re os age of the pueblo viejo epithermal deposit dominican republic re os analyses of pyrite from the pueblo viejo high sulfidation epithermal deposit both monte negro and moore ore zones yield an isochron age of 111 9 3 7 ma and initial 187 os 188 os ratio of 0 22 0 18 mean square of weighted deviates 84 n 21 isochrons grouped by pyrite setting layered versus vein or locations yield ages within uncertainty of the overall isochron and with each other and as such do not yield temporally distinguishable ages sphalerite vein samples as well as shale source rocks show evidence of open system behavior and loss of re and therefore are not included in the above regression these results confirm that the pueblo viejo deposit is early cretaceous in age and coeval with the volcanic los ranchos formation which hosts the deposit recent paleontological age assignments for the hatillo formation which overlies the los ranchos formation show that it is only slightly younger opening the possibility that mineralization in the hatillo formation could also be related to late stages of los ranchos magmatism whether these revised los ranchos hatillo relations provide sufficient cover to account for the paragenetically late formation of pyrophyllite at pueblo viejo remains unclear recognition that pueblo viejo is coeval with the los ranchos formation shows that large scale high sulfidation epithermal mineralization can form from basal island arc tholeiitic magmatism a review of available experimental data confirms that these magmas can generate hydrothermal solutions of the necessary composition favorability of the los ranchos formation might have been enhanced by its bimodal composition these results suggest that exploration for epithermal precious metal deposits should include basal island arc tholeiite terranes

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

submarine hydrothermal activity along the mid kermadec arc new zealand large scale effects on venting 1 the 2 500 km kermadec tonga arc is the longest submarine arc on the planet here we report on the second of a series of cruises designed to investigate large scale controls on active hydrothermal venting on this arc the 2002 nzaplume ii cruise surveyed 12 submarine volcanic centers along 580 km of the middle kermadec arc mka extending a 1999 cruise that surveyed 260 km of the southern kermadec arc ska average spacing between volcanic centers increases northward from 30 km on backarc crust along the ska to 45 km on backarc crust along the southern mka to 58 km where the mka joins the kermadec ridge volcanic cones dominate in the backarc and calderas dominate the kermadec ridge the incidence of venting is higher along the mka 83 10 of 12 volcanic centers than the ska 67 8 of 12 but the relative intensity of venting as given by plume thickness areal extent and concentration of dissolved gases and ionic species is generally weaker in the mka this pattern may reflect subduction of the 17 km thick oceanic hikurangi plateau beneath the ska subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the ska conversely volcanic centers in the southern mka are starved of magma replenishment and so their hydrothermal systems are waning farther north where the mka centers merge with the kermadec ridge fewer but larger magma bodies accumulate in the thicker older crust ensuring more widely separated caldera dominated volcanic centers

recent developments in element concentration and isotope ratio analysis of individual fluid inclusions by laser ablation single and multiple collector icp ms abstract laser ablation inductively coupled plasma mass spectrometry la icp ms has become a most powerful technique for the elemental analysis of individual polyphase inclusions completely enclosed in minerals be they solid or a solid liquid gas mixture at the time of measurement simultaneous accurate quantification of major to ultra trace element concentrations from li to u by well controlled ablation of the entire fluid or melt inclusion content and successful use of largely matrix independent external calibration protocols are unique features of this method this contribution reviews fluid inclusion fundamentals relevant for their bulk analysis by la icp ms and discusses key aspects of the analytical protocol emphasis is on figures of merit precision accuracy obtained from the analysis of individual inclusions and fluid inclusion assemblages and procedures and technical developments to improving data quality are elaborated a new equation for the calculation of detection limits for la icp ms analysis is presented which closely follows iupac conventions applications are reviewed with emphasis on the use of synthetic fluid inclusions in constraining metal solubility and distribution between co existing phases new data for natural bismuth fluid inclusions document the seamless transition to melt inclusion analysis by la icp ms thus highlighting the fact that the procedures presented here are generally applicable to the analysis of inclusions in complex host minerals isotope ratio analysis of individual fluid inclusions by multicollector icp ms mc icp ms is a recent development that requires fast transient signals to be accurately recorded by instrumentation designed for high precision static measurements of long lasting stable ion beams we address the general principles based on pb isotopes and review a first application to the bingham canyon porphyry cu au mo deposit a pilot study using about 50 synthetic fluid inclusions containing srm 987 sr and variable nacl ca and rb concentrations demonstrates that accurate 87 sr 86 sr isotope ratios can be obtained on an individual rb poor fluid inclusion at absolute 2 precisions of 0 0003 to 0 002 a residual trend in 87 sr 86 sr as a function of the rb sr abundance ratio in the fluid inclusions suggests that interference correction of 87 rb on mass 87 assuming identical mass bias coefficients for the two elements may be inaccurate however the offset can be accurately corrected for by regressing the data to zero 87 rb the versatility and detection power of la icp ms makes this technique the method of choice for solute abundance and isotope ratio analysis of individual fluid inclusions significant future progress can be achieved by improvements in ion production transmission and data recording efficiency and by further improving control on inclusion ablation by pulsed laser beams data quantification strategies may also have to be further refined to keep pace with instrumental progress and innovation

an authoritative global database for active submarine hydrothermal vent fields 1 the interridge vents database is available online as the authoritative reference for locations of active submarine hydrothermal vent fields here we describe the revision of the database to an open source content management system and conduct a meta analysis of the global distribution of known active vent fields the number of known active vent fields has almost doubled in the past decade 521 as of year 2009 with about half visually confirmed and others inferred active from physical and chemical clues although previously known mainly from mid ocean ridges mors active vent fields at mors now comprise only half of the total known with about a quarter each now known at volcanic arcs and back arc spreading centers discoveries in arc and back arc settings resulted in an increase in known vent fields within exclusive economic zones consequently reducing the proportion known in high seas to one third the increase in known vent fields reflects a number of factors including increased national and commercial interests in seafloor hydrothermal deposits as mineral resources the purpose of the database now extends beyond academic research and education and into marine policy and management with at least 18 of known vent fields in areas granted or pending applications for mineral prospecting and 8 in marine protected areas

distributions of hydrothermal activity along the mid atlantic ridge interplay of magmatic and tectonic controls abstract hydrothermal activity has been investigated along three different sections of the slow spreading mid atlantic ridge mar 11 20 30 n 36 38 n and the reykjanes ridge 57 45 63 06 n when considered in total the incidence of venting along these three sections of the mar compares well with the predictions of a model in which frequency of venting is linearly related to ridge crest spreading rate at the scale of individual study areas however departure from the model is observed by up to an order of magnitude venting is anomalously rare along the reykjanes ridge but anomalously abundant along the mar 36 38 n whilst such variability may be within the error of the linear spreading rate model we note that the interplay between magmatic and tectonic processes also differs between the three study areas in the case of the reykjanes ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling investigative strategy because the style of venting which predominates may not give rise to conventional black smoker hydrothermal plumes along the oblique and broadly segmented mar 36 38 n we propose that vigorous hydrothermal venting in broad segment end non transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

paragenetic relationships between low and high grade gold mineralization in the cripple creek au te deposit colorado trace element studies of pyrite abstract the cripple creek alkaline igneous rock related low sulfidation epithermal gold telluride deposit is hosted in a 10 km wide oligocene alkaline volcanic diatreme in proterozoic rocks periods of phreatic eruptions and progressively more mafic alkaline magma emplacement created the diatreme and breccias that host the gold ore followed by breccia pipe creation and later by the emplacement of phonolite and later lamprophyre dikes throughout the diatreme pervasive potassic alteration affected much of the diatreme and is spatially related to the late stage veins containing native gold and precious metal tellurides previous studies have mainly focused on the high grade gold veins primarily located toward the center of the diatreme the current study evaluates the mineralogy and paragenesis of low grade mineralization in the currently mined wild horse extension whex globe hill and schist island pits and the galena embayment along the margin of the diatreme next to the whex pit as well as high grade mineralization in the whex and cresson pits given the variable paragenesis of mineralizing stages in different locations in the cripple creek deposit a simplified three stage paragenesis stages 1 2 and 3 is recognized here involving low and high grade gold mineralization scanning electron microscopy electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses are used to compare the trace element compositions of pyrite in low and high grade gold mineralization these analyses reveal a complex trace element pattern with variability in ag as au co ni sb and te in the rims cores and zones area between the core and the rim of stages 2 py2 and 3 pyrite py3 and spatially between the pits currently being mined trace element concentrations in stage 1 pyrite py1 are lower than those in py2 and py3 and the patterns are uniform trace elements reside within the lattice structure of pyrite and as micro and nano inclusions e g up to 48 220 ppm as 6983 ppm co 12 250 ppm ni and 10 860 ppm sb the trace element patterns from the final stage of pyrite growth in high grade gold telluride veins indicate enrichment in ag as au cu pb sb and te the paragenesis and compositional zoning of pyrite associated with high grade ore contrast with those associated with pyrite from low grade ore which preceded the formation of high grade gold telluride veins this suggests that metallic mineralization occurred in multiple episodes and that low grade ore did not form from the same hydrothermal fluids that precipitated the high grade ore early pyrite formed under passive non boiling conditions followed by a transition to vigorous boiling in py2 and py3 ending with non boiling conditions recorded in the au and as rich rims of py3 this enrichment in au and as on the rims of pyrite is consistent with other alkaline igneous rock related epithermal au deposits as are the concentrations of trace elements throughout the pyrite grains the presence of as ag co cu mo ni sb te and tl in pyrite are considered vectors to high grade au ore in the cripple creek deposit

hydrothermal venting in magma deserts the ultraslow spreading gakkel and southwest indian ridges 1 detailed hydrothermal surveys over ridges with spreading rates of 50 150 mm yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting but the validity of this relation at slow and ultraslow ridges is unproved here we compare hydrothermal plume surveys along three sections of the gakkel ridge arctic ocean and the southwest indian ridge swir to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges along the gakkel ridge most apparent vent sites occur on volcanic highs and the extraordinarily weak vertical density gradient of the deep arctic permits plumes to rise above the axial bathymetry individual plumes can thus be extensively dispersed along axis to distances 200 km and 75 of the total axial length surveyed is overlain by plumes detailed mapping of these plumes points to only 9 10 active sites in 850 km however yielding a site frequency fs sites 100 km of ridge length of 1 1 1 2 plumes detected along the swir are considerably less extensive for two reasons an apparent paucity of active vent fields on volcanic highs and a normal deep ocean density gradient that prevents extended plume rise along a western swir section 10 23 e we identify 3 8 sites so fs 0 3 0 8 along a previously surveyed 440 km section of the eastern swir 58 66 e 6 sites yield fs 1 3 plotting spreading rate us versus fs the ultraslow ridges and eight other ridge sections spanning the global range of spreading rate establish a robust linear trend fs 0 98 0 015us implying that the long term heat supply is the first order control on the global distribution of hydrothermal activity normalizing fs to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster spreading ridges in supporting active vent fields this increased efficiency could derive from some combination of three dimensional magma focusing at volcanic centers deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle and nonmagmatic heat supplied by exothermic serpentinization

evolution of magmatic vapor to gold rich epithermal liquid the porphyry to epithermal transition at nevados de famatina northwest argentina fluid inclusions in the subeconomic porphyry cu mo au system at nevados de famatina and closely associated high sulfidation epithermal cu au ag as sb te veins at la mejicana northwest argentina were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment field geology vein petrography fluid inclusion microthermometry and single inclusion microanalysis by laser ablation inductively coupled plasma mass spectrometry la icp ms were combined to determine the evolution of pressure temperature and ore metal concentrations in the fluids cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations aqueous liquid inclusions of 5 wt percent nacl equiv salinity trapped in quartz sericite pyrite qsp veins between 360 and 325 c contain unusually high concentrations of cu 400 5 000 ppm as 200 ppm sb and te both up to 100 ppm and au several ppm along with other ore forming elements these veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor and high sulfidation epithermal veins with high au grades which are preserved along a high ridge adjacent to the porphyry stock low density vapor and hypersaline liquid i e brine inclusions were trapped in early formed quartz of the porphyry stockwork veins between 450 and 600 c the brine contains lower cu and au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins the latter may be equivalent to a parental fluid the low and intermediate density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz sericite pyrite veins fluid inclusions and geologic time relations indicate that the transitional quartz sericite pyrite veins were the channelways for metal rich aqueous liquids that generated the high sulfidation epithermal cu au deposit these mineralizing liquids are interpreted to have evolved by continuous density increase contraction from a low salinity s rich magmatic fluid of low to intermediate density this vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic hydrothermal complex and probably underwent minor brine separation prior to reaching the present level of exposure epithermal mineral precipitation occurred upon dilution of the low salinity magmatic fluid with meteoric water which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent

genesis of the telescoped eocene silver and oligocene gold san dimas deposits sierra madre occidental mexico constraints from fluid inclusions oxygen deuterium and noble gases isotopes abstract the san dimas district is a world class ag au deposit developed as a telescoped eocene oligocene ag au mineralization located in the sierra madre occidental smo of western mexico san dimas exhibits multiple mineralization events during different magmatic and tectonic episodes from late cretaceous to early oligocene the well preserved magmatic hydrothermal system provides an excellent opportunity to determine the source of silver and gold the evolution of the hydrothermal fluids and the controls on the mineralization precipitation mineralogical fluid inclusions fi stable and noble gases isotope analyses suggest that the san dimas deposit consist of two different mineralization styles 1 ag dominant epithermal eocene veins that occurred at temperatures up to 350 c developed at ca 2 3 km depth associated to the final stages of intrusion of the piaxtla batholith with fi dominated by a crustal component and 2 epithermal low sulfidation au dominant oligocene veins which were developed at 250 c at shallower depths

non transform offsets along the mid atlantic ridge south of the azores 38 n 34 n ultramafic exposures and hosting of hydrothermal vents ten contiguous non transform offsets ntos along the mid atlantic ridge mar south of the azores between 38 n and 35 40 n have been studied in detail using swath bathymetric acoustic backscatter and deep tow high resolution sidescan sonar tobi data in contrast with discontinuities studied elsewhere at slow spreading ridges these left lateral ntos are consistently broader and larger with complex structural fabrics accommodating the offset they are characterized by a range of elevated and faulted massifs detached from their segment flanks with an irregular acoustic backscatter pattern some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks and sometimes associated with high temperature hydrothermal venting water column surveys adjacent to these massifs show high ch4 and low tdm total dissolvable manganese concentrations possibly resulting from the process of serpentinization of ultramafic rocks the correlation between the shallow dome like shaped massifs and the high concentrations of ch4 associated with low levels of mn is of particular interest to predict the outcrop of ultramafic rocks within the ntos where no geological data are available the exposure of the ultramafic massifs within the ntos is favored by low magmatic supply and low angle detachment faulting occurring at segment ends the pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high temperature vent sites

metallogenesis and mineralization of intraoceanic arcs i kermadec arc introduction the kermadec intraoceanic arc marks the southernmost section of the pacific ring of fire in the western pacific region a chain of mostly underwater volcanoes that stretches from north of japan southward to new zealand the kermadec arc striking for 1 200 km from monowai volcano to the north island of new zealand represents the southern portion of the contiguous 2 500 km kermadec tonga arc fig 1 de ronde et al 2003 the kermadec arc is populated by almost 30 volcanic centers comprising single large caldera or cone volcanoes or less commonly groups of relatively smaller cones de ronde et al 2001 2007 graham et al 2008 all of these volcanic centers lie to the west of the kermadec ridge in the southern and mid parts of the arc by up to 70 km in the case of rumble ii west then merge with the kermadec ridge near the kermadec islands around 30 s the volcanoes then diverge away westward from the kermadec ridge in the northern reaches of the arc fig 1 fig 1 location map for the kermadec arc and the various volcanoes referred to in the papers of this special volume the kermadec arc havre trough system is made up from east to west of a number of tectonic elements including the kermadec trench the kermadec ridge the havre trough or kermadec back arc and the colville ridge the volcanoes of the active arc front lie immediately west of the kermadec ridge in the southern and mid parts of the arc then merge with the kermadec ridge to the north the underlying bathymetry in shaded relief is derived from the satellite altimetry grid of smith and sandwell 1997 with more up to date bathymetry over top especially in the southern

fault geometry and permeability contrast control vent temperatures at the logatchev 1 hydrothermal field mid atlantic ridge high temperature 300 c off axis hydrothermal systems found along the slow spreading mid atlantic ridge are apparently consistently located at outcropping fault zones while preferential flow of hot fluids along highly permeable fractured rocks seems intuitive such efficient flow inevitably leads to the entrainment of cold ambient seawater the temperature drop this should cause is difficult to reconcile with the observed high temperature black smoker activity and formation of associated massive sulfide ore deposits here we combine newly acquired seismological data from the high temperature off axis logatchev 1 hydrothermal field lhf1 with numerical modeling of hydrothermal flow to solve this apparent contradiction the data show intense off axis seismicity with focal mechanisms suggesting a fault zone dipping from lhf1 toward the ridge axis our simulations predict high temperature venting at lhf1 only for a limited range of fault widths and permeability contrasts expressed as the fault s relative transmissibility the product of the two parameters the relative transmissibility must be sufficient to capture a rising hydrothermal plume and redirect it toward lhf1 but low enough to prevent extensive mixing with ambient cold fluids furthermore the temperature drop associated with any high permeability zone in heterogeneous crust may explain why a significant part of hydrothermal discharge along slow spreading ridges occurs at low temperatures

microseismicity of the mid atlantic ridge at 7 s 8 15 s and at the logatchev massif oceanic core complex at 14 40 n 14 50 n lithospheric formation at slow spreading rates is heterogeneous with multiple modalities favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate we report microseismicity from three deployments of seismic networks at the mid atlantic ridge mar two networks surveyed the mar near 7 degrees s in the vicinity of the ascension transform fault three inside corner high settings were investigated however they remained seismically largely inactive and major seismic activity occurred along the center of the median valley in contrast at the logatchev massif core complex at 14 degrees 45n seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted logatchev hydrothermal vent field to the north and south of the massif however seismic activity occurred along the ridge axis emphasizing the asymmetry of seismicity at the logatchev segment focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3 5 km depth which we interpret to reflect volume expansion accompanying serpentinization at shallower depth of 2 4 km some earthquakes in the vicinity of the vent field showed normal faulting behavior suggesting that normal faults facilitates hydrothermal circulation feeding the vent field further a second set of cross cutting faults occurred indicating that the surface location of the field is controlled by local fault systems

vent fluid chemistry of the rainbow hydrothermal system 36 n mar phase equilibria and in situ ph controls on subseafloor alteration processes abstract the rainbow hydrothermal field is located at 36 13 8 n 33 54 15 w at 2300 m depth on the western flank of a non volcanic ridge between the south amar and amar segments of the mid atlantic ridge the hydrothermal field consists of 10 15 active chimneys that emit high temperature 365 c fluid in july 2008 vent fluids were sampled during cruise knox18rr providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at rainbow data suggest that the mg concentration of the hydrothermal end member is not zero but rather 1 5 2 mmol kg this surprising result may be caused by a combination of factors including moderately low dissolved silica low ph and elevated chloride of the hydrothermal fluid combining end member mg data with analogous data for dissolved fe si al ca and h2 permits calculation of mineral saturation states for minerals thought appropriate for ultramafic hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations these data indicate that chlorite solid solution talc and magnetite achieve saturation in rainbow vent fluid at a similar ph t p 400 c 500 bar of approximately 4 95 while higher ph values are indicated for serpentine suggesting that serpentine may not coexist with the former assemblage at depth at rainbow the high fe mg ratio of the rainbow vent fluid notwithstanding the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0 78 and 0 22 respectively in situ ph measurements made at rainbow vents are in good agreement with ph t p values estimated from mineral solubility calculations when the in situ ph data are adjusted for temperature and pressure calculations further indicate that ph t p and dissolved h2 are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution fluid equilibria indeed the predicted correlation between dissolved silica and h2 defines a trend that is in good agreement with vent fluid data from rainbow and other high temperature ultramafic hosted hydrothermal systems we speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions which in turn are variably replaced by chlorite magnetite talc tremolite with important implications for ph lowering dissolved sulfide concentrations and metal mobility

porphyry gold copper mineralisation in the cadia district eastern lachlan fold belt new south wales and its relationship to shoshonitic magmatism the cadia porphyry gold copper district is the largest hydrothermal intrusion related gold deposit in eastern australia discovered in 1992 by newcrest mining limited pre mine resources in the district were in excess of 585 t au and 2 35 mt cu the cadia district lies within shoshonitic subaqueous volcanic rocks of the late ordovician molong volcanic belt in the eastern lachlan fold belt lfb of new south wales an island arc tectonic setting is envisaged for the formation of these volcanic rocks but the nature of the arc basement and geometry of the subduction zone s is debated mineralisation occurs in four principal porphyry deposits cadia hill cadia ridgeway cadia east cadia far east and cadia quarry as sheeted and stockwork quartz sulfide veins and locally as broadly stratabound disseminated mineralisation cadia east and skarn big cadia and little cadia all of the porphyry deposits show a close spatial association with shoshonitic monzodiorite to quartz monzonite dykes and stocks of the cadia intrusive complex cic gold copper mineralisation is hosted by these intrusions and also by the enclosing forest reefs volcanics frv wall rocks hydrothermal alteration associated with mineralisation is potassic which is overprinted by selectively pervasive propylitic and silica albite assemblages petrological studies and major and trace element analysis of rocks of the cic and frv as well as of other intrusions in the cadia district that appear unrelated to mineralisation have been conducted unaltered samples from the cic are characterised by high k2o contents up to 6 5 wt and molecular k na ratios consistently 1 confirming the alkalic and shoshonitic nature of the complex the cic samples also fall just within the shoshonite field on ce yb ta yb and th yb ta yb plots although immediately adjacent to the calc alkaline field the frv are geochemically similar to the cic although lower in p2o5 and ce lree dacitic and hornblende porphyry intrusions from within the district but spatially unrelated to mineralisation have a distinctive geochemical signature and are not shoshonitic in composition the close spatial association between gold copper mineralisation and the shoshonitic monzonitic cic argues strongly for a genetic link between the two a similar association occurs at the economic goonumbla porphyry gold copper deposits in the eastern lfb while sub economic gold copper mineralisation at cargo and copper hill is associated with calc alkaline quartz diorite and dacitic intrusions the cadia district is considered to be an excellent example of the association of alkaline potassic magmas and gold copper porphyry style mineralisation

geochemical evidence for relict degassing pathways preserved in andesite andesitic arc volcanoes degas large quantities of volatiles evidence for vapour transport in the erupted lavas is rarely preserved and poorly understood but is crucial for understanding eruption style we present geochemical evidence for the transport of metal bearing vapour in shear zones preserved in lavas erupted from soufriere hills volcano montserrat textural evidence suggests that shear induced brittle failure occurred in a narrow zone at the conduit wall or in the lava dome elevated metal concentrations cu au ag pb zn within the zones indicate that the fractures acted as a transient pathway for metal bearing magmatic gases during slip frictional heating to temperatures of 1000 c caused partial melting at the slip surface resorption of volatiles and metals into the partial melt preserved the geochemical signature of magmatic vapour in the shear zone cordierite which is highly unusual in volcanic rocks crystallised from the peraluminous partial melt with metal bearing sulphides and oxides the shear zones provide the first geochemical evidence for vapour segregation and transport through viscous andesite magmas and provide an insight into controls on eruption style

fluxes to sediments underlying the rainbow hydrothermal plume at 36 14 n on the mid atlantic ridge abstract a geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the rainbow hydrothermal field as well as a large biogenic component 80 caco3 these sediments record clear enrichments of the elements fe cu mn v p and as from hydrothermal plume fallout but only minor detrital background material systematic variations in the abundances of hydrothermal elements are observed at increasing distance from the vent site consistent with chemical evolution of the dispersing plume further pronounced ni and cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near field locations radiocarbon dating reveals mean holocene accumulation rates for all four cores of 2 7 to 3 7 cm kyr 1 with surface mixed layers 7 to 10 cm thick from which a history of deposition from the rainbow hydrothermal plume can be deduced deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced e g fe mn cu or scavenged from seawater via the hydrothermal plume e g v p as holocene fluxes into to the cores surface mixed layers are presented which typically are an order of magnitude greater than background authigenic fluxes from the open north atlantic one core collected closest to the vent site indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14c kyr ago the preferred explanation is that this variation reflects the initiation intensification of hydrothermal venting at the rainbow hydrothermal field at this time perhaps linked to some specific tectonic event in this fault controlled hydrothermal setting

the relationship between near axis hydrothermal cooling and the spreading rate of mid ocean ridges recent seismic observations and new crustal thermal models suggest that near axis 1 km from the spreading axis hydrothermal cooling increases linearly with increasing spreading rate we present here the first corroborating evidence from near axis hydrothermal plume observations comprehensive surveys of plume distributions have now been conducted on multi segment portions of ridge crest with full spreading rates from 20 to 150 km myr these surveys find thatph the fraction of ridge crest length overlain by hydrothermal plumes and full spreading rate us are related byph us where 0 004 myr km we recast this large scale spatial variability into long term temporal variability by postulating that any vent field site is hydrothermally active for an us fraction of geologic time we then use this relationship and the mean of eight studies that have determined instantaneous heat flux ha at vent field scales 75 45 mw km to estimate the time averaged heat flux ha asha us applying these scaled values ofha to the distribution of mid ocean ridge spreading rates yields a global near axis heat flux ha of 8 8 1011 w this value is 10 of the total oceanic hydrothermal heat flux and agrees with a crustal cooling model derived value of ha of 9 1011 w our estimate of ha implies a mean global3he heat ratio of 2 4 8 3 10 13 cm3 j stp and high temperature hydrothermal fluid flow of 1 8 1013 kg yr

where are the undiscovered hydrothermal vents on oceanic spreading ridges abstract in nearly four decades since the discovery of deep sea vents one third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits as well as by academic research here we incorporate recent data for back arc spreading centers and ultraslow and slow spreading mid ocean ridges mors to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate we apply this equation globally to predict a total number of vent fields on spreading ridges which suggests that 900 vent fields remain to be discovered almost half of these undiscovered vent fields comparable to the total of all vent fields discovered during 35 years of research are likely to occur at mors with full spreading rates less than 60 mm yr we then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction with the majority expected to occur outside of states exclusive economic zones we hope that these predictions will prove useful to the community in the future in helping to shape continuing ridge crest exploration

is gold solubility subject to pressure variations in ascending arc magmas magmas play a key role in the genesis of epithermal and porphyry ore deposits notably by providing the bulk of ore metals to the hydrothermal fluid phase it has been long shown that the formation of major deposits requires a multi stage process including the concentration of metals in silicate melts at depth and their transfer into the exsolved ore fluid in more superficial environments both aspects have been intensively studied for most of noble metals in subsurface conditions whereas the effect of pressure on the concentration i e solubility of those metals in magmas ascending from the sublithospheric mantle to the shallow arc crust has been quite neglected here we present new experimental data aiming to constrain the processes of gold au dissolution in subduction linked magmas along a range of depth we have conducted hydrous melting experiments on two dacitic adakitic magmas at 0 9 and 1 4 gpa and 1000 c in an end loaded piston cylinder apparatus under fo2 conditions close to nno as measured by solid co pd o sensors experimental charges were carried out in pure au containers the latter serving as the source of gold in presence of variable amounts of h2o and for half of the charges with elemental sulfur s so as to reach sulfide saturation au concentrations in melt quenched to glass were determined by la icpms when compared to previous data obtained at lower pressures and variable redox conditions our results show that in both s free and sulfide saturated systems pressure has no direct detectable effect on melt au solubility nevertheless pressure has a strong negative effect on sulfur solubility since gold dissolution is closely related to the behaviour of sulfur in reducing and moderately oxidizing conditions pressure has therefore a significant but indirect effect on au solubility the present study confirms that au dissolution is mainly controlled by fo2 in s free melts and by a complex interplay of fo2 and melt s2 concentration in sulfide saturated melts at given temperature in addition we propose that the transition from sulfide s2 to sulfate s6 species in melt is shifted towards more oxidizing conditions when pressure and the degree of melt polymerization increase if this is true this may have important consequences during mantle melting and magma ascent in particular if mantle melting occurred in moderately oxidizing conditions a small degree of partial melting would allow the primary melts to become au enriched but the relatively high pressure would move the sulfide sulfate transition to more oxidizing conditions making the primary melts saturated with sulfide phases that would sequester gold from the melt during magma ascent the decreasing pressure would favour the destabilization of sulfides and the release of gold to the silicate melt however at shallow levels decreasing pressure magma evolution and varying redox conditions would be continuously competing to concentrate or fractionate gold

hydrothermal activity on the southern mid atlantic ridge tectonically and volcanically controlled venting at 4 5 s we report results from an investigation of the geologic processes controlling hydrothermal activity along the previously unstudied southern mid atlantic ridge 3 7 s our study employed the noc uk deep tow sidescan sonar instrument tobi in concert with the whoi usa autonomous underwater vehicle abe to collect information concerning hydrothermal plume distributions in the water column co registered with geologic investigations of the underlying seafloor two areas of high temperature hydrothermal venting were identified the first was situated in a non transform discontinuity ntd between two adjacent second order ridge segments near 4 02 s distant from any neovolcanic activity this geologic setting is very similar to that of the ultramafic hosted and tectonically controlled rainbow vent site on the northern mid atlantic ridge the second site was located at 4 48 s at the axial summit centre of a second order ridge segment there high temperature venting is hosted in an 18 km2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre existing chemosynthetic vent fauna in both appearance and extent these lava flows are directly reminiscent of those emplaced in winter 2005 06 at the east pacific rise 9 50 n and reference to global seismic catalogues reveals that a swarm of large m 4 6 5 6 seismic events was centred on the 5 s segment over a 24 h period in late june 2002 perhaps indicating the precise timing of this volcanic eruptive episode temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5 s mar turtle pits have subsequently revealed vent fluids that are actively phase separating under conditions very close to the critical point for seawater at 3000 m depth and 407 c the hottest vent fluids yet reported from anywhere along the global ridge crest

geology sulfide geochemistry and supercritical venting at the beebe hydrothermal vent field cayman trough the beebe vent field bvf is the world s deepest known hydrothermal system at 4960m below sea level located on the mid cayman spreading centre caribbean the bvf hosts high temperature 401 c black smoker vents that build cu zn and au rich sulphide mounds and chimneys the bvf is highly gold rich with au values up to 93 ppm and an average au ag ratio of 0 15 gold precipitation is directly associated with diffuse flow through beehive chimneys significant mass wasting of sulphide material at the bvf accompanied by changes in metal content results in metaliferous talus and sediment deposits situated on very thin 2 3km thick oceanic crust at an ultraslow spreading centre the hydrothermal system circulates fluids to a depth of 1 8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies we suggest hydrothermal interaction with chalcophile bearing sulphides in the mantle rocks together with precipitation of au in beehive chimney structures has resulted in the formation of a au rich volcanogenic massive sulphide vms deposit with its spatial distribution of deposit materials and metal contents the bvf represents a modern day analogue for basalt hosted au rich vms systems this article is protected by copyright all rights reserved

stability and abundance of the trisulfur radical ion s 3 in hydrothermal fluids the interpretation of sulfur behavior in geological fluids and melts is based on a long standing paradigm that sulfate sulfide and sulfur dioxide are the major sulfur compounds this paradigm was recently challenged by the discovery of the trisulfur ion s 3 in aqueous s bearing fluids from laboratory experiments at elevated temperatures however the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions here we used in situ raman spectroscopy on model thiosulfate sulfide and sulfate aqueous solutions across a wide range of sulfur concentration 0 5 10 0 wt acidity ph 3 8 temperature 200 500 degrees c and pressure 15 1500 bar to identify the different sulfur species and determine their concentrations results show that in the low density 0 2 g cm 3 vapor phase h2s is the only detectable sulfur form by contrast in the denser liquid and supercritical fluid phase together with sulfide and sulfate the trisulfur radical ion s 3 is a ubiquitous and thermodynamically stable species from 200 degrees c to at least 500 degrees c in addition the disulfur radical ion s is detected at 450 500 degrees c in most solutions and polymeric molecular sulfur with a maximum abundance around 300 degrees c in s rich solutions these results combined with revised literature data allow the thermodynamic properties of s 3 to be constrained enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids these predictions suggest that s 3 may account for up to 10 of total dissolved sulfur s tot at 300 500 degrees c in fluids from arc related magmatic hydrothermal systems and more than 50 s tot at 600 700 degrees c in s rich fluids produced via prograde metamorphism of pyrite bearing rocks the trisulfur ion may favor the mobility of sulfur itself and associated metals au cu pt mo in geological fluids over a large range of depth and provide the source of these elements for orogenic au and porphyry epithermal cu au mo deposits furthermore the ubiquity of s 3 in aqueous sulfate sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass dependent and mass independent fractionation of sulfur isotopes

young volcanism and related hydrothermal activity at 5 s on the slow spreading southern mid atlantic ridge the effect of volcanic activity on submarine hydrothermal systems has been well documented along fast and intermediate spreading centers but not from slow spreading ridges indeed volcanic eruptions are expected to be rare on slow spreading axes here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated apparently magmatically robust segment center on the slow spreading southern mid atlantic ridge near 5 s three high temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase separated vapor type fluids exit temperatures at one of the fields reach up to 407 c at conditions of the critical point of seawater the highest temperatures ever recorded from the seafloor fluid and vent field characteristics show a large variability between the vent fields a variation that is not expected within such a limited area we conclude from mineralogical investigations of hydrothermal precipitates that vent fluid compositions have evolved recently from relatively oxidizing to more reducing conditions a shift that could also be related to renewed magmatic activity in the area current high exit temperatures reducing conditions low silica contents and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source probably related to a young volcanic eruption event nearby in which basaltic magma is actively crystallizing this is the first reported evidence for direct magmatic hydrothermal interaction on a slow spreading mid ocean ridge

synthetic fluid inclusions in natural quartz i compositional types synthesized and applications to experimental geochemistry synthetic fluid inclusions having a wide range of geologically applicable compositions in the c o h s na k ca cl cu fe system have been formed by healing fractures in inclusion free natural quartz and by precipitating new quartz overgrowths on an original seed crystal inclusion types synthesized include 1 1 liquid rich pure h2o inclusions of primary and secondary origin 2 2 pure h2o inclusions with the critical density 3 3 liquid rich inclusions containing undersaturated aqueous solutions of nacl or kcl or cacl2 or mixtures of the three salts 4 4 h2o nacl inclusions containing halite daughter minerals 5 5 h2o nacl kcl inclusions containing halite and sylvite daughter minerals 6 6 h2o co2 inclusions of various compositions containing liquid h2o and either co2 vapor or co2 liquid or both at 25 c 7 7 h2o co2 nacl inclusions containing an aqueous phase liquid and vapor co2 and halite at 25 c 8 8 c o h inclusions containing liquid h2o a co2 ch4 liquid phase and graphite daughter minerals at 25 c 9 9 h2o nacl inclusions that contain a chalcopyrite daughter mineral and 10 10 inclusions representing trapping of the coexisting immiscible fluids in the h2o nacl h2o co2 nacl and na c o h systems the inclusions exhibit uniform phase ratios at room temperature and the temperatures of various phase changes within individual inclusions agree with those predicted from experimental and theoretical data indicating that the inclusion fluid has the same composition and density as the parent solution these miniature autoclaves thus may be used to study various problems related to fluid inclusion research to calibrate analytical equipment used to study natural inclusions and to study phase equilibria solubility and pvt relations of a variety of chemical systems

sustained volcanically hosted venting at ultraslow ridges piccard hydrothermal field mid cayman rise at slow spreading mid ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically controlled hydrothermal circulation unrelated to young volcanic activity here by contrast we show that the piccard hydrothermal field phf on the ultraslow spreading mid cayman rise represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting the lateral extent and apparent longevity associated with the phf are comparable to some of the largest tectonically hosted vent sites known along the slow spreading mid atlantic ridge if such systems recur along all ultraslow ridges which comprise 20 of the 55 000 km global ridge crest potential implications would include i a higher probability of locating large economically valuable mineral deposits along ultraslow ridges together with ii larger fluxes than previously anticipated of chemicals released from high temperature venting entering the oceans along the atlantic indian ocean sectors of the deep ocean thermohaline conveyor

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

distributed mass balance modelling on two neighbouring glaciers in ortles cevedale italy from 2004 to 2009 a 6 year application of an enhanced temperature index mass balance model to careser and la mare glaciers eastern italian alps is presented the two glaciers exhibit very different characteristics and a comprehensive dataset of distributed mass balance measurements was used to test the model performance the model was run using meteorological data acquired outside the glaciers the work was focused on two main aspects 1 the development of a morphological redistribution procedure for snow and 2 the comparison of three different melt algorithms proposed in the literature the results show that the simple method proposed for snow redistribution can greatly improve simulation of winter balance and further improvements would be achievable by collecting data on inaccessible and high altitude areas all three melt formulations displayed a good skill level and very similar results in modelling the mass balance distribution over glacier areas with slightly better results from a multiplicative algorithm in capturing the vertical balance gradient the simulation errors are related to aspect and elevation and tend to be spatially aggregated some assumptions concerning the spatial and temporal distribution of air temperature and incoming solar radiation although reasonable and widely used in the literature may be responsible for this aggregation hence there is a need to further investigate the processes that regulate the distribution of melt energy and that appear to control the current deglaciation phase in this area

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

characteristics of magma driven hydrothermal systems at oceanic spreading centers 1 we use one and two limb single pass models to de termine vent field characteristics such as mass flow rate q bulk permeability in the discharge zone kd thickness of the conductive boundary layer at the base of the system magma replenishment rate and residence time in the discharge zone data on vent temperature vent field area heat output and the surface area and depth of the subaxial magma chamber amc constrain the models the results give q 100 kg s kd 10 13 m2 and 10 m essentially independent of spreading rate and detailed characteristics of the amc in addition we find no correlation between heat output at individual vent fields and spreading rate or depth to the amc we conclude that high temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of amc depth using data on partitioning of heat flow between focused and diffuse flow we find that 80 90 of the hydrothermal heat output is derived from high temperature fluid even though much of the heat output discharges as low temperature fluid in some cases diffuse flow fluids may exhibit considerable conductive cooling or heating by assuming conservative mixing of diffuse flow fluids at east pacific rise 9 50 n we find that most transport of metals such as fe and mn occurs in diffuse flow and that co2 h2 and ch4 are taken up by microbial activity

geochemical controls on hydrothermal fluids from the kairei and edmond vent fields 23 25 s central indian ridge prior to 2000 no mid ocean ridge hydrothermal systems were identified and sampled in the indian ocean the kairei field central indian ridge was discovered and sampled in 2000 gamo et al 2001 hashimoto et al 2001 this paper reports the chemical composition of hydrothermal fluids collected in 2001 from both the kairei and newly discovered edmond hydrothermal systems data are used to infer subsurface processes and conditions at the two sites as well as to place them within the global context of known hydrothermal sites on the mid ocean ridge system while both sites are located on the intermediate spreading central indian ridge their hydrologic systems have affinities with those observed on slow spreading ridges although the compositions of fluids from the four individual vents sampled at edmond and the three individual vents sampled at kairei vary respectively our interpretation is that a single source fluid at each site is subsequently modified by processes including phase separation subsurface mixing with seawater and deposition and or remobilization of metal sulfide deposits to generate the observed range of compositions both fields are located 6 km from the neovolcanic zone on steps on the east wall of the axial valley with the edmond field 160 km north of kairei and almost 1000 m deeper 3300 versus 2450 m the edmond fluids are extremely hot with maximum measured temperatures of 382 c compared to 365 c for kairei all of the fluids sampled have chlorinities greater than local ambient seawater due to phase separation of seawater at supercritical conditions visual observations suggest both sites have been active for a substantial period of time again similar to observations of slow spreading ridges our chemical data suggest that the substrate underlying kairei is highly altered while that at edmond is not significant albitization appears to be occurring below the edmond field but not at kairei the edmond fluids are by far the hottest brines yet observed venting from a mid ocean ridge hydrothermal site resulting in unusually high concentrations of several transition metals

copper partitioning between felsic melt and h2o co2 bearing saline fluids abstract analysis of fluid inclusions from porphyry copper deposits reveals that magmatic vapor and brine are vital for the removal of copper from arc magmas and its transport to the site of ore deposition experiments in melt vapor brine systems allow for investigation of the partitioning of copper between silicate melts and volatile phases at magmatic conditions the presence of co2 affects both the pressure at which a melt saturates with respect to volatile phases therefore the partitioning of copper among felsic rhyolitic melt vapor and brine in co2 bearing experiments was examined to provide insights into copper partitioning and the generation of porphyry copper and related deposits we present results from experiments performed at 800 c and 100 mpa in co2 bearing melt vapor brine systems with x co 2 v b 0 10 and 0 38 the compositions of vapor and brine inclusions and run product glasses were determined during the course of this investigation microthermometric measurements of fluid inclusions show an increase in the salinity of the magmatic brine 65 to 70 wt nacleq and decrease in the salinity of the vapor 3 5 to 1 wt nacleq as x co 2 is increased from 0 10 to 0 38 the partitioning of copper between brine and vapor d cu b v 2 increases from 25 6 at x co 2 0 10 to 100 30 at x co 2 0 38 the partitioning of copper between vapor and melt d cu v m 2 decreases from 9 6 3 3 at x co 2 0 10 to 2 0 8 at x co 2 0 38 these data demonstrate that copper partitioning in sulfur free co2 bearing systems is controlled by the changes in the salinity of the vapor and brine that in turn are functions of x co 2 no change in the apparent equilibrium constants for cu na exchange was observed in fe bearing experiments which supports a salinity dependent model for copper partitioning an existing model mvpart for ore metal partitioning between melt and volatile phases was modified to incorporate partitioning data from co2 bearing experiments the model can be used to predict trends in efficiency of removal of copper from melts into exsolving co2 bearing magmatic volatile phases at a fixed pressure and temperature the co2 mvpart model predicts that co2 rich x co 2 0 38 magmatic vapor will remove half of the available copper from the melt without a contribution from the brine compared to low co2 x co 2 0 10 systems thus periods of co2 rich vapor exsolution are not expected to be associated with efficient removal of copper from shallow felsic melts

hydrothermal activity along the southwest indian ridge twenty years after the discovery of sea floor hot springs vast stretches of the global mid ocean ridge system remain unexplored for hydrothermal venting the southwest indian ridge is a particularly intriguing region as it is both the slowest spreading of the main ridges1 and the sole modern migration pathway between the diverse vent fauna of the atlantic and pacific oceans2 a recent model postulates that a linear relation exists between vent frequency and spreading rate3 and predicts vent fields to be scarcest along the slowest spreading ridge sections thus impeding migration and enhancing faunal diversity2 here however we report evidence of hydrothermal plumes at six locations within two 200 km long sections of the southwest indian ridge indicating a higher frequency of venting than expected these results suggest that fluxes of heat and chemicals from slow spreading ridges may be greater than previously thought and that faunal migration along the southwest indian ridge may serve as an important corridor for gene flow between pacific and atlantic hydrothermal fields

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

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

chlorine partitioning between a basaltic melt and h2o co2 fluids at mount etna abstract partitioning experiments between a basaltic melt from mt etna and a low density hydrous fluid or vapor containing h 2 o or h 2 o co 2 were performed at 1200 1260 c at pressures between 1 and 200 mpa either near the nickel nickel oxide nno buffer or at two log units above it nno 2 and with different chloride concentrations most of the experiments were done at chloride brine undersaturated conditions although at the highest cl concentrations explored brine saturation might have been reached the average partition coefficients d cl fluid melt over the range of cl concentrations were derived on a weight basis by plotting the calculated concentrations of cl in the fluid phase versus the measured ones in the melt for h 2 o cl experiments in which the cl concentration in the melt was 0 4 wt a negative dependence between d cl f m and pressure is observed d cl fluid melt in h 2 o cl bearing experiments ranges between 11 14 at 1 and 25 mpa to 6 at 200 mpa at nno and between 4 at 50 mpa and 13 at 100 mpa at nno 2 addition of co 2 at nno yields lower partition coefficients than in co 2 free conditions over the pressure range investigated the negative pressure dependence observed for h 2 o cl experiments disappears when co 2 is present in the system overall once co 2 is introduced in the system cl fugacity in the silicate melt tends to increase thus resulting in a decrease of d cl f m application to mt etna shows that the composition of the volcanic plume in terms of cl records very shallow pressures of magma equilibration with its exsolved fluid

new insights into mid ocean ridge volcanic processes from the 2005 2006 eruption of the east pacific rise 9 46 n 9 56 n digital seafloor imagery collected on 37 camera tows and alvin dives in which we identify 186 contacts between new and old lava are used to create the most detailed map of a mid ocean ridge mor eruption to date lava flows erupted in 2005 2006 at the east pacific rise epr covered an area of 14 6 km2 along 18 km of the epr crest between 9 46 and 9 56 n the 2005 2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors and pillow forms at terminal flow fronts numerous lava channels 10 50 m wide and 1 5 m deep trending approximately east west served as distributory pathways eruptions were sourced from fissures within the epr axial summit trough as well as fissures located on an off axis fissure mound 600 m east of the epr axis between 9 52 and 9 56 n portions of the lava flow reached as far as 2 km east of the axis near 9 51 2 n using a conservative estimate of 1 5 m for the average flow thickness implies that the 2005 2006 eruptions produced 22 106 m3 of lava 4 5 times larger than estimated volumes of 1991 1992 epr lava flows estimated lava volume for the 2005 2006 eruptions represents 15 of the magma available in the axial magma chamber

hydrothermal activity on the reykjanes ridge the steinaholl vent field at 63 06 n we have completed a systematic survey for hydrothermal activity along the reykjanes ridge from 57 45 n to 63 09 n shipboard determinations of total dissolvable mn tdmn and dissolved si ch4 and h2 have revealed evidence for only one site of hydrothermal activity along this section of ridge crest the steinaholl vent field at 63 06 n 1 our measurements confirm this site to be a stable hydrothermal convection cell of at least two years duration the site is situated in just 250 350 m of seawater and unlike deeper sited vent fields e g tag 3650 m 26 n mar is notable for the formation of bubble rich plumes which have been imaged using a high frequency 38 khz echo sounder high dissolved gas concentrations of up to 18 nmol l ch4 and 30 nmol l h2 coincide with these bubble rich plumes the steinaholl plume is also characterised by high total dissolvable mn tdmn anomalies up to 60 nmol l and dissolved si anomalies 2 mol l yielding molar tdmn ch4 and tdmn si ratios of 3 5 and 0 03 respectively directly comparable to the tag hydrothermal plume the absence of any indication of hydrothermal activity along the remainder of the reykjanes ridge is intriguing between 11 n and 40 n on the mid atlantic ridge previous studies have revealed evidence for hydrothermally active sites approximately every 150 km for the 300 km of ridge crest studied in detail here 750 km total only one site has been found thus despite its proximity to the iceland hot spot the incidence of hydrothermal activity along this section of ridge crest appears to be only 20 50 of that measured elsewhere along the slow spreading mid atlantic ridge

constraints on water depth of massive sulfide formation evidence from modern seafloor hydrothermal systems in arc related settings the results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment the shallowest hydrothermal systems in subduction related settings are hosted by arc volcanoes these shallow vent sites have a number of features in common with subaerial epithermal systems massive sulfide occurrences in arc related rifts the most likely setting for many ancient analogs are generally restricted to water depths from 700 to 2 000 m with rifts developing within old arc crust at the deeper end of this range back arc spreading centers proximal to arcs host massive sulfide deposits at depths of 1 500 to 2 000 m the deepest hydrothermal systems occur along mature back arc spreading centers distal to volcanic arcs where water depths range from 2 000 to 3 700 m these deeper vent sites probably represent the best modern analogues of ophiolite hosted massive sulfide deposits boiling of the hydrothermal fluids is common at volcanic arcs and in arc related rifts in these environments elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss by contrast the high ambient pressures in deep marine hydrothermal systems along mature back arc spreading centers prevent fluids from boiling during their ascent to the seafloor boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and therefore influences the metal content of seafloor sulfide deposits copper rich massive sulfides typically occur at water depths exceeding 1 000 m whereas zn and pb rich occurrences may form at any water depth boiling can be an important control on ag and au grades but is not the only factor controlling precious metal enrichment in massive sulfides shallow marine hot spring deposits can be highly enriched in trace metals such as as hg and sb submarine volcanic arc and back arc settings are geologically complex and significant variations in water depth can occur over short distances paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth the relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic hosted massive sulfide deposits

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

the cooling history and the depth of detachment faulting at the atlantis massif oceanic core complex 1 oceanic core complexes occs are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults we combine previously reported u pb zircon crystallization ages with u th he zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the atlantis massif occ 30 n mar and help establish cooling rates as well as depths of detachment faulting and gabbro emplacement we present nine new u th he zircon ages for samples from iodp hole u1309d ranging from 40 to 1415 m below seafloor these data paired with u pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at atlantis massif as 2895 1276 1162 c myr 1 from 780 c to 250 c the lower 600 m of the borehole cooled more slowly at mean rates of 500 125 102 c myr 1 from 780 c to present day temperatures rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of 300 c myr 1 possibly due to hydrothermal circulation to 4 km depth through the detachment fault zone assuming a fault slip rate of 20 mm yr from u pb zircon ages of surface samples and a rolling hinge model for the sub surface fault geometry we predict that the 780 c isotherm lies at 7 km below the axial valley floor likely corresponding both to the depth at which the semi brittle detachment fault roots and the probable upper limit of significant gabbro emplacement

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

how fast does gold trickle out of volcanoes the enormous hydrothermal gold ore deposit on lihir island papua guinea may have formed in less than 55 000 years

sulphur solubility in andesitic to basaltic melts implications for hekla volcano the solubility of sulphur in sulphide saturated h2o bearing basaltic andesitic and basaltic melts from hekla volcano iceland has been determined experimentally at 1 050 c 300 and 200 mpa and redox conditions with oxygen fugacity logfo2 between qfm 1 2 and qfm 1 1 qfm is a quartz fayalite magnetite oxygen buffer in the systems containing various amounts of s and h2o the s content of the h2o rich glasses saturated with pyrrhotite decreases from 2 500 ppm in basalt to 1 500 ppm in basaltic andesite at the investigated conditions furthermore the reduction of water content in the melt at pyrrhotite saturation and fixed t p and redox conditions leads to a decrease in s concentration from 2 500 to 1 400 ppm for basaltic experiments for h2o decrease from 7 8 to 1 4 wt and from 1 500 to 900 ppm for h2o decrease from 6 7 to 1 7 wt for basaltic andesitic experiments our experimental data combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas indicate that the parental basaltic melts of hekla were not saturated with respect to sulphide during magmatic differentiation the s content in the residual melts increased and might have reached sulphide saturation with 2 500 ppm dissolved s with further magma crystallization the s concentration in the melt was controlled by the sulphide saturation of the magma decreasing from 2 500 to 900 ppm s

a new experimental approach to study fluid rock equilibria at the slab mantle interface based on the synthetic fluid inclusion technique the role of high pressure aqueous fluids in mass transfer processes during slab dehydration has been recognized for a long time however the quantitative assessment of their material transport capacity in complex natural systems remains poorly understood mainly as a consequence of their unquenchable nature and current experimental limitations a new experimental approach has been developed to investigate complex fluid rock equilibria at high pressure and temperature conditions relevant for slab dehydration processes aqueous fluids pre equilibrated with high pressure mineral assemblages were sampled at run conditions in the form of synthetic fluid inclusions sfi in quartz and subsequently analyzed by laser ablation icpms la icp ms the main innovation introduced in the experiments is that the quartz crystal was fractured in situ during the run only after chemical equilibrium between phases has been achieved thus allowing the entrapment of fluid inclusions that sample true equilibrium compositions an efficient fracturing of quartz at high pressure and temperature conditions was achieved by crossing the quartz coesite reaction boundary which occurs at pressures of the sub arc slab mantle interface an experimental methodology has been developed to implement this strategy and experiments in the eclogite water system were conducted to demonstrate the feasibility and advantage of the method the results demonstrate that secondary fluid inclusions formed early in pre fractured quartz are systematically diluted compared to secondary inclusions formed after in situ fracturing of quartz particularly for elements such as sr zr nb ti and mg these observations demonstrate that early entrapment of fluids in pre fractured quartz do not represent equilibrium fluids at high pressure temperature conditions

high cu concentrations in vapor type fluid inclusions an artifact abstract studies of natural fluid inclusions have shown that cu commonly occurs at higher concentrations in vapor type inclusions than in coexisting brine inclusions a phenomenon which has been interpreted to arise from copper partitioning into the vapor phase in the first part of this study we attempted to experimentally reproduce this behavior in hydrothermal experiments covering a range of p t conditions 600 800 c 70 130 mpa fluid compositions nacl kcl fecl 2 0 5 6 6 wt s fluid acidities quench fluid ph 0 3 10 and sulfur speciation h 2 s dominated to so 2 dominated however as in several other studies we did not succeed in reproducing conditions under which cu partitions into the vapor phase in view of recent observations that quartz hosted fluid inclusions can diffusively loose or gain cu after entrapment we set out to determine if the evidence from natural fluid inclusions could be compromised for this purpose we synthesized vapor and brine inclusions from cu h 2 o nacl s fluids at 800 c 130 mpa and re equilibrated them in slightly different fluids at 800 c 70 mpa measuring some inclusions by la icp ms after each step vapor inclusions indeed experienced a dramatic increase in cu from 0 3 0 03 to 5 7 3 3 wt while brine inclusions remained largely unmodified leading to a change in the partition coefficient d cu vap brine from a true value i e before re equilibration of 0 4 0 05 to an apparent value of 8 3 4 9 the requirements for substantial diffusional gain of cu in fluid inclusions are a change in the ph of the surrounding fluid from 1 to more neutral and the presence of s in the pre existing fluid inclusions these requirements are also fulfilled in nature cooling magmatic hydrothermal fluids experience a change from acidic to more neutral ph due to buffering along the feldspar mica join and natural vapor inclusions typically contain significant amounts of sulfur a reversal experiment performed on natural quartz hosted fluid inclusions from the erongo granite namibia showed that this process can be reversed with the measured d cu vap brine value of 11 9 3 being modified to 0 06 0 04 thus d cu vap brine values 1 measured on natural fluid inclusions in quartz are likely a secondary feature caused by post entrapment copper diffusion realistic d cu vap brine values in porphyry cu environments are between 0 11 and 0 15 and reconstructed vapor brine mass ratios are in the order of 4 9 this suggests that the main transporting agent of cu at the porphyry level are brines and that models based on copper into the vapor phase are incorrect

early cretaceous bimodal magmatism related epithermal mineralization a case study of the gaosongshan gold deposit in the northern lesser xing an range ne china abstract low sulfidation ls gold mineralization is partially associated with bimodal volcanism in origin sources of ore mineralized fluids and relevant dynamic processes remain poorly understood to better understand these issues a study including deposit geology petrology zircon u pb dating and element and sr nd hf isotope geochemistry of bimodal volcanic rocks at gaosongshan was conducted the bimodal volcanic rocks are composed of andesitic rocks basalt andesite andesite trachyandesite andesitic pyroclastic of the meifeng formation and overlying rhyolite lava of the fuminhe formation cut by equivalent porphyry stocks which contain pyrite bearing granular textured quartz cavity with a sugary appearance with trapped single phase magmatic fluids the andesitic suite formed at approximately 106 108 ma prior to eruption of the younger rhyolitic magmas 105 6 1 7 ma n 13 mswd 1 2 the andesites show calc alkaline affinities which is a subduction related characteristic and show moderate la ybn 7 91 11 36 and weakly negative europium anomalies 0 80 1 02 indicating a source in the asthenospheric mantle with varying contamination by both subduction and continental materials a type rhyolitic rocks are geochemically comparable to the andesitic suite and their higher la ybn 13 01 30 05 and lower eu eu 0 66 0 73 indicate evolved products of andesitic magmas the estimated oxygen fugacity of dry 1 2 water rhyolite lava is fmq 1 87 whereas rhyolite porphyry is more oxidized fmq 0 12 and water rich 2 5 2 8 with probably being water saturated at 2 0 2 4 km depths therefore it is assumed that the essential constraints of bimodal magmatism on epithermal gold mineralization are sequential processes from ascending and gradual differentiation of crustal level mantle derived basaltic magmas to releases of hydrothermal solutions from the silica rich magma reservoir at shallower levels most importantly the oxygen fugacity of and the fluid fluxes from dry rhyolitic melts are critical restricting parameters for the limited reserves at gaosongshan thus this research contributes objective evidence for investigations of deposit genesis and prospecting targets

geotectonic setting of hydrothermal activity on the summit of lucky strike seamount 37 17 n mid atlantic ridge we have investigated the relations between volcanic tectonic and hydrothermal activity on lucky strike seamount 37 17 n mid atlantic ridge using a nested survey strategy involving collection of data from different deep sea mapping systems the highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic pillow lava and hydrothermal sulfide rubble and hydrothermally cemented breccias substrates the large volume of sulfide rubble together with the nature of the sulfide structures indicates that hydrothermal activity has been episodic but ongoing for a long period of time hundreds to thousands of years on the basis of the distribution of hydrothermal deposits we propose a model of alternation between tectonic and volcanic control on hydrothermalism at lucky strike seamount midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high temperature hydrothermal activity over long time periods during periods of amagmatic extension active faulting within the summit depression provides multiple near surface fluid flow pathways for discharge of high temperature fluids and widespread deposition of massive sulfides during eruptive events rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits the new sheet flows form a cap on the hydrothermal system and fluid upflow is reorganized discharge of high temperature fluids is restricted to isolated sites with relatively high permeability for example the edges of the lava lake much of the upwelling hydrothermal fluid pools in the subsurface conductively cools and mixes with entrained seawater before discharging as widespread low temperature diffuse flow hyaloclastites become cemented further augmenting the sealing of the system present day activity at lucky strike seamount represents this locally volcanically controlled phase of activity despite the segment as a whole being dominantly amagmatic

discovery of abundant hydrothermal venting on the ultraslow spreading gakkel ridge in the arctic ocean submarine hydrothermal venting along mid ocean ridges is an important contributor to ridge thermal structure and the global distribution of such vents has implications for heat and mass fluxes from the earth s crust and mantle and for the biogeography of vent endemic organisms previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow spreading ridges ridges with full spreading rates 2 cm yr 1 which make up 25 per cent of the global ridge length and that such vent systems would be hosted in ultramafic in addition to volcanic rocks here we present evidence for active hydrothermal venting on the gakkel ridge which is the slowest spreading 0 6 1 3 cm yr 1 and least explored mid ocean ridge on the basis of water column profiles of light scattering temperature and manganese concentration along 1 100 km of the rift valley we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites our discovery of such abundant venting and its apparent localization near volcanic centres requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow spreading ridges

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

first active hydrothermal vents on an ultraslow spreading center southwest indian ridge the ultraslow spreading southwest indian ridge is a major tectonic province representing one of the important end member mid ocean ridge types for its very slow and oblique spreading and providing the only known route for migration of chemosynthetic deep sea vent fauna between the atlantic and indian oceans we report the investigation of the first active high temperature hydrothermal field found on any ultraslow mid ocean ridge worldwide located on southwest indian ridge at 37 47 s 49 39 e it consists of three zones extending 1000 m laterally and it is one of four recently discovered active and inactive vent sites within a 250 km long magmatically robust section our results provide the first direct evidence for potentially widespread distribution of hydrothermal activity along ultraslow spreading ridges at least along magmatically robust segments this implies that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities it is surprising that the special vent fauna appear to indicate some complex affinity to those on the central indian ridge southern mid atlantic ridge and the southwest pacific ocean

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

amphibole and apatite insights into the evolution and mass balance of cl and s in magmas associated with porphyry copper deposits chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic hydrothermal systems such as the coroccohuayco fe cu au porphyry skarn deposit peru here we used recent partitioning models to determine the cl and s concentration of the melts from the coroccohuayco magmatic suite using apatite and amphibole chemical analyses the pre mineralization gabbrodiorite complex hosts s poor apatite while the syn and post ore dacitic porphyries host s rich apatite our apatite data on the coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity from the gabbrodiorite complex to the porphyries causing the dominant sulfur species to shift from s2 to s6 at upper crustal pressure where the magmas were emplaced we suggest that this change in sulfur speciation could have favored s degassing rather than its sequestration in magmatic sulfides using available partitioning models for apatite from the porphyries pre degassing s melt concentration was 20 200 ppm estimates of absolute magmatic cl concentrations using amphibole and apatite gave highly contrasting results cl melt concentrations obtained from apatite 0 60 wt for the gabbrodiorite complex 0 2 0 3 wt for the porphyries seems much more reasonable than those obtained from amphibole which are very low 0 37 wt for the gabbrodiorite complex 0 10 wt for the porphyries in turn relative variations of the cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the coroccohuayco magmatic suite this confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100 200 mpa finally mass balance constraints on s cu and cl were used to estimate the minimum volume of magma required to form the coroccohuayco deposit these three estimates are remarkably consistent among each other ca 100 km3 and suggest that the cl melt concentration is at least as critical as that of cu and s to form an economic mineralization

the role of sulfur in the formation of magmatic hydrothermal copper gold deposits abstract essential resources of many rare metals including copper zinc molybdenum silver and gold occur in natural sulfide mineral deposits understanding the origin of these metal resources has been limited by a lack of data about the geochemistry of sulfur the most important and abundant element of ore deposits we report the first directly measured sulfur concentrations in high temperature fluids together with their ore metal contents using a new method for sulfur quantification in fluid inclusions by laser ablation inductively coupled plasma mass spectrometry la icp ms co genetic brine and vapor inclusions from magmatic hydrothermal ore deposits and granitic intrusions show an excess of sulfur over ore metals as required for efficient ore mineral precipitation the results demonstrate that s cu and au are highly enriched in vapor like magmatic fluids implying that such low salinity fluids are the key agent for the formation of porphyry copper and epithermal gold deposits

volatility of se and te during subduction related distillation and the geochemistry of epithermal ores of the western united states subduction partial melting of mantle and magma generation and ultimately magma degassing can lead to the preferential mobilization of volatile elements and compounds research indicates that devolatilization reactions in a subducted slab can cause the overlying mantle to be enriched in volatile ore forming constituents water co2 sulfur metal loid s this process of mantle enrichment plays an important role in subsequent magma generation and initiation of ore genesis furthermore the relative volatility of metal loid s appears to play a role in the ultimate geochemistry of shallow epithermal epizonal hydrothermal ores in the western united states for example there is a general west to east geographic zonation of tertiary ores from se rich to te rich it is proposed that this geographic geochemical zonation is the direct result of the higher geochemical volatility of se as compared to te previously au ag te se hg as sb and tl have been referred to as the epithermal suite of elements it is suggested here that their geochemical cycles are linked by their similar volatile behavior through subduction partial melting in the mantle and subsequent magma degassing processes essentially specific tectonomagmatic events that affected the western united states e g oligocene eocene magmatism in the great basin and southern rocky mountains mid miocene magmatism associated with the inception of the yellowstone hotspot in the pacific northwest were distillation events that drove devolatilization of fertile mantle after the laramide shallow slab event furthermore the metal loid volatility behavior likely occurs in other regions affected by subduction and may play a fundamental role in the metal endowment of these regions

hydrothermal circulation and the dike gabbro transition in the detachment mode of slow seafloor spreading one of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast spreading ocean crust this boundary marks the brittle ductile transition at the ridge crest and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi steady state melt lens in contrast at slow spreading ridges the crustal structure appears chaotic with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor recent work suggests that as much as 50 of the atlantic ocean crust formed by a detachment mode of seafloor spreading including the formation of oceanic core complexes capped by long lived convex upward detachment faults these detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall thus explaining the link between faulting and black smoker systems we suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike gabbro transition in the detachment mode of spreading inevitably leading to exposure of gabbros on the seafloor through continued faulting this concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates

fluid evolution of the neoarchean pampalo orogenic gold deposit e finland constraints from la icpms fluid inclusion microanalysis abstract the sources of fluids and precious metals in orogenic gold deposits remain controversial and a key question is whether auriferous magmatic hydrothermal fluids are a decisive ingredient for economic orogenic gold mineralization contributions of magmatic hydrothermal fluids have been proposed based on indirect arguments such as isotope data or emplacement of granites coevally with gold deposits by contrast complete elemental concentration data for orogenic gold fluids which could be used to compare them directly with those of magmatic hydrothermal fluids are not yet available this study reports the major and trace element chemistry of fluid inclusions from the neoarchean pampalo orogenic gold deposit located in the hattu schist belt in eastern finland three successive generations of gold mineralized and barren veins provide the paragenetic and chronological framework for establishing the fluid evolution history of the pampalo deposit based on petrography microthermometry and la icpms fluid inclusion microanalysis five major fluid types were identified the data shows that all fluid types despite characteristic differences carry the signature of orogenic gold fluids i e low to intermediate salinity presence of co 2 ch 4 or n 2 high s contents enrichment in b as sb and w and low pb and zn concentrations gold concentrations vary significantly between the oldest ca 0 6 g g fluid related to the zones with highest gold grades in the deposit and later fluid types

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

zircon u pb and molybdenite re os geochronology and geochemistry of jinchang porphyry gold copper deposit ne china two phase mineralization and the tectonic setting abstract whether multiple mineralization styles in porphyry deposits are the result of one or multiple pulses of magmatism is unknown in this paper we present new age and geochemical data on the jinchang porphyry gold copper au cu deposit an important deposit in ne china and in the eastern central asian orogenic belt caob field geological study suggests that both the mineralization and ore causative magmatism at jinchang are multiphase la icp ms zircon u pb dating of the monzogranite ore host yielded 197 1 ma which is considerably older than the ore causative monzonite 115 3 ma and granodiorite 107 2 ma molybdenite re os dating on the breccia pipe hosted au cu ores yielded a well defined isochron age of 114 3 ma coeval to the monzonite emplacement the au cu mineralization was followed by another phase of crypto explosive breccia pipe hosted disseminated vein type au mineralization this au only mineralization phase was auriferous pyrite re os dated to be 103 101 ma coeval to the granodiorite emplacement geochemically the monzonite and granodiorite are enriched in liles and lrees but depleted in hfses and hrees the ore causative rocks are i type and display continental arc geochemical affinities we suggest that the monzonite and granodiorite were generated in an intracontinental extensional setting probably caused by the early cretaceous subduction roll back of the paleo pacific plate

gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit the ladolam hydrothermal system on lihir island papua new guinea hosts one of the youngest and largest gold deposits in the world several deep more than 1 kilometer geothermal wells were drilled beneath the ore bodies to extract water at 275 c and to facilitate open pit mining using a titanium down hole sampler we determined that the deep geothermal brine of magmatic origin contains 15 parts per billion gold at the current gold flux of 24 kilograms per year this deposit could have formed within 55 000 years the combination of sustained metal flux and efficient metal precipitation led to the formation of a giant hydrothermal gold deposit in a short period

mineralogy and geochemistry of hydrothermal sediments from the serpentinite hosted saldanha hydrothermal field 36 34 n 33 26 w at mar abstract the saldanha hydrothermal field is located at the top of a serpentinized massif mount saldanha ms at a non transform offset nto5 along the mid atlantic ridge mar south of the azores it is one of the rare known sites on a worldwide basis where direct evidence of low temperature 7 9 c hydrothermal activity has been provided by direct observation of hydrothermal fluid venting through small orifices in the ocean floor sedimentary cover this study focuses on the mineralogy and geochemistry of 14 sediment cores collected at ms for comparison four samples collected at the rainbow site nto6 were also studied mount saldanha hydrothermal sediments are highly diluted within a dominant foraminiferal nanofossiliferous ooze with small fragments of underlying rocks the mineral assemblage of the hydrothermal component is characterized by sulphides nontronite smectites poorly crystallized mn oxyhydroxides and amorphous material cu zn and fe sulphides mn mg oxy hydroxides and putative manganobrucite were also identified in one sample collected at an orifice vent in this sample micro chimneys conduits composed of isocubanite and sphalerite were also identified mount saldanha sediments show a clear enrichment in elements such as mn mg fe cu p and v derived from hydrothermal fluids and ni cr and co derived from ultramafic rocks the geochemical data together with the observed mineral assemblage suggest that the hydrothermal fluids are at a higher temperature than those measured at the escape orifices 7 9 c and a strong enrichment in mg mainly at the top of the mount agrees with extensive mixing of the hydrothermal fluid with unmodified seawater nevertheless the mineral assemblage of ms sediments is consistent with the precipitation from hydrothermal fluids at much lower temperatures than at rainbow the presence of serpentinized and steatitized talcshist ultramafic rocks and the occurrence of a strong methane anomaly within the overlying water column collectively suggest that the hydrothermal circulation at ms is driven by exothermic reactions closely associated with the serpentinization process rainbow sediments have a higher concentration in transition metals and consequently an enrichment in sulphides these differences are likely to be a consequence of the higher temperature of hydrothermal fluids reflected in the composition of hydrothermal solutions and of a stronger hydrothermal flux at the rainbow site

enhanced hydrothermal activity on an ultraslow spreading supersegment with a seismically detected melting anomaly abstract seafloor hydrothermal venting fields occur on all ocean spreading ridges osrs regardless of spreading rates however the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow spreading osrs are poorly known chinese dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system with an array of turbidity sensors and a near bottom camera along the ultraslow spreading southwest indian ridge here we report the discovery of multiple hydrothermal plumes overlying segments 28 29 and 30 between the indomed and gallieni fracture zones from these data and earlier explorations in segments 25 27 we identify nine active venting sites the spatial density fs sites 100 km of active sites along the 394 km of ridge axis in our study area is thus 2 8 nearly 3 higher than predicted by the global trend of fs for ultraslow osrs in the interridge database previous studies concluded that an enhanced magma supply to the central indomed gallieni supersegment 11 8 ma is now limited to segment 27 our results indicate that although hydrothermal activity may be most concentrated in segment 27 the discoveries of active venting in segments 25 30 implies the presence of additional magma bodies across a broad extent of the indomed gallieni supersegment

hydrothermal exploration of mid ocean ridges where might the largest sulfide deposits be forming highlights black smokers on slow spreading ridges can host larger seafloor massive sulfide sms deposits than on fast ridges the largest slow ridge sms deposits are typically associated with long lived tectonic fractures sustained fluid flow ultramafic tectonic hosted sites exhibit high average cu 10wt and au 3ppm contents in their surface sms samples hydrothermal plume surveys are consistent with equal abundances of magmatic vs tectonic vent control along ultraslow ridges the first black smokers on any ultraslow ridges exhibit high cu au sms contents at both magmatic and tectonic vent sites abstract here we review the relationship between the distribution of modern day seafloor hydrothermal activity along the global mid ocean ridge crest and the nature of the mineral deposits being formed at those sites since the first discovery of seafloor venting a sustained body of exploration has now prospected for one form of hydrothermal activity in particular high temperature black smoker venting along 30 of the global mid ocean ridge crest while that still leaves most of that 60 000 km continuous network to be explored some important trends have already emerged first it is now known that submarine venting can occur along all mid ocean ridges regardless of spreading rate and in all ocean basins further to a first approximation the abundance of currently active venting as deduced from water column plume signals can be scaled linearly with seafloor spreading rate a simple proxy for magmatic heat flux what can also be recognized however is that there is an excess of high temperature venting along slow and ultra slow spreading ridges when compared to what was originally predicted from seafloor spreading magmatic heat budget models an examination of hydrothermal systems tracked to source on the slow spreading mid atlantic ridge reveals that no more than half of the sites responsible for the black smoker plume signals observed in the overlying water column are associated with magmatic systems comparable to those known from fast spreading ridges the other half of all currently known active high temperature submarine systems on the mid atlantic ridge are hosted under tectonic control these systems appear both to be longer lived than and to give rise to much larger sulfide deposits than their magmatic counterparts presumably as a result of sustained fluid flow a majority of these tectonic hosted systems also involve water rock interaction with ultramafic sources importantly from a mineral resource perspective this subset of tectonic hosted vent sites also represents the only actively forming seafloor massive sulfide deposits on mid ocean ridges that exhibit high concentrations of cu and au in their surface samples 10 wt average cu content and 3 ppm average au along ultraslow spreading ridges first detailed examinations of hydrothermally active sites suggest that sulfide deposit formation at those sites may depart even further from the spreading rate model than slow spreading ridges do hydrothermal plume distributions along ultraslow ridges follow the same 50 50 distribution of black smoker plume signals between magmatic and tectonic settings as the slow spreading mar however the first three black smoker sites tracked to source on any ultra slow ridges have all revealed high temperature vent sites that host large polymetallic sulfide deposits in both magmatic as well as tectonic settings further deposits in both types of setting have now been revealed to exhibit moderate to high concentrations of cu and au respectively an important implication is that ultra slow ridges may represent the strongest mineral resource potential for the global ridge crest despite being host to the lowest magmatic heat budget

the behavior of metals pb zn as mo cu during crystallization and degassing of rhyolites from the okataina volcanic center taupo volcanic zone new zealand tthe taupo volcanic zone tvz new zealand is a region of voluminous and frequent rhyolitic volcanism and widespread geothermal activity additionally the hydrothermal systems of the tvz contain relatively high concentrations of base and precious metals here we present an extensive dataset of major element volatile and trace element including pb zn as mo cu abundances in melt inclusions pumice glasses and minerals from eight eruptions within the okataina volcanic center ovc of the tvz to investigate the behavior of metals during melt evolution the high sio2 rhyolites of the ovc contain high concentrations of volatiles 6 wt h2o 0 25 wt cl and underwent significant degassing prior to and during eruption the ovc melts contain moderate concentrations of metals 11 24 ppm pb 20 50 ppm zn 2 7 ppm as 2 5 ppm mo 5 ppm cu ferromagnesian minerals amphibole biotite and orthopyroxene in the ovc pumice have high concentrations of zn 1500 ppm and plagioclase and biotite contain moderate amounts of pb 11 ppm the melt inclusion and pumice glass trace element data reveal complex histories of magma mixing and mingling prior to eruption however discrete melt batches are easily identified based on trace element geochemistry variations in incompatible trace elements within these melt batches suggest that the ovc rhyolites underwent at least 20 25 fractional crystallization during quartz crystallization and melt inclusion entrapment at pressures of 100 200 mpa and little to no crystallization 5 during ascent and eruption comparison of melt inclusion metal and incompatible element e g u concentrations reveals that melt pb mo and as increase whereas melt zn decreases during fractional crystallization at depth 100 200 mpa these observations can be explained by minor partitioning of the metals pb mo and as into the fractionating minerals and stronger partitioning of zn into the ferromagnesian phases supported by calculated metal d values and analyzed metal concentrations in ovc minerals interestingly throughout both deep vapor saturated crystallization and during extensive degassing during magma ascent and eruption as recorded by pumice glasses the metals analyzed here do not appear to partition appreciably into the vapor we propose that the lack of volatility of the metals analyzed in this study can be attributed to a combination of several factors first vapor melt partitioning requires the presence of ligands commonly cl s and oh with which the metals may complex given the low cl h2o ratios in the ovc melts and the extensive degassing of h2o compared with cl it seems likely that the rhyolites would have exsolved h2o rich vapor with insufficient cl to transport metals in particular pb and zn into the vapor phase either at depth or during magma ascent second the overall small volumes of vapor present during crystallization at pressures of 100 200 mpa would have impeded significant vapor melt partitioning of the metals finally the estimated very rapid ascent of the ovc melts from depths of 4 8 km suggests that there was insufficient time at low pressure for diffusion of metals out of the melt these results imply that there may be an indirect connection between the rhyolites and the metals of the hydrothermal systems of the tvz as the metals and other species such as cl remain in the rhyolitic magmas upon eruption they are available in the large volumes of rhyolite emplaced in the upper crust of the tvz for leaching by heated meteoric waters

separation of molybdenum and copper in porphyry deposits the roles of sulfur redox and ph in ore mineral deposition at bingham canyon the giant bingham canyon porphyry cu mo au deposit utah is associated with eocene subvolcanic intrusions it shows a distinct metal zonation above a barren core with dominantly shallow cu au mineralization cu stage following the early quartz monzonite porphyry qmp intrusion and spatially deeper mo mineralization mo stage occurring in a separate vein set exclusively after a late quartz latite porphyry qlp intrusion that truncates earlier cu au veins to understand this metal separation and the geochemical process of molybdenite mineralization we investigated fluid inclusions by microthermometry raman spectroscopy and laser ablation inductively couple plasma mass spectrometry la icp ms microanalysis in low and high grade quartz veins of both mineralization stages in deep low grade quartz veins interpreted to represent the root zone of the cu stage we found high concentrations of cu s and mo in the fluid inclusions whereas in low grade mo stage veins we found lower cu but similar concentrations of s and mo compared to the inferred input fluids to the cu stage sulfur and copper concentrations were similar in intermediate density type fluid inclusions in deep low grade cu stage samples whereas intermediate density type inclusions in low grade mo stage veins have s contents that exceed their cu contents in high grade mo stage vein we found large variations of mo concentrations in coexisting brine and vapor inclusions compared to the p t conditions of the cu precipitation stage 90 260 bars and 320 430 c the mo precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360 to 580 c mass balance calculation based on the compositions of intermediate density inclusions and brine vapor assemblages interpreted to be derived by phase separation during decompression of the ascending single phase intermediate density fluid indicate that the mass of vapor phase exceeded that of brine by about 9 1 in both mineralization stages combining this mass balance with the analyzed vapor brine partitioning data indicates that more than 70 of mo and s by mass in the deposit were deposited from the vapor phase earlier cu au deposition was similarly dominated by vapor but recently published data about postentrapment cu diffusion in and out of fluid inclusions cast doubt on previous quantifications suggesting that almost none of the copper was deposited by brine mo is less likely to be modified by selective diffusion and high mo contents max 0 0054 mo na in intermediate density 380 g g mo in brine in the hydrothermal fluids were maintained from the early cu stage to the late mo stage this indicates that mo concentration was not the decisive factor for separate precipitation of late mo ore at bingham canyon instead the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids i e a large subvolcanic magma reservoir this is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions a tentative difference in the fe mn ratio in fluids of both veining stages incipient muscovite alteration along high temperature molybdenite veins and an increasing tendency for mo to fractionate from brine to vapor we suggest that the early cu stage fluids were slightly more oxidized and neutral allowing cu fe sulfides to saturate first while molybdenite saturation was suppressed and mo was lost from the early ore stage by contrast during the later mo stage the fluids were more reduced and acidic thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two phase fluid consistent with textural observations this interpretation may imply more generally that small differences in redox potential and acid base balance of the magmatic source of porphyry mineralizing systems may be decisive in the temporal and spatial separation of the two metals

oceanic detachment faults focus very large volumes of black smoker fluids it is generally assumed that the seawater derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of 1 3 km we present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300 400 c were focused along a low angle detachment fault at 15 45 n near the mid atlantic ridge isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids not conductively heated seawater the fault zone protolith was predominantly ultramafic but also included mafic rocks with metasomatic alteration to talc tremolite chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow fluids in equilibrium with this assemblage would be similar to ultramafic hosted black smoker fluids we present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted tag type to footwall ultramafic hosted rainbow type to low temperature ultramafic hosted lost city type key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis