major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

replenishment crystal accumulation and floor aggradation in the megacrystic kameruka suite australia detailed field evidence indicates that the kameruka suite plutons of the bega batholith eastern australia grew by crystal accumulation on the floor of a magma chamber depositional features in the plutons including mafic enclave channels asymmetric enclave pillows and exotic rafts load casts and flame structures and graded and trough cross beds indicate that the pluton built progressively upward the general eastward dip of depositional features in the main pluton implies a lower western and upper eastern contact consistent with a basal granite migmatite contact in the west and a sharp hornfelsic sidewall contact in the east mafic felsic and composite dykes most common near and below the basal western contact are interpreted as conduits for magma chamber replenishment and imply open system behaviour during pluton construction textural relations are also consistent with an open system cumulate origin typically centimetre scale grains of quartz plagioclase and megacrystic alkali feldspar form a touching framework with interstices filled with smaller biotite flakes and smaller intercumulus quartz and feldspar crystals alkali feldspar megacrysts vary from euhedral and unzoned to mantled and partially replaced by plagioclase to ovoid and completely pseudomorphed by quartz albite aggregates the common occurrence of mantled and pseudomorphed alkali feldspar in mafic enclaves and in hybrid tonalitic rocks forming the matrix to enclave swarms suggests that replacement or resorption of granitic primocrysts was associated with mafic replenishments the occurrence of all megacryst types at outcrop scale implies extended alkali feldspar crystallization in different parts of the chamber thorough stirring during or after periodic replenishment and final settling in a cumulate mush the bulk composition of the cumulate mush represented by granodiorite cannot represent the emplaced magma compositional variation can be modelled by variable degrees of crystal accumulation from a parental silica rich melt represented by the silicic dykes as dykes periodically fed the magma chamber crystals accumulated on the floor and more evolved melts probably erupted from its roof thus the average composition of the magma and the cumulus minerals may have remained relatively constant and the sublinear chemical trends that typify the kameruka suite simply reflect differing proportions of melt and cumulate material sublinear chemical trends can also be explained by a restite model however the distinctive ba light rare earth element and zr spikes at high silica can be explained only by a cumulate model which also explains why the low silica granites of the suite share the same chemical characteristics as the high silica granites

absolute timing of sulfide and gold mineralization a comparison of re os molybdenite and ar ar mica methods from the tintina gold belt alaska new re os molybdenite dates from two lode gold deposits of the tintina gold belt alaska provide direct timing constraints for sulfide and gold mineralization at fort knox the re os molybdenite date is identical to the u pb zircon age for the host intrusion supporting an intrusive related origin for the deposit however 40ar 39ar dates from hydrothermal and igneous mica are considerably younger at the pogo deposit re os molybdenite dates are also much older than 40ar 39ar dates from hydrothermal mica but dissimilar to the age of local granites these age relationships indicate that the re os molybdenite method records the timing of sulfide and gold mineralization whereas much younger 40ar 39ar dates are affected by post ore thermal events slow cooling and or systemic analytical effects the results of this study complement a growing body of evidence to indicate that the re os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems

a melt inclusion study of the toba tuffs sumatra indonesia the toba caldera in northern sumatra indonesia is the site of earth s largest quaternary volcanic eruption this eruption dated at 74 ka produced the 2800 km3 youngest toba tuff ytt and ash fall quartz bearing silicic pumices ranging from 68 to 77 wt sio2 indicate that the ytt magma was zoned compositionally and mineralogically prior to the ytt eruption two other silicic quartz bearing tuffs known as the middle toba tuff mtt and the oldest toba tuff ott were erupted from toba at 0 501 ma and 0 840 ma respectively although the volatile contents of the toba magmas are poorly constrained aerosols generated by the ytt eruption are generally thought to have caused a global volcanic winter an evaluation of the pre eruptive dissolved gas contents of the ytt magma is fundamental to understanding the global effects of this eruption we used melt inclusions in quartz crystals to determine the dissolved h2o co2 s cl and f contents of the ytt mtt and ott magmas quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data melt inclusions from the ytt mtt and ott are rhyolitic in composition 73 77 wt sio2 and have overlapping indistinguishable major element trends inclusions from the large eruptions ytt and ott have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses in contrast the least evolved melt inclusions occur in the most silicic samples and either overlap with or are less evolved than their matrix glasses these patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt while those in the most silicic melt formed in situ several lines of evidence suggest that crystal settling of early formed quartz crystals in long lived magma bodies was an important process in the ytt and ott magmas trace element data from the ytt melt inclusions have wide ranges rb 250 830 ppm ba 10 485 ppm and mostly support these models however incompatible trace elements are enriched 2 3 over their matrix glass compositions in all samples in the smaller mtt melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in situ crystallization ftir analyses indicate that the ytt mtt and ott melts contained about 4 0 5 5 2 0 5 5 and 2 0 5 5 wt h2o respectively co2 contents in all units are generally 100 ppm and cl is 2000 ppm water gradients occur in all 3 tuffs cl gradients are evident only in the ytt and the least silicic samples from all units had no detectable co2 sulfur contents in melt inclusions are low in all samples 32 ppm and overlap with matrix glass s contents applying broad degassing constraints about 1014 g of h2so4 aerosols were likely loaded into the atmosphere during the ytt eruption 2 orders of magnitude less than previous petrologic estimates considerably more cl and f about 1015 g of each may have been part of the ytt aerosol cloud

re os isotopic dating of molybdenite and pyrite in the baishan mo re deposit eastern tianshan nw china and its geological significance the baishan mo re deposit is located in the eastern section of the eastern tianshan orogenic belt nw china the deposit has a grade of 0 06 mo and a high content of rhenium of 1 4 g t rhenium and osmium isotopes in sulfide minerals from the baishan deposit are used to determine the age of mineralization rhenium concentrations in molybdenite samples are between 74 and 250 g g analysis of eight molybdenite samples yields an isochron age of 224 8 4 5 ma 2 pyrite samples have rhenium and osmium concentrations varying in the range 33 4 330 6 ng g and 0 08 0 81 ng g respectively isotope data on seven pyrite samples yield an isochron age of 225 12 ma 2 on the 187re 188os versus 187os 188os plot and an age of 233 14 ma 2 on the 187os versus 187re correlation diagram the ages of molybdenite and pyrite are consistent within the analytical errors combined with field observations the data indicate that mo re mineralization in the baishan deposit is produced by a magmatic hydrothermal event in an intracontinental extensional setting after late paleozoic orogeny the initial 187os 188os ratio of pyrite is 0 3 0 07 the 34s values of molybdenite vary from 0 5 to 3 6 both data indicate that mineralization is derived mainly from a mantle source

chemical differentiation of the bishop tuff and other high silica magmas through crystallization processes re examination of petrochemical data for high silica ash flow tuffs suggests that crystal liquid fractionation plays a major role in the differentiation of high silica magmas for the high silica bishop tuff the enrichment and depletion of 27 major and trace elements correlates with the bulk crystal liquid partition coefficients calculated from the data of hildreth in ways that are consistent with crystal fractionation comparison of the bishop tuff with other high silica tuffs and rhyolites with different compositional variations and different phenocryst assemblages shows the correspondence of chemical gradients with mineralogy crystal zoning studies in the cordillera paine granite in southern chile suggest that the evolution of differentiated liquids similar to those in the bishop tuff results from crystal fractionation the physical mechanisms of separation of liquid and crystals for the paine granite and bishop tuff were quite different however differentiation of the high silica ash flows might result from advection of a less dense liquid from a crystallizing boundary into the top of the magma chamber

precise sm nd and u pb isotopic dating of the supergiant shizhuyuan polymetallic deposit and its host granite se china the supergiant shizhuyuan w sn bi mo deposit is hosted by the qianlishan granite a small highly fractionated granitic pluton 10 km 2 with multiple phases of intrusions within the early yanshanian granitoid province of se china strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and devonian limestone is responsible for the polymetallic mineralizations shrimp u pb zircon analysis indicates that the two early phases of the qianlishan granite formed contemporaneously at 152 2 ma metasomatic minerals garnet fluorite and wolframite separated from the skarn and greisen yield a sm nd isochron age of 149 2 ma that is interpreted as the formation age of the shizhuyuan deposit therefore the mineralization of the supergiant shizhuyuan polymetallic deposit formed contemporaneously with or very shortly after the intrusion of the small highly fractionated qianlishan granite

age and genesis of the w bi cu f au nui phao deposit northeast vietnam constrains from u pb and ar ar geochronology fluid inclusions study s o isotope systematic and scheelite geochemistry abstract northeast vietnam comprises several tectonic units which host number of metallic deposits of different types from orogenic gold to intrusion hosted pge deposits the lo gam domain being part of the ne vietnam hosts the lang vai gold district a several pb zn deposits and the recently discovered super large w bi cu f au nui phao deposit the nui phao deposit is 87 9 mt at 0 19 wo3 7 95 caf2 0 18 cu 0 19 g t au and 0 09 bi while the measured resource is 25 1 mt at 0 25 wo3 8 4 caf2 0 23 cu 0 26 g t au and 0 108 bi the nui phao deposit is a very interesting object for research due to location of the orebodies between of two granite massifs of the cretaceous da lien and triassic nui phao age consequently the temporal and genetic relationships of mineralization with a certain magmatic events are unclear here we present mineralogical compositional and geochronological data of the nui phao deposit in conjunction with physical chemical researches to understand the ore genesis and its genetic link to a certain magmatic events according to our observations at least three different mineral associations are distinguished at the nui phao deposit which include greisens skarns and quartz sulfide ore skarn and sulfide ore mineral assemblages generally contain abundant scheelite associated with pyrrhotite chalcopyrite and native bismuth the greisens consist of fluorite muscovite and rare molybdenite the results of fluid inclusions study indicate that at the nui phao deposit the initial ore forming fluid was represented by the reduced co2 ch4 h2o nacl cacl2 system characterized by high temperatures and moderate salinities the fluid boiling is the dominant mechanism for ore deposition at the nui phao deposit relatively homogeneous isotopic compositions of oxygen 18oh2o 6 73 to 9 24 and sulfur 34s 1 6 to 3 5 that closely correspond to magmatic source suggest that the ore forming fluid was generated from cooling granite pluton without contribution meteoric water the systematic studies of scheelite composition show that ree contents decrease from early to late scheelite generations which suggest gradual ree depletion of the ore forming fluid without significant fractionation between lree mree and hree the rees of scheelite are mainly controlled by the substitution mechanisms ca2 w6 ree3 nb5 and 3ca2 2ree3 ca by combining ree features y ho ratios and other characteristics of the nui phao deposit we propose a single magmatic source for hydrothermal ore firming fluids without fluid mixing a significantly contribution of material during fluid rock interaction is also unlikely the combination of mineralogical and geochemical features as well as complex studies of the formation conditions allowed us to conclude that gold at the nui phao deposit occur as impurities in the native bismuth the ages of the da lien and nui phao granite massifs and the nui phao deposit have been estimated to be ca 84 ma ar ar 244 ma u pb and ca 85 ma ar ar respectively indicating a genetic relationship between the mineralization and the cretaceous magmatic event

rhyolite melts vs derp newer does not make it better a comment on the effect of anorthite content and water on quartz feldspar cotectic compositions in the rhyolitic system and implications for geobarometry by wilke et al 2017 journal of petrology 58 789 818

ar ar ages in phlogopites from marble hosted ruby deposits in northern vietnam evidence for cenozoic ruby formation abstract ruby growth in phlogopite bearing marbles has been indirectly dated using the 40 ar 39 ar laser stepwise heating technique on purified syngenetic phlogopite and other micas from ruby deposits in yen bai luc yen and quy chau mining districts in northern vietnam the principal results indicate the following 1 across the red river shear zone the phlogopites from the yen bai deposits yielded miocene cooling ages between 23 2 and 24 4 ma identical to those previously published using the same dating method on magmatic and metamorphic rocks from the day nui con voi range 2 luc yen ruby deposits in the lo gam zone on the eastern flank of the red river shear zone yielded oligocene cooling 40 ar 39 ar mica ages between 30 8 and 34 0 ma regarding the age of ruby crystallisation itself the most plausible hypothesis is that all rubies in both zones formed during the period 40 to 35 ma diachronism of cooling in adjacent zones leads to the conclusion that around 35 ma the ductile deformation in the lo gam zone ended and the ruby bearing marbles cooled rapidly while the high temperature deformation remained in the red river shear zone resulting in cooling through blocking temperature some 15 ma later 3 the quy chau ruby deposit is restricted to the quy chau shear zone that bounds the eastern part of the oligocene miocene bu khang dome phlogopite and biotite samples reveal miocene cooling ages between 21 and 22 5 ma which are minimum ages for ruby formation these ages could be linked with the end of the extension of the bu khang dome vietnamese ruby formation is linked to cenozoic tectonics resulting from continental collision between the asian and eurasian plates as for other marble hosted ruby deposits in central and southeast asia

internal triggering of volcanic eruptions tracking overpressure regimes for giant magma bodies abstract understanding silicic eruption triggers is paramount for deciphering explosive volcanism and its potential societal hazards here we use phase equilibria modeling to determine the potential role of internal triggering wherein magmas naturally evolve to a state in which eruption is inevitable in rhyolitic magma bodies whole rock compositions from five large to super sized rhyolitic deposits are modeled using rhyolite melts by running simulations with varying water contents we can track crystallization and bubble exsolution during magma solidification we use simulations with variable enthalpy and fixed pressure for the five compositions the interplay between bubble exsolution and crystallization can lead to an increase in the system volume which can lead to magma overpressurization we find that internal triggering is possible for high silica rhyolite magmas crystallizing at pressures below 300 mpa 50 wt which makes magma immobile high silica rhyolite eruptions from such depths would require external triggering but examples are scarce or entirely absent calculated crystallinities at which the critical overpressure threshold is reached compare favorably with observed crystal contents in natural samples for many systems suggesting that internal evolution plays a critical role in triggering eruptions systems in which fluid saturation happens late relative to crystallization or in which degassing is effective can delay or avoid internal triggering we argue that priming by crystallization and bubble exsolution is critical for magma eruption and external triggering serves simply as the final blow rather than being the driving force of explosive rhyolitic eruptions

re os isotopic chronology of molybdenites in huangshaping lead zinc deposit southeast hunan and its geological implications the huangshaping lead zinc polymetallic deposit in southeast hunan is genetically related to the intrusion of huangshaping granitic rocks and occur in the contact zone between the granodioritic porphyry and the limestone of lower carboniferous shidengzi formation as well as in the neighboring bedding plane fault zone previously the metallogenic epoch of the deposit was indirectly deduced by the ages of such nearby granitic rocks as granite porphyry and quartz porphyry or altered minerals based on the re os isochron method of molybdenites this paper measured directly the ore forming age from skarn ore in the huangshaping deposit and got a re os age of 153 8 4 8 ma which is in accordance with the previous k ar age of 153 157 ma obtained from phlogopite in the ore bearing skarn of the deposit combined with related rare earth element and trace element geochemistry of huangshaping granites and ore bearing skarn it is concluded in this paper that the age of 154 ma should represent the metallogenic epoch of the deposit and that there existed a close relationship between the ore forming process of the deposit and the granite porphyry which were both formed during the peak stage of early yanshanian large scale rock forming and ore forming activities in south china

geophysical investigations of the geology and structure of the pangushan tieshanlong tungsten ore field south jiangxi china evidence for site selection of the 2000 m nanling scientific drilling project sp nlsd 2 abstract the pangushan tieshanlong tungsten ore field located in the center of southern jiangxi province eastern nanling region contains many granitic plutons both exposed and concealed and hosts numerous tungsten polymetallic ore deposits despite many previous geological studies the deep geology and structure of the field are poorly known because few geophysical measurements have been undertaken in this study we carried out a variety of geophysical measurements including gravity magnetic field detection magnetotelluric sounding mt and high resolution seismic reflection to better outline the nature of the ore field and to aid in site selection for the 2000 m nanling scientific drilling project sp nlsd 2 to be carried out by the chinese sinoprobe program four granitic intrusives the tangcun pangushan huangsha and bai e plutons crop out independently at the surface but were found to be parts of a large batholith at depth the tangcun composite pluton which at the surface consists chiefly of caledonian granite and minor early yanshanian intrusive material was found to be chiefly early yanshanian granite concealed beneath the lower part of the caledonian granite the bai e pluton initially decreases in size with depth and then expands as it merges with the major batholith the pangushan and huangsha plutons are domes on the inferred batholith four granitic magma channels and a considerable number of faults have been identified due to its favorable geological conditions the top of the pangushan pluton is predicted to be the best candidate for ore prospecting it is basically a granite cupola with many cross cutting and parallel faults in addition it is characterized by a high gravity anomaly a high magnetic anomaly and locally a low electrical resistivity anomaly

identification of a kulshan caldera correlative tephra in the palouse loess of washington state northwest usa abstract the kulshan caldera formed at 1 15 ma on the present day site of mt baker washington state northwest usa and erupted a compositionally zoned dacite rhyolite magma and a correlative eruptive the lake tapps tephra this tephra has previously been described but only from the puget lowland of nw washington here an occurrence of a kulshan caldera correlative tephra is described from the quaternary palouse loess at the washtucna site wa 3 site wa 3 is located in east central washington 340 km southeast of the kulshan caldera and 300 km east southeast of the lake tapps occurrence in the puget lowland major and trace element chemistry and location of the deposit at washtucna within reversed polarity sediments indicates that it is not correlative with the mesa falls rockland bishop ash lava creek b or huckleberry ridge tephras instead the washtucna deposit is related to the lake tapps tephra by fractional crystallisation but is chemically distinct a consequence of its eruption from a compositionally zoned magma chamber the correlation of the washtucna occurrence to the kulshan caldera forming eruption indicates that it had an eruptive volume exceeding 100 km3 and that its tephra could provide a valuable early pleistocene chronostratigraphic marker in the pacific northwest

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 1 principles procedures and evaluation of the method constraining the pressure of crystallization of magmas is an important but elusive task in this work we present a method to derive crystallization pressures for rocks that preserve glass compositions either glass inclusions or matrix glass representative of equilibration between melt quartz and 1 or 2 feldspars the method relies on the well known shift of the quartz feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure the critical realization for development of the method is the fact that melt quartz and feldspars need to be in equilibrium at the liquidus for the melt composition the method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite melts over a range of pressures and searching for the pressure at which the expected assemblage quartz 1 feldspar or quartz 2 feldspars is found at the liquidus we evaluate errors resulting from uncertainties in glass composition using a series of monte carlo simulations for a quartz hosted glass inclusion composition from the bishop tuff which reveal errors on the order of 20 45 mpa for the quartz 2 feldspars constraint and on the order of 25 100 mpa for the quartz 1 feldspar constraint we suggest actual errors are closer to the lower bounds of these ranges we investigate the effect of fluid saturation in two ways 1 by applying our procedure over a range of water contents for three glass compositions we show that the effect of fluid saturation is more important at higher pressures 300 mpa than at lower pressures 100 mpa but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant h2o concentrations 3 wt and 2 by performing the same type of pressure determinations with a preliminary version of rhyolite melts that includes a h2o co2 mixed fluid phase we use a range of h2o and co2 concentrations for two compositions characteristic of early erupted and late erupted bishop tuff glass inclusions and demonstrate that calculated pressures are largely independent of co2 concentration for co2 1 000 ppm at least for relatively high h2o contents as expected in most natural magmas such that co2 concentration can be effectively neglected for application of our method finally we demonstrate that pressures calculated using the rhyolite melts geobarometer compare well with those resulting from h2o co2 glass inclusion barometry and al in hornblende barometry for an array of natural systems for which data have been compiled from the literature the agreement is best for quartz hosted glass inclusions while matrix glass yields systematically lower rhyolite melts pressures suggestive of melt evolution during eruptive decompression

the role of hydrothermal alteration in tungsten mineralization at the dahutang tungsten deposit south china abstract the giant dahutang tungsten deposit has total reserves of more than 1 31 mt of wo3 with a scheelite wolframite ratio of 1 and is mainly hosted by the neoproterozoic jiuling granodiorite batholith 820 ma the deposit is characterized by four types of alteration including biotite alteration phyllic alteration greisenization and silicification whole rock geochemical analyses showed that the elements ti ni v sc and lu exhibited immobility during the four alteration processes the mobile element geochemistry effectively differentiated among four distinct hydrothermal alteration styles during the biotite mineralization there were mass gains in al2o3 fe2o3 mno mgo k2o p2o5 and w and depletions in sio2 cao and na2o the phyllic alteration exhibited mass gains in sio2 fe2o3 mgo and w and depletions in cao na2o and k2o but al2o3 mno and p2o5 were immobile the weak greisenization exhibited mass gains in sio2 fe2o3 k2o p2o5 and w and depletions in na2o mgo and cao whereas al2o3 and mno remained immobile the silicification exhibited mass gains in sio2 and w and depletions in al2o3 fe2o3 mgo cao na2o and k2o but mno exhibited immobility these alterations were related to at least three major hydrothermal fluid systems firstly a hydrothermal fluid caused biotitization zones and fe mn w mineralization mostly biotite at temperatures ranging from 560 to 450 c and the magmatic hydrothermal fluids were derived from the jurassic porphyritic biotite granite 150 ma and characterized by alkaline oxidized moderate pressure and low salinity features secondly a hydrothermal fluid was formed by the mixing of magmatic fluid with meteoric water and was responsible for the phyllic weak greisenization alteration zones at temperatures ranging from 440 to 450 c and characterized by alkaline weakly oxidized moderate pressure and low salinity features thirdly a hydrothermal fluid caused greisenization and silicification zones at temperatures ranging from 440 to 160 c which were characterized by acidic reduced high pressure and moderate to low salinity features derived from the cretaceous fine grained biotite granite 144 ma and associated with the main w mineralization at dahutang the characteristics of fe enriched biotite in the biotitized and greisenized rock decreasing temperature high pressure and high log fh2o fhcl could facilitate the formation of scheelite deposits but the high f suppressed it the decreasing temperature and f o2 and the fe mn released from biotite contributed to the formation and precipitation of wolframite during alkaline alteration biotite and apatite acted as storage places for fe mn and ca which were subsequently released by acidic alteration to form scheelite and wolframite there was no enough ca was released from biotitized rock for the acidic alteration to provide the wo42 to form scheelite meanwhile fe was added and mn was released to form the scheelite wolframite deposit based on all the results we have developed a genetic model for tungsten mineralization including superimposed alteration processes alkaline overprinted by acidic alteration which led to the gain and loss of elements and corresponding to two magmatic events the jurassic porphyritic biotite granite and the cretaceous fine grained biotite granite the superposition of alterations played an important role in the mineralization of the dahutang giant tungsten deposit

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 2 application to taupo volcanic zone rhyolites constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems however it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems in this study we use the recently developed rhyolite melts geobarometer to constrain the crystallisation pressures of rhyolites from the taupo volcanic zone tvz as sanidine is absent from the studied deposits we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass the quartz 1 feldspar constraint of gualda and ghiorso contrib mineral petrol 168 1033 doi 10 1007 s00410 014 1033 3 2014 we use glass compositions matrix glass and melt inclusions from seven eruptive deposits dated between 320 and 0 7 ka from four distinct calderas in the central tvz and we discuss advantages and limitations of the rhyolite melts geobarometer in comparison with other geobarometers applied to the same eruptive deposits overall there is good agreement with other pressure estimates from the literature amphibole geobarometry and h2o co2 solubility models one of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions regardless of volatile saturation the examples presented also emphasise the utility of this method to filter out spurious glass compositions pressure estimates obtained with the new rhyolite melts geobarometer range between 250 to 50 mpa with a large majority at 100 mpa these results confirm that the tvz hosts some of the shallowest rhyolitic magma bodies on the planet resulting from the extensional tectonic regime and thinning of the crust distinct populations with different equilibration pressures are also recognised which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events

mesozoic metallogeny in east china and corresponding geodynamic settings an introduction to the special issue abstract the giant east china mesozoic metallogenic province hosts some of the world s largest resources of tungsten tin molybdenum antimony and bismuth ores of gold silver mercury lead zinc copper uranium and iron are also of major importance the province and its constituent metallogenic belts or regions south china middle lower yangtze river valley east qinling dabie interior of north china craton yan liao and north east china are the products of several pulses of igneous activity and mineralisation between 240 and 80 ma each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the mesozoic this geodynamic evolution is linked to a complex series of tectonic events involving far field subduction plate collisions crustal thickening post collision collapse and rifting

low pressure origin of high silica rhyolites and granites abstracthigh silica rhyolites and granites 75 wt sio2 anhydrous basis are common features of the crust as part of both the volcanic and the plutonic records while low crystallization pressure 500 mpa we use glass compositions derived from the earthchem portal selected natural examples from the literature and rhyolite melts calculations to show that the phase relations in the quartz albite orthoclase ternary dictate the silica content of silicic melts in particular we show that silica content of melts increases with decreasing pressure as a result of the displacement of the quartz sanidine cotectic toward the qz apex with decreasing pressure it follows from our analysis that 1 the crust is expected to be stratified in terms of the silica content of residing melts 2 high silica glasses form at low pressure requiring shallow level cryst

geochemistry of the highly evolved sn w mo bearing mount douglas granite new brunswick canada implications for origin and mineralization abstract the late devonian 368 1 ma post orogenic peraluminous mount douglas granite located in southwestern new brunswick canada forms the eastern part of the saint george batholith the batholith was emplaced following the accretion of the gander and avalon zones of the northern appalachians during the neoacadian orogeny the mount douglas granite is divided into three units dmd1 dmd2 and dmd3 that formed by progressive high degrees of fractional crystallization the two most fractionated units i e dmd2 and dmd3 are locally associated with greisen sheeted veins related endogranitic sn w mo mineralization whole rock 18o values of 6 0 to 7 3 high initial 87sr 86sr ratios of 0 70550 to 0 71665 slightly positive end 368 ma values 0 3 to 1 1 and pb isotopic data indicate that the granite was derived dominantly from partial melting of juvenile avalonian crust contaminated by supracrustal rocks petrochemical features support the idea of multiple periods of fractional crystallization at lower temperatures producing compositionally more evolved magmas this is supported by estimated zircon saturation temperatures that decrease from dmd1 747 826 c dmd2 733 817 c dmd3 729 816 c the estimated crystallization pressure based on normative quartz and feldspar albite orthoclase contents also decreases with increasing silica content from 2 3 kbar 7 7 km in dmd1 to 0 3 kbar 1 km in dmd3 whole rock geochemical characteristics of this system show that unit dmd3 is the most highly evolved unit it has the highest sio2 avg 76 4 wt lile contents e g li rb cs rb sr mean 42 certain hfse ta th u including y 138 ppm and ree and has the most pronounced negative eu anomalies avg eu eu 0 08 and the lowest k rb 70 127 nb ta mean 4 9 and zr hf mean 23 5 therefore unit dmd3 with the highest content of incompatible elements is the most prospective for sn w zn bi and u mineralization and then dmd2

re os ages for the shameika porphyry mo deposit and the lipovy log rare metal pegmatite central urals russia the ages for pegmatite rare metal and beryl emerald deposits as well as porphyry mo deposits in the hercynian uralide orogen are not well known five molybdenite samples from the lipovy log pegmatite ta nb mo deposit and 11 molybdenite samples from the shameika porphyry mo deposit were selected for re os dating both mineral occurrences are spatial temporally associated with the adui composite granite pluton a well known rare metal related granite intrusion a re os isochron age of 262 0 7 3 ma was obtained for the lipovy log pegmatite ta nb mo deposit the shameika porphyry mo deposit associated with the malyshevo leucogranitic stock and surrounding hornfels provided isochron ages of 273 5 and 282 6 ma for two groups of molybdenite within stock and within hornfels all of these re os ages are consistent with presumed hercynian ages for the granite intrusions formed in a post collisional setting within the uralide orogen

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

geology geochemistry and age of the hukeng tungsten deposit southern china abstract the hukeng tungsten deposit located in the wugongshan area in central part of jiangxi province south china is a large scale quartz vein wolframite deposit it is hosted in the hukeng granitic intrusion based on the mineral assemblage and crosscutting relationship of the veins three mineralization stages are identified including 1 quartz wolframite stage 2 quartz fluorite wolframite stage and 3 quartz pyrite sphalerite wolframite stage the homogenization temperatures of fluid inclusions in vein quartz vary from 220 to 320 c and the salinities are from 0 to 10 wt nacl equiv corresponding densities range from 0 7 to 1 g cm 3 these features indicated that the ore forming fluids in the hukeng tungsten deposit have medium temperature low density and low salinity the 18 osmow values of quartz range from 10 8 to 14 4 with corresponding 18 o fluid values of 3 7 to 7 7 and d values of fluid inclusions of between 70 and 55 the combined isotopic data indicate that the ore forming fluids of the hukeng tungsten deposit were mainly derived from magmatic water with some minor input from meteoric water we have carried out molybdenite re os and muscovite 40 ar 39 ar dating to constrain the timing of mineralization re os dating of six molybdenite samples yielded model ages ranging from 149 1 2 0 to 150 7 3 7 ma with an average of 150 0 ma the re os analyses give a well defined 187 re 187 os isochron with an age of 150 2 2 2 ma mswd 0 60 hydrothermal muscovite yields a plateau 40 ar 39 ar age of 147 2 1 4 ma 40 ar 39 ar age is in good agreement with the re os age these ages show that the timing of tungsten mineralization occurred at about 150 ma our new data when combined with published geochronological results from the other major deposits in this region suggest that widespread w mineralization occurred in the late jurassic throughout south china

deep and shallow sources for the lusi mud eruption revealed by surface deformation the lusi mud eruption in east java indonesia began in may 2006 and continues to the present previous analyses of surface deformation data suggested an exponential decay of the pressure in the mud source but did not constrain the location geometry and evolution of the possible source s of the erupting mud and fluids to map the surface deformation we employ multitemporal interferometric synthetic aperture radar and analyze a well populated l band data set acquired by the advanced land observing satellite alos between may 2006 and april 2011 we then apply a time dependent inverse modeling scheme volume changes occur in two regions beneath lusi at 0 3 2 0 km and 3 5 4 75 km depth the cumulative volume change within the shallow source is 2 3 times larger than that of the deep source the observation and model suggest that a shallow source plays a key role by supplying the erupting mud but that additional fluids do ascend from depths 4 km on eruptive timescales

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

magma emplacement differentiation and cooling in the middle crust integrated zircon geochronological geochemical constraints from the bergell intrusion central alps abstract u th pb zircon geochronology is an essential tool for quantifying the emplacement differentiation and thermal evolution of crustal magmatic systems however the power of u pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry as this permits more detailed interpretations of geochronological datasets than conventionally achieved our approach to better relating zircon dates and geological processes consists of a multi method analytical workflow including cathodoluminescence imaging cl in situ la icpms epma zircon geochemistry u pb zircon id tims geochronology and solution icpms zircon trace element analysis u pb tims tea these methods are here applied to zircon from the bergell intrusion a composite alpine pluton preserving a 10 km mid crustal transect hand samples of tonalite granodiorite and hybridized granitoid each record 250 700 kyr of autocrystic zircon growth bergell zircons are ubiquitously zoned with ca 10 4 10 6 yr growth histories as evidenced by id tims analysis of microsampled fragments from single crystals u pb tims tea data exhibit compositional trends on multiple spatiotemporal scales including the handsample scale representing in situ differentiation at the emplacement level e g th u lithology scale defining trajectories corresponding to the production of tonalitic versus granodioritic magmas lu hf and pluton scale indicating increasingly evolved melts over 1 6 myr of pluton assembly zr hf these absolute tims tea temporal trends are corroborated by relative la icpms epma core to rim geochemistry we compare records of trace element evolution from tims tea bergell whole rock geochemistry and a global compilation of whole rock geochemical data these findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust

diverse late stage crystallization and storage conditions in melt domains from the youngest toba tuff revealed by age and compositional heterogeneity in the last increment of accessory phase growth the chemical record contained within the final increment of growth on crystals is utilized to reveal the dynamics and timescales of magma assembly and storage before eruption of the cataclysmic 2800 km3 youngest toba tuff ytt indonesia in situ u th disequilibrium dates and trace element concentrations were obtained via secondary ionization mass spectrometry sims on unsectioned and unpolished faces of individual zircon and allanite crystals the six high silica 73 wt sio2 pumices from which crystals were derived are among the more evolved and lower crystallinity 25 wt pumices from the ytt eruption and likely represent the melt dominated portion of the magma system discrete sims measurement cycles were coupled with statistical treatments to detect zircon and allanite surface zoning domains at the 1 m scale coupled r melts and accessory phase saturation modeling indicates that at the granite ternary minimum or eutectoid conditions that define this portion of the ytt zircon and allanite crystallization is dependent on and proportionate to major phase crystallization and is more limited than at pre eutectoid conditions a lower proportion of near eruption zircon surface ages in the comparatively cool and wet ytt relative to other hotter and drier voluminous silicic eruptions could reflect the influence of eutectoid storage conditions on magmatic responses to remobilization related magmatic recharge

fitful and protracted magma assembly leading to a giant eruption youngest toba tuff indonesia the paroxysmal eruption of the 74 ka youngest toba tuff ytt of northern sumatra produced an extraordinary 2800 km3 of nonwelded to densely welded ignimbrite and coignimbrite ashfall we report insights into the duration of ytt magma assembly obtained from ion microprobe u th and u pb dates including continuous age spectra over 50 of final zircon growth for pumices and a welded tuff spanning the compositional range of the ytt a relatively large subpopulation of zircon crystals nucleated before the penultimate caldera related eruption at 501 ka but most zircons yielded interior dates 100 300 ka thereafter zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of 100 to 500 ka final zircon growth is evident as thin rim plateaus that are in th u chemical equilibrium with hosts and that give crystallization ages within tens of ka of eruption the longevity and chemical characteristics of the ytt zircons as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge is especially favored at the cool and wet eutectoid conditions that characterize at least half of the ytt wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals repeated magma recharge may have contributed to the development of compositional zoning in the ytt but considered together with limited allanite quartz and other mineral dating and geospeedometry regular perturbations to the magma reservoir over 400 ka did not lead to eruption until 74 ka ago

reos dating of molybdenites from ore deposits in japan implication for the closure temperature of the reos system for molybdenite and the cooling history of molybdenum ore deposits abstract rhenium and osmium abundances were determined on molybdenites from eighteen ore deposits in japan fifteen vein type a skarn type a greisen type and a pegmatite type deposit re os ages obtained for molybdenites from northeastern central and southwestern japan are 100 130 ma 50 75 ma and ca 10 ma respectively reflecting the ages of regional igneous activities in individual areas for vein type ore deposits re os ages for molybdenites agree with rb sr whole rock ages of host granitoids while they are 3 12 ma older than k ar mineral ages of the host rocks on the other hand re os ages of molybdenites from skarn pegmatite and greisen type ore deposits agree with k ar ages of the host rocks the comparison of re os ages for molybdenites with rb sr and k ar ages of host rocks in vein type deposits suggests that re os closure temperature for vein molybdenite is close to whole rock rb sr closure temperature for host granitic rocks i e roughly estimated to be around 500 c and higher than k ar closure temperature for host granodiorites i e 230 370 c if the thermal history of vein molybdenites is essentially equivalent to that of host rocks one possible explanation for age discrepancies of 3 12 ma between re os ages for molybdenites and k ar ages for the wall rocks observed in vein type molybdenum deposits is that later thermal events after formation of molybdenum ores may reset k ar ages hydrothermal alteration might have occurred in such a temperature condition as to disturb k ar mineral systems in wall rocks and made them younger but not the re os system in molybdenite the other explanation might be that the age discrepancies correspond to a cooling interval from ca 500 to 300 c of host granitoid on the other hand the agreement of re os ages of molybdenites and k ar ages of host rocks in skarn pegmatite and greisen type deposits in this study indicates no later disturbance for k ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits

re os ages for archean molybdenite and pyrite kuittila kivisuo finland and proterozoic molybdenite kabeliai lithuania testing the chronometer in a metamorphic and metasomatic setting seven 187re 187os ages were determined for molybdenite and pyrite samples from two well dated precambrian intrusions in fennoscandia to examine the sustainability of the re os chronometer in a metamorphic and metasomatic setting using a new 187re decay constant 1 666 10 11y 1 with a much improved uncertainty 0 31 we determined replicate re os ages for molybdenite and pyrite from the kuittila and kivisuo prospects in easternmost finland and for molybdenite from the kabeliai prospect in southernmost lithuania these two localities contain some of the oldest and youngest plutonic activity in fennoscandia and are associated with newly discovered economic au mineralization ilomantsi finland and a cu mo prospect kabeliai lithuania two re os ages for vein hosted kabeliai molybdenite average 1486 5 ma in excellent agreement with a 1505 11 ma u pb zircon age for the hosting kabeliai granite pluton the slightly younger age suggests the introduction of cu mo mineralization by a later phase of the kabeliai magmatic system mean re os ages of 2778 8 ma and 2781 8 ma for kuittila and kivisuo molybdenites respectively are in reasonable agreement with a 2753 5 ma weighted mean u pb zircon age for hosting kuittila tonalite these re os ages agree well with less precise ages of 2789 290 ma for a rb sr whole rock isochron and 2771 75 ma for the average of six sm nd tdm model ages for kuittila tonalite three re os analyses of a single pyrite mineral separate from the same sample of kuittila pluton that yielded a molybdenite separate provide individual model ages of 2710 27 2777 28 and 2830 28 ma re 17 4 12 1 and 8 4 ppb respectively with a mean value of 2770 120 ma in agreement with the kuittila molybdenite age the re and 187os abundances in these three pyrite splits are highly correlated r 0 9994 and provide a 187re 187os isochron age of 2607 47 ma with an intercept of 21 ppt 187os mswd 1 1 it appears that the re os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187os intercept reflects the in situ decay of 187re during the 160 to 170 m y interval from 2778 ma time of molybdenite pyrite deposition to 2607 ma time of pyrite resetting when the re os data for molybdenites from the nearby kivisuo prospect are plotted together with the kuittila molybdenite and pyrite data a well constrained five point isochron with an age of 2780 8 ma and a 187os intercept 2 4 3 8 ppt of essentially zero results mswd 1 5 we suggest that the pyrite isochron age records a regional metamorphic and or hydrothermal event possibly the time of au mineralization a proposed re os age of 2607 ma for au mineralization is in good agreement with radiometric ages by other methods that address the timing of archean au mineralization in deposits worldwide so called late au model molybdenite in contrast provides a robust re os chronometer retaining its original formation age of 2780 ma despite subsequent metamorphic disturbances in archean and proterozoic time

age constraints on felsic intrusions metamorphism and gold mineralisation in the palaeoproterozoic rio itapicuru greenstone belt ne bahia state brazil u pb sensitive high resolution ion microprobe mass spectrometer shrimp ages of zircon monazite and xenotime crystals from felsic intrusive rocks from the rio itapicuru greenstone belt show two development stages between 2 152 and 2 130 ma and between 2 130 and 2 080 ma the older intrusions yielded ages of 2 152 6 ma in monazite crystals and 2 155 9 ma in zircon crystals derived from the trilhado granodiorite and ages of 2 130 7 ma and 2 128 8 ma in zircon crystals derived from the teofil ndia tonalite the emplacement age of the syntectonic ambrosio dome as indicated by a 2 080 2 ma xenotime age for a granite dyke probably marks the end of the felsic magmatism this age shows good agreement with the ar ar plateau age of 2 080 5 ma obtained in hornblendes from an amphibolite and with a u pb shrimp age of 2 076 10 ma in detrital zircon crystals from a quartzite interpreted as the age of the peak of the metamorphism the predominance of inherited zircons in the syntectonic ambrosio dome suggests that the basement of the supracrustal rocks was composed of archaean continental crust with components of 2 937 16 3 111 13 and 3 162 13 ma ar ar plateau ages of 2 050 4 ma and 2 054 2 ma on hydrothermal muscovite samples from the fazenda brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition the ar ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism or episodically between 2 080 ma and 2 050 ma during uplift and exhumation of the orogen

contrasting stratified plutons exposed in tilt blocks eldorado mountains colorado river rift nv usa abstract roof to floor exposures of mid miocene plutons in tilt blocks south of las vegas nv reveal distinct but strongly contrasting magma chamber statigraphy the searchlight and aztec wash plutons are well exposed stratified intrusions that show a similar broad range in composition from 45 75 wt sio 2 homogeneous granites that comprise about one third of each intrusion are virtually identical in texture and elemental and isotopic chemistry mafic rocks that are present in both plutons document basaltic input into felsic magma chambers isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination whereas more felsic rocks are hybrids that are either juvenile basaltic magma crustal melt mixtures or products of anatexis of ancient crust young mesozoic or miocene mafic intraplate despite general similarities the two plutons differ markedly in dimensions and lithologic stratigraphy the searchlight pluton is much thicker 10 vs 3 km and has thick quartz monzonite zones at its roof and floor that are absent in the aztec wash pluton isotopic and elemental data from searchlight pluton suggest that the upper and lower zones are cogenetic with the granite we interpret the finer grained slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate in contrast the upper part of the aztec wash pluton is granite and a heterogeneous mafic rich injection zone with distinct isotopic chemistry forms the lower two thirds of the intrusion similar mafic rocks are relatively sparse in searchlight pluton and do not appear to have played a central role in construction of the pluton large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the aztec wash pluton but absent in the searchlight pluton thus although both intrusions were filled by similar magmas and both developed internal stratification the two intrusions evolved very differently the distinctions may be attributable to scale and resulting longevity and or to subtle differences in tectonic setting

reliable re os age for molybdenite abstract re os ages for four molybdenites were determined by our newly developed method with isotope dilution mass spectrometry special caution was given to achieve osmium isotopic equilibration between sample and spike during chemical treatment re os data for molybdenites from mt mulgine australia matasvaara finland preissac canada and climax usa define ages of 2790 ma 2600 ma 2680 ma and 26 6 ma respectively the reproducibilities of our re os age determination are shown to be within 2 by the replicate analyses for mt mulgine matasvaara and preissac samples the ages obtained are in good agreement with reference ages estimated for wall rocks of molybdenite deposits concerned thus with careful experimental procedure it appears that the re os geochronometer can be used to reliably determine the age of molybdenite

the invisible hand tectonic triggering and modulation of a rhyolitic supereruption identifying the processes that initiate and control large explosive eruptions on time scales of human interest is challenging we present evidence for syneruptive subsurface interactions between rhyolite magmas derived from distinctive sources that reflect tectonic processes starting then temporarily stopping the 27 ka oruanui supereruption 530 km 3 magma taupo new zealand subordinate 3 16 biotite bearing pumice lapilli occur in fall deposits from phases 1 and 2 of 10 among the biotite free normal pumices in the oruanui deposits the biotite bearing magma is sourced to an independent magmatic system 10 15 km northeast of taupo lateral migration of the biotite bearing magma was initiated during a rifting episode that also triggered the oruanui eruption limited physical mingling indicates that the biotite bearing and oruanui biotite free magmas interacted in the conduit simultaneously with eruption after phase 1 a time break of months elapsed before phase 2 explosive activity resumed from the same vent again with intermixed biotite bearing magma a rift related tectonic event with its accompanying stress change is inferred to have initiated diking from northeast to southwest transporting biotite bearing magma and triggering the oruanui eruption stress relaxation following the rifting event terminated phase 1 before a tectonic event caused renewed diking and activity in phase 2 tectonics may trigger large silicic eruptions through rupturing of shallow crustal magma chambers and lateral magma transport especially in extensional regimes without necessarily leaving any discernible geological imprint only the presence of a foreign magma in the oruanui deposits allows the tectonic control to be identified

the world class nanling metallogenic belt jiangxi china w and sn deposition at 160 ma followed by 30 m y of hydrothermal metal redistribution abstract the w sn maoping deposit and related w sn deposits from the world class nanling range in southeast china formed at ca 160 ma and experienced several phases of metal addition and metal redistribution between 160 and 130 ma isotopic dating of ore mineral wolframite of maoping demonstrates that w was deposited first during a hydrothermal event at ca 160 ma successive fluid episodes at ca 156 ma mo and sn rich fluid and 152 ma ree rich fluid resulted in the formation of ree and fe cu zn sulfide minerals a last fluid event occurring at ca 130 ma was responsible for the deposition of zr ree nb ta minerals that are attributed to magmatic fluids derived from unexposed magmatic bodies the three episodes of fluid circulation post dating w deposition resulted in partial to complete resetting of the isotopic systems mica wolframite xenotime which are conventionally used for dating such deposits we show that w and sn mineralization in the nanling range formed during a unique fluid event at ca 160 ma later fluid episodes redistributed the previously deposited metals and sequentially introduced additional metals including mo sn fe cu zn and zr ree nb ta direct dating of paragenetically well constrained minerals is therefore critical for determining the age and the duration of mineralizing processes and for characterizing the fluid evolution of magmatic hydrothermal systems as exemplified in the nanling range

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

extensional tectonics within a subduction type orogen the case study of the wugongshan dome jiangxi province southeastern china abstract in southeastern china the wugongshan massif consists of metamorphic rocks and granites generally considered as basement rocks formed during an early paleozoic orogeny our structural and radiometric reappraisal of this massif shows that it results from a mesozoic doming in the metamorphic rocks the foliation defines an e w elliptical shape and bears a n s stretching lineation the axial zone of the dome is occupied by oriented plutons the outer part of the dome consists of devonian quartzite carboniferous permian limestone and triassic jurassic sandstone ductile deformation observed in the permian carbonates and older units develops contrasting kinematics the northern and southern flanks present top to the north and top to the south motions respectively the same divergent pattern of motion is recorded from brittle structures up to the jurassic sandstone the late cretaceous red beds are undeformed in the axial zone coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates along the n and s flanks of the dome post folial recumbent folds are symmetrically overturned to the n and s respectively but at the two extremities of the dome the fold axes are parallel to the stretching lineation 40 ar 39 ar ages of biotite and muscovite from mylonite and granite suggest a late triassic age for the ductile deformation and an early cretaceous age for the final doming the wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime a model which emphasizes plutonism is discussed from permian to early cretaceous southeastern china was a subduction type orogen the crust of which was thickened by accretion of mantle derived magmas the emplacement of a large amount of mesozoic granitoids in southeastern china promoted thermal softening and gravitational instability that allowed the crust to extend the formation of the wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology

emerald dating through 40ar 39ar step heating and laser spot analysis of syngenetic phlogopite emerald occurring in k metasomatic rocks developed at the contact of the carnaiba leucogranite with serpentinite bahia state brazil has been dated using an original 40ar 39ar procedure it combines step heating and spot fusion experiments on two types of phlogopite crystals 1 bulk samples and individual grains extracted from the enclosing k metasomatic host rocks and 2 syngenetic solid inclusions precipitated along growing zones of the emerald host crystals the second procedure uses in situ laser probe experiments on rock sections in spite of the huge amounts of excess 40ar detected in adjacent emerald we could measure reliable ages of 1951 8 ma and 1934 8 ma for the trecho velho and braulia occurrences respectively spot fusion data had higher discrepancy than the step heating data but minute crystals of phlogopite included in emeralds bearing excess argon do not reveal excess argon a muscovite belonging to the same granite hydrothermal complex gave a plateau age of 1976 8 ma which may correspond to a higher closure temperature of the kar system during the cooling of the whole pluton and associated hydrothermal halo these accurate measurements lead to the following conclusions 1 direct emerald dating is possible 2 in spite of a polyphase history during the transamazonian orogenesis 2 ga combined step heating and spot fusion experiments give a better precision for granite related emerald mineralization than the scattered ages obtained by rb sr and k ar methods 3 the late transamazonian tectonothermal retrograde event which probably caused the dispersion of previous rb sr and k ar data is not revealed by our procedure 4 the emerald mineralization and k metamorphism appear to be linked with the thermal history of the leucogranite 5 in addition to its use in polyphase crustal domains accurate 40ar 39ar dating is of major interest in the field of metallogenic models even for instance for mineralizations characterized by disturbed isotopic systems which record effects as excess argon

precise molybdenite re os and mica ar ar dating of the mesozoic yaogangxian tungsten deposit central nanling district south china the yaogangxian deposit in the central nanling region south china consists of vein type ore bodies hosted in cambrian to jurassic strata and mesozoic granitic intrusions wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz feldspar phlogopite and muscovite we have carried out molybdenite re os and phlogopite and muscovite 40ar 39ar dating to better understand the timing and genesis of mineralization re os dating of eight molybdenite samples yielded model ages ranging from 152 0 3 5 to 161 1 4 5 ma with an average of 156 0 ma the re os analyses give a well defined 187re 187os isochron with an age of 154 9 2 6 ma mswd 2 4 hydrothermal phlogopite and muscovite display extremely flat 40ar 39ar age spectra phlogopite yields a 40ar 39ar plateau age of 153 0 1 1 ma whereas muscovite yields a plateau age of 155 1 1 1 ma both 40ar 39ar ages are in good agreement with the re os ages placing the timing of tungsten mineralization at about 154 ma this age is consistent with the field relationships our new data when combined with published geochronological results from other major deposits in this region suggest that large scale w sn mineralization occurred throughout the central nanling region in the late jurassic

mafic injections in situ hybridization and crystal accumulation in the pyramid peak granite california located in the northern sierra nevada the middle jurassic pyramid peak granite and associated dioritic rocks display superbly exposed interlayered sheets of mafic felsic and hybrid rocks the mafic hybrid layers show a consistent asymmetry sharply chilled on one side and gradational through hybrids to granite on the other and dip to the southwest load casts and silicic pipes along the chilled mafic northeastern margins of the sheets indicate that depositional way up is to the southwest and that the adjacent mount tallac roof pendant was the original floor of the intrusion orientations of the pipes indicate that the layers were tilted after deposition and solidification we interpret the sheets to be intramagmatic flows that formed by injections of mafic magma into a silicic magma chamber followed by flow across the crystal rich base of that chamber the granitic and hybrid rocks interlayered with the mafic sheets represent material that accumulated on the chamber floor between episodes of mafic replenishment most contamination of the mafic sheets probably occurred because of flow front instabilities generated as the mafic magma entered and spread across the chamber floor the upward gradation from diorite to granite with decreasing abundance and size of mafic enclaves can best be explained by convective stirring at the upper boundaries of the dioritic replenishments the pyramid peak granite was probably initiated as a thin concordant sheet intruding into gently dipping jurassic supracrustal rocks the pluton was constructed incrementally with many replenishments of felsic and mafic magma and dominantly solidified by accumulation of crystals on the floor of the chamber the present steep dips of the layers suggest that downward warping of the chamber floor may have been a factor in accommodating the incrementally constructed pluton

re os isotopic dating of molybdenites in the xiaoliugou w mo deposit in the northern qilian mountains and its geological significance abstract the xiaoliugou is a large scale w mo deposit newly explored in the northern qilian mountains caledonian fold belt northwestern china it is a vein and skarn type deposit genetically associated with the xiaoliugou caledonian granodiorite stock re os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 13 ma 2 and model ages of 436 to 496 ma these results suggest that w mo mineralization occurred before the collision of plates and mineralized substances mainly originated from the crust

impact of additional dead carbon on the circulation estimation of thermal springs exposed from deep seated faults in the dongguan basin southern china abstract in hydrothermal systems the dissolved carbon of thermal groundwater may contain components derived from mineral alterations under high temperatures metamorphic source and or from magmatic gases additionally from carbonate dissolution most common these components are known as additional dead carbon which dilutes the modern carbon used for dating leading to the overestimation of 14c ages four typical geothermal springs hosted by deep seated faults and eighteen non thermal springs and wells from the dongguan basin of southern china were investigated to assess the circulation times of the hydrothermal system based on their radiocarbon contents for the thermal springs the circulation depth approximately 3648 m the apparent 14c ages 20 15 to 12 97 ka and the 13c calibrated 14c ages 16 02 to 8 55 ka were estimated the circulation speed of the thermal groundwater was as low as 0 46 m a which was too slow for a fracture hydrothermal system hosted by deep faults the analysis of the helium and carbon isotopes in the thermal groundwater revealed the existence of mantle derived carbon and thermally altered carbon from limestones a 13c 3he 4he illustration was proposed to correct for the additional dead carbon content according to the reassessed 14c ages the groundwater circulation time was overestimated and the circulation speed of the thermal groundwater was underestimated in hydrothermal system the reassessed 14c ages can provide a more reasonable result than the initial slow estimate in the context of a fracture hydrothermal system hosted by deep faults we conclude that measured 14c contents should be reassessed during the use of radiocarbon dating for thermal groundwater if additional dead carbon exists in the water

micro analytical perspectives on the bishop tuff and its magma chamber new in situ major and trace element analytical data are presented for crystals sanidine plagioclase biotite orthopyroxene clinopyroxene and matrix glasses from juvenile materials representing the full bishop tuff sequence from the earliest fall unit f1 to the latest ignimbrite package ig2nc these data are combined with published information to investigate the nature and zonation of the pre eruptive bishop magma chamber our data confirm that this magma chamber was a single unitary body that was thermally and compositionally zoned the zonation was largely established prior to the growth of crystals and also prior to mixing in the lower parts of the chamber induced by late stage intrusion of a magma of contrasting composition and slightly higher temperature the bright rim magma sparse mixed swirly and dacitic pumices show enrichments in ba sr and ti that identify these pumices as possible representatives of the bright rim magma a model revised from previously published work for the pre eruptive magma chamber comprises three main parts 1 an upper volumetrically dominant 2 3 relatively unzoned region that was the source of the earlier eastern erupted ignimbrite units and their coeval fall units 2 a volumetrically minor transition zone that shows evidence for minor degrees of mixing and was the dominant source for the latest eastern erupted part of ig1eb sherwin subunit and the earlier part of the northern erupted ignimbrite ig 2na 3 a lower volumetrically subordinate 1 3 region that was affected by mixing with the bright rim invasive magma in the lead up to the eruption and fed later northern erupted units ingress of the bright rim magma introduced orthopyroxene and bright rimmed zircon crystals and induced partial resorption then overgrowth of rims enriched in ti sr and ba on sanidine and quartz and development of zoning in clinopyroxene based on pumice proportions and associated crystal and glass chemistries through the eruptive sequence we infer that the roof and floor of the magma chamber were stepped down to the north such that the transition zone magma formed the floor of the southern part of the melt dominant chamber and the roof of the northern part our data reinforce the previous concept of a single compositionally and thermally zoned bishop magma chamber and additionally support a temporally constrained role for pre eruptive magma mixing and the introduction of melts and minerals with contrasting compositions to the resident bishop magma

the formation of the world class zhuxi scheelite skarn deposit implications from the petrogenesis of scheelite bearing anorthosite abstract a quartz free scheelite bearing fine to medium grained anorthosite occurs as a dike in the world class zhuxi scheelite skarn deposit of south china the anorthosite mainly comprises an rich plagioclase anavg 91 90 vol scheelite 3 vol apatite 2 5 vol ilmenite 1 5 vol titanite 1 vol as well as minor 2 vol fluorite prehnite chalcopyrite pyrrhotite pyrite sphalerite rutile and uraninite this paper reports the first occurrence of scheelite bearing anorthosite of hydrothermal origin in a w deposit field textural mineralogical and geochemical evidence suggest a hydrothermal genesis for the scheelite bearing anorthosite in the zhuxi deposit the scheelite bearing anorthosite formed when the al si p w and f rich residual magma fluid was contaminated by pure limestone in a reducing environment the formation of the scheelite bearing anorthosite requires fertile w rich metasedimentary rocks as a magmatic source which formed granitic magma emplaced at a great depth that emanated w enriched magmatic hydrothermal fluids during the latest stage of magma crystallization thus producing the large scale tungsten mineralization in the zhuxi mine

rhenium osmium evidence for regional mineralization in southwestern north america more than 40 base metal porphyry ore deposits in southwestern north america are associated with the laramide orogeny about 90 million to 50 million years ago rhenium osmium dates on molybdenite a rhenium enriched sulfide common in many of the deposits reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception magnitude or duration of magmatism deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization rhenium osmium ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago most deposits that formed in the oldest interval are within the older precambrian basement of northwestern arizona whereas the younger deposits are restricted to the younger precambrian basement in southern arizona and northern mexico synchronous widespread mineralization indicates that similar crust mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

timing of skarn deposit formation of the tonglushan ore district southeastern hubei province middle lower yangtze river valley metallogenic belt and its implications abstract the tonglushan ore district in the middle lower yangtze river valley metallogenic belt includes the tonglushan cu fe the jiguanzui au cu and the taohuazui au cu skarn deposits they are characterized by ne striking ore bodies and hosted at the contact of triassic carbonate rocks and late mesozoic granitoid deposits new sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb molybdenite re os and phlogopite 40 ar 39 ar ages indicate that these skarn deposits formed between 140 3 1 1 and 137 3 2 4 ma these dates are identical to the zircon u pb ages for host quartz diorites ranging from 140 2 to 139 1 ma these results confirm that both skarn mineralization and related intrusions were initiated during the early cretaceous the high rhenium contents 261 4 1152 g g of molybdenites indicate that a metasomatic mantle fluid was involved in the ore forming process of these skarn ore systems this conclusion is consistent with previously published constraints from sulfur deuterium and oxygen isotope compositions and the geochemical signatures and sr nd isotopic data of the mineralization hosting intrusions geological and geochronological evidence demonstrates that there were two igneous events in the tonglushan ore district the first resulted in the emplacement of quartz diorite during the early cretaceous 140 2 to 139 1 ma and the second is characterized by the eruption of volcanic rocks during the mid early cretaceous 130 2 to 124 2 ma the former is spatially temporally and genetically associated with skarn gold bearing mineralization 140 3 1 1 to 137 3 2 4 ma the recognition of these two igneous events invalidates previous models that proposed continuous magmatism and associated mineral deposits in the middle lower yangtze river valley metallogenic belt

rhyolite melts vs derp reply to comment by gualda et al on the effect of anorthite content and water on quartz feldspar cotectic compositions in the rhyolitic system and implications for geobarometry by wilke et al 2017 journal of petrology 58 no 4 789 818

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

zircon u pb molybdenite re os and muscovite ar ar geochronology of the yashan w mo and xiatongling w mo be deposits insights for the duration and cooling history of magmatism and mineralization in the wugongshan district jiangxi south china abstract to test the impact of cooling history and prolonged hydrothermal activity on the grade and total endowment of the tungsten deposits we obtained new and detailed geochronology from the yashan w mo and the xiatongling w mo be deposits in the wugongshan mining district in jiangxi province south china both deposits contain stockwork and greisen ores within contact zones between granitic plutons and wall rock metasandstone and mudstone and within the granitic plutons new shrimp u pb zircon analyses yielded weighted mean 206 pb 238 u ages of 231 0 5 0 ma and 161 5 1 8 ma for the monzogranite and granite in the yashan w mo deposit and 154 7 1 7 ma for the granite in the xiatongling w mo be deposit three molybdenite samples from stockwork ores in the yashan deposit and four molybdenite samples from greisen and stockwork ores in the xiatongling w mo be deposit yielded re os isochron ages of 156 0 2 8 ma and 156 4 8 7 ma respectively two muscovite samples from greisen w mo ores and w bearing stockwork in the yashan deposit gave 40 ar 39 ar plateau ages of 152 77 0 78 ma and 152 36 0 78 ma and two muscovite samples from greisen w mo be ores and w mo bearing muscovite quartz stockwork ores in the xiatongling deposit gave 40 ar 39 ar plateau ages of 146 85 0 8 ma and 146 31 0 75 ma respectively the age data indicate that the ore forming granitic magmas were emplaced earlier at the yashan deposit than the xiatongling deposit but mo mineralization was concurrent at both deposits and the last hydrothermal ore forming alteration started earlier again in the yashan deposit the u pb re os and ar ar geochronological data indicate that both deposits had similar durations of magmatic hydrothermal process 9 1 1 9 m y for the yashan deposit and 8 6 1 8 m y for the xiatongling deposit but with different cooling path rate the yashan deposit experienced a slow cooling rate 36 4 c m y followed by a moderately rapid cooling rate 48 6 c m y whereas the xiatongling deposit had an early rapid cooling rate 74 1 c m y followed by a slow cooling rate 29 7 c m y unlike porphyry cu systems the longevity of magmatic hydrothermal process is not likely to be a key controlling factor for the total endowment and grade of w deposits however our analyses show that rapid cooling rates might facilitate formation of high grade stockwork and greisen w deposits

petrogenesis of late mesozoic high ba sr granites in the qiushuwan cumo orefield implications for the distribution of porphyry mo mineralization in the east qinling area of central china abstract the qinling dabie orogenic belt of china is well known for its large mo resources there have been many studies of the genesis of the mo deposits but the reasons for the intensive mineralization during the late mesozoic and the spatially uneven distribution of the resulting ore deposits are not fully understood deposits with reserves of 6 mt of mo occur predominantly in the southern margin of the north china craton ncc and are mostly late mesozoic porphyry deposits in comparison a few relatively insignificant and cu dominated porphyry ore deposits have been identified in the north and south qinling terranes this study presents new whole rock geochemical isotopic and zircon u pb age data for ore bearing granites associated with the qiushuwan cu mo deposit the largest porphyry deposit in the north qinling nq terrane which provide insights into the asymmetric distribution of mo mineralization in this region the rocks display high k calc alkaline to shoshonitic affinities and are metaluminous to weakly peraluminous with high ba and sr contents low rb nb and ta contents and depletion in heavy rare earth elements and y which are features typical of high ba sr granites zircon u pb dating indicates that the granites crystallized at ca 145 ma their narrow ranges of whole rock initial 87sr 86sr ratios 0 7056 0 7069 and end t 7 4 to 9 7 zircon ehf t 2 49 to 5 13 and 18o 5 58 to 6 49 values together with the presence of significant nd hf isotopic decoupling hf 7 61 to 11 3 indicate that the parent magma was derived from partial melting of enriched subcontinental lithospheric mantle with minor assimilation of the lower crust in contrast late mesozoic granitic rocks in the southern margin of the ncc including both mineralized and barren intrusions are geochemically associated with the crust of the northern margin of the yangtze block yb the yb contains mo rich shales that are thought to have been subducted beneath the southern margin of the ncc during the triassic melting of these mo rich black shales would have mobilized mo a process that enabled the formation of the extensive mo mineralization associated with the late mesozoic granites of the southern margin of the ncc this indicates that the extensive mo mineralization within the southern margin of the ncc is genetically related to a mo rich source that was generated by the stagnation of subducted yb continental crust beneath the southern margin of the ncc in comparison the less significant mo mineralization within the east qinling orogenic belt might reflect the variable removal of the subducted yb continental slab during post collisional intracontinental orogenesis