magma degassing buffered by vapor flow through brecciated conduit margins obsidian pyroclasts a common component of rhyolitic tephra preserve a range of volatile contents which has been used to infer syneruptive conditions of magmatic degassing here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins obsidian clasts often contain xenoliths sheared bands of lithic powder and textures consistent with magma autobrecciation these features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent we use these observations to develop a degassing model for pyroclastic obsidian from the a d 1340 mono craters california eruptions we suggest that degassing was buffered by continual flux of vapor through highly permeable brecciated magma along conduit walls continuous reequilibration of magma with vapor of relatively constant composition not only explains the co 2 h 2 o and d h 2 o data from mono craters pyroclastic obsidian but also requires much lower magmatic co 2 values than the commonly accepted model of closed system degassing taken together the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite

two forsterite bearing fun inclusions in the allende meteorite we have discovered two fun inclusions cg 14 and te among a group of five forsterite rich inclusions in allende two of which are described for the first time herein all five consist of euhedral forsterite and spinel crystals poikilitically enclosed by fassaite forsterite and spinel are usually segregated from one another sometimes into a spinel rich mantle and a forsterite rich core some inclusions contain vesicles indicating that they were once molten the crystallization sequence inferred from textures is spinel forsterite fassaite and finally mg rich melilite one concentrically zoned inclusion contains melilite in its mantle whose composition lies on the opposite side of the liquidus minimum in the melilite binary from that in its core this suggests that segregation of forsterite from spinel in all of these inclusions could be due to volatilization of mgo and sio2 relative to al2o3 and cao from the outsides of droplets cg 14 is relatively uniformly enriched in refractory elements relative to cl chondrites by a factor similar to that for ca al rich coarse grained inclusions except for ca al and hf which are unusually low no ce anomaly such as found in fun inclusions cl and hal is present in cg 14 whole rock samples of cg 14 and te are more strongly mass fractionated in oxygen relative to normal allende inclusions than the fun inclusion ek 1 4 1 and less so than cl relative to bulk allende both inclusions have strongly massfractionated magnesium and silicon and 25mg excesses or deficits of 24mg or 26mg cg 14 has a 29si excess or a deficit of 28si or 30si volatilization loss cannot be responsible for the magnesium and silicon isotope fractionations as this would require prohibitively large mass loss from these magnesium rich inclusions the remarkable similarity in textures between fun and non fun inclusions implies similar thermal histories arguing against different rates of evaporative loss of major elements sputtering alone may be insufficient to account for the magnitude and direction of oxygen isotope fractionation in fun inclusions

bajo de la alumbrera copper gold deposit stable isotope evidence for a porphyry related hydrothermal system dominated by magmatic aqueous fluids alteration zones at the gold rich bajo de la alumbrera porphyry copper deposit in northwestern argentina are centered on several porphyritic intrusions they are zoned from a central copper iron sulfide and gold mineralized potassic biotite k feldspar quartz core outward to propylitic chlorite illite epidote calcite assemblages a mineralized intermediate argillic alteration assemblage chlorite illite pyrite has overprinted the potassic alteration zone across the top and sides of the deposit and is itself zoned outward into phyllic quartzinuscovite illite pyrite alteration this study contributes new data to previously reported delta 18 o and delta d compositions of fluids responsible for the alteration at bajo de la alumbrera and the data are used to infer likely ore forming processes measured and calculated delta 18 o and delta d values of fluids 8 3 to 10 2 and 33 to 81 parts per thousand respectively confirm a primary magmatic origin for the earliest potassic alteration phase lower temperature potassic alteration formed from magmatic fluids with lower delta d values down to 123 parts per thousand these depleted compositions are distinct from meteoric water and consistent with degassing and volatile exsolution of magmatic fluids derived from an underlying magma variability in the calculated composition of fluid associated with potassic alteration is explained in terms of phase separation or boiling if copper iron sulfide deposition occurred during cooling as proposed elsewhere this cooling was largely a result of phase separation magmatic water was directly involved in the formation of overprinting intermediate argillic alteration assemblages at bajo de la alumbrera calculated delta 18 o and delta d values of fluids associated with this alteration range from 4 8 to 8 1 and 31 to 71 per mil respectively compositions determined for fluids associated with phyllic alteration 0 8 to 10 2 and 31 to 119 parts per thousand overlap with the values determined for the intermediate argillic alteration we infer that phyllic alteration assemblages developed during two stages the first was a high temperature 400 degrees 300 degrees c stage with d depleted water delta d 66 to 119 parts per thousand this compositional range may have resulted from magma degassing and or the injection of new magmatic water into a compositionally evolved hydrothermal system the isotopic variations also can be explained by increased fluid rock interaction the second stage of phyllic alteration occurred at a lower temperature similar to 200 degrees c and variations in the modeled isotopic compositions imply mixing of magmatic and meteoric waters ore deposition that occurred late in the evolution of the hydrothermal system was probably associated with further cooling of the magmatic fluid in part caused by fluid rock interaction and phase separation changing ph and or oxygen fuoracity may have caused additional ore deposition the ingress of meteoric water appears to postdate the bulk of mineralization and occurred as the system at bajo de la alumbrera waned

oxidation of methane at the ch4 h2o co2 transition zone in the external part of the central alps switzerland evidence from stable isotope investigations abstract with the aim of understanding the mechanisms that control the metamorphic transition from the ch 4 to the h 2 o co 2 dominated fluid zone in the helvetic domain of the central alps of switzerland fluid inclusions in quartz illite crystallinity index vitrinite reflectance and the stable isotope compositions of vein and whole rock minerals and fluids trapped in quartz were investigated along four cross sections increasing temperature during prograde metamorphism led to the formation of dry gas by hydrocarbon cracking in the ch 4 zone fluid immiscibility in the h 2 o ch 4 co 2 nacl system resulted in cogenetic ch 4 and h 2 o dominated fluid inclusions in the ch 4 zone fluids were trapped at temperatures 270 5 c the end of the ch 4 zone is marked by a sudden increase of co 2 content in the gas phase of fluid inclusions at temperatures 270 5 c in the h 2 o zone the total amount of volatiles within the fluid decreased below 1 mol with no immiscibility this resulted in total homogenization temperatures of h 2 o co 2 ch 4 nacl inclusions below 180 c hydrogen isotope compositions of methane in fluid inclusion have d values of less than 100 in the ch 4 zone typical for an origin through cracking of higher hydrocarbons but where the methane has not equilibrated with the pore water d values of fluid inclusion water are around 40 in isotopic equilibrium with phyllosilicates of the whole rocks within the ch 4 to h 2 o co 2 transition zone d h 2 o values in fluid inclusions decrease to 130 interpreted to reflect the contribution of deuterium depleted water from methane oxidation in the h 2 o zone d h 2 o values increase again towards an average of 30 which is again consistent with isotopic equilibrium with host rock phyllosilicates 13 c values of methane in fluid inclusions from the ch 4 zone are around 27 in isotopic equilibrium with calcite in veins and whole rocks the 13 c ch 4 values decrease to less than 35 at the transition to the h 2 o zone and are no longer in equilibrium with the carbonates in the whole rocks 13 c values of co 2 are variable but too low to be in equilibrium with the wall rock fluids compatible with a contribution of co 2 from closed system oxidation of methane differences in isotopic composition between host rock and alpine fissure carbonate are generally small suggesting that the amount of co 2 produced by oxidation of methane was small compared to the c budget in the rocks and local pore fluids were buffered by the wall rocks during precipitation of calcite within the fissures

a review of the occurrence form and origin of c bearing species in the khibiny alkaline igneous complex kola peninsula nw russia abstract the khibiny complex hosts a wide variety of carbon bearing species that include both oxidized and reduced varieties oxidised varieties include carbonate minerals especially in the carbonatite complex at the eastern end of the pluton and na carbonate phases reduced varieties include abiogenic hydrocarbon gases particularly methane and ethane dispersed bitumens solid organic substances and graphite the majority of the carbon in the khibiny complex is hosted in either the carbonatite complex or within the so called central arch the central arch is a ring shaped structure which separates khibinites coarse grained eudialite bearing nepheline syenites in the outer part of the complex from lyavochorrites medium grained nepheline syenites and foyaites in the inner part the central arch is petrologically diverse and hosts the major ree enriched apatite nepheline deposits for which the complex is best known it also hosts zones with elevated hydrocarbon dominantly methane gas content and zones of hydrothermally deposited na carbonate mineralisation the hydrocarbon gases are most likely the product of a series of post magmatic abiogenic reactions it is likely that the concentration of apatite nepheline deposits hydrocarbon gases and na carbonate mineralisation is a function of long lived fluid percolation through the central arch fluid migration was facilitated by stress release during cooling and uplift of the khibiny complex as a result carbon with a mantle signature was concentrated into a narrow ring shaped zone

excess methane in continental hydrothermal emissions is abiogenic thermogenic hydrocarbons entirely deriving from the thermal degradation of organic matter usually exhibit methane to ethane plus propane ratios smaller than 100 we present hydrocarbon distribution data of continental hydrothermal gases whose methane has been independently identified to derive from the abiogenic reduction of co2 we find that excess amounts of methane with respect to thermogenic hydrocarbon distributions are characteristic for the investigated gases a similar pattern is observed for well discharges whose temperatures are too high to support any microbially mediated methanogenesis these findings strongly suggest that abiogenic methane production in continental hydrothermal systems is a more widespread process than previously assumed the maximum contribution of such emissions to the modern atmospheric ch4 budget is estimated at 1

copper deposition during quartz dissolution by cooling magmatic hydrothermal fluids the bingham porphyry abstract scanning electron microscope cathodoluminescence imaging is used to map successive generations of fluid inclusions in texturally complex quartz veinlets representing the main stage of ore metal introduction into the porphyry cu au mo deposit at bingham utah following conventional fluid inclusion microthermometry laser ablation inductively coupled plasma mass spectrometry la icpms is applied to quantify copper and other major and trace element concentrations in the evolving fluid with the aim of identifying the ore forming processes textures visible in cathodoluminescence consistently show that the bulk of vein quartz q1 characterized by bright luminescence crystallized early in the vein history cu fe sulfides are precipitated later in these veins in a microfracture network finally filled with a second generation of dull luminescing q2 quartz mapping of brine and vapor inclusion assemblages in these successive quartz generations in combination with la icpms microanalysis shows that the fluids trapped before and after cu fe sulfide precipitation are very similar with respect to their major and minor element composition except for copper copper concentrations in inclusions associated with ore formation drop by two orders of magnitude in a tight pressure temperature interval between 21 and 14 mpa and 425 350 c several hundred degrees below the temperature of fluid exsolution from the magma copper deposition occurs within a limited p t region in which sulfide solubility shows strong normal temperature dependence while quartz solubility is retrograde this permits copper sulfide deposition while secondary vein permeability is generated by quartz dissolution the brittle to ductile transition of the quartz feldspar rich host rocks occurs in the same temperature range which further enhances vein reactivation and promotes cooling and expansion of fluids ascending across the transition from lithostatic to hydrostatic conditions

depositional ages of the stratiform base metal deposits at mount isa and mcarthur river australia based on u pb zircon dating of concordant tuff horizons potassium rich tuff beds occur interbedded with major stratiform pb zn mineralization in shales and dolomitic shales of the mount isa group northwest queensland and the mcarthur group northern territory these deposits are some 600 km apart yet they exhibit many similarities which together with regional correlations suggest that the deposits formed by similar processes and that the successions developed more or less contemporaneously zircon is sparsely distributed as tiny crystal fragments or phenocrysts in the tuffs and its morphology and u pb systematics affirm that it is primarily of igneous origin u pb zircon data from a tuff in the mineralized h y c pyritic shale member of the mcarthur group give an age of the tuff of 1 690 super 29 sub 25 m y and from a tuff in the mineralized urquhart shale of the mount isa group an age of 1 670 super 20 sub 17 m y these results are consistent with all other geologic controls and are regarded as the best estimates of the depositional ages of the groups the ages of the mount isa and mcarthur groups are thus the same to within the present experimental uncertainties essentially confirming geologic synchrony of these important sedimentary successions pooled whole rock data from several of the mount isa tuff marker beds give an updated rb sr age of about 1 480 m y interpreted as indicating that isotopic exchange and regional metamorphism had essentially ceased by this time

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

diversity of primary cl textures in quartz from porphyry environments implication for origin of quartz eyes porphyry style mineralization is related to the intrusion and crystallization of small stocks which can be of different compositions from intermediate to felsic and can intrude into different host rocks from magmatic to sedimentary we used cathodoluminescence and electron probe microanalysis to study the internal textures of more than 300 quartz eyes from six porphyry deposits panguna papua new guinea far southeast porphyry philippines batu hijau indonesia antapaccay peru rio blanco chile and climax usa significant diversity of the internal textures in quartz eyes was revealed sometimes even within a single sample quartz grains with ti rich cores surrounded by ti poor mantles were found next to the grains showing the opposite ti distribution or only slight ti fluctuations we propose that diversity of the internal patterns in quartz eyes can actually reflect in situ crystallization history and that prolonged crystallization after magma emplacement under conditions of continuous cooling can account for the observed features of internal textures formation of quartz eyes begins at high temperatures with crystallization of high titanium quartz 1 which as the melt becomes more and more evolved and cooler is overgrown by low ti quartz 2 subsequent fluid exsolution brings about dramatic change in the melt composition oh alkalis and other cl complexed elements partition into the fluid phase whereas ti stays in the melt contributing to a rapid increase in ti activity separation of the fluid and its further cooling causes disequilibrium in the system and the quartz 2 becomes partially resorbed exsolution of the fluid gradually builds up the pressure until it exceeds the yield strength of the host rocks and they then fracture this pressure release most likely triggers crystallization of quartz 3 which is higher in ti than quartz 2 because ti activity in the melt is higher and pressure of crystallization is lower as a result of the reaction between the exsolved fluid and quartz a new phase a so called heavy fluid forms from this phase quartz 4 crystallizes this phase has extremely high metal carrying capacity and may give a rise to mineralizing fluids finally on the brink of the subsolidus stage groundmass quartz crystallizes prolonged crystallization under conditions of continuous cooling accounts better for the diversity of cl textures than crystallization in different parts of a deep magma chamber it is also in a better agreement with the existing model for formation of porphyry style deposits

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

the magmatic hydrothermal transition evidence from quartz phenocryst textures and endoskarn abundance in cu zn skarns at the empire mine idaho usa information about the magmatic to hydrothermal transition is preserved in late stage features of quartz phenocrysts and endoskarn alteration in some cu zn skarn deposits such as the empire mine in idaho important features include 1 quartz phenocrysts with strong resorption textures such as vermicular zones of igneous groundmass cutting primary quartz cathodoluminescence banding 2 anomalous amounts of endoskarn more than 50 of mineralized rock 3 high f activities as evidenced by fluorite as an accessory mineral in igneous rocks in alteration assemblages and in fluid inclusions and by high f in hydroxyl sites in igneous biotite and amphibole and 4 direct association of zn which normally is deposited distally at low temperature with cu in proximal locations and in endoskarn these features are explained by the following model 1 f lowers the solidus temperature of the magma thus changing the timing temperature and duration of hydrothermal fluid exsolution 2 upon magmatic vapor saturation the f rich hydrothermal fluids form bubbles that adhere to quartz phenocrysts and chemically corrode tunnel into the quartz forming vermicular resorption textures 3 f rich hydrothermal fluids also promote the formation of endoskarn silicic rocks are attacked by f rich fluids in the same sense that carbonate wall rocks are dissolved by weakly to moderately acidic hydrothermal fluids 4 low fluid exsolution temperature facilitated by high f activity promotes high zn cu ratios in proximal locations due to the solubility of zn relative to cu at lower temperatures this model may be applicable at other localities such as the world class cu zn skarn antamina mine as well as some tin and rapakivi granites

rbsb age determinations on precambrian carbonate rocks of the carpentarian mcarthur basin northern territories australia abstract the applicability and the uncertainty range of rb sr age determinations on carbonate rocks are discussed briefly a study of clay minerals isolated from carbonate rocks of the mcminn formation provides an isochron age of 1429 31 ma and thus the age for cessation of the proterozoic mcarthur basin sedimentation the eukaryots recently described in this area are therefore the oldest occurrences found in earth history a second sequence of measurements on a homogeneous dolomite siltstone ca 23 km southeast of the mcarthur river h y c pb zn deposits yields an rb sr age of 1537 52 ma for the upper barney creek formation in broad agreement with recent age determinations on feldspar beds of this horizon these data are compared with previous isotopic results from data presently available it can be concluded that the sediments in the mcarthur basin the type locality of the carpentarian were deposited between 1800 1400 ma

combined raman microprobe spectrometry and microthermometry of fluid inclusions in minerals from igneous rocks of the gardar province south greenland abstract raman microprobe spectrometry has been applied to analysis of the gaseous phase in individual fluid inclusions in minerals from alkalic igneous rock types and associated hydrothermal veins from the gardar province south greenland quantitative analyses are given for co2 ch4 c2h6 h2 and h2s traces of co were observed in one co2 inclusion in conjunction with data from microthermometry the results permit calculation of the bulk chemical composition of entrapped fluids and evaluation of the possible pressure temperature oxygen fugacity and sulphur fugacity during entrapment of the individual fluid inclusions calculated oxygen fugacity values correspond to those of the nno and qfm buffer reactions at temperatures of 300 400 c and pressures of 0 5 1 0 kbar calculated sulphur fugacity values are within the range of normal ore forming solutions log fs2 6 to 10

methane origin and oxygen fugacity evolution of the baogutu reduced porphyry cu deposit in the west junggar terrain china most porphyry copper deposits worldwide contain magnetite hematite and anhydrite in equilibrium with hypogene copper iron sulfides chalcopyrite bornite and have fluid inclusions with co2 ch4 that are indicative of high fo2 in contrast the baogutu porphyry cu deposit in the west junggar terrain xinjiang china lacks hematite and anhydrite contains abundant pyrrhotite and ilmenite in equilibrium with copper iron sulfides chalcopyrite and has fluid inclusions with ch4 co2 that are indicative of low fo2 the mineralized intrusive phases at baogutu include the main stage diorite stock and minor late stage diorite porphyry dikes the main stage stock underwent fractional crystallization and country rock assimilation contamination and consists of dominant diorite and minor gabbro and tonalite porphyry the country rocks contain organic carbons 0 21 0 79 wt the 13cvpdb values of the whole rocks 23 1 to 25 8 in the wall rocks suggest a sedimentary organic carbon source the 13cvpdb values of ch4 28 2 to 36 0 and co2 6 8 to 20 0 in fluid inclusions require an organic source of external carbon and equilibration of their 13cco2 ch4 values 8 2 25 0 at elevated temperatures 294 830 c suggesting a significant contribution of thermogenic ch4 mineral composition data on the main stage intrusions such as clinopyroxene hornblende biotite magnetite ilmenite sphene apatite and pyrrhotite suggest that the primary magma at baogutu was oxidized and became reduced after emplacement by contamination with country rocks mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from logfo2 fmq 1 in the magma stage to logfo2 fmq in the hydrothermal stage though oxidized magma was emplaced initially assimilation contamination of carbonaceous country rocks decreased its fo2 such that exsolved fluids contained abundant ch4 and deposited a reduced assemblage of minerals

sulfur bearing magmatic accessory minerals almost all magmas contain sulfide or sulfate bearing phases in most natural samples the sulfur bearing phase is a sulfide which typically is pyrrhotite fe1 x s or pyrite fes2 although chalcopyrite cufes2 pentlandite fe ni 9s8 sphalerite zns or molybdenite mos2 may be present as well sulfate minerals are rare at magmatic conditions and anhydrite caso4 is the most common other magmatic so4 bearing minerals include the sodalite group minerals hauyne simplified formula na ca 4 8 al6si6 o s 24 so4 cl 1 2 scapolite minerals silvialite ca na 4al6si6o24 so4 co3 and s bearing apatite ca5 po4 3 f cl oh barite baso4 has been mentioned in rare cases marchev 1991 sulfur bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals the crystallization of sulfide sulfate and s bearing minerals strongly depends on melt composition temperature and pressure and the s speciation in melt which in turn is strongly dependent on the prevailing oxygen fugacity baker and moretti 2011 this volume wilke et al 2011 this volume irrespectively of the low abundance of s bearing minerals the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates these minerals are critical for estimating the activity of various sulfur bearing species in magmas and can be used to constrain the oxygen fugacity and the s concentration in the melt e g pre eruptive sulfur concentration in melts the presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are s2 or s6 respectively typically one of these species is dominant but there are conditions where s2

a reassessment of models for hydrocarbon generation in the khibiny nepheline syenite complex kola peninsula russia although hydrocarbon bearing fluids have been known from the alkaline igneous rocks of the khibiny intrusion for many years their origin remains enigmatic a recently proposed model of post magmatic hc generation through fischer tropsch type reactions suggests a primary co2 fluid this hypothesis is based upon textural data gas bulk and isotopic composition and low entrapment temperature of fluid inclusions that indicate an abiogenic origin for the hc gases however new petrographic microthermometric laser raman bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of hc generation the gas phase is dominated by ch4 with only minor proportions of higher hydrocarbons no remnants of the proposed primary co2 rich fluid are found anywhere in the complex primary and secondary fluid inclusions show similar compositions and entrapment conditions the majority of the fluid inclusions are of secondary nature and trapped in healed microfractures this indicates a high fluid flux after magma crystallisation entrapment condtions for primary fluid inclusions are 450 550 c at 2 8 4 5 kbar these temperatures are too high for hc gas generation through the ft reaction chemical analyses of rims of fe rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation furthermore there is no clear relationship between the presence of fe rich minerals and fluid inclusion abundance as reported elsewhere fluid inclusion planes generally start and terminate at mineral boundaries 13c values for methane range from 22 4 to 5 4 confirming also an abiogenic origin for the gas these data support an abiogenic hc origin but are not consistent with a post magmatic ft type reaction the presence of primary ch4 dominated fluid and melt inclusions which contain a methane rich gas phase indicate that a magmatic origin is more likely an increase in methane content together with a decrease in 13c isotope values towards the intrusion margin suggest that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks

xiongcun tibet a telescoped system of veinlet disseminated cu au mineralization and late vein style au ag polymetallic mineralization in a continental collision zone abstract the xiongcun cu au deposit occurs in the western part of the gangdese porphyry and skarn cu au mineralization belt in the himalayan tibetan orogen and was formed in an intracontinental orogenic environment related to the india asia collision sericite 40 ar 39 ar dating of the xiongcun deposit indicates that ore formation occurred around 38 11 0 90 ma much earlier than the main ore forming ages 20 to 14 ma of the gangdese cu au metallogenic belt the potassium feldspar k ar age of the sulfide bearing pegmatoid dyke in the xiongcun district is 47 62 0 70 ma these ages indicate that the mineralization in xiongcun is related to relaxation ca 52 to 40 ma and subsequent uplift 40 to 38 ma of the orogen in the xiongcun deposit mineralization hosted by jurassic pyroclastic rocks and controlled by nw striking faults can be divided into two styles i e veinlet disseminated cu au mineralization and vein style au ag polymetallic mineralization the former is associated with local remnant k silicate alteration extensive phyllic alteration and peripheral widespread propylitization the latter overprinting the former is associated with strong silicification chloritization and kaolinization the early veinlet disseminated cu au mineralization is similar to porphyry type mineralization and has a mineral assemblage of quartz sericite pyrite chalcopyrite bornite magnetite pyrrhotite sphalerite rutile anhydrite the late vein style au ag polymetallic mineralization characterized by a mineral assemblage of quartz pyrite pyrrhotite sphalerite rich in fe chalcopyrite galena native gold electrum chlorite kaolinite barite calcite siderite hematite belongs to a sulfide rich low sulfidation type epithermal system the h o and s isotopic compositions and high salinities of fluid inclusions indicate a magmatic contribution and increasing contents of meteoric water in the mineralized system from early to late stage the wide range of homogenization temperatures 380 to 120 c homogenization pressures 45 92 to 1 94 bar and salinities 36 61 to 1 23 wt nacl equiv suggest rapid uplift and are consistent with the geological setting because of uplift the hydrothermal system of xiongcun evolved from a porphyry system to an epithermal system resulting in an immature porphyry system and the telescoping of late vein style au ag polymetallic mineralization onto the early veinlet disseminated cu au mineralization

helium neon and argon systematics of the european subcontinental mantle implications for its geochemical evolution in this study we present a comprehensive noble gas study of mantle xenoliths from various european cenozoic volcanic provinces the main body of samples is from the massif central france and the eifel germany smaller subsets of samples are from spitsbergen and the graz basin austria in all the helium neon and argon isotopic abundances of a total of forty five mantle xenoliths phenocrysts and xenocrysts were determined the 3he 4he ratios within each volcanic province are very uniform irrespective of the diverse lithologies and p t conditions which are represented by our sample suite mean 3he 4he ratios of the massif central eifel spitsbergen and kapfenstein are 6 53 25 6 03 14 6 65 25 and 6 1 7 1 times the atmospheric ratio ra respectively the strontium and neodymium isotopic composition of some of the cpx separates are highly variable and therefore in contrast to the uniform he signature we thus conclude that he and probably also the other noble gases in the xenoliths are effectively decoupled from the non volatile elements therefore the he signature that is preserved in the xenoliths is actually that of their host magmas published strontium neodymium and lead isotope data of the unevolved host magmas of the xenoliths correlate well with our xenolith he data the position of the fields of the investigated volcanic provinces in hesr hend and hepb variation diagrams depict ternary mixtures between dmm em himu endmembers as the source the host magmas and their volatiles the neon isotopic composition of the gases released from the xenoliths is in most cases atmospheric and probably reflects atmospheric contamination only a few samples reveal indications for morb type ne or evidence of mass fractionation the 40ar 36ar ratios of the xenoliths are mostly radiogenic with the highest ratio being 17 000 however all samples have suffered a certain degree of atmospheric contamination we calculate that the uncontaminated 40ar 36ar ratio of the european subcontinental mantle is 30 000 the4he 40ar ratios have been fractionated since with values around 0 25 they are below both the current production ratio and the morb ratio they cannot be explained by atmospheric contamination as the samples with high 40ar 36ar ratios also have low 4he 40ar ratios helium and ar were probably fractionated during initial closed system degassing of the host magma we calculate the original unfractionated 4he 40ar ratios of the massif central and eifel magma source which are 2 56 18 and 2 71 17 respectively the noble gas systematics are compatible with the notion that addition of crustal material has modified a former morb source signature we use our mean 3he 4 he ratios and reconstructed 4he 40ar ratios to model the time and degree of contamination of a morb like source with crustal material combining our model with evidence from published oxygen strontium neodymium and lead isotope data we judge that the source of the massif central and eifel magmas became enriched in k u and th shortly prior or during the variscan orogeny pre collisional collisional in this case the enrichment in these elements is therefore most probably related to subduction of crustal material in a destructive continental margin setting

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

apparent polar wander and reversal stratigraphy of the palaeo mesoproterozoic southeastern mcarthur basin australia abstract approximately 1800 samples from volcanic clastic and carbonate sequences of the southeastern mcarthur basin in northern australia were analysed palaeomagnetically to define in detail the australian apparent polar wander path apwp for a period centred at 1670 ma and to develop a proterozoic reversal stratigraphy for correlation and dating twelve palaeomagnetic poles are interpreted as primary including two that are preliminary these increase the australian palaeomagnetic database for the proterozoic by 30 another ten poles record three or possibly four periods of overprinting the study also yields one of the oldest reversal records reported so far the ten better established primary poles were obtained from the lower three of the four main stratigraphic sub divisions of the basin the tawallah mcarthur and nathan groups these poles define an angular trajectory of about 120 for a time span of at least 75 m y and possibly exceeding 100 m y they confirm earlier preliminary results that suggested a major revision of the proterozoic apwp for australia the two preliminary poles were obtained from the youngest sub division of the basin the roper group and confirm a large age gap between the nathan and roper groups the poles for the oldest overprints were from igneous units in the tawallah group and appear to record metasomatism at the end of the last major period of volcanism in the basin the pole for the next overprint falls on the path near the apex of a hairpin bend it coincides with a likely period of rifting and high heat flow that accompanied the deposition of the upper mcarthur group the third overprint apparently acquired during the long time break separating the nathan and roper groups was found mostly in red dolomitic rocks the geomagnetic reversal record from this study represents an accumulated stratigraphic thickness of 2100 m excluding gaps where the polarity remained undetermined because of overprinting unstable remanences intervals not sampled or other causes reversal patterns from duplicate sections at different localities were compared for two formations and were found to be consistent several parts of the reversal column appear to be distinctive enough for use in stratigraphic correlation

melt and fluid inclusions in hydrothermal veins the magmatic to hydrothermal evolution of the elatsite porphyry cu au deposit bulgaria the magmatic to hydrothermal transition in the late cretaceous elatsite porphyry cu au mo platinum group element deposit has been studied in a suite of samples with clear timing relations between porphyry dikes magmatic hydrothermal veins silicate melt inclusions in quartz veins fluid inclusion generations and ore minerals ore mineralization occurs late at temperatures 200 to 300 c below those of the early multistage interplay between magmatic and hydrothermal processes at near magmatic temperature pressure conditions crosscutting relations and petrography indicate that shortly after the intrusion of the earliest monzodioritic dikes fluids precipitated a first generation of granular quartz veins accompanied by potassic alteration the second vein generation with crystalline quartz textures and k feldspar alteration halos formed at the same time as the second granodioritic pulse of porphyry intrusions cathodoluminescence imaging of quartz growth textures reveals that the earliest fluid inclusions in the crystalline quartz veins are of intermediate density and 8 wt nacl equiv salinity probably trapped at near lithostatic pressures of 1 200 to 1 300 bars at near magmatic temperatures 730 c and implies a depth of 4 to 5 km depressurization led to phase separation indicated by a first generation of coexisting brine and vapor inclusions trapped at temperatures of 640 c and suprahydrostatic pressures of 920 bars a second quartz generation in crystalline quartz veins precipitated during progressive depressurization and hosts assemblages of coexisting brine vapor and silicate melt inclusions trapped at temperatures in excess of 600 c and suprahydrostatic pressures of 630 to 880 bars some open spaced quartz veins were filled with aplite during this stage field relations and geochemical evidence suggest that the aplites as well as the silicate melt inclusions in hydrothermal quartz veins represent volumetrically minor residual melts that evolved directly from granodiorite porphyries at the level of deposit formation and do not represent aliquots of metal supplying magma at depth fluid inclusions coexisting with the silicate melt inclusions are metal rich but these fluids predate sulfide precipitation and are therefore not the dominant fluids responsible for the cu au mineralization at elatsite melt fluid metal separation processes recorded in these co trapped silicate melt and fluid inclusions in vein quartz are small scale local phenomena and do not appear to be suited for understanding and quantifying metal segregation in the deeper source microthermometry data of fluid inclusions trapped during the later stages of the formation of the second quartz generation show further depressurization to 260 to 325 bars and cooling to 460 c representing a hot hydrostatic regime bornite chalcopyrite and magnetite seem to be slightly later precipitated at temperatures 460 c in separate veins or opened spaces of preexisting veins dissolution textures of the second quartz generation indicate a subsequent local redissolution of vein quartz most likely as a result of passing through a window of retrograde quartz solubility upon further cooling below 460 c the next economically most important chalcopyrite pyrite stage is largely devoid of quartz precipitation and is mostly expressed as paint veins due to the lack of fluid inclusions the temperature pressure conditions of formation of these veins could not be constrained the waning phase of hydrothermal activity is represented by a quartz carbonate zeolite stage formed at low temperature 145 c as indicated by microthermometry of fluid inclusions trapped in quartz from this stage laser ablation inductively coupled plasma mass spectrometry analyses of fluid inclusions show very high cu contents in early intermediate density fluid inclusions and in the first vapor inclusion generation followed by a drastic decrease in the second generation of vapor inclusions the decrease correlates with the appearance of anhydrite inclusions in the second quartz generation and may indicate that a lack of sulfur in these later vapor inclusions led to cu partitioning into the brine alternatively it is also possible that unlike early vapor inclusions the later vapor inclusions were not susceptible to postentrapment copper enrichment in accordance with recent experiments a progressive cu enrichment in the brine phase correlates well with depressurization prior to mineralization mass balance considerations based on analyzed fluid components and fluid phase relations indicate that brine was volumetrically minor and therefore likely stagnant but this may have prepared ore precipitation by accumulating cu stripped from ascending vapor

tappania bearing association of the siberian platform biodiversity stratigraphic position and geochronological constraints abstract a new fossil microbiota yielding eukaryotic tappania is described from the mesoproterozoic kamo group of the central angara basin siberian craton the assemblage also includes valeria satka lophosphaeridium pulvinosphaeridium miroedichia osculosphaera tasmanites obruchevella glomovertella as well as branching and anastomosing filamentous microfossils of putative fungal affinities and peculiar short trichomes that together substantially increase the biodiversity of the tappania association stratigraphic position and geochronological constraints indicate a relatively young late mesoproterozoic age of the tappania bearing sediments there appears to be a continuity between the tappania association and the neoproterozoic microbiota the latter being firmly rooted in the late mesoproterozoic

petroleum geology and geochemistry of middle proterozoic mcarthur basin northern australia ii assessment of source rock potential five potential source rocks have been discovered in the middle proterozoic of the mcarthur basin the lacustrine barney creek formation mcarthur group and the marine velkerri formation roper group compare favorably in thickness and potential with demonstrated petroleum source rocks in the phanerozoic the former contains up to 7 total organic carbon toc and kerogen types i and ii toc values in the latter range up to 6 5 and the kerogen is type ii petrographic examination shows the organic matter is mostly lamalginite which on maturation yields nonfluorescent lamalginite and bitumen extractable hydrocarbon yields from mature samples indicate that good to excellent source rocks are present and there is ample evidence of migration having occurred tmax values derived from rock eval analysis may be used to define maturation levels in much the same way as in phanerozoic sediments at low maturation levels the reflectivities of lamalginite and bitumen are lower than vitrinite reflectance values calculated from methylphenanthrene indices but converge in the mature zone this convergence reflects both the inherent lower reflectivity of hydrogen rich macerals and the source control on methylphenanthrene indices at low maturity maturation levels in mcarthur group sediments vary from marginally mature to overmature with abrupt changes over short distances associated with faults hydrocarbon generation occurred prior to the deposition of the younger proterozoic sediments of the roper group in contrast the source rocks in the roper group re marginally mature to mature throughout the study area and are overmature only where they are affected by igneous intrusions considerations of burial history suggest that hydrocarbon generation may have occurred during deposition of the roper group although an early paleozoic timing cannot be excluded

reduced fluids in porphyry copper gold systems reflect the occurrence of the wall rock thermogenic process an example from the no 1 deposit in the xiongcun district tibet china abstract high fo2 conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals such as magnetite and anhydrite in contrast the no 1 porphyry cu au deposit in the xiongcun district tibet china has abundant pyrrhotite reduced fluids ch4 co2 and a relative lack of highly oxidized minerals which are indicative of low fo2 conditions scanning electron microscopy fluid inclusion c h o he ar isotopes and whole rock organic carbon contents and isotope analysis were used to constrain the evolution of ore forming fluid the origin of ch4 and metal deposition mechanisms for the no 1 deposit the he ar isotopic compositions 3he 4he 0 11 0 96 ra 40ar 36ar 418 7 2920 2 suggest that the ore forming fluids predominantly derived from crust source with minor mantle input the h o isotopic analysis results 18oh2o 1 8 to 5 2 d 106 to 89 9 indicate that the ore forming fluids were derived from a magmatic source that mixed with some meteoric waters the element compositions of zircons and fluid melt inclusions from the mineralized middle jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log fo2 nno conditions the quartz diorite porphyry intruded into the carbon bearing wall rocks produced abundant ch4 by thermal decomposition of organic matter 13cch4 26 3 to 28 5 which changed the redox state of the porphyry copper system from oxidized to reduced condition ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure

the strangways impact structure northern territory australia geological setting and laser probe 40ar 39ar geochronology abstract the strangways impact structure occurs within the mcarthur sedimentary basin northern territory australia the target rocks are primarily quartzites and subordinate shale siltstone sequences of the mesoproterozoic roper group these are primarily underlain by granitoid gneiss which is exposed in a 9 11 km diameter central uplift exposure is relatively limited and the upper parts of the impact structure have been removed by erosion based on the extent of the disturbance of the regional geology the diameter of strangways is estimated to be 24 26 km it is thus a complex crater and probably a central peak basin stratigraphic constraints place the time of impact as post roper group 145 ma in order to better define the time of impact infrared laser spot fusion 40 ar 39 ar data have been obtained from two impact melt lithologies and a highly shocked breccia clast these yield a combined age of 646 42 ma biotite dated from the granitoid gneiss central uplift yields 1686 1237 ma with the younger age reflecting the effects of shock induced resetting and or hydrothermal alteration due to hot melt sheet water interaction following impact the new age of 646 42 ma for strangways places it firmly in the neoproterozoic rather than being ordovician or younger as previously proposed

characterisation of fluid inclusion oil in a mid proterozoic sandstone and dolerite roper superbasin australia detailed geochemical analysis has been carried out on fluid inclusion oils trapped for over 1 billion years in the roper superbasin northern australia in two different geological settings in healed microfractures in a quartz arenite and within partially altered plagioclase laths in a dolerite sill the oil inclusions and co occurring solid bitumens record multiple oil migration events postdating an intrusive episode ca 1300 ma most likely during neoproterozoic basin inversion solid bitumen represents the oldest migration event and occurs in secondary pore space within the bessie creek sandstone and in veins cross cutting the dolerite sill it is interpreted to represent thermally altered oil that may have at least partly been generated by contact metamorphism near the igneous intrusion inclusion oils in both the dolerite feldspar and in the transgranular fractures in the bessie creek sandstone are pristine nonbiodegraded fluids with a great similarity to other bitumens oils and sediment extracts from the roper superbasin biomarkers indicate that the oils were mainly sourced from prokaryotic cyanobacterial organic matter with a small contribution from eukaryotes based on migration distance organic richness geochemistry and similarity of source rock maturity the velkerri formation which directly overlies the dolerite sill and the bessie creek sandstone is the most likely source rock for the higher molecular weight hydrocarbons preserved in the fluid inclusion oils geochemical data indicate that the oil inclusions in clusters in albitised zones of labradorite crystals in the dolerite matrix represent a different migration event from oil inclusions in transcrystalline fractures in vein calcite in the dolerite sill and in transgranular fractures in quartz in the bessie creek sandstone the bessie creek sandstone fluid inclusion oil was generated in the peak to late oil window 0 9 1 0 vre the higher molecular weight c13 proportion of the dolerite feldspar fluid inclusion oil is of lower maturity 0 8 vre than the same molecular weigh range fluid in the bessie creek sandstone fluid inclusion oil both of these oils were most likely generated in the overlying velkerri formation however the volumetrically dominant low molecular weight hydrocarbons in the dolerite feldspar fluid inclusion oils have a higher maturity 1 3 vre and may have been generated from the deeper ca 1600 ma barney creek formation in the underlying mcarthur group this study shows that oil trapped in fluid inclusions has a greater potential to preserve pristine organic geochemical information than porosity bound oils and dispersed kerogen if oil inclusions occur in sufficient abundance the analysis of these tiny oil encapsulations is technically feasible and can give valuable information on the composition of the earth s early biosphere and precambrian petroleum systems

spatial and temporal trends in precambrian nitrogen cycling a mesoproterozoic offshore nitrate minimum abstract fixed nitrogen is an essential nutrient for eukaryotes as n 2 fixation and assimilation of nitrate are catalyzed by metalloenzymes it has been hypothesized that in mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses restricting eukaryotes to near shore environments and limiting their evolutionary innovation to date this hypothesis has only been tested in the belt supergroup 1 4 ga with results that support an onshore offshore nitrate gradient as a potential control on eukaryote ecology here we present bulk nitrogen and organic carbon isotopic data from non isochronous cross basinal facies across the bangemall 1 5 ga and the roper 1 4 1 5 ga basins to better understand the extent and variability of onshore offshore nitrogen isotope gradients in the mesoproterozoic both basins show an average 1 2 enrichment in 15 n bulk from deep to shallow facies with a maximum range from 1 offshore to 7 5 onshore unlike the belt basin the bangemall and roper basins show some offshore 15 n bulk values that are enriched beyond the isotopic range associated with biological n 2 fixation alone this suggests a mixture of aerobic and anaerobic metabolisms offshore in shallow waters where 15 n bulk enrichment peaks an aerobic nitrogen cycle was evidently operating even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the bangemall and roper basins our data are consistent with a lateral gradient in nitrate availability within the photic zone with higher concentrations in near shore environments than offshore the variability in 15 n bulk values in each depositional environment and the consistently low 15 n bulk values from mesoproterozoic units compared to the paleoproterozoic and neoproterozoic suggest that nitrate concentrations in the global ocean were likely low this trend is now seen in all three mesoproterozoic basins so far examined and contrasts with the paleoproterozoic and neoproterozoic where nearly all 15 n bulk data plot above the n 2 fixation window thus we propose that the mesoproterozoic ocean was characterized by a nitrate minimum with the lowest concentrations in offshore environments this inference is consistent with a mesoproterozoic o 2 decline following a temporary paleoproterozoic o 2 peak and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near shore environments delaying their rise to ecological dominance until the neoproterozoic era

multiple origins of methane in the earth methane occurrences in the earth s crust are predominantly of biogenic origin i e their ultimate source is biologically formed organic matter methane can also form through inorganic reactions and is consequently termed abiogenic biogenic methanes can either form through bacterial or thermogenic processes bacterial processes follow a co2 reduction and or fermentation pathway the fermentation processes are quantitatively more important in recent fresh sediments and swamps methane formed by co2 reduction however is most common in older sediments and commercial gasfields temperature organic substrate and age may be the major factors controlling the relative importance of the two pathways stable isotope concentrations in thermogenic methanes seem to be controlled by the extent of conversion of organic matter the timing of gas expulsion and trapping the different character of methane in individual sedimentary basins may be a result of the geologic history geothermal methanes are most likely derived from pyrolysis of organic matter abiogenic methane occurs in hydrothermal vents and ophiolite complexes inorganic reactions either surficial or deep seated are the likely source of such methanes a uniform mantle origin of methane is not supported by the observed isotope variations in naturally occurring methanes

geochemistry of oil in fluid inclusions in a middle proterozoic igneous intrusion implications for the source of hydrocarbons in crystalline rocks abstract the ca 1280 ma dolerite sill within the mesoproterozoic roper group in the roper superbasin australia contains evidence for at least two episodes of hydrocarbon migration represented respectively by solid bitumen with a ketone rich extract and a mixture of a high maturity gas condensate and a lower maturity oil within oil bearing fluid inclusions the ketone isomers are formed by flash pyrolysis of kerogen during the intrusion of the dolerite sill org geochem 21 1994 829 and represent the first and oldest phase of hydrocarbon migration the gas condensate and oil were subsequently trapped as a mixture within fluid inclusions at diagenetic temperatures and pressures of around 110 c and 250 bars significantly after cooling of the sill and likely during the neoproterozoic reactivation of the roper superbasin either 1 these fluids migrated together and mixed in the reservoir or 2 an earlier oil charge was flushed by a later condensate charge and the oil condensate mixture was trapped within single fluid inclusions oil inclusions occur chiefly within albitised zones of labradorite laths within the dolerite matrix and within transcrystalline microfractures cutting vein calcite and rarely vein quartz oil inclusions trapped in the vein calcite are accompanied by hypersaline ca mg brines gas chromatography mass spectrometry of oil extracted from inclusions within the dolerite matrix shows that the oil is non biodegraded and was therefore trapped relatively quickly within the host minerals trace amounts of biomarkers indicate that the inclusion oil is of a biogenic origin and excludes any abiotic processes that are apparent sources of hydrocarbons in many crystalline rocks monomethylalkanes pentacyclic terpanes chiefly comprising hopanes and diahopanes and very low concentrations of steranes and diasteranes indicate input from cyanobacterial organic matter with a minor contribution from eukaryotes the hydrocarbons are likely derived from proterozoic source rocks such as the directly overlying velkerri formation and or the underlying barney creek formation from the mcarthur group the study has implications for the source of hydrocarbons in non sedimentary rocks and suggests that fluid inclusions can be used not only to distinguish between biogenic and abiogenic sources of hydrocarbons but also to provide critical information about the composition of the biomass fluid migration events and conditions under which hydrocarbons in different geological settings are trapped

the paleoecology and significance of the gunflint type microbial assemblages from the frere formation early proterozoic nabberu basin western australia abstract eight distinct microbial assemblages are preserved in the frere formation which was deposited in the nabberu basin western australia these assemblages contain taxa animikiea eoastrion gunflintia and huroniospora typical of gunflint type microbial assemblages which have been described from australia canada and south africa four assemblages are interpreted as primary composed of benthic microorganisms and four as secondary containing components that were transported before deposition these microbial assemblages have similar paleoecologies to other gunflint type microbial communities all gunflint type microbial assemblages including the frere assemblages were deposited in a marine subtidal environment the apparent worldwide distribution of these similar microbial assemblages supports the hypothesis that gunflint type microbiotas were characteristic of the biota living in subtidal settings during the early proterozoic however this hypothesis does not account for the lack of these assemblages in post early proterozoic rocks this may be explained by the absence of primary inorganic silica precipitation in open marine settings during the middle and late proterozoic primary silica precipitation may be necessary for the preservation of well preserved microbial assemblages in marine settings of normal salinity silica precipitation in early proterozoic open marine settings is commonly associated with banded iron formation deposition which is confined mainly to 2500 2000 ma ago

constraints on origin and evolution of red sea brines from helium and argon isotopes brines from three depressions along the axis of the red sea the atlantis ii the discovery and the kebrit deep were sampled and analyzed for helium and argon isotopes we identified two principally different geochemical fingerprints that reflect the geological setting of the deeps the atlantis ii and the discovery brines originating from locations in the central red sea show 4 he concentrations up to 1 2u10 35 cm 3 stp g 31 and a 3 he 4 he ratio of 1 27u10 35 the morb like 3 he 4 he ratio is typical of an active hydrothermal vent system and clearly indicates a mantle origin of the helium component within the brines 40 ar 36 ar ratios are as high as 305 implying that mantle derived 40 ar excesses of up to 3 of the total argon concentration are present in the brines and transported along with the mantle helium signal the mean 4 he 40 ar excess ratio of 2 1 is in agreement with the theoretical mantle production ratio in the kebrit deep located in the northern red sea we found a helium excess of 5 7u10 37 cm 3 stp g 31 the low 3 he 4 he ratio of 1u10 36 points to a predominantly radiogenic source of the helium excess with only a minor mantle contribution of approximately 9 we propose a new scenario assuming that the kebrit brine accumulates a diffusive helium flux that migrates from deeper sedimentary or crustal horizons in contrast to the atlantis ii and discovery deep the kebrit brine shows no sign of an active hydrothermal input fl 2001 elsevier science b v all rights reserved

jurassic u pb and re os ages for the newly discovered xietongmen cu au porphyry district tibet prc implications for metallogenic epochs in the southern gangdese belt the gangdese belt in southern tibet includes the cretaceous to early eocene gangdese continental magmatic arc a newly recognized middle jurassic magmatic arc and post gangdese arc magmatism of miocene age that is well known for its association with important copper molybdenum deposits although the tectonic history of the gangdese continental arc is relatively well constrained the tectonomagmatic setting of the jurassic and miocene magmatic events was poorly constrained previously geochronological analysis of samples from the xietongmen copper gold porphyry district has identified a previously unknown metallogenic event related to middle jurassic arc magmatism the occurrence of several metallogenic epochs within the southern gangdese belt highlights the importance of systematic metallogenic studies of this belt

proterozoic source rocks sedimentology and organic characteristics of the velkerri formation northern territory australia the velkerri formation 1 43 ga in northern australia is one of the organically richest pro terozoic successions in australia and over the last 10 yr has been a target for oil exploration this formation is predominantly a glauconitic marine shale deposited in restricted to anoxic conditions in a deep water pro delta slope to basin environment the velkerri formation is composed of superimposed sediment couplets made up of laminated black organic rich mud shales total organic carbon toc 4 6 and laminated gray green organically lean mud shales toc 2 the couplets stack at scales ranging from centimeters to meters of particular interest are three black organic rich mud shales in the middle velkerri member tens of meters thick and correlatable over much of the drilled area of the basin their high organic contents reflect changes in organic productivity vs the rate of sediment supply and not fluctuating oxygen levels in the bottom waters or changes in water depth extractability and rock eval data show more efficient expulsion of hydrocarbons from the upper two of the three organic rich units in the middle velkerri evidence also exists for local intraformational migration of hydrocarbons into siltstone layers nevertheless the middle velkerri has very low permeability and porosity and still retains a significant fraction of its generated hydrocarbons especially in the lower portions of the middle velkerri where a volcanogenic provenance led to the early blocking of pore throats by clay diagenesis during early burial this study emphasizes the high quality of middle proterozoic source rocks for petroleum exploration in northern australia the main problem is finding traps that can preserve hydrocarbons over the long periods of time since the rocks became charged with hydrocarbons

hydrocarbon shows and petroleum source rocks in sediments as old as 1 7 109 years we report the discovery of indigenous live oil in 1 4 gyr old rocks in the mcarthur basin of northern australia previously reported occurrences of indigenous precambrian oil are 1 gyr old1 5 potential petroleum source rocks in the mcarthur basin range in age from 1 4 to 1 7 gyr and were deposited in marine and lacustrine environments in parts of the basin they have been buried sufficiently deeply to have generated hydrocarbons they span the period corresponding to the appearance of eukaryotic organisms and because of their low degree of thermal alteration they provide a valuable resource for the study of primitive biota through their hydrocarbon biomarkers the hydrocarbon composition of the oil is consistent with a derivation from organic matter of prokaryotic origin these results show that exploration of previously ignored mid proterozoic sediments may lead to the discovery of new reserves of oil

kotuikan formation assemblage a diverse organic walled microbiota in the mesoproterozoic anabar succession northern siberia abstract abundant and diverse microfossils from shales of the lower member of the 1500 ma old kotuikan formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the kotuikan assemblage is dominated by prokaryotic cyanobacteria both filamentous oscillatorialeans and nostocaleans which are represented primarily by cellular trichomes and by empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens however unlike mesoproterozoic microbiota of shallow water settings the kotuikan compressed organic walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans as well as other morphologically complex microorganisms the kotuikan assemblage contains numerous taxa previously recorded from neoproterozoic successions e g elatera eosolena palaeastrum pterospermopsimorpha rugosoopsis and navifusa but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits the kotuikan assemblage documents the status of the marine biosphere at an early mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic microbiota as reported in this study 35 taxa were identified in the kotuikan assemblage of which five forms are described informally that are assigned to 18 genera of microscopic prokaryotes and eukaryotes two new genera and five new species are proposed hirudiforma lancetica gen and sp nov h simmetrica gen and sp nov lineaforma elongata gen and sp nov elatera minor sp nov eosolena minuta sp nov

differential exhumation histories between qulong and xiongcun porphyry copper deposits in the gangdese copper metallogenic belt insights from low temperature thermochronology abstract the qulong and xiongcun porphyry deposits in the gangdese copper molybdenum metallogenic belt gcmb are both giant porphyry deposits they were located in the eastern and the central gcmb about 400 km apart and were generated in the miocene and the jurassic respectively in order to reveal why the two asynchronous deposits could occur in the same metallogenic belt the in situ zircon u pb zircon and apatite fission track zft and aft and apatite u th he ahe dating were conducted their associated exhumation amounts and histories were compared and finally the prospecting significance was evaluated ahe ages of the qulong deposit indicate it underwent an amount of at least 1 3 2 8 km exhumation since 13 8 13 1 ma however aft data and modeling results reveal that the xiongcun deposit experienced two stages of rapid cooling at 20 13 ma and 13 8 ma respectively the temperature decreases from 160 c to 40 c after 20 ma and the corresponding exhumation amount is about 4 6 km integrated the regional thermochronological data exhumation amounts of 4 6 km and

geology of the jiama porphyry copper polymetallic system lhasa region china abstract the jiama deposit located in the eastern part of the well known gangdese metallogenic belt on the tibetan plateau is the largest porphyry cu polymetallic system in the region with the largest exploration budget and is economically viable in the gangdese belt to undergo large scale development the deposit is well preserved and has experienced little erosion the proven resources of the deposit are 7 4 mt cu 0 6 mt mo 1 8 mt pb zn 6 65 moz au and 360 32 moz ag the results presented in this paper are based on geological and tectonic mapping geological logging and other exploration work performed by members of the jiama exploration project team over a period of 6 years we propose that the jiama porphyry cu polymetallic system is composed of skarn cu polymetallic hornfels cu mo porphyry mo cu and distal au mineralization the development of skarn cu polymetallic orebodies at the jiama deposit was controlled mainly by the contact zone between porphyries and marbles an interlayer detachment zone and the front zone of a gliding nappe structure the hornfels cu mo and porphyry mo cu orebodies were controlled mainly by a fracture system related to intrusions and the distal au mineralization resulted from late stage hydrothermal alteration on the basis of field geological logging optical microscopy and chemical analysis we verify that the alteration zones in the jiama deposit include potassic phyllic propylitic and argillic alteration with a local lithocap as well as endoskarn and exoskarn zones the endoskarn occurs mainly as epidote alteration in quartz diorite porphyry and granite porphyry and is cut by massive andradite veins the exoskarn includes garnet pyroxene and wollastonite skarn in which the mineralogy and mineral chemical compositions display an outward zonation with respect to the source porphyry from the proximal skarn to the intermediate skarn to the distal skarn the garnet pyroxene ratio varies from 20 1 to 10 1 to 5 1 the garnet color varies from red brown to brown green to green yellow and the average composition of garnet varies from ad80 1gr18 9 sp py 1 0 to ad76 3gr23 sp py 0 7 to ad59 5gr39 5 sp py 1 0 respectively the pyroxene is not as variable in composition as the garnet and is primarily light green to white diopside with a maximum hedenbergite content of 20 and an average composition of di88 6hd8 9jo2 5 from the proximal skarn to the intermediate skarn to the distal skarn the mineralization changes from cu mo to cu mo to pb zn cu au ores respectively the wollastonite skarn displays no zonation and hosts mainly bornite mineralization the cu and mo mineralization is closely related to the potassic and phyllic zones in the porphyry hornfels zircons from four mineralized porphyries yield u pb ages of 15 96 0 5 ma 15 72 0 14 ma 15 59 0 09 ma and 15 48 0 08 ma the re os ages of molybdenite from the skarn hornfels and porphyry are 15 37 0 15 ma 14 67 0 37 ma and 14 66 0 27 ma respectively the present results are consistent with the findings of previous research on fluid inclusions isotopes and other such aspects on the basis of the combined evidence we propose a porphyry cu polymetallic system model for the jiama deposit and suggest a regional exploration strategy that can be applied to prospecting for porphyry skarn mineralization in the lhasa area

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

mineralogical evidence for crystallization conditions and petrogenesis of ilmenite series i type granitoids at the baogutu reduced porphyry cu deposit western junggar nw china mossbauer spectroscopy epm and la mc icpms analyses abstract primary ore forming minerals retain geochemical signatures of magmatic crystallization information and can reveal the petrochemical conditions prevalent at the time of their formation the baogutu deposit is a typical reduced porphyry cu deposit amphibole and biotite fe 3 fe ratios minerals feldspar biotite amphibole zircon and apatite in situ elemental and apatite nd isotopic compositions were determined by mossbauer spectroscopy electron probe microanalysis and laser ablation multiple collection inductively coupled plasma mass spectrometry respectively to investigate the magma oxidation state petrogenesis source features and to constrain the carbon species at magmatic stages for the intrusive phases the results show that the primary plagioclase and amphibole in the mineralized diorite to granodiorite porphyry and post ore hornblende diorite porphyry are distinct an 26 55 versus an 60 69 mg hornblende versus tschermakite in particular the amphibole shows distinct major and trace element compositions with light rare earth element enrichments and negative eu anomalies in mg hornblende and light rare earth element depletions and no eu anomalies in tschermakite all the analyzed biotites are primary igneous phases with a biotite phenocryst profile showing significant variations of zn cr sc and sr from core to rim these results may indicate the occurrence of mixing between two distinct magmas during mineral formation titanium in zircon and si in amphibole thermometries indicate that magma crystallized at 900 c and continued to 650 c in situ apatite nd isotope end t 5 6 7 6 t dm2 620 460 ma indicate absence of significant reduced sedimentary contamination and the source of juvenile lower crust slightly decreasing fe 3 fe ratios from biotite and amphibole to whole rock indicate decreasing oxygen fugacity during magma crystallization recalculated biotite compositions according to fe 3 fe ratios indicate f o 2 values of less than ni nio buffer nno which show slightly lower values than that estimated according to zircon melt distribution coefficients ce anomalies nno 0 6 these values are consistent with the features of reduced porphyry cu deposits crystallization of other mineral phases significantly affects the reliability of oxybarometer of zircon melt distribution coefficients eu anomalies and mn contents in apatite this oxidation state suggests that only co 2 was present at the magmatic stage and implies that ch 4 formed during co 2 reduction occurring later hydrothermal alteration the alteration of primary amphibole to actinolite released ti al fe mn na and k to the fluid with later precipitation of titanite albite and minor ilmenite and magnetite during actinolite alteration

collision related genesis of the sharang porphyry molybdenum deposit tibet evidence from zircon u pb ages re os ages and lu hf isotopes article i nfo sharang is a low fluorine calc alkaline porphyry mo deposit hosted mainly in a granite porphyry of a multi stage plutonic complex in the northern gangdese metallogenic belt largely with stockwork and ribbon textured mineralization the observed age estimates suggest that the formation of the magmatic host complex 52 9 51 6 ma and the ore deposit itself 52 3 ma occurred during the main stage of the india asia collision the host rocks are characterized by lower zircon ehf t values than those of the pre ore and post ore rocks this suggests that the lhasa terrane basement might play an important role in the formation of sharang ore forming intrusions in view of the framework of magmatic metallogenic events we suggest that slab roll back may have induced melting of juvenile crust and ancient continental complexes during the india asia collision this proposal focuses exploration for additional molybdenum deposits on the collision zone

the use of rbsr and kar dating methods as a stratigraphic tool applied to sedimentary rocks and minerals abstract rb sr and k ar methods can be used for direct numerical chronostratigraphy of sediments using the clay silicates they contain the interpretation of the ages calculated depends on the satisfaction of the following requirements 1 1 the series studied must include diagenetic clay sediments coarsely detrital or alightly metamorphic formations must be excluded studies in progress concerning decarbonatation may lead to the application of direct dating of sediments being extended to carbonate sediments 2 2 careful mineralogical study of the clay minerals must prove early diagenetic recrystallization or new formation this part of the study must precede any isotopic work 3 3 the isotopic study must involve the use of both rb sr and k ar methods the extreme sensibility of the k ar isotopic system to late diagenesis helps to choose between early or late diagenesis a younger k ar age may generally be regarded as that of the late diagenesis whereas the rb sr age may be that of the early diagenesis 4 4 the initial 87sr 86sr isotopic ratio is not always a reliable criterion owing to the effects of the dilution of the radiogenic 87sr by the great amounts of primary sr contained in carbonates 5 5 the regularly decreasing succession of isotopic ages must follow the stratigraphic succession when this condition is satisfied there is a high probability that the calculated ages are indeed those of the deposition dates

geological characteristics and genesis of the jurassic no i porphyry cu au deposit in the xiongcun district gangdese porphyry copper belt tibet abstract the xiongcun district located in the western segment of the gangdese porphyry copper belt gpcb hosts the only known jurassic mineralization in the gpcb tibet prc the no i deposit in the xiongcun district is related to the middle jurassic quartz diorite porphyry 167 161 ma and the mineralization was formed at ca 161 5 2 7 ma ore bearing middle jurassic quartz diorite porphyry emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation veinlets and disseminated mineralization developed within the middle jurassic quartz diorite porphyry and the surrounding metamorphosed tuff hosting a measured and indicated resource of 1 04 mt copper 143 31 t gold and 900 43 t silver with an average grade of 0 48 copper 0 66 g t gold and 4 19 g t silver the mineralization can be assigned to four stages including three main stages of hypogene mineralization and one epigenetic stage the main alteration associated with mineralization is potassic seven mineralization related hydrothermal veins have been recognized including quartz sulfide biotite sulfide magnetite sulfide quartz molybdenite sulfide chalcopyrite pyrite pyrrhotite pyrite and polymetallic veins the s and pb isotopic compositions of the ore sulfides and the re contents of the molybdenite suggest a mantle source for the ore forming materials with minor contamination from the subducted sediments hydrogen and oxygen isotope compositions of quartz in the ores suggest that both magmatic and meteoric waters were involved in the ore forming process the ore bearing porphyry 167 161 ma and ore forming 161 5 2 7 ma ages of the no i deposit correspond to the time of northward subduction of neo tethys oceanic slab the geochemical data of the ore bearing porphyry indicate that the no i deposit formed in an intra oceanic island arc setting and the ore bearing porphyry originated from the partial melting of mantle with limited contribution of subducted sediments the genesis of the ore bearing porphyry and no i deposit is interpreted as being related to northward intra oceanic subduction of neo tethys oceanic slab in the middle jurassic time 167 161 ma

effects of cation exchange treatment and acid leaching on the rb sr system of illite from fithian illinois size fractions 5 m of fithian illite were treated with 1 n hcl h from cation exchange resin and 5 nh4 edta solution and analyzed for their rb sr isotopic composition in comparison to the effects of hcl the ion exchange resin leached the same amount of sr but significantly less rb and radiogenic 87sr from poorly crystallized 1 m illite an age of 284 8 ma i r 0 7109 6 is more or less in agreement with the missourian stephanian age of deposition the agreement is fortuitous because of the presence of small amounts of detrital 2 m illite and kaolinite thus the true age of diagenesis might be somewhat less 87sr86sr ratio measurements of tithonian solnhofen limestone demonstrate that even though the silicate component constitutes less than 5 of the total the majority of the radiogenic 87sr is leached from broken bond surfaces of the nearly pure kaolinite residues during the hcl treatment ion exchange and edta treatment can be useful in this situation to obtain a precise separation of environmental sr in carbonate from radiogenic 87sr in silicates and an adequate spread of rbsr values from argillaceous fine fractions in the construction of isochrons

genesis of oxidized and reduced type granites the genesis of oxidized type magnetite series and reduced type ilmenite series granites is discussed on the basis of thermodynamic considerations we show that so 2 in the magma can oxidize augite to form magnetite with decreasing temperature because the so 2 h 2 s buffer line intersects with the augite magnetite quartz buffer line on a 1 t f sub o 2 diagram when the magma contains so 2 above 250 ppm and the f sub o 2 is constrained by the so 2 h 2 s buffer the magma forms oxidized type granites 0 2 modal magnetite on the other hand when the so 2 content is less than 250 ppm the magma solidifies as a reduced type granite 0 2 modal magnetite if the f sub o 2 of the magmas is buffered by h 2 o h 2 co 2 co or ch 4 co 2 no magnetite will crystallize and reduced type granites will be formed as a result the genesis of oxidized type and reduced type granites can be attributed to the difference in volatile components in magmas

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

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

oxidation of ch4 to co2 and h2o by chloritization of detrital biotite at 270 5 c in the external part of the central alps switzerland abstract the upper end of the ch 4 zone in the external part of the central alps of switzerland results from an oxidation reaction of methane into water and carbon dioxide at 270 5 c tarantola a mullis j vennemann t dubessy j de capitani c 2007 oxidation of methane at the ch 4 h 2 o co 2 transition zone in the external part of the central alps switzerland evidences from stable isotope investigations chemical geology 237 329 357 petrographic investigations of the north helvetic flysch revealed the presence of detrital biotite muscovite and chlorite in the ch 4 zone at the transition from the methane to the water zone the sudden increase of co 2 in the fluid is associated with the formation of chlorite and muscovite in replacement of detrital biotite in the h 2 o zone biotite is entirely replaced by chlorite muscovite interlayered stacks mossbauer spectroscopy showed a minimum decrease of x fe 3 from 0 18 to 0 09 within the sheet silicate fraction at the transition from the ch 4 to the h 2 o co 2 dominated fluid zone detrital biotite of the investigated host rocks incorporates at the same conditions larger amounts of fe 3 than chlorite in its structure and x fe 3 biotite x fe 3 new chlorite x fe 3 detrital chlorite the reaction of chloritization of detrital biotite is interpreted as a fluid controlled mineralogical isograde at 270 5 c marking the transition from the low to the high grade anchizone and allowing the oxidation of up to 36 moles of ch 4 per cubic meter of rock

hydrothermal evolution of the el teniente deposit chile porphyry cu mo ore deposition from low salinity magmatic fluids the magmatic hydrothermal evolution of the el teniente porphyry cu mo deposit in the central andes in chile is reconstructed based on field relationships scanning electron microscopy cathodoluminescence petrography and fluid inclusion analysis by microthermometry and laser ablation inductively coupled plasma mass spectrometry la icpms three major stages of cu mo mineralization are observed following the barren hydrothermal stage 1 the stage 2 mineralization is characterized by quartz anhydrite stockwork veins and breccias with chalcopyrite bornite and molybdenite both stages 1 and 2 are associated with pervasive potassic alteration quartz anhydrite veins with chalcopyrite bornite and molybdenite associated with phyllic alteration represent stage 3 mineralization stage 4 mineralization linked to the formation of the large braden diatreme is characterized by breccias and rare veins containing a lower temperature assemblage with tourmaline sericite and lesser tennantite bornite and chalcopyrite with late gypsum in local vugs ten fluid types are distinguished in this study based on petrographic and microthermometric criteria such as phase proportions daughter minerals homogenization behavior and salinity the hydrothermal evolution across the stages of cu deposition is characterized by the contraction of a vapor phase originating by phase separation during stage 2 overall cooling of the system at pressures fluctuating around the two phase fluid surface led to a transition from a two phase fluid state dominated by vapor at 410 c and 300 bars in stage 2 to a single phase low salinity fluid derived from cooling and contraction of magmatic vapor to a liquid which dominates during stage 3 mineralization at 350 c and 200 bars copper mineralization mainly formed from the vapor phase and its low salinity liquid derivatives representing a large volume of fluid with an initially high cu content 1 2 0 4 wt cu copper sulfides precipitated upon cooling between 410 and 320 c indicated by a drop of cu na k mn fe ratios of over four orders of magnitude through the evolution of the deposit from stage 2 to stage 4 the highest mo concentrations occur in residual brines resulting from extreme boiling as indicated by concurrent halite saturation recent geochronology cannell 2005 maksaev et al 2004 suggests a relatively long lived magmatic hydrothermal system at el teniente our fluid chemical data show no evidence for major crystal fractionation in a large fluid generating upper crustal magma chamber because cs na k mn fe is constant from pre ore to all syn ore fluids however the initiation of copper mineralization was associated with a 4 to 10 fold increase in the concentration of cu mo li and fe in the inferred main ore forming input fluid compared with pre ore fluids of intermediate salinity and otherwise very similar major and trace element ratios these data indicate that injection from depth of an exceptionally cu mo li and probably also s rich volatile phase into an already actively evolving upper crustal magmatic hydrothermal system triggered the formation of this unusually large and rich copper deposit

zircon ce4 ce3 ratios and ages for yulong ore bearing porphyries in eastern tibet yulong ore bearing porphyries along the northwestern extension of the red river ailao shan fault system in eastern tibet consist of five porphyry deposits containing a total of more than 8 million tons of copper resources u th pb laser inductively coupled plasma mass spectrometry dating of zircon shows that the porphyries were emplaced in early tertiary 41 2 36 9 ma covering a period of 4 3 ma with formation ages decreasing systematically from northwest to southeast the start of porphyry magmatism coincided with the onset of transpressional movement along the red river ailao shan fault system implying a close link between these two events age sequence in intrusions can be plausibly explained by assuming that a region of melting in the lower northwestern plate moved southeasternward along the tuoba mangkang fault relative to the upper plate zircon grains from the yulong ore bearing porphyries have higher ce4 ce3 than those from barren porphyries in the region this suggests that the ore bearing porphyries crystallized from a relatively oxidized magma which has important implications for future ore exploration in the region and other cu deposits in convergent margin environments in general

sedimentology and geochemistry of a mid proterozoic lacustrine unit from northern australia abstract the wollogorang formation is an unmetamorphosed middle proterozoic sedimentary unit ca 100 m thick in the highly mineralized mcarthur basin of northern australia based on sedimentology three of the six lithofacies in this unit can be confidently interpreted to be of continental origin facies 1 evaporitic red beds deposited in highly saline mudflats on an irregular volcanic landsurface at the base of the formation facies 3 varved organic rich black shales deposited in a quiet water distal lacustrine setting in the middle of the formation and facies 5 conglomeratic cross bedded poorly sorted dolomitic sandstones deposited in braided fluvial environments near the top of the formation a detailed geochemical study mainly of facies 3 was carried out to see if the two studies would complement each other and to show what geochemical criteria may be useful in defining proterozoic depositional environments dolomite concretions which are present in the carbonate poor black shales of facies 3 are shown to be early diagenetic the 87 sr 86 sr ratio from one of these concretions is 0 7139 significantly above the accepted range for marine carbonates of this age 0 705 0 708 these concretions and surrounding black shales often contain relatively high pyrite contents the concretions have 13 c values similar to the average host dolomite value of 0 6 and pyrite 34 values range from 3 5 to 5 8 c av 2 4 0 these data agree with the sedimentology and indicate a low sulfate lacustrine environment with little activity from the sulfate reducing bacteria disseminated pyrite 34 s values from facies 2 and 4 up to 11 8 0 suggest similar low sulfate environments this combined with carbonate 18 o values of around 23 facies 1 4 suggests there was only one water source present during deposition diagenesis and lithification when these data are viewed in the light of the regional stratigraphic setting and the gradational facies relationships they suggest a non marine setting for the bulk of the wollogorang formation and support previous observations of an abundance of rift related continental environments in this highly mineralized region the relatively high pyrite s organic c ratios found during this study are interpreted as indicating some addition of hydrothermally formed pyrite this result is similar to that found in the black shales from the younger more highly mineralized rocks of the mcarthur basin and may support the originally perceived mineral ootential of units such as the wollogorang formation jackson 1981a

baogutu an example of reduced porphyry cu deposit in western junggar abstract classic oxidized porphyry copper deposits opcd are characterized by high tonnage and elevated f o 2 and contain highly oxidized minerals including primary anhydrite and hematite in contrast the baogutu deposit contains abundant hypogene pyrrhotite and methane rich fluid inclusions characteristic of lower tonnage reduced porphyry copper deposits rpcd mineral paragenesis and mineral chemistry studies reveal two mineralization periods magmatic and hydrothermal with the hydrothermal period further subdivided into three paragenetic stages including ca na alteration potassic alteration and propylitization locally overlapped by phyllic alteration several independent geothermometers indicate that the temperatures of the magma potassic and propylitization alteration are estimated to be 600 900 c 200 400 c and 200 300 c respectively multiple indicators including mineral assemblages apatite so 3 content whole rock fe 2 o 3 feo ratios and fluid compositions indicate the f o 2 of the magma and hydrothermal fluid to be nno and nno nno 2 respectively hydrothermal fluids associated with primary biotite yield log f h 2 o f hcl fluids values of 4 8 6 2 for diorite and 4 1 4 5 for granodiorite porphyry the log f s2 of the magmatic and potassic alteration are estimated to be 0 7 to 3 0 and 5 5 to 11 0 respectively based on pyrrhotite and sulfide assemblages mineral assemblage and hessite composition suggest the log f te2 of the potassic alteration is 8 5 to 14 5 the low tonnage baogutu deposit displays significantly lower f o 2 than opcds nno 2 and on this basis could be classified as a rpcd other physicochemical conditions including t f s2 log f h2o f hcl fluid do not show obvious differences to those of opcds we deduce that the low f o 2 of baogutu metallogenic granitoids and aqueous fluids likely produced the unusual pyrrhotite arsenopyrite mineral assemblage and the nacl h 2 o ch 4 co 2 ore forming fluids and small tonnage typical of rpcds previous obtained fluid inclusion h o isotope data and sulfide s pb isotope data suggest the methane rich ore forming fluids were derived from deep mantle source with little contamination from sedimentary components however detailed studies are needed to clarify the origin of the ch 4

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

evidence from fluid inclusions for microbial methanogenesis in the early archaean era methanogenic microbes may be one of the most primitive organisms although it is uncertain when methanogens first appeared on earth during the archaean era before 2 5 gyr ago methanogens may have been important in regulating climate because they could have provided sufficient amounts of the greenhouse gas methane to mitigate a severely frozen condition that could have resulted from lower solar luminosity during these times nevertheless no direct geological evidence has hitherto been available in support of the existence of methanogens in the archaean period although circumstantial evidence is available in the form of approximately 2 8 gyr old carbon isotope depleted kerogen here we report crushing extraction and carbon isotope analysis of methane bearing fluid inclusions in approximately 3 5 gyr old hydrothermal precipitates from pilbara craton australia our results indicate that the extracted fluids contain microbial methane with carbon isotopic compositions of less than 56 per thousand included within original precipitates this provides the oldest evidence of methanogen 3 46 gyr ago pre dating previous geochemical evidence by about 700 million years

depositional environment of the middle proterozoic velkerri formation in northern australia geochemical evidence the organic matter rich shales of the middle proterozoic velkerri formation in the mcarthur basin of northern australia are part of the essentially unmetamorphosed roper group which has many units showing remarkable lithological consistency over vast areas jackson et al reported the discovery of indigenous live oil in the velkerri formation thereby raising considerable interest in its depositional environment based on sedimentology alone this unit was suggested to have formed in a deep water low energy open marine environment a geochemical study of the silty mudstones of the velkerri formation from urapunga 3 corehole which intersects the upper half of this unit was carried out to examine some aspects of this depositional model organic c contents in the core are up to 7 1 and maturity determinations indicate that the sediments have experienced a relatively low organic metamorphic grade pyrite s organic c ratios vary from very low 0 01 in the bottom section of the core to high up to 1 52 in the top part of the core disseminated pyrite 34s values range from 3 6 to 34 4 the geochemical results from the upper half of the velkerri formation indicate an initial lowsulfate oxidizing environment this environment may have been a large lake or a bay which became barred early in its depositional history and later became sulfate poor as a result of the activities of sulfate reducing bacteria some influx es of seawater resulted in increased organic c and pyrite s contents in the anoxic sediments while the lake or barred bay eventually became euxinic with the preservation of significant amounts of organic matter these data indicate the need for an integrated approach to the interpretation of proterozoic sedimentary environments

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

helium isotopes as tracers of trapped hydrothermal fluids in ocean floor sulfides helium isotopes have been measured in fluids trapped in minerals from three mid ocean ridge hydrothermal systems at bare rock sites and at the sediment covered site at middle valley northern juan de fuca ridge fluid inclusion 3 he 4 he ratios from the bare rock sites are indistinguishable from those of the vent fluids and are close to the local basalt values values for 34 s of the host sulfide are consistent with variable contributions of seawater sulfate s to the basalt derived s in the hydrothermal fluids this does not affect inclusion hosted 3 he 4 he because of the high he concentration of the hydrothermal fluids relative to seawater fluid inclusion 3 he 4 he ratios from middle valley are a mixture of basalt derived he and radiogenic he acquired from pore fluids in the overlying pleistocene turbidite sediments after seawater basalt interaction the radiogenic he accompanies isotopicalty heavy s demonstrating that he isotopes may be used to distinguish pore fluids from seawater in hydrothermal systems

vapor phase exsolution as a controlling factor in hydrogen isotope variation in granitic rocks the notch peak granitic stock utah abstract the notch peak granitic stock western utah is comprised of three concentric sequentially intruded rock types from granite at the rim to quartz monzonite i to quartz monzonite ii at the core the 18 o values of whole rocks vary about an average of 9 4 smow irrespective of the rock type and position relative to contact suggesting that the three magmas had the same parent the whole rock d values in the stock range from 100 to 55 d values increase toward the cores of both quartz monzonite i and quartz monzonite ii resulting in concentric contours the d contours of quartz monzonite ii cross cut those of quartz monzonite i suggesting little isotopic interaction between these bodies and the absence of a late pervasive fluid phase there is a positive correlation between d values and water content of the samples where samples from each body define a distinct field the positive correlation is explained by isotopic fractionation attendant on vapor exsolution from the crystallizing magma an observed increase in d with the degree of chloritization a trend opposite to that observed in systems where participation of meteoric water has been demonstrated is the result of subsolidus interaction with the exsolved fluids these results show that large variations in the hydrogen isotope ratios of a granitoid can arise by exsolution of a vapor phase from the melt on crystallization in general magmas with larger modal amount of primary hydrous phases will tend to have higher d values than those with small amounts of hydrous phases furthermore the relatively high d values of chlorites at notch peak confirm the applicability of classical concepts of closed system deuteric alteration to some granitoid bodies thus meteoric water interaction need not be always invoked to explain hydrogen isotope variation and deuteric alteration in granitoids

alkali metals control the release of gold from volatile rich magmas magmatic hydrothermal ore deposits are a major resource of gold volatiles exsolving from magmas rising through the earth s crust are the key agent for enrichment of gold in these deposits we report the first experimental determination of gold solubilities in the magmatic volatile phases at conditions typical of most magmas associated with gold deposits t 1000 degrees c p 150 mpa we show that gold hydrosulfide complexes supersede gold chloride complexes and more importantly that the stability of gold hydrosulfide complexes is greatly increased by the presence of minute concentrations of kc or nacl 0 1 0 5 mol kg h2o the amplifying effect of alkali chlorides on the solubility of gold in h2s bearing volatiles may explain the preferential association of many giant hydrothermal gold deposits with high potassium alkaline mafic to intermediate igneous rocks which exsolve volatiles that simultaneously contain both h2s and alkali chlorides in significant concentrations c 2010 elsevier b v all rights reserved

the miocene gangdese porphyry copper belt generated during post collisional extension in the tibetan orogen abstract the mid miocene gangdese orogenic belt in southern tibet contains porphyry cu deposits that developed after the indian asian continental collision in the early cenozoic field work confirms that these porphyry cu deposits are temporally and spatially associated with mid miocene monzogranitic and quartz monzonitic stocks which intruded mesozoic cenozoic gangdese granitoid batholiths and surrounding triassic tertiary volcano sedimentary sequences in the lhasa terrane and formed a 50 km wide 400 km long e w belt magmatism occurred during the interval 11 2 to 19 7 ma peaking at 16 ma for the porphyry stocks this is consistent with the age 10 to 18 ma of the east west extension and subsequent ns striking normal fault systems 13 5 ma in the tibetan orogen the stocks typically occur as multiple small diameter intrusions 50 km breakdown of hornblende in the juvenile lower crust source during melting is regarded as fundamental for the formation of the fertile adakite and porphyry cu systems in south tibet the gangdese belt contains four large and three intermediate and small sized porphyry cu mo deposits and shows a continuous spectrum from typical porphyry type cu mo to skarn type zn pb mineralization twenty three samples collected from four deposits yielded a limited range of molybdenite re os isochron ages from 15 2 to 16 0 ma similar to previously reported re os ages 13 8 to 14 9 ma for other porphyry cu mo deposits in the belt suggesting that cu mo mineralization in the belt occurred in a post collisional extension environment the characteristics of porphyry cu mo deposits in the collisional zone are similar in many aspects to those in arc settings i e mineralization style alteration zoning and metal association preliminary fluid inclusion studies indicate that the early hydrothermal fluids are preserved as high temperature 450 c high salinity 40 to 47 wt nacl equiv and high pressure 1100 to 1500 bar fluid inclusions these fluids are interpreted to represent an orthomagmatic fluid that boiled episodically due to fluid overpressuring and hydrofracturing resulting in potassic alteration and precipitation of chalcopyrite molybdenite assemblages on the basis of comparison with porphyry cu deposits in arc settings we proposed that melting of thickened newly formed basaltic crust resulted in generation of cu bearing magmatic systems in the tibetan collision zone and both orogen transverse normal faults and orogen parallel thrust zones during the miocene post collisional extension constrain the temporal and spatial localization of the porphyry cu systems in the collision zone

early jurassic volcanic rocks in the xiongcun district southern lhasa subterrane tibet implications for the tectono magmatic events associated with the early evolution of the neo tethys ocean abstract early mesozoic late triassic to middle jurassic igneous rocks in the southern lhasa subterrane tibet record important information on the early tectono magmatic evolution of the neo tethys ocean this paper presents petrological geochronological and geochemical data for the xiongcun formation ignimbrites to constrain the petrogenesis and tectonic setting of these rocks in the southern lhasa subterrane the xiongcun formation is dominated by andesitic and dacitic rocks these rocks yield early jurassic 195 176 ma zircon u pb ages and display typical arc like geochemical signatures being enriched in large ion lithophile elements e g rb ba and u and depleted in high field strength elements e g nb ta and ti sr nd pb hf isotopic compositions are highly depleted these chemical and isotopic data suggest that the xiongcun formation ignimbrites probably formed in an intra oceanic island arc setting related to the northward subduction of the neo tethys oceanic slab the parental magmas were derived from a depleted mantle source modified by fluids released from the neo tethys oceanic slab combined with previously reported data we suggest that the volcanic rocks in the early jurassic xiongcun and yeba formations of the southern lhasa subterrane were generated by a single subduction system but developed on oceanic and continental crust respectively

re os and mo isotope systematics of black shales from the middle proterozoic velkerri and wollogorang formations mcarthur basin northern australia joint application of the mo isotope paleoredox proxy and re os deposition age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time here we report new re os and mo isotope data for the mesoproterozoic velkerri formation roper group and paleoproterozoic wollogorang formation tawallah group mcarthur basin northern australia new re os ages of 1361 21 ma 2 n 14 mean square of weighted deviates mswd 1 3 model 1 and 1417 29 ma 2 n 12 mswd 1 3 model 1 constrain the depositional age of the velkerri formation and its contained biomarkers as well as acritarchs and microfossils from the roper group black shales from the upper velkerri formation have high mo abundances 105 119 ppm and degree of pyritization dop values 0 90 0 92 implying quantitative conversion of molybdate moo42 to thiomolybdate mos42 in overlying bottom waters the average 97 95mo 0 72 0 10 2 n 6 of these shales is consistent with previous data but represents a significantly more precise determination for global seawater 97 95mo at 1 4 ga this value is lighter than present day seawater by 0 85 and reflects expanded strongly euxinic deep ocean conditions h2s aq 11 m relative to oxic suboxic and weakly intermittently euxinic h2s aq 11 m marine deposition in the 1 4 ga oceans mass balance modelling suggests mo removal into strongly euxinic and oxic sediments may have comprised 30 70 and less than 15 respectively of the oceanic mo sink at 1 4 ga as opposed to 5 and 35 today respectively the re os radioisotope system in organic rich shales serves as a test for post depositional alteration that could affect the usefulness of paleoredox tracers such as mo stable isotopes re os isotope data for the wollogorang formation black shales are scattered and yield a highly imprecise date of 1359 150 ma 2 n 21 mswd 85 model 3 this age is younger than u pb zircon ages from interbedded tuffs that constrain the age of deposition at ca 1730 ma in conjunction with previous petrological geochemical and paleomagnetic data the re os isotope data suggest hydrothermal fluid flow through the wollogorang formation possibly associated with formation of the ca 1640 ma mcarthur river pb zn ag sedimentary exhalative deposit resulted in post depositional mobilization of re and os based on the degree of deviation of the re os data from a 1730 ma reference line open system behavior of re and os was greatest near the base of the black shale unit wollogorang formation black shales are enriched in mo 41 58 ppm but are characterized by variable 97 95mo 0 3 0 8 and dop 0 57 0 92 the lightest 97 95mo values occur near the base of the black shale unit based on the re os systematics hydrothermal fluids have probably overprinted the authigenic 97 95mo signature in those shales however the heaviest 97 95mo values in the wollogorang formation come from stratigraphically higher shales and are similar to those observed for the velkerri formation and thus may reflect seawater 97 95mo at 1 73 ga

geochemistry of hydrothermal fluids from an intraplate ocean island everman volcano socorro island mexico abstract socorro island mexico still volcanically active is an alkaline and peralkaline volcanic island in the eastern pacific ocean located on a mid ocean ridge spreading center that was abandoned at 3 5 ma gas and water samples collected on the island in 1999 from the dome fumaroles and hot springs of everman volcano have been analyzed for major components h o c s he and ar isotopic composition and noble gas abundances the boiling point temperature steam vents and low salinity thermal springs discharge cl free meteoric water and are typical surface manifestations of a high temperature hydrothermal system high h 2 and ch 4 contents up to 14 and 3 5 vol in dry gas respectively and an enrichment of ch 4 in 13 c 13 c ch 4 18 may suggest that these reduced gases were at least partially produced by the interaction of ultramafic rocks with water inside volcanic edifice due to serpentinization of olivine the 3 he 4 he ratios 6 0 6 6 r a which are lower than the morb values and the high he ne ratios 4000 and he ar ratios up to 4 indicate a contribution of radiogenic he from the oceanic crust or shallow magma chamber like at a low 3 he hot spot the maximum of the 40 ar 36 ar values 311 is close to the maximum value found in the socorro volcanic rocks noble gas abundances are close to values for the partially degassed air saturated water however with some excess ne concentrations an enrichment in 132 xe or depletion in 84 kr of the least air contaminated gas sample he ne 4000 relative to air saturated water can be related to the contribution of noble gases from a non atmospheric source probably oceanic crustal component assimilated in a shallow magma reservoir

two episodes of mineralization in the mengya a deposit and implications for the evolution and intensity of pb zn ag mineralization in the lhasa terrane tibet abstract mengya a is a typical pb zn ag deposit in the lhasa terrane which is located in the eastern part of the central lhasa subterrane tibet two different types of pb zn ag mineralization skarn and porphyry like styles have been identified in the mengya a mining district the skarn type mineralization occurs as layered or lenticular units in the laigu formation and contains nearly all the pb zn ore resources whereas the porphyry like mineralization exists mainly as veins in granite porphyry 40 ar 39 ar dating of muscovite suggests that the skarn orebodies formed at 54 8 0 4 ma whereas zircon u pb dating of granite porphyry 13 2 0 2 ma indicates that the porphyry like mineralization formed in the miocene as such the primary mineralization event skarn type took place during the main indo asia collision the ore forming mineralization and alteration characteristics of the mengya a deposit are consistent with other examples of regional pb zn ag mineralization in the lhasa terrane the present results combined with previous studies indicate that multiple pb zn ag mineralization events occurred in the lhasa terrane including during the cretaceous paleocene and miocene all of which were associated with felsic intrusions sulfur and lead isotopes suggest that the pb zn ag mineralization events were mainly magmatic in origin and were closely associated with the ancient basement of the central lhasa subterrane whole rock major and trace elements and zircon hf isotope data of the ore forming intrusions indicate that these intrusions are s type granites produced mainly by partial melting of the ancient crust which underwent extensive fractionation therefore the presence of moderately to strongly fractionated s types granites along with the ancient crustal magma source were the key factors in generating the pb zn ag mineralization in the central lhasa subterrane crustal melting in the central lhasa subterrane peaked in the paleocene and resulted in the most intense pb zn ag mineralization during the initial stages of indo asia continental collision 65 50 ma during the cretaceous the scale of crust melting was considerable creating large pb zn ag deposits limited miocene crustal melting and magmatism in the central lhasa subterrane resulted in the miocene pb zn ag mineralization being smaller in scale than that in the paleocene and cretaceous

sm nd dating of fig tree clay minerals of the barberton greenstone belt south africa sm nd isotopic data from carbonate derived clay minerals of the 3 22 3 25 ga fig tree group barberton greenstone belt south africa form a linear array corresponding to an age of 3102 64 ma making these minerals the oldest dated clays on earth the obtained age is 120 160 m y younger than the depositional age determined by zircon geochronology nd model ages for the clays range from approximately 3 39 to 3 44 ga and almost cover the age variation of the barberton greenstone belt rocks consistent with independent evidence that the clay minerals are derived from material of the belt the combined isotopic and mineralogical data provide evidence for a cryptic thermal overprint in the sediments of the belt however the highest temperature reached by the samples since the time of clay mineral formation was 300 degrees c lower than virtually any known early archean supracrustal sequence

subdivision and correlation of late precambrian sequences in australia abstract the diversity of the australian precambrian record is used to empirically assess the application of all available techniques of correlation of late precambrian rocks lithofacies isotopic ages palaeontology and palaeomagnetism within the framework of latest lithostratigraphic geochronologic correlation charts lithostratigraphy is the main tool for intrabasinal correlation and mapping but isotopic ages will continue for some time yet to provide the main criteria for interregional correlation lithostratigraphic correlation is demonstrated over hundreds of kilometres but is generally constrained by tectonic controls to individual major crustal blocks climatic markers such as tillites may be correlated interregionally over thousands of kilometres the most exciting recent development in platform geochronology is the application of u pb zircon dating to fine grained tuffs illite may provide reliable minimum estimates but can be shown to be up to 150 ma too young glauconite and rb sr total rock shale ages have provided useful broad age estimates of sequences for which better methods are unavailable but only when based on cautious assessment and multiple sampling their ages may be either minimum or maximum stromatolites continue to demonstrate their potential for broad correlation when based on rigorous taxonomic study but the framework of time ranges of known taxa must be expanded for stromatolites to achieve their full potential especially for rocks older than 1000 ma acritarchs in black shales provide diverse and complex assemblages with excellent correlation potential but once again the australian taxonomic framework is still inadequate black cherts contain an evolutionarily conservative biota with only limited correlation potential recent palaeomagnetic studies based on both apwp and magnetostratigraphy demonstrate excellent potential for detailed correlations down to at least 1700 ma old however the present chronologically calibrated data base is sparse and must be supplemented by many more detailed studies through thick continuous rock sequences spanning substantial intervals of time the value of multidisciplinary correlation is elegantly demonstrated by very detailed and unambiguous correlation of combined palaeomagnetic isotopic age and stromatolite data across the 1700 ma mcarthur basin previous regional lithostratigraphic correlations have been conclusively and significantly revised with major implications to regional palaeogeography

the contribution of fluid geochemistry to the volcano monitoring of stromboli abstract the persistent open vent mild explosive activity of stromboli volcano is episodically interrupted by more violent and dangerous explosive events major explosions and paroxysms according to the nature of erupted products paroxysms can be related either to the explosion of overpressured gas pockets located in the proximity of the magma column or to the sudden uprise of hot gas rich magma from a deep part of the plumbing system in both cases these more energetic explosive events should be preceded by an escape or a preferential uprise of the highly mobile volatiles which in turn should produce gas leakage anomalies at the surface in sites of high vertical permeability such as deep reaching faults in order to identify such gas leaking sites a systematic co2 soil flux survey has been carried out on the island using an accumulation chamber four hundred sixty one points have been measured with a high density in the summit crater area where high flux values have been found 10 3 10 2 cm s anomalous points are concentrated along the main ne sw axial feeding system of the volcano co2 soil flux decreases from the crater zone to the base of the volcanic cone where however an interesting gas leakage anomaly occurs pizzillo mofette in the pizzillo area thermal water wells also occur whose chemistry indicates an origin by sea water heated by hot gas soil gases have been sampled in the sites with the highest co2 flux in the crater area and at the base of the cone chemical and isotopic analyses indicate the presence of a deep gas component especially in the crater zone samples high co2 appreciable contents of he and h2 3he 4he values up to 3 55 13c of co2 2 the sc5 low flux fumarole on the crater rim has shown during six years a remarkable persistence of temperature 93 95 c with only minor fluctuations of chemistry appearance of anomalous peaks of h2 in correspondence with strombolian explosions was observed during a 2 h 30 min experiment of continuous recording of hydrogen content of the fumarole together with previous data on h2o co2 and he this experiment confirms that strombolian blasts produce rapid fluctuations in the fumarolic gas composition in the light of this study sc5 fumarole and pizzillo mofette look promising sites for the testing of a continuous geochemical monitoring system of stromboli volcano

reduced porphyry copper gold deposits a new variation on an old theme it is a widely accepted fact that porphyry cu au deposits form from highly oxidized ore fluids despite this assertion several otherwise typical porphyry cu au deposits exhibit clear evidence of having formed from relatively reduced ore fluids these reduced porphyry cu au deposits lack primary hematite magnetite and sulfate minerals i e anhydrite but contain abundant hypogene pyrrhotite commonly have carbonic rich ore fluids with substantial ch 4 and are associated with ilmenite bearing reduced i type granitoids based on a synthesis of theoretical experimental and field data a variation on the classic porphyry cu model is proposed to explain the formation of reduced porphyry cu au deposits and their relatively cu poor but au rich nature the proposed reduced porphyry cu au model does not contradict the current understanding of porphyry cu au formation rather it adds yet another variation on the theme of porphyry cu au genesis

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved