interface the ooid deposit is essentially pure to a minimum depth of 0 5 m and covers an area of at least 1000 m 2 ooids are composed of concentric accretionary layers of limonite admixed with amorphous silica precipitated around andesitic rock fragments the smallest layers forming the ooids are about 10 min thickness the iron rich precipitate is 50 iron by weight ooid formation is likely promoted by precipitation of iron and silica from exhalative fluids rising up through the substrate concentric layering is thought to result from constant agitation of ooids associated with currents and expulsion of gas from the sediment