modeling the role of sodic alteration in the genesis of iron oxide copper gold deposits eastern mount isa block australia liberation of iron and potassium by widespread postmetamorphic albitization of country rocks was one of the likely contributing processes in the formation of both barren and mineralized magnetite chalcopyrite biotite gold hematite clinopyroxene actinolite apatite ironstones in the cloncurry district of the proterozoic mount isa block whole rock geochemical data indicate nearly immobile al ga ti zr during transformation of a variety of least altered rocks toward albitite the data indicate that the addition of na from a brine to the rock accompanied the loss of fe k ba rb ca sr co v mn pb and zn from those altered rocks and enrichment in the brine but that cu was not systematically stripped from a variety of wall rocks during albitization conversely the formation of metasomatic ironstones the immediate hosts to some cu au ores involves addition of most of the same elements that were lost during albitization the correlation between intensity of alteration its distribution and timing e g in breccias cored by ca 1530 1500 ma granitoids and convergence of all rock types toward magmatic stable isotope values provides strong evidence for a substantial component of igneous derived fluid simulations of the albitization process were carried out isothermally at 550 c and 350 mpa and polythermally from these conditions down to 400 c and 200 mpa using the gibbs minimization method with hch software and the unitherm database both simple fluid rock mixing models and more complicated reactor style algorithms used a range of input fluids from fluids equilibrated fluid with two feldspar granite through to na k ratios consistent with fluid inclusion results and geochemical data for initially unaltered wall rocks the apparent paradox of widespread albitization resulting from fluid released by two feldspar granites can be explained by relatively small shifts away from the k feldspar albite equilibrium curve and even isothermal model fluids derived from two feldspar granites produce albitites in calc silicate rocks outboard of granite proximal k feldspar clinopyroxene skarns matching field patterns those models with fluid na k ratios similar to those of fluid inclusions produced the most realistic alteration assemblages dominated by albite for both isothermal conditions and decreasing temperature which approximate those observed in the field pixe and microthermometric data on fluid inclusions from quartz in two feldspar quartz monzonite and pegmatite at the top of the mount angelay pluton indicate bulk na k molar ratios in the fluid of between 10 and 20 considerably higher than our thermodynamically calculated values for fluid in equilibrium with two feldspar granite of around 3 at 550 c 350 mpa such shifts may have been attained by admixture of magmatic hydrothermal fluid with small amounts of nacl brines trapped along grain boundaries in scapolite bearing calc silicate wall rocks by fluid immiscibility due to high initial co2 contents in the felsic intrusions by contributions from mafic magmas or from dissolution of salt rich layers into the intrusions prior to crystallization and fluid release with increasing amounts of fluid rock interaction in the models the fluids were enriched in k fe and ca approaching compositions observed in fluid inclusions in the ore deposits these fluids reacted with pelitic rocks which are common ore hosts would produce magnetite clinopyroxene biotite actinolite alteration at high temperature similar to the proximal alteration around ore deposits we infer that precipitation of sulfides in the cu au deposits was the result of mixing of cu bearing brine of ultimately magmatic origin but modified extensively via albitization with sulfur bearing fluids or reaction of the brine with sulfur bearing rocks when cu was absent from the initial magmatic fluid barren ironstones may have been the result

the stable isotope signature of kilometre scale fracturedominated metamorphic fluid pathways mary kathleen australia abstract large calcite veins and pods in the proterozoic corella formation of the mount isa inlier provide evidence for kilometre scale fluid transport during amphibolite facies metamorphism these 10 to 100 m scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 10 km mary kathleen fold belt despite the isotopic heterogeneity of the surrounding wallrocks the fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks the pods have 13ccalcite values of 2 to 7 and 18ocalcite values of 10 5 to 12 5 away from the pods metadolerite wallrocks have 18owhole rock values of 3 5 to 7 and unaltered banded calc silicate and marble wallrocks have 13ccalcite of 1 6 to 0 6 and 18ocalcite of 18 to 21 in the alteration zones adjacent to the pods the 18o values of both metadolerite and calc silicate rocks approach those of the pods large calcite pods hosted entirely in calc silicates show little difference in isotopic composition from pods hosted entirely in metadolerite thus 100 to 500 m scale isotopic exchange with the surrounding metadolerites and calc silicates does not explain the observation that the 18o values of the pods are intermediate between these two rock types pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc silicates these data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the corella formation at the presently exposed crustal level although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc silicates or thermal fluctuations attending deformation and metamorphism the overall l shaped trend of the data on plots of 13c vs 18o is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc silicate rocks localization of the vein systems in dilatant sites around metadolerite calc silicate boundaries indicates a strong structural control on fluid flow and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km the ultimate source for the external fluid is uncertain but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism

the lightning creek sill complex cloncurry district northwest queensland a source of fluids for fe oxide cu au mineralization and sodic calcic alteration the lightning creek cu au prospect is hosted by a cogenetic suite of plutonic i type granitoids the dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks including monzogranite and alkali feldspar granite a series of flat lying sills are interpreted to be late stage differentiates based on their timing mineralogy and chemistry in parts of the prospect there is pervasive sodic calcic alteration pyroxene after amphibole albite after k feldspar and oligoclase of the plutonic rocks this alteration predates sill emplacement and is unrelated to veining or fracturing of any kind the presence of small amounts of carbonate in the altered rocks suggests that the fluids were co2 bearing quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz monzodiorite suggesting that the fluids were hot and of magmatic composition sodium and ca were added and k fe cl and cu were stripped during what is interpreted as an autometasomatic event the sills display considerable textural and mineralogical complexity and evolved from equigranular quartzofeldspathic rocks aplites with magmatic chemistry to unusual fe rich rocks albite magnetite quartz that exhibit a range of bizarre spherulitic textures some of the albite and magnetite in the sills is secondary albite forms pseudomorphs after k feldspar na fe ca alteration along sill margins and within sills at the contacts between different textural zones halos of disseminated magnetite clinopyroxene fe ca na alteration are developed adjacent to early magnetite veins fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500 c and at pressures in excess of 1 5 kbar the range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase this magmatic fluid phase was dominated by h2o co2 and chlorine and underwent phase separation into a co2 rich vapor and a hypersaline brine 33 55 wt nacl equiv the hypersaline fluid was enriched in fe 10 wt and cu 1 wt pixe analysis in addition to na k and ca where this fluid was retained within fe rich portions of the sills it caused ca fe na alteration pyroxene albite magnetite growth at the expense of quartz where the fluid was expelled from the sills it produced quartz magnetite clinopyroxene albite veins broadly coeval with the early magnetite veins although rich in cu these granitoid derived magmatic fluids did not generate significant cu au mineralization perhaps because of the high temperatures involved and or a lack of reduced sulfur in the fluids or host rock however the amount of iron present is estimated from the aeromagnetic anomaly to be in excess of 2 000 million tonnes mt a later generation of calcite chlorite pyrite chalcopyrite veins contain traces of cu au mineralization fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler 200 c more dilute 15 28 wt nacl equiv fluids perhaps generated by the admixture of a meteoric component the conclusions reached in this study have implications for understanding the genesis of fe oxide cu au deposits and related sodic calcic alteration the study indicates the potential for co2 rich granitoid magmas to evolve hypersaline fe and cu rich fluids capable of causing intense magnetite veining and cu au mineralization autometasomatic sodic calcic alteration of the granitoids may be an important precursor to mineralization contributing fe k cu and cl to the magmatic fluids

br cl geochemistry of hydrothermal brines associated with proterozoic metasediment hosted copper mineralization at mount isa northern australia abstract instrumental neutron activation analyses inaa of small bulk samples of vein quartz are used to estimate the br cl ratio of saline fluid inclusions associated with late metamorphic copper mineralization in quartz dolomite breccias at mount isa the results are compared with similar data from veins associated with district scale metasomatism in potential copper source rocks near mount isa and with quartz samples from other locations in northern australia which may have trapped fluids approaching the br cl composition of paleoproterozoic halite evaporites br cl ratios in some of the evaporite related samples are similar to present day seawater 0 0035 while others range up to 0 015 all samples from the mount isa copper ores fluids with total salinities mostly between 10 and 25 wt nacl eq have consistently high br cl ratios between 0 008 and 0 018 on average 3 5 times higher than modern seawater fluids associated with district scale alteration of metabasalts up to 30 km away from the deposit have high br cl ratios identical to those of the ore fluids this indicates a common source of the salts and supports a tentative correlation between mine and district scale alteration events and may ultimately help to trace potential fluid flow paths to or from the site of ore deposition the observed fluid compositions are contrary to the low br cl ratios that would be expected from a simple model of ascending fluids derived by prograde metamorphism of salt bearing paleoproterozoic metasediments mass balance calculations based on mine and district scale alteration suggest that the high br cl fluids may have originated above the present level of erosion as basin brines which infiltrated the terrain during the waning stages of regional metamorphism they may have formed in an inferred evaporitic cover sequence of mesoproterozoic age as residual bitterns or by diagenesis and partial dissolution of halite similar to modern br enriched basin brines secular variation in oceanic br cl preferential chlorine removal from evaporite derived metamorphic fluids by fluid rock interaction or an unusually br rich magma source beneath the deposit could provide alternative explanations for the high br cl ratios of the mount isa brines but are considered to be less likely processes in light of the available geologic data

brine history indicated by argon krypton chlorine bromine and iodine analyses of fluid inclusions from the mississippi valley type lead fluorite barite deposits at hansonburg new mexico argon krypton chlorine bromine and iodine were measured in a homogeneous population of high salinity hydrothermal fluid inclusions from the tertiary age mississippi valley type mvt lead fluorite barite deposits at hansonburg new mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts noble gases and halogens in fluid inclusions containing 10 10 10 9 l of brine cl 3 molal were analyzed by laser microprobe noble gas mass spectrometry lmngms on neutron irradiated samples the concentrations of36ar 4 7 10 8 molal and84kr1 8 10 9 molal in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately20 5 the mole ratios ofbr cl 1 2 10 4 andi cl 1 2 10 6 are among the lowest measured in any natural waters similar to those of modern brines formed by dissolution of permian nacl bearing evaporites in southeast new mexico 40ar 36ar ratios 600 are twice that of air and indicate that the fluid inclusions had excess radiogenic40ar 1 4 10 5 molal when trapped the amount of excess40ar appears to be too large to have been acquired with cl by congruent dissolution of halite bearing evaporites and possibly too small to have been acquired with pb by congruent dissolution of granitic basement rocks with proterozoic kar ages from thelmngms data combined with published pb and s isotope data we infer the following sequence of events in the history of the hansonburg mvt hydrothermal brine 1 the brine originated as relatively dilute meteoric water and it did not gain or lose atmospheric ar or kr after recharge 2 the originally dilute fluid acquired the bulk of its cl and sulfate in the subsurface after recharge by dissolving halite bearing permian marine evaporites 3 the high salinity brine then acquired most of its pb and excess radiogenic40ar from interactions with aquifer rocks other than evaporites possibly clastic sedimentary rocks or basement rocks with phanerozoic kar ages and 4 the brine deposited fluorite without having boiled or degassed

geochemistry of post 1540 ma granites in the cloncurry district northwest queensland proterozoic fe oxide cu au co mineralization in the cloncuriy district is spatially and temporally related to intrusion of voluminous postpeak metamorphic granites that form most of the williams and naraku batholiths detailed mapping and geochemical study of these granites in the southern part of the district have identified two groups of intrusions that are referred to here as the cloncurry supersuite and eureka supersuite the cloncurry supersuite is composed mainly of monzogranite and syenogranite which consist of quartz plagioclase alkali feldspar hornblende biotite with accessory magnetite titanite apatite and zircon the timing of intrusion of the wiley igneous complex is interpreted to be pre or syn d 3 based on overprinting relationships while hornblende geobarometry suggests an emplacement depth of approximately 14 km the granites are subalkaline high k metaluminous rocks and range from 59 to 74 wt percent sio 2 increasing sio 2 is accompanied by increasing k 2 o rb and th and decreasing mgo fe 2 o 3 feo tio 2 cao p 2 o 5 sr eu zr cu zn ni v and sc this is consistent with fractionation of observed phases including plagioclase amphibole biotite magnetite titanite and apatite the eureka supersuite ranges from diorite to syenogranite in composition more mafic varieties contain pyroxene and hornblende while more felsic varieties contain biotite as the only ferromagnesian silicate sodiccalcic alteration events are closely linked in space and time to intrusion of eureka supersuite magmas the timing of intrusion is interpreted to be pre and post d 3 based on overprinting relationships between different plutons and mylonitic fabrics hornblende geobarometry suggests an emplacement depth of approximately 10 km for the mount angelay igneous complex eureka supersuite granites in the mount angelay igneous complex range from 51 to 76 wt percent sio 2 the granites are alkaline and k 2 o rich leading to their classification as a shoshonite series with increasing sio 2 content tio 2 al 2 o 3 fe 2 o 3 t mno mgo cao p 2 o 5 sr eu cu zn ni v and sc decrease indicating fractionation of observed minerals including apatite magnetite pyroxene hornblende plagioclase and biotite some samples are characterized by very high abundances of k 2 o p 2 o 5 sr ba and light rare earth elements lree compared to other eureka supersuite samples this may reflect their derivation from a source that was variably enriched in incompatible elements compared with many other shoshonitic rocks high k eureka supersuite samples are strongly depleted in sr and to a lesser extent eu and enriched in y and heavy rare earth elements hree this suggests that garnet was not a stable phase in the source region that the source contained plagioclase as a stable phase and or the granites evolved by extensive plagioclase fractionation partial melting in the lower crust to generate post 1540 ma granites was most likely caused by intrusion of basic magma into the crust eureka supersuite magmas were probably derived from partial melting of variably incompatible element enriched gabbroic rocks while cloncurry supersuite magmas were most likely derived from partial melting of more felsic material of dioritic to tonalitic composition

nd isotope and petrogenetic constraints for the origin of the mount angelay igneous complex implications for the origin of intrusions in the cloncurry district ne australia abstract the mesoproterozoic mount angelay igneous complex contained intrusions that were emplaced into amphibolite facies metasedimentary rocks during two periods of 1550 and post 1540 ma magmatism sm nd isotopic analysis together with mineralogical and chemical considerations suggest that the intrusions were produced from a paleoproterozoic crustal source with a t 2 model age 2200 ma on geochemical and petrological grounds the 1550 ma trondhjemitic intrusions are interpreted to have been produced by melting of amphibolite under garnet stable conditions 8 10 kbar the late syn to post peak metamorphic timing of these intrusions suggested that they were associated with the tectono thermal event that produced regional peak metamorphic mineral assemblages the post 1540 ma intrusions are k rich and consist of two groups of synchronously emplaced intrusions 1 a high k monzodiorite and monzogranite suite that range between 51 and 77 wt sio 2 and 2 a high k na enriched hornblende monzonite the chemistry and mineralogy of these intrusions suggested that they were derived via plagioclase stable and garnet unstable melting 2 o p 2 o 5 cr v and zn compared with slightly younger high k monzonite which is interpreted to have formed via one of two mechanisms 1 melting of a low k amphibole and plagioclase rich source or 2 melting of residual material that produces a potassic and incompatible element rich melt these magmas likely contained mantle derived material particularly the k rich intrusions of mafic composition the heat required for the production of post 1540 ma intrusions appears to have been generated by the intrusion of high t mantle derived mafic material into the crust 25 30 km 8 10 kbar this model is consistent with the synchronous emplacement of mafic and felsic magma and the lack of a consanguineous regional metamorphic association and suggests high t high degree partial melting in localised pockets within fertile source regions in the crust an increase in sm nd model source age and decrease in end with increasing sio 2 in the k rich intrusions suggests the incorporation of juvenile material in the more mafic rocks the origin of this component is unknown but it may represent either the incorporation of mantle derived material during melting or the partial melting of crust with a younger mafic component on a district scale the 30 million year period over which the k rich post 1540 ma intrusions were emplaced suggested that mantle derived material continued to be injected into the crust a mantle component to these rocks and the global distribution of proterozoic intrusions with similar geochemical affinities strongly suggests a world wide period of mantle induced crustal melting at that time the dominant paleoproterozoic isotopic composition of most of these intrusions suggests melting of similarly composed and matured source rocks

simulated airborne particle size distributions over greenland during last glacial maximum polar ice cores indicate that the deposition of dust from the atmosphere was strongly enhanced during last glacial maximum lgm the concentration of dust in the ice sheets and in the overlaying atmosphere are not proportional to each other but are dependent among other things on the relative magnitudes of dry and wet deposition which change with climate observed dust particle size distributions in the greenland ice sheet are shifted toward larger particles during lgm by applying common theories for particle removal processes we show that the airborne particle size distributions over greenland probably remained the same in the two different climates this leads to the conclusion that the airborne dust concentration was even higher during lgm than indicated by the enhancement in deposition flux we suggest a lgm pre industrial current climate aerosol ratio including the soluble fraction over greenland of about 90 125 by mass and 75 100 by number

effect of time evolving age and convergence rate of the subducting plate on the cenozoic adakites and boninites partial melting of subducting oceanic crust expressed as high mg volcanic rocks such as adakites and boninites has been actively studied for decades and lee and king 2010 reported that time evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust e g aleutians however few subduction model experiments have considered time evolving subduction parameters posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones therefore we constructed two dimensional kinematic dynamic subduction models for the izu bonin mariana northeast japan kuril tonga java sunda and aleutian subduction zones that account for the last 50 myr of their evolution the models include the time evolving age and convergence rate of the incoming oceanic plate so the effect of time evolving subduction parameters on transient partial melting of oceanic crust can be evaluated our model calculations revealed that adakites and boninites in the izu bonin and aleutian subduction zones resulted from transient partial melting of oceanic crust however the steady state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones indicating that time evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites our model calculations confirm that other geological processes such as forearc extension back arc opening mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the mariana northeast japan tonga and southeastern java sunda subduction zones

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

hydrothermal fluid origins in a fluorite rich mississippi valley type district combined noble gas he ar kr and halogen cl br i analysis of fluid inclusions from the south pennine ore field united kingdom the south pennine peak district ore field united kingdom is host to several fluorite rich mississippi valley type deposits and was chosen for study as an analogue to the illinois kentucky fluorspar district united states the study was undertaken to test the idea that the large scale carbonate dissolution and fluorite rich mineralogy characteristic of both districts may require a magmatic source of hf the data also provide constraints for water rock interactions and potential fluid sources relevant to the local geology of the south pennine ore field samples were studied from two of the largest and most economically important deposits within the south pennine ore field the hucklow edge and dirtlow rakes in vacuo crushing of irradiated calcite samples released inclusion fluids that were analyzed for noble gas isotopes ar ar kr and the halogens cl br i this was achieved simultaneously by noble gas mass spectrometry ar ar methodology and was combined with fluid inclusion thermometric salinity data to determine noble gas concentrations in addition he isotope analyses were obtained from unirradiated fluorite the he he ratio of the pennine ore fluids 0 1 ra where ra atmospheric he he ratio of 1 4 10 is significantly lower than the maximum value recorded in the illinois kentucky fluorspar district 0 35 ra kendrick et al 2002 this volume indicating a crustal fluid source and supporting existing basinal brine models the low ar ar ratios with most 50 to 60 percent halite dissolution water br cl mol ratio 0 57 0 92 10 the salinity of fluids included in the main stage of the hucklow edge mineralization avg of 22 wt nacl equiv places an upper limit on the extent of meteoric recharge at 15 vol percent a secondary vein present at hucklow edge has a lower salinity of 40 m y a time scale compatible with models in which fluids were derived from small local basins such as the widmerpool and edale gulfs

metasomatic albitites and related biotite rich schists from a low pressure polymetamorphic terrane snake creek anticline mount isa inlier north eastern australia microstructures and p t d paths rocks of the snake creek anticline are mainly pelitic schists psammitic schists and quartzites that were metamorphosed during multiple high t low p events extending from d1 to d5 with the metamorphic peak occurring late to post d3 albitites are widespread but are concentrated in five areas they are typically fine to medium grained and consist of albite with or without combinations of quartz biotite staurolite cordierite garnet andalusite sillimanite kyanite gedrite and tourmaline from the presence or absence of albite inclusions in porphyroblasts the albitites are interpreted as forming early in the d3 event as a result of infiltration of external fluids psammitic schists and quartzites were preferentially altered but pelitic schists were also albitized in localities where the alteration was more extreme with the replacement of muscovite total and the replacement of quartz and biotite variable structural controls on albitization include fracturing and syn d3 shear zones in fold hinges biotite schists with abundant porphyroblasts combinations of staurolite garnet andalusite and cordierite occur adjacent to albitites and it is argued that they formed by the addition of fe and mg sourced from the albitites in several albitite rich areas cordierite grew early in d3 and was partly or entirely replaced during or after d3 by combinations of biotite andalusite tourmaline staurolite and sillimanite a postulated p t d path involved an increase in pressure with or without a decrease in temperature subsequent to early d3 albitization followed by an increase in temperature up to the metamorphic peak late d3 to early d4 the metamorphism was contemporary in part with the emplacement of the williams batholith c 1550 1500 ma which probably supplied the na rich fluids

age of the barramundi orogeny in northern australia by means of ion microprobe and conventional u pb zircon studies abstract u pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust forming event in early proterozoic terranes of northern australia magmatism and tectonic processes associated with this event the barramundi orogeny appear similar over very wide areas about one third of the australian continent the barramundi event marks the deformation metamorphism and cratonization of the earliest proterozoic 2 0 1 9 ga basinal successions and it was followed by and closely linked to extensive orogenic felsic magmatism the yaringa metamorphics are an inlier of paragneiss and schist that pre dates 1865 3 ma leichhardt felsic magmatism and younger basin formation in the mount isa inlier from an ion microprobe u pb zircon study of massive and migmatitic gneiss the time of yaringa metamorphism is established at 1890 8 ma recognized younger proterozoic deformations in the interval 1610 1550 ma are generally not evident in zircon u pb systems but have totally reset local rb sr whole rock values conventional u pb work on multi grain zircon samples of yaringa gneiss gives a geologically meaningless age 2090 40 ma that is an artifact of complex zircon growth part of which was initially inherited from a 2 55 to 2 2 ga provenance and the other major part of which was formed in situ during the 1890 ma high grade metamorphism predictably for simple zircon systems in low grade metavolcanic terranes conventional and ion microprobe zircon ages are in close agreement dating of volcaniclastic sequences above and below the barramundi tectonic break in the pine creek inlier brackets the orogeny in this terrane to a similar time frame 1885 1870 ma some zircons which crystallized at this time form selvedges around older cores whose age of 2100 2000 ma may be relics of an earlier deep crustal underplating the uniform chemistry of post barramundi felsic magmatism and the similar tectonic setting of the areally extensive barramundi event are mirrored by their temporal coherency cycles of geosynclinal deposition burial metamorphism and uplift took place during intervals of several million years or less implying comparable rates for early proterozoic orogenesis to those established for tertiary and quaternary settings

petrology geochronology and isotope geochemistry of the post 1820 ma granites of the mount isa inlier mechanisms for the generation of proterozoic anorogenic granites four distinct episodes of granitic intrusion occurred in the mount isa inlier at 1820 1800 ma 1760 1740 ma 1700 1670 ma and 1560 1480 ma combined these granites cover at least 4700 km2 and they contrast in composition and intrusive style with an earlier major felsic magmatic event which occurred between 1870 and 1840 ma post 1820 ma granite batholiths are of two types large major coarse grained anorogenic i type intrusives and small minor microgranites which are probably s type the i type granites are all pink to red colour and most contain fluorite rapakivi textures are common they have a high ratio of fe mg and are more enriched in tio2 zr nb y and u than the 1870 1840 ma felsic igneous rocks the post 1820 ma i types are interpreted to have formed by small degrees of partial melting of a mafic underplate the smaller microgranites are relatively k2o enriched and are sometimes muscovite bearing they are interpreted to be derived from proterozoic feldspathic metasediments the first three episodes of granite magmatism are coeval with major extensional sedimentary sequences but they are generally spatially displaced from the maximum area of surface rift development this separation is predicted by the asymmetric detachment model of continental extension the tectonic relationship of the youngest granite episode is not clearly defined the 143nd 144nd initial ratios of the post 1820 ma granites fall into two groups one having positive relatively primitive nd values of 1 8 to 3 5 the other having negative nd values ranging from 2 9 to 3 7 samples with positive nd values are interpreted as partial melts of mafic sources formed post 1870 ma whilst the group with negative nd values are derived from significantly older sources formed probably around 2300 2000 ma the sources of these granites are produced by significant underplating events that may occur during either extension or compression

fluid inclusion noble gas and halogen evidence on the origin of cu porphyry mineralising fluids the naturally occurring noble gas isotopes 40ar 36ar 84kr and 129xe and halogens cl br i have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits pcd bingham canyon utah and silverbell ray mission pinto valley and globe miami in arizona in addition analyses of 3he 4he have been obtained from sulphide minerals hosted by the quartz veins at silverbell ray pinto valley and globe miami the majority of pcd fluids studied have br cl and i cl ratios that overlap those of fluids included in mantle diamond suggesting that the salinity had a juvenile origin the high i cl 121 10 6 mole m in samples from the propylitic zone of silverbell is attributed to the presence of sedimentary formation water 3he 4he ratios have r ra values in the range 0 3 to 1 72 and provide evidence for the involvement of a crustal component in addition to mantle volatiles 40ar 36ar ratios vary from meteoric values of 317 in the propylitic zone of bingham canyon and 323 in the skarn alteration of mission up to 3225 in the potassic zone of pinto valley fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end member comprising meteoric water and air and a high salinity end member consisting of a mixed mantle and crustal fluid the 40ar cl ratio of fluid inclusions at pinto valley 10 4 m is similar to values obtained previously for mantle fluids the 40ar cl value is two orders of magnitude lower at bingham canyon where a depleted 36ar concentration 0 2 10 6 cm3 g below that of air saturated water asw and a range of highly fractionated noble gas compositions f84kr 13 and f129xe 160 indicate that boiling and pulsed fluid flow have occurred

high salinity fluids the result of retrograde metamorphism in thrust zones abstract highly saline fluid inclusions in vein quartz from caledonian thrust zones of northern norway are associated with late orogenic retrograde metamorphic reactions several retrograde reactions have been observed in pelitic lithologies and all involve the rehydration of earlier higher grade metamorphic assemblages the most common products of these reactions are chlorite and sericite which have formed at the expense of garnet kyanite staurolite and hornblende chlorite geothermometry suggests temperatures between 330 and 370 c for the formation of the retrograde chlorite a range of total fluid salinities has been determined by microthermometry between 27 and 50 wt these high fluid salinities result from relatively rapid rehydration reactions during limited infiltration of the thrust zones by aqueous fluids crush leach analyses show the fluids to be multi component fluids whose electrolyte contents are na and cl dominated but with appreciable contents of ca k fe and sr geochemical modelling of the fluid compositions suggests that the concentrations of the major elements in solution were buffered by the retrograde rehydration reactions the source of the infiltrating fluids which caused the retrogression is not identifiable from their major element chemistry surface derived meteoric sea or sedimentary formation waters fluids may have utilised pre existing thrust zones as fluid pathways and are considered to have been important in the retrogression in the structurally higher parts of the caledonian allochthon at deeper levels however metamorphic fluids produced by devolatilisation reactions in underthrust units may have migrated upwards into the thrust zones and resulted in the retrogression of the hangingwalls of overlying higher grade nappes

age constraints on the tectonothermal evolution of the selwyn zone eastern fold belt mount isa inlier the selwyn zone is characterized by multiple ductile deformation and metamorphic events there is general consensus that early in the isan orogeny there were both extensional and thrust events that produced bedding parallel foliation sbp we propose that these were followed by the d1 event that resulted in a regional steep east west foliation and that the snake creek anticline and the dominant s2a regional foliation formed as upright north south structures local overturning of d2 structures was due to subsequent localized deformation d2b that resulted in structures with flat lying axial planes d3 and d4 produced steep upright folds and crenulations with north northwest and northeast trending axial surfaces metamorphism largely controlled by the emplacement of mafic rocks is dominantly low pressure but the p t t path is complex early growth of cordierite m1 followed by andalusite was succeeded by a clockwise medium pressure loop of ca 500 600 mpa in the northern snake creek anticline as indicated by early d2a kyanite growth the metamorphic peak during d2 was around 580 c and 400 mpa at the sillimanite isograd and reached a maximum of around 670 c and 400 500 mpa in the migmatitic sillimanite k feldspar zone at the cannington and osborne mines previous age determinations for the metamorphic peak of 1580 1600 ma are confirmed with new electron microprobe dating of monazite whereas an age grouping of 1600 1630 ma may represent the d1 m2 event additional ages in the range 1640 1680 ma are recorded in albitites from the soldiers cap group and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills this is supported by shrimp dating of zircon from a trondhjemite lens in a largely mafic sill complex in the snake creek anticline that provided an age of 1686 8 ma also indicating that at least part of the soldiers cap group is older than previously indicated a zircon shrimp age of 1527 4 ma was obtained for a sample of albitized saxby granite this age also represents the d4 m6 event and is the timing of the extensive brecciation and albitization of calc silicate rocks of the corella formation in the selwyn zone

magmatic hydrothermal albite actinolite apatite rich rocks from the cloncurry district nw queensland australia abstract albite quartz actinolite apatite rich rocks with accessory titanite form a carapace that caps a small dome like intrusion of roxmere pluton and provide evidence of the accumulation and release of magmatic fluids that may have contributed to regionally extensive na ca alteration in the proterozoic cloncurry district australia the roxmere pluton was emplaced after the peak amphibolite facies metamorphism and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc silicate rich mary kathleen group na ca hydrothermal assemblages closely spatially related to the roxmere pluton suggest that na ca alteration in this case is inconsistent with excepted up temperature metamorphic fluid circulation models the carapace rocks comprise two variants aplite and pegmatite both of which were affected by alteration and brecciation aplitic material predominates and the carapace has a brecciated top and a distinct textural zonation from an aplite rich pegmatite poor base to a relatively aplite poor pegmatite rich top this zonation is interpreted to represent the progressive accumulation of volatile phases toward the top of the carapace which culminated in fluid overpressuring and brecciation of the granitoid stock the convoluted and ptygmatic habit of the aplitic and pegmatitic material within the carapace is similar in nature to the comb quartz unidirectional solidification textures that are developed in the roof zones of some porphyry stocks these complex textures are interpreted to have formed from a single phase of fluid saturation the roxmere pluton or a deeper equivalent has a na mg and ca rich composition and a clinopyroxene amphibole rich mineralogy that is consistent with the formation of the contained albite actinolite apatite rich phases however the source of the rocks within the carapace is equivocal given that their contact relationships are obscured the 18 o composition of minerals within the carapace has the following ranges albite 7 1 8 7 actinolite 8 0 8 5 and quartz 8 9 11 4 albite and quartz 18 o equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of 510 c to 540 c the calculated 18 o composition of a fluid in equilibrium with these minerals 6 0 to 8 0 at temperatures between 450 c and 550 c is consistent with a magmatic origin these estimated temperatures and 18 o fluid composition are comparable to estimates for regional na ca alteration in contrast d analysis of actinolite 136 to 150 within the carapace demonstrates that these fluids have a significantly lower d signature compared to na ca alteration assemblages associated with regional alteration 70 to 90 the low values in the carapace could either be interpreted as being produced from meteoric fluids or from degassed magmatic fluids although a meteoric fluid model is deemed improbable as meteoric fluids in the district have a documented d composition of ca 0 0 as a consequence an open system degassed magmatic hydrothermal fluid origin is considered to be the most plausible explanation for the low isotopic signatures and is also consistent with the complex textural relationships described within the carapace

hydrothermal fluid origins in mississippi valley type ore districts combined noble gas he ar kr and halogen cl br i analysis of fluid inclusions from the illinois kentucky fluorspar district viburnum trend and tri state districts midcontinent united states samples were selected from three of the classic mississippi valley type districts in the midcontinent area of north america the illinois kentucky fluorspar district belongs to the fluoritic subtype of mississippi valleytype districts and the ozark districts of the viburnum trend and tri state are representative of the pb rich and more typical zn rich subtypes of mississippi valley type mineralization respectively noble gas ar kr and halogen cl br i data have been obtained simultaneously by noble gas mass spectrometry of inclusion fluids released by in vacuo crushing of irradiated fluorite quartz carbonate and sphalerite samples additionally he analyses have been obtained from nonirradiated fluorite from the illinois kentucky fluorspar district fluorite from the illinois kentucky fluorspar district has a maximum 3 he 4 he ratio of 0 35 ra where ra atmospheric 3 he 4 he ratio of 1 4 10 6 higher than values typical of crustal fluids and confirming the presence of a minor mantle component equal to less than 6 percent of the total he in the fluids of the illinois kentucky fluorspar district the br cl mol ratios of all the deposits are with one exception higher than the value of seawater 1 54 10 3 ranging from 1 69 to 3 70 10 3 and indicating acquisition of salinity by the evaporation of seawater beyond the point of halite saturation late quartz from the tri state district contains a very minor component of halite dissolution water and has a br cl mol ratio of 1 46 10 3 the i cl mol ratios are typical of oil field brines and are ubiquitously higher than what is attainable by the evaporation of seawater alone i cl mol ratios are in the range of 6 to 270 10 6 indicating that the fluids have interacted with i rich organic matter present in sedimentary rocks in the ozark districts 40 ar 36 ar ratios vary between 320 and 345 in the tri state and between 350 and 420 in the viburnum trend in the illinois kentucky fluorspar district 40 ar 36 ar ratios reach values of 1 200 the corresponding concentrations of 40 arexcess 40 ar not attributable to radiogenic decay of 40 k or an atmospheric source are similarly elevated in the main stage fluorite mineralization of the illinois kentucky fluorspar district 6 8 18 2 10 4 cm 3 cm 3 h2o relative to the viburnum trend and tri state districts mostly 1 3 3 3 10 4 cm 3 cm 3 h2o and 0 3 1 4 10 4 cm 3 cm 3 h2o respectively together the noble gas and halogen data indicate the existence of three brine provinces 1 a regional tristate brine present throughout the ozark region 2 a viburnum trend brine and 3 an illinois kentucky brine furthermore in the viburnum trend the halogen composition of main stage dolomite is distinct from that of the main stage sphalerite but identical to the regional hydrothermal dolomite seen in the tri state district the data imply that mixing relationships in the viburnum trend are highly complex but are compatible with regional models in which the tri state brine present throughout the ozark region was sourced in the arkoma basin

n2 and co2 in deep crustal fluids evidence from the caledonides of norway abstract high pressure metamorphic rocks eclogites and high pressure granulites occur along the entire length of the norwegian caledonides and have formed from a variety of protoliths in some cases the relationship between protoliths high pressure rocks and their later retrogression products have been preserved in situ fluid inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition 1 eclogites and high pressure granulites contain n 2 bearing fluids pure n 2 or mixtures with co 2 or h 2 o with x n 2 5 in some eclogite facies rocks co 2 1bn 2 inclusions are associated with aqueous brine inclusions ca 30 wt nacl the two compositions representing immiscible fluids at peak metamorphic conditions 2 granulite facies protoliths and eclogites reworked in the granulite facies contain pure co 2 or co 2 dominated fluids with less than 2 5 n 2 3 rocks retrograded in the amphibolite facies contain h 2 o 1bnacl fluids immiscibility between brine and anhydrous n 2 1bco 2 fluid and between anhydrous fluid and waterbearing aluminosilicate melt have taken place in some eclogites during high grade metamorphism nitrogen may be incorporated in minerals as nh 4 substituting for k or it may occur as n 2 in the free fluid phase the partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism low a h 2 o and or high f o 2 partitioning nitrogen to the fluid phase a rock interacting with a carbonic fluid at granulite facies pt conditions will be depleted in mineralogically bound nitrogen in cases where the protoliths of high pressure rocks have been through a previous granulite facies event a local source for the nitrogen contained in high pressure fluid is therefore unlikely

mineralogical and chemical evolution of the ernest henry fe oxide cu au ore system cloncurry district northwest queensland australia the ernest henry cu au deposit was formed within a zoned post peak metamorphic hydrothermal system that overprinted metamorphosed dacite andesite and diorite ca 1740 1660 ma the ernest henry hydrothermal system was formed by two cycles of sodic and potassic alteration where biotite magnetite alteration produced in the first cycle formed ca 1514 24 ma whereas paragenetically later na ca veining formed ca 1529 11 8 ma these new u pbtitanite age dates support textural evidence for incursion of hydrothermal fluids after the metamorphic peak and overlap with earlier estimates for the timing of cu au mineralization ca 1540 1500 ma a distal to proximal potassic alteration zone correlates with a large up to 1 5 km k fe mn ba enriched alteration zone that overprints earlier sodic alteration mass balance analysis indicates that k fe mn ba alteration largely produced during pre ore biotite and magnetite rich alteration is associated with k rb cl ba fe mn and as enrichment and na ca and sr depletion the aforementioned chemical exchange almost precisely counterbalances the mass changes associated with regional na ca alteration this initial transition from sodic to potassic alteration may have been formed during the evolution of a single fluid that evolved via alkali exchange during progressive fluid rock interaction cu au ore dominated by co precipitated magnetite minor specular hematite and chalcopyrite as breccia matrix forms a pipe like body at the core of a proximal alteration zone dominated by k feldspar alteration both the core and k feldspar alteration overprint na ca alteration and biotite magnetite k fe alteration ore was associated with the concentration of a diverse range of elements e g cu au fe mo u sb w sn bi ag f ree k s as co ba and ca mineralization also involved the deposition of significant barite k ba feldspar calcite fluorite and complexly zoned pyrite the complexly zoned pyrite and variable k ba feldspar versus barite associations are interpreted to indicate fluctuating sulphur and or barium supply together with the alteration zonation geochemistry and overprinting criteria these data are interpreted to indicate that cu au mineralization occurred as a result of fluid mixing during dilation and brecciation in the location of the most intense initial potassic alteration a link between early alteration na ca and k fe and the later k feldspathization and the cu au ore is possible however the ore related enrichments in particular elements especially ba mn as mo ag u sb and bi are so extreme compared with earlier alteration that another fluid possibly magmatic in origin contributed the diverse element suite geochemically independently of the earlier stages structural focussing of successive stages produced the distinctive alteration zoning providing a basis both for exploration for similar deposits and for an understanding of ore genesis

alteration mineralization and fluid evolution at the eloise cu au deposit cloncurry district northwest queensland australia the eloise deposit 3 1 million metric tons mt 5 5 cu 1 4 g t au and 16 g t ag is hosted by proterozoic rocks of the soldiers cap group in the eastern fold belt of the mount isa inlier the deposit is characterized by very high grade chalcopyrite pyrrhotite rich mineralization hosted by mafic silicate alteration alteration and mineralization lie within and adjacent to a major shear zone the main metasomatism comprised three stages early pervasive albite alteration stage i was overprinted by stage ii hornblende biotite quartz veins and alteration stage iii chalcopyrite pyrrhotite magnetite pyrite calcite quartz chlorite and actinolite replaced and brecciated the earlier mafic silicates the lodes occupy a narrow portion of a zoned mineralized system that is at least 2 km in length magnetite pyrite rich mineralization occurs in the south with chalcopyrite pyrrhotite rich mineralization in the central lode zone and pyrrhotite rich mineralization in the north the system has a distinct chemistry with enriched cu au ag co ni and zn fluid inclusions in pre syn and late to postmineralization veins indicate an evolving fluid history primary inclusions and associated hornblende biotite assemblages in premineralization veins indicate a high temperature fluid 450 degrees 600 degrees c and salinity estimates suggest it was an ultrasaline brine 32 68 wt total salts fluid inclusions in stage iii quartz in mineralized veins display a wide range of homogenization temperatures approximately 100 degrees 500 degrees c and gangue assemblages actinolite chlorite muscovite suggest mineralization was a cooler event 200 degrees 450 degrees c inclusions associated with the mineralization have a lower salinity 30 47 wt total salts than do premineralization inclusions and variable fluid inclusion types occur within the same clusters which may reflect fluid mixing the ultrasaline nature of the fluids and preliminary sulfur isotope results 0 4 2 0ppm suggest the ore fluid was predominantly magmatic metals were carried as chlorocomplexes and h 2 s was the dominant sulfur species important factors in ore deposition were a decrease in temperature and salinity and sulfidation of fe silicates eloise is interpreted to be a structurally controlled distal magmatic hydrothermal vein deposit

carbon and oxygen isotope constraints on fluid sources and fluid wallrock interaction in regional alteration and iron oxide copper gold mineralisation eastern mt isa block australia the source of metasomatic fluids in iron oxide copper gold districts is contentious with models for magmatic and other fluid sources having been proposed for this study 18o and 13c ratios were measured from carbonate mineral separates in the proterozoic eastern mt isa block of northwest queensland australia isotopic analyses are supported by petrography mineral chemistry and cathodoluminescence imagery marine meta carbonate rocks ca 20 5 18o and 0 5 13c calcite and graphitic meta sedimentary rocks ca 14 18o and 18 13c calcite are the main supracrustal reservoirs of carbon and oxygen in the district the isotopic ratios for calcite from the cores of na ca alteration systems strongly cluster around 11 18o and 7 13c with shifts towards higher 18o values and higher and lower 13c values reflecting interaction with different hostrocks na ca rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks however igneous rocks in the eastern mt isa block contain negligible carbon and are incapable of buffering the 13c signatures of co2 rich metasomatic fluids associated with na ca alteration in contrast plutons in the eastern mt isa block have been documented as having exsolved saline co2 rich fluids and represent the most probable fluid source for na ca alteration intrusion proximal skarn like cu au orebodies that lack significant k and fe enrichment e g mt elliott display isotopic ratios that cluster around values of 11 18o and 7 13c calcite indicating an isotopically similar fluid source as for na ca alteration and that significant fluid wallrock interaction was not required in the genesis of these deposits in contrast k and fe rich intrusion distal deposits e g ernest henry record significant shifts in 18o and 13c towards values characteristic of the broader hostrocks to the deposits reflecting fluid wallrock equilibration before mineralisation low temperature low salinity low 18o 10 calcite and co2 poor fluids are documented in retrograde metasomatic assemblages but these fluids are paragenetically late and have not contributed significantly to the mass budgets of cu au mineralisation

evaporitic source model for igneous related fe oxide ree cu au u mineralization we propose that many igneous related fe oxide rich ree cu au u bearing deposits form by hydrothermal processes involving evaporitic ligand sources either coeval salars or older evaporites these deposits are abundant in both phanerozoic and proterozoic extensional continental and continent margin settings they commonly form in global arid zones but they also occur where magmatism is superimposed upon older evaporites magmatic compositions exert only second order control mainly on alteration mineralogy and on element abundances hot s poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments siderophile lithophile and hydrothermal alteration sodic locally alkaline found in these systems this model contrasts with immiscible oxide melt and magmatic hydrothermal origins commonly proposed for these deposits although all three mechanisms can occur 31 refs 3 figs 1 tab

noble gas and halogen constraints on regionally extensive mid crustal na ca metasomatism the proterozoic eastern mount isa block australia fluid inclusions in late isan quartz veins associated with regional na ca alteration albitisation in the mary kathleen fold belt and the cloncurry district of the eastern mt isa block have been analysed for naturally occurring and neutron produced isotopes of ar kr and xe the noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types including variably saline aqueous liquid carbon dioxide and mixed aqueous carbonic varieties the variably saline 5 65 wt aqueous fluid inclusions dominate and have 40ar 36ar values of less than 2700 in most of the samples from across the region these fluid inclusions have extremely variable molar br cl values of 0 3 4 10 3 and i cl values of 0 2 35 10 6 and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite or scapolite to achieve their ultra high salinity fluid inclusions in a sample from the snake creek anticline in the cloncurry district have the highest 40ar 36ar value of 25 000 demonstrating a deep magmatic or metamorphic fluid origin in this case a magmatic origin is favoured because this fluid is very similar to a fluid involved in iron oxide copper gold iocg mineralization at ernest henry that was also interpreted to have a magmatic origin and because the aqueous fluid inclusions have br cl of 1 2 10 3 and i cl of 10 10 6 that are similar to mantle derived igneous rocks carbon dioxide fluid inclusions dominate samples from the knobby quarry in the mary kathleen fold belt and have a maximum 40ar 36ar value of 6000 7000 these fluid inclusions are estimated to have a 36ar concentration of 1 4 ppb that is similar to the range determined for aqueous fluid inclusions in all the other samples in addition aqueous and carbonic fluid inclusions in all the samples have similar 129xe 36ar plus 84kr 36ar values that are unfractionated and close to the air air saturated water asw range these data are interpreted to indicate an independent and dominantly metamorphic origin for co2 and the presence of mixed aqueous carbonic fluid inclusions are attributed to fluid mixing and or mingling rather than fluid unmixing however the data do not preclude the presence of a minor magmatic co2 component in the samples from the knobby quarry fluid inclusions in most of the samples from the mary kathleen fold belt have higher br cl and i cl values higher 36ar concentrations and lower maximum 40ar 36ar values than fluid inclusions in samples from the cloncurry district this suggests na ca alteration in these different parts of the eastern mt isa block occurred independently however fluid inclusions associated with na ca alteration in the cloncurry district have a very similar composition to fluid inclusions in iocg mineralization stage quartz veins from ernest henry these data are therefore compatible with a genetic relationship between regional na ca alteration and iocg mineralization in the cloncurry district in both cases the ultra saline hydrothermal fluids had a dominant origin from sedimentary formation water but are interpreted to contain a magmatic component sourced from the late isan williams naraku batholiths which may have driven fluid convection

iron mobility during synmetamorphic alteration in the selwyn range area nw queensland implications for the origin of ironstone hosted au cu deposits the proterozoic metamorphic rocks of the selwyn range area in western queensland contain a variety of styles of au cu deposit including some e g selwyn mine and osborne prospect that are spatially associated with magnetite rich ironstones the area is characterized by extensive alkali chloride metasomatism that occurred during the later stages of regional metamorphism existing experimental data indicate that hot saline fluids will dissolve and transport iron as they move through temperature pressure gradients in the vicinity of the amphibolite greenschist facies transition and predict that circulation cells will move iron and concentrate it in relatively cool dilute and or oxidized parts of such systems new geological mapping coupled with geochemical data from the amphibolite facies zone of the selwyn range area suggest such processes occurred on a large scale and that large masses of fe were mobilized particularly in association with na metasomatism of biotite bearing metasediments and high fe tholeiitic dolerites field and textural features of ironstones and other fe rich lithologies demonstrate that some of them at least formed by replacement of other rocks during metamorphism and deformation mineralization localized within such iron stones must itself be epigenetic

internal control of fluid compositions in amphibolite facies scapolitic calc silicates mary kathleen australia unaltered metasediments of the mary kathleen fold belt are composed predominantly of layered amphibolite facies scapolitic calc silicate rocks in which minimal infiltration of externally derived fluids occurred during regional metamorphism there were substantial differences in volatile activities between different layers in the layered sequences in particular a co2 a h 2 o inferred from reaction progress estimates and analysis of biotite clinopyroxene fluid phase relations a nacl a h 2 o inferred from scapolite compositions and a hcl a h 2 o inferred from biotite compositions in one outcrop in which a clinopyroxene producing reaction dominated differences in approximate x co 2of up to 0 25 occurred between several samples collected over 50 metres variations in a h 2 o a hcl of up to one order of magnitude are inferred at 1 to 50 m scales from biotite cl contents and variations in nacl contents of scapolite from 0 0 to up to 0 6 cl atoms in the cl co3 so4 site reflect a large variation of a nacl in the coexisting fluid at similar scales most calcsilicate layers internally buffered fluid compositions in the h2o co2 nacl hcl system local occurrences of nacl rich scapolite suggest that some layers may have been in equilibrium with halite during early prograde metamorphism at peak metamorphic temperatures disolution of halite was complete but layers containing high nacl scapolite continued to buffer fluid at high values of a nacl fluid immiscibility does not appear to have affected the progress of the devolatilization reactions although fluid was predominantly internally buffered moderate quantities of fluid were released by prograde mineral reactions in many layers up to 30 cm3 fluid per 100 cm3 rock numerous episodes of fluid escape were required probably via microfractures such that the released fluid did not obviously influence reaction progress in the layers through which it passed the anomaly of beautifully preserved internal buffering signatures and the requirement for produced fluid locally to pass across layers in a deforming rock sequence suggest that the escaping fluid did not leave any readily observable tracks this is explained by rapid rates of fracture propogation and fluid migration therein this internally buffered system contrasts strongly with adjacent calc silicate rocks that show evidence for infiltration of externally derived fluids at high fluid rock ratios and highlights the broad range of fluid behaviour that can be expected in deforming heterogeneous rock sequences

sodic calcic alteration in fe oxide cu au districts an origin via unmixing of magmatic h 2 o co 2 nacl cacl 2 kcl fluids iron oxide cu au deposits particularly those formed in deeper level plutonic environments are commonly characterized by regional scale sodic calcic alteration which typically formed pre or syn cu au mineralization the sodic calcic assemblages include albite scapolite pyroxene actinolite apatite titanite epidote and calcite the consistent presence of coexisting hypersaline aqueous and co2 rich fluids in minerals from sodic calcic alteration and associated fe oxide cu au deposits is the result of unmixing of h2o co2 nacl cacl2 kcl magmatic fluids experimental evidence indicates that the na na k ratio of fluids in equilibrium with two alkali feldspars in co3 2 bearing parent fluids would be significantly higher than in unmixed chloride bearing aqueous fluids therefore fluid unmixing caused by decreases in temperature and or pressure will result in albitization of wall rocks as is observed in most deeper level fe oxide cu au deposits this alteration style may be succeeded by k feldspathization with decreasing temperature because of the increase in equilibrium na na k in chloride bearing fluids buffered by alkali feldspars

stable isotope evidence for thermochemical sulfate reduction in the dugald river australia strata bound shale hosted zinclead deposit abstract sulfur isotopes vary along strike and down dip in the mid proterozoic dugald river strata bound shale hosted zinc lead ore deposit australia although the metal grade has been substantially increased at its southern end by tectonism geological relationships indicate that mineralisation occurred during early diagenesis probably in an organic rich shallow water to evaporitic setting muir 1983 cu cu cu pb zn and pb pb zn decrease in the mineralisation northwards independent of the main structural thickening and so are likely to represent a pre deformational primary geochemical dispersion average 34s values in pyrite and sphalerite range from 1 in the south coincident with the highest copper content of the ores to 8 in the north accompanying deposit thinning in contrast adjacent footwall and hangingwall iron sulfide does not exhibit isotopic zonation and has a 34s peak between 3 and 4 n 33 with some values as light as 14 5 whereas overlying dolomites contain 34 s py 5 5 to 17 5 each population is attributed to varying degrees of closed system biogenic sulfate reduction two populations of carbonate carbon isotopes are present the first 13 c 11 to 5 mainly characterises graphitic footwall limestone of the lady clayre dolomite and is interpreted to reflect the metamorphic equilibration of carbonate and organic carbon isotopes the second 13 c 25 to 15 characterises the sulphide lens and its immediate sedimentary host and is most consistent with formation by the oxidation of organic carbon thermochemical sulfate reduction tsr by organic matter is preferred to account for the 34s zonation because of 1 mass balance calculations 2 the 13c values of ore and near ore carbonate 3 the high temperatures likely for ore formation 150 250 c which would have prevented biogenic sulfur reduction but promoted tsr and 4 the trend toward 34sseawater the proposed mineralisation model involves an h2s and metal bearing fluid with 34 s 3 to 0 this fluid ascended into the dugald basin in the south permeating laterally northward through carbonaceous sediments below an evaporitic to shallow marine carbonate platform during this migration the hydrothermal fluid reacted with organic matter and sulfate to form 34s rich sulfide 34 s 8 to 10 by tsr reaction was catalysed by deep sourced h2s biogenic h2s and h2s that evolved by hydrothermal cracking of organic matter the sources of sulfate for the reaction were 1 evaporites locally within the sequence and 2 dissolved sulfate diffusing from surface waters into the diagenetic zone

the source and significance of argon isotopes in fluid inclusions from areas of mineralization argon isotopes in fluid inclusions in quartz veins associated with granite hosted tungsten mineralization in the southwest and north of england have been investigated in detail by the40ar 39ar technique the natural argon is present as a number of discrete components which can be identified through correlations with39ar 38ar and 37ar induced by neutron bombardment of potassium chlorine and calcium the potassium correlated component arises principally from in situ decay of potassium in solid phases in the inclusions in the case of the hemerdon tungsten deposit of southwest england the phases responsible are small 25 m captive authigenic micas which are shown to have been deposited from a fluid 268 20 ma ago shortly after the emplacement of the host granite the chlorine correlated component is present in the brines which constitute the fluid phase of the inclusions the argon in these hydrothermal fluids is made up in part of parentless or excess 40ar leached from surrounding crustal rocks and in part of dissolved ancient atmospheric argon absolute concentrations of both atmospheric and excess components in the brine can be estimated from 40arcl ratios and independent determinations of the salinity of the inclusions the absolute concentrations of the atmospheric argon are close to those found in modern meteoric water while those of the excess component can be interpreted in terms of the degree of interaction betwen the circulating fluids and country rock a calcium correlated component with a much higher ratio of excess to atmospheric argon than that in the brine was found to be a dominant phase in one sample from the hemerdon deposit indicating the presence of a solid phase probably a caso4 daughter mineral inclusions of this composition represent fluids which have had a more prolonged interaction with crustal rocks the results obtained from this study provide a systematization and a framework for future multi component argon studies of fluid inclusions together with an indication of the wide range of information which can be inferred

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