geochemistry and zircon geochronology of the permian a type hasanrobat granite sanandaj sirjan belt a new record of the gondwana break up in iran the sanandaj sirjan metamorphic plutonic belt ssb in west central iran is a polyphase metamorphic terrain composed of dominantly greenschist grade metasedimentary and metavolcanic rocks and felsic to mafic plutons of neoproterozoic phanerozoic ages the hasanrobat granite in central ssb occurs as a single pluton 20 km 2 surface area with relatively consistent mineralogy and chemistry quartz alkali feldspars microcline and perthite sodic plagioclases and biotite are the main constituents commonly associated with minor amphibole accessory phases include zircon allanite apatite and magnetite the country rocks are upper carboniferous lower permian sandstones and dolomitic limestones scattered patches of skarn type assemblages dominated by tremolite and talc occur in the dolomitic limestones and sandstones are recrystallized to a coarse grained quartzite at contact with the granite the granite is metaluminous to slightly peraluminous and is distinguished by high feo t mgo ratios typical of ferroan a type granites the a type affinity is also reflected by high na 2o k 2o high ga al ratios high contents of large ion lithophile elements lile high field strength elements hfse and rare earth elements ree as well as low contents of sr and distinct negative eu anomalies the biotites are aluminous fe rich and plot near the siderophyllite corner in the quadrilateral biotite diagram they are further distinguished by high fluorine contents 0 61 to 1 33 wt amphibole is ferrohastingsite in composition ion microprobe analyses of zircon grains separated from a representative granite sample yielded concordant u pb ages with weighted mean 206 pb 238 u age of 288 3 3 6 ma the granite and the country rocks are cut by a set of mafic dykes with asthenosphere like geochemical signatures such association suggests anorogenic intraplate magmatism in lower permian in the region this provides further evidence for and significantly constrains timing of a major extension in upper paleozoic in iran previously inferred from the rock record the extension led to the gondwana break up and the opening of neotethys ocean between sanandaj sirjan and zagros in permian

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

petrogenesis of silicic magmatism related to the 2 44 ga rifting of archean crust in koillismaa eastern finland abstract early paleoproterozoic extension in the archean craton of the fennoscandian shield led to the emplacement of several 2 44 ga layered gabbroic intrusive complexes in northern finland and adjacent russia closely associated with them are felsic rocks of similar age 1 the sirnio group volcanic rocks on top of the koillismaa layered complex 2 a quartz alkali feldspar syenite at kynsijarvi near the koillismaa complex and 3 an aluminous a type granite at nuorunen near the oulanka layered complex in the koillismaa area the ruptured archean crust consists of ortho and paragneisses that were intruded and migmatized by somewhat younger granites u pb zircon data indicate that the gneisses are at least 2 8 ga old and that the granites were crystallized at 2 7 ga both rock types show a common monazite age of 2695 ma that registers the peak of granulite facies metamorphism and possibly the intrusion of the granites the local neoarchean crust has nd at 2440 ma values between 5 and 8 5 the mafic rocks of the koillismaa complex show initial nd at 2440 ma values around 1 5 and those of the oulanka complex range from 2 1 to 0 the nd value 4 8 and t dm model age 2 9 3 0 ga of the kynsijarvi quartz alkali feldspar syenite are within the limits of the evolution path of the local archean crust the corresponding values for the nuorunen granite are 2 0 and 2 76 ga and are thus closer to those of the mafic rocks the volcanic rocks of the sirnio group show more scatter with initial nd at 2440 ma values of 1 1 to 5 3 the lowest nd values probably reflect later disturbance magmatic values cluster around 2 major and trace element modeling shows that fractional crystallization of the koillismaa complex parental magma or partial melting of the archean crust cannot account for the 2 44 ga silicic rocks of koillismaa the geochemical and nd isotope characteristics of the volcanic rocks and the kynsijarvi quartz alkali feldspar syenite are best explained by a combined assimilation fractional crystallization model involving a contaminated komatiitic parental magma and intermediate mafic lower crustal assimilant differences in the initial nd isotope composition of the 2 44 ga rocks reflect varying source components and evolutionary history for the archean lithosphere in eastern fennoscandia

archean rapakivi granite anorthosite rhyolite complex in the witwatersrand basin hinterland southern africa the gaborone granite suite and the kanye formation formed during a single magmatic event in the late archean evolution of the kaapvaal craton southern africa and may be the oldest rapakivi granite anorthosite rhyolite suite in the world the gaborone granite suite underlies an area of 6000 km2 in the northwestern part of the craton and comprises a type rapakivi granite leucogranite granophyric microgranite and minor anorthosite it is partly surrounded by the kanye formation a 1000 m thick pile of pyroclastic and flow banded rhyolitic lava precise u pb dating of granitic and granophyric components of the gaborone suite and rhyolite of the kanye formation shows that all three rock types are the same age within error 2783 2785 ma emplaced in the source area during upper witwatersrand supergroup sedimentation the gaborone kanye event has important implications for the tectonic and magmatic evolution of the northern kaapvaal craton and possibly also played an important role in the development of the adjacent auriferous sediments

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

mineralogy geochemistry and the origin of high phosphorus oolitic iron ores of aswan egypt abstract the coniacian santonian high phosphorus oolitic iron ore at aswan area is one of the major iron ore deposits in egypt however there are no reports on its geochemistry which includes trace and rare earth elements evaluation texture mineralogy and origin of phosphorus that represents the main impurity in these ore deposits have not been discussed in previous studies in this investigation iron ores from three localities were subjected to petrographic mineralogical and geochemical analyses the aswan oolitic iron ores consist of uniform size ooids with snowball like texture and tangentially arranged laminae of hematite and chamosite the ores also possess detrital quartz apatite and fine grained ferruginous chamosite groundmass in addition to fe 2 o 3 the studied iron ores show relatively high contents of sio 2 and al 2 o 3 due to the abundance of quartz and chamosite p 2 o 5 ranges from 0 3 to 3 4 wt showing strong positive correlation with cao and suggesting the occurrence of p mainly as apatite x ray diffraction analysis confirmed the occurrence of this apatite as hydroxyapatite under the optical microscope and scanning electron microscope hydroxyapatite occurred as massive and structureless grains of undefined outlines and variable size 5 150 m inside the ooids and or in the ferruginous groundmass among trace elements v ba sr co zr y ni zn and cu occurred in relatively high concentrations 62 240 ppm in comparison to other trace elements most of these trace elements exhibit positive correlations with sio 2 al 2 o 3 and tio 2 suggesting their occurrence in the detrital fraction which includes the clay minerals ree ranges between 129 5 and 617 ppm with strong positive correlations with p 2 o 5 indicating the occurrence of ree in the apatite chondrite normalized ree patterns showed lree enrichment over hree la yb n 2 3 5 4 and negative eu anomalies eu eu 0 75 0 89 the oolitic texture of the studied ores forms as direct precipitation of iron rich minerals from sea water in open space near the sediment water interface by accretion of feo sio 2 and al 2 o 3 around suspended solid particles such as quartz and parts of broken ooliths the fairly uniform size of the ooids reflects sorting due to the current action the geochemistry of major and trace elements in the ores reflects their hydrogenous origin the oolitic iron ores of the timsha formation represent a transgressive phase of the tethys into southern egypt during the coniacian santonian between the non marine turonian abu agag and santonian campanian um barmil formations the abundance of detrital quartz positive correlations between trace elements and tio 2 and al 2 o 3 and the abundance mudstone intervals within the iron ores supports the detrital source of fe this prediction is due to the weathering of adjacent land masses from cambrian to late cretaceous the texture of the apatite and the ree patterns which occurs entirely in the apatite exhibits a pattern similar to those in the granite thus suggesting a detrital origin of the hydroxyapatite that was probably derived from the precambrian igneous rocks determining the mode of occurrence and grain size of hydroxyapatite assists in the maximum utilization of both physical and biological separation of apatite from the aswan iron ores and hence encourages the use of these ores as raw materials in the iron making industry

geochemistry of oolitic iron ores an electron microprobe study oolitic iron ores are made up of silicate oxide and carbonate minerals that are part of solid solution series the compositions of the minerals can depart from pure end members and show variation among deposits this variability can be quantified using electron microprobe techniques iron silicates are represented by chamositic clays both berthierine and chamosite glauconite was not seen in any of these samples berthierine predominates in younger deposits whereas the paleozoic ores studied contained only chamosite compositions of the chamositic clays however were independent of age iron oxides are mostly goethite in younger deposits or hematite in older deposits both minerals lack appreciable al in solid solution most analyses show small amounts of al but usually balanced by si in the same proportion as in chamosite indicating a slight admixture of clay iron carbonates are siderite with appreciable substitution of ca and mg but not of mn apatite is common but no iron phosphates were found the compositions of the silicates and carbonates are close to those reported for early diagenetic minerals from other types of sedimentary rocks indicating that ironstone formation does not require unusual chemical conditions other than a large supply of iron the analytical results can be used to argue against several hypotheses of ironstone genesis the lack of al in the goethitic and hematitic ooliths is inconsistent with an origin either by redeposition of soil ooliths or by oxidation and leaching of earlier chamositic ooliths the presence of high al contents in the chamositic ooliths shows that they cannot have formed from earlier aragonitic ooliths and the presence of small amounts of chamositic clay and apatite as discrete laminae in goethitic and hematitic ooliths suggests that this type of oolith could also not have formed by replacement of aragonite times of ironstone deposition show a correlation with climate and sea level stands that suggests a mechanism for generating locally high iron concentrations in seawater a combination of a warm humid climate and low average continental relief would intensify lateritic weathering and the formation of iron enriched detritus coincidence of this increased detrital iron supply with intensification and shallowing of the oxygen minimum layer in the ocean could lead to mobilization of iron from offshore shelf clastics to be redeposited near the shoreline in intertidal to shallow subtidal environments

synsedimentary formation of ooidal ironstone an example from the jurassic deposits of se central iran abstract the study of the upper lower to middle jurassic badamu formation in se of central iran represents the first report of jurassic ooidal ironstone in iran and exhibits a record of ferruginous radial ooid dominated ironstone the studies showed that this chamosite or ferric oxide chamosite type ooidal ironstone is similar to minette type ironstones a shoreface zone in an epeiric or epicontinental sea is proposed for formation and accumulation of the ferruginous ooids and the ooidal ironstone fe derived from lateritic weathering has been transported into the shoreface zone via a fluvial current during the synsedimentary formation of the ooidal ironstone during the periods with moderate to lower energy or weakly agitated and quiet environmental conditions suspended ferruginous small radial ooids and fe were increased in the lower shoreface subzone higher environmental energy resulted in the development of large concentric ooids and limited distribution of fe in the upper shoreface subzone higher environmental energy also prevented the development of radial ooids and restricted fe precipitation on carbonate ooids variations in environmental energy in the middle shoreface subzone were unfavorable for increasing ferruginous ooids and fe in the environment although short term periods with weakly agitated and quiet or moderate to lower energy conditions could had been existed in the middle shoreface subzone but precipitation of fe on a few small radial ooids was allowed environmental energy in the shoreface zone was the major factor controlling precipitation of fe especially suspended river fe loads on the ooids however precipitation of dissolved fe could have been related to oxidizing conditions in the environment also the probable presence of fe oxidizing bacteria in the cortexes of the ferruginous small radial ooids indicates that bacteria could have possibly contributed to precipitation of fe on the ooids in a calm environment

magnetic anisotropy of the redencao granite eastern amazonian craton brazil implications for the emplacement of a type plutons abstract a magnetic fabric study was performed on the redencao pluton in an attempt to understand its emplacement history the redencao pluton is part of the 1 88 ga anorogenic a type jamon suite that intruded 2 97 2 86 ga old archean granitoids of the rio maria granite greenstone terrane in the eastern amazonian craton northern brazil previous gravity survey indicates that the pluton is a 6 km thick tabular intrusion it is characterized by a concentric distribution of facies with rings of seriated and porphyritic granite that cut across the main facies of even grained monzogranites the whole set is intruded by leucogranites that occupy the center of the pluton petrographic examination magnetic susceptibilities coercivity spectra and thermomagnetic curves indicate that the magnetic fabric is primarily carried by coarse grained multidomain magnetite this is reinforced by the coincidence of magnetic susceptibility and remanence anisotropy principal axes the absence of solid state deformation features and the low anisotropy degrees indicate that the magnetic fabric is magmatic in origin the magnetic fabric displays a systematic pattern with all facies including the rings of porphyritic granite being characterized by concentric gently dipping foliations associated with gently plunging lineations only the central leucogranitic facies shows a slightly discordant pattern with steeply dipping fabrics at its northeastern sector an emplacement model by vertical stacking of successive magma batches is proposed for the construction of the redencao pluton which reconciles the tabular shape of the intrusion the petrographic and geochemical zoning and the magnetic fabric pattern initially two magma batches were emplaced as sills first the even grained monzogranite then the seriated and porphyritic granites which formed by mingling of a leucogranitic melt with the host biotite monzogranitic magma as attested by geochemical data and field evidence the final shape of the pluton was acquired after the intrusion and inflation of the central leucogranite giving raise to the concentric pattern of facies in map view

ferrimagnetic iron sulfide formation and methane venting across the paleocene eocene thermal maximum in shallow marine sediments ancient west siberian sea

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

ti in zircon thermometry applied to contrasting archean metamorphic and igneous systems abstract ti in zircon thermometry with shrimp ii multi collector has been applied to two well documented archean igneous and metamorphic samples from southern west greenland zircons from 2 71 ga partial melt segregation g03 38 formed in a small 1000 c yield a low temperature focused normal distribution with a mean of 683 32 c further demonstrated by high resolution ti mapping of two individual grains this distribution is interpreted to represent the temperature of the residual magma at zircon saturation late in the crystallization history of the tonalite hypothetically ti in zircon thermometry on eoarchaean detrital zircons sourced from such a high temperature tonalite would present a low temperature biased image of the host magma which could be misconstrued as being a minimum melt granite multiple analyses from individual zircons can yield complex ti distributions and associated apparent temperature patterns reflecting cooling history and local chemical environments in large magma chambers in addition to inclusions and crystal imperfections which can yield apparent high temperature anomalies zircon surfaces can also record extreme 1000 c apparent ti temperatures in our studies these were traced to 49ti or a molecular isobaric interference contamination derived from the double sided adhesive tape used in sample preparation and should not be assigned geological significance

petrogenetic and mineralization processes in paleo to mesoproterozoic rapakivi granites examples from pitinga and goias brazil abstract the 1 8 ga pitinga granites agua boa and madeira massifs in amazonas northern brazil are within plate shallow level rapakivi granites associated with an extensional fracture system they comprise an early facies of pyterlitic to wiborgitic rapakivi granite a fine to medium grained biotite granite as well as topaz granite agua boa massif and albite granite madeira massif the granites are usually metaluminous to peraluminous the albite granite however is peralkaline they are enriched in sio2 k2o na2o f rb th nb y zr and the rare earth element ree and impoverished in mgo tio2 p2o5 and sr as the majority of sn mineralized granites sn contents range from 1 ppm in the rapakivi facies to 3400 ppm in the albite granite end at 1 8 ga values vary from 2 2 to 0 4 and nd model ages lie between 2 4 and 2 1 ga mineralization in the madeira massif includes disseminated magmatic cryolite zircon cassiterite pyrochlore columbite tantalite and xenotime and massive cryolite bodies in the f rich peralkaline albite granite in the agua boa massif sn mineralization is associated with greisens and episyenite the 1 77 ga g1 and 1 58 1 57 ga g2 rapakivi granites of goias central brazil are coeval with the arai rift basin granites of the g1 suite are metaluminous and alkaline while the g2 suite is metaluminous to peraluminous both are enriched in f sn rb y th nb ga and the ree primary micas range from fe rich biotite to zinnwaldite the micas of the more evolved granites and the metasomatic micas are strongly enriched in f with f li between 2 and 10 the initial end values of both suites show a considerable range 14 to 0 and indicate substantial compositional variation in source sn deposits in goias are hosted mainly by greisens indium is concentrated in quartz topaz rock and albitized g2d granite of the mangabeira massif and is always related to a cassiterite sulfide association this quartz topaz rock is of metasomatic origin probably generated by a hydrothermal fluid derived from the topaz albite granite mineralization in the studied deposits was essentially associated with f enrichment in the peralkaline madeira albite granite extremely high f contents favored disseminated mineralization while in the goias tin province gtp and agua boa massif greisenization was related to early fluid saturation the gtp and pitinga granites display tectonic petrogenetic geochemical isotopic and metallogenic similarities that can be applied in search of sn and rare metal deposits

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

methane seepage inferred from the porewater geochemistry of shallow sediments in the beikang basin of the southern south china sea abstract several tens of methane ch4 seepage sites have been discovered in the northern south china sea scs during the investigation and exploration of gas hydrates and cold seeps over the past decade however our knowledge about methane seepage in the southern scs remains scarce especially in the beikang basin on the southern slope of the scs four piston cores bh cl18 bh cl23 bh cl29 and bh cl35 were collected in the beikang basin to understand ch4 cycling in shallow sediments the extrapolated depths of the sulfate methane transition zones smtz of the four cores are between 5 3 and 8 8 mbsf by simulating the porewater profiles of so42 ch4 dic and po43 using the reaction transport model we estimated that the dissolved so42 was predominantly consumed by the anaerobic oxidation of methane aom at rates of 27 5 mmol m 2 yr 1 in bh cl18 28 1 mmol m 2 yr 1 in bh cl23 43 1 mmol m 2 yr 1 in bh cl29 and 39 4 mmol m 2 yr 1 in bh cl35 respectively due to intense aom almost all the methane sourced from subsurface sediments was depleted within the smtz resulting in negligible benthic methane fluxes the kink type depth so42 profile of bh cl23 was apparently attributable to an enhanced methane flux which was not captured by the steady state modelling furthermore highly depleted 13c values of the dissolved inorganic carbon dic in bh cl18 43 6 bh cl23 44 and bh cl29 51 2 also indicated a methane derived dic in contrast relatively higher 13c values in bh cl35 suggested a mixture of 13c enriched dic produced by methanogenesis as shown by the reversal toward more 13c enriched dic below 4 2 mbsf in bh cl29 to our knowledge this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern scs and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern scs

grenville age a type and related magmatism in southern laurentia texas and new mexico u s a abstract large grenville age plutons are known to be present in the 1340 1370 ma basement in texas and eastern new mexico but their distribution and compositional ranges are poorly known new u pb zircon dates shrimp rg stanford university on subsurface samples have identified widespread presence of compositionally diverse grenville age 1070 1110 ma plutonic rocks in the texas panhandle core from a 175 m thick sill like gabbroic intrusion yielded a date of 1081 8 3 ma in situ differentiation of this tholeiitic magma led to a 7 fold increase in incompatible element concentrations as ree patterns remained essentially flat trace element data suggest an e morb like source to the south alkali feldspar granite with a type affinities was recovered from drill core from the abilene gravity minimum near albany texas it yielded a date of 1078 23 ma similar to undeformed post orogenic granites in the llano uplift further west an anorthosite xenolith from the eocene three sisters intrusion in el paso texas yielded a date of 1068 30 ma which is within error of the 1110 19 ma age determined for the main stage of the nearby a type red bluff granitic suite xenoliths from potrillo maar volcano in the center of the southern rio grande rift include monzonitic xenoliths with granulitic texture which yielded dates of 1072 ma in contrast potrillo maar xenoliths with igneous textures yielded u pb dates of 27 ma development of the granulitic texture is interpreted to reflect granulite facies metamorphism during tertiary time 26 27 ma on the basis of age zoning and u concentrations in the zircon crystals our results show that grenville age magmatism in the texas and new mexico subsurface was widespread and was coeval with syn and post deformation granites in the llano uplift of central texas the compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were a type these magmatic affinities are thought to be associated with crustal extension rather than with subduction moreover the presence of anorthosite xenoliths in the el paso kilbourne hole region the broadly tholeiitic character of mafic rocks and the a type nature of the granitic rocks are broadly similar to amcg anorthosite mangerite charnockite granite suites present in the grenville province of eastern laurentia

anomalous subsidence history of the west siberian basin as an indicator for episodes of mantle induced dynamic topography abstract vertical intraplate motions are not easily related to plate tectonics understanding their underlying mechanism is further complicated by poor constraints on the magnitude and timing of these motions western siberia is a prominent intraplate area affected by vertical tectonics shortly after the assemblage of pangaea the region experienced substantial uplift followed by a long period of subsidence that led to formation of the world s largest sedimentary basin the west siberian basin its sedimentary cover has been extensively explored and described in the soviet literature here we digitise compilations of stratigraphic data to calculate the basin s subsidence history by backstripping analysis our results confirm the prolonged and high amplitude regional subsidence that has been noted before and constitutes the basin s first order vertical motion however our backstripping results also reveal a secondary mode of shorter spatial and temporal scales induced by the migration of the maximum subsidence across the basin importantly the generally slow subsidence of the basin was interrupted by uplift events in the early and late cretaceous and in the middle oligocene while the secondary mode of vertical motion is not easily understood in terms of traditional subsidence models restricted to lithospheric cooling after stretching it is consistent with rapid uplift events that have been reported for other locations such as elevated passive continental margins we discuss the geodynamic implications and conclude that the basin may hold important constraints on dynamic topography induced by sublithospheric mantle flow processes

stratiform siderites from the mesoproterozoic xiamaling formation in north china genesis and environmental implications abstract stratiform or layered siderites are much less common than siderite concretions in the stratigraphic record in the middle part of the mesoproterozoic xiamaling formation ca 1 40 1 35 ga layered siderites are widespread across the north china platform facies analysis indicates that the xiamaling siderites were formed in shallow water environments above the storm wave base high fe al and v al ratios and the lack of pyrites in siderite bearing strata indicate ferruginous bottom water conditions during the formation of siderites the low thermal maturity of organic matter and clay mineral structures suggest that the layered siderites were formed at burial temperatures 90 c the negative 13c values 24 to 12 of siderites and the presence of m scale hematite inclusions in siderite grains suggest that dissimilatory iron reduction dir during early diagenesis has played critical roles in siderite formation the source of fe oxyhydroxides required for dir could be from fe ii oxidation in shallow seawater above the redoxcline in response to enhanced hydrothermal activities or alternatively directly from continental fe oxide input the positive cerium ce and europium eu anomalies of clay minerals in siderite bearing strata and their positive correlation with total fe contents tfe2o3 seem to favor a hydrothermal origin of iron syndepositional hyalophane concretions intermittent tuff layers and remarkable enrichment of cu and zn in some of the siderite hosting rocks provide further support to hydrothermally sourced iron the enhanced hydrothermal activity for the development of stratiform siderites in the xiamaling formation probably related to the final breakup of the north china craton from the columbia supercontinent during which active hydrothermal systems provided sufficient fe ii for fe redox recycling and formation of layered siderites

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

warming the fuel for the fire evidence for the thermal dissociation of methane hydrate during the paleocene eocene thermal maximum dramatic warming and upheaval of the carbon system at the end of the paleocene epoch have been linked to massive dissociation of sedimentary methane hydrate however testing the paleocene eocene thermal maximum hydrate dissociation hypothesis has been hindered by the inability of available proxy records to resolve the initial sequence of events the cause of the paleocene eocene thermal maximum carbon isotope excursion remains speculative primarily due to uncertainties in the timing and duration of the paleocene eocene thermal maximum we present new high resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells from ocean drilling program site 690 weddell sea southern ocean demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface water warming both of these findings support the thermal dissociation of methane hydrate as the cause of the paleocene eocene thermal maximum carbon isotope excursion furthermore the data reveal that the methane derived carbon was mixed from the surface ocean downward suggesting that a significant fraction of the initial dissociated hydrate methane reached the atmosphere prior to oxidation

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

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

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

petrological and geochemical evolution of the kymi stock a topaz granite cupola within the wiborg rapakivi batholith finland abstract the 6 3 km kymi monzogranite stock represents the apical part of an epizonal late stage pluton that was emplaced within the 1 65 to 1 63 ga wiborg rapakivi batholith the stock has a well developed zonal structure from the rim to the center stockscheider pegmatite equigranular topaz granite porphyritic topaz granite the contact between the two granites is usually gradational within a few centimeters but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later hydrothermal greisen and quartz veins some of which contain genthelvite beryl wolframite cassiterite and sulfides cut the granites of the stock and the surrounding country rocks the equigranular granite contains 1 to 4 vol topaz and its biotite is lithian siderophyllite the porphyritic granite has 0 to 3 vol topaz and the mica is siderophyllite the equigranular granite is geochemically highly evolved with elevated li rb ga ta and f and very low ba sr ti and zr the ree patterns show deep negative eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid rock interaction the zonal structure of the stock is interpreted as a result of differentiation within the magma chamber internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved volatile rich magma in the apical part of the chamber crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite the underlying crystal mush solidified as the porphyritic granite much of the crystallization took place from volatile saturated melt and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization

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

seafloor oxygen consumption fuelled by methane from cold seeps the leakage of cold methane rich fluids from subsurface reservoirs to the sea floor sustains some of the richest ecosystems on the sea bed these cold seep communities consume around two orders of magnitude more oxygen than the surrounding sea floor as a result of the microbial consumption of seep methane

petrography mineralogy and geochemistry of the late eocene oolitic ironstones of the jebel ank southern tunisian atlas abstract the oolitic ironstones ore deposit of jebel ank central tunisia is a simply folded stratiform ore body of about 2 5 8 m thickness located in the upper part of the epicontinental souar formation late eocene and is covered by the continental segui formation mio pliocene the deposit contains about 20 mt of ore with an average grade of 50 fe generally oolitic iron deposition occurs in shallow water lagoonal environments the jebel ank deposit lies between two regional disconformities late eocene and miocene and is evidence of a transitional stage at the end of regional regression before renewed transgression the footwall of the oolitic iron ore bearing bed consists of a fine grained sandstone bed 10 20 cm thick pinching out laterally westward into green clays the hanging wall is composed of thin bedded limestone and clay alternations 2 3 5 m thick iron occurs in the form of cryptocrystalline goethite with limited al fe substitution the goethite contains around 48 fe 5 al and up to 1 5 p jarosite alunite and manganese minerals cryptomelane psilomelane and manjiorite are supergene secondary minerals probably related to descending surface fluids these manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies local cement space filling and free centimeter sized nodules fe oolites in the deposit are similar to those documented in many other oolitic ironstone deposits the dominant fe oolite type 90 has a concentrically laminated cortex with no nucleus the nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains major elements in high grade samples fe 2 o 3 65 vary within a limited range and show higher concentrations of sio 2 average 7 85 and al 2 o 3 average 5 1 with minor tio 2 mno mgo na 2 o k 2 o and so 3 less than 1 paas normalized trace elements of bulk samples and fe oolite generally show similar behavior both are enriched in v co ni mo as zn and y and are depleted in cu rb zr nb ba and hf anomalous v cr ni zn and ree y are correlated with goethite paas normalized ree y patterns of both bulk samples and fe oolite show slight hree enrichment positive ce with negative y anomalies the mineralogy goethite and cryptomelane along with the geochemistry si vs al as cu mo pb v zn vs ni co binary plots zn ni co triangular diagram ree y content and patterns and ce ce vs nd and ce ce vs y n ho n binary plots of the studied oolitic ironstone are congruent with a hydrogenetic type while two possible sources of iron for jebel ank ironstone can be proposed i submarine weathering of glauconite rich sandstone and ii detrital iron from adjacent continental hinterland the later is the more plausible source of iron based on paleogeographic setting the occurrence of fine sandstone underlying the iron level occurrence of mn ores in the lower part of the fe ores succession high phosphorous zinc ree y concentrations and y ho ratios and low la ce ratios

the distinctive compositional evolution of glauconite in the cretaceous ukra hill member kutch basin india and its implications abstract an integrated study involving sedimentology mineral chemistry and spectroscopy highlights a distinctive compositional evolution of cretaceous glauconite within the ukra hill member glauconite occurs at the top part of transgressive systems tract deposits built on a marine shelf the concentration of glauconite steadily increases towards the maximum flooding surface maximizing around 50 and sharply falls at the onset of progradation unlike most phanerozoic examples ukra glauconite forms by the variable degree of replacement of quartz feldspar and mica grains xrd feg sem and spectroscopy of glauconite pellets indicate an evolved stage of maturation mossbauer spectroscopy reflects a minor substitution of al3 fe3 total in tetrahedral sites and significant substitution of the same in octahedral sites a consistently high value of k2o as well as fe2o3 contradicts the two popular theories layer lattice and verdissement and support replacement origin of glauconite in a high asi and high ak pore water environment incipiently formed glauconite records a marginal increase in k2o content accompanied by release of al2o3 and sio2 to form evolved glauconite pellets those forming within quartz grains involve an addition of fe2o3 total content during maturation the minimal increase in k2o content of incipiently formed glauconite best exhibited by those formed within quartz grains is possibly related to stratigraphic condensation compositional evolution like this is exhibited by precambrian glauconites involving abiotic substrates but is unusual for the phanerozoic original k2o and fe2o3 total content of glauconites is reduced around peripheries and fractures during diagenesis adding to compositional variability

palaeozoic oolitic ironstones on the north american craton abstract more than 40 mostly minor palaeozoic oolitic ironstones oi accumulated on low latitude cratonic north america almost entirely in usa a few middle cambrian oi among the oldest anywhere were deposited on the western cratonic shelf of usa widely scattered late cambrian ones developed on the southwestern southeastern and northeastern flanks of the transcontinental arch middle ordovician oi were deposited on the cratonic interior and on the southern and southeastern cratonic margins in latest ordovician time neda oi spread across north central usa east of the arch and south of the laurentian upland minor ones developed in the southern part of the taconian foreland basin after major orogeny early and middle silurian clinton oi flourished throughout the foreland basin in eastern usa producing the largest oi deposit on the north american craton minor early late silurian oi accumulated in the central part of the basin middle and late devonian oi in the acadian foreland basin in northeastern cratonic usa developed progressively westward of the encroaching catskill delta an isolated middle devonian one accumulated in southwestern cratonic usa and latest devonian ones in north central usa east of the arch during this palaeozoic episode sites of oi deposition shifted eastward across the craton most of the oi were deposited during a hiatus in normal sedimentation accumulating in the upper part of shoaling upward sequences some early palaeozoic ones occur in glauconitic siliciclastic carbonate facies and contain mostly spherical hematitic ooids these suggest derivation from iron rich soils developed on glauconitic deposits oi in wholly siliciclastic facies containing distorted chamositic ooids with cores of mud peloids were common in later palaeozoic time these oi like many other phanerozoic ones suggest a synsedimentary to early diagenetic origin

geology and age of the late archean keivy alkaline province northeastern baltic shield abstract the keivy alkaline province kola peninsula consists of six peralkaline granite bodies alkaline syenogranite dikes and two nepheline syenite intrusions emplaced between the tonalite trondhjemite granodiorite basement and the supracrustal keivy terrane u pb zircon ages for the keivy alkaline province range from 2613 to 2682 ma and for the spatially associated gabbro anorthosite complexes from 2659 to 2668 ma the keivy alkaline granitoids are geochemically different from other late archean alkaline rocks inferred to have been emplaced in subduction zone settings the keivy alkaline province may represent the earliest example of magmatism associated with continental rifting and the involvement of a mantle plume with oceanic island basalt like characteristics

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

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

contrasting mineralogical geochemical signatures of manganese oxides guides to metallogenesis manganese oxides have a strong sorption capacity for cationic species in the depositional fluid this sorption capacity is reflected in the chemistry of the oxides which is different from that for the oxides precipitated from fresh water seawater and hydrothermal fluids characteristic mineralogical geochemical enrichment and geochemical association defined statistically signatures in conjunction with several diagnostic plots can be employed to distinguish between the following different genetic types of manganese deposits supergene hydrothermal supergene marine terrestrial supergene dubhite and supergene marine hydrothermal sedimentary exhalative sedex dubhites are manganese oxides derived from the weathering of a mineralized sequence i e base and precious ore minerals and gangue hydrothermal deposits characteristically show geochemical enrichments in the assemblage as ba cu li mo pb sb sr v zn and an mn as geochemical association supergene marine deposits show na k ca mg sr and co cu ni geochemical enrichments whereas supergene terrestrial deposits tend to have very high ba contents and an mn ba association dubhites contain high levels of pb and zn show an mn pb base metal association and are composed of significant quantities of base metal bearing manganese oxides bixbyite braunite hausmannite huebnerite jacobsite and pyrochroite occur predominantly only in hydrothermal deposits whereas chalcophanite coronadite crednerite delta mno 2 groutite hollandite lithiophorite manganite nsutite quenselite ramsdellite romanechite todorokite and woodruffite are generally of supergene origin in addition to established plots presently used to identify marine sedimentary exhalative deposits the following diagnostic graphs were devised na vs mg distinguishes deposits of marine and fresh water origin co ni vs as cu mo pb v zn discriminates between hydrothermal and supergene oxides and pb vs zn can be used to recognize dubhites and therefore prospective sequences

geochemistry and mineralogy of rare earth elements in high phosphorus ooidal ironstones a case study of the kamysh burun deposit azov black sea iron province abstract this study is aimed at characterizing the distribution and speciation of the rare earth elements and yttrium ree y in ooidal ironstones from the kamysh burun deposit kerch peninsula with implications for the depositional environments and contributions of different ree y carriers to the ore budget the lower pliocene ooidal ironstone sequence of the so called kerch ores up to 15 m thick occupies an area of 28 km2 the ore sequence lies between miocene lower pliocene shell limestones and upper pliocene pleistocene sandy and clayey sediments and consists of horizontal ooidal ironstone beds composed mainly of goethite and x ray amorphous fe3 oxy hydroxides intercalated with siderite and or rhodochrosite beds iron is a main component in ooidal ironstones 50 03 66 19 wt fe2o3 and 1 61 wt feo as well as in fresh carbonate ores 34 wt feo and 14 70 wt fe2o3 the kerch ores contain up to 4 59 wt p2o5 and show significant positive correlation between p and fe r 0 75 the bulk ore samples and their coarse 1 10 mm fractions are goethite dominated and have similar phase major and trace element and ree compositions the fine la nd ca pr sm the ree content in fresh carbonate ores is below that in paas xav 103 ppm but approaches the latter in oxidized crusts xav 178 ppm the kerch ironstones also contain ree poor early diagenetic fe2 phosphate vivianite ree xav 0 93 ppm and ca fe2 phosphate anapaite ree xav 6 95 ppm with typical seawater ree y distributions the ooidal ironstones show mree enriched patterns with distinct negative y anomalies which reveals fe3 oxy hydroxides as chief carriers of adsorbed ree mainly mree the kerch ironstones accumulated ree progressively due to hysteretic dissolution and precipitation of fe3 oxy hydroxides in oscillating redox conditions the diagenetic source of ree stored in ooidal goethite ironstones was inferred from the paas normalized values cen cen vs nd and cen cen vs yn hon discrimination diagrams in general the ree y budget of the kerch ooidal ironstones mainly formed during early diagenesis and scavenging from pore water with a minor contribution of siliciclastic inputs in the absence of hydrothermalism

ooidal ironstones from ordovician gondwana a review abstract ooidal ironstones occur widely in the ordovician shelf sequences of sw europe the western european platform the avalonian terranes and north africa they form the most important group of deposits of the earlier of the two major periods of phanerozoic ooidal ironstone generation ordovician devonian and jurassic paleogene individual ooidal ironstone horizons are generally thin less than 2 m but may have a very wide lateral extent the ooidal ironstones represent sediments formed under extremely low sedimentation rates and were most commonly developed as the initial deposit above a disconformity ooidal ironstone producing events occurred almost synchronously over wide areas of the gondwanan shelf reflecting the uniform sedimentary conditions across large areas of the shallow low gradient shelf clastic sediments deposited during periods of normal sedimentation show similar remarkable uniformity over large areas this geometry together with the abundant fauna both micro and macro in the ironstone facies means that these thin ironstone beds form important marker horizons for correlation within the region the stratigraphic distribution of the ironstones shows strong correlation with periods of high relative sea level sedimentological study of examples in sw europe indicate that they separate transgressive systems tracts with parasequences bounded by phosphorites from the early part of the following highstand systems tracts without significant parasequences and are therefore interpreted to lie on maximum flooding surfaces such maximum flooding surfaces occur in the early llavirn the late llanvirn the early caradoc and the early ashgill of sw europe outside sw europe the ooidal ironstones may have a different relationship to the sequence stratigraphy possibly reflecting the influence of absolute water depth

phanerozoic oolitic ironstones geologic record and facies model oolitic ironstonesl composed predominantly of ooidsl of ferric oxide and chamosite accumulated throughout much of phanerozoic time figure 1 and some of them have been important ore deposits they are built of two very common products of weathering iron oxide and clay but they constitute a distinctive and subordinate sedimentary facies for which there is no simple modem analogue as a result considerable disagreement has attended more than 125 years of discussion about their origin early analyses of these rocks for example the minette are of france focused mainly on their chemical and mineral composition and on processes of diagenesis more recent work has described their sedimentary textures and structures as well as their general facies and depositional environments ad eleye 1973 brookfield 1973 corbin 1980 hallam 1975 karpov et a11967

trace element behaviour at cold seeps and the potential export of dissolved iron to the ocean seawater samples were collected by submersible above methane seeps in the gulf of guinea regab and baboon pockmarks in order to investigate the behaviour of iron fe manganese mn and rare earth elements ree during fluid seepage our aim was to determine whether cold seeps may represent potential sources of dissolved chemical species to the ocean dissolved 0 45 m filtered samples and total dissolvable unfiltered samples concentrations were determined over 50 m long vertical transects above the seafloor and at various discrete locations within the pockmarks we show that substantial amounts of fe and mn are released into seawater during seepage of methane rich fluids mn is exported almost quantitatively in the dissolved form more than 90 of total mn mean mndiss 12 11 nmol kg 12 11 nmol kg although a significant fraction of fe is bound to particulate phases the dissolved iron pool still accounts on average for approximately 20 percent of total iron flux at vent sites mean fediss 22 11 nmol kg 22 11 nmol kg this dissolved fe fraction also appears to remain stable in the water column in contrast there was no evidence for any significant benthic fluxes of pore water ree associated with fluid seepage at the studied sites overall our results point towards distinct trace element behaviour during fluid seepage with potential implications for the marine geochemical budget the absence of any dissolved ree enrichments in bottom waters clearly indicates effective removal in sub surface sediments most likely precipitation of authigenic mineral phases at cold seeps i e carbonates represents a net sink for these elements while mn appears to behave near conservatively during fluid seepage the observed relative stability of dissolved fe in the water column above seepage sites could be explained by complexation with strong organic ligands and or the presence of fe bearing sulfide nanoparticles as reported previously for submarine hydrothermal systems considering the ubiquitous occurrence of methane vents at ocean margins cold seeps could represent a previously unsuspected source of dissolved fe to the deep ocean

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

origin and sequence stratigraphic implications of high alumina glauconite within the lower quartzite vindhyan supergroup abstract an integrated field petrographical and geochemical investigation reveals a shallow marine dys oxic depositional setting for the glauconite at the top of the mesoproterozoic lower quartzite member of the vindhyan supergroup a thorough facies and paleogeographic analysis indicates a storm influenced shallow subtidal depositional condition for the glauconitic sandstone occupying the mid level of a transgressive systems tract tst deposit which is capped by the maximum flooding zone mfz the x ray diffractional parameters confirm the evolved character of the glauconite the mineral chemical analysis indicates the glauconite is enriched in k2o mgo and al2o3 and depleted in fe2o3 total compared to the phanerozoic variety it formed by the pseudomorphic replacement of the k feldspar by the addition of fe2o3 at consistently high content of k2o unlike most phanerozoic examples the elevated level of h3sio4 cl k na mg and fe possibly derived by the continental weathering and or contemporary volcanism facilitated the glauconitization process the high content of mg in the glauconite corresponds to the mg rich mesoproterozoic seawater a dys oxic depositional condition associated with a major marine flooding surface favoured the formation of glauconite while the onset of anoxic regime caused the disappearance of glauconite and the deposition of pyritiferous and carbonaceous shale in the overlying condensed zone deposits

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

diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems mineral magnetic results and electron microscope observations from gas hydrate bearing marine sediments cored at southern hydrate ridge during ocean drilling program leg 204 sites 1244 to 1252 cascadia margin offshore oregon demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially mediated diagenetic reactions in the sulphate the anaerobic oxidation of methane aom and the methanic gas hydrate zones geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide whether it derives from microbially driven sulphate reduction in the sulphate zone in the aom zone or in deep sediments undergoing aom so that pyritization reactions are not driven to completion our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times which is likely to compromise paleomagnetic results from greigite and pyrrhotite bearing marine sediments

differentiation of ironstone types by using rare earth elements and yttrium geochemistry a case study from the bahariya region egypt abstract this paper deals with the geological and geochemical characterization of ironstone deposits encountered in two different sedimentary successions upper cretaceous and lower cenozoic in northern bahariya egypt the ironstones occur as uneconomic thin bands lenses and concretionary beds in cenomanian clastic rocks of the bahariya formation and as economic iron ore associated with eocene carbonate depositional units the ironstones contain similar iron bearing minerals mainly goethite and hematite which display a variety of fabrics i e concretionary massive stromatolitic like oolitic pisolitic reniform aggregates boxwork liesegang geode like and brecciated the iron rich minerals preferentially replaced and or cemented the primary and diagenetic mainly dolomite carbonates preservation of fabrics sedimentary structures and thickness of the precursor carbonates is conspicuous whole rock composition of the cretaceous ironstones shows lower fe2o3 and mno contents and relative enrichment in detrital derived elements namely al zr and nb when compared with the eocene ironstones the latter showing enrichment in fe mn si and ba oxides as well as cu zn ni and sr trace elements total ree content in the cretaceous ironstones ranges widely from 70 to 348 ppm whilst the eocene ironstones show quite low content mostly from 1 96 to 31 ppm post archean australian shale paas normalized patterns of the upper cretaceous ironstones display flat ree patterns with small positive eu and pr slightly negative y and ce anomalies which are close to unity and intermediate nd concentrations between 10 and 100 ppm in contrast paas normalized ree y patterns of the eocene ironstones display lree positive slope with enriched flat hree trend as well as negative ce anomaly positive y and eu anomalies and low nd concentration in general less than 10 ppm geochemical data along with sedimentary features of the upper cretaceous and eocene ironstones in bahariya point to different origins in the two ironstone types the former having been originated diagenetically whereas the latter were constrained by hydrothermal fluids a syngenetic marine origin is ruled out for any of the studied ironstones higher amount of detrital derived elements as well as higher ree concentration in the upper cretaceous ironstones than in the eocene ironstones suggest some contribution from the associated clastic sediments the negative ce anomaly determined in both types indicates anoxic iron rich solution that passed to oxidized surface the presence of positive y and eu anomalies in the eocene ironstones suggests that iron precipitation was favoured by hydrothermal reducing conditions when slightly acidic fluids reached oxidizing alkaline waters

compositional variation of glauconites in upper cretaceous paleogene sedimentary iron ore deposits in south eastern western siberia abstract glauconite occurs either as unaltered greenish or as altered brownish variety in upper cretaceous palaeocene sediments in the southeastern corner of western siberia studied section within the bakchar iron ore deposit includes ipatovo slavgorod gan kino and lyulinvor formations which are represented by sandstones siltstones claystones and oolitic ironstones of coastal marine facies the origin of unaltered glauconite is explained by the verdissement theory transgressions during lower coniacian santonian and campanian favored the formation of unaltered glauconites in dysoxic to anoxic conditions subaerial exposure of glauconite resulted in leaching of potassium oxidation of iron and formation of iron hydroxides in upper coniacian maastrichtian and palaeocene glauconite ultimately converts to leptochlorite and hydrogoethite by this alteration abundant microscopic gold inclusions besides sulphides sulphates oxides and silicates characterize this glauconite mineral inclusions include precious rare metals and non ferrous metals the concentration of gold in glauconite may be as high as 42 9 ppb abundant inclusions of various compositions in glauconites indicate enrichment of marine sediments in precious and non precious metals while major element composition of glauconites is affected by subaerial exposure the broadly similar micro inclusions in both altered and unaltered varieties are possibly related to the comparatively immobile nature of ree and trace elements

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

petrogenesis of the igneous mucajai amg complex northern amazonian craton geochemical u pb geochronological and nd hf o isotopic constraints abstract the ca 1525 ma igneous mucajai anorthosite monzonite granite amg complex in northern brazil is a rare manifestation of mesoproterozoic intraplate magmatism in the northern amazonian craton the complex comprises a two phase rapakivi granite batholith with subordinate quartz fayalite monzonites and syenites and the closely associated repartimento anorthosite zircon u pb id tims geochronology reveals that the anorthosite 1526 2 ma monzonite 1526 2 ma and the main phase biotite hornblende granite 1527 2 ma of the complex intruded the paleoproterozoic 1 94 ga country rocks simultaneously at 1526 ma and that the more evolved biotite granite is marginally younger at 1519 2 ma intraplate magmatism in the mucajai region was relatively short lived and lasted 12 million years 1529 1517 ma at maximum the nd whole rock id tims e nd from 1 9 to 2 8 hf zircon lam icp ms e hf from 2 0 to 3 1 and o zircon sims 18 o from 6 1 to 7 0 isotopic compositions of the studied rocks are fairly uniform but still reveal a small degree of isotopic heterogeneity in the paleoproterozoic crust enclosing the complex the small isotopic differences observed in the two types of rapakivi granites biotite hornblende granite and biotite granite may result either from an isotopically heterogeneous lower crustal source or more likely from contamination of the granitic magma derived from a lower crustal source during prolonged residence at upper crustal levels

himalayan ironstones superplumes and the breakup of gondwana oolitic ironstones or glauco phosphorites were deposited all along the northern india continental margin during major tectonically enhanced transgressions in frasnian permian norian callovian early cretaceous and paleocene time these regional events of starvation and drowning were commonly associated with alkalic to tholeiitic magmatism and invariably coincided in time with multiple rifting events that punctuated the progressive disintegration of gondwana deposition of major condensed horizons enriched in iron silica and phosphorus was fostered by global factors such as eustatic rise warm greenhouse climates sluggish circulation increased fertility and poor ventilation in the oceans such anomalous sedimentary conditions recurred synchronously during magnetic quiet zones pointing to successive episodes of global fast spreading and accelerated terrestrial heat loss multiple pulsations of earth are inferred to have triggered supercontinent breakup and widespread volcanism thus altering the atmosphere and hydrosphere and dramatically affecting sedimentary and biological systems

residual liquid origin for a monzonitic intrusion in a mid proterozoic anorthosite complex the sybille intrusion laramie anorthosite complex wyoming the sybille intrusion 100 km 2 is one of three large monzonitic intrusions in the 1 43 ga laramie anorthosite complex of southeastern wyoming the petrographic geochemical isotopic and geophysical characteristics of sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates ferrodiorites and contamination by archean wall rocks the exposed part of the intrusion is composed mainly of coarse grained monzosyenites with abundant alkali feldspar phenocrysts the monzosyenites preserve mineralogical evidence for high crystallization temperatures 1000 c mid crustal emplacement pressures 3 kbar relatively reduced crystallization conditions 2 log units below the fayalite magnetite quartz fmq oxygen buffer and they crystallized in the presence of a co2 rich fluid phase fuhrman et al 1988 frost and touret 1989 the eastern monzosyenites those adjacent to contemporaneous anorthosite are distinguished by an anhydrous mineral assemblage fo16 fo8 olivine high ca pyroxene lacking modal quartz silica contents of 60 wt and smaller eu anomalies eu eu 1 2 to 1 3 abundant xenoliths of archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber we propose that the monzosyenites represent a relatively thin 0 5 1 0 km thick roof to a magma chamber dominated by dense ferrodioritic cumulates at depth extensive open system fractionation of a ferrodioritic parent magma residual after crystallization of anorthosite produced fe enriched monzodioritic and or monzonitic magma in the upper part of the chamber and complementary fe and ti rich cumulates in the lower levels we have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium crystallization experiments the presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the sybille intrusion hodge et al 1973 in the upper parts of the chamber the fractionated monzodioritic and or monzonitic magmas eventually became saturated in alkali feldspar owing to density contrasts the alkali feldspar phenocrysts floated to the roof of the chamber thus producing the exposed porphyritic monzosyenites in addition the roof of the chamber was the site of significant melting of archean gneiss and locally metapelite the sr and nd isotopic compositions of the monzosyenites with sr isotopic ratios becoming increasingly radiogenic from east i sr 0 7059 and initial nd 2 5 to west i sr 0 7092 and initial nd 2 6 are consistent with a 5 to 15 addition of archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks the stratigraphic and compositional similarity of the sybille monzosyenites to mangerites in the bjerkreim sokndal intrusion of the rogaland anorthosite complex southern norway indicates that similar open system magmatic processes are capable of having produced high temperature k rich monzonitic rocks in other proterozoic anorthosite complexes

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

deposition and diagenesis of clinton ironstones silurian in the appalachian foreland basin of pennsylvania clinton ironstones mid silurian on the southeastern margin of the appalachian foreland basin in central pennsylvania consist largely of skeletal grainstone storm beds whose skeletal fragments are extensively impregnated and replaced by ferric oxide other forms of iron mineralization consist of ooids superficial ooids cavity fillings and cements composed of ferric oxide and or chamositic clay and two generations of iron rich carbonate spar true oolite is rare small proportions of ferruginized grains are present in coarse lags at tops of some small scale 1 to 5 m coarsening upward sequences which are the record of mid shelf shoals from which the skeletal grainstone storm beds were derived ooids and superficial ooids are likely to have formed in dysaerobic conditions where the pycnocline of a highly stratified sea intersected shoal flanks ferrous iron from anaerobic deeper waters precipitated on quartz and skeletal fragment nuclei in weakly reducing conditions with episodic gentle agitation several stages of early diagenetic ferruginization affected skeletal grainstone storm beds and sequence capping coarse lags chamositic clay precipitated in pores and cavities of skeletal fragments within quartz sand adjacent to crinoid ossicles and as bed parallel connectors of large skeletal fragments as diagenesis progressed with further burial many skeletal fragments were extensively replaced by ferric oxide most chamositic clay altered to ferric oxide other chamositic clay matured from berthierine to chamosite and clusters of bladed specularite replaced parts of primary grains and secondary carbonate cements paleomagnetic and fluid inclusion analysis indicates that many late diagenetic changes took place at elevated temperatures during deen burial in an early phase of the alleghanian orogeny

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

compositional variability of glauconites within the upper cretaceous karai shale formation cauvery basin india implications for evaluation of stratigraphic condensation abstract a detailed mineral chemical investigation of glauconite within the condensed section deposits of the cretaceous karai shale formation cauvery basin india reflects a wide spectrum in chemical composition related to origin and evolution in different substrates stratigraphic condensation and post depositional alteration fe and mg rich glauconite comprising up to 60 of the sedimentary rocks occurs as replaced forms of fecal pellets as infillings within pores and chambers of bioclasts including those of foraminifera ostracoda bryozoa and algae and as altered forms of mica exhibiting vermiforms authigenic precipitation of k and fe poor glauconite followed by addition of fe and k into the lattice and concomitant release of al and si explains the origin of glauconite pellets and infillings the origin of glauconite vermiforms in partly degraded mica involves only the second stage of fe and k addition glauconite pellets and vermiforms exhibit sharply defined alteration zones along peripheries to form rims and in proximity to cracks or cleavages with reduced k2o and fe2o3 total and enhanced al2o3 and sio2 related to late stage meteoric water actions cores of glauconite pellets and unaltered zones of vermiforms reflect evolved characteristics with 6 k2o typical of a condensed section while other glauconite varieties occurring at the same stratigraphic level exhibit slightly evolved nature not consonant with stratigraphic condensation increasing abundance of glauconite pellets from the bottom to the top of the transgressive systems tract accompanied by slight increase in k2o within their cores reflects the effect of stratigraphic condensation on the evolution of glauconite high fe2o3 total content of glauconite in the karai shale formation may be related to upwelling although the fe may be contributed partly by the biotite substrate mossbauer spectroscopy of glauconites reveals significant total fe substitution in both tetrahedral and octahedral sites detailed mineral chemical analysis enables us to distinguish stratigraphically significant glauconite within the karai shale formation from the rest of the glauconite notwithstanding its wide compositional range

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