evolution of seawater 87sr86sr over the last 400 ka the absence of glacial interglacial cycles abstract fluctuations in the 87 sr 86 sr ratio of seawater accompanying glacial interglacial climate changes have been reported in recent studies but remain highly controversial to investigate these potential fluctuations we present very high precision 13 ppm 87 sr 86 sr measurements on planktonic foraminifera from indian ocean and pacific cores 87 sr 86 sr ratios from three different foraminiferal species are indistinguishable from one another in both core top and 50 ka samples demonstrating that changes due to diagenesis or contamination do not influence the measurements average 87 sr 86 sr ratios for pacific ocean and indian ocean samples are also indistinguishable at three intervals core top 50 ka and 300 370 ka demonstrating that the oceans have remained well mixed with respect to sr also 87 sr 86 sr ratios are not affected by changes in the precleaning of samples measurements from pacific core v28 238 that used in the study of dia et al 1 do not reproduce the cycles seen in the previous study and are statistically well explained by a linear increase in the seawater 87 sr 86 sr ratio it seems likely that an analytical artefact caused the cycles in the previous study measurements from two key sections of core from odp site 758 that used in the study of clemens et al 2 do not reproduce the largest shifts seen in the previous study despite the considerably better precision reported here the apparent cyclicity in the clemens et al 2 data is also suggested to be due to an unknown analytical artefact the 48 measurements presented here are a statistically good fit to a straight line defined by 87sr ppm 0 0613 age ka at the 13 ppm level of precision no evidence for a glacial interglacial variation in the seawater 87 sr 86 sr ratio is seen glacial interglacial variation may still exist but this data constraints its maximum amplitude to 6 9 ppm which would correspond to changes in the riverine sr flux of 30

in situ elemental and sr isotope characteristics of magmatic to hydrothermal minerals from the black mountain porphyry deposit baguio district philippines at the black mountain porphyry cu au deposit in the baguio district northern luzon philippines pre and synmineralized rocks preserve magmatic and hydrothermal minerals e g plagioclase amphibole titanite and epidote spanning the complete paragenesis of the deposit strontium isotope values in early crystallized plagioclase phenocrysts from all felsic porphyries can be divided into two types the type i plagioclase crystals show relatively homogeneous sr isotope values 0 7035 0 7038 1 0 0001 indicating crystallization from a relatively stable and long lived felsic magma chamber the type ii plagioclase grains have a wider range of sr isotope compositions 0 7032 0 7039 1 0 0001 indicating mafic and or felsic magma recharge in magmatic titanite nb ta values are higher than those in the whole rock while zr hf and y ho values are lower in hydrothermal titanite and epidote the ratios are similar to those in the whole rock these patterns reflect crystallization effects imposed during the magmatic stage but an absence of differentiation during the hydrothermal stage the consistent gradual decrease in total rare earth element y zr and u contents in both hydrothermal titanite and epidote from early to late stages indicates the effect of hydrothermal fluid evolution with decreasing temperature the variation of 87sr 86sr values in magmatic amphibole plagioclase and hydrothermal epidote in felsic and mafic rocks indicates the addition of mafic magma derived fluid into the felsic magma derived fluid one extra source of fluid probably derived from wall rock limestone was required to generate the highly radiogenic 87sr 86sr values of some epidote 0 7038 0 7053 thus in situ elemental and sr isotope variation in minerals from different paragenetic stages can be used to interpret formation process and source for both magmas and hydrothermal fluids

fe isotope fractionation during fe ii oxidation by the marine photoferrotroph rhodovulum iodosum in the presence of si implications for precambrian iron formation deposition abstract the iron fe isotopic composition of precambrian iron formations ifs besides providing geological context through its mineralogical properties was suggested to function as a biosignature that can be used to infer a potential microbial role in the formation of the deposited fe minerals anoxygenic phototrophic fe ii oxidizing bacteria photoferrotrophs capable of oxidizing fe ii anoxically using light energy were potentially involved in fe ii oxidation in anoxic or suboxic precambrian oceans the effect of si on fe isotopic fractionation between aqueous fe ii and fe si co precipitates has been investigated before however it is currently unknown how stable fe isotopes are fractionated during enzymatic fe ii oxidation under marine hydrogeochemical conditions and particularly how the presence of si affects the fe isotope composition and the isotopic exchange among different fe phases we therefore studied fe isotope fractionation during fe ii oxidation by the marine photoferrotroph rhodovulum iodosum in simulated precambrian seawater amended with 1 mm dissolved si our results show that the change in the fe isotope compositions over time for both the initial aqueous fe ii fe aq and the fe iii precipitates fe ppt follow a rayleigh distillation model moreover the fractionation e 56 fe ppt aq determined independently from either 56 fe aq or 56 fe ppt data resulted in a value of 2 3 0 3 2sd n 6 this value differs from the fractionation factor determined previously for fe ii oxidation by r iodosum in the absence of si where the fractionation calculated from 56 fe aq i e 0 96 1 18 was different from that calculated from 56 fe ppt 1 96 1 98 this difference was attributed to isotopic exchange processes with soluble and sorbed fe species the present study suggests that si present in precambrian oceans retards fe isotopic exchange likely through combined effects of complexation of dissolved fe species by si and sorption of si to fe iii minerals thus lowering sorption of fe ii to the fe iii minerals which is necessary for isotopic exchange in summary our data suggests that si in ancient oceans played a key role for the fe isotope composition of fe iii minerals that were deposited by photoferrotrophic iron oxidation in precambrian oceans by minimizing subsequent isotope exchange and recrystallization processes with aqueous fe ii

evidence for magmatic hydrothermal fluids and ore forming processes in epithermal and porphyry deposits of the baguio district philippines the baguio district contains a diverse array of epithermal porphyry and skarn deposits together with a large broadly strata bound advanced argillic lithocap magmatism mineralization and alteration occurred in response to subduction of the south china sea plate and the scarborough ridge beneath northern luzon over the past 3 m y rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry cu au deposits most of the epithermal au ag deposits of the baguio district including antamok and acupan the two largest au deposits are intermediate sulfidation state quartz carbonate adularia illite base metal sulfide veins that contain electrum and minor au ag tellurides in contrast high sulfidation mineralization at kelly includes enargite tennantite electrum and precious metal tellurides and is associated with advanced argillic alteration although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type the epithermal mineralizing fluids were dilute generally 600 c hypersaline brines 30 to 70 wt nacl equiv and low density vapor sulfur isotope compositions of sulfides in the porphyry skarn and intermediate sulfidation epithermal veins of the southern and central baguio district are mostly between 1 and 6 per mil consistent with a predominance of h2s in the mineralizing fluids i e reducing conditions in contrast sulfides from the high sulfidation porphyry and intermediate sulfidation deposits located adjacent to the baguio lithocap mostly have negative sulfur isotope values 6 9 to 0 8 consistent with oxidizing so42 predominant mineralizing fluids intermediate sulfidation epithermal veins at acupan have crosscut a well mineralized porphyry cu au stock work at ampucao the two deposits cannot be distinguished on the basis of radiometric age determinations ampucao 0 51 0 26 ma acupan 0 65 0 07 ma and are interpreted to be cogenetic with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction measured 34ssulfide 1 1 to 6 6 34ssulfate 10 4 to 31 8 values and initial strontium ratios of anhydrite 0 70378 0 70385 are consistent with identical and predominantly magmatic sources of these components for the ampucao porphyry and acupan epithermal veins helium isotopes provide further evidence of mantle derived components in the epithermal veins r ra values of 6 0 and 6 7 oxygen deuterium and carbon isotopes provide evidence for predominantly magmatic water at ampucao and for hybrid magmatic meteoric waters at acupan that precipitated precious metals due to boiling the proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at acupan the common observation of cross cutting relationships between porphyry and epithermal veins observed throughout the baguio district imply that the evolution of porphyry style to intermediate sulfidation style mineralization was a common phenomenon in this region and contributed significantly to its rich metal endowment

characteristic microseismicity during the development process of intermittent rockburst in a deep railway tunnel abstract during deep tunnel excavation more than one rockburst sometimes occur in a particular area which seriously threatens construction safety in this work the characteristic microseismicity occurring during the development process of intermittent rockburst more than one rockburst occurring in a particular area and single rockburst only one rockburst occurring in a particular area and the associated mechanism of formation are studied data was collected during the construction of a railway tunnel in southwestern china microseismicity was recorded and analyzed before and after each rockburst for both intermittent and single rockbursts the schmidt number declines heavily with time the cumulative apparent volume rises sharply with time and the seismic activity is high before the rockburst however this characteristic microseismicity can also be found with no rockburst occurrence between each rockburst in an intermittent rockburst the ms events mostly have low ms energy i e whose logarithm measured in joules is smaller than 3 5 and the energy stored in the rockmass of the rockburst zone is not adequately released therefore there is likely to be another rockburst after the blast when high in situ stress is present after the last rockburst of an intermittent rockburst and the single rockburst some ms events of high ms energy i e whose logarithm measured in joules is larger than 3 5 occur in the deep surrounding rockmass and the energy stored in the rockmass of the rockburst zone is gradually released to a certain degree the stress is transferred to the deep surrounding rockmass whose stability is relatively high as a result a subsequent rockburst is hardly likely to occur again

composition and evolution of ore fluids in a magmatic hydrothermal skarn deposit the chemistry of brine vapor and low salinity fluid inclusions measured by proton induced x ray emission from the bismark skarn deposit mexico is consistent with an evolving magmatic hydrothermal system with no evidence for external fluid inputs the results support a model that invokes early phase separation of magmatic fluids into brine and vapor two phase field at high temperature and lithostatic pressure followed by the entrapment of a low salinity magmatic fluid one phase field at lower temperature and hydrostatic pressure the early brine and vapor inclusions contain high pb and zn concentrations and low cu however the vapor contains significantly more cu than the brine and was likely transported as a sulfur complex the fluid phase changes observed and behavior of cu are comparable to those of porphyry cu systems the later low salinity fluid at bismark represents a distinct pulse of magmatic fluid with a high k ca ratio in which base metals including cu were transported by chloride complexes paragenetic relationships and variations in the relative concentrations of cu pb and zn suggest that this fluid was primarily responsible for ore deposition the relative cu pb and zn concentrations in fluid inclusions at bismark are consistent with those measured from base metal ores in high temperature carbonate replacement deposits throughout mexico and suggest that the bulk metal budget of these deposits is primarily controlled by magmatic hydrothermal fluids deficient in cu

evaluation and application of controlling parameters for seismic events in hard rock mines a method is proposed for quantitative assessment and interpretation of the seismic hazard of planned excavation in a hard rock mine the quantitative assessment is derived from three dimensional elastic boundary element modelling of the host rock mass parameters obtained from the modelling are associated with historical observations of seismicity in the mine to generate probabilistic relations between seismic event occurrence and event strength the parameters used in the event spatial occurrence relations and the event strength estimates are the factor of safety against seismic failure for different types of seismic events which are inferred from back analysis of field observations and the modelled ground work the estimates of these parameters are related to the controlling quantities for a seismic event which are a local state of stress sufficient to cause rock mass failure and an unstable local concentration of strain energy i e energy in excess of that which can be dissipated non violently in rock mass failure application of the method is illustrated by reference to several case studies of mine seismicity

rockburst mechanism and prediction based on microseismic monitoring abstract in this study the rockburst definition is comprehensively described and several main rockburst theories the strength theory the energy theory the bursting liability theory the rigidity theory and the instability theory are analyzed the two body interaction theory based on the newton s law can explain the rockburst mechanism most reasonably the rockburst development process is a static process and the rockburst occurrence process is within the scope of dynamics in addition some precursors exist before rockburst occurrence reflected by the consistency between acoustic emissions and damage variables in rock mass which can be summarised as three seismogenic models according to the tempo spatial distribution by utilizing the 3s stress buildup stress shadow and stress transfer principle in seismology three rockburst criteria are proposed in this paper namely the stress tempo spatial evolution the magnitude energy and concentration degree of microseismic events sudden change of apparent volume finally by comparing the results obtained by microseismic monitoring and landsonar positioning it is shown that the root cause of rockburst occurrence is rock heterogeneity and the existence of joints and fractures the fracture intersection points are the locations for rockburst initiation it is proven that the two body interaction principle can perfectly explain the rockburst mechanism

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

the morphology of the reaction front of the dissolution precipitation reaction rutile wollastonite titanite in time series experiments at 600 c 400 mpa fluid assisted mass transport reactions by dissolution precipitation where a precursor mineral reacts with a fluid play an important role in metamorphism and metasomatism we investigated titanite growth on rutile in time series experiments between one and 107 days at constant p t conditions of 600 c and 400 mpa in the system tio 2 cao sio 2 na 2 o hcl h 2 o a two capsule assemblage allows for transport of ca and si from dissolving wollastonite to dissolving rutile the ti source in a nacl bearing aqueous fluid according to the general reaction casio 3 tio 2 catisio 5 complete overgrowth of rutile by titanite occurred after just one day of experiment fine grained lozenge shaped titanite crystals of short time runs up to 14 days reorganize to larger predominantly prismatic crystals after 14 days after investigation by scanning electron microscopy the titanite overgrowth was removed from the rutile by hydrofluoric acid to provide a three dimensional view of the dissolution reaction front on the rutile surface the morphology of the rutile surface is dominated by humps or ridges beneath the central region of a titanite crystal and valleys at the grain boundaries between adjacent titanite crystals the dissolution pattern on the rutile surface mimics the titanite overgrowth and changes with changing grain size and shape of the titanite with longer run times the preferred dissolution of rutile in the valleys is clearly linked to the position of the titanite grain boundaries which served as pathways for fluid assisted element transport rutile titanite and titanite titanite boundaries show a significant porosity in transmission electron microscopy images of foils prepared by focused ion beam milling the large scale dissolution pattern on the rutile surface is independent of the crystallographic orientation of the rutile and entirely dominated by the arrangement of titanite crystals in the overgrowth dissolution features on a scale smaller than 1 m are dominated by stepwise dissolution and etch pits following the crystallographic orientation of the rutile similar observations were made in experiments with an additional al source although these experiments result in a different overgrowth pattern i e an exposed rutile surface is always present and solitary titanite crystals are accompanied by partial overgrowths quantitative characterization of the surface morphology by white light interference microscopy demonstrates that with increasing grain size of titanite dissolution of rutile is strongly enhanced at the titanite grain boundaries natural examples of titanite overgrowths on rutile show the same relations between element pathways arrangement of titanite crystals and 3d dissolution pattern on rutile as in the experimental systems we conclude that the transport of ti away from the rutile and of ca si into the reaction rim occurred in a grain boundary fluid the composition of which must have been strongly different from the composition of the bulk fluid in the experiment as well as in the natural system the reaction progress depends on the availability of a fluid and relicts of rutile in titanite indicate restricted availability of fluid in the natural system e g a fluid pulse that was consumed by the reactions the reaction examined here can serve as a proxy for other reactions of the conversion of oxide minerals e g spinel or corundum into silicates

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

redistribution of hfse elements during rutile replacement by titanite titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon we investigated an amphibolite sample from the tromso eclogite facies terrain in northern norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite patchy compositional zoning of al ti and f in titanite relates to the presence of a fluid with variable ti al and or f during its growth laser ablation icp ms and electron microprobe data for high field strength elements hfse nb zr ta and hf of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal 8 mm and a systematic decrease towards uniform hfse contents in the large core hfse contents of smaller rutile grains 0 5 mm and rutile inclusions 100 m in the titanite overgrowth are similar or higher than in the rims of large rutile crystals element profiles from the rim inward demonstrate that hfse enrichment in rutile is controlled by diffusion hfse ratios in diffusion altered rutile show systematic variations compared with the uniform core composition of the large rutile modelling of zr and nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems variations of hfse contents in the newly formed titanite show no systematic spatial distribution hfse ratios in titanite and the rims of rutile are different indicating different solid fluid distribution coefficients in these minerals element fractionation by diffusion into the relict rutile and during fluid mediated growth of new titanite could substantially change the hfse budget of these minerals and could affect their use for geochemical tracing and other applications such as zr based geothermobarometry

magma mixing recharge and eruption histories recorded in plagioclase phenocrysts from el chichon volcano mexico operative throughout the 200 ky history of the el chichon magma consistent core to rim decreases of sr sr ratios and coincident system increases in sr concentrations in plagioclase phenocrysts of varying size 1 cm to 2 mm are reported from samples of the 1982 and pre 1982 200 ka eruptions of el chichon volcano maximum sr sr ratios of 0 7054 significantly higher than the wholerock isotopic ratios 0 7040 0 7045 are found in the cores

porphyry copper deposit formation by sub volcanic sulphur dioxide flux and chemisorption the processes that create economic grade accumulations of metals above magma chambers are unclear high temperature laboratory experiments show that rapid reactions between magmatic gases and earth s crust can trigger efficient metal deposition

melt composition control of zr hf fractionation in magmatic processes zircon zrsio4 and hafnon hfsio4 solubilities in water saturated granitic melts have been determined as a function of melt composition at 800 and 1035 c at 200 mpa the solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt moreover the molar ratio of zircon and hafnon solubility is a function of melt composition although the solubilities are nearly identical in peralkaline melts zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts accordingly calculated partition coefficients of zr and hf between zircon and melt are nearly equal for the peralkaline melts whereas for metaluminous and peraluminous melts dhf dzr for zircon is 0 5 to 0 2 consequently zircon fractionation will strongly decrease zr hf in some granites whereas it has little effect on the zr hf ratio in alkaline melts or similar depolymerized melt compositions the ratio of the molar solubilities of zircon and hafnon for a given melt composition temperature and pressure is proportional to the hf zr activity coefficient ratio in the melt the data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts however it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached for most ferromagnesian minerals in equilibrium with a depolymerized melt dhf dzr typical values of dhf dzr range from 1 5 to 2 5 for clinopyroxene amphibole and titanite because of the change in the hf zr activity ratio in the melt the relative fractionation of zr and hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite it is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in zr hf whereas fractional crystallization of depolymerized melts tends to increase zr hf there is no need to invoke fluid metasomatism to explain these effects results demonstrate that for ions with identical charge and nearly identical radius crystal chemistry does not alone determine relative compatibilities rather the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice

porphyry and epithermal deposits and 40ar 39ar geochronology of the baguio district philippines the baguio district is located in the central cordillera of northern luzon philippines it contains numerous mineralized porphyry copper gold epithermal gold silver and skarn gold lead zinc deposits the district is floored by cretaceous eocene metavolcanic and metasedimentary rocks which are overlain by marine to terrestrial sedimentary and volcanic rocks of early miocene to pliocene ages tertiary arc magmatism related to east directed subduction of the south china sea plate along the manila trench beneath northern luzon produced a major batholith the central cordillera intrusive complex which defines the eastern boundary of the baguio district the intrusive complex was emplaced in the early miocene based on 40ar 39ar age determinations for the lucbuban gabbro 22 6 0 5 ma and virac granodiorite 20 23 0 38 and 20 2 0 7 ma recent exploration has led to the discovery of several new porphyry copper gold and skarn prospects in the western baguio district mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4 55 0 15 to 3 45 0 19 ma porphyry copper gold and skarn deposits at black mountain and mexico on the western side of the district formed between 3 09 0 15 and 2 81 0 24 ma the santo tomas ii cu au pd porphyry deposit was emplaced into the central southern part of the district at about 1 5 ma based on secondary biotite 40ar 39ar ages of 1 48 0 05 ma and 1 47 0 05 ma on the eastern side of the district porphyry style mineralization at the hartwell and ampucao porphyry cu au prospects returned 40ar 39ar ages of 1 09 0 10 and 0 51 0 26 ma respectively quartz carbonate base metal sulfide style epithermal gold silver veins are well developed on the eastern side of the baguio district and crosscut porphyry style mineralization at the acupan baguio gold and nugget hill deposits epithermal veining is inferred to have occurred in the past million years based on a reported k ar age determination of 0 65 0 07 ma from the acupan gold mine a large advanced argillic alteration zone the baguio lithocap crops out in the northwest part of the district and formed between 1 4 and 0 9 ma based on previous k ar dating pliocene pleistocene mineralization in the baguio district was triggered by the east directed subduction of the scarborough ridge ridge subduction caused the subduction angle along the manila trench beneath northern luzon to decrease which facilitated crustal thickening and exhumation ridge subduction also promoted the development of northwest trending faults in the upper plate that interacted with major arc parallel north and arc normal northeast trending fault sets associated with the philippine fault system fertile magmas were emplaced into transtensional strike slip relay basins resulting in the accumulation of more than 35 moz of gold and several million tonnes of copper over approximately 3 m y

isotope geochemistry of the northeast zone mount polley alkalic cu au ag porphyry deposit british columbia a case for carbonate assimilation mount polley is a late triassic 205 ma alkalic porphyry cu au ag deposit 226 3 thousand tonnes t cu 21 5 t au and 65 1 t ag hosted by silica undersaturated to silica saturated monzonitic intrusions of the mount polley complex located in british columbia canada the northeast ore zone at mount polley is hosted by magmatic hydrothermal breccia copper and precious metals occur in sulfide minerals primarily as coarse to fine grained breccia cement local wall rocks include equigranular to porphyritic diorite monzodiorite and monzonite alteration breccia cement and veins of the northeast ore zone formed in five paragenetic stages prebreccia stage 1 brecciation and main stage mineralization stage 2 late stage mineralization stage 3 unmineralized postbreccia dikes and veins stage 4 and epithermal style veins stage 5 intense pervasive k and fe metasomatism calcite and calc silicate alteration occurred prior to brecciation caused by the intrusion of megacrystic k feldspar phyric monzonite stage 2 fluids were silica undersaturated high temperature 350 c co2 enriched and of near neutral to alkaline ph potassic sodic and calc potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4 evidence for more acidic and lower temperature conditions is preserved in stage 5 veins the 34ssulfide isotope compositions of stages 2 and 3 chalcopyrite pyrite and bornite range from 7 1 to 1 4 sulfur isotope compositions of anhydrite and gypsum are mostly between 6 2 and 9 8 these values together with the presence of hematite are consistent with deposition from an oxidized sulfate dominant high temperature magmatic hydrothermal fluid limited sulfur isotope geothermometry indicates that cu sulfides precipitated at temperatures from 480 to 250 c hydrothermal calcite occurs in all paragenetic stages at mount polley calcite 13c values range from 0 2 to 10 5 and 18o values from 4 0 to 20 9 the enriched c o isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid interaction of the fluid and or magma with limestone is considered a likely process to explain the c and o isotope signature lead isotope data suggest mixing of mantle and crustal sources during mineralization main stage chalcopyrite and pyrite as well as late stage galena have 206 204pb values of 18 77 to 18 92 207 204pb of 15 56 to 15 59 and 208 204pb of 38 22 to 38 32 strontium isotope data 0 70331 0 70371 provide evidence of a strongly depleted mantle source of sr with minor crustal input epsilon nd values for main stage apatite range between 5 9 and 6 5 also indicating a depleted mantle source stage 5 carbonate 206 204pb values of 18 96 to 19 04 207 204pb of 15 57 to 15 59 and 208 204pb of 38 26 to 38 36 suggest superposition of an epithermal system onto the northeast ore zone potentially as late as 100 m y after breccia formation the data presented are consistent with the hypothesis that the silica undersaturated alkalic mount polley complex formed due to carbonate assimilation prior to mineralization this process can explain the 13c 18o isotope data calcite precipitation concurrent with cu au mineralization and silica undersaturation of the magma the co2 released during assimilation of carbonate also could have promoted magmatic hydrothermal brecciation silica undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate bearing substrate

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

magmatic process recorded in plagioclase at the baogutu reduced porphyry cu deposit western junggar nw china abstract despite the fact that porphyry cu deposits contain large amounts of cu in one or more small stocks few studies have discussed evidence for significant magma recharge in porphyry cu deposits a systematic elemental and sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the baogutu reduced porphyry copper deposit western junggar nw china large compositional changes in an 12 24 mol are observed along with strong positive correlations between an and feo significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in an and feo and complex sr isotope variations three types of crystals with different core to rim sr isotope variations are recognized type i crystals have core to rim increases in 87sr 86sr i that could be explained by diffusion for example sr isotope variations recorded in bck2 1 2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100 500 years with the proximate value of 300 years type ii crystals with different variation trends in 87sr 86sr i on opposite sides of the core could be produced either by multi stage crystallization or by analytical uncertainty whereas type iii crystals with complex core to rim variations in 87sr 86sr i may record repeated magma recharge events all these results suggest repeated recharging of the magma chamber by hotter more mafic and less radiogenic sr isotope melts therefore more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber which ultimately results in cu mineralization at the baogutu reduced porphyry copper deposit

igneous geochemistry of mineralized rocks of the baguio district philippines implications for tectonic evolution and the genesis of porphyry style mineralization the baguio district of the philippines is one of the world s premier mineral provinces containing 35 million ounces moz of gold and 2 7 million metric tons mt of copper in epithermal porphyry and skarn deposits that formed in the last 3 5 m y pliocene and pleistocene magmatic rocks of the baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium k intrusions some of which have adakitic affinities and a suite of mafic to intermediate medium k to shoshonitic hornblende phyric dikes the geochemical and isotopic characteristics of the dikes are consistent with primitive mantle derived melts that underwent minimal crustal contamination as they ascended through the arc crust in contrast the intermediate to felsic suite has been contaminated by young arc crust suggesting ponding and fractionation within shallow crustal magma chambers the philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones eastward directed subduction of the scarborough ridge along the manila trench is currently associated with flattening of the downgoing slab the formation of the mafic dike complex is broadly coeval with the onset of subduction of the scarborough ridge and slab flattening the extinct scarborough ridge would have been younger than the downgoing plate and consequently more susceptible to melting these melts can account for the isotopic recharge of the pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the baguio district the interaction between primitive mafic melts and the more felsic calc alkaline rocks has

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

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

geo and cosmochemistry of the twin elements yttrium and holmium we have analyzed the y ho ratios in bulk chondrites chondrules and four ca and al rich inclusions cais from carbonaceous and unequilibrated ordinary and enstatite chondrites ec by laser ablation inductively coupled mass spectrometry la icpms we demonstrate that bulk rock sample preparation by containerless melting is a suitable method for preparation of bulk rock samples for high precision la icpms bulk chondrites have variable y ho ratios carbonaceous chondrites ci1 cm2 cv3 and ck4 have a common y ho ratio 25 94 0 08 2 that is regarded as the solar system y ho ratio the y ho ratio increases from carbonaceous through ordinary ll l h to enstatite chondrites el6 which show the highest y ho ratio of 27 25 we discuss the result with respect to the origin of fractionation of re and os between chondrite groups within analytical error y and ho show a good correlation in oc and cv3 chondrules and define an y ho ratio of 26 22 0 40 2 y ho fractionation in ca and al rich inclusions is related to differences in volatility the bulk silicate earth is suggested to have a solar y ho ratio and links the earth with carbonaceous chondrites y ho variations in primitive and differentiated terrestrial igneous rocks are discussed in framework of incompatibility of y and ho during partial melting applicability of y ho as tracer for or against a sedimentary origin of the putative host rock of the earth s oldest traces of life from the island of akilia is briefly discussed

in situ la mc icp ms trace element and nd isotopic compositions and genesis of polygenetic titanite from the baogutu reduced porphyry cu deposit western junggar nw china abstract titanite sphene catisio 5 is sensitive to changes in temperature oxygen and water fugacity and fluid composition in order to understand formation processes and the nature of hydrothermal fluids various types of titanite from cu ores at the baogutu reduced porphyry cu deposit were chosen for detailed study magmatic titanite is associated with biotite plagioclase and k feldspar whereas hydrothermal titanite occurs with k feldspar chlorite actinolite and calcite the formation of hydrothermal titanite was related to hydration of igneous minerals under high f h 2 o whereas the widespread replacement of ilmenite by titanite without magnetite indicates a relatively low oxygen fugacity magmatic titanite has low al high fe y sn zr nb and ree contents relative to hydrothermal titanite on the basis of the zr in titanite and al in chlorite geothermometers formation temperatures for magmatic and hydrothermal titanite are estimated to be 687 739 c and 250 670 c respectively the gradual decrease in ree y and sn contents from magmatic to late hydrothermal titanite was probably caused by precipitation of ree bearing minerals magmatic and hydrothermal titanites have similar chondrite normalized ree patterns with negative eu anomalies and relatively flat hree randomly selected titanites have nd isotopic compositions similar to the host rocks thus both magmatic and hydrothermal titanite are believed to have been predominantly derived from a mantle source