the origin of hematite in high grade iron ores based on infrared microscopy and fluid inclusion studies the example of the conceicao mine quadrilatero ferrifero brazil petrographic and textural analysis combined with fluid inclusion studies by infrared microscopy of high grade 65 fe hematite ore samples from the conceicao deposit in the northeastern part of the quadrilatero ferrifero brazil indicate a complex process of oxidation and mineralization during two orogenic events each developed under different conditions and involving distinct fluids the earliest mineralization formed massive magnetite rich orebodies under relatively reducing conditions in the early stages of the transamazonian orogeny magnetite was oxidized martitized with the development of porous hematite crystals hematite i possibly during this stage new hematite crystals were also formed from low temperature low to medium salinity fluids as indicated by two phase fluid inclusions the origin of these fluids is still uncertain but tentatively interpreted as being modified surface water the fluids were transported along normal faults and fractures during post tectonic collapse following the transamazonian orogeny 2 1 2 0 ga and creation of the dome and keel structural pattern of the quadrilatero ferrifero these solutions were also likely responsible for the initial oxidation of the iron formations and the development of hematite i subsequent uplifted hot basement rocks or post tectonic plutons were probable heat sources for the regional metamorphism and development of a granoblastic fabric of hematite ii grains in the iron formations and high grade orebodies however the ore was only partially recrystallized as several relics of the early magnetite martite and hematite are still preserved in the granular hematite ii crystals during the brasiliano pan african orogeny 0 8 0 6 ga high salinity fluids with temperatures varying from 120 to a maximum of approximately 350 c penetrated the iron formations along shear zones crystallizing initially tabular and thereafter platy hematite crystals hematite iii and specularite forming schistose orebodies quartz veins that cut across the ore and envelop specularite plates and ore fragments formed from late stage high temperature and low salinity fluids containing co2 these later fluids did not alter the ore each of these stages of mineralization produced orebodies with distinct features recurrent hydrothermal mineralization is thought to have been responsible for the development of giant high grade iron ore deposits in structurally favorable sites fold hinges with enhanced permeability and deep faults able to conduct the fluids to the surface repeatedly over time should be important targets for exploration of new resources

multiple metal sources in the glaciomarine facies of the neoproterozoic jacadigo iron formation in the santa cruz deposit corumba brazil abstract the rapitan type banded iron formation bif in the banda alta formation fm of the neoproterozoic jacadigo group in brazil was deposited in a redox stratified marine sub basin which was strongly influenced by glacial advance retraction cycles with temporary influx of continental freshwater and upwelling metal enriched seawater from deeper anoxic parts these new finding are based on new stratigraphic whole rock geochemical and stable fe and c isotope data from the santa cruz hematite deposit near corumba mato grosso do sul where a stratigraphy of lower and upper dolomite rich and intermediate chert rich bif facies with up to three intercalated diamictites is revealed the ca mg fe mn carbonate chert hematite and chert hematite bif 30 45 wt fe of the lower dolomite rich facies shows chemical signatures consistent with well oxidized seawater i e low pr yb paas strong negative cepaas and positive gdpaas and ypaas anomalies as well as negative 13c carbonate typical for neoproterozoic glaciogenic carbonates sedimentation in a rather shallow water depth during relatively warm interglacial periods was likely influenced by abundant freshwater from fluviatile runoff and melting icebergs in such conditions abundant microbial activity accommodated co2 sequestration in carbonates as spheroids and mats and fractionated 57fe 2 6 and 1 2 in primary fe hydroxides in contrast the intermediate chert rich facies characterized by chert hematite bif 35 55 wt fe and isolated hematite chert and hematite mud recorded trace element signatures of non oxidized absence of ce anomalies and variable ypaas anomalies more metal enriched fe mn si ni zn pb u seawater thus deposition was within and below the shallow level redoxcline during ice cover colder water and isolation from sunlight reduced microbial activity and thus an almost non fractionated 57fe 0 7 to 0 0 fluid signature reveals that low temperature based on almost absence of eu anomaly in bif hydrothermal mor vents or sub seafloor alteration of mafic or felsic rocks or shales fertilized seawater with metals all results lead to the model that the jacadigo group formed during one major marine transgression regression cycle and bif facies in the banda alta fm were a response to first and second order periodic variations of the depth of the redoxcline induced by the juxtaposition of glacial advance retraction cycles active graben tectonics and glacial isostatic adjustment or eustatic water level changes the chert rich bif facies marked the maximum of transgression age and tectonic setting of the jacadigo basin remains contentious it may represent a continental back arc basin of the brasiliano collision zone 590 ma in which submarine alteration was possibly related to low temperature hydrothermal fluids moving through the active graben system as an alternative to basin wide benthic pore water flux this would place the jacadigo group to the suite of ediacarian gaskier glaciations although a relationship to the marinoan glaciation 660 635 ma mainly based on the published sedimentation age bracket of 706 587 ma is also possible

pull aparts shear fractures and pressure solution vein arrays are often composed of pull aparts which are linked by shear fractures good examples of which occur in the lower jurassic limestones of somerset southwestern england such pull apart arrays have displacement distance characteristics which are similar to fault zones with maximum displacement indicated by the largest pull apart widths near the centre of the array and with displacement decreasing towards the tips pull apart arrays usually die out into enechelon or pinnate veins evidence for pressure solution along the shear fractures which connect pull aparts include their dark and braided nature their obliquity to the displacement direction the high dihedral angles often 90 between conjugate shear fractures and the dissolution of earlier structures a range of geometries occurs with varying relative amounts of veins and pressure solution being related to varying amounts of transtension or transpression there is a general trend for an increase in the angle between vein segments and the shear fractures as contraction increases there is therefore a trend for increased pressure solution on the shear fractures in more contractional arrays the concentration of insoluble material along shear fractures has important implications for the mechanics and sealing of faults

structural control and hydrothermal evolution model of unusual high grade metasandstone hosted iron deposits mesoproterozoic eastern chapada diamantina brazil abstract the ibicoara high grade hypogene hematite deposits are located in the sao francisco craton in the chapada diamantina fold thrust belt iron mineralization host rocks are quartz and arkose sandstones from the tombador formation which present planar stratification regionally three deformational phases are identified the first d1 phase developed kink with subordinate chevron folds shear zones related to flexural slip with s c structures stretch lines lx1 slickenlines and slickensides the second phase d2 is represented by dextral and sinistral reverse high angle shear zones with nnw sse and nw se orientation trends respectively high angle tension fractures with an ene wsw orientation during the third deformation phase d3 dextral and sinistral shear zones with ene wsw and ne sw orientations are developed the richest hematite domains are massive having fe2o3total contents to 98 these irregular massive domains are distributed along the primary sedimentary foliation plane parallel stratification and tangential cross bedding of d2 shear zones or tension fractures generated during d2 the irregular massive hematite domains grade laterally into platy and stringer domains in addition there are vein stockwork and breccia mineralization styles that crosscut irregular massive domains or host rock chemical analyses of hematite in irregular massive domains show that fe2o3 correlates negatively with sio2 zr th sr nb hf and ba on the other hand it correlates positively with zn v u cu co and with the sum of rare earth elements ree when normalized to the post archean australian shale paas the distribution of ree generally show negative eu anomalies and a weak enrichment of light rare earth elements lree compared to the heavy rare earth elements hree the paragenetic succession shows the existence of three groups of hematite groups 1 and 2 are mainly related to the irregular massive domains controlled by rock stratification and the s0 s2 foliation in shear zones the distinction between them is given by the finer grain size of the first generation of hematite and the presence of breccia domain in the second group the third group is related to massive vein domains for each group the hematite grain size increases successively and there is a relative increase in the hree in relation to the lree furthermore similarities between the chemical signatures of the first groups 1 and 2 hematite generation with the signatures of the host rock reveal an important influence of the chemical composition of the original rock this is not the case for the late hematite of massive domains as well as vein style hematite suggesting high fluid rock conditions the hematite hydrothermal alteration at ibicoara has structural control with dilational structures associated with d2 shear zones the host lithotype was also a contributing factor due to its porosity and permeability and the presence of sedimentary structures that favored flow of the hydrothermal fluid from shear zones replacement and direct precipitation took place and are responsible for the transfer of solutions and formation of hematite rich areas the hydrothermal fluid probably leached components from the espinhaco supergroup metasedimentary units and dikes that are regionally present and carried these during the deformational phases that inverted the paramirim aulacogen the espinhaco supergroup is characterized by a sequence of siliciclastic rocks predominantly metasandstones and felsic metavolcanic rocks sterile in iron which were deposited between the statherian and stenian periods in the chapada diamantina area the espinhaco supergroup is comprised from base to top by the serra da gameleira formation and the rio dos remedios paraguacu and chapada diamantina groups the chapada diamantina group contains the tombador formation that hosts the high grade iron deposits investigated in the present study in light of the tectonic context and existence of shear related gold and baryte hydrothermal deposits nucleated during the paramirim aulacogen inversion the chapada diamantina thrust and fold belt is considered a potential regional exploration target

replacement origin for hematite in 2 5 ga banded iron formation evidence for postdepositional oxidation of iron bearing minerals banded iron formations bifs are central to interpretations about the composition of the precambrian ocean atmosphere and biosphere hematite is an important component of many bifs and its presence has been used as evidence for the former presence of hydrous ferric oxyhydroxides that formed from the oxidation of dissolved ferrous iron in seawater however textural evidence for the origin of hematite is equivocal new petrographic results show that hematite in unmineralized bif from the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia including morphologies previously interpreted to represent ferric oxyhydroxide precipitates formed via fluid mediated replacement of iron silicates and iron carbonates along sedimentary layering the lateral transition from stilpnomelane and siderite rich laminae to hematite dominated laminae is interpreted to reflect progressive stages of in situ alteration of reduced mineral assemblages by oxygen bearing fluids rather than changes in the chemistry of the water column during deposition although morphologies previously ascribed to primary hematite are present they are related to mineral replacement reactions raising doubts about the petrographic criteria used to identify original hematite hematite replacement in unmineralized bif postdated deposition and possibly metamorphism and predated modern weathering from a regional perspective it appears to be a distal signature of the processes that were responsible for iron ore mineralization which involved the deep infiltration of oxygen bearing meteoric fluids the mineral replacement reactions recorded in the dales gorge member are unlikely to be unique and probably occurred in bifs elsewhere at some point in their history the observation that at least some of the hematite in unmineralized bif did not form directly from ferric oxyhydroxides implies that hematite is not a reliable proxy for the composition of the precursor sediment or the redox chemistry of the ocean the oxidation of ferrous rich phases after deposition suggests that the precursor sediments of bif originally had a more reduced bulk composition this raises the possibility that in an ocean with negligible molecular oxygen and elevated si and fe the growth of iron rich clay minerals was favored over hematite

the anatomy of shallow crustal transpressional structures insights from the archaean carajas fault zone amazon brazil abstract the carajas fault zone cfz is the most prominent structure in the e w trending carajas strike slip system casss an ancient upper crustal fault network of probable late archaean age that cuts across the precambrian amazonian craton in brazil the subvertical faults reactivated pre existing high grade basement shear zone fabrics itacaiunas shear zone and display a complex long lived history of movement 1 ga dominated by oblique and strike slip displacements a postulated early regional phase of dextral movement downfaulted units of low grade to unmetamorphosed basement rocks into dilational bends and offsets in the casss subsequent sinistral transpression ca 2 6 ga led to faulting and folding during partial inversion of the cover rocks in these dilational sites with much of the associated deformation focused in the immediate vicinity

iron oxide mineralization at the contact zone between phyllite and itabirite of the pau branco deposit quadrilatero ferrifero brazil implications for fluid rock interaction during iron ore formation this study examines the whole rock geochemistry and laser ablation inductively coupled plasma mass spectrometry la icp ms iron oxide chemistry of itabirite and related iron ore as well as underlying phyllitic wall rock of the pau branco iron ore deposit quadrilatero ferrifero brazil the phyllite and iron ore of pau branco extend from hypogene deep seated into supergene weathered zones the phyllitic sequence that underlies the itabirite hosted iron ore along a sheared contact includes from the bottom to top carbonaceous and carbonate chlorite and fe rich zones which likely reflect distal to proximal alteration halos the fe rich phyllite and iron ore comprise a complex iron oxide mineralogy 1 magnetite martite is hosted in itabirite and related medium grade iron ore and present as disseminated minerals in phyllite 2 microplaty hematite replaces chlorite and carbonate in the phyllite 3 granoblastic hematite is a recrystallization precipitation product and the dominant iron oxide of the hard iron ore 4 schistose specular hematite overgrows earlier iron oxides and 5 goethite replaces earlier hematite and amphibole within the weathering horizon whole rock geochemistry and related mass balance calculations reveal that unlike the itabirite hosted iron ore interpreted to be the result of a solely residual iron enrichment via depletion of most major oxides and trace elements the iron enrichment of the phyllite is caused by addition of fe 2 o 3 the fe 2 o 3 is likely sourced from the overlying itabirite iron ore and is mineralogically reflected in the replacement of carbonate and chlorite by hematite the rare earth elements ree abundances of the different ore and phyllite zones reveal seawater like ree signatures and y ho ratios of the itabirite hypogene and supergene iron ore shale atypical ree patterns in the unaltered phyllite and eu fractionation trends this suggests a distinct input of hydrothermal fluids during the fe enrichment of the phyllite locally restricted ce fractionation is attributed to distinct e g mn rich lithostratigraphic intercalations within the itabirite laser ablation icp ms mineral chemistry of phyllite hosted martite and microplaty hematite suggests a strong chemical inheritance by the host rock and precursor mineral positive ce anomalies are restricted to phyllite and itabirite hosted martite only and therefore may reflect the highly oxidative conditions during martitization at pau branco the fluid rock interaction affected not only the itabirite but also the underlying phyllite resulting in distinct alteration halos that extend beyond the itabirite toward the footwall the significant chemical influence and importance of country rock lithologies as a possible elemental source for example must be taken into account when interpreting whole rock geochemical data and investigating an itabirite banded iron formation hosted iron ore system

petrographic and geochemical evidence for hydrothermal evolution of the north deposit mt tom price western australia high grade iron mineralisation 65 fe in the north deposit occurs as an e w trending synclinal sheet within banded iron formation bif of the early proterozoic dales gorge member and consists of martite microplaty hematite ore three hypogene alteration zones between unmineralised bif and high grade iron ore are observed 1 distal magnetite siderite iron silicate 2 intermediate hematite ankerite magnetite and 3 proximal martite microplaty hematite apatite alteration zones fluid inclusions trapped in ankerite within ankerite hematite veins in the hematite ankerite magnetite alteration zone revealed mostly h2o cacl2 pseudosecondary and secondary inclusions with salinities of 23 9 1 5 1 n 38 and 24 4 1 5 1 n 66 eq wt cacl2 respectively pseudosecondary inclusions homogenised at 253 59 9 c 1 n 34 and secondary inclusions at 117 10 0 c 1 n 66 the decrepitation of pseudosecondary inclusions above 350 c suggests that their trapping temperatures are likely to be higher i e 400 c hypogene siderite and ankerite from magnetite siderite iron silicate and hematite ankerite magnetite alteration zones have similar oxygen isotope compositions but increasingly enriched carbon isotopes from magnetite siderite iron silicate alteration 8 8 0 7 1 n 17 to hematite ankerite magnetite alteration zones 4 9 2 2 1 n 17 when compared to the dolomite in the wittenoom formation 0 9 0 7 1 n 15 that underlies the deposit a two stage hydrothermal supergene model is proposed for the formation of the north deposit early 1a hypogene alteration involved the upward movement of hydrothermal cacl2 rich brines 150 250 c likely from the carbonate rich wittenoom formation 13c signature of 0 9 0 7 1 n 15 within large scale folds of the dales gorge member fluid rock reactions transformed unmineralised bif to magnetite siderite iron silicate bif with subsequent desilicification of the chert bands stage 1b hypogene alteration is characterised by an increase in temperature possibly to 400 c depleted 13c signature of 4 9 2 2 1 n 17 and the formation of hematite ankerite magnetite alteration and finally the crystallisation of microplaty hematite late stage 1c hypogene alteration involved the interaction of low temperature 120 c basinal brines with the hematite ankerite magnetite hydrothermal assemblage leaving a porous martite microplaty hematite apatite mineral assemblage stage 2 supergene enrichment in the tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay

u pb rb sr and sm nd chronology of granitic basement hydrothermal albitites and uranium mineralization lagoa real south bahia brazil the granites orthogneisses hydrothermal albitities and rocks which have suffered uranium mineralization from the lagoa real district south state of bahia brazil have been investigated by u pb rb sr and sm nd techniques u pb values on zircons from the granitic protolith give an age of 1725 ma u pb on u mineralization dates the primary mineralization at 1395 ma and indicates a reworking at 480 ma which may represent the age of the thrusting of the lagoa real complex over espinhaco metasediments during the brazilian orogeny these two dates are given by rb sr on albitites but from sparse information and are not supported by unequivocal arguments the uranium deposition and sodium metasomatism however cannot be linked either with the thermal history of the granite or with brazilian thrusting sm nd gives scattered results which are suggestive of autochtonous reworkings of rees these results lead to the following suggestions 1 magmatic activity of subalkaline affinity existed within the sao francisco craton at about 1 7 ga 2 such subalkaline plutons are likely sources for u mineralization 3 at about 1 4 ga an unknown event caused hydrothermal activity leading to u deposition at lagoa real 4 brazilian overthrusting at about 480 ma did not play any genetic role in the hydrothermal activity

development of different types of pull apart microstructures in mylonites an experimental investigation abstract intragranular fracturing of feldspar porphyroclasts in granite mylonites of a ductile shear zone has led to development of diverse types of pull apart and bookshelf structures porphyroclasts with centrally located fractures show pull aparts with parallel walls type 1 whereas those with off centered fractures display pull aparts with both parallel type 1 and non parallel type 2 geometry analog model experiments performed by embedding segmented rigid elliptical objects within a viscous matrix indicate that the orientation and location of the fracture and the aspect ratio of the porphyroclast are the principal parameters in controlling the development of bookshelf structures and pull aparts in the case of centrally located fractures porphyroclasts of moderate aspect ratios develop either type 1 pull apart or bookshelf depending upon the fracture orientations whereas those of large aspect ratios 3 form only type 1 pull aparts irrespective of the fracture orientation off centered fracturing of porphyroclasts gives rise to fragments of unequal size which rotate independently at equal or contrasting velocities forming type 1 or type 2 v pull aparts respectively in the latter case depending upon the orientation of fracture the smaller fragment rotates faster or slower than the larger fragment showing relative tilt synthetic type 2a or antithetic type 2b to the bulk shear sense respectively type 2b geometry generally develops when the fracture angle with respect to the short axis of porphyroclasts is low antithetic to the shear direction and the long axis of the porphyroclast is at a high angle to the shear plane

sedimentary purpurins oxidative transformation products of chlorophylls examination of sediments from three contemporary lakes priest pot loch ness and lake baikal reveals the presence of oxidative transformation products of chlorophyll a in two of the settings purpurin 18 phytyl ester not previously reported in sediments occurs in the two lakes that have deep water columns the free acid counterpart purpurin 18 occurs in only one of the samples that from loch ness although where present the abundance of purpurin 18 phytyl ester is similar to that of individual chlorin steryl esters there appears not to be a correlation between their abundances in the sediments suggesting that formation of the purpurins is not mediated by herbivory the presence of oxidative transformation products of chlorophyll in contemporary lake bottom sediments indicates their formation during the earliest stages in chlorophyll degradation their preservation in sediments demonstrates that oxidative transformation reactions in the aquatic environment lead not only to destruction of the chlorophyll macrocycle but also to structures modified in ring e this lends support to the view that sedimentary aetioporphyrins originate mainly though not exclusively from transformations of chlorophylls that involve oxidative cleavage of the five membered exocyclic ring

3d visualization of sheath folds in ancient roman marble wall coverings from ephesos turkey abstract archaeological excavations and restoration of a palatial roman housing complex in ephesos turkey yielded 40 wall decorating plates of folded mylonitic marble cipollino verde derived from the internal hellenides near karystos greece cipollino verde was commonly used for decoration purposes in roman buildings the plates were serial sectioned from a single quarried block of 1 25 m3 and provided a research opportunity for detailed reconstruction of the 3d geometry of meterscale folds in mylonitized marble a gocad model is used to visualize the internal fold structures of the marble comprising curtain folds and multilayered sheath folds the sheath folds are unusual in that they have their intermediate axis normal to the parent layering this agrees with regional tectonic studies which suggest that cipollino verde structures formed by local constrictional non coaxial flow sheath fold cross section geometry exposed on the surface of a plate or outcrop is found to be independent of the intersection angle of the fold structure with the studied plane consequently a single surface cannot be used as an indicator of the three dimensional geometry of transected sheath folds

geochemical evolution of the banded iron formation hosted high grade iron ore system in the koolyanobbing greenstone belt western australia the banded iron formation bif hosted iron ore deposits in the lower greenstone succession of the koolyanobbing greenstone belt 50 km north of southern cross in western australia are a 200 mt high grade fe 58 pre mining resource and represents one of the most important iron ore districts in the yilgarn craton four hypogene alteration ore forming stages and one supergene upgrading event took place 1 during ore stage 1 lree depleted transition metal enriched mg fe ca carbonates replaced quartz in bifs the deposit scale alteration was most likely induced by devolatilization of sea floor altered ca si depleted mafic rocks in the vicinity of the bif during early regional syn d 1 very low to low grade metamorphism and was most strongly developed on reactivated bif basalt contacts 2 ore stage 2 involved the formation of patchy magnetite ore by a syn d 2 to d 4 dissolution of early carbonate enrichment of fe 2o 3total in magnetite iron ore was by a factor of 2 to 2 4 and compatible trace elements in magnetite such as ga v and al were immobile a subdeposit scale ferroan talc footprint proximal to magnetite iron ore in the largest deposit k deposit was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the bif under greenschist facies conditions and potentially high fluid rock ratio 3 magnetite growth during ore stage 3 forming granular magnetite martite ore is related to a subsequent hydrothermal event occurring locally throughout the belt especially in d 2b faults 4 ore stage 4 was associated with fe ca p l ree y enriched hydrothermal fluids possibly from a magmatic source such as the postmetamorphic lake seabrook granite that crops out about 10 km west of the koolyanobbing deposits and at the southern margin of the greenstone belt these ca enriched fluids interacted with distal metamorphosed mafic rock and influenced the bif ore system in a small number of deposits they were channelled through regional d 4 faults and caused specularitedolomite quartz alteration resulting in fe grades of up to 68 5 supergene upgrade ore stage 5 by further gangue leaching in the weathering zone was most effective in carbonate altered bifs and magnetite ore this process together with supergene martitization and goethite replacement of magnetite led to the formation of high grade locally at k deposit high p goethite martite ore at koolyanobbing the two geochemically distinct stages of archean carbonate alteration clearly controlled the formation of hypogene magnetite specularite martite rich ore and recent supergene modification including the further upgrade of fe ore 2012 by economic geology

the bif hosted high grade iron ore deposits in the archean koolyanobbing greenstone belt western australia structural control on synorogenic and weathering related magnetite hematite and goethite rich iron ore the koolyanobbing banded iron formation bif hosted iron ore deposits total premining resources 150 million metric tons mt indicated reserves 32 mt are located in the mesoarchean lower succession bif of the koolyanobbing greenstone belt younami terrane yilgarn craton in western australia in the koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the southern cross domain includes d 1 structures mainly small scale f 1a and f 1b folds formed in a north south to northwest southeast compressional regime a ductile to brittle deformation sequence d 2 to d 4 generated during east west compression and a late stage brittle segmentation of bif and reactivation of faults attributed to d 5 the formation of the seven known medium 45 58 wt fe to high grade 58 68 wt fe magnetite martite specularite and goethite bearing orebodies can be subdivided into four archean stages and one weathering related upgrade from the permian and or mesozoic to recent times the archean ore forming stages comprise 1 early fe mg ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz magnetite bif protolith 2 sequential syn d 2a coaxial to syn d 4 transpressional tight folding driven removal of carbonate quartz and minor ferroan talc by solution and mechanical transfer producing residual enrichment of medium to high grade magnetite ore 3 magnetite mineralization in syn d 2b and syn d 4 breccias and fractures forming medium grade ore zones or overprint magnetite in bif and first stage magnetite ore and 4 mineralization of hydrothermal specularite and locally associated ferroan dolomite quartz alteration and local oxidation of magnetite in and near brittle d 4 faults fractures and reactivated f 1 and f 2a fold cores modern weathering related leaching of carbonate and minor quartz pseudomorphic goethite replacement of existing iron oxides and gangue and coeval or subsequent to oxidation in the vadose zone formed goethite martite ore with local relics of specularite or magnetite and or kenomagnetite the intensity and localization of this supergene modification is in most deposits at koolyanobbing controlled by existing hypogene magnetite specularite rich medium to high grade ore zones and or carbonate altered bif at depth the existence of high grade ore below the weathering horizons suggests the possibility of further concealed magnetite and or specularite rich orebodies within the deposits and region

genesis of high grade hematite orebodies of the hamersley province western australia a reply sir we are very thankful for the constructive comments provided by kneeshaw and kepert unfortunately the competitive ownership structure of the iron ore mining industry in the hamersley province limits our personal knowledge of the mount whaleback deposit to occasional mine visits and to the available literature published and unpublished we do however have access to the data on the smaller giles mini and lido deposits 50 and 10 km west of mount whaleback respectively and to extensive exploration mapping in the ophthalmia ranges to the north direct proof for rotation of the orebody as it is preserved at paraburdoo in the approximately 50 dip of the overlying proterozoic hematite conglomerates of the mount mcgrath formation is not preserved at mount whaleback however we believe that there is sufficient information available in the literature to think that a rotational model for a large part of the deposit is perfectly plausible first and above all as stated in taylor et al 2001 f2 folds are more upright with axial planes dipping about 50 s in the southern and western part of the deposit away from the whaleback fault e g as shown on section no 315120e fig 4 in powell et al 1999 interestingly the ore controlling faults are also more upright in this part of the deposit which suggests an intimate link between the dip of these fault planes and dip of the axial planes of f2 folds secondly

high grade iron ore at windarling yilgarn craton a product of syn orogenic deformation hypogene hydrothermal alteration and supergene modification in an archean bif basalt lithostratigraphy banded iron formation bif hosted iron ore deposits in the windarling range are located in the lower greenstone succession of the marda diemals greenstone belt southern cross domain yilgarn craton and constitute a total hematite martite goethite ore resource of minimum 52 mt at 60 wt fe 0 07 p banded iron formation is interlayered with high mg basalts at windarling and precipitated during episodes of volcanic quiescence trace element content and the rare earth element ree ratios y ho 42 to 45 sm yb 1 5 together with positive la and gd anomalies in least altered hematite magnetite metachert bif indicate the precipitation from archean seawater that was fertilised by hydrothermal vent fluids with a basaltic hree y signature hypogene iron ore in sub greenschist facies metamorphosed bif formed during three distinct stages ore stage 1 was a syn to post metamorphic syn d1 fe ca mg ni co p ree metasomatism that produced local ni ree rich fe dolomite magnetite alteration in bif hydrothermal alteration was induced by hot fluid flow controlled by brittle ductile reactivation of bif basalt margins and crosscutting d1 faults the ni co rich content of dolomite and a shift in ree ratios in carbonate altered bif towards archean mafic rock signature y ho to 31 to 40 sm yb to 1 to 2 and gd gd to 1 2 to 1 4 suggest that high mg basalts in the windarling range were the primary source of introduced metals during ore stage 2 a syn deformational and likely acidic and oxidised fluid flow along bif basalt margins and within d1 faults leached carbonate and precipitated lepidoblastic and anhedral granoblastic hematite high grade magnetite hematite ore is formed during this stage ore stage 3 hydrothermal specular hematite spch fe dolomite quartz alteration was controlled by a late orogenic brittle compressional transpressional stage d4 the regional scale shear zone related d3 is not preserved in windarling this minor event remobilised iron oxides carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore ore stage 4 involved mesozoic to recent supergene oxidation and hydration in a weathering environment reaching down to depths of 100 to maximum 200 m below surface supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation important ground preparation for supergene modification and upgrade were mainly the formation of steep d1 to d4 structures steep bif basalt margins and particularly the syn d1 to syn d2 carbonate alteration of bif that is most susceptible to supergene dissolution the windarling deposits are structurally controlled supergene modified hydrothermal iron ore systems that share comparable physical chemical and ore forming characteristics to other iron ore deposits in the yilgarn craton e g koolyanobbing beebyn in the weld range mt gibson however the remarkable variety in pre syn and post deformational ore textures relative to d1 and d2 has not been described elsewhere in the yilgarn and are similar to the ore deposits in high strain zones such as of brazil quadrilatero ferrifero or iron quadrangle and nigeria the overall similarity of alteration stages i e the sequence of hydrothermal carbonate introduction and hypogene leaching with other greenstone belt hosted iron ore deposits supports the interpretation that syn orogenic bif alteration and upgrade was crucial in the formation of hypogene supergene iron ore deposits in the yilgarn craton and possibly in other archean paleoproterozoic greenstone belt settings worldwide

nonredox transformations of magnetite hematite in hydrothermal systems the transformation of magnetite to hematite or hematite to magnetite in nature has generally been considered a redox reaction and linked to a specific redox state of fluid however a nonredox reaction fe2o3 hm fe2 h2o fe3o4 mt 2h may have been the principal mechanism for the transformations of iron oxides in nature especially in hydrothermal environments for example the transformation of goethite and or hematite primary precipitates to magnetite in banded iron formations bifs probably occurred through nonredox reactions with fe2 bearing hydrothermal fluids during the accumulation of a bif sequence rather than through redox reactions involving organic matter during and or after the bif deposition the proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite rich secondary ores extending the target for orebodies to much deeper zones below the paleosurface another important implication of the proposed mechanism is that the presence or absence of magnetite and or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid

laboratory simulation of explosive volcanic loading and implications for the cause of the k t boundary intense explosive volcanism was simulated by preheating samples to 600 c and subjecting them to dynamic stress pulses of 0 9 and 1 3 gpa the resultant ejecta was examined optically and by transmission electron microscopy tem deformation and failure was entirely by brittle fracture and most fractures were irregular and intergranular none of the features typical of k t boundary sediments and of ejecta from meteorite impact craters planar deformation features amorphization mosaicism were observed there were also no signs of intracrystalline plasticity we conclude that explosive volcanism is incapable of producing the microstructural features seen at the k t boundary and that they must have been caused by impact of a large meteorite

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

the evolution of the geothermal gradient from lambert graben and mahanadi basin a contribution to the indo antarctic rift debate abstract the east antarctic lambert graben and the indian mahanadi basin are considered to represent segments of an intra gondwanan rift structure that was active at least since the paleozoic fission track analyses of apatites from a comprehensive data set across the shoulders of both grabens were used to compare their low temperature history and to estimate the paleo geothermal gradients before the onset of the last denudation rifting stage late jurassic the paleo geothermal gradients of both juxtaposed gondwana margins similarly increase from the basement towards the respective rift shoulder from 15 20 ckm 1 to 25 30 ckm 1 this trend of increasing paleo geothermal gradients together with a denudation episode commencing in the early cretaceous and coeval igneous activity indicates a common rifting stage accompanying the breakup of gondwana in the india antarctica sector

structural and hydrothermal evolution of the shear zone related iron oxide enrichment in metavolcano sedimentary rocks of the intracontinental aracuai orogen the case of the espirito santo iron deposit abstract the rio sao joao metavolcano sedimentary sequence is located in the eastern border of the northern espinhaco fold thrust belt in the intracontinental aracuai west congo orogen mafic schist quartz amphibole and carbonate itabirites as well as a carbonate silicate rock occur in the study area these contain mineral assemblages compatible with the amphibolite facies metamorphic conditions magnetite and hematite rich itabirites form a series of iron deposits in the region near igapora caetite bahia state brazil the espirito santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol of the rock volume siliciclastic and mafic rocks also host iron rich domains variably enriched in magnetite and hematite the deposit is hosted in compressional duplexes with structural top towards the sw these structures have an sn 1 sn foliation represented by compositional banding and parallel schistosity that is observed at all scales other common features are s c c boudins pinch and swell structures as well as sheath syn tectonic folds and a mineral stretching lineation lxn hydrothermal alteration was contemporaneous to the development of duplex and the sn 1 sn foliation a distal alteration zone is dominated by a potassic muscovite and or biotite chlorite assemblage and the intermediate zone by carbonate development the proximal alteration zone constitutes the iron oxide magnetite and hematite domains sulfide minerals as well as quartz veins silicification in association with other phases also formed hydrothermal magnetite is associated with c structures which grows either including silicates and skeletal carbonates or forming corrosion borders in these minerals lamellar hematite forms polycrystalline aggregates characterizing the s c structure in the sn 1 sn foliation as well as the axial planar foliation sn in intrafolial folds and the mineral stretching line lxn data obtained by the la icpms technique show that in general in quartz rich and amphibole itabirites hydrothermal magnetite is richer in ree than an early stage magnetite hydrothermal magnetite ree pattern is close to that of the country rock of the mineralization the formation of hematite and magnetite rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates quartz and the carbonates of all host rocks and precipitated new hydrothermal phases taking advantage of the ediacaran shear structures

structural control hydrothermal alteration zonation and fluid chemistry of the concealed high grade 4ee iron orebody at the paraburdoo 4e deposit hamersley province western australia high grade iron ore of the 4ee orebody of the 4e deposit 200 mt at 63 5 wt fe occurs as a southerly dipping sheet within banded iron formation bif of the paleoproterozoic dales gorge and joffre members of the brockman iron formation structural reconstruction of the 4e deposit shows that reactivation of the 18e fault and development of the nw striking steeply sw dipping 4e and 4ee normal faults resulted in preservation of the 4ee orebody below the 4e deposit and 400 m below the modern topographic surface three hypogene alteration zones between low grade bif and high grade iron ore are observed 1 distal magnetite quartz dolomite stilpnomelane hematite pyrite 2 intermediate magnetite dolomite hematitechlorite quartz stilpnomelane and 3 proximal hematite dolomite chlorite pyrite magnetite hydrothermal alteration is temporally and spatially constrained by nw trending dolerite dikes that intruded the 4e and 4ee faults prior to hypogene alteration six vein types v1 v6 are recognized at the 4e deposit the veins both cut and parallel the primary bif layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low grade bif to high grade iron ore our integrated structural hydrothermal alteration and fluid flow model proposes that during early stage 1a hypogene fluid flow in the 4e orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the paraburdoo range heated basinal brines were focused by the nw striking steeply sw dipping 4e and 4ee normal faults and reacted with bif of the dales gorge and joffre members the warm to hot 160 255 c ca rich 26 6 31 9 equiv wt cacl2 basinal brine interacted with magnetite chert layers transforming them into magnetite quartz dolomite stilpnomelane hematite pyrite bif the iron rich brine up to 2 8 wt fe likely originated from evaporated seawater that had lost mg and na and gained li and ca through fluid rock reactions with volcaniclastic rocks and carbonate successions within the wittenoom formation the first incursion of deeply circulating low salinity 5 8 9 5 wt nacl equiv heated 106 201 c modified meteoric water is recorded in late stage 1a minerals this modified meteoric water had lost some of its na through wall rock interaction with plagioclase possibly by interaction with dolerite of the weeli wooli formation that directly overlies the joffre and dales gorge members stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite quartz dolomite stilpnomelane hematite pyrite bif and produced both the intermediate magnetite dolomite hematite chlorite pyrite and the proximal hematite dolomite magnetite stilpnomelane alteration assemblages microplaty 10 80 m platy 100 250 m and anhedral hematite increasingly replace magnetite in the intermediate alteration zone forming the proximal alteration zones that consist of microplaty platy anhedral hematite and magnetite the intermediate and proximal alteration zones represent the mixing of a hot 250 400 c high salinity ca rich 30 40 wt cacl2 equiv sr rich basinal brine with low temperature and low salinity 5 wt nacl equiv modified meteoric water that was heated 100 200 c during its descent into the upper crust heterogeneous mixing of the two end member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures up to 200 c and salinities up to 25 wt nacl equiv stage 1c of the hypogene hydrothermal fluid is characterized by low temperature 110 c low salinity 5 wt nacl meteoric water that interacted with the proximal hematite dolomite magnetite stilpnomelane altered bif leaving a porous hematite apatite high grade ore supergene alteration affected the orebody since the cretaceous and produced a hematite goethite alteration assemblage resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering discovery of the concealed 4ee orebody of the 4e deposit demonstrates that structural geology plays a critical role in the exploration for high grade iron orebodies structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist

nature and evolution of fluids associated with specularite bearing fe and au pge jacutinga mineralization during the brasiliano orogeny in the eastern sao francisco craton minas gerais brazil abstract hydrothermal fluids related to the development of the aracuai orogen during the brasiliano cycle have affected paleoproterozoic banded iron formation bearing sequences along the eastern border of the sao francisco craton controversy surround the origin of specularite in high grade schistose iron ore bodies in the serpentina range serpentina type ore versus specularite rich s free au pge quartz veins jacutinga type that occur at the eastern border of the quadrilatero ferrifero mining district chemical and thermobarometric characteristics of fluid inclusions from serpentina type quartz specularite veins show that metamorphic aqueous saline fluids of low to moderate temperature i e salinity between 6 3 and 13 7 wt nacl eqv and homogenization temperatures from 91 to 228 c were responsible for silica leaching and fe enrichment along shear zones in the statherian serra da serpentina group these serpentina type fluids present a distinct pattern with na ca k the fluids are depleted in li na ba ca k cu and mg and enriched in as and sr in comparison to the jacutinga type fluids in contrast other two distinct fluids and chemical processes were involved to precipitate fe oxides with au pge in jacutinga type veins the first type is an aqueous carbonic fluid from the catas altas and itabira deposits these jacutinga type fluids present a pattern with na k ca mg and k li ba sr the fluids are interpreted to be of brasiliano age and are typically associated with anatectic pegmatites from santa maria de itabira the high salinity of these brines 23 2 25 3 wt nacl eqv from the itabira ore likely originated from their percolation through evaporitic layers from salt concentration during deformation or from fluid rock interactions mixing between high temperature aqueous carbonic pegmatite forming fluids and low temperature aqueous metamorphic brines could have induced an abrupt change in the physical chemical conditions of the system which led to au and pge precipitation in the itabira jacutinga type deposit a second jacutinga type fluid is observed at the gongo soco and barao de cocais deposits this fluid is interpreted as an aqueous saline metamorphic fluid with the pattern na ca k mg low to moderate salinity 4 3 16 4 wt nacl eqv and temperatures of 77 to 305 c it is relatively enriched in b depleted in li ba na cu zn and k and follows the trend k li ba in both types of fluids responsible for jacutinga type mineralization gold was transported as chlorine complexes under acidic and oxidizing conditions the metal precipitation may have occurred due to a rapid depressurization during failure of shear zones operating in the upper levels of the crust or variations in the ph and redox conditions during the interaction with the host dolomitic itabirite

neoproterozoic glacial dynamics revealed by provenance of diamictites of the bebedouro formation sao francisco craton central eastern brazil one of the main questions on neoproterozoic geology regards the extent and dynamics of the glacial systems that are recorded in all continents we present evidence for short transport distances and localized sediment sources for the bebedouro formation which records neoproterozoic glaciomarine sedimentation in the central eastern sao francisco craton sfc brazil new data are presented on clast composition based on point counting in thin section and shrimp dating of pebbles and detrital zircon cluster analysis of clast compositional data revealed a pronounced spatial variability of clast composition on diamictite indicating the presence of individual glaciers or ice streams feeding the basin detrital zircon ages reveal distinct populations of archean and palaeoproterozoic age the youngest detrital zircon dated at 874 9 ma constrains the maximum depositional age of these diamictites we interpret the provenance of the glacial diamictites to be restricted to sources inside the sfc suggesting deposition in an environment similar to ice streams from modern high latitude glaciers

domainal fabrics of hematite in schistose shear zone hosted high grade fe ores the product of the interplay between deformation and mineralization abstract schistose high grade hematite orebodies 64 wt fe in the iron quadrangle minas gerais were formed in shear zones by hydrothermal alteration of the paleoproterozoic caue bif during the transamazonian orogenesis the ore is comprised of platy hematite specularite grains that define the foliation and overprint a relict banded martite hematite fabric resembling at first sight a mylonite the ebsd analyses of a m scale schistose orebody from the pau branco mine show that specularite grew as elongated plates with the 00 1 plane parallel to the foliation the population of the measured grain aspect ratio gar is homogenous in different scales and the longest axes of the crystals align with the stretching lineation l x building continuous domains or anastomose around stretched iron oxide aggregates and rootless fold hinges the pole figure of the 00 1 plane shows usually a maximum centered on the pole of the foliation z often elongated on a girdle perpendicular to the lineation l the 10 4 pole figure has the configuration of a symmetric cleft girdle and the corresponding 11 0 and 10 0 pole figures present well developed girdles parallel to the foliation with an elongated maximum centered on x microstructures associated with crystal plastic behavior and dynamic recrystallization are missing and the fabric of the orebody probably results from precipitation of strain controlled oriented hematite plates and anisotropic syntaxial growth of favorably oriented grains with the intervention of hydrothermal fluids during fe enrichment the shear zone provided pathways for the percolation of mineralizing fluids under temperatures that varied from 140 to 350 c or higher under ductile or ductile brittle conditions the orthorhombic fabric and cpo crystallographic preferred orientation of the ore nevertheless contrast with the asymmetry of simple shear as observed in the torsion experiments by siemes et al 2010 siemes et al 2011 probably due to volume loss and possibly a flattening component of deformation in the ore zone

fabric development in brittle to ductile shear zones brittle to ductile shear zones from two separate geological settings are shown to have nucleated on zones of predominantly brittle deformation the shear zones are not simply foliated cataclasites since they contain abundant evidence of dynamic recrystallization of constituent minerals a small quartz diorite lens in the borrego springs shear zone southern california contains centimeter scale cataclasite zones that exhibit a gradual transition into foliated rock alteration of magnesiohornblende to actinolite feldspar to white mica plus quartz and biotite to chlorite produced elongate minerals that define the foliation during the later stages of deformation intracrystalline slip and dynamic recrystallization of quartz and feldspar were important deformation mechanisms