gold in porphyry copper deposits its abundance and fate abstract porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust cu au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5 000 000 with a median near 40 000 which separates gold rich and gold poor deposits gold is found in porphyry copper deposits in solid solution in cu fe and cu sulfides and as small grains of native gold usually along boundaries of bornite sims ion probe analyses of ore minerals from the gold rich batu hijau kingking and skouries porphyry copper deposits show that bornite contains about 1 ppm au whereas chalcopyrite contains about an order of magnitude less chalcocite and covellite contain 10 20 ppm au but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits the amount of gold presently in solid solution in cu fe sulfides is not adequate to account for all the gold in porphyry copper deposits and the remainder is present as micron scale grains of native gold experiments in the cu fe s au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600 700 c and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200 300 c in contrast cu au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary potassic ore and alteration are thought to form this indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in cu fe sulfides when the deposit forms at high temperature and that gold is not commonly added later from other sources although it can be redistributed during cooling or later events the experimental data also suggest that high temperature 600 700 c vapors can extract considerably more gold from porphyry copper systems than can low temperature 300 c alteration comparison to cu au ratios of volcanic emissions suggests further that high temperature processes remove copper relative to gold from porphyry copper systems whereas low temperature processes remove gold preferentially and that this can account for deposits with extremely low and high cu au ratios respectively deposits with cu au ratios between about 20 000 and 100 000 however probably reflect different degrees of removal of gold or copper by immiscible sulfides

geology of the don manuel igneous complex central chile implications for igneous processes in porphyry copper systems the don manuel igneous complex and associated porphyry copper mineralization in the andean cordillera of central chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics the don manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes which intrude the older units mineralization is associated with the biotite tonalite and intermediate porphyries which also contain the greatest abundance of mafic enclaves zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing new zircon u pb ages and whole rock ar ar ages indicate that the don manuel igneous complex was emplaced between ca 4 and 3 6 ma the time scale for the episodic intrusion of the don manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes observations from the don manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement the observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation

melting of plagioclase quartz assemblages at 2 kbar water pressure the melting of plagioclase and quartz has been investigated at ph2o 2kb a single crystal of plagioclase was surrounded by quartz powder and water a reaction rim consisting of glass and of an rich plagioclase developed around unchanged starting plagioclase microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase for example at 850 c the approximate xan of melt is 0 3 and that of plagioclase is 0 8 the temperature interval between beginning of melting and complete melting of cotectic compositions is 100 c or more for quartz plagioclase mixtures with plagioclases between an 40 and an 75 in comparison to the system ab an h2o the plagioclase melting loop is somewhat wider in the investigated system qz ab an h2o but the temperature interval is much smaller 100 c than in the system ab an h2o 200 c the solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments the results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi component rocks than in the pure plagioclase system this finding is important for the development of albite rich rocks from more basic compositions

gold speciation and transport in geological fluids insights from experiments and physical chemical modelling this contribution provides an overview of available experimental thermodynamic and molecular data on au aqueous speciation solubility and partitioning in major types of geological fluids in the earth s crust from low temperature aqueous solution to supercritical hydrothermal magmatic fluids vapours and silicate melts critical revisions of these data allow generation of a set of thermodynamic properties of the auoh aucl 2 auhs and au hs 2 complexes dominant in aqueous hydrothermal solutions however other complexes involving different sulphur forms chloride and alkali metals may operate in high temperature sulphur rich fluids vapours and melts the large affinity of au for reduced sulphur is responsible for au enrichment in s rich vapours and sulphide melts which are important gold sources for hydrothermal deposits thermo dynamic speciation and partitioning data and their comparison with au and s contents in natural fluid inclusions from magmatic hydrothermal gold deposits provide new constraints on the major physical chemical parameters temperature pressure salinity acidity redox and ubiquitous fluid components sulphur carbon dioxide arsenic affecting au concentration transport precipitation and fractionation from other metals in the crust the availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations

magma evolution and the formation of porphyry cu au ore fluids evidence from silicate and sulfide melt inclusions silicate and sulfide melt inclusions from the andesitic farallon negro volcanic complex in nw argentina were analyzed by laser ablation icpms to track the behavior of cu and au during magma evolution and to identify the processes in the source of fluids responsible for porphyry cu au mineralization at the 600 mt bajo de la alumbrera deposit the combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle derived mafic magma that contained approximately 6 wt h2o and 200 ppm cu this magma and a rhyodacitic magma mixed in an upper crustal magma chamber feeding the volcanic systems and associated subvolcanic intrusions over 2 6 million years generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps 1 generation of a sulfide melt upon magma mixing in some parts of the magma chamber 2 partitioning of cu and au into the sulfide melt enrichment factor of 10 000 for cu leading to cu and au concentrations of several wt or ppm respectively 3 a change in the tectonic regime from local extension to compression at the end of protracted volcanism 4 intrusion of a dacitic magma stock from the upper part of the layered magma chamber 5 volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber generating a fluid with a cu au ratio equal to that of the precursor sulfide 6 focused fluid transport and precipitation of the two metals in the porphyry yielding an ore body containing au and cu in the proportions dictated by the magmatic fluid source the cu s ratio in the sulfide melt inclusions requires that approximately 4 000 ppm sulfur is extracted from the andesitic magma upon mixing this exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for s the extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma it appears thus that unusually sulfur rich not cu rich magmas are the key to the formation of porphyry type ore deposits our observations imply that dacitic intrusions hosting the porphyry cu au mineralization are not representative of the magma from which the ore fluid exsolved the source of the ore fluid is the underlying more mafic magma and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated at alumbrera these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole sulfide inclusions in similar dykes of other less explored magmatic complexes may be used to predict the au cu ratio of potential ore forming fluids and the expected metal ratio in any undiscovered porphyry deposit

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

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

physicochemical evolution of a porphyry breccia system a laser ablation icp ms study of fluid inclusions in the mount leyshon au deposit queensland australia the evolution of magmatism brecciation alteration and mineralization is described for the mount leyshon intrusive complex queensland australia using p t v x analysis and major to trace element analysis of single fluid inclusions using laser ablation icp ms the 290 ma mount leyshon intrusive complex is a subvolcanic porphyry breccia system mined for 97 metric tons t au 70 mt at 1 4 g t au between 1986 and 2000 the mount leyshon intrusive complex is characterized by several generations of genetically related breccias and magmatic intrusions both alteration and mineralization can be traced to specific intrusive events the earliest phase of brecciation was followed by widespread biotite magnetite alteration stage 1 fluid inclusions associated with stage 1 were trapped at 350 c and 145 bars and were anomalously k and fe rich this stage was followed by the emplacement of rhyolite to dacite stocks and coeval breccia pipes stage 2 fluids exsolved from these magmas simultaneously exsolved na k fe mn rich brine 60 wt nacl equiv and dilute vapor at 500 bars 2 km paleodepth and 600 c pressures likely oscillated between lithostatic and sublithostatic conditions in response to the transient rupture and sealing of intrusions and wall rocks hydrothermal fluids infiltrating breccia pore space produced base metal mineralization stage 3 stage 3 fluid inclusions contain 2 to 10 wt percent nacl equiv and were trapped at 300 to 400 c below 500 bars stage 3 fluids were depleted in k fe and mn and enriched in ca sr and ba relative to stage 2 brines this trend is consistent with the transition from potassic alteration in stage 2 to feldspar destructive phyllic alteration in stage 3 cu zn and pb were conservative elements above 350 c and precipitated nonselectively as sulfides below 350 c in a bulk ratio very similar to that measured in fluid inclusions subsequent emplacement of andesite to rhyodacite dikes stage 4 was marked by au and additional base metal sulfide mineralization in veins and breccia cavities stage 4 fluids are chemically similar to stage 3 but contain several mol percent co2 perhaps suggesting a more mafic co2 charged magma source relative to earlier intrusive pulses in addition to fluid cooling the destabilization of au bisulfide complexes attending phase separation of h2o co2 nacl fluids below 400 c and 400 bars was the likely cause for au precipitation mount leyshon appears to indicate that co2 can play a crucial role in the transport of gold bearing solutions from a magmatic source at depth into much shallower hydrothermal environments

the partitioning behavior of silver in a vapor brine rhyolite melt assemblage abstract the partitioning of silver in a sulfur free rhyolite melt vapor brine assemblage has been quantified at 800 c pressures of 100 and 140 mpa and f o 2 nno nickel nickel oxide silver solubility 2 in rhyolite increases 5 fold from 105 21 to 675 98 g g as pressure increases from 100 to 140 mpa nernst type partition coefficients d ag i j 2 describing the mass transfer of silver at 100 mpa between vapor and melt brine and melt and vapor and brine are 32 30 1151 238 and 0 026 0 004 respectively at 140 mpa values for d ag i j 2 for vapor and melt brine and melt and vapor and brine are 32 10 413 172 and 0 06 0 03 respectively apparent equilibrium constant values 2 describing the exchange of silver and sodium between vapor and melt k ag na v m at 100 and 140 mpa are 105 68 and 14 6 the average values 2 for silver and sodium exchange between brine and melt k ag na b m at 100 and 140 mpa are 313 288 and 65 12 these data indicate that the mass transfer of silver from rhyolite melt to an exsolved volatile phase s is enhanced at 100 mpa relative to 140 mpa suggesting that decompression increases the silver ore generative potential of an evolving silicate magma model calculations using the new data suggest that the evolution of low density aqueous fluid i e vapor may be responsible for the the silver tonnage of many porphyry type and perhaps epithermal type ore deposits for example halter et al halter w e pettke t and heinrich c a 2002 the origin of cu au ratios in porphyry type ore deposits science 296 1842 1844 used detailed silicate and sulfide melt inclusion and vapor and brine fluid inclusions analyses to estimate a melt volume on the order of 15 km3 to satisfy the copper budget at the bajo de la alumbrera copper gold silver ore deposit using their melt volume estimate with the data presented here model calculations for a 15 km3 felsic melt saturated with pyrrhotite and magnetite suggest that a low salinity magmatic vapor may scavenge on the order of 7 1012 g of silver from the melt this quantity of silver exceeds the discovered 2 109 g of ag at alumbrera calculated tonnages for numerous other deposits yield similar results the excess silver in the vapor remaining after porphyry formation is then available to precipitate at lower ptconditions in the stratigraphically higher epithermal environment these data suggest that silver and perhaps other ore metals in the porphyry epithermal continuum may be derived solely from the time integrated flux of dominantly low salinity vapor exsolved from a series of sequential magma batches

the role of magmatic sulfides and mafic alkaline magmas in the bingham and tintic mining districts utah this paper reports the existence of abundant magmatic sulfides in introduction latite lava vitrophyres and vent facies dikes from the bingham and the eruption of sulfur rich magmas from el chichon tintic districts in central utah these dikes and flows appear to luhr et al 1984 and mt pinatubo westrich gerlach be comagmatic with the ore related monzonite intrusions in addition 1992 in recent years has attracted attention to the limited analytical data suggest that the magmatic sulfides contain most of solubility of sulfur in intermediate magmas and the factors the cu and ag present in the latites magmatic sulfides are preserved potentially responsible for the formation of magmatic only in the least oxidized and least degassed lavas and vitrophyres sulfides and anhydrite however the occurrence of magpetrography of comagmatic dikes and intrusions suggests that matic sulfides in intermediate magmas is not universal resorption and oxidation s degassing of the globules may make many volcanic suites appear to be sulfide unsaturated the metal content available to a magmatic ore fluid minette and inasmuch as magmatic sulfides may sequester chalcophile shoshonite lavas occur in subordinate amounts in the latitic volcanic metals what would their presence signify in magmatic fields associated with latite monzonite intrusions and hydrothermal systems that formed porphyry cu and mesothermal vein ore deposits of the tintic ag pb zn cu au and bingham deposits do they sequester a significant portion of the porphyry cu au mo districts utah the ages chemical and magmatic cu pb zn ag and au thereby restricting modal compositions and field relations of the mafic alkaline lavas metal availability of subsequent ore forming fluids suggest that such magmas mixed and fractionated at shallow crustal we have examined the volcanic rocks from the tintic levels to form the ore related latites and monzonites the unaltered and bingham mining districts fig 1 utah and found mafic alkaline lavas in these districts are some of the most cu rich that almost all of the volcanic units contain magmatic lavas in the great basin province sulfides some quenched dikes and intrusion margins also have preserved magmatic sulfides comparable with those in the volcanic rocks several lines of evidence indicate that the volcanic and intrusive rocks are comagmatic keith et al 1989 1991 waite 1996 the tintic district and east tintic sub district in utah have recorded over

gold transporting hydrothermal fluids in the earth s crust an introduction hydrothermal ore deposits that are exploited for gold include both gold only deposits such as orogenic deposits and gold bearing examples of the common hydrothermal deposit types that are formed around upper crustal magmatic centres in particular porphyry and epithermal deposits fluid inclusion data have shown that ore fluids of gold only deposits are compositionally distinct compared to fluids of other deposit types fluids of the latter deposit types are in contrast not distinctive and recent research has been focussed on relating gold content to geochemical and petrological aspects of related magmatic systems and to fluid evolution on migration through the crust this special publication includes an up to date summary of thermodynamic parameters of aqueous au species at high temperatures and pressures a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world several comprehensive case studies of different types of gold deposits and their fluids from the usa brazil egypt slovakia and bulgaria and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales historical and disciplinary context the majority of gold deposits of the earth s crust are generated by fluxes of hot aqueous hydrothermal fluids that are efficiently focussed within relatively small volumes of rock a proportion of this gold is extracted from hydrothermal deposit types such as iron oxide copper gold iocg deposits porphyry deposits and lowand high sulphidation epithermal deposits as a co product with other metals most commonly with copper or silver the other group 12 elements a similar proportion is however a product of gold only hydrothermal deposits including orogenic gold intrusion related gold irgd and carlin type gold deposits in these gold only deposit types au is in most cases not only the only commodity of economic interest in the ore but we can infer from the available multi element compositional data on fluid inclusions that it is the only commodity of interest transported in the hydrothermal fluids of these deposits e g yardley et al 1993 su et al 2009 gold only hydrothermal deposits are thus environments in which gold is very effectively fractionated from other metals presumably as a consequence of gold s unique chemical properties such as high electronegativity and the tendency to form bonds with strong covalent character e g crerar et al 1985 and the strength of relativistic effects on bonding behaviour e g pyykko 2012 the affinity of gold with reduced sulphur in hydrothermal fluids was first experimentally demonstrated by seward 1973 and has been confirmed in all subsequent experimental studies see pokrovski et al 2014 gold only deposits including orogenic gold deposits do appear to be products of hydrothermal fluids that are chemically distinct compared to fluids of other higher temperature hydrothermal ore deposit types including porphyry deposits epithermal deposits and greisens hypersaline fluids are absent at almost all deposits the dominant fluid contains sufficient co2 to form a carbonic phase in fluid inclusions and has relatively low salinity in contrast the fluids of gold rich epithermal and porphyry deposits are not distinct from those of typical deposits of these types at least with respect to compositional fluid parameters that can be constrained through routine microthermometry it is difficult to pinpoint who first recognized that co2 bearing and relatively low salinity ore fluids are characteristic and almost ubiquitous in orogenic gold deposits roedder 1973 in his early comprehensive summary of knowledge of fluid inclusion compositions reported that kravtsov fridman 1965 recorded high co2 in fluid inclusions in gold quartz veins and suggested that its abundance might be an effective exploration tool for gold boyer et al 1967 recognized co2bearing fluid inclusions in quartz veins and summarized the results of other similar studies to suggest a clear relation direct or indirect between gold and co2 in another early work coveney kelly from garofalo p s ridley j r eds gold transporting hydrothermal fluids in the earth s crust geological society london special publications 402 http dx doi org 10 1144 sp402 9 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics at universita di bologna on june 30 2014 http sp lyellcollection org downloaded from

crystallisation ages in coeval silicic magma bodies 238u 230th disequilibrium evidence from the rotoiti and earthquake flat eruption deposits taupo volcanic zone new zealand abstract the timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent successive eruption deposits in the taupo volcanic zone new zealand the earlier caldera forming rotoiti eruption 100 km3 magma at okataina volcano was followed within months at most by the earthquake flat eruption 10 km3 magma from nearby kapenga volcano both generated non welded ignimbrite and coeval widespread fall deposits the rotoiti and earthquake flat deposits are both crystal rich high silica rhyolites with sparse glass bearing granitoid fragments also occurring in rotoiti lag breccias generated during caldera collapse here we report 238u 230th disequilibrium data on whole rocks and mineral separates from representative rotoiti and earthquake flat pumices and the co eruptive rotoiti granitoid fragments using tims and in situ zircon analyses by sims multiple grain zircon controlled crystallisation ages measured by tims from the rotoiti pumice range from 69 3 ka 350 ka with a pronounced peak at 70 90 ka the weighted mean of isochrons is 83 14 ka in accord with the tims data one glass bearing rotoiti granitoid clast yielded an age of 57 8 ka by tims controlled by th rich phases that however are not apparently present in the juvenile pumices another glass bearing rotoiti granitoid yielded sims zircon model ages peaking at 60 90 ka having a similar age distribution to the pumice age data from pumices are consistent with a published 64 4 ka eruptive age now modified to 62 2 ka but chemical and or mineralogical data imply that the granitoid lithics are not largely crystalline rotoiti rhyolite but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies similarly although the earthquake flat eruption immediately followed and probably was triggered by the rotoiti event age data from juvenile material are significantly different a multiple grain zircon controlled crystallisation age measured by tims from a representative pumice is 173 5 ka while sims model ages range from 70 26 34 ka to 350 ka with a peak at 105 ka these age data coupled with previously published geochemical and isotopic data show that the rotoiti and earthquake flat deposits were erupted from independent unconnected magma bodies

zircon crystallization and the lifetimes of ore forming magmatic hydrothermal systems magmatic hydrothermal copper ore formation involves multiple pulses of subvolcanic porphyry intrusion vein opening and hydrothermal ore deposition it is driven by larger subjacent magma reservoirs acting as the source of fluid and ore forming components high precision u pb ages of individual zircon crystals from porphyries immediately predating and postdating cu au mineralization at bingham canyon utah united states and bajo de la alumbrera northwestern argentina show a significant spread of reliably concordant ages this demonstrates zircon crystal formation over a protracted period of 1 m y which is interpreted to record the lifetime of the magma reservoir from which porphyries and ore fluids were extracted the youngest zircons in all pre ore and post ore intrusions overlap within a much shorter time interval of 0 32 m y at bingham canyon and 0 090 m y at alumbrera these youngest zircons of each intrusion are interpreted to bracket the maximum duration of porphyry emplacement and ore formation to short periods consistent with thermal constraints this study illustrates that age brackets based on individual magmatic zircon grains are geologically more informative than the calculation of means and standard deviations based on apparently normal age distributions in zircon populations

syneruptive mixing degassing and crystallization at redoubt volcano eruption of december 1989 to may 1990 abstract the 1989 1990 eruption of redoubt volcano included initial energetic vertical venting of pumiceous tephra followed by shedding of dense pyroclastic flows from successive unstable domes the suite of quenched magmatic materials thus produced coupled with detailed seismic observations provides a record of establishment and subsequent evolution of an active magma column in the upper crust the initial eruptive event involved a dacite magma with rhyolite melt and an andesite magma with dacite melt the dacite magma contains reaction rim free hornblende high cl concentrations 0 12 0 02 wt of melt and is microlite free all evidence for rapid ascent from depth in contrast the andesite component is microlite rich though this characteristic may have arisen by mixing between the two magmas rather than by slow ascent abundant dense juvenile lava like clasts with low h2o 0 1 0 3 wt were ejected in the initial eruption event implying that the magma column had reached the near surface and its upper portion had undergone degassing and bubble collapse prior to explosive disruption later eruptive events contain evidence of progressive blending of the two magmatic components as well as mixing between fresh reservoir derived and old conduit resident magma magma apparently ascended to the surface from the 6 10 km deep source chamber in a series of pulses rather than by steady flow or gradual emptying of a shallowly intruded body increased shallow long period seismicity which may indicate subsurface degassing from the flowing magma column occurred 5 days before each major eruptive event petrologic evidence indicates that each eruption contained some magma that ascended quickly from the chamber with time the proportion of new magma in eruptive batches may have decreased however even the final dome contains magma that left the chamber no more than a few days before extrusion together the pulses apparently record passage of an andesite plume through redoubt s dacite magma reservoir

the occurrence and distribution of mo and molybdenite in unaltered peralkaline rhyolites from pantelleria italy small hexagonal and triangular platelets of molybdenite mos2 5 to 25 m in diameter were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from pantelleria italy the mos2 occurs commonly in pantellerites peralkaline rhyolites rarely in pantelleritic trachytes and never in trachytes the occurrence of euhedral mos2 platelets in all phenocryst phases in matrix glass and even in some melt inclusions indicates that mos2 precipitated directly from the peralkaline melt despite mos2 saturation the melt glass contains greater than 95 of the mo in pantellerian rocks x ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less mo than corresponding matrix glasses the differences being in proportion to phenocryst abundances the mo contents increase with differentiation from trachytes 2 12 ppm to pantellerites 15 25 ppm and correlate positively with incompatible elements such as th y and nb the mo concentrations as determined by secondary ion mass spectrometry are essentially the same in matrix glasses and melt inclusions showing that mo did not partition strongly into a volatile fluid phase during outgassing the high mo contents of the pantellerites relative to metaluminous magmas with 1 5 ppm may be due to several factors 1 the enhanced stability of highly charged cations such as mo6 u4 and zr4 in peralkaline melts 2 the rarity of fe ti oxides and litanite into which mo might normally partition 3 reduced volatility of mo in low fo2 h2o poor 1 2 wt peralkaline magmas geochemical modeling indicates that the precipitation of mos2 can be explained simply by the drop in temperature during magmatic differentiation the occurrence of mos2 in pantellerites may result from their high mo concentrations and low redox state ni nio 2 5 relative to metaluminous magmas causing them to reach mos2 saturation at magmatic temperatures the apparent absence of mos2 microphenocrysts in more oxidized metaluminous rhyolites may indicate that mo is dissolved primarily as a hexavalent ion in those magmas

generation of metal rich felsic magmas during crustal anatexis we show that partial melting of a preexisting mineral deposit during regional anatexis is likely to lead to incorporation of sulfide melt into silicate melt simple dissolution of metals in silicate melt during anatexis leads to minor metal addition to escaping magma but physical entrainment of sulfide melt can lead to extreme enrichment and higher total metal incorporation metal enrichment is enhanced if melt segregation is structurally driven only dissolved metal is incorporated if initial segregation is driven by buoyancy alone however once encapsulated within a larger volume of silicate melt immiscible globules of sulfide melt are physically carried by buoyantly rising magma gradual dissolution of entrained sulfide melt globules is expected as the silicate magma migrates from the lower crust this process may lead to significant variations in metal contents of silicate magmas above regions that underwent anatexis

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

major to trace element analysis of melt inclusions by laser ablation icp ms methods of quantification current techniques for the quantification of melt inclusion chemistry require that inclusions are compositionally homogeneous and that post entrapment devitrification or crystallization onto the inclusion walls could be reversed by appropriate re melting laser ablation icp ms provides a technique by which single heterogeneous inclusions can be analysed thus avoiding the above prerequisites because host mineral is ablated with the inclusion quantification of the melt composition necessitates deconvolution of the mixed signal by an internal standard this can be obtained in various ways including 1 a fixed pre determined concentration of a given element in the melt 2 whole rock differentiation trends in a given igneous suite 3 a constant measured distribution coefficient between the host and the inclusion melt and 4 determination of the volume ratios between the inclusion and total ablated volume these four approaches were tested on a large set of cogenetic inclusions from a single plagioclase crystal in a rhyodacitic intrusion results suggest that quantification through whole rock differentiation trends is the most widely applicable the most accurate and the least time consuming technique provided that the resulting data are critically interpreted with regard to the underlying assumptions uncertainties on the calculated element concentrations in the inclusions depend on the mass ratio between the melt inclusion and the host for a given ablation they are of the order of 10 if the melt inclusion contributes more than 20 to the bulk analytical signal of a particular element calculated limits of detection for spherical 10 m melt inclusions are of the order of a few ppm for elements strongly enriched in the melt relative to the host crystal concentrations in the melt inclusions can be determined even for elements enriched in the host mineral but in this case uncertainties and calculated limits of detection increase with the concentration in the host the uncertainty on the melt composition from a set of cogenetic inclusions can be commonly decreased by calculating of an uncertainty weighted average of the concentration and their uncertainty

white island new zealand volcanic hydrothermal system represents the geochemical environment of high sulfidation cu and au ore deposition the white island volcanic hydrothermal system new zealand is thought to closely represent the chemical conditions that lead to the formation of high sulfidation cu au ore deposits the amounts of cu and au produced over a 10 ka period of activity largely from degassing magma are calculated to be 106 and 45 t respectively altered andesite blocks ejected from recent vents contain alunite anhydrite and pyrite their s isotopic composition indicates vein filling at 380 c at this temperature cu and au are highly soluble in acid solutions which may explain the depletion of cu and absence of au in the ejecta mass balance calculations however suggest that cu and au are precipitated in cooler zones before the acid solutions discharge at the surface

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

fluid evolution in a volcanic hosted epithermal carbonate base metal gold vein system alto de la blenda farallon negro argentina alto de la blenda is a 6 6 ma intermediate sulphidation epithermal vein system in the farallon negro volcanic complex which also hosts the 7 1 ma porphyry cu au deposit of bajo de la alumbrera the epithermal vein system is characterised by a large extent and continuity 2 km 400 m open to depth 6 m maximum width and an average gold grade of 8 g t the vein is best developed within an intrusion of a fine grained equigranular monzonite interpreted as the central conduit of a stratovolcano whose extrusive activity ended prior to porphyry cu au emplacement at bajo de la alumbrera which is in turn cut by minor epithermal veins the alto de la blenda vein consists predominantly of variably mn rich carbonates and quartz with a few percent of pyrite sphalerite galena and other sulphide and sulphosalt minerals four phases of vein opening hydrothermal mineralisation and repeated brecciation can be correlated between different vein segments stages 2 and 3 contain the greatest fraction of sulphide and gold they are separated by the emplacement of a polymictic breccia containing clasts of quartz feldspar porphyry as well as basement rocks fluid inclusions in quartz related to stages 2 to 4 are liquid rich with 2 4 wt nacl eq they homogenise between 160 and 300 c with very consistent values within each assemblage vapour inclusions are practically absent in the epithermal vein quartz fragments in the polymictic breccia contain inclusions of intermediate to vapour like density and similar low salinity 3 wt nacl eq besides rare brine inclusions containing halite laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of epithermal inclusions indicate high concentrations of k fe as sb cs and pb that significantly vary within and through subsequent vein stages careful consideration of detection limits for individual inclusions shows high gold concentrations of 0 5 to 3 ppm dissolved in the ore fluid which contains variably high sulphur concentrations in excess over fe and other chalcophile metals compositional variations are interpreted to reflect cooling and contraction of lower density magmatic fluids at depth like those preserved in porphyry clasts that were mechanically transported up by the polymictic breccia ore mineral precipitation from the magmatic fluid occurred by further cooling and possibly minor mixing with surface derived water leading to sulphide saturation de sulphidation of the magmatic fluid and consequent gold precipitation the absence of flash boiling and or reduction by carbonaceous host rocks has led to relatively modest but constant gold grades in the carbonate base metal gold veins of alto de la blenda

hydrothermal evolution in the calabona porphyry copper system sardinia italy the path to an uneconomic deposit the calabona porphyry copper system is developed in a small relatively deep seated nearly equal 5 km dacitic intrusion located in northwestern sardinia italy early hydrothermal alteration produced a potassic assemblage in the deep and central parts of the complex and a peripheral propylitic halo sodic alteration was subsequently superimposed on the potassic zone and volumetrically dominant phyllic alteration overprinted the apical parts of the intrusion hypogene copper mineralization chalcopyrite and minor bornite was associated with potassic alteration the earliest fluid that circulated in the calabona porphyry complex had high salinity 40 60 wt nacl equiv and is interpreted to have been exsolved directly from the crystallizing magma however major entrapment of this fluid only occurred after it had cooled to temperatures of about 400 degrees c this fluid was responsible for potassic alteration and for precipitation of chalcopyrite and bornite in thin discontinuous group 1 veins and irregular and widely spaced group 2 veins the average cu grade in the potassic zone is between 0 05 and 0 1 percent the circulation of a lower temperature 270 degrees 330 degrees c ca enriched fluid of meteoric origin in the peripheral parts of the system caused propylitic alteration at an intermediate stage of hydrothermal evolution waters of external origin entered the central parts of the system along a network of late fractures group 3 veins or reopened group 2 veins partial mixing of this meteoric formational water with the high salinity fluid already circulating in the system created a fluid characterized by salinities ranging from 2 to 23 wt percent nacl equiv circulation of this mixed fluid at relatively low fluid rock ratios along a prograde thermal path caused sodic alteration the continuous inflow of meteoric water and the general temperature decrease in the system produced progressively more oxidized and acidic fluids which caused phyllic alteration and intense copper leaching late boiling in the apical parts of the phyllic alteration zone favored deposition of chalcopyrite and bornite but did not add significant copper cu grade 0 03 supergene enrichment led to copper grades higher than those in the phyllic zone 0 07 but not enough to permit mining the very low concentration of cu in the calabona deposits is surprising in view of wall rock alteration style of mineralization and a fluid evolution typical of those of many productive porphyry copper systems we propose that this is mainly a consequence of the raltive deep level of emplacement of the intrusion and the dacitic composition of the magma these factors are interpreted to have combined to retard melt saturation with alkali chlorife enriched fluids until late stages of crystallization which restricted the amount of exsolved fluid and cu extracted form the melt as a result overpressuring in the apical parts of the system was limited the related fracture density was low and the system therefore failed to provide the focus for the mineralizing fluids needed to permit bulk concentration of copper to economic levels further reasons for the uneconomic nature of the calabona porphyry were the lack of multiple intrusive events and the sulfide destructive cu leaching effects of phyllic alteration

magmatic sulfides and au cu ratios in porphyry deposits an experimental study of copper and gold partitioning at 850 c 100 mpa in a haplogranitic melt pyrrhotite intermediate solid solution gold metal assemblage at gas saturation abstract the behavior of copper and gold in the phase assemblage pyrrhotite po intermediate solid solution iss haplogranitic melt gold metal capsule at 850 c 100 mpa and gas saturation ah2o 0 9 has been investigated the results are presented as nernst partition coefficients the partition coefficient dcupo melt 2 6 0 3 103 1 was determined for po and melt in equilibrium with an iss of composition cu 19 1 fe 30 1 s 51 1 2 for gold the partition coefficients are dauiss melt 5 7 2 2 103 1 between iss and melt and daupo melt 140 40 1 between po and melt these imply dauiss po 42 9 1 between iss and po the oxygen fugacity for the experiments log f o 2 13 2 0 3 1 was determined by using the hydrogen sensor technique the sulfur fugacity log f s 2 1 0 0 7 1 was calculated from the composition of po in our experiments melts at iss saturation have cu fe by weight of ca 1 100 the results indicate that crystallization of iss may deplete a melt in gold and crystallization of po may deplete a melt in copper however a melt saturated with iss at a given bulk composition temperature pressure oxygen and sulfur fugacity and feo concentration will have a fixed concentration of cu preventing strong depletion of the melt in cu until iss is fractionated from the melt or changes in environmental conditions destabilize iss a thermodynamic model for the partitioning of cu and au between po and melt in terms of oxygen and sulfur fugacity indicates that dcupo melt and daupo melt may increase greatly with increasing sulfur fugacity and decreasing oxygen fugacity within the po stability field the model also suggests that destabilization of both iss and po relative to magnetite due to changes in oxygen and sulfur fugacities can result in the release of cu and au from the sulfides to the melt

from andesitic volcanism to the formation of a porphyry cu au mineralizing magma chamber the farallon negro volcanic complex northwestern argentina abstract the geological relations and geochemical composition of extrusive and subvolcanic intrusive rocks of the upper miocene farallon negro volcanic complex nw argentina were studied to reconstruct the evolution of a deeply eroded andesitic volcano over most its life history of some 3 5 my this paper is part of an integrated study of the processes that generate magmatic hydrothermal fluids responsible for the formation of porphyry cu au deposits here we present new data on bulk rock chemical and nd sr and pb isotopic compositions of unaltered volcanic and subvolcanic intrusive rocks for which geological relations and ages are constrained by field based stratigraphy and extensive ar ar geochronology the combined results indicate that voluminous volcanism started at ca 9 7 ma with the eruption of basaltic andesite to andesite lava flows and pyroclastic deposits this basic to intermediate volcanism persisted until 7 5 ma and was followed at 7 35 ma by the eruption of dacites as the last recorded major extrusive event the oldest andesitic intrusions emplaced at 9 0 ma were followed by hypabyssal intrusions of basaltic andesite andesite and dacite composition which intruded the volcanic sequence between 8 3 and 6 1 ma intrusions at 6 8 0 2 ma led to the formation of over 800 mt of porphyry cu au ore at bajo de la alumbrera it was formed by a large flux of magmatic hydrothermal fluids channeled through several phases of fractured dacitic porphyry which together with immediately following barren andesitic intrusions make up a composite stock after this magmatic hydrothermal event only minor rhyodacitic to rhyolitic intrusions were emplaced until magmatic activity ceased at 6 1 ma geochemical data for the magmatic rocks indicate that the evolution of the system was dominated by mixing of a mafic magma with a felsic magma that contained a component of crustal material as indicated by linear compositional correlations between compatible and incompatible elements initial strontium isotopic ratios vary between 0 7047 and 0 7118 while 143nd 144nd range from 0 5127 to 0 5124 and both isotopic variations correlate with each other and with sio2 content of the rocks after 8 5 ma a continuous increase in the silica content of intrusive rocks suggests the gradual formation of a magma chamber a substantial magma reservoir 7 km3 must have been established by the time of porphyry cu au mineralization to allow rapid release of a large volume of ore forming magmatic brine as a result of wholesale crystallization and de volatilization of the magma chamber

first data on volume and type of deflated sediment from southern puna plateau and its role as source of the chaco pampean loess abstract this paper presents the first attempt at qualitatively and quantitatively evaluating the sedimentary contribution of the puna plateau to the loess and soils of the argentinean pampas the origin of the pampean sediment has been traditionally ascribed to northern patagonia but new evidence supports a dual supply western central andes north patagonia and puna altiplano plateau three different lines of evidence show here that puna dust supply is active and produces significant sedimentary deposition up to places like buenos aires city the first evidence is meteorological and we show here one event related to an abnormal rainfall that deposited large quantities of a reddish brownish silt eroded out from the puna calling the attention of the environmental agency of buenos aires city the second evidence is the sampling of active wind transport collecting extremely coarse material carried out in the saltation layer suggesting the fine grained material escapes the area as fugitive dust this also demonstrates that observed gravel bearing bedforms of the puna are active and that the transport system seems to be activated by polar fronts only results show that the saltation curtain is abnormally thick and coarse suggesting that the kinetic energy peaks ca 7 10 times higher than for normal saltation while the transported grain size coincides with the lagged sediment in this basin composing different bedforms varying between sandy gravel and gravelly sand the third evidence is the quantification of the exported material by defining a reference surface as the initial time of evolution and the present day surface the initial surface is an alluvial surface of an estimated age of 11 12 ka bp volume balance was achieved by a detailed topographic stratigraphic and geophysical survey reconstructing old alluvial and present surfaces two gridding methods were used and more conservative results of the volume balance indicate a total erosion of ca 3 386 hm 3 of sediment in 72 ha spliced in 0 963 hm 3 of sediment that was left within the place forming gravel dunes while the remaining volume of ca 2 423 hm 3 is sediment truly exported from the surveyed area results imply a continuous blanket of ca 4 m thick of sediment exported out of this basin in the holocene these findings prove that the puna contribution to the chaco pampean plain is significant although not unique thus true proportions of sediment contribution to soils and loess from these multiple sources should be better evaluated in future studies

laser ablation icp ms analysis of silicate and sulfide melt inclusions in an andesitic complex i analytical approach and data evaluation quantitative microanalysis of entire silicate and sulfide melt inclusions by excimer laser ablation inductively coupled plasma mass spectrometry la icpms has been applied to extrusive and shallow intrusive rocks from the andesitic farallon negro volcanic complex northwestern argentina silicate melts are trapped in pyroxene amphibole plagioclase and quartz and sulfide melts are trapped in amphibole details of the analytical approach and the quantification procedure are given and the results are evaluated to test the accuracy of the technique and the validity of the interpretation of analytical signals similar compositions of silicate melt inclusions trapped in truly co precipitating minerals show that the quantification approach of melt inclusion compositions from la icpms signals through an internal standard is valid this correspondence also shows that melt inclusions investigated in this study are not significantly influenced by the boundary layer around a growing crystal or by post entrapment modifications post entrapment diffusive re equilibration only affected the fe and mg content of melt inclusions in mafic phases thus melt inclusions are representative samples of the melt from which the host mineral crystallized with regard to most major and trace elements sulfide melt inclusions present as pyrrhotite with exsolution of au and cu in phases separated during cooling were analyzed for their bulk fe cu and au content and the abundance of these elements was quantified using a silicate glass as external standard the validity of this calibration was tested by comparing electron microprobe analyses of fe cu ni and co in homogeneous sulfide minerals with la icpms results identical results within calculated uncertainty one standard deviation of five to nine analyses mostly between 1 and 5 wt rsd demonstrate that for these elements measured element ratios are independent of the matrix using our analytical setup

evidence for a copper bearing fluid in magma erupted at the valley of ten thousand smokes alaska several lines of evidence point to the existence of a pre eruptive fluid phase in the magmas erupted in 1912 at the valley of ten thousand smokes alaska the discovery of rare cu rich bubbles in some melt inclusions is best explained by the random entrapment of a low density fluid phase within growing phenocrysts the h2o s and cl concentrations of the high silica rhyolite are also consistent with fluid saturation in the rhyolite water contents as determined by infrared spectroscopy on doubly polished melt inclusions cluster between 3 5 and 4 5 wt h2o without any apparent differences between magmas vented explosively and effusively these concentrations would be sufficient for h2o saturation at a pressure of 100 mpa though the presence of other volatiles such as co2 and so2 could allow saturation with respect to a fluid phase at higher pressures the s cu and cl contents of the phenocryst assemblages as determined by xrf analyses are too low to account for the decrease in the concentrations of s and cu in the melt and only modest increase in cl with differentiation therefore the behavior of these volatile elements was controlled either by eryptic fractionation if phenocryst phasese or more likely by partitioning into a coexisting fluid phase the presence of this low density phase apparently provided metals such as cu with a volatile phase into which they could partition the concentration of cu in this fluid reached tens to hundreds of times that of the coexisting silicate melt and may have been as rich as 0 05 wt cu the distribution of cu in the magma was also controlled by sulfides such as intermediate solid solution and pyrrhotite which cyrstallized directly from the silicate melt

u th pb and 230th 238u disequilibrium isotope systematics precise accessory mineral chronology and melt evolution tracing in the alpine bergell intrusion in order to investigate the potential of combined th u pb isotope and 230th 238u disequilibrium systematics for tracing magmatic crystallization and melt evolution conventional high resolution single crystal tims techniques have been applied to zircon titanite and fragments of geochemically characterized growth zones of allanite these minerals were extracted from a single tonalite specimen collected from the feeder zone of the tertiary bergell pluton southern steep belt s switzerland n italy the isotopic results document an extended history of crystallization and melt evolution of at least 5 ma with well resolved zircon ages defining an early interval of 33 0 to 32 0 ma followed by crystallization of zoned allanite from 32 0 to 28 0 ma and formation of magmatic epidote possibly as late as 26 ma trace and major element patterns in zoned allanite closely mirror melt evolution characterized by increase of u concentration and sharp decrease of th and lree reflecting early crystallization of phases low in u and in particular the dominating control by allanite precipitation preservation of substantial quantities of excess 206pb derived from initial excess 230th in all analyzed allanite grains indicates that their isotopic systems have not been reset by loss of radiogenic pb during prolonged residence at magmatic conditions and regional metamorphic cooling and that the measured sequence of 208pb 232th dates translates into a real age sequence major loss of radiogenic pb from compositionally zoned allanite by volume diffusion would have resulted in a data pattern grossly different from the observed one as demonstrated by numerical modeling of 232th 208pb 238u 230th 206pb isotopic evolution the results therefore suggest closure temperatures 700 c for magmatic allanite quantification of 230th 238u disequilibrium relationships reveals a smooth initially steep decrease of th u in the magma from values of 2 9 at 32 0 ma to 0 1 at 28 0 ma in equilibrium with sequential allanite zoning comparison of calculated th u magma with measured total rock th u 0 79 requires fractional crystallization of allanite at an early stage this process of removal of allanite and thus th from the melt was operative at least until 31 5 ma providing an upper limit for the time of emplacement of the studied magma batch the fact that bulk zircon and a large part of allanite crystallization predate emplacement cautions against equating ages determined on refractory minerals from deep seated plutons with intrusion ages the persistence of magmatic conditions over a period of 5 ma concentrated in a narrow collision zone between the european and the african plate has been a major controlling factor for the oligocene evolution of the adjacent insubric fault the main tectonic lineament of the alps being active from late cretaceous to miocene times

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

a physical model for metal extraction and transport in shallow magmatic systems 1 the highest concentrations of metals e g cu au ag mo in the earth s crust are found in porphyry type deposits the metals are ultimately sourced from magmas and appear to be concentrated hundred to thousand fold from typical magmatic contents ppm ppb in the exsolved volatile phase to better quantify the purging and transport of metals we develop a physical model of volatile evolution in an incrementally built upper crustal magma reservoirs that considers 1 partitioning of metals from the melt to an exsolved volatile phase and 2 advection of the buoyant volatile phase using a single dimensionless parameter the peclet number pe ratio of advection rate over diffusion rate we propose that metal extraction and segregation from magmas can occur in 3 stages with different pe 1 during exsolution of the magmatic volatile phase in shallow crystal poor magma bodies slow volatile advection pe 1 2 during the growth of volatile channels that develop in the reservoir as crystallinity increases pe 1 and 3 during advection in connected channels rapid volatile advection high pe 1 for each stage a metal enrichment factor can be calculated allowing insight into the optimal conditions to maximize metal mass flux into the overlying hydrothermal system the model predicts that the most efficient purging of metals occurs for magmas with intermediate volatile contents and is enhanced during late stage magmatic activity as the reservoirs reach high crystallinity and are not disturbed by volcanic venting in agreement with natural observations suggesting that ore formation post dates volcanic activity

anhydrite pyrrhotite and sulfur rich apatite tracing the sulfur evolution of an oligocene andesite eagle mountain co usa abstract the oldest known occurrence of magmatic anhydrite in a shallow sub volcanic rock is at the oligocene andesitic eagle mountain volcanic center huerto formation san juan volcanic field which immediately postdates the pyrrhotite bearing fish canyon tuff estimates of intensive parameters derived from the compositions of coexisting minerals hornblende plagioclase anhydrite pyrrhotite s rich apatite augite hypersthene feti oxides indicate that the eagle mountain magma was wet oxidized and s rich it was much like k rich andesitic and dacitic anhydrite bearing magmas recently erupted from the subduction related volcanoes el chichon mt pinatubo and lascar although the oligocene geodynamic setting of the san juan volcanic field is poorly constrained and its enormous volume is not easily reconciled with an arc setting the eagle mountain occurrence emphasizes how closely some of its magmas resemble those from active continental margin arcs s rich magmas similar to the eagle mountain andesite also may have contributed to the formation of some hydrothermal ore deposits of the san juan volcanic field

potential reconciliation of devils hole and deep sea pleistocene chronologies the standard model of pleistocene climate fluctuations being driven by orbital insolation changes has been challenged by results from a high precision radiometrically dated deposit from devils hole nevada although the devils hole record agrees with a previously derived ice volume record for most of the last 500 000 years clear indication of warming prior to the orbitally dated onset of the last interglacial forms the prime basis for this conclusion i suggest that early warming at devils hole needs to be understood in light of the prior very extensive glaciation at 150 000 b p marine records in both the north pacific and north atlantic from the same latitude as devils hole indicate that the subpolar fronts during the penultimate glaciation 150 000 years ago were 5 father south than during the last glaciation continental glaciation may also have been more extensive at this time the first phase of ice retreat 145 000 years ago is associated with a northward shift of these fronts which results in full interglacial sea surface temperatures in the north atlantic the early warming at devils hole may reflect the northward movement of the ice margins and associated fronts if so both the devils hole and deep sea chronologies may be correct with differences between 130 000 and 150 000 years ago reflecting a regionally varying response to climate shifts associated with a very extensive glaciation

fluid melt trace element partitioning behaviour between evolved melts and aqueous fluids experimental constraints on the magmatic hydrothermal transport of metals abstract the fluid melt partition coefficients of ten trace metals li mn cu zn rb sr mo w and pb along with k between hydrous evolved melts and coexisting cl bearing fluid s have been experimentally determined at 850 1050 c 0 6 2 2 kbar and o2 nno 0 5 to nno 2 log units the measured compositions of the quenched fluid phases vary from low salinity 2 wt cl to 19 wt cl with maximum starting fluid concentrations of 21 wt cl the results of 15 partitioning experiments confirm that dmetalfluid melt values depend strongly on the melt and fluid composition with the greatest metal affinities for the fluid phase shown in the experiments with high cl fluids and the most fractionated melts five experiments conducted in au96cu4 capsule alloy show dcufluid melt values ranging from 40 to 80 and a strong correlation with melt feo wt contents at equivalent fluid cl contents in contrast li shows only weak partitioning in favour of coexisting fluids across all melt compositions with dlifluid melt ranging from 0 16 in low salinity fluids to 1 17 in the highest cl fluid 19 15 wt cl a procedure was employed to redissolve and analyse the aluminosilicate precipitates that form during quenching of the experiments this procedure allows for metal contents of the final quench fluids to be constrained by both mass balance and direct measurement by solution icp ms analysis a comparison of the mass balance calculated vs directly measured fluid melt partition coefficients generally gives good agreement within an order of magnitude for most trace elements however discrepancies between these methods arise due to errors inherent in the mass balance calculations for metals with low abundances in the fluids e g sr and especially those showing affinities to alloy with the experiment capsule material e g mo these discrepancies in constraining experimental fluid trace element budgets also suggest that the precious metal capsule alloys cannot always be considered completely inert containers in experimental investigations and semi quantitative la icp ms analysis of the raw capsule alloys confirms the detectable presence of several of the trace elements investigated here integrating these new partitioning data with previous experimental and natural studies confirms that aqueous cl bearing fluids exsolved by hydrous calc alkaline melts stored at mid to upper crustal conditions are capable of efficiently sequestering and transporting some economically important trace metals e g mn cu zn however the new fluid melt li partitioning data in conjunction with evidence of very late post eruptive li modification suggests that li is moderately strongly fluid immobile at depth and suffers from significant post eruptive re equilibration this has implications for deciphering the signatures of volatile fluxing in arc magmas that are preserved in mineral and melt inclusion trace element concentrations and the development of mineralised stockwork regions in porphyry style metal deposits

platinum solubility and partitioning in a felsic melt vapor brine assemblage abstract the solubility and partitioning of pt in a s free vapor brine rhyolite melt pt metal assemblage has been quantified at 800 c f o 2 nno and pressures of 100 and 140 mpa vapor and brine were sampled at run conditions by trapping these phases as glass hosted fluid inclusions as the melt cooled through the glass transition temperature the vapor and brine were in equilibrium with the melt at the time of trapping and thus represent fluids which were sampled at the termination of each experimental run the microthermometrically determined salinities of vapor and brine are 2 and 63 wt nacl eq and 9 and 43 wt nacl eq at 100 and 140 mpa respectively platinum solubilities in vapor brine and glass i e quenched melt were quantified by using laser ablation inductively coupled plasma mass spectrometry la icp ms equilibrium is discussed with reference to the major and trace element concentrations of glass hosted fluid inclusions as well as the silicate melt over run times that varied from 110 to 377 h at 140 mpa and 159 to 564 h at 100 mpa platinum solubility values 1 in h2o saturated felsic melt are 0 28 0 13 g g and 0 38 0 06 g g at 140 and 100 mpa respectively platinum solubility values 1 x at 140 and 100 mpa respectively in aqueous vapor are 0 91 0 29 g g and 0 37 0 17 g g and in are brine 16 10 g g and 3 3 1 0 g g the measured solubility data were used to calculate nernst type partition coefficients for pt between vapor melt brine melt and vapor brine the partition coefficient values 1 x for vapor melt brine melt and vapor brine at 140 mpa are 2 9 1 0 67 27 and 0 13 0 05 and at 100 mpa are 1 0 0 2 6 8 2 4 and 0 15 0 05 the partitioning data were used to model the pt scavenging capacity of vapor and brine during the crystallization driven degassing i e second boiling of a felsic silicate melt over a depth range i e 3 6 km consistent with the evolution of magmatic hydrothermal ore deposits model calculations suggest that aqueous vapor and brine can scavenge sufficient quantities of pt and by analogy other platinum group elements pge to produce economically important pge rich magmatic hydrothermal ore deposits in earth s upper continental crust

fluid immiscibility in volcanic environments volatile components enter the atmosphere oceans and other surface waters through pre syn and post eruptive volcanic processes that involve a variety of fluids aluminosilicate poor fluids include aqueous or carbonic to sulfide sulfate chloride fluoride carbonate and phosphate rich compositions in volcanic environments but other more complex combinations of these constituents may be involved giggenbach 1977 roedder 1984 1992 lowenstern 1995 other rare gases and dissolved constituents e g h2 n2 he ar h3bo3 hg ch4 and other hydrocarbon compounds metals and metalloids are also present but in general they are not sufficiently concentrated to form their own phases or control bulk fluid composition and volcanic processes as a result of their diverse compositions volcanic fluids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity dielectric behavior viscosity and other chemical and physical characteristics geiger et al 2006a the density of a saline liquid in the system nacl h2o at 50 mpa and 800 c for example is more than 7 times that of the coexisting aqueous vapor e g 0 2 gm cm3 henley and mcnabb 1978 multiple fluids move mix and or unmix in magma chambers and volcanic conduits in the root zone of fumaroles and in the convective hydrothermal systems that are ubiquitous to volcanic environments hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits hydrothermal processes involving two or more fluid phases operate through a variety of geologically relevant shallow crustal pressure temperature composition conditions volcanic fluids occur at supersolidus to subsolidus conditions meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present this chapter addresses the evidence for and occurrence of two or more low density aluminosilicate poor fluids

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma

apatite fission track thermochronology of the sierras pampeanas central western argentina implications for the mechanism of plateau uplift in the andes over much of its length the andean orogen is characterized by a generally east vergent geometry and a progressive eastward cratonward migration of individual arc orogenic elements a departure from this model occurs in the sierras pampeanas region of central western argentina where a terrane of active basement uplifts is currently developing to the east of the main cordillera apatite fission track data from uplifted precambrian and phanerozoic basement rocks of the sierras pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late miocene two pre andean cooling events during the carboniferous permian ca 300 280 ma and the early jurassic middle jurassic ca 200 174 ma have been identified the onset of andean deformation is represented by a cooling event during the late paleocene middle eocene this cooling was followed by a period of middle miocene late miocene reheating during foreland basin style sedimentation exhumation and possible westward migration of the exhumation front in the sierras pampeanas commenced during the late miocene pliocene to the east of the dominantly east vergent precordillera fold and thrust belt the apparent convergence of deformation in these two terranes and the progressive closure of miocene pliocene intermontane basins in the sierras pampeanas may reflect the early stages of andean plateau uplift the relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central bolivia ca 20 s furthermore the time space coincidence between basement uplift and flattening of the subducted slab beneath the sierras pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction

geology mineralization stable isotope and fluid inclusion characteristics of the vostok 2 reduced w cu skarn and au w bi as stockwork deposit sikhote alin russia abstract the large 180 kt wo 3 and at least 10 15 t au vostok 2 deposit is situated in a metallogenic belt of w sn w au and au w deposits formed in late to post collisional tectonic environment after cessation of active subduction the deposit is related to an ilmenite series high k calc alkaline plutonic suite that by its petrologic signatures is transitional between those at w dominant and au dominant reduced intrusion related deposits consistently besides large w cu skarns of the reduced type the deposit incorporates quartz stockworks with significant au w bi mineralization also formed in a reduced environment the hydrothermal stages include prograde and retrograde essentially pyroxene skarns hydrosilicate amphibole chlorite quartz alteration and phyllic quartz sericite albite apatite and carbonate alteration assemblages these assemblages contain abundant scheelite associated with pyrrhotite chalcopyrite and at the phyllic stage also with bi minerals as bi sb te pb zn sulfides and sulfosalts as well as au mineralization the fluid evolution included hot high pressure 420 460 c 1 1 1 2 kbar low salinity 5 4 6 0 wt nacl equiv aqueous fluids at the retrograde skarn stage followed by lower temperature cyclic releases of high carbonic low salinity to non carbonic moderate salinity aqueous fluids at the hydrosilicate stage a high carbonic ch 4 dominated hot 350 380 c low salinity fluid was followed by cooler 300 350 c non carbonic moderate salinity 5 7 14 9 wt nacl equiv fluid at the phyllic stage a high carbonic co 2 dominated moderately hot 330 355 c 0 9 kbar low salinity fluid was followed by cooler 230 265 c non carbonic moderate salinity 6 6 12 0 wt nacl equiv fluid a homogenized magmatic source of water 18 o h2o 8 3 to 8 7 and a sedimentary source of sulfur 34 s 6 9 to 6 2 and carbon 13 c fluid 20 1 to 14 9 at the hydrosilicate stage are suggested a magmatic source of water 18 o 8 6 to 9 2 and a sedimentary source of sulfur 34 s 9 3 to 4 1 but a magmatic mantle to crustal derived source of carbon 13 c fluid 6 9 to 5 2 are envisaged for fluids that formed the early mineral assemblage of the phyllic stage then the role of sedimentary carbon again increased toward the intermediate 13 c fluid 16 4 to 14 5 and late 13 c fluid 16 3 to 14 7 phyllic mineral assemblages the magmatic differentiation was responsible for the fluid enrichment in w whereas au and bi could also have been sourced from mafic magma the decreasing temperatures together with elevated ca content in non boiling fluids promoted scheelite deposition at the early hydrothermal stages the most intense scheelite deposition at the phyllic stage was caused by co 2 removal due to boiling of co 2 rich fluids further cooling of non boiling fluids favoured joint deposition of scheelite bi and au

petrology of the yerington batholith nevada evidence for evolution of porphyry copper ore fluids the jurassic yerington batholith western nevada is a composite pluton that contains several centers of porphyry copper mineralization and is exposed in structural cross section at paleodepths ranging from 0 to 8 km within these exposures the mcleod hill quartz monzodiorite bear quartz monzonite and luhr hill granite form successive intrusions that are in turn volumetrically smaller approximately 75 19 and 6 vol respectively more deeply emplaced tops at 65 km 3 luhr hill granite the low cu content approximately 10 ppm of the granite would allow extraction of approximately 50 ppm cu from it during crystallization these data suggest an orthomagmatic model in which both high and low density saline aqueous fluids formed during exsolution of water from the crystallizing luhr hill granite and from which salts cu fe and s strongly partitioned into the high density ore fluid low density fluids may have risen from the magma as a vapor plume mineralization occurred when fluid overpressures caused fracturing of wall rock and upward intrusion of granite porphyry dikes and high density saline ore fluids both the f sub o 2 path during subsolidus cooling and the deposition of most s as chalcopyrite pyrite in ore suggest that reduction of magmatic s to sulfide may have been a mechanism for oxidation of magmatic ferromagnesian silicate and fe ti oxide minerals

mo rich source and protracted crystallization of late mesozoic granites in the east qinling porphyry mo belt central china constraints from zircon u pb ages and hf o isotopes abstract in the east qinling orogenic belt central china there are numerous mo deposits with over 6 mt mo metal the giant nannihu shangfanggou and yuchiling porphyry mo deposits are hosted in porphyry stocks associated with the heyu batholith whereas the giant donggou porphyry mo deposit is hosted in a stock associated with the taishanmiao batholith zircon grains from the heyu batholith have concordant 238u 206pb ages scattered from 150 3 to 130 2 ma with an age interval of nearly 20 m y whereas ore hosting granitic porphyry stocks from the nannihu shangfanggou and yuchiling deposits have 206pb 238u ages ranging from 151 1 to 135 1 ma 143 1 to 132 1 ma and 143 5 to 131 5 ma respectively it is likely that the heyu batholith has a prolonged history of incremental assembly and the ore bearing granitic porphyry stocks may have originated from the same magma reservoir and emplaced in different stages likewise zircon grains from the taishanmiao batholith have concordant 238u 206pb ages spanning in a period from 130 2 to 111 3 ma whereas those from the donggou granitic porphyry stock have 238u 206pb ages ranging from 125 2 to 110 3 ma nearly coeval with the taishanmiao batholith the molybdenite re os model ages for each deposit are coincident with relatively young zircon 238u 206pb ages of the host stock indicating that mo mineralization is likely related to late stage magmatism zircon grains from the batholiths and granitic porphyry stocks have ehf t ranging from 10 to 30 tdm2 ages from 1 6 to 2 5 ga and 18o from 5 0 to 8 7 which are taken to suggest the involvement of the subducted continental crust of the yangtze block in the source we thus propose that the extensive mo mineralization in the east qinling porphyry mo belt is genetically related to the prolonged granitic magmatism in the late mesozoic and a mo rich source that was related to the subducted continental crust of the yangtze block stagnating beneath the southern margin of the north china craton

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data