geochemical evolution of the precambrian old rag granite virginia u s a testing a uth exploration model abstract the old rag granite is one of several recognized units within the virginia blue ridge complex generally composed of grenville age gneisses and granitoid intrusive rocks chemical variation in the old rag granite indicates simple orthomagmatic crystallization from the borders to the core of the pluton fractional crystallization of plagioclase oxides apatite and zircon accounts for much of the variation some trace elements however show anomalous behavior in the central part of the pluton y and sr and possibly ba and nb are enriched whereas rb and total ree are depleted in the central part of the pluton late crystallization of monazite could contribute to the observed ree patterns it could not however account for rb depletion and sr enrichment and no clear relationship between ree data and th u ratios are observed the geochemical patterns indicate overall orthomagmatic crystallization overlain by a late stage hydrothermal event an exploration model for uraniferous granites based upon the uranium deposits at rossing and bokan mountain included the precambrian basement of central and northern virginia in a list of proposed target areas geological mineralogical and whole rock chemical characteristics show a close association between the old rag granite and the rossing type deposits the uranium mean 6 9 ppm range 0 1 19 8 ppm and thorium mean 43 ppm range 3 5 114 ppm values are anomalous th u ratios average 7 5 and are widely variable indicating a decoupling of uranium from thorium neither u nor th are lognormally distributed within the old rag granite requiring an explanation beyond simple removal of u by weathering rayleigh fractionation modeling shows that both u and th although probably distributed originally by closed system fractionation have been subsequently mobilized uranium can be lost in an oxidized form by secondary processes such as weathering or hydrothermal alteration thorium however will be affected only by hydrothermal processes it is concluded that although primary fractionation was orthomagmatic the old rag granite was affected by late hydrothermal alteration whereby u was lost from the body and th was redistributed three primary avenues for u migration are possible 1 the rocks of the saddleback mountain intrusive suite which may have been contemporaneous with the old rag granite 2 paleozoic fault zones that cut the old rag pluton and 3 quartz veins pegmatites cupolas and roof pendants of the country rock nellysford gneiss that have been lost to erosion the last option is preferred on geological and geochemical grounds it is possible that the sedimentary uranium deposits of the adjacent triassic culpepper basin were derived from eroded upper levels of the old rag granite and like intrusives of the virginia blue ridge complex

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

oxidized magnetite series rapakivi type granites of carajas brazil implications for classification and petrogenesis of a type granites abstract the varying geochemical and petrogenetic nature of a type granites is a controversial issue the oxidized magnetite series a type granites defined by anderson and bender anderson j l bender e e 1989 nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america lithos 23 19 52 are the most problematic as they do not strictly follow the original definition of a type granites and approach calc alkaline and i type granites in some aspects the oxidized jamon suite a type granites of the carajas province of the amazonian craton are compared with the magnetite series granites of laurentia and other representative a type granites including finnish rapakivi and lachlan fold belt a type granites as well as with calc alkaline i type orogenic granites the geochemistry and petrogenesis of different groups of a types granites are discussed with an emphasis on oxidized a type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis oxidized a type granites are clearly distinguished from calc alkaline cordilleran granites not only regarding trace element composition as previously demonstrated but also in their major element geochemistry oxidized a type granites have high whole rock feo t feo t mgo tio 2 mgo and k 2 o na 2 o and low al 2 o 3 and cao compared to calc alkaline granites the contrast of al 2 o 3 contents in these two granite groups is remarkable the cao feo t mgo tio 2 vs cao al 2 o 3 and cao feo t mgo tio 2 vs al 2 o 3 diagrams are proposed to distinguish a type and calc alkaline granites whole rock feo t feo t mgo and the feo t feo t mgo vs al 2 o 3 and feo t feo t mgo vs al 2 o 3 k 2 o na 2 o diagrams are suggested for discrimination of oxidized and reduced a type granites experimental data indicate that besides pressure the nature of a type granites is dependent of o 2 conditions and the water content of magma sources oxidized a type magmas are considered to be derived from melts with appreciable water contents 4 wt originating from lower crustal quartz feldspathic igneous sources under oxidizing conditions and which had clinopyroxene as an important residual phase reduced a type granites may be derived from quartz feldspathic igneous sources with a metasedimentary component or alternatively from differentiated tholeiitic sources the imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct a type granite groups assuming conditions near the nno buffer as a minimum for oxidized granites magnetite bearing granites formed near fmq buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced a type granites and respectively magnetite series and ilmenite series granites is not always observed

synorogenic hydrothermal origin for giant hamersley iron oxide ore bodies geologic mapping basin analysis and calculated fluid compositions indicate that giant orebodies of microplaty hematite and possibly martite goethite in the hamersley province of western australia were formed by heated fluids driven by early paleoproterozoic orogenesis detrital grains of microplaty hematite in the mcgrath trough a foreland basin in front of the northward advancing ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to 200 c and locally up to 400 c were involved regional circulation of hydrothermal fluids including heated surface water through reduced banded iron formations occurred during or soon after the ophthalmian orogeny we speculate that martite goethite orebodies previously considered mesozoic cenozoic could also be related to heated paleoproterozoic meteoric fluids migrating northward away from the ophthalmian fold belt

oxygen isotopes in iron iii oxides abstract quartz sio 2 and hematite fe 2 o 3 possess considerably different gram formula weights and different stoichiometric coefficients for their oxygen these differences mean that the numerical values of sio 2 and fe 2 o 3 compositions when reported as weight percent are not the same as the numerical values of x o i e oxygen in the mineral as a mole percent of the total oxygen in a binary quartz hematite system consequently end member quartz and hematite 18 o values which are determined from linear extrapolations of 18 o vs weight percent fe 2 o 3 data from binary mixtures can be in error by as much as several per mil extrapolation to end member 18 o values in binary quartz hematite systems should always be performed using mole fraction of oxygen as the compositional variable the hematite water fractionation curve of yapp and the quartz water curve of knauth and epstein were used together with the kinetic isotope exchange model of criss et al to constrain the depositional environment of the late proterozoic quartz hematite banded iron formation deposits of urucum brazil the depositional temperatures permitted by the model assumptions employed here range from 0 to 35 c or 54 c the permitted range of depositional water 18 o values is from 6 6 or 8 9 to 0 0 while the range of permitted depositional conditions is rather large it does encompass the possible cool temperatures and brackish or fresh waters which are indicated by sedimentologic evidence in these deposits

experimental determination of the compositional limits of immiscibility in the system cacl2h2oco2 at high temperatures and pressures using synthetic fluid inclusions abstract using the synthetic fluid inclusion method the temperature pressure and compositional limits of fluid immiscibility in the binary system cacl 2 h 2 o have been determined at 600 and 700 c at pressures between 1 and 2 kbar the limits of the immiscible regions were determined by observation of two different inclusion types in the samples and by measurements of the depression of ice melting temperatures the region of immiscibility in this system extends to much higher pressures than that of the nacl h 2 o system with similar compositional ranges a similar study was done on the cacl 2 h 2 o co 2 system at 500 600 and 700 c at pressures of 1 1 5 2 and 3 kbar in the ternary system the presence of two different inclusion types and the measurement of clathrate melting temperatures were used to delineate the compositional regions of immiscibility the presence of co 2 extends the regions of immiscibility shown in the binary system study to much higher pressures tie lines in the two phase regions were determined by measuring the homogenization temperatures of co 2 vapor and liquid phases and the volume ratios of inclusion bubble to total inclusion in vapor rich type inclusions the locations of these tie lines indicate that the cacl 2 is heavily partitioned towards the more liquid rich phase the experimental results show that with even relatively small amounts of a divalent salt such as cacl 2 immiscibility can exist to very high temperatures and pressures from these results immiscibility should be a very common phenomenon in geologic processes ranging from sedimentary to quite high grade metamorphic environments and that it should have a very important effect on reactions between hydrothermal fluids and rocks and the resulting mineral assemblages

geochronological constraints on the genesis of high grade iron ore in the gongchangling bifs from the anshan benxi area north china craton abstract the gongchangling iron deposit located in the northeast of the north china craton is hosted in late neoarchean algoma type bifs it is famous for the major production of high grade iron ore in the gongchangling no 2 mining area in china with regard to the genesis of high grade iron ore more and more evidences indicate that it was related with hydrothermal enrichment of bifs however the hydrothermal nature was argued for meteoric metamorphic or migmatitic fluid in this study the trace element compositions of garnet from the altered wall rock of high grade iron ore obtained by la icp ms show compositional zoning indicating it is a metamorphic origin these garnets yield a sm nd isochron age of 1888 77 ma interpreted as the time of metamorphism in this area la icp ms u pb dating of zircon from the upper migmatite zone shows that the migmatitic granite was formed at 2478 36 ma since the ree patterns of garnet suggest that it was formed in the nearly neutral fluid rather than the acid meteoric fluid in the paleoproterozoic hence combined the ages of metamorphism and migmatization with high grade iron ore of 1840 7 or 1860 7 ma obtained by previous studies it indicates that the genesis of the high grade iron ore derived from the enrichment of bifs by metamorphic hydrothermal fluid

regional scale proterozoic iocg mineralized breccia systems examples from the wernecke mountains yukon canada a large scale proterozoic breccia system consisting of numerous individual breccia bodies collectively known as wernecke breccia occurs in north central yukon territory canada breccias cut early proterozoic wernecke supergroup sedimentary rocks and occur throughout the approximately 13 km thick deformed and weakly metamorphosed sequence iron oxide copper gold uranium cobalt mineralization is associated with the breccia bodies and occurs as veins and disseminations within breccia and surrounding rocks and locally forms the breccia matrix extensive sodic and potassic metasomatic alteration occurs within and around breccia bodies and is overprinted by pervasive calcite and dolomite ankerite and locally siderite alteration respectively multiple phases of brecciation alteration and mineralization are evident breccia bodies are spatially associated with regional scale faults and breccia emplacement made use of pre existing crustal weaknesses and permeable zones new evidence indicates the presence of metaevaporitic rocks in lower wsg that may be intimately related to breccia formation no evidence of breccia age magmatism has been found to date

tourmaline b isotopes fingerprint marine evaporites as the source of high salinity ore fluids in iron oxide copper gold deposits carajas mineral province brazil the carajas mineral province in northern brazil contains a variety of world class 100 mt ore iron oxide copper gold iocg deposits including the only archean examples of this deposit class e g the igarape bahia alemao and salobo deposits tourmaline of schorldravite composition a common gangue mineral in these deposits precipitated shortly prior to and after the ore assemblage a boron isotope study of texturally different tourmaline from three iocg deposits igarape bahia salobo and sossego using secondary ion mass spectrometry sims provides new evidence in the long standing debate of magmatic versus non magmatic sources for the high salinity up to 50 wt nacl equiv of ore fl uids in these deposits values of 11 b from 14 to 26 5 for the igarape bahia and salobo deposits confi rm marine evaporite derived brines in the ore fl uids whereas lower 11 b values for the igarape bahia deposit 5 8 to 8 8 suggest that these fl uids may have mixed with an isotopically different hydrothermal fl uid or one that had a mixed boron source more variable and isotopically lighter boron in tourmaline from the sossego deposit 8 to 11 is attributed to mixed sources including light boron leached from felsic intrusive and volcanic host rocks and heavy boron derived from marine evaporites the boron isotope data indicate that the characteristic high salinity of the ore fl uids in the carajas mineral province was acquired by the interaction of hydrothermal fl uids with marine evaporites for iocg deposits that contain tourmaline as a common gangue mineral boron isotopes offer a valuable tool to constrain the high salinity source problem which is a critical issue in metallogenesis of iocg deposits worldwide

geology and shrimp u pb geochronology of the igarape bahia deposit carajas copper gold belt brazil an archean 2 57 ga example of iron oxide cu au u ree mineralization a striking feature of the carajas region brazil is the clustering of a variety of different types of cu au deposits the most abundant in the belt are the 200 million metric tons mt of fe oxide cu au u ree deposits which despite the variety of host rocks and different orebody morphologies share a number of diagnostic features including 1 intense fe metasomatism leading to the formation of grunerite fayalite and or fe oxides magnetite and or hematite 2 intense carbonate alteration mainly siderite 3 sulfur poor ore mineralogy chalcopyrite and bornite 4 quartz deficient gangue 5 extreme low ree enrichment and 6 enrichment in u and co the igarape bahia deposit is perhaps the best documented fe oxide cu au u ree deposit of the belt containing about 219 mt at 1 4 percent cu and 0 86 g t au the cu au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano sedimentary rocks shrimp ii zircon dating of the host metavolcanic rocks gives a 207pb 206pb age of 2748 34 ma this suggests a correlation between the igarape bahia volcano sedimentary sequence and the grao para volcanic rocks which have published ages of ca 2 75 ga shrimp dating of monazite from the matrix of ore bearing magnetite breccias gives a 207pb 206pb age of 2575 12 ma confirming the epigenetic nature of the mineralization and placing it 175 m y after accumulation of the host volcano sedimentary sequence the 2575 12 ma shrimp age of hydrothermal monazite from the igarape bahia mineralization is indistinguishable from published conventional 207pb 206pb ages for zircons from the archean a type granites of the carajas belt indicating that mineralization processes at igarape bahia were temporally related to these a type archean granites the wide range of highly radiogenic 87sr 86sr ratios 0 714 0 755 of carbonates from the igarape bahia deposit suggests multiple crustal sources consistent with a magmatic hydrothermal origin shrimp dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207pb 206pb age of 2670 ma consistent with field evidence and the age of host rocks but does not unequivocably constrain the age of the ores the styles of hydrothermal alteration mineralogy and geochemistry of the igarape bahia ore as well as published fluid inclusion and stable isotope data support its classification as a member of the world class olympic dam type fe oxide cu au u ree group of deposits as previously argued by several authors the shrimp age of 2575 12 ma for hydrothermal monazite indicates that igarape bahia is an archean example of this deposit group

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

fluid types and their genetic meaning for the bif hosted iron ores krivoy rog ukraine abstract this paper contributes to the understanding of the genesis of epigenetic hypogene bif hosted iron deposits situated in the eastern part of ukrainian shield it presents new data from the krivoy rog iron mining district skelevatske magnetitove deposit frunze underground mine and balka severnaya krasnaya outcrop and focuses on the investigation of ore genesis through application of fluid inclusion petrography microthermometry raman spectroscopy and baro acoustic decrepitation of fluid inclusions the study investigates inclusions preserved in quartz and magnetite associated with the low grade iron ores 31 37 fe and iron rich quartzites 38 45 fe of the saksaganskaya suite as well as magnetite from the locally named high grade iron ores 52 56 fe these high grade ores resulted from alteration of iron quartzites in the saksaganskiy thrust footwall saksaganskiy tectonic block and were a precursor to supergene martite high grade ores 60 70 fe based on the new data two stages of iron ore formation metamorphic and metasomatic are proposed the metamorphic stage resulting in formation of quartz veins within the low grade iron ore and iron rich quartzites involved fluids of four different compositions co 2 rich h 2 o h 2 o co 2 n 2 ch 4 nacl nahco 3 and h 2 o co 2 n 2 ch 4 nacl the salinities of these fluids were relatively low up to 7 mass nacl equiv as these fluids were derived from dehydration and decarbonation of the bif rocks however the origin of the nahcolite nahco 3 remains unresolved the minimum p t conditions for the formation of these veins inferred from microthermometry are t min 219 246 c and p min 130 158 mpa the baro acoustic decrepitation analyses of magnetite bands indicated that the low grade iron ore from the skelevatske magnetitove deposit was metamorphosed at t 530 c the metasomatic stage post dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high grade iron ores the genesis of these ores which are located in the saksaganskiy tectonic block saksaganskiy ore field and the factors controlling iron ore forming processes are highly controversial according to the study of quartz hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow simultaneous with thrusting and deformation during the 1 st episode three types of fluids were introduced co 2 ch 4 n 2 c co 2 n 2 ch 4 and low salinity h 2 o n 2 ch 4 nacl 6 38 7 1 mass nacl equiv the 2 nd episode included expulsion of the aqueous fluids h 2 o n 2 ch 4 nacl co 2 c of moderate salinities 15 22 16 76 mass nacl equiv whereas the 3 rd event involved high salinity fluids h 2 o nacl c 20 35 mass nacl equiv the fluids most probably interacted with country rocks e g schists supplying them with ch 4 and n 2 the high salinity fluids were most likely either magmatic hydrothermal fluids derived from the saksaganskiy igneous body or heated basinal brines and they may have caused pervasive leaching of fe from metavolcanic and or the bif rocks the baro acoustic decrepitation analyses of magnetite comprising the high grade iron ore showed formation t 430 500 c the fluid inclusion data suggest that the upgrade to high grade fe ores might be a result of the krivoy rog bif alteration by multiple flows of structurally controlled metamorphic and magmatic hydrothermal fluids or heated basinal brines

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

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

oxygen and hydrogen isotope variations among goethites α feooh and the determination of paleotemperatures abstract a plot of d vs 18o values from a diverse group of natural goethites defines a data array which is approximately parallel to the meteoric water line the 18o values range from 9 5 to 4 4 per mil while the corresponding d values vary from 202 to 94 per mil there is considerable scatter in the d vs 18o array which can be attributed primarily to different temperatures of mineral formation as determined previously yapp and pedley 1985 and confirmed in this study the goethite water d h fractionation factor is essentially independent of temperature over the range from 25 c to 145 c however the goethitewater 18 o 16 o fractionation factor does vary with temperature as indicated by initial goethite synthesis experiments conducted at 25 c 44 c and 62 c the synthetic goethite 18 g w values suggest that the natural goethites in this study formed at temperatures that may range from about 8 c to 66 c support for the validity of these approximate calculated temperatures is provided by a natural goethite sample from the atlantis ii deep in the red sea these results suggest that goethite can be used as a single mineral low temperature geothermometer perhaps more importantly goethite can be used in low temperature oxygen isotope mineral pair geothermometers such as chert goethite calcite goethite phosphate goethite etc

dating hypogene iron mineralization events in archean bif at weld range western australia insights into the tectonomagmatic history of the northern margin of the yilgarn craton this study is the first to constrain the absolute timing of hypogene iron mineralization in archean bif located in the yilgarn craton in situ shrimp u th pb geochronology on xenotime and monazite grains has been used to constrain the age of hypogene magnetite replacement ores at the beebyn deposit and hypogene magnetite vein ores at the madoonga deposit in the weld range study area the beebyn magnetite replacement ores c 2627 ma are younger than the published maximum depositional age of the bif hosts of the wilgie mia formation 2792 9 ma and partially overlaps crystallization ages of granitic rocks 2757 2606 ma that intrude supracrustal rocks throughout the study area these plutons and their probable subvolcanic expressions are considered to be likely sources of energy and fluids responsible for magnetite replacement ores in contrast madoonga magnetite veins record multiple dates the first at 2857 41 ma followed by events at c 2775 ma through 1812 ma the two oldest monazite dates of 2857 41 and 2832 51 ma are interpreted to be mineralization ages for magnetite veins possibly related to subseafloor volcanism and base metal vms systems whereas the younger dates i e 2775 1812 ma probably represent multiple episodes of phosphate mineral precipitation related to reactivation of structures and overprinting by discrete pulses of hydrothermal fluids the madoonga bifs are genetically distinct from the beebyn bifs and are the oldest dated bifs at weld range being older than c 2857 ma but younger than the c 2970 ma felsic volcanic rocks in the stratigraphic footwall to the madoonga bifs hydrothermal events at the madoonga deposit 2215 2120 ma coincide with published dates for rifting mafic ultramafic magmatism and basin development along the northern margin of the yilgarn craton 2215 2145 ma whereas the younger phosphate mineral dates correspond with the glenburgh 2002 1947 ma and capricorn orogenies 1817 1772 ma tectonic activity along the northern margin of the yilgarn craton coincides with reported ages for hematite mineralization in bif hosted iron ore deposits in the hamersley basin and pilbara craton suggesting the far reaching effects of tectonism along paleocratonic boundaries as drivers for iron mineralization in bif

sulfur and carbon isotopes in archean banded iron formations implications for sulfur sources abstract sulfur isotope ratios of pyrite from two late archean banded iron formations were used to constrain possible sources of sulfur the iron formations studied were the temagami iron formation which hosts the sherman mine near temagami and the boston iron formation including the adams mine south of kirkland lake both in ontario in both cases algomantype oxide facies magnetite chert ores predominate the sulfur studies were supplemented by limited carbon isotopic data for the sherman mine iron formation in the sherman mine occurs on both limbs of a regional synformal structure each iron formation unit caps a thick calc alkaline andesite dominated volcanic sequence with one band hosted in sedimentary rocks of turbidite origin the adams mine includes four major iron formation bands interbedded with mafic and ultramafic volcanic rocks and greywacke of turbidite origin the sulfur isotope statistics for averages of pyrites from these three units are sherman mine volcanic hosted suite 34 s 1 7 4 6 n 14 sherman mine sediment hosted suite 34 s 1 4 1 6 n 43 and adams mine 34 s 1 8 2 9 n 17 these averages differ by from juvenile sulfur and together with their limited dispersions suggest a predominantly unfractionated source of sulfur it is argued that this sulfur has been derived from the associated volcanic piles most of the pyrite occurs as disseminated grains in oxide facies iron formation and interbedded clastic sediments a distinctive nodular pyrite in the sherman mine sediment hosted suite occurs in cherty graphitic sulfide facies iron formation while the sulfur isotopic properties of this subset 34 s 0 0 0 8 are not suggestive of biogenic sulfur the geological setting and carbon isotopic compositions of associated graphite 13 c 34 imply that anaerobic bacterial processes were active in this reducing environment the apparent lack of biogenic sulfide is rationalized by models suggesting that archean marine basins differed from phanerozoic analogues in various parameters which influence sulfur isotopic characteristics it is argued that biogenic sulfide is probably a significant component of the sulfur in sherman mine nodular pyrite

reflectance spectroradiometry applied to a semi quantitative analysis of the mineralogy of the n4ws deposit carajas mineral province para brazil abstract quantifying the abundance and physicochemical properties of minerals using reflectance spectroradiometry in the visible near infrared and shortwave infrared 400 2500 nm regions is an important tool in mineral exploration in this study the reflectance spectra of drill cores from the world class n4ws iron deposit located in the carajas mineral province brazil were obtained these spectra were validated using x ray fluorescence xrf geochemical analyses and thin sections the reflectance spectra were collected using a fieldspec 3 spectroradiometer asd boulder colorado usa in 10 drill cores the mineralogy of the deposit is mainly hematite with lesser amounts of magnetite goethite quartz kaolinite gibbsite smectite talc carbonate and chlorite the mineralogy of the iron deposit was extracted from the spectral data using the geometry depth and wavelength of absorption features across the reflectance spectrum removed from the continuum the depth of the absorption features is proportional to the mineral abundance and the wavelength is proportional to the mineral chemical composition the diagnostic absorption features of each mineral were used to determine the mineral abundance and composition the final products include the abundance of iron hydro oxide 11 6 root mean square error rmse fe2o3 abundance of aluminous clays rmse 6 al2o3 abundance of talc 8 rmse mgo identification of clay type kaolinite montmorillonite or gibbsite composition of carbonate dolomite vs calcite and composition of chlorite mg vs fe the mineral abundance and composition results provided an effective characterisation of the ore protore and host rocks and showed variations within the ore body

fe solubilities in very saline hydrothermal fluids their relation to zoning in some ore deposits the fe concentrations in fluids present in very saline fluid inclusions from the king island w skarn hole 16 f sn w deposit and the mary kathleen u rare earth element skarn have been determined using the phase deposit volume method fe bearing daughter minerals include magnetite pyrite amarantite fe super 3 so 4 oh 3h 2 o and hydrated ferrous chloride the fluid inclusion liquid compositions are inferred from a combined analysis of first melting temperatures eutectic temperatures of the salt system daughter crystal identification by scanning electron microscopy in conjunction with an energy dispersive x ray analyzer and relevant published experimental salt solubilities maximum fe solubilities show an exponential increase to 9 wt percent fe with increasing total anion content of the fluid especially in the 20 to 30 wt percent interval fe solubility is mainly dependent on total chloride contents with fluoride contributing very little to increasing the fe solubility temperature f sub o 2 and f sub s 2 appear to have a minor influence on the total fe solubilities some of the samples contain solid fe bearing crystals which coexisted with the fluid inclusions and probably buffered fe at the appropriate a fe values during genesis but the fe contents of most fluids were probably undersaturated with respect to fe bearing minerals over the range for which the total anion content of the fluid is 25 to 35 wt percent the apparent maximum fe content of fluids tends to decrease in the following order when the fe bearing daughter crystals are present magnetite pyrite hydrated ferrous chloride raquo amarantite although anomalies to this trend occur the fe solubility data are used to explain some fe mineral overprints stages ii and iii of skarns the magnetite fe chlorite cassiterite veins in tin granites and the zonal pattern present in porphyry copper systems the fe data suggest that the highly saline fe bearing solutions present in the cores of porphyry systems and other near contact deposits are not a result of increased salinity of hydrothermal fluids due to boiling of meteoric solutions but are inherent from an orthomagmatic source

polymetamorphic relations in iron ores from the iron quadrangle brazil the correlation of oxygen isotope variations with deformation history this study deals with the oxygen isotope composition of hematite rich ore bodies in the iron quadrangle brazil the area studied can be divided into two different regions a western w zone of greenschist facies assemblages and an eastern e zone of amphibolite facies with transitions into granulite facies assemblages

contrasting behavior of oxygen and iron isotopes in banded iron formations revealed by in situ isotopic analysis abstract in situ o and fe isotope measurements of magnetite and hematite in banded iron formations bifs from the 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia document distinct fine scale isotopic zonation for hematite o 18 values vsmow range from 7 1 to 0 6 and fe 56 values range from 0 50 to 1 53 magnetite has a o 18 range of 5 6 to 7 0 and a fe 56 range of 0 76 to 1 33 notably magnetite shows significant o isotope variability at a o 18 values exists between low si core and si rich overgrowth iron isotope compositions are homogeneous to 0 2 in fe 56 values within individual grains or layers hematite is always isotopically heavier than co existing magnetite by 0 1 0 4 in fe 56 within individual samples and there is a large variation 2 in fe 56 values between bif samples at the m scale e g between meso or macro bands of the dales gorge member in situ isotopic results highlight the distinct behavior of o and fe isotopes during interaction with post deposition diagenetic or metamorphic fluids the large variations in o 18 values of iron oxides likely reflect exchange with fluids during post depositional events at temperatures up to ca 280 c and the lower o 18 values of fe oxides are interpreted to reflect less isotopic modification and lower temperatures o 18 variations support a model where initial fe oh 3 precipitates that formed in the photic zone were converted to hematite during very early diagenesis followed by production of magnetite over a range of temperatures during later diagenesis metamorphism in contrast magnetite and hematite seem to have preserved the initial fe 56 values of early fe oh 3 precipitates despite significant changes in o isotope compositions in magnetite the consistency in apparent fe isotope fractionation between co existing hematite and magnetite suggests that they share a common precursor where magnetite was formed in soft sediment through in situ reduction by microbial processes and possibly including reaction with aqueous fe ii combined in situ fe o isotope analyses allow distinctions to be made between near primary and post depositional signatures in bifs which bears on the use of bifs as paleo environmental proxies and recorders of microbial processes

petrology and petrogenesis of the bokan granite complex southeastern alaska the bokan granite complex is a peralka line ring dike complex emplaced into marine shales volcanic flows and tuffs and plutonic rocks mineralogically and chemically the complex is composed of i type granites aegirine and riebeckite bearing granite aplites porphyries and pegmatites comprise twelve distinct intrusive episodes the aegirine bearing rocks occur in an outer annular zone formed during early crystallization subsequent rocks are riebeckite bearing due to devolatilization of the magma chamber during a collapse ring dike emplacement event early crystallization of alkali feldspar occurred during magma ascension from a lower crustal upper mantle source at shallow depths subsolvus crystallization allowed microcline and albite to dominate local bodies of aegirine syenite were formed during the early collapse ring dike emplacement in response to the magma devolatilization the riebeckite granites reflect lower po2 and possibly declining peralkalinity the granitic rocks at bokan all exhibit na2o contents greater than k2o lithophile elements are concentrated in all the rocks especially in zones where hydrothermal albite and chlorite formed rb sr ratios increase in progressively younger rocks in the complex agpaitic ratios vary from 0 92 to 2 08 for the granitic rocks

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

iron isotope and ree y composition of the caue banded iron formation and related iron ores of the quadrilatero ferrifero brazil the minas supergroup banded iron formations bifs of the brazilian quadrilatero ferrifero qf mineral province experienced multiple deformational events synchronous with hypogene mineralization which resulted in the metamorphism of bifs to itabirites and their upgrade to high grade iron ore here we present rare earth element and yttrium ree y compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during bif deposition and the effects of hypogene iron enrichment the ree y characteristics of itabirites positive eu anomaly and lree depletion indicate hydrothermal iron contribution to the minas basin iron isotope data and ce anomalies suggest bifs were precipitated by a combination of anoxic biological mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen such as an oxygen oasis perhaps related to increase in oxygen concentrations before the great oxidation event goe the similarity of the ree y composition of the itabirites from the different qf deformational domains as well as to other superior type bifs indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original bifs however iron isotope compositions of iron ore vary systematically between deformational domains of the qf likely reflecting the specific mineralization features in each domain

origin of fluids in iron oxide copper gold deposits constraints from δ 37 cl 87 sr 86 sr i and cl br the origin of the hypersaline fluids magmatic or basinal brine associated with iron oxide cu u au ree deposits is controversial we report the first chlorine and strontium isotope data combined with cl br ratios of fluid inclusions from selected iron oxide copper gold iocg deposits candelaria raul condestable sossego a deposit considered to represent a magmatic end member of the iocg class of deposit gameleira and a magnetite apatite deposit el romeral from south america our data indicate mixing of a high 37cl magmatic fluid with near 0 37cl basinal brines in the candelaria raul condestable and sossego iocg deposits and leaching of a few weight percent of evaporites by magmatic hydrothermal fluids at gameleira and el romeral the sr isotopic composition of the inclusion fluids of candelaria raul condestable and el romeral confirms the presence of a non magmatic fluid component in these deposits the heavy chlorine isotope signatures of fluids from the iocg deposits candelaria raul condestable sossego reflecting the magmatic hydrothermal component of these fluids contrast with the near 0 37cl values of porphyry copper fluids known from the literature the heavy chlorine isotope compositions of fluids of the investigated iocg deposits may indicate a prevailing mantle cl component in contrast to porphyry copper fluids an argument also supported by os isotopes or could result from differential cl isotope fractionation processes e g phase separation in fluids of iocg and porphyry cu deposits

can high alumina arc basalt be derived from low alumina arc basalt evidence from kanaga island aleutian arc alaska the coexistence of both high mgo low alumina basalt lab and low mgo high alumina basalt hab magmatic inclusions within a single andesitic host lava on kanaga island provides the opportunity to rigorously evaluate the presence or absence of a fractionation relation between the two basalt magma types traditional mass balance calculations pearce ratio arguments comparative phenocryst modes and compositions and incompatible element rb ba zr abundances all indicate that selective clinopyroxene and orthopyroxene fractionation from the plagioclase clinopyroxene orthopyroxene titanomagnetite saturated lab magma yielded the plagioclase phyric hab magma the same data are consistent with a model of low pressure hab generation calling upon selective mafic crystal fractionation along the margins of a crystallizing and convecting lab magma chamber the ability to demonstrate that nonliquid hab magma can be produced from lab through crystal fractionation eliminates many of the arguments used in support of a primary eclogite melting origin for hab

paleobasinal to band scale ree y distribution in iron formations from carajas amazon craton brazil abstract banded iron formations bifs from the carajas mineral province cmp southeastern amazonian craton are the protolith of the world class iron deposits of the cmp besides being one of the largest iron ore deposits in the world they recorded important paleoenvironmental and regional geodynamic markers across the neoarchean paleoproterozoic transition we investigate the secular variation of geochemical signature from several deposit of carajas province the low contents of al2o3 and of 19 other immobile trace elements e g zr th hf ti in 26 high resolution detritus free bif samples indicate their detritus free nature their positive shale normalized subscript sn la and gd anomalies are similar to those of modern seawater whereas their y ho ratios are higher these bif samples also display large positive chondrite normalized subscript cn eu anomalies reflecting the influence of high temperature hydrothermal vents and bottom solutions in contrast to modern seawater and to most other neoarchean paleoproterozoic detritus free bifs the carajas bifs do not show the typical heavy rare earth element ree enrichment relative to light ree instead they do show variable light to heavy ree fractionation trends ybsn prsn ratios 0 05 4 91 the th u ratios of both bulk bifs and mesobands are generally lower than the upper crustal ratio although the bulk bifs show mostly chondritic values the individual fe and si bands exhibit distinctly higher ratios the general absence of negative cesn anomalies attests for rather reducing paleoenvironmental conditions unlike the great majority of the data which lack negative anomalies of ce a few samples do display such anomalies which may be compatible to the oxidizing conditions shown by the low th u ratios of most the carajas bifs such low th u ratios may register terrestrial weathering conditions which cause preferential u input into the oceans in response to the oxidation of immobile u4 to more mobile u6

constraints of sr isotopic compositions of apatite and carbonates on the origin of fe and cu mineralizing fluids in the lala fe cu mo lree deposit sw china abstract numerous fe cu deposits with mineralization styles similar to iron oxide copper gold iocg deposits form the kangdian fe cu metallogenic province southwestern sw china as one of the largest deposits in the region the 1 0 ga lala fe cu deposit is hosted in a paleoproterozoic volcanic sedimentary succession named the hekou group which is alternately intruded by 1 0 ga doleritic plutons this deposit has a paragenetic sequence evolving from stage i of na alteration to stage ii of fe mineralization and finally to stage iii of cu mo ree mineralization coeval with mafic felsic intra plate magmatism in the region this study conducted in situ sr isotopic analyses on apatite and carbonate aiming to resolve the long controversial issue regarding the origin of the fe and cu mineralizing fluids in the deposit apatite of stage ii has 87sr 86sr ratios varying from 0 71380 to 0 72733 much higher than those of synchronous igneous rocks in the region 0 7074 to 0 7091 but similar to the paleoproterozoic host rocks 0 71368 to 0 71837 at 1 0 ga this similarity indicates that radiogenic sr of the fe mineralizing fluid was dominantly sourced from the host rocks apatite and calcites of stage iii have 87sr 86sr ratios 0 75758 0 79293 much higher than apatite of stage ii and the host rocks but similar to the archean basement rocks as high as 0 80 at 1 0 ga beneath the cover of the yangtze block suggesting that the highly radiogenic sr isotopic composition of the cu mineralizing fluid was mainly inherited from the old basement rocks in combination with previous c o s isotopic data indicating a magma hydrothermal origin it was suggested that the fe mineralizing fluid was exsolved from a mafic magma that generated the 1 0 ga doleritic plutons and inherited radiogenic sr from the host rocks during fluid rock interaction by contrast the cu mineralizing fluid might have been sourced from another pulse of magmatic cu mo ree and co2 rich fluid which have once interacted with archean basement rocks prior to mineralization the source of such a cu mo ree rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma we propose that intrusions of the bimodal magmas in kangdian are responsible for regional hydrothermal circulation which led to fe cu mo ree mineralization in the kangdian province

new evidence on magnetite oxygen isotope geothermometers at 175 and 112 c in wairakei steam pipelines new zealand abstract millimeter deep surface layers of magnetite occur in the steam pipelines of the wairakei geothermal power station as part of serious corrosion discovered after 15 years of service these quasi experimental growths provide the following oxygen isotope calibration points of magnetite and h 2 o below the previously available experimental range 175 c 7 9 0 5 and 112 c 3 7 0 5 the data support the earliest proposed magnetite h 2 o fractionation curve by j r o neil and r n clayton with an isotopic cross over near 50 c and could indicate a relatively low temperature origin for some precambrian iron formations

the evolution of a porphyry cu au deposit based on la icp ms analysis of fluid inclusions bajo de la alumbrera argentina the chemical and physical evolution of magmatic to hydrothermal processes in the porphyry cu au deposit of bajo de la alumbrera northwestern argentina has been reconstructed with a quantitative fluid inclusion study fluid inclusion petrography microthermometry and single inclusion microanalysis by excimer laser ablation inductively coupled plasma mass spectrometry la icp ms are combined to determine the evolution of pressure temperature and ore metal concentrations including cu and au in the fluids complementary hydrogen and oxygen isotope analyses are used to further constrain the water sources in the evolving system the combined data provide a new level of insight into the mechanisms of metal sourcing and ore mineral precipitation in a porphyry style magmatic hydrothermal system based on previously reported observations of the igneous geology alteration geochemistry and veining history of the subvolcanic porphyries at alumbrera the distribution of fluid inclusion types in space and time is documented six major inclusion types are distinguished the highest temperature brine inclusions up to 750 c p 1 kbar are mainly recorded in barren quartz magnetite veins in the core of the alteration system these polyphase brine inclusions halite sylvite multiple opaque and transparent daughter crystals are interpreted as the most primitive magmatic fluid recorded at the level of the deposit they are of moderately high salinity 50 60 wt nacl equiv dominated by nacl kcl and fecl 2 and contain on average 0 33 wt percent cu and 0 55 ppm au upon cooling and decompression these saline liquids exsolve a vapor phase which is preferentially enriched in cu relative to its main salt components but probably plays a minor role in the formation of this particular deposit because of the inferred small mass fraction of vapor cooling and decompression from the highest initial p t conditions down to about 450 c causes magnetite k silicate alteration but no saturation in au or cu sulfides as recorded by continually high ore metal concentrations in the fluid inclusions coprecipitation of cu and au as chalcopyrite and native gold some early bornite occurs over a narrow range of decreasing fluid temperature with cooling from 400 to 305 c the cu concentration in the brine drops by about one order of magnitude to less than 0 07 wt percent without a proportional decrease in major salt components ore mineral precipitation extracts 85 percent of the cu and au from the fluid it is associated with potassic alteration as shown by a concomitant decrease in the k na ratio of the cooling magmatic brine and by an increase in its ba and sr concentrations elements which are probably liberated in the destruction of calcic igneous minerals the fluid chemical data demonstrate that the metal ratios in this and probably many other porphyry style ore deposits are primarily controlled by the magmatic source of the ore brines on the other hand the final hypogene ore grade of the deposit is controlled by the efficiency of ore mineral precipitation at alumbrera metal extraction is governed by the efficiency of cooling a high flux of magmatic fluid within a small rock volume dilution of residual magmatic fluids as recorded by aqueous fluid inclusions of decreasing salinities and temperatures below 295 c follows after the main stage of copper introduction and is associated with feldspar destructive phyllic alteration geometric relationships fluid analyses and stable isotope data together indicate that phyllic alteration results from postmineralization hydrothermal activity involving minor mixing between meteoric water residual brine and a waning input of magmatic vapor

u pb geochronology of archean magmatism and basement reactivation in the carajas area amazon shield brazil abstract the archean carajas subprovince is a metallogenic district bif bauxite cu ni mn au located in the easternmost amazonian craton in brazil the main conclusions from a u pb study of the itacaiunas supergroup and a locality in the xingu complex are the following the xingu basement complex the oldest unit underwent migmatization at 2859 2 ma and also grew new zircon at 2851 4 ma the luanga mafic intrusion is 2763 6 ma old and coeval with the 2759 2 ma felsic volcanics of the grao para group amphibolites from the salobo group contain 2851 ma zircons were metamorphosed 2761 3 ma and intruded by granitic veins ca 2758 ma at the time of grao para magmatism and also grew new zircon at 2732 2742 ma the data indicate that the salobo group is part of the xingu complex and not of the itacaiunas supergroup as heretofore believed the pojuca group possibly coeval with the salobo group was metamorphosed at 2732 3 ma a late archean reactivation event of regional extent is marked by titanite growth at 2581 5 ma in the mirim area intrusion of the old salobo granite at 2573 2 ma new zircon growth in salobo group amphibolites at 2555 4 3 ma and monazite growth in a metamorphosed iron formation at 2551 2 ma in addition k metasomatism associated with shear zones formed titanite in salobo group amphibolites at 2497 5 ma and new titanite at 2519 5 ma in the xingu complex events in the 2581 2497 ma interval are probably associated with the latest activity of a wnw ese sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano sedimentary sequence of the grao para bahia and rio fresco groups finally undeformed proterozoic granitoid intrusions were dated at 1883 2 ma cigano 1883 5 2 ma musa 1880 2 ma carajas and 1874 2 ma pojuca

geology and uranium thorium mineral deposits of the bokan mountain granite complex southeastern alaska abstract the bokan mountain kendrick bay uranium thorium deposits are associated with a late jurassic peralkaline granite ring dike complex the uranium thorium deposits form pipe like bodies along contacts or occur as pods in en echelon northwest striking shear zones the host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite aplite aegirine granite porphyry and eight other lesser rock types the ore is closely associated with an albitized aegirine syenite the ore consists of thorium and rare earth rich rock containing uranothorite uraninite and generally less than 2 of sulfide species about 100 000 tons 89 000 t of ore have been mined at an average grade of about 1 u3o8 and nearly 3 tho2 mainly from the ross adams pipe the pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry it is as much as 24 m across and was mined along strike for over 300 m the ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite fluorite calcite quartz sericite and tourmaline hematite is present in the outer distal parts of the ore zones the minerals produced to date occur within the outer annular zone of the bokan granite complex in which aegirine is present as the major alkali ferro magnesian mineral ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber the pod like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events subsequent magma crystallization yielded riebeckite granites that were depleted in u and th filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331 c with pressure corrections increasing the temperature of formation to 420 c or higher depending on dissolved gas content sulfur isotopic analyses on pyrite galena sphalerite and pyrrhotite indicate disequilibrium but 34sh2s is estimated at 7 6 carbon and oxygen isotopic analyses on ore stage calcite yield 18oh2o of 6 8 to 8 1 and 13c c of 4 3 to 7 0 the oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium uranium deposits

fluid immiscibility in natural processes use and misuse of fluid inclusion data ii interpretation of fluid inclusion data in terms of immiscibility abstract phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility in part i following from that point the major limits on the accuracy and even the validity of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed the practical implications of the general constraints temperature pressure topology of the fluid systems are envisaged emphasis is laid on some important consequences such as the use of isochore intercepts and the possible resulting interpretation of fluid mixing rather than unmixing the case of highly saline inclusions the identification and interpretation of heterogeneous trapping the composition and density constraints on coexisting fluids are presented and illustrated by natural examples taking into account all the measurable parameters in fluid inclusions volume temperature and nature of phase transitions more or less complete individual spectroscopic analyses all the available experimental data and all the theoretical constraints may be long and difficult however it is most generally very informative and productive although part of these data is often sufficient to deny unmixing nevertheless a final example on metamorphic fluids demonstrates how such an approach can prove and also characterize a fluid unmixing during a geologic process

chlorine isotopes in fluid inclusions determination of the origins of salinity in magmatic fluids abstract thermal ionisation mass spectrometry tims of the cs 2 cl ion has been used to determine the 37 cl value of paleofluids trapped in fluid inclusions modifications to the existing published method have lowered the precision by a factor of 2 0 09 1 thereby allowing real differences to be measured with greater certainty the method has been applied to determine 37 cl of two sets of magmatic fluids trapped in fluid inclusions samples were studied from the capitan pluton in new mexico and the sw england batholith both of which have high temperature fluids whose d and 18 o signatures are distinctly magmatic relative to standard mean ocean chloride smoc the 37 cl values of the capitan samples cluster around 0 whereas those from sw england cluster around 1 8 the br cl ratios of the capitan fluid inclusions are low indicative of halite dissolution and are comparable to local high salinity waters derived from leaching permian evaporites the br cl ratios of the sw england fluid inclusions are much higher and within the range of values normally considered to be magmatic we conclude that the 37 cl value of the capitan magmatic fluids confirms the evidence from br cl ratios that cl was derived from an evaporite source whereas the 37 cl value of the sw england fluids appears to be more representative of a deep magmatic source

stratigraphy and mineralogy of the manganese ores of urucum mato grosso brazil preliminar results of geological mapping and microscopic studies are presented the mineral deposits of urucum are clearly different from the rest of the banded iron formations in brazil urucum is essentially younger and probably late proterozoic the sequence is unmetamorphic two main types of manganese ore exist in urucum namely

partial melting fractional crystallisation liquid immiscibility and hydrothermal mobilisation a recipe for the formation of economic a type granite hosted hfse deposits abstract a number of a type granites host resources of the high field strength elements hfse including the rare earth elements ree that have been considered for economic exploitation the best known of them is the strange lake pluton canada with a resource of 20 mt grading 1 44 wt ree2o3 of which 50 are heavy rare earth oxides in this paper we examine the various processes that are likely to be responsible for the hyper enrichment of the hfse starting with the generation of a parental magma through a very low degree of partial melting of a mantle plume or a metasomatically enriched mantle in both cases below a continental crust continental rift we trace the evolution of this magma olivine nephelinite through fractional crystallisation of olivine and clinopyroxene followed by plagioclase k rich alkali feldspar and arfvedsonite to the final residues of hyper enriched hfse granitic pegmatites in so doing we show that at least two additional steps are essential to the process of hfse enrichment the first of these is the abrupt termination of plagioclase crystallisation prior to the final consumption of ca or assimilation of a ca source and the resulting exsolution of an immiscible ca fluoride melt that sequesters the lree and y the second of these steps is prolonged crystallisation of arfvedsonite which delays saturation of the magma with an aqueous phase and ensures the development of an agpaitic assemblage of na bearing hfse minerals e g na zirconosilicates and na titanosilicates finally we show that saturation of the magma with an aqueous carbonic fluid which evolves to a very acidic composition is an essential ingredient in the recipe for the formation of an economic a type granite hosted hfse deposit

diffusion reaction in a thermal gradient implications for the genesis of anorthitic plagioclase high alumina basalt and igneous mineral layering abstract piston cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt rock reaction processes occurring during magma differentiation in the crust in two experiments juxtaposing basaltic andesite and gabbro at 0 5 gpa pressure for durations of either 13 or 26 days diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase specifically the basaltic andesite gains al2o3 mgo and cao from the gabbro and loses na2o k2o sio2 and feo to it with a plagioclase rich layer developing at the interface between the two materials in a process termed diffusion reaction the percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores up to an90 that abruptly shift in composition to thin rims that are in na ca exchange equilibrium with the co existing melt both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion reaction isotopic tracers 45ca 6li 84sr and 136ba initially deposited at the basaltic andesite gabbro interface in the 13 day experiment were detected in the cores of the anorthitic plagioclase after the experiment demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion reaction experiment the formation of anorthitic plagioclase during diffusion reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra calcic melts textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 c for instance apatite fenis olivine and almost pure albite occur at distinct specific horizons in the gabbro within the 26 day experiment because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro equilibration between minerals and fluids melts must be rapid the overall effect of the diffusion reaction process is to make an ascending magma more primitive in composition and in this case produce anorthitic plagioclase while making surrounding crustal wall rocks more evolved several observations within igneous rocks support the occurrence of this process suggesting that the genesis of porphyritic high alumina basalt ubiquitously observed at convergent margins could reflect a diffusion reaction process in the crust

using oxygen isotope chemistry to track hydrothermal processes and fluid sources in itabirite hosted iron ore deposits in the quadrilatero ferrifero minas gerais brazil the quadrilatero ferrifero brazil is presently the largest accumulation of single itabirite hosted iron ore bodies worldwide detailed petrography of selected hypogene high grade iron ore bodies at e g the aguas claras conceicao pau branco and pico deposits revealed different iron oxide generations from oldest to youngest magnetite martite hematite pseudomorph after magnetite granoblastic recrystallised microplaty fine grained 100 m hematite laser fluorination oxygen isotope analyses of selected iron ore species showed that the 18o composition of ore hosted martite ranges between 4 4 and 0 9 and is up to 11 depleted in 18o relative to hematite of the host itabirite during the modification of iron ore and the formation of new iron oxide generations e g microplaty and specular hematite an increase of up to 8 in 18o values is recorded calculated 18o values of hydrothermal fluids in equilibrium with the iron oxide species indicate 1 the involvement of isotopically light fluids e g meteoric water or brines during the upgrade from itabirite hosted hematite to high grade iron ore hosted martite and 2 a minor positive shift in 18ofluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated 18o values and or the infiltration of small volumes of isotopically heavy metamorphic and or magmatic fluids into the iron ore system the circulation of large fluid volumes that cause the systematic decrease of 18o 16o ratios from itabirite to high grade iron ore requires the presence of e g extensive faults and or large scale folds

iron deposits hosted by banded iron formations in the yilgarn craton products of sequential iron enrichment by magmatic marine and meteoric fluids abstract mesoarchean algoma type bif in the yilgarn craton are locally enriched by secondary processes to iron concentrations approaching 72 wt fe while their near surface goethite hematite ores are unequivocally the product of supergene fluid alteration their deeper magnetite and crystalline hematite martite ores have a contentious origin this study shows that magnetite ores formed after diagenesis and peak greenschist facies metamorphism of bif coinciding with regional deformation events the ores formed by a multi stage process involving initial replacement of primary quartz bands in bif by hypogene carbonate minerals followed by the dissolution of the carbonate and concentration of magnetite at the beebyn deposit the earliest fluids were hot 440 c co2 vapor rich and were likely derived from felsic magmatic fluids in contrast the fluids responsible for carbonate alteration at the matthew ridge koolyanobbing and windarling deposits were cooler 366 to 125 c and aqueous rich with salinities from 0 3 to 43 9 wt nacl equiv their cl br and c o isotopic ratios suggest they are the product of mixing between magmatic fluids and heated seawater in all deposits later cooling 178 to 98 c of these fluids led to dissolution of carbonate minerals and concentration of magnetite dated monazite and xenotime from magnetite ores demonstrates that fluid alteration of bif began in the mesoarchean and continued episodically through to the paleoproterozoic later specular hematite rich veins that cut and modify magnetite ores crystallized from aqueous rich fluids that are more oxidized with salinities from 0 to 28 7 wt nacl equiv and crystallization temperatures of 243 to 85 c their cl br and c o isotopic fluid ratios indicate that they formed from mixing between heated seawater and meteoric waters the bif hosted iron deposits in the yilgarn craton show remarkable similarities with the well studied iron deposits in the carajas mineral province and hamersley basin they all show the same transition through time from magnetite to hematite rich ores subsequent episodic tectonically driven fluid alteration events led to further iron enrichment in these deposits the transition in ore types most likely corresponds to a common guiding process namely the uplift of crustal rocks containing bif resulting in the progressive shallowing of fluid alteration pathways and the greater influence of cooler more oxidized hydrothermal fluids unlike iron deposits hosted by algoma type bif in the yilgarn and carajas iron deposits hosted by superior type bif in the hamersley basin display a greater influence of oxidized marine and meteoric waters without involvement of magmatic fluids

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

geochemistry and geochronology of the banxi sb deposit implications for fluid origin and the evolution of sb mineralization in central western hunan south china abstract central western hunan in south china hosts the largest antimony belt in the world with two types of sb deposits identified sb au woxi type and sb only xikuangshan type banxi is the most representative deposit in the region with vein type sb mineralization hosted in neoproterozoic clastic rocks stibnite developing in ores and arsenopyrite mainly occurring in altered country rocks trace element contents including rare earth elements ree and isotopic ratios s pb sr nd he and ar in stibnite and or arsenopyrite were analyzed to determine the timing of ore formation and to elucidate fluid origin and evolution processes most sulfides have high ree contents 45 103 ppm moderate fractionation between lree and hree lree hree 6 38 11 56 and slightly negative eu anomalies eu eu 0 53 0 72 the 34 s values are 3 88 5 81 for stibnite and 9 25 11 82 for arsenopyrite the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios range from 18 617 to 18 635 15 695 to 15 739 and 38 965 to 38 981 for stibnite and from 18 594 to 18 609 from 15 587 to 15 698 and from 38 893 to 38 926 for arsenopyrite the 87 rb 86 sr and 87 sr 86 sr ratios are 0 3016 3 538 and 0 711463 0 717591 respectively for stibnite and 0 2251 2 214 and 0 711244 0 711565 respectively for arsenopyrite the 147 sm 144 nd and 143 nd 144 nd ratios are 0 1174 0 9816 and 0 511942 0 512768 for stibnite with end t values t 130 ma ranging from 12 4 to 6 6 and two stage model ages t 2dm ranging from 1457 to 1932 ma the 40 ar 36 ar and r ra ratios where ra is the atmospheric 3 he 4 he ratio of 1 4 10 6 are 409 545 and 0 0088 0 0348 respectively for stibnite the rb sr and sm nd isotopic analyses of the sulfides yield isochron ages of 129 4 2 4 ma 2 mswd 1 3 and 130 4 1 9 ma 2 mswd 1 6 respectively the combined trace element and isotopic data set indicates that the ore forming fluids were a mixture of dominantly deep basement derived solute rich parent water and a small amount of dilute heated meteoric water the decreases in ree 34 s 206 pb 204 pb 207 pb 204 pb 208 pb 204 pb 87 rb 86 sr 87 sr 86 sr 147 sm 144 nd and 143 nd 144 nd and increases in 40 ar 36 ar and 4 he contents in the stibnite from the earlier quartz stibnite mineralization stage stage ii through to the later stibnite mineralization stage stage iii indicate that the influx of meteoric water increased during sb mineralization the deep sourced relatively high temperature rock reacted fluid mixed with increasing amounts of more oxidized low temperature meteoric fluids causing the precipitation of arsenopyrite and stibnite sequentially at the banxi deposit a two period genetic model has been proposed for the world class sb mineralization in the central western hunan region the woxi type sb au deposits were derived from orogenic fluids generated from folding and shearing during the caledonian and indosinian orogeny whereas the xikuangshan type sb deposits were formed by mixing heated basement derived hydrothermal fluid circulating during the flare up of magmatism in south china with lower temperature meteoric water

the evolution of high alumina basalts of the klyuchevskoy volcano kamchatka russia based on microprobe analyses of mineral inclusions abstract the origin of calc alkaline high alumina basalts hab of the klyuchevskoy volcano kamchatka was examined using electron microprobe analyses of phenocrysts and mineral phases included in the phenocrysts continuous trends on major element variation diagrams suggest the hab were derived from high magnesia basalt hmb by fractional crystallization phenocrysts in the hab are strongly zoned olivine mg 91 64 clinopyroxene wo45 38en40 51fs5 20 and chrome spinel magnetite inclusions in them cr2o3 45 0 wt tio2 0 5 11 microprobe analyses of minerals included in the phenocrysts provide additional constraints on the mineral crystallization trends in the hab fe mg partitioning data when applied to the phenocrysts cores show they crystallized from a hmb the similarity of phenocryst core compositions in hab with those in hmb strongly suggests a genetic relationship between the two magma types

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

sulfur isotopic profile through the oceanic crust sulfur mobility and seawater crustal sulfur exchange during hydrothermal alteration measurements of the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust fluxes of 2 5 10 12 g yr s 34 s 2 7 from the volcanic section to seawater and 2 1 10 12 g yr of seawater s 21 into the crust result in an insignificant change in the s content of altered ocean crust these are clearly second order fluxes compared to the riverine input of s to the oceans and the output of sedimentary pyrite most of the anhydrite that forms in the high temperature axial convection cell must later be dissolved at lower temperatures during off axis circulation the mean 34 s of altered oceanic basement is 0 9 only slightly changed from the primary value of 0 1 unless s is preferentially mobilized from 34 s rich sulfide deposits or mineralized zones subduction of altered basaltic crust is unlikely to be the source of high 34 s values of arc volcanic rocks 4 but could contribute to isotopic heterogeneities of s in the mantle

the early jurassic bokan mountain peralkaline granitic complex southeastern alaska geochemistry petrogenesis and rare metal mineralization abstract the early jurassic ca 177 ma bokan mountain granitic complex located on southern prince of wales island southernmost alaska cross cuts paleozoic igneous and metasedimentary rocks of the alexander terrane of the north american cordillera and was emplaced during a rifting event the complex is a circular body 3 km in diameter of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite all the rock forming minerals typically record a two stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma the bokan granites and related dikes have sio2 from 72 to 78 wt high iron feo tot 3 4 5 wt and alkali 8 10 wt concentrations with high feo tot feo tot mgo ratios typically 0 95 and the molar al2o3 na2o k2o ratio

fluid inclusion and isotopic constraints on the origin of the paleoproterozoic yinachang fe cu ree deposit southwest china the yinachang deposit in the kangdian fe cu metallogenic province southwest china contains approximately 20 mt ore 41 9 to 44 5 wt fe and 15 mt ore 0 85 to 0 97 wt cu with potentially significant rees orebodies are hosted in the late paleoproterozoic dongchuan group and consist mainly of massive and banded replacement ores the paragenetic sequence of this deposit includes pre ore na fe alteration stage i fe ree mineralization dominated by magnetite and siderite with subsidiary apatite and fluorite stage ii and cu ree mineralization with chalcopyrite ankerite biotite and subordinate apatite fluorite allanite and synchysite stage iii this sequence is similar to those of many iron oxide copper gold iocg deposits elsewhere there are two distinct types of fluid inclusions within apatite of both stages ii and iii type 1 fluid inclusions with liquid and vapor phases have relatively low homogenization temperatures 98 345 c and salinities 4 7 16 2 wt nacl equiv whereas type 2 fluid inclusions with one or more solid phases a liquid phase and a vapor phase have higher homogenization temperatures 273 500 c and salinities 36 2 59 8 wt nacl equiv fluids in sulfur isotope equilibrium with chalcopyrite of stages iii have 34 s cdt values ranging from 2 8 to 2 7 consistent with a magmatic origin for the sulfur thus sulfur isotopie compositions imply that magmatic hydrothermal fluids probably represented by the hot and saline type 2 fluid inclusions were involved in the ore forming process although 34 s close to 0 might also mean that sulfur was leached from igneous rocks by nonmagmatic fluids siderite of stage ii and ankerite of stage iii have 13 c pdb values ranging from 12 4 to 7 5 and from 8 5 to 4 4 respectively the large variation of 13 c pdb values reveals a strong interaction between the magmatic hydrothermal fluids and the dolostone in the ore hosting sequence calculated 18 o smow values of the fluids in stages ii and iii are broadly similar and vary from 11 1 to 17 1 avg 13 5 such values are higher than that of magmatic hydrothermal fluids and thus may indicate extensive fluid wall rock interaction and or mixing of magmatic hydrothermal fluids with shallow level nonmagmatic fluids this interpretation is consistent with the radiogenic sr isotope compositions of bulk ores 87 sr 86 sr i 0 710404 0 734034 and fluorite separated from ores 87 sr 86 sr i 0 709851 0 723658 these nonmagmatic fluids are probably representative of the type 1 fluid inclusions bulk ores and mineral separates have nd t values ranging from 8 1 to 0 4 mostly 0 9 to 0 4 similar to the coeval mafic intrusions in the region 2 0 to 2 8 this similarity suggests that rees in the yinachang deposit might have been derived dominantly from mantle derived magmas on the other hand relatively low nd t values 5 5 and 8 1 of some samples indicate insignificant ree contributions from wall rocks the yinachang deposit is temporally associated with regional mantle derived mafic magmatism and may have a genetic relationship with it fluids derived from deep seated magmas may have supplied abundant sulfur and carbon as well as some of the ore metals in this deposit the mafic magmatism could also have provided heat to induce the shallow level nonmagmatic fluids to interact with the sedimentary volcanic sequence thus leaching additional ore metals into the ore forming system fluid mixing can effectively trigger saturation and deposition of ore minerals our work highlights that nonmagmatic fluids are not a prerequisite for cu au ree mineralization but may facilitate metal deposition in magmatic hydrothermal iocg systems

from the roots to the roof an integrated model for the neoarchean carajas iocg system brazil abstract the carajas mineral province located in northern brazil comprises the oldest iron oxide copper gold mineralizing system on earth and is one of the world s most significant cluster of large tonnage iocg deposits based on a critical assessment of the literature a new integrated model for the carajas neoarchean iocg deposits the model considers the hydrothermal alteration footprint and the rock textures the geometry of the mineralization and the ore assemblage and the isotopic and geochronological signature the majority of the deposits form in the hypozonal domain pervasive and well developed sodic magnesian and calcic iron hydrothermal alteration halos form through magmatic derived hydrothermal fluids the occurrence of hydrothermal orthopyroxene marialite in the ni rich gt 34 and salobo deposits extend further the p t boundaries to temperatures of up to 700 c and 5 kbars thus co is proposedrresponding to the root zone of the system epizonal deposits conversely are characterized by low temperature potassic biotite phlogopite propylitic and chloritic hydrolytic hydrothermal assemblages towards shallower crustal levels although a mantle contribution to the hydrothermal fluids is recognized the o b stable isotopic signature reveals the participation of surface derived basinal brines in the ore forming process the occurrence of polymetallic ooids in the alemao orebody igarape bahia deposit hosting identical hydrothermal alteration mineralogy to the chalcopyrite breccias of other iocg deposits in the carajas mineral province suggests that the mineralizing fluids may have reached the paleo surface of the carajas basin in the neoarchean representing the roof of the iocg system conversely the ore mineralogy is consistent throughout the vertical scheme magnetite is the only iron oxide formed and precipitate from iron rich hydrothermal alteration accompanying the calcic alteration similarly chalcopyrite represents the cu sulfide there is no transition to more oxidized assemblages towards shallower zones supporting that the overall system was relatively reduced since more oxidizing phases hematite bornite are not observed as part of the neoarchean iocg system thus the neoarchean carajas iocg system represents a crustal scale continuum of a vertical paleo hydrothermal system formed coevally to the voluminous bimodal magmatism and the formation of the world class banded iron formation hosted iron ore

oxygen isotope compositions of iron oxides from high grade bif hosted iron ore deposits of the central hamersley province western australia constraints on the evolution of hydrothermal fluids the hamersley province of northwest western australia is one of the world s premier iron ore regions with high grade martite microplaty hematite iron ore deposits mostly hosted within banded iron formation bif sequences of the brockman iron formations of the hamersley group these high grade iron ores contain between 60 and 68 wt percent fe and formed by the multistage interaction of hydrothermal fluids with the host bif formation the oxygen isotope compositions of magnetite and hematite from bif hydrothermal alteration assemblages and high grade iron ore were analyzed from the mount tom price paraburdoo and channar iron ore deposits the 18o values of magnetite and hematite from hydrothermal alteration assemblages and high grade iron ore range from 9 0 to 2 9 per mil a depletion of 5 to 15 per mil relative to the host bif the 18o values are spatially controlled by faults within the deposits a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids the oxygen isotope composition of hydrothermal fluids 18ofluid indicates that the decrease in the 18o content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original bif late stage talc bearing ore at the mount tom price deposit formed in the presence of a pulse of 18o enriched basinal brine indicating that hydrothermal fluids may have repeatedly interacted with the bifs during the paleoproterozoic

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

constraints on the origins of fluids forming irish zn pb ba deposits evidence from the composition of fluid inclusions the numerous zn pb deposits in the irish midlands together with the quantity and grade of the ore make this a world class base metal province despite significant exploration and research there is still disagreement on the origin of the mineralizing fluids in this study we have measured the composition of fluid inclusions using a crush leach technique these data constrain the possible sources of the fluids both during and after mineralization chloride and br concentrations indicate that the main ore fluid at tynagh and silvermines was seawater that had evaporated until the salinity was between 12 and 18 wt percent however na k and li data show that water rock interactions have resulted in the depletion of na and the enrichment of k and li relative to the concentrations expected for evaporated seawater the fluids that produced postore dolomitization were also derived from seawater that had evaporated to higher salinities and had precipitated halite the concentrations of na k and li are what would be expected for seawater at this degree of evaporation and show that these fluids did not interact with the same lithologies as the ore fluids the major change to their fluid composition was exchange of mg for ca during dolomitization we suggest that the ore fluids could only have reached their high salinity by evaporation of seawater on the extensive shallow water shelf regions that existed over much of the irish midlands

isotopic evidence for holocene climate change in the northern rockies from a goethite rich ferricrete chronosequence abstract climate proxy records often show regional heterogeneity due to the complex interplay of large scale atmospheric circulation patterns and local topographic features this is especially prevalent in the mountainous regions of the western united states where the interaction of two seasonally distinct weather systems can result in locally abrupt climate boundaries here we establish oxygen isotope ratios of goethite from ferricrete chronosequences formed by natural acid rock drainage as a new source of spatially small scale holocene climate information systematic study of these deposits may prove useful in understanding regional variations in response to global holocene climate change oxygen isotope ratios of goethites from the mountainous northeastern yellowstone region show an increase of 3 over the past 9000 years we attribute this isotopic change to a local increase of isotopically heavier summer precipitation since the early holocene possibly augmented by seasonal temperature differences brought on by orbitally forced insolation changes over the past 9000 years our interpretation is compatible with palynology studies as well as holocene climate simulations which indicate a decrease in monsoon intensity from 9000 years b p to the present unlike the southwestern united states this change may have resulted in wetter summers in our study area

poisson magnetization to density ratio and magnetization inclination properties of banded iron formations of the carajas mineral province from processing airborne gravity and magnetic data

mineralogical constraints on the petrogenesis of trachytic inclusions carpenter ridge tuff central san juan volcanic field colorado although bulk rock normative analyses of the trachytic inclusions from the carpenter ridge tuff yield abundant quartz and minor corundum a portion of the phenocryst assemblage is indicative of an alkaline parentage sanidine and biotite contain up to 8 and 5 wt bao respectively in addition both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica undersaturated magma amphibole is magnesiohastingsite with 3 wt tio2 and less than 0 3 mole fraction vacancies in the a site clinopyroxene compositions straddle the calcic augite salite boundary chrondite normalized ree patterns are similar for both inclusions and rhyolites the inclusions are slightly poorer in ree and have a positive eu anomaly versus the negative anomaly of the rhyolites the similarity in ree patterns would seem to indicate that the two rock types are genetically related with the positive eu anomalies resulting from feldspar accumulation however this possibility is denied by the antithetic alkaline and subalkaline phenocryst assemblages of the two rock types we suggest that the best explanation for these discrepancies is that a mildly silica undersaturated magma was the parent for the phenocrysts this magma intruded the carpenter ridge chamber and because the crystallization temperatures of both magma overlapped the alkaline magma mixed with the ambient rhyolite to form a hybrid this hybrid consisted of a portion of the phenocryst assemblage from the alkaline magma but the bulk rock chemistry depended upon the proportions of the endmember liquids the abundance of normative quartz the minor normative corundum and the similarity of ree patterns indicates that the inclusions are mixtures dominated by the rhyolitic component additional processes such as liquid state diffusion crystal accumulation and alkali loss may have contributed to obscure the compositions of the initial liquids involved in the inferred mixing process

macflincor and its application to fluids in archean lode gold deposits abstract macflincor was developed to process laboratory data gathered on fluid inclusions and to calculate isochores for geologically important fluids composed of h2o c02 ch4 nacl and n2 within the program interactive diagrams are available to describe those chemical systems which cannot be adequately constrained numerically the user can choose from various published equations of state describing fluid behavior and can easily compare the results obtained by using different equations the program once the basic fluid parameters have been established allows the data to be used to determine the p t trapping conditions to evaluate possible pressure corrections and to compare different depths of formation based on the pressure calculations and given geothermal gradients for illustrative purposes macflincor has been utilized here to constrain different physical chemical parameters of fluid inclusions trapped in ore related quartz veins and breccias of archean lode gold deposits in western australia inferred pressures from individual deposits show a wide range of values which may be due to post entrapment modifications and or observational uncertainties including estimation of liquid vapor ratios

new geologic fluid inclusion and stable isotope studies on the controversial igarape bahia cu au deposit carajas province brazil the igarape bahia cu au deposit in the carajas province brazil is hosted by steeply dipping metavolcano sedimentary rocks of the igarape bahia group this group consists of a low greenschist grade unit of the archean 2 750 ma itacaiunas supergroup in which other important cu au and iron ore deposits of the carajas region are also hosted the orebody at igarape bahia is a fragmental rock unit situated between chloritized basalt with associated hyaloclastite banded iron formation bif and chert in the footwall and mainly coarse to fine grained turbidites in the hanging wall the fragmental rock unit is a nearly concordant 2 km long and 30 250 m thick orebody made up of heterolithic usually matrix supported rocks composed mainly of coarse basalt bif and chert clasts derived from the footwall unit mineralization is confined to the fine grained matrix and comprises disseminated to massive chalcopyrite accompanied by magnetite gold u and light rare earth element lree minerals and minor other sulfides like bornite molybdenite cobaltite digenite and pyrite gangue minerals include siderite chlorite amphibole tourmaline quartz stilpnomelane epidote and apatite a less important mineralization style at igarape bahia is represented by late quartz chalcopyrite calcite veins that crosscut all rocks in the deposit area fluid inclusions trapped in a quartz cavity in the ore unit indicate that saline aqueous fluids 5 to 45 wt nacl cacl2 equiv together with carbonic co2 ch4 and low salinity aqueous carbonic 6 wt nacl equiv fluids were involved in the mineralization process carbonates from the fragmental layer have 13c values from 6 7 to 13 4 per mil that indicate their origin from organic and possibly also from magmatic carbon the 34s values for chalcopyrite range from 1 1 to 5 6 per mil with an outlier at 10 8 per mil implying that most sulfur is magmatic or leached from magmatic rocks whereas a limited contribution of reduced and oxydized sulfur is also evident oxygen isotopic ratios in magnetite quartz and siderite yield calculated temperatures of 400 c and 18o enriched compositions 5 to 16 5 per mil for the ore forming fluids that suggest a magmatic input and or an interaction with 18o rich probably sedimentary rocks the late veins of the igarape bahia deposit area were formed from saline aqueous fluids 2 to 60 wt nacl cacl2 equiv with 18ofluid compositions around 0 per mil that indicate contribution from meteoric fluids with respect to geological features igarape bahia bears similarity with syngenetic volcanic hosted massive sulfide vhms type deposits as indicated by the volcano sedimentary geological context stratabound character and association with submarine volcanic flows hyaloclastite and exhalative beds such as bif and chert on the other hand the highly saline ore fluids and the mineral assemblage dominated by magnetite and chalcopyrite with associated gold u and lree minerals and scarce pyrite indicate that igarape bahia belongs to the fe oxide cu au iocg group of deposits the available geochronologic data used to attest syngenetic or epigenetic origins for the mineralization are either imprecise or may not represent the main mineralization episode but a later superimposed event the c s and o isotopic results obtained in this study do not clearly discriminate between fluid sources however recent b isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit xavier wiedenbeck dreher rhede monteiro araujo chemical and boron isotopic composition of tourmaline from archean and paleoproterozoic cu au deposits in the carajas mineral province 1 simposio brasileiro de metalogenia gramado brazil extended abstracts cd rom 2005 provide strong evidence of the involvement of a marine evaporitic source in the hydrothermal system of igarape bahia evaporite derived fluids may explain the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit evaporite derived fluids also exclude a significant participation of magmatic or mantle derived fluids reinforcing the role of nonmagmatic brines in the genesis of igarape bahia considering this aspect and the geological features the possibility that the deposit was generated by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated

petrogenesis of the paleoproterozoic rapakivi a type granites of the archean carajas metallogenic province brazil three paleoproterozoic a type rapakivi granite suites jamon serra dos carajas and velho guilherme are found in the carajas metallogenic province eastern amazonian craton liquidus temperatures in the 900 870 c range characterize the jamon suite those for serra dos carajas and velho guilherme are somewhat lower pressures of emplacement decrease from jamon 3 2 0 7 kbar through serra dos carajas 2 0 1 0 kbar to velho guilherme 1 0 0 5 kbar oxidizing conditions nno 0 5 characterized the crystallization of the jamon magma the velho guilherme magmas were reducing marginally below fmq and the serra dos carajas magmas were intermediate between the two in this respect the three granite suites have archean tdm model ages and strongly negative nd values 12 to 8 at 1880 ma and they were derived from archean crust the jamon granite suite may have been derived from a quartz dioritic source and the velho guilherme granites from k feldspar bearing granitoid rocks with some sedimentary input the serra dos carajas granites either had a somewhat more mafic source than velho guilherme or were derived by a larger degree of melting underplating of mafic magma was probably the heat source for the melting the petrological and geochemical characteristics of the carajas granite suites imply considerable compositional variation in the archean of the eastern amazonian craton the oxidized jamon suite granites are similar to the mesoproterozoic magnetite series granites of laurentia and they were derived from archean igneous sources that were more oxidized than the sources of the fennoscandian rapakivi granites the serra dos carajas and velho guilherme granites approach the classic reduced rapakivi series of fennoscandia and laurentia no counterparts of the mesoproterozoic two mica granites of laurentia have been found however following the model of hoffman hoffman p 1989 speculations on laurentia s first gigayear 2 0 to 1 0 ga geology 17 135 138 the origin of the 1 88 ga carajas granites is related to a mantle superswell beneath the trans amazonian supercontinent this caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of a type granite magmas the paleoproterozoic continent that included the archean and trans amazonian domains of the amazonian craton was assembled at 2 0 ga its disruption was initiated at 1 88 ga at least 200 ma earlier than in laurentia and fennoscandia the carajas granites were related to the breakup of the supercontinent not to subduction processes

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

spatial and temporal zoning of hydrothermal alteration and mineralization in the sossego iron oxide copper gold deposit carajas mineral province brazil paragenesis and stable isotope constraints the sossego iron oxide copper gold deposit 245 mt 1 1 cu 0 28 g t au in the carajas mineral province of brazil consists of two major groups of orebodies pista sequeirinho baiano and sossego curral with distinct alteration assemblages that are separated from each other by a major high angle fault the deposit is located along a regional wnw ese striking shear zone that defines the contact between metavolcano sedimentary units of the 2 76 ga itacaiunas supergroup and tonalitic to trondhjemitic gneisses and migmatites of the 2 8 ga xingu complex the deposit is hosted by granite granophyric granite gabbro and felsic metavolcanic rocks the pista sequeirinho baiano orebodies have undergone regional sodic albite hematite alteration and later sodic calcic actinolite rich alteration associated with the formation of massive magnetite apatite bodies both these alteration assemblages display ductile to ductile brittle fabrics they are cut by spatially restricted zones of potassic biotite and potassium feldspar alteration that grades outward to chlorite rich assemblages the sossego curral orebodies contain weakly developed early albitic alteration and very poorly developed subsequent calcic sodic alteration these orebodies contain well developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation of breccia bodies breccia matrix commonly displays coarse mineral infill suggestive of growth into open space sulfides in both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite quartz epidote chlorite in the sequeirinho orebodies sulfides range from undeformed to deformed sulfides in the sossego curral orebodies are undeformed very late weakly mineralized hydrolytic alteration is present in the sossego currral orebodies the sulfide assemblage is dominated by chalcopyrite with subsidiary siegenite and millerite pyrrhotite and pyrite are minor constituents of ore in the sequerinho orebodies while pyrite is relatively abundant in the sossego curral bodies oxygen isotope partitioning between mineral pairs constrains temperatures in the deposit spatially and through time in the sequeirinho orebody the early sodic calcic alteration stage was characterized by temperatures exceeding 500 c and delta text 18 text o text h text 2 text o values for the alteration fluid of 6 9 0 9 temperature declines outward and upward from the zone of most intense alteration paragenetically later copper gold mineralization displays markedly lower temperatures 300 c and was characterized by the introduction of 18o depleted hydrothermal fluids 1 8 3 4 the calculated dh2o and delta text 18 text o text h text 2 text o values suggest that the fluids that formed the early calcic sodic alteration assemblage were of formational metamorphic or magmatic origin the decrease of delta text 18 text o text h text 2 text o values through time may reflect influx of surficially derived waters during later alteration and mineralization events influx of such fluids could be related to episodic fluid overpressure resulting in dilution and cooling of the metalliferous fluid causing deposition of metals transported as metal chloride complexes

fluid inclusion evidence for p v t x evolution of hydrothermal solutions in late alpine gold quartz veins at brusson val d ayas northwest italian alps

the age origin and tectonics of the grao para group and associated rocks serra dos carajas brazil archean continental volcanism and rifting the grao para group which is host to the large serra dos carajas fe deposits is located in the eastern part of the central brazil shield ssw of belem brazil it is composed of a lower sequence of metavolcanic rocks a middle sequence of iron formation carajas formation and an upper sequence of mixed metavolcanic and metasedimentary rocks the grao para group is overlain by an unnamed sedimentary dominantly sandstone unit and unconformably overlies the older xingu complex the lower metavolcanic sequence of the grao para in the region of the serra norte fe deposits is bimodal with dominant metabasalts metabasaltic andesites and metatrachyandesites and subordinate 10 15 metarhyolitic tuffs and flows most of the metavolcanic rocks are at low to medium metamorphic grade with more intense metamorphism and deformation along the northern contact with the xingu complex the thickness of the metavolcanic section is not known but may be 4 6 km the mafic metavolcanic rocks are enriched in rb ba and k with respect to oceanic basalts and basalts of greenstone belts and are relatively depleted in th sr ta ti and p the mafic rocks also show an enrichment in lrees with flat hree patterns the hrees are correlated with mg number u pb zircon analyses of the metarhyolites from the grao para group yield an age of 2758 39 ma rb sr wholerock analyses of the mafic rocks yield an isochron with an age of 2687 54 ma with an initial ratio of 0 7057 0 0010 the results indicate a late archean age of volcanism in the grao para group much older than previously thought rb sr results from the metarhyolites have been partially reset yielding an age 2497 62 ma sm nd analyses from the grao para metavolcanic rocks were too scattered to yield an isochron but nd values relative to chur at 2758 ma indicate values similar to depleted mantle at this time although with some scatter the trace element data and the sr and nd isotopic results from the mafic rocks are consistent with contamination of mantle derived melts by significant but variable amounts of diverse older continental crust the stratigraphy geochemical and isotopic compositions show that the grao para group originated on extending older continental crust during the archean an age terrane boundary must exist between this archean continental terrane and the apparently ensimatic early proterozoic greenstone belts of the northern amazonian craton similarities to other sequences e g the fortescue group australia suggest that similar tectonic settings were widespread at this time many major iron formations were deposited within these environments indicating significant iron formation development on sialic crust in the late archean u ph zircon analyses from the serra dos carajas granite which intrudes the unnamed sedimentary unit overlying the grao para group yield an age of 1820 49 ma in agreement with previous age determinations this age is similar to the ages for the uatuma supergroup volcanic and shallow plutonic rocks which occur throughout the amazonian craton

arc volcanism in an extensional regime at the initiation of subduction a geochemical study of hahajima bonin islands japan hahajima represents an eocene forearc volcanic centre produced during the initiation of subduction within an intra oceanic domain of the western pacific the eocene oligocene izu bonin arc forearc is envisaged as a broad region of volcanism generated in an extensional tectonic regime the forearc is likely to have consisted of a series of small rift basins punctuated and bordered by coherent volcanic edifices hahajima represents one such edifice at the southern termination of the chichijima mukojima rift basin major element trace element and isotopic data are presented for representative volcanic rocks from hahajima the volcanic sequence comprises porphyritic basalt and andesite breccias and flows intercalated with tuffaceous sandstones all lavas contain high proportions of plagioclase phenocrysts 20 36 and comparatively low mafic mineral contents the andesite suite is typified by the presence of magnetite and orthopyroxene phenocrysts and pseudomorphs after olivine basalts are dominated by an assemblage of plagioclase clinopyroxene and olivine nodules and xenocrysts typical of the andesites are found in the basalts and vice versa suggesting an intimate relationship between the two magmas during volcanism the basaltic suite follows a tholeiitic fractionation path iron enrichment while the andesites are more representative of calc alkaline magma evolution these trends are found to be clearer when corrections are made for the excess proportions of plagioclase in the whole rock trace element differences between the suites can not be reconciled by a straightforward crystallization model but can be related to a similar parental magma if the shallow level magma evolution involves a combination of open and closed system fractionation processes comparisons of volcanic suites within and between each site sampled along the izu bonin forearc indicate that each eruptive centre has an individual trace element signature this suggests that variations within each site can be explained by melting and or shallow level fractionation processes while differences between localities are related to heterogeneities in the composition trace element and isotopic enrichment and thermal characteristics of the source the hahajima volcanic rocks contrast with the more recent arc magmas of the izu bonin system in that they share some features such as high mg and elevated zr y with the contemporaneous boninitic lavas