statistical microthermometry of synthetic fluid inclusions in quartz during decompression reequilibration fluid inclusion microthermometric data are often reported as homogenization temperature frequency histograms interpretation of such histograms for a single fluid inclusion assemblage fia of non reequilibrated fluid inclusions is usually straightforward and provides an accurate determination of the original density th of that fia however interpretation of such histograms for reequilibrated inclusions is more problematic decompression experiments using synthetic inclusions in natural quartz and conducted at 2 5 kbar and 600 700 c with a maximum internal overpressure of 2 kbar indicate that histogram shape reflects the sample s p t history our results further indicate that the mean mode range standard deviation extreme values etc all have a significance with respect to the p t history of the sample thus a mound shaped unimodal histogram with low range is indicative of a nearly isochoric cooling p t path a unimodal histogram that is slightly skewed to the right and with a low standard deviation but high range results from inclusion deformation in the plastic regime high temperature low strain rates fluid inclusions deformed plastically show no correlation between size and density histogram outliers should not be ignored and may be used to determine an isochore that passes close to the conditions of entrapment minimum th or close to the final reequilibration conditions maximum th the histogram mean th value corresponds to an isochore that represents the internal overpressure about 1 kbar that can be maintained over geologic time by a majority of reequilibrated fluid inclusions a multimodal histogram with high range and high standard deviation indicates inclusion brittle deformation low temperature high strain environments fluid inclusions deformed in a brittle manner show strong positive correlation between size and density histograms produced in the laboratory show many similarities to histograms for natural samples offering the hope that laboratory results may be used to interpret p t histories of natural samples

chemical analyses of halite trend inclusions from the granisle porphyry copper deposit british columbia high salinity fluid inclusions from many porphyry copper and skarn deposits fall on linear trends in the h 2 o nacl kcl system called the halite trend and they have been interpreted to result from separation of kcl bearing halite before entrapment of the fluid in the inclusion in addition to na k and cl halite trend inclusions can also contain abundant fe ca and s all of which could have an effect on the orientation of the trend in the h 2 o nacl kcl system making halite subtraction a less likely explanation for the observed compositional trend we have attempted to evaluate this possibility by analysis of inclusion daughter minerals decrepitates and leachates as well as phase volume measurements for daughter minerals in fluid inclusions from the granisle porphyry copper deposit these observations show that fecl 2 composes 31 to 45 wt percent of the inclusion and that the resulting halite trend inclusion salinities range from 70 to 84 wt percent of nacl kcl fecl 2 cacl 2 and mncl 2 are minor components the liquid contains smaller but significant amounts of sulfur probably as hso super 4 and its complexes as well as co 2 co and ch 4 in minor amouns trends established by these data point toward nacl in multicomponent space thus supporting halite or halite solid solution subtraction over the alternative mechanisms proposed to explain the halite trend

factors controlling copper solubility and chalcopyrite deposition in the sungun porphyry copper deposit iran the sungun porphyry copper deposit is hosted in a diorite granodioritic to quartz monzonitic stock that intruded eocene volcanosedimentary and cretaceous carbonate rocks copper mineralization is associated mainly with potassic alteration and to a lesser extent with sericitic alteration based on previously published fluid inclusion and isotopic data by hezarkhani and williams jones most of the copper is interpreted to have deposited during the waning stages of orthomagmatic hydrothermal activity at temperatures of 400 to 300 c these data also indicate that the hydrothermal system involved meteoric waters and boiled extensively in this work thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function of fs2 fo2 and ph the solubility of chalcopyrite was evaluated in this range of conditions using recently published experimental data during early potassic alteration 450 c copper solubility is calculated to have been 50 000 ppm whereas the copper content of the initial fluid responsible for ore deposition is estimated from fluid inclusion data to have been 1200 3800 ppm this indicates that initially the fluid was highly undersaturated with respect to chalcopyrite which agrees with the observation that veins formed at t 400 c contain molybdenite but rarely chalcopyrite copper solubility drops rapidly with decreasing temperature and at 400 c is approximately 1000 ppm within the range estimated from fluid inclusion data whereas at 350 c it is only 25 ppm these calculations are consistent with observations that the bulk of the chalcopyrite deposited at sungun is hosted by veins formed at temperatures of 360 60 c other factors that in principle may reduce chalcopyrite solubility are increases in ph and decreases in fo2 and acl our analysis shows however that most of the change in ph occurred at high temperature when chalcopyrite was grossly undersaturated in the fluid and that the direction of change in fo2 increased chalcopyrite solubility we propose that the sungun deposit formed mainly in response to the sharp temperature decrease that accompanied boiling and partly as a result of the additional heat loss and decrease in acl which occurred as a result of mixing of acidic cu bearing magmatic waters with cooler meteoric waters of lower salinity

controls of alteration and mineralization in the sungun porphyry copper deposit iran evidence from fluid inclusions and stable isotopes the sungun porphyry copper deposit is located in northwestern iran azarbaijan province and is associated with diorite granodiorite to quartz monzonite of miocene age which intruded eocene volcano sedimentary and cretaceous carbonate rocks copper mineralization was accompanied by both potassic and phyllic alteration field observations and petrographic studies demonstrate that emplacement of the sungun stock took place in several intrusive pulses each with associated hydrothermal activity molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper somewhat later four main vein groups have been identified i quartz molybdenite anhydrite or k feldspar with minor pyrite chalcopyrite and bornite ii quartz chalcopyrite pyrite or molybdenite iii quartz pyrite calcite or chalcopyrite or anhydrite gypsum or molybdenite and iv quartz or calcite or gypsum or pyrite early hydrothermal alteration produced a potassic assemblage orthoclase biotite in the central part of the stock propylitic alteration occurred contemporaneously with potassic alteration but in the peripheral parts of the stock and phyllic alteration occurred later overprinting the earlier alteration the early hydrothermal fluids are represented by high temperature 340 degrees 500 degrees c high salinity up to 60 wt nacl equiv liquid rich fluid inclusions and high temperature 320 degrees 550 degrees c low salinity vapor rich inclusions these fluids are interpreted to represent an orthomagmatic fluid which boiled episodically the brines are interpreted to have caused potassic alteration and deposition of group i and ii quartz veins containing molybdenite and chalcopyrite propylitic alteration is attributed to a liquid rich lower temperature 240 degrees 330 degrees c ca rich evolved meteoric fluid influx of meteoric water into the central part of the system and mixing with magmatic fluid produced deep albitization transition alteration and shallow phyllic alteration this influx also caused dissolution of early formed copper sulfides and remobilization of cu into the sericitic zone where it was redeposited in response to a boiling induced decrease in temperature supergene alteration was minor and restricted to a thin blanket of cu sulfides below an argillic cap

synthetic fluid inclusions in natural quartz iii determination of phase equilibrium properties in the system h2o nacl to 1000 c and 1500 bars phase equilibria in the system h2o nacl have been determined to 1000 c and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion free brazilian quartz in the presence of the two coexisting immiscible h2o nacl fluids at various temperatures and pressures petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present or mixtures of the two salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single homogeneous fluid phase results of the present study are consistent with previously published data on the h2o nacl system at lower temperatures and pressures and indicate that the two phase field extends well into the p t range of most shallow magmatic hydrothermal activity as a consequence chloride brines exsolved from many epizonal plutons during the process of second boiling should immediately separate into a high salinity liquid phase and a lower salinity vapor phase and produce coexisting halite bearing and vapor rich fluid inclusions this observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions particularly the porphyry copper deposits in which halite bearing and coexisting vapor rich inclusions are commonly associated with the earliest stages of magmatic hydrothermal activity

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

a synvolcanic origin for magnetite rich orebodies hosted by bif in the weld range district western australia abstract iron ore deposits hosted by archean banded iron formation bif in the weld range greenstone belt are representative of most of the documented iron ore deposits in the yilgarn craton they include near surface supergene goethite hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores the cenozoic goethite hematite rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting bif in the near surface supergene environment however the deeper and likely older magnetite and specular hematite rich orebodies have a more contentious origin this study is the first to present a fluid alteration model for hypogene iron mineralization in the yilgarn craton that uses fluid inclusion and mineral chemistry data to constrain the physical chemical characteristics and source of hypogene fluids responsible for mineralization high grade 57 wt fe magnetite rich iron ore at the beebyn deposit defines a discontinuous series of 440 c and co2 rich paired o and c stable isotope data for stage 1 ferroan dolomite suggest that these fluids had a magmatic source while stage 1 magnetite chemistry e g enrichments in mg mn ca and na indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite the presence of monophase carbonic fluid inclusions in stage 1 ferroan dolomite suggest that phase separation of a bicarbonate rich aqueous fluid took place in deeper parts of the hydrothermal system which led to the separation of the resultant volatile rich and brine phases during transport of the stage 1 fluid to shallower crustal levels cooling of the hydrothermal system during the stage 2 fluid event involved i an early brine 275 327 c 36 40 equiv wt nacl with cl br and c and o isotopes values that overlap the ranges for magmatic fluids with minor involvement of archean seawater and extensive chemical exchange with country rocks followed by ii pulses of moderate and lower temperature stage 2 brines 125 260 c 2 24 equiv wt cacl2 with cl br o and c isotope na br and ca ca na signatures that suggest cooling of magmatic derived fluids that mixed with archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways the last stage of formation of magnetite rich ore at the beebyn deposit involved the flow of stage 3 fluids through the existing fault network that controlled earlier fluids stage 3 fluids are lower temperature 98 175 c low to high salinity brines with cl br values that overlap reported ranges for 3 2 ga vent fluids and seawater thus they are likely the product of heated archean seawater that was chemically modified through interaction with mafic country rocks magnetite talc veins at the madoonga deposit have fluid halogen ratios for fluid inclusions hosted by magnetite that are compatible with a range of possible sources including low grade metamorphic fluids geothermal brines or oil field formation waters although their common spatial association with semi massive sulfides suggests their likely precipitation from brines derived from heated archean seawater more locally developed specular hematite quartz veins that cut folded magnetite rich ores at the beebyn and madoonga deposits are the product of iron redistribution in bif by heated meteoric fluids or seawater with precipitation of specular hematite as a consequence of oxidation and cooling of the fluid

prospectivity of western australian iron ore from geophysical data using a reject option classifier abstract there has recently been a rapid growth in the amount and quality of digital geological and geophysical data for the majority of the australian continent coupled with an increase in computational power and the rising importance of computational methods there are new possibilities for a large scale low expenditure digital exploration of mineral deposits here we use a multivariate analysis of geophysical datasets to develop a methodology that utilises machine learning algorithms to build and train two class classifiers for provincial scale greenfield mineral exploration we use iron ore in western australia as a case study and our selected classifier a mixture of a gaussian classifier with reject option successfully identifies 88 of iron ore locations and 92 of non iron ore locations parameter optimisation allows the user to choose the suite of variables or parameters such as classifier and degree of dimensionality reduction that provide the best classification result we use randomised hold out to ensure the generalisation of our classifier and test it against known ground truth information to demonstrate its ability to detect iron ore and non iron ore locations our classification strategy is based on the heterogeneous nature of the data where a well defined target iron ore class is to be separated from a poorly defined non target class we apply a classifier with reject option to known data to create a discriminant function that best separates sampled data while simultaneously protecting against new unseen data by closing the domain in feature space occupied by the target class this shows a substantial 4 improvement in classification performance our predictive confidence maps successfully identify known areas of iron ore deposits through the yilgarn craton an area that is not heavily sampled in training as well as suggesting areas for further exploration throughout the yilgarn craton these areas tend to be more concentrated in the north and west of the yilgarn craton such as around the twin peaks mine 27 s 116 e and a series of lineaments running east west at 25 s within the pilbara craton potential areas for further expansion occur throughout the marble bar vicinity between the existing spinifex ridge and abydos mines 21 s 119 121 e as well as small isolated areas north of the hamersley group at 21 5 s 118 e we also test the usefulness of radiometric data for province scale iron ore exploration while our selected classifier makes no use of the radiometric data we demonstrate that there is no performance penalty from including redundant data and features suggesting that where possible all potentially pertinent data should be included within a data driven analysis this methodology lends itself to large scale reconnaissance mineral explorations and through varying the datasets used and the commodity being targeted predictive confidence maps for a wide range of minerals can be produced

geology and alteration geochemistry of the porphyry cu au deposit at bajo de la alumbrera argentina this paper describes the igneous geology alteration mineralogy and geochemistry as well as the ore mineralization history of bajo de la alumbrera a world class porphyry copper gold deposit in northwestern argentina the deposit occurs in the k rich calc alkaline farallon negro volcanic complex located 200 km east of the main andean porphyry copper belt of chile the deposit consists of a composite stock of dacitic porphyry intrusions and extends into the surrounding andesitic volcanic rocks two of the earliest intrusions now occupying the center of the intrusive complex show intense hydrothermal alteration and mineralization these two porphyries host the highest ore grades and were subsequently intruded by several weakly mineralized or barren porphyries intense quartz magnetite k feldspar alteration pervades the first mineralized porphyry whereas similar but lower intensity alteration affected the second laterally this alteration grades into potassic alteration secondary k feldspar and biotite which affects all porphyries except the latest dikes andesitic wall rocks within a hundred meters of the intrusions are dominated by dark hydrothermal biotite an outer halo of propylitic alteration epidote chlorite albite calcite extends up to 1 km into the andesites feldspar destructive alteration sericite pyrite clay minerals gypsum overprints the potassic and propylitic alteration in the volcanic rocks and all porphyries the alteration is locally controlled by faults and late fractures and developed most pervasively upon the outer part of the potassic zone and toward the transition to propylitic alteration in the upper part of the deposit mass balance calculations show small positive changes in net volume for the potassic alteration and minor volume decreases for the chlorite epidote and feldspar destructive alteration the intensely quartz magnetite veined sample of quartz magnetite alteration indicates a total volume increase of 350 percent copper and gold are intimately associated at all scales they occur as composite gold sulfide grains chalcopyrite rarely bornite show a close correlation of ore grades in sample assays and have a closely overlapping distribution on the mine scale a well developed ore shell coincides with potassic alteration and partly with the distribution of intense quartz magnetite veins even though the bulk of the ore minerals now occupy texturally late positions in vugs and fractures a barren zone with intense quartz magnetite potassic alteration and veins but little or no sulfides is interpreted as the focus of upflow of high temperature ore fluids prior to ore mineral saturation and coprecipitation of copper sulfides and gold in the ore shell

on postmidnight low latitude ionospheric irregularities during solar minimum 2 c nofs observations and comparisons with the equatorial atmosphere radar 1 a detailed comparison between the observations of the communication navigation outage forecasting system c nofs satellite and the 47 mhz equatorial atmosphere radar ear in west sumatra indonesia 10 36 s dip latitude on the postmidnight irregularities is presented the zonal and meridional e b drift velocities measured by the vector electric field instrument on the c nofs are consistent with the westward propagation of backscatter echoes and the line of sight doppler velocities observed with the ear respectively the plasma density depletions are observed in the postmidnight sector for several consecutive orbits which suggests the depletions grow slowly during the premidnight period and reach the spacecraft altitude around local midnight the convergence of the equatorward wind which could be responsible for the midnight temperature maximum may produce a preferable condition for the growth of the rayleigh taylor instability around midnight electric field fluctuations of medium scale traveling ionospheric disturbances may play an important role in seeding the instability both equatorial and midlatitude type plasma instabilities could be operational at the ear latitude sector which together would foster a high occurrence of postmidnight irregularities during solar minimum

the role of deformation in the formation of banded iron formation hosted high grade iron ore deposits hamersley province australia abstract the hamersley province western australia hosts some of the world s largest iron ore deposits but despite decades of research their genesis is still extensively debated many iron ore deposits are hosted in complexly deformed archean to paleoproterozoic banded iron formations comprising thin chert and iron oxide bands interlayered with silicate rich shales and carbonates current iron ore genesis models have identified a strong structural control on ore formation linked to extensive hypogene and supergene fluid circulation along fault structures these fluid pathways facilitate the removal of vast amounts of gangue minerals leading to enrichment of the iron oxide residue to iron ore however the evolution of the associated structures has not yet been considered as a key element in ore genesis here we show through multiscale structural analyses that deformation not only forms suitable fluid channels but that folding and shearing also result in significant synkinematic removal of gangue minerals our multidisciplinary investigation of the structural evolution of the mount tom price deposit combines microtectonic field geology and 3d implicit modelling techniques to establish a link between deformation structures at various scales microscale shear bands and outcrop scale asymmetric parasitic folds share striking similarities in their evolution and their controlling mechanisms both features record substantial non coaxial deformation accompanied by volume changes due to stress induced silica remobilisation the closely spaced layering of rheologically different lithologies within hamersley province strata plays a crucial role in complex multilayer deformation which resulted in extensive strain partitioning our study suggests that deformation was of major significance in the upgrading of banded iron formation to iron ore and was active from the early stages of banded iron formation during diagenesis deformation structures also established a micro to deposit scale lateral and vertical fluid network which enabled infiltration by hypogene and supergene fluids during or after deformation these new insights have important implications for iron ore genesis models structural applications in the mine environment and for understanding complex multilayer deformation with volume loss

evolution of hydrothermal fluid characteristics at the santa rita new mexico porphyry copper deposit mineralogic and fluid inclusion investigations conducted in the laboratory combined with in situ observations of temporal relationships among geologic features demonstrate that systematic changes in hydrothermal fluid characteristics occurred with time during evolution of mineralization and alteration of the granodiorite stock at the santa rita porphyry copper deposit the earliest fluids to circulate are recorded in potassically altered orthoclase and biotite stable veins as vapor rich h 2 o co 2 inclusions and hypersaline 30 70 equiv wt nacl or kcl inclusions which homogenize over a wide temperature range from 240 degrees to greater than 800 degrees c the high salinity fluid inclusions are complex chloride brines with significant concentrations of sodium potassium copper iron and sulfur but sulfide minerals are not associated with the early potassic assemblage produced by this fluid oxygen isotope ratios of quartz in the orthoclase and biotite stable veins are identical to phenocryst quartz in the intrusion 8 2ppm later solutions circulated through newly formed fractures and older reopened veins and are recorded in low salinity less than 15 equiv wt nacl fluid inclusions which homogenize primarily in the temperature range of 220 degrees to 360 degrees c these hydrothermal fluids formed two distinct alteration assemblages as they reacted with potassically altered rock chlorite is the dominant silicate gangue mineral of the earlier chalcopyrite bearing orthoclase and clay stable assemblages and sericite characterizes the later phyllic alteration assemblage with quartz and pyrite oxygen isotope ratios of quartz of the later assemblages 8 6 10 4ppm are heavier than quartz of the earlier potassic assemblages it is concluded that the earliest hydrothermal fluids to circulate within the stock localizing the orebody at santa rita could have been derived by phase separations of fluids exsolved from crystallizing magma into condensed hypersaline liquids and h 2 o co 2 vapors as these fluids moved through fractures potassic alteration of rock occurred without concomitant deposition of sulfides later solutions of dominantly meteoric origin progressively formed hypogene copper mineralization associated with chlorite stable alteration and then phyllic alteration

microplaty hematite ore in the yilgarn province of western australia the geology and genesis of the wiluna west iron ore deposits abstract the wiluna west small 130 mt high grade bedded hematite ore deposits consisting of anhedral hematite mesobands interbedded with porous layers of acicular hematite show similar textural and mineralogical properties to the premium high grade low phosphorous direct shipping ore from pilbara sites such as mt tom price mt whaleback etc in the hamersley province and goldsworthy shay gap and yarrie on the northern margin of the pilbara craton both margins of the pilbara craton and the northern margin of the yilgarn craton were subjected to sub aerial erosion in the paleoproterozoic era followed by marine transgressions but unlike the hamersley basin the jfgb was covered by comparatively thin epeirogenic sediments and not subjected to proterozoic deformation or burial metamorphism the joyner s find greenstone belt jfgb in the yilgarn region of western australia was exhumed by middle to late cenozoic erosion of a cover of unmetamorphosed and relatively undeformed paleoproterozoic epeirogenic sedimentary rocks that preserved the jfgb unaltered for nearly 2 ga thus providing a unique snapshot of the early proterozoic environment acicular hematite pseudomorphous after acicular iron silicate is only found in iron ore and bif that was exposed to subaerial deep weathering in early paleoproterozoic times pre 2 2 ga and in the overlying unconformable paleoproterozoic conglomerate derived from these rocks and is absent from unweathered rocks lascelles 2002 high grade ore and bif weathered during later subaerial erosion cycles contain anhedral hematite and acicular pseudomorphous goethite the acicular hematite was formed from goethite pseudomorphs of silicate minerals by dehydration in the vadose zone under extreme aridity during early paleoproterozoic subaerial weathering the principal high grade hematite deposits at wiluna west are interpreted as bedded ore bodies that formed from bif by loss of chert bands during diagenesis and have been locally enriched to massive hematite by the introduction of hydrothermal specular hematite no trace of chert bands are present in the deep saprolitic hematite and hematite goethite ore in direct contrast to shallow supergene ore in which the trace of chert bands is clearly defined by goethite replacement voids and detrital fill abundant hydrothermal microplaty hematite at wiluna west is readily distinguished by its crystallinity the genesis of the premium ore from the pilbara region has been much discussed in the literature and the discovery at wiluna west provides a unique opportunity to compare the features that are common to both districts and to test genetic models

compositional re equilibration of fluid inclusions in quartz compositional modifications to salt water fluid inclusions in quartz were observed following exposure to disequilibrium conditions in a series of laboratory experiments in which samples containing inclusions of known composition were annealed at 3 kbar and 600 t 825 c in the presence of fluids having different compositions for time intervals ranging from a few days to one month changes in inclusion compositions following re equilibration were monitored using salt crystal dissolution temperatures and or ir infra red spectroscopy the behaviors of both synthetic and natural fluid inclusions were studied the synthetic samples were re equilibrated under pint pconf conditions to minimize stress in the crystal surrounding the inclusions and were subjected to both fh2ointfh2oconf and fh2ointfh2oconf after re equilibration for four days at t 600 c most inclusions displayed significant compositional changes without decrepitation salinity variations as large as 25 wt were inferred for brine inclusions exposed to fh2oint fh2oconf for 16 days at 825 c the majority of our observations are consistent with the net diffusion of water toward the reservior having the lowest h2o i e synthetic nacl h2o fluid inclusions exhibited increased tm nacl s implying lower relative h2o contents when re equilibrated in the presence of fluids having lower h2o whereas similar and in some cases the same inclusions exhibited decreased tm nacl s implying higher h2o contents after exposure to fluids having higher h2o the behavior of natural salt water fluid inclusions during re equilibration was generally consistent with corresponding observations on synthetic samples verifying that compositional changes are not restricted to synthetic inclusions our results clearly show that there was chemical communication between fluids trapped as inclusions in quartz and the external fluid reservoir additionally it is evident that although applied stress can significantly enhance the re equilibration rate strain in the crystal host around the inclusions resulting from large pressure differentials between the internal and confining fluids is not a necessary prerequisite for compositional change finally because significant compositional changes can be induced in brine inclusions in quartz during shortterm exposure to non equilibrium conditions at 600 t 825 c in the laboratory it is likely that similar changes may result at much lower temperatures during exposure of natural rocks to non equilibrium conditions over geologic time

geochemistry and pb pb geochronology of the neoarchean nkout west metamorphosed banded iron formation southern cameroon the nkout west iron deposit of the northern edge of the congo craton in southern cameroon forms part of the northwest plunging isoclinal mega fold composed of metamorphosed banded iron formations bifs which have been deformed and metamorphosed resulting in recrystallized magnetite quartz and silicates here we present whole rock geochemistry together with pb pb isotopic compositions and dating of metamorphosed bif with the aim to constrain the genesis and elucidate the metamorphic resetting age based on the mineral composition two types of bif magnetite bif garnet bif have been distinguished the magnetite bif shows high sio2 and fe2o3 and low al2o3 tio2 hfse and ree contents similar to detritus free chemical sediments in contrast their high concentrations in garnet bif indicate clay rich contaminated input the co zn ratios are consistent with trace metals of hydrothermal sources 0 14 for magnetite bif or with minor hydrogenous component 0 27 for garnet bif the paas normalized ree y patterns of the studied metamorphosed bifs show lree depletion relative to hree and positive eu anomalies the magnetite bif exhibits positive y anomaly y y paas 1 3 which is the signature inherited from sea water but its y ho 39 93 ratio suggests a mixture of hydrothermal solution with sea water the garnet bif shows positive eu negative y anomalies and chondritic y ho ratio 27 49 which is consistent with hydrothermal solution with significant clastic contamination whole rock pb concentration is roughly on average of 2 0 6 0 ppm suggesting a hydrothermal pb source the pb pb dating yielded an age of 2679 ma interpreted as the neoarchean metamorphic event in the ntem complex

regional variations in bulk chemistry mineralogy and the compositions of mafic and accessory minerals in the batholiths of california we define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole rock geochemistry in the batholiths of california in so doing we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano plutonic arcs of active continental margins little studied metaluminous to strongly peraluminous granites containing fe rich biotite with log x mg x fe f oh and mn in biotite and amphibole increase on a regional scale from western i wc types to eastern i mc and i sc types parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons in contrast the belts of western i scr granites and eastern i wc quartz diorites and granodiorites disrupt the regional west to east systematics in both mineral and whole rock geochemistry spatial variations in the al content of amphibole are regional in scale and reflect pressures of pluton crystallization we conclude that significant previously unrecognized complexity exists in regional geochemical systematics in the california batholiths

contrasting evolutions of hydrothermal alteration in quartz monzonite and quartz diorite wall rocks at the sierrita porphyry copper deposit arizona two wall rock types of contrasting chemical composition host the causative ruby star quartz monzonite porphyry intrusion and hypogene mineralization at the sierrita porphyry copper deposit vein related hydrothermal alteration in the harris ranch quartz monzonite and biotite quartz diorite wall rocks consists of several mineralogically discrete assemblages temporal evolution of different alteration assemblages was established in part using petrographic relations within and crosscutting relations among individual veins temperature and salinity characteristics of hydrothermal fluids responsible for filling of individual veins were determined using primary fluid inclusions in vein filling quartz each generation of primary vein filling introduced characteristic secondary fluid inclusions into earlier developed veins as well histograms of homogenization temperatures of primary and secondary fluid inclusions from different veins and accompanying salinity data permitted temporal correlations to be drawn between veins which either did not exhibit crosscutting relations in individual samples or which formed in different wall rocks and thus exhibited different alteration mineralogies evolution of hydrothermal activity in the area sampled commenced with potassic alteration in both quartz monzonite and quartz diorite wall rocks from 10 to 12 molal 37 41 wt nacl equivalent fluids inclusions of which homogenize by halite dissolution in the approximate temperature range 300 degrees to 370 degrees c salinities of later fluids were in the range 2 to 3 molal 10 15 wt nacl equivalent with only minor salinity variations for the remaining time span monitored by this study homogenization temperatures of primary fluid inclusions in veins formed from these lower salinity fluids began near 400 degrees c and increased initially to approximately 430 degrees c where boiling occurred the pressure defined by boiling of these fluids is about 330 bars a continuous decrease in fluid inclusion homogenization temperatures followed down to about 300 degrees c during which time deposition of quartz and k feldspar with accessory biotite and or hematite occurred in new and reopened veins in the quartz monzonite wall rock simultaneously in the quartz diorite a sequence of veins and adjacent alteration halos formed each consisting of an early assemblage of potassic affinity quartz biotite k feldspar albite which evolved to a propylitic assemblage quartz epidote chlorite as vein filling proceeded with continued cooling of the solution below about 300 degrees c muscovite took the place of k feldspar as the stable potassium bearing mineral in the quartz monzonite an analogue to this late stage quartz muscovite veining in the quartz monzonite could not be established in the quartz diorite but may consist of zeolite stilbite anhydrite the bulk of hypogene copper mineralization in both wall rock types was associated with approximately 2 molal 10 wt nacl equivalent solutions in the quartz diorite significant chalcopyrite deposition is associated with fluid inclusions homogenizing from 370 degrees down to about 320 degrees c and always occurs with the later stage propylitic minerals in each vein in the quartz monzonite wall rock chalcopyrite was deposited during the transition from potassic and into phyllic alteration fluid inclusion homogenization temperatures for this mineralization range from about 330 degrees down to 200 degrees c no primary chalcopyrite was seen to occur with earlier potassic veining formed from either high or low salinity fluids in the quartz monzonite a very late stage of deposition of chalcopyrite pyrite and minor bornite filled the center of late phyllic veins in the quartz monzonite correlated fluid inclusions have salinities from 1 to 5 molal 5 23 wt nacl equivalent and homogenization temperatures in the range 140 degrees to 160 degrees c the different alteration and sulfide mineral assemblages interpreted to have formed simultaneously in the two wall rocks of contrasting chemical character can be reasonably assigned to different chemical interactions of each rock type with similar or equivalent hydrothermal fluids

upper regime parallel lamination as the result of turbulent sediment transport and low amplitude bed forms a series of laboratory experiments has been carried out in which parallel laminated deposits were produced from an upper regime plane bed the laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area 0 3 m by 1 5m fluctuations in bed elevation were measured both during deposition and at equilibrium much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence as most theories for lamina formation would require we suggest instead that extremely low amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination all the lamination produced in the laboratory experiments was normally graded using high speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain diameters thick followed by the slow sieving out of a well packed surface layer of finer sand the initial deposition is the result of small scale turbulent fluctuations in boundary shear stress the sieving out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition we term this process glazing the alignment of small scale turbulent scour and fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination

initial gold contents of silicic volcanic rocks bearing on the behavior of gold in magmatic systems many fresh silicic volcanic rocks have markedly lower initial gold contents than previously recognized of 129 carefully selected glassy silicic volcanic rocks analyzed 113 contain f o 2 more readily accommodate gold the mean of 23 relatively silicic intermediate rocks is 0 54 ppb au with tholeiitic andesites icelandites generally higher in gold than calc alkalic types there is little evidence that particular geologic regions are intrinsically richer in gold than others bulk composition melt structure and the amount and timing of separation of vapor mineral and sulfide and or metal melt phases would appear to largely determine the gold contents of silicic magmas the various lines of evidence for the removal of gold from magma by the separation of vapor crystalline and immiscible melt phases indicate that fresh volcanic rocks provide only minimum limits on magmatic gold concentrations in most cases magmatic values will be much higher elevated gold contents 0 3 to 1 0 ppm of some porphyry deposits suggest the existence of salic magmas containing gold in concentrations appreciably greater than the 1 2 ppb concentrations of basalts

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

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

metal fractionation between magmatic brine and vapor determined by microanalysis of fluid inclusions the major and trace element compositions of individual fluid inclusions from a range of magmatic hydrothermal ore deposits were analyzed by laser ablation inductively coupled plasma mass spectrometry to explore the behavior of ore forming components during fluid phase separation boiling in high temperature saline fluid systems data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior na k fe mn zn rb cs ag sn pb and tl preferentially partition into the brine probably as cl complexes whereas cu as au probably as hs complexes and b selectively partition into the vapor fluid phase separation is probably a major previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic hydrothermal systems from deep plutons through porphyry style and greisen deposits to epithermal mineralization and volcanic fumaroles

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

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

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

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

endeavour 26 north a porphyry copper gold deposit in the late ordovician shoshonitic goonumbla volcanic complex new south wales australia endeavour 26 north is the largest of 12 pipelike porphyry cu au deposits found near parkes in central west new south wales within the late ordovician goonumbla volcanic complex the dominant volcanic rocks of the complex the goonumbla volcanics are latitic flows and volcanic breccias which are overlain by the trachytic wombin volcanics the volcanic rocks are characterized by high k 2 o and high k 2 o na 2 o ratios which are typical of the shoshonite association as well as an enrichment in large ion lithophile elements and a low concentration of high field strength elements the nelungaloo volcanics form an early ordovician nonshoshonitic volcanic substrate to the complex which initially developed in a submarine and subsequently subaerial setting the 87 sr 66 sr epsilon nd and pb isotope data indicate the magmas were derived from mantle sources the shoshonitic magmas may be products of high pressure fractionation in the subcontinental lithosphere the porphyry deposits are associated with a gravity low which is taken to reflect a subvolcanic chamber beneath the center of the complex all deposits are located within a pronounced circular feature in the aeromagnetic data that is interpreted as the margin of a collapsed caldera dioritic to monzonitic stocks which in part define the caldera rim represent a resurgent phase of magmatism the endeavour 31 stock which was part of this phase is ringed by six porphyry cu au deposits including endeavour 26 north the deposits are centered on fingerlike quartz monzonite porphyries which cut the endeavour 31 stock at depth geochemical anomalies of cu and zn in the bedrock define the size of the alteration system associated with the stock in part the distribution of the cu au deposits is controlled by the endeavour lineament in comparison to most other volcanic and intrusive rocks of the goonumbla volcanic complex the endeavour

postsunset vortex in equatorial f region plasma drifts and implications for bottomside spread f recent high resolution incoherent scatter drift measurements conducted at jicamarca during solar minimum conditions indicate that a vortical plasma flow pattern accompanies the well known prereversal enhancement events observed in the equatorial f region the vortex is centered at 250 km altitude and about 2000 3000 km east of the evening terminator and is characterized by upward and downward flows to the west and to the east and eastward and westward flows on the top and the bottom respectively postsunset bottomside spread f events are observed to commence in the interior of the vortex in regions of westward plasma drifts the formation of the vortex implies localized charging of the postsunset bottomside f region in response to eastward neutral wind driven f region dynamo action

the origin of terrestrial pisoliths and pisolitic iron ore deposits raindrops and sheetwash in a semi arid environment abstract ooliths evidently form by chemical precipitation in limnic paralic fluvial and marine environments pisoliths however appear to be restricted to terrestrial environments typically composed of iron aluminium and manganese sesquioxides with minor admixtures of quartz and kaolinite they are widely distributed in tropical to subtropical regions overlying deeply weathered soil profiles although iron aluminium and manganese rich end members are important sources of these metals their genesis is still enigmatic their formation has never been observed or produced experimentally and current models for their origin are little more than guesses a new model is presented based on a unique personal observation in which pisoliths are formed by the action of charged raindrops during thunderstorms impacting on dry deeply weathered powdery soils the pisoliths are transported across pediments by sheetwash accumulating as thick deposits in the valley floors pisolites are characteristically unfossiliferous and typically clearly pedogenic the absence of fine depositional layering fossil seeds leaves and pollen in pisolites is explained by bioturbation and the action of soil organisms during extended pedogenesis while the major coarse bedding features derive from erosional and depositional events in the evolution of the pediment pisolitic iron ores aka channel iron deposits cid are a special case of transported pisolitic ferricrust that form an important resource of medium grade iron ore 57 60 wt fe in the pilbara region of western australia apart from minor deposits in the northern yilgarn province of western australia they have not been found elsewhere they differ from normal transported ferricrust and terrestrial pisolites not only in the exceptionally high iron and low alumina and silica content but also in containing abundant fossilised wood particles

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

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

hydrothermal origin for the 2 billion year old mount tom price giant iron ore deposit hamersley province western australia giant iron ore deposits such as those in the hamersley province of northwestern australia may contain more than a billion tonnes of almost pure iron oxides and are the world s major source of iron it is generally accepted that these deposits result from supergene oxidation of host banded iron formation bif accompanied by leaching of silicate and carbonate minerals new textural evidence however shows that formation of iron ore at one of those deposits mount tom price involved initial high temperature crystallisation of magnetite siderite iron silicate assemblages this was followed by development of hematite and ferroan dolomite bearing assemblages with subsequent oxidation of magnetite leaching of carbonates and silicates and crystallisation of further hematite preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids initial hematite crystallisation occurred at about 250 c and high fluid pressures and continued as temperatures decreased although the largely hydrothermal origin for mineralisation at mount tom price is in conflict with previously proposed supergene models it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation

the genesis of the hope downs iron ore deposit hamersley province western australia the banded iron formation bif hosted hope downs high grade hematite ore deposits are situated within the marra mamba iron formation with subsidiary deposits in the brockman iron formation of the archean to proterozoic hamersley group of western australia the main orebody extends to 260 m below the surface and is unusually rich in martite pseudomorphous hematite after magnetite and poor in limonite and goethite compared to other ore deposits of the marra mamba iron formation the high grade hematite ore is mainly within the newman member but also occurs in parts of the nammuldi member together with low grade limonitic ore that becomes high grade after calcining karst erosion of the overlying wittenoom formation has produced steep sided buried valleys adjacent to the in situ orebodies that contain thick deposits 160 m of goethitic and sideritic sediments including remnants of robe pisolite formation bedded siderite hematite gravels red ochreous detrital material and enriched scree deposits that are additional sources of ore the ore consists of low phosphorous martite limonite goethite derived from chert free bif by supergene weathering no evidence of the complete carbonate replacement of chert has been found at hope downs nor were any traces of preexisting chert bands seen in the ore despite the abundance of chert bands in bif elsewhere a variety of textures and composition shown by cherty bif adjacent to the orebodies is described from which the origin of the chert free bif is inferred including sedimentary structures consistent with density current deposition a model is presented for the origin of the host iron formation and the ore deposits in which density currents transported reworked iron silicates and hydroxides in colloidal suspension onto an unstable sea floor the amorphous silica produced during diagenesis of al poor iron silicates formed the characteristic chert bands of bif but some of the hydrous amorphous silica was lost prior to lithification to form chert free bif weathering of the chert free bif produced the high grade hematite ore that is exposed today

the relation between iron formation and low temperature hydrothermal alteration in an archean volcanic environment abstract the uppermost section of the hunter mine group hmg 2728 ma a bimodal volcanic complex in the abitibi greenstone belt contains both oxide and carbonate facies banded iron formation bif this paper explores the relationship between volcanic activity and the development of the two types of iron formation the oxide facies represented by chert jasper magnetite iron formation is widespread and recurs at several stratigraphic levels field evidence suggests it deposited directly on the sea floor during periods of volcanic quiescence and is most probably derived from fluids seeping from volcaniclastic sediments and the compaction of felsic shard rich tuffs seeping occurred along synsedimentary and synvolcanic faults and then along bedding planes carbonate siderite facies iron formation in contrast formed locally below a silica cap rock by in situ low temperature hydrothermal replacement of chert tuff beds and even of the oxide formation thus the two types of iron formation although spatially and stratigraphically juxtaposed are not the result of the same process and only the carbonate facies appears to be directly related to a low temperature hydrothermal volcanic process the contrasting genesis of oxide and carbonate iron formations indicates that current simplistic models relating banded iron formation to volcanic massive sulphide deposits need to be re evaluated the notion that oxide iron formation represents a distal manifestation of volcano generated hydrothermal activity may be the exception and not the rule because as indicated by this study bif occurs in the central part of a volcanic edifice rather than on the flanks

origin of hydrothermal fluids at olympic dam preliminary results from fluid inclusions and stable isotopes fluid inclusions and stable isotope data provide evidence of the temperature composition and sources of hydrothermal fluids associated with formation of the fe rich breccias that host cu u au ag ore at olympic dam early magnetite associated with pyrite and siderite precipitated from fluids of high delta 18 o content approx 10ppm at temperatures near 400 degrees c and magnetite in k feldspar apatite veins at the nearby acropolis deposit probably formed from similar fluids in contrast hematite associated with ore bearing breccias formed at lower temperatures 200 degrees 400 degrees c from fluids of lower delta 18 o content 9ppm fluid inclusion homogenization temperatures in quartz fragments within hematite breccias overlap with and are lower than those indicated by isotope geothermometry two phase liquid plus vapor inclusions mainly homogenize at temperatures between 130 degrees and 280 degrees c with a mode at 170 degrees to 190 degrees c quartz fragments within rare laminated barite clasts display a larger range of homogenization temperatures from 110 degrees to 420 degrees c and may have preserved evidence of early high temperature hydrothermal events measurement of freezing point depressions in fluid inclusions provides evidence of widely varying salinities and complex solute chemistry indicated salinities vary from a minimum of 7 3 wt percent nacl equiv 1 3 m to a maximum of 23 7 wt percent nacl equiv 4 2 m in two phase fluid inclusions and up to 42 wt percent nacl equiv in halite saturated three phase inclusions all the fluid inclusions studied displayed evidence of additional solutes in addition to nacl all required a high degree of supercooling to induce freezing and displayed initial ice melting at temperatures below 23 degrees c and many formed distinctive brown granular ice in addition several inclusions also formed co 2 clathrates and other metastable solids qualitative assessment of these characteristics indicates fluids carrying multiple solutes probably including nacl kcl cacl 2 fecl 2 caf and co 2 these preliminary results suggest that at least two sources of fluids of contrasting temperature composition and oxygen isotope characteristics were involved in the formation of olympic dam early magnetite was deposited from hydrothermal fluids of deep seated possibly magmatic origin but magnetite bearing rocks are not significant hosts for copper uranium mineralization later hematite associated with ore bearing breccias was deposited by fluids of significantly lower temperature and lower delta 18 o values the geologic setting of the deposit a fault bounded depression at or near the palcosurface implies that waters of surficial origin seawater closed basin water or ground water may have been involved in the later stages of hydrothermal activity that formed the hematite breccias although our preliminary isotopic data do not require such waters they are consistent with this conclusion

gold in porphyry copper deposits experimental determination of the distribution of gold in the cu fe s system at 400 to 700 c experiments in the system au cu fe s were carried out at temperatures of 400 to 700 c to determine how much gold could be accommodated by bornite and chalcopyrite the two most common ore minerals in porphyry copper gold deposits our results show that for all temperatures bornite contains one order of magnitude more gold than chalcopyrite or intermediate solid solution iss its high temperature equivalent the range of gold concentrations in bornite and chalcopyrite or iss decreases with decreasing temperature from 1 280 to 8 200 ppm au in bornite and 100 to 125 ppm au in iss at 600 c to 235 to 364 ppm au in bornite and 5 to 16 ppm au in chalcopyrite or iss at 500 c and to 13 to 80 ppm au in bornite and 2 to 4 ppm au in chalcopyrite or iss at 400 c the amount of gold in bornite is also strongly dependent on the composition of bornite being highest in stoichiometric bornite compositions cu5fes4 and decreasing toward cu rich and cu poor compositions phase equilibrium constraints for solutions with geologically reasonable reduced sulfur contents indicate that high temperature porphyry copper gold deposits will contain bornite and magnetite whereas lower temperature deposits whether primary or overprinted by phyllic alteration will contain chalcopyrite and pyrite if gold is present in the ore forming solutions more of it will be deposited in high temperature porphyry copper gold deposits where it will be closely associated with bornite coexisting magnetite in these deposits should generate magnetic anomalies lower temperature deposits will contain less gold which is hosted by pyrite as well as chalcopyrite and will lack magnetic anomalies comparison of the amount of gold hosted by natural porphyry copper gold ores to that hosted by bornite and chalcopyrite in our experiments suggests that significant amounts of gold can be lost from these deposits into surrounding hydrothermal systems

hydrothermal alteration and stable isotope systematics of the babine porphyry cu deposits british columbia implications for fluid evolution of porphyry systems the eocene babine intrusive suite of west central british columbia hosts a number of porphyry copper deposits most significantly bell granisle and morrison all deposits feature central potassic zones containing the ore zones and peripheral propylitic zones in addition granisle and bell feature superimposed sericite carbonate zones between the potassic and propylitic zones and the bell deposit also possesses a superimposed phyllic stockwork zone which hosts most of the ore calculated fluid compositions from potassic zone biotites of all deposits yield a range from delta 18 o sub h 2 o 6 to 8 per mil whereas plagioclases yield delta 18 o sub h 2 o 1 to 7 per mil since plagioclase samples yield more 18 o depleted fluids than coexisting biotites and delta sub plagioclase biotite values are very small it appears that isotopically lighter fluids were present in later stages of potassic alteration oxygen isotope values from the sericite carbonate and phyllic alteration zones yield fluid compositions more 18 o depleted than magmatic fluids requiring the incorporation of isotopically light ground waters calculated hydrogen isotope values of potassic propylitic and sericite carbonate fluids fall in the range of 40 to 110 per mil these results in combination with the delta 18 o data suggest that the alteration events were caused by mixtures of magmatic and meteoric waters phyllic and argillic alteration fluids delta d sub h 2 o 100 to 130 and 140ppm respectively indicate fluids dominantly of meteoric origin the stable isotope data in combination with the petrologic and field relations suggest that early alteration fluids were derived from the magma and in peripheral areas fluids evolved at very low water rock ratios incorporation of external fluids in later alteration was responsible for the texturally destructive cation leaching alteration and remobilization of the ores differences in the stable isotope systematics and salinities of porphyry copper deposits compared to meteoric hydrothermal systems and epithermal ore deposits indicate that the systems are fundamentally distinct the strong density contrast between the high salinity magmatic fluids and low salinity meteoric waters most likely inhibits mixing resulting in separation of the hydrothermal systems

direct evolution of brine from a crystallizing silicic melt at the questa new mexico molybdenum deposit molybdenum mineralization in the southwest orebody at the questa deposit is associated with crystallization of aplite porphyry explosive brecciation and exsolution of magmatic hydrothermal fluids quartz k feldspar fluorophlogopite and molybdenite precipitated from exsolving aqueous fluids and cemented aplite porphyry and andesite wall rock clasts forming a magmatic hydrothermal breccia ore fluid inclusions in quartz in the breccia matrix trapped aqueous fluids that transported and precipitated molybdenum two fluid inclusion populations associated with molybdenite mineralization were identified the largest population commonly contains liquid vapor halite sylvite and one or more opaque daughter crystals these inclusions homogenize between 200 degrees and 500 degrees c and have a prominent mode at 360 degrees to 400 degrees c salinities vary from 31 to 57 wt percent nacl equiv low first ice melting temperatures suggest that additional fluid components possibly calcium or magnesium chlorides may be present approximately 80 percent of these inclusions homogenize by halite dissolution phase equilibria constraints require that these inclusions trapped fluids in the vapor absent field the remaining saline inclusions homogenize to liquid at temperatures above halite dissolution temperatures significantly the saline fluid did not coexist with a low density aqueous fluid a second population of inclusions contains fluids with salinities of 2 7 to 25 0 wt percent nacl equiv and homogenizes by vapor bubble disappearance vapor bubble expansion or critical behavior textural relationships and phase behavior during microthermometry indicate that these inclusions constitute a single population which formed as pressure and temperature fluctuated at near critical conditions pressure changes related to brecciation followed by crystallization and system resealing produced the observed fluids saline fluids were trapped as the system sealed following a major brecciation event these high salinity fluids were produced as the melt under pressure that increased from hydrostatic to lithostatic partitioned increasing amounts of chlorine into the exsolving aqueous fluid renewed fracturing and abrupt pressure decrease to hydrostatic conditions reduced the mass of chlorine partitioning to the exsolving fluid lower salinity fluids exsolved at near critical conditions and were trapped by inclusions that homogenize to liquid to vapor or by critical behavior the lack of cogenetic liquid and vapor rich fluid inclusions and phase equilibria constraints indicate that high salinity fluids or 57 wt nacl equiv were generated directly by the crystallizing magma and were not a product of aqueous fluid immiscibility

causes for large scale metal zonation around mineralized plutons fluid inclusion la icp ms evidence from the mole granite australia detailed reconstruction of the fluid evolution in several ore deposits associated with the sn w and base metal mineralized mole granite shows that multiple factors contributed to the zonal metal distribution around this intrusion information about the mechanism of ore formation was obtained from field relationships detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry la icp ms microanalysis of single fluid inclusions premineralization fluid inclusions in sn deposits are characterized by high sn w ratios whereas early fluid inclusions from w deposits contain higher w concentrations and comparatively low sn w ratios this indicates that the metal ratio in these deposits already was predefined by the composition of the magmatic input fluid however in both types of deposits the input fluid actually contained much higher concentrations of base metals pb zn cu etc than sn and w demonstrating that metal precipitation was highly selective the metal zonation around the mole granite therefore reflects both a compositional variation of the source fluid as well as sequential metal precipitation from it an additional process contributing to metal fractionation occurred by phase separation of the hydrothermal fluid into saline brine and low density vapor whereby cu b li as s ag la preferentially partitioned into the vapor phase analyses on six assemblages of unambiguously coexisting vapor and brine inclusions allowed direct determination of vapor brine distribution coefficients for 24 elements

constraints on magma degassing beneath the far southeast porphyry cu au deposit philippines k silicate alteration in the far southeast fse porphyry cu au introduction deposit formed at 1 4 ma concentric to dikes of quartz diorite magmas become saturated with volatile components as porphyry at 2 km paleodepth the hydrothermal system consisted they ascend in the crust and either erupt or crystallize of magmatic hypersaline liquid and vapor at 550 c and 50 at depth saturated magmas that remain at depth exsolve mpa lithostatic pressure advanced argillic alteration formed at an aqueous fluid that forms a hydrothermal system on the same time over the deposit at 1 km paleodepth from acidic separation from the magma chamber burnham 1979 condensates of the vapor at 1 3 ma k silicate alteration was although part of the exsolved fluid a low density vapor overprinted by 350 c magmatic liquid 5 wt nacl equiv may discharge to the surface from passively degassing at hydrostatic pressure sericite alteration and much of the cu au volcanoes hedenquist 1995 part will remain at depth mineralization formed at this later stage evolution of the magmatic and this dense residual magmatic liquid may be asfluid composition was simulated with a magma chamber cryssociated with hydrothermal mineralization in the portallization model homogeneous crystallization during early stage phyry ore environment hedenquist lowenstern convection is assumed whereas at 50 vol crystals the chamber 1994 becomes stagnant and crystallizes from rim to core over a narrow in this paper we use evidence from a porphyry ore crystallization interval the model calculation based on a magma deposit in the philippines to determine the composition chamber with 2 km thickness at 6 km depth 150 mpa and of the fluid that exsolved from the deeper parent magma 800 c saturated melt composition at 30 vol crystals 5 the evolving composition of the magmatic fluid at the wt h2o 0 2 wt cl and dd of 40 can reproduce the depth of the ore body can then be used to place constraints chemical and isotopic compositions of the early and late magmatic on the nature of the evolved magma chamber far below fluids the most critical factor controlling the compositional evolution the porphyry ore body this approach differs from that of the model hydrothermal system is the transition from convective of other studies where a portion of the parent magma to stagnant magma chamber crystallization there is also a sharp chamber is available to study directly e g at yerington decrease in the rate of fluid exsolution associated with this transition nevada dilles 1987 which can account for the thermal collapse of the fse porphyry the far southeast fse porphyry cu au deposit system from k silicate to sericite alteration and overlying lepanto epithermal cu au ore body are

the formation of magnetite in the early archean oceans abstract banded iron formations bifs are iron and silica rich chemical sedimentary rocks that were deposited throughout much of the precambrian the biological oxidation of dissolved fe ii led to the precipitation of a ferric oxyhydroxide phase such as ferrihydrite in the marine photic zone upon burial ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological fe iii reduction coupled to the oxidation of buried microbial biomass however it has always been intriguing as to why the oldest bifs are characteristically magnetite rich while bifs formed after the neoarchean are dominated by hematite here we propose that some magnetite in early archean bif could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot fe ii rich hydrothermal fluids that existed in the deeper waters we conducted experiments that showed the reaction of fe ii with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite our model further posits that with the progressive cooling and oxidation of the earth s oceans the above reaction shuts off and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism

c nofs and radar observations during a convective ionospheric storm event over south america 1 the development of a convective ionospheric storm is studied using three radars the c nofs satellite airglow instrumentation and a numerical model first detected in the form of convective plumes over the eastern pacific plasma irregularities airglow signatures plumes and irregularities were also detected over brazil and then peru dynamo conditions were such that a modest prereversal enhancement was recorded at both christmas island and peru and probably over brazil as well no prereversal enhancement occurred during the next two days and no plumes were detected the numerical model reproduced the results quite well over peru evidence for seeding by both gravity waves and the kelvin helmholtz instability is presented

the mount gibson banded iron formation hosted magnetite deposit two distinct processes for the origin of high grade iron ore the mount gibson banded iron formation lies within the windanning formation of the luke creek group which is found in almost all greenstone belts throughout the murchison province the banded iron formation bif typically consists of alternating bands of magnetite and microcrystalline quartz chert with rare carbonaceous and iron silicate rich shale partings and layers and rare thin fine grained tuff bands owing to repetition by isoclinal folding and attenuation by faulting the true thickness of the mount gibson bif is unknown but appears to be on the order of 100 m although the mount gibson bif is typical of many algoma type iron formations situated within a greenstone belt on an archean craton it is also similar to hamersley type bif in petrology areal extent and ore genesis high grade hematite deposits formed within bif were thought to have formed by the supergene leaching of chert from typical cherty bif recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite magnetite carbonate bif in which there is clear evidence of hydrothermal replacement of chert by carbonate forms distinctive magnetite goethite ore with magnetite locally persisting to the surface mount gibson shows clear evidence of the formation of high grade ore by this process but also contains high grade hematite ore and chert free bif that show no evidence of the hypogene replacement of chert high grade hematite occurrences up to 1 km in strike length are found within the weathered zone overlying the magnetite bif at mount gibson and continue into unweathered chert free bif at depth that show no evidence of hydrothermal carbonate or supergene enrichment the cherty bif shows sharp contacts against chert free bif and high grade ore even when strongly weathered this suggests that deep saprolitic in situ high grade ore may be produced by different processes including hydrothermal replacement of chert mesobands by carbonates with subsequent supergene leaching of the carbonate and by the oxidation of chert free bif in which chert bands either never developed or were apparently removed during diagenesis neither model requires supergene selective leaching of quartz chert during deep weathering

meteoric water component in magmatic fluids from porphyry copper mineralization babine lake area british columbia coexisting vapor rich and high salinity magmatic fluid inclusions from mineralized quartz carbonate veins in porphyry cu systems of the babine lake area are depleted in deuterium d relative to accepted magmatic isotopic values the d compositions of high salinity fluids from early mineralized veins range from 100 to 135 standard mean ocean water whereas fluid inclusions in late veins and breccias have d values between 138 to 153 and unmineralized regional veins have d values from 94 to 150 multiple d analyses of fluids from inclusions are within 5 for 14 of 17 duplicate analyses which suggests that mixing of fluid inclusion populations did not contribute significantly to the d depleted values a correlation between 13 c values of vein carbonate and d values of inclusion fluids indicates that postentrapment reequilibration did not significantly alter isotopic compositions of fluid inclusions incorporation of externally derived low d material into the mineralizing intrusions either from the influx into the melt of deeply convected evolved meteoric fluids or by crustal assimilation of d depleted country rock is required to explain the observed d depleted values of the magmatic fluids

oxygen and hydrogen isotope characteristics of hydrothermal alteration at the ann mason porphyry copper deposit yerington nevada late cenozoic normal faulting has tilted a 3 by 5 km block containing the mid jurassic ann mason porphyry copper deposit 90o w and exposed the hydrothermal alteration pattern in cross section from 1 to 6 km palcodepth oxygen and hydrogen isotopes have been used to deduce sources of hydrothermal fluids fresh samples of the host yerington batholith pluton have typical calc alkaline magmatic isotopic compositions 18 o whole rock 6 7 7 0 d biotite 85 to 88 the various types of hydrothermally altered rocks have similar d and 18 o values only slightly isotopically shifted from original magmatic values except along an axial zone within and above the orebody in this zone altered rocks are enriched in 18 o by 1 to 3 5 per mil and 18 o values increase with decreasing palcodepth the 18 o values of the whole rock and or feldspar and the d values of biotite chlorite and or actinolite in the various alteration zones are as follows propylitic alteration is widespread and is isotopically similar to fresh rock 18 o 6 2 to 7 2 per mil d 82 to 92 per mil potassic alteration characterized by 100 percent alteration of hornblende to biotite and cu mineralization occurs at 3 5 km palcodepth along the axis of the mineralizing granite porphyry dike swarm 18 o 6 5 to 8 4 per mil d 68 to 96 per mil sodic calcic alteration characterized by hydrothermal oligoclase actinolite sphene occurs laterally alongside and below the ore zone at 3 to 6 km palcodepth 18 o 5 7 to 8 4 per mil d 67 to 90 per mil late stage sodic albite chlorite alteration occurs at 1 to 4 km palcodepth in the vicinity of the axial zone 18 o 6 7 to 9 6 per mil d 79 to 85 per mil the youngest late stage sericitic alteration occurs at the shallowest levels primarily at 1 to 2 km palcodepth and displays a distinct 18 o and d enrichment 18 o 9 8 to 10 4 per mil d 61 per mil integration of stable isotope data geologic field relations and phase equilibria constraints indicates that the high salinity fluids responsible for potassic alteration and cu mineralization were of internal magmatic origin but that synchronous deep convecting saline sodic calcic fluids were largely derived from external nonmagmatic sources a third type of fluid also external but lower in temperature and much more dilute was prevalent at shallow levels and caused latestage sericitic alteration this was either an isotopically heavy coastal meteoric water or a seawater

evolution of hydrothermal fluids in the park premier stock central wasatch mountains utah park premier is a low grade gold rich porphyry copper system located about 3 km east of precious and base metal veins of the park city district utah the park premier stock consists of five small intrusions that were emplaced into coeval volcanic rocks at depths less than 1 km two of the intrusions are microaplitic granodiorite porphyries that developed strong hydrothermal alteration stockwork quartz veins and low grade disseminated copper gold mineralization early alteration of these intrusions included biotite replacement of hornblende deposition of disseminated magnetite and actinolite local sodic alteration of plagioclase and formation of 1 quartz magnetite actinolite or na plagioclase and 2 quartz k feldspar or magnetite or chalcopyrite veins mineral compositions fluid inclusion data and inferred alteration reactions indicate that early alteration occurred at temperatures or 600 degrees c from hydrothermal fluids with unusually high salinities or 82 wt total salts and low k na molar ratios or 700 degrees c and low pressure or 200 bars quartz pyrite or sericite veins and local alteration of high temperature minerals to chlorite sericite calcite and clay minerals occurred at much lower temperatures or 270 degrees c by the influx of dilute 0 3 equiv wt nacl boiling meteoric water large areas of the igneous wall rocks of the microaplitic porphyries are altered to a variety of hydrolytic assemblages including albite sericite sericite kaolinite dickite pyrophyllite and alunite all quartz or pyrite albite sericite sericite and local kaolinite dickite alteration probably formed from chloride dominated fluids derived from the microaplitic porphyries pyrophyllite and alunite alteration postdate all other types of alteration and formed along major northeast trending fracture zones from fluids that were probably sulfate rich the hydrothermal system at park premier contains several features that are unusual in other well described porphyry copper systems but which may be common in gold rich porphyry systems they include 1 early extremely saline fluids that only migrated locally 2 abundant magnetite actinolite and sodic plagioclase and relatively minor biotite k feldspar and sulfide minerals 3 sparse quartz vein formation at temperatures 500 degrees c 4 little interaction of the microaplitic porphyries with dilute fluids until the intrusions cooled to temperatures or 270 degrees c 5 early albitization of wall rocks adjacent to the microaplitic porphyries and 6 relatively high au contents and high au cu ratios of the microaplitic porphyries these differences probably arose primarily from the very shallow depths of emplacement small size of the intrusions and low sulfur contents of the magmas

can economic porphyry copper mineralization be generated by a typical calc alkaline melt numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that typical calc alkaline magmas contain sufficient copper chlorine and water to produce economic porphyry copper mineralization neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles the most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper the ratio of initial water in the melt to the water saturation level and the initial chlorine water ratio of the melt the absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume the bulk salinity of the exsolved fluid may vary from 2 0 wt nacl to saturation levels 84 wt nacl at 700 c indicating that boiling is not necessary to produce high salinity brines at appropriate p t xnacl conditions the magmatic aqueous fluid separates into a saline liquid which transports most of the copper and a low salinity vapor the salinities of the two immiscible phases are governed by the p t conditions while the bulk fluid salinity determines the mass fractions of liquid and vapor formed pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid this results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt as copper is transported from the melt by the earliest exsolving fluids in deep 2 0 kbar systems and by late exsolving fluids in shallow 0 5 kbar systems the relative timing of pressure quenching aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures model results incorporating petrologic constraints determined for yerington nevada are in good agreement with observed mineralization