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

u pb zircon and geochemical constraints on the age origin and evolution of paleozoic arc magmas in the oyu tolgoi porphyry cu au district southern mongolia abstract uranium pb zircon ages are linked with geochemical data for porphyry intrusions associated with giant porphyry cu au systems at oyu tolgoi to place those rocks within the petrochemical framework of devonian and carboniferous rocks of southern mongolia in this part of the gurvansayhan terrane within the central asian orogenic belt the transition from devonian tholeiitic marine rocks to unconformably overlying carboniferous calc alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation radiogenic nd and pb isotopic compositions e nd t range from 3 1 to 7 5 and 206 pb 204 pb values for feldspars range from 17 97 to 18 72 as well as low high field strength element hfse contents of most rocks mafic rocks typically have 2 are consistent with magma derivation from depleted mantle in an intra oceanic volcanic arc the late devonian and carboniferous felsic rocks are dominantly medium to high k calc alkaline and characterized by a decrease in sr y ratios through time with the carboniferous rocks being more felsic than those of devonian age porphyry cu au related intrusions were emplaced in the late devonian during the transition from tholeiitic to calc alkaline arc magmatism uranium pb zircon geochronology indicates that the late devonian pre to syn mineral quartz monzodiorite intrusions associated with the porphyry cu au deposits are 372 ma whereas granodiorite intrusions that post date major shortening and are associated with less well developed porphyry cu au mineralization are 366 ma trace element geochemistry of zircons in the late devonian intrusions associated with the porphyry cu au systems contain distinct th u and yb gd ratios as well as hf and y concentrations that reflect mixing of magma of distinct compositions these characteristics are missing in the unmineralized carboniferous intrusions high sr y and evidence for magma mixing in syn to late mineral intrusions distinguish the late devonian rocks associated with giant cu au deposits from younger magmatic suites in the district

satellite based constraints on explosive so2 release from soufriere hills volcano montserrat 1 numerous episodes of explosive degassing have punctuated the 1995 2009 eruption of soufriere hills volcano shv montserrat often following major lava dome collapses we use ultraviolet uv and infrared ir satellite measurements to quantify sulfur dioxide so2 released by explosive degassing which is not captured by routine ground based and airborne gas monitoring we find a total explosive so2 release of 0 5 tg which represents 6 of total so2 emissions from shv since july 1995 the majority of this so2 0 4 tg was vented following the most voluminous shv dome collapses in july 2003 and may 2006 based on our analysis we suggest that the so2 burden measured following explosive disruption of lava domes depends on several factors including the instantaneous lava effusion rate dome height above the conduit and the vertical component of directed explosions space based so2 measurements merit inclusion in routine gas monitoring at shv and other dome forming volcanoes

zircon u pb and hf o isotopes trace the architecture of polymetallic deposits a case study of the jurassic ore forming porphyries in the qin hang metallogenic belt china abstract the qin hang intra continental porphyry skarn cu polymetallic belt qhmb is among the economically important metallogenic belts in south china the significant differences in the size and metal assemblage of the jurassic magmatic hydrothermal ore deposits in this belt remain as an enigma here we employ zircon u pb and hf o isotopes of the tongshanling and baoshan cu pb zn deposits in the central part of the qhmb to investigate the contrasting metallogenic architecture our sims zircon u pb data indicate that the tongshanling and baoshan granodiorite formed at 160 ma these rocks show high mg values and negative zircon e hf t and high 18 o values suggesting that the magmas of the granodiorite porphyries were mainly generated through the anatexis of older crustal components triggered by the input of mantle derived magma the minor content of amphibole phenocrysts low sr y ratios negative eu anomaly and low zircon ce 4 ce 3 ratios indicate that the porphyries are relatively less oxidized with less water content compared with the ore bearing porphyries in the dexing and yuanzhuding porphyry cu deposits in the northern and southern part of the qhmb suggesting that high magmatic water content and oxidation state are important prerequisites for the formation of large size porphyry skarn copper deposits in the qhmb the positive correlation between zircon e hf t values with the cu reserves as well as zircon 18 o values with the cu cu pb zn ratios of the deposits indicate that the magmatic sources exerted a first order control on the volume and metal assemblage of deposits in the qhmb the hf and nd isotope contour maps indicate that the central part of the qhmb has high potential for pb zn dominated magmatic hydrothermal deposits whereas the northern and southern part of the qhmb are prospective for large cu deposits our results have important implications in formulating regional exploration strategies for jurassic porphyry skarn cu pb zn deposits in the qin hang belt

origin of the xiaohekou skarn copper deposit and related granitoids in the zha shan ore cluster area south qinling china abstract the xiaohekou copper deposit is one of the few typical skarn type commercial deposits in the zha shan ore cluster area in the eastern part of south qinling the ore related granitoids in the xiaohekou mining district include a number of small plutons of granodiorite porphyry and granite porphyry as well as numerous dykes they are characterized by relatively low sio2 65 9 73 3 wt and mgo 0 26 1 72 wt contents variable mg 29 4 64 0 values high k2o 3 11 5 75 wt content and negligible eu 1 00 1 15 values the samples also exhibit depletion in cr ni nb ta and ti content and high sr y 46 6 64 7 and la yb 17 0 25 0 ratios these features are similar to those of high k calc alkaline adakite like magma formed by partial melting of thickened lower crust in continental collision zones or intracontinental settings the majority of the samples have tdm2 hf of 1 5 1 2 ga corresponding to weakly negative ehf t values 4 75 to 0 13 typical of crustal material but more than one quarter have tdm1 hf of 0 80 0 71 ga corresponding to positive ehf t values of 0 07 to 2 45 indicating that the granitoids resulted from partial melting of mesoproterozoic lower crustal rocks with significant input from enriched lithospheric mantle derived magmas zircons from the xiaohekou granitoids have fmq values concentrated around 0 62 to 5 54 most of which show the magma fo2 of fmq to hm in accordance with the widespread occurrence of magnetite hematite and specularite in the xiaohekou skarn system which is an indicator of relatively high fo2 for a magmatic hydrothermal system the granitoids exhibit similar or even higher ce4 ce3 average of 452 and eu eu average of 0 72 values in comparison with those of fertile cu deposits worldwide indicating that the xiaohekou granitoids may possess the potential to form a large magmatic hydrothermal cu deposit zircons of the granitoids show obvious positive correlations between oxidation indices ce and eu anomalies ce4 ce3 ratios and fmq values and ehf t values implying the involvement of an oxidized mantle component which might have caused the fo2 elevation and played an important role in the formation of cu deposits the zircon u pb ages 141 3 1 3 and 138 1 2 0 ma of the xiaohekou ore related granitoids obtained in this study are similar to the zircon u pb and molybdenite re os ages yielded from porphyry mo mineralization at the chigou and lengshuigou deposits in the zha shan ore district indicating that they are possibly part of a single tectonic magmatic and metallogenic event in the yanshan orogeny the ore related granitoids in the xiaohekou area were formed in a post collisional compression extension transition regime during the early cretaceous qinling intracontinental orogeny the orogeny can be attributed to the far field effect of a switch from pre mesozoic tethyan tectonics to late mesozoic pacific tectonics this triggered a stress field transition from n s to e w trending and from intracontinental subduction thickening to large scale lithospheric delamination and thinning

proterozoic reactivation of the southern superior province and its role in the evolution of the midcontinent rift major reverse faults associated with the late compressional phase of the 1 1 ga midcontinent rift in the western lake superior region appear to cut across the rift at the eastern end of the lake and join with reverse faults on the eastern shoreline defined on the basis of geological and potential field data the continuation of the faults across eastern lake superior is inferred on evidence drawn from nearshore shipborne magnetic surveys together with new interpretations of published bathymetric and glimpce aeromagnetic data in the archean superior province about 100 km east of lake superior paleomagnetic and petrographic data from the 2 45 ga matachewan dyke swarm show that the kapuskasing zone a narrow belt of uplifted crust can be extended to within 50 km of the lake superior shoreline and has bounding reverse faults that are almost continuous with two faults of similar dip and sense of displacement that define the inversion of the midcontinent rift in the central and western parts of the lake si

post rift deformation of the midcontinent rift under grenville tectonism abstract using a thin viscous sheet model we investigate the hypothesis that compressive stresses resulting from the grenville orogeny caused uplift along the midcontinent rift the objective of this study is to investigate factors involved in shaping the post rift structure of the midcontinent rift including the size and location of the colliding terranes and the convergence angle between the colliding terranes and the north american craton we find a linear relationship between the size of the terrane and the deformation within the midcontinent rift the calculations predict comparable deformation along both the eastern and western arms of the midcontinent rift our results demonstrate that the geologically indicated ne sw deformation can be produced without a multistage compression event because of the nonlinear interaction of far field compressive stress and the mechanically weak rift therefore it is not possible to determine which compression direction is representative of the compression associated with the grenvillian orogenic event based on the post rift pattern of deformation

zircon u pb and lu hf systematics of the major terranes of the western superior craton canada mantle crust interaction and mechanism s of craton formation abstract paleo to neoarchean granitoid gneisses ca 3 30 to 2 49 ga are well preserved in the western superior craton protoliths of these gneisses are mainly i type granitoids characterized by high sr y and la yb ratios and low mg consistent with archean tonalite trondhjemite granodiorites zircons from granitoid gneisses commonly contain three growth phases inherited cores zircon i magmatic rims zircon ii and outer rims that have undergone pb loss zircon iii the 3 12 ga to 2 86 ga zircon i represent early crustal material that was captured in younger zircons zircon ii preserve crustal re working and younger crustal additions that are constrained between 2 85 to 2 72 and 2 69 to 2 65 ga zircon ii contains both positive and negative ehf t values 6 3 to 8 1 with both depleted mantle and older crustal signatures half of the magmatic rims ii are characterized by depleted mantle signatures with positive ehf t values representing juvenile crust forming events whereas the other half are characterized by recycled crustal signatures with negative ehf t values ehf t results show that the north caribou and the island lake terranes and the northern uchi domain are isotopically more enriched than the southern uchi english river wabigoon and winnipeg river terranes suggesting the northern uchi margin represents a major terrane boundary based on mass balance calculations large volumes of juvenile material at circa 3 0 ga mixed with smaller amounts of older crust the vast majority of the granites were derived from a source with about 50 mantle material during the peak crust formation events after 2 8 ga the decline in the volume of felsic magmatism in the later archean is coeval with a reduced supply of both heat and material from depleted mantle sources combined with previously published geochemical geochronological and isotopic data this suggests an evolution in felsic magma sources consistent with crustal thickening

rapid transition to long lived deep crustal magmatic maturation and the formation of giant porphyry related mineralization yanacocha peru abstract the yanacocha magmatic field northern peru hosts the largest high sulfidation gold deposit on earth mineralization is associated with porphyritic intrusions distributed along a ne trending magmatic structural corridor eight of these intrusions investigated in this study range in age from 12 4 to 8 4 ma and show systematic chemical and isotopic changes through time they are interpreted to derive from hydrous mafic magmas evolving through amphibole clinopyroxene garnet fractionation and lower crust melting leaving a garnet residue at deeper levels which led to variably strong adakite like signatures and through plagioclase amphibole fractionation at shallower levels both accompanied by crustal assimilation and recharge recharge assimilation fractional crystallization rafc processes systematic geochemical and isotopic changes with intrusion ages coupled with plagioclase zoning and amphibole geobarometry suggest that the evolution of the magmatic system occurred through interaction of mantle derived melts with an increasing length of the crustal column and propagation from deep towards shallower crustal levels through time this was probably the result of a steadily increasing compression that has progressively slowed down magma ascent forcing magmas to evolve at a series of intermediate level chambers between the lower and upper crust increased compression might have been related to the onset of subduction of the buoyant inca oceanic plateau estimated to occur at 12 ma i e the same time of the onset of the rapid transition from normal to adakite like signatures the giant yanacocha ore system developed in coincidence with the 3 6 4 0 ma long intrusion of the adakite like magmas 12 4 12 0 8 4 ma formed by the above processes into a small upper crustal volume and peaked during the last 2 4 ma 10 8 8 4 ma of magmatic activity after a 1 4 ma long 12 4 11 0 ma maturation of magmas at deep crustal levels further investigation is needed to understand whether the association of adakite like magmas with ore which is typical of other giant porphyry systems is the result of the build up of incompatible volatiles and metals in oxidized magmas that evolve under high pressure conditions of recycling of lower crustal sulfide rich cumulates and or of a long lived focused transfer of magmas from deep to shallow crustal levels

an experimental study of trace element partitioning between zircon and melt as a function of oxygen fugacity abstract the partitioning of p sc ti sr y nb the rare earth elements ree hf ta th and u between zircon and a synthetic andesitic melt was determined as a function of oxygen fugacity f o 2 over a range of fourteen log units from qfm 8 to qfm 6 where qfm is the quartz fayalite magnetite oxygen buffer at 1300 c and 1 atm using sims and la icp ms the partition coefficients of ce and eu were found to vary systematically with f o 2 relative to those of the other ree producing ce and eu anomalies similar to those that are characteristic of natural igneous zircon these anomalies coexist at terrestrial f o 2 s and were used to quantify eu 3 eu 2 eu 3 and ce 4 ce 3 ce 4 in the melt the partition coefficients of the redox invariant trivalent cations are in excellent agreement with lattice strain theory the values of d ree are related to those determined for natural samples by the expected dependence on temperature the incorporation of ree 3 in zircon was independent of the presence of p d u d th was found to vary systematically with f o 2 indicating the presence of u 4 u 5 and u 6 in the melt at terrestrial conditions d ti was independent of f o 2 despite an expectation of significant ti 3 in the most reduced experiments

relative oxidation states of magmas inferred from ce iv ce iii in zircon application to porphyry copper deposits of northern chile major and trace element compositions of zircons and whole rocks from 14 barren and seven ore bearing calc alkaline intrusions from the chuquicamata el abra porphyry copper belt of northern chile have been measured in situ by excimer laser ablation ela icp ms these data permit the ce iv ce iii ratio within zircon to be calculated using a lattice strain model for mineral melt partitioning of ce iv and ce iii zircon ce iv ce iii and eun eun ratios and by inference magmatic oxidation states generally increase from older mafic to younger felsic units within this sequence porphyry copper mineralization is directly associated only with intrusions with zircon ce iv ce iii 300 and eun eun 0 4 such trends can be understood in terms of interdependent relations between oxygen fugacity sulfur speciation and solubility and chalcophile element partitioning in silicate magmas because zircon occurs in most calc alkaline intrusions and is resistant to subsolidus alteration zircon ce iv ce iii ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic hydrothermal cu au mineralization the approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks

1300 m high rising bubbles from mud volcanoes at 2080m in the black sea hydroacoustic characteristics and temporal variability a mud volcano area in the deep waters 2000 m of the black sea was studied by hydroacoustic measurements during several cruises between january 2002 and june 2004 gas bubbles in the water column give strong backscatter signals and thus can be detected even in great water depths by echosounders as the 38 khz ek500 scientific split beam system that was used during the surveys because of their shape in echograms and to differentiate against geochemical plumes and real upwelling bubble water plumes we call these hydroacoustic manifestations of bubbles in the water column flares digital recording and processing of the data allows a 3d visualization and data comparison over the entire observation period without artefacts caused by changing system settings during our surveys we discovered bubble release from three separate mud volcanoes dvurechenskiy dmv vodianitskiy vmv and the nameless seep site nss in about 2080 m water depth simultaneously bubble release was observed between 9 june 2003 and 5 june 2004 the most frequently surveyed dmv was found to be inactive during very intensive studies in january 2002 the first activity was observed on 27 june 2002 which finally ceased between 5 and 15 june 2004 after a period of continuously decreasing activity this observed 2 yr bubble release period at a mud volcano may give an indication for the duration of active periods the absence of short term variations within days or hours may indicate that the bubble release from the observed mud volcanoes does not undergo rapid changes the recorded echograms show that bubbles rise about 1300 m high through the water column to a final water depth of about 770 m which is 75 m below the phase boundary of pure methane hydrate in the black sea with a release depth from 2068 m and a detected rise height of 1300 m the flare at vmv is among the deepest and highest reported so far and gives evidence of highly extended bubble life times up to 108 min in deep marine environments to better understand how a methane bubble gas analyses of the pore water and gas hydrate gave 99 4 methane can rise so high without dissolving we applied a recently developed bubble dissolution model that takes into account a decreased mass transfer due to an immediately formed gas hydrate rim using the hydroacoustically determined bubble rising speeds 19 22 cm s at the bottom 12 14 cm s at the flare top and the relation between the rising speed of dirty gas hydrate rimmed bubbles and the bubble size we could validate that a gas hydrate rimmed bubble with a diameter of 9 mm could survive the 1300 m rise through the water column before it is finally dissolved a diameter of about 9 mm is reasonable for bubbles released at seep sites and the coincidence between the observed bubble rising speed and the model approach of a 9 mm bubble supports the assumption of gas hydrate rimmed bubbles

natural marine seepage blowout contribution to atmospheric methane the release of methane sequestered within deep sea methane hydrates is postulated as a mechanism for abrupt climate change however whether emitted seabed methane reaches the atmosphere is debatable we observed methane emissions for a blowout from a shallow 22 m hydrocarbon seep the emission from the blowout was determined from atmospheric plume measurements simulations suggest a 1 1 gas loss to dissolution compared to 10 loss for a typical low flux bubble plume transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge this mechanism could allow methane suddenly released from deeper 250 m waters to contribute significantly to atmospheric methane budgets copyright 2006 by the american geophysical union

shrimp u pb and 40ar 39ar age constraints for relating plutonism and mineralization in the boulder batholith region montana the composite boulder batholith montana hosts a variety of mineral deposit types including important silver rich polymetallic quartz vein districts in the northern part of the batholith and the giant butte porphyry copper molybdenum pre main stage system and crosscutting copper rich main stage vein system in the southern part of the batholith previous dating studies have identified ambiguous relationships among igneous and mineralizing events mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at butte have all been considered to be late stage differentiates of the boulder batholith however previous dating studies indicated that the boulder batholith plutons cooled from about 78 to 72 ma whereas copper rich main stage veins at butte were dated at about 61 ma recent efforts to date the porphyry copper molybdenum pre main stage deposits at butte resulted in conflicting estimates of both 64 and 76 ma for the mineralizing events silver rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously to resolve this controversy we used the u s geological survey stanford shrimp rg ion microprobe to date single age domains within zircons from plutonic rock samples and 40 ar 39 ar geochronology to date white mica biotite and k feldspar from mineral deposits u pb zircon ages are rader creek granodiorite 80 4 1 2 ma unionville granodiorite 78 2 0 8 ma pulpit rock granite 76 5 0 8 ma butte granite 74 5 0 9 ma altered steward type quartz porphyry dike i 15 roadcut 66 5 1 0 ma altered steward type quartz porphyry dike continental pit 65 7 0 9 ma and quartz monzodiorite of boulder baldy big belt mountains 66 2 0 9 ma zircons from rader creek granodiorite and quartz porphyry dike samples contain archean inheritance the 40 ar 39 ar ages are muscovite silver rich polymetallic quartz vein basin district 74 4 0 3 ma muscovite silver rich polymetallic quartz vein boulder district 74 4 1 2 ma muscovite early dark micaceous vein continental pit 63 6 0 2 ma biotite early dark micaceous vein continental pit 63 6 0 2 ma potassium feldspar early dark micaceous vein continental pit 63 to 59 ma and biotite biotite breccia dike continental pit 63 6 0 2 ma outlying silver rich polymetallic quartz veins of the basin and boulder mining districts probably are directly related to the 74 5 ma butte granite whereas steward type east west quartz porphyry dikes and butte pre main stage deposits are parts of a 66 to 64 ma magmatic mineralization system unrelated to emplacement of the boulder batholith the age of the crosscutting main stage veins may be about 61 ma as originally reported but can only be bracketed as younger than the 64 ma pre main stage mineralization and older than the about 50 ma eocene lowland creek intrusions the 66 ma age for the quartz monzodiorite of boulder baldy and consideration of previous dating studies in the region indicate that small ca 66 ma plutonic systems may be common in the boulder batholith region and especially to the east the approximately 64 ma porphyry copper systems at butte and gold mineralization at miller mountain are indicative of regionally important mineralizing systems of this age in the boulder batholith region resolution of the age and probable magmatic source of the butte pre main stage porphyry copper molybdenum system and of the silver rich polymetallic quartz vein systems in the northern part of the boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events

magma evolution leading to porphyry au cu mineralization at the ok tedi deposit papua new guinea trace element geochemistry and high precision geochronology of igneous zircon

comment on paleozoic reactivation structures in the appalachian ouachita marathon foreland far field deformation across pangea by j craddock d malone r porter j compton j luczaj a konstantinou j day and s johnston earth science reviews 169 2017 1 34

high sr y magmas reflect arc maturity high magmatic water content and porphyry cu mo au potential examples from the tethyan arcs of central and eastern iran and western pakistan the temporal and geochemical evolution of arc magmatism that culminated in porphyry cu mo au deposit formation has been studied in three separate neo tethyan arc systems in central and eastern iran and western pakistan porphyry cu au deposits in the lut block of eastern iran formed in the middle eocene at the end of a period of extensive paleocene eocene volcanism porphyry cu mo deposits in the kerman belt of central iran formed in the middle miocene at the end of a period of voluminous eocene oligocene volcanism and porphyry cu au deposits in the chagai belt of western pakistan formed in four pulses during the eocene early miocene middle late miocene and late miocene pliocene after a prolonged period of arc magmatism that began in the late cretaceous and is still active in each region the late porphyry related magmas are more geochemically evolved and more hydrous as evidenced by the presence of hornblende phenocrysts than the preceding volcanic rocks we suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years leading to the generation of more evolved volatile rich magmas at later stages of the arc s history high magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore forming subvolcanic magmatic hydrothermal systems it is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites later suites are more prospective noting the common presence of hornblende or biotite phenocrysts indicating high magmatic water contents and through lithogeochemical fingerprinting of magmatic fractionation processes relatively high sr y and la yb ratios and eun eu ratios 1 indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas

sulfur partition coefficient between apatite and rhyolite the role of bulk s content experimental studies have been performed to constrain sulfur behavior during apatite crystallization and to determine sulfur partition coefficient between apatite and melt kd s apatite melt at oxidizing conditions crystallization experiments have been conducted with a hydrous rhyolitic melt and different bulk sulfur contents 0 15 to 2 wt s at 900 and 1 000 c 200 mpa and log f rm o 2 nno 3 6 the sulfur content in the glass increases with increasing amount of added s anhydrite crystallizes for s added 0 75 wt 0 10 and 0 13 wt so3 in glass at 900 and 1 000 c respectively the amount of anhydrite increases and the amount of apatite decreases with increasing amount of added sulfur the sulfur exchange reaction in apatite is influenced by the bulk composition of the melt e g p content however changing melt composition has only little effect on kd s apatite melt for the investigated rhyolitic composition the kd s apatite melt does not depend directly on temperature decreases from 14 2 to 2 7 with increasing s content in glass from so3 0 03 to 0 19 wt respectively and can be predicted from the following equation ln kd 0 0025 s in melt in ppm 2 9178 the combination of experimental data obtained for rhyolitic and andesitic melts reveals that the sulfur partition coefficient tends toward a value of 2 for high sulfur content in the glass 0 2 wt so3 using s in apatite as proxy for determining s content in melt is promising but additional experimental data are needed to clarify the individual effects of t f rm s 2 and p and ca content in the melt on s partitioning

redox controlled generation of the giant porphyry cu au deposit at pulang southwest china some porphyry cu au deposits with relatively reduced ore assemblages characterized by high hydrothermal pyrrhotite contents and a lack of primary hematite and magnetite are generally considered to be associated with reduced i type granitoids however the role of magmatic oxygen fugacity fo2 in controlling cu au mineralization in such reduced porphyry deposits is poorly understood the giant late triassic ca 216 ma pulang porphyry cu au deposit of southwest china shows typical reduced ore assemblages this study reported the systematical variation of upper crustal magmatic fo2 of pulang deposit based on detailed investigations of mineral crystallization sequences and compositional features of the mineralization related porphyries early p1 and late p2 porphyry results indicate that magma of the mineralization related porphyries experienced complex fo2 fluctuations during its upper crustal evolution the early primary magma had very high initial fo2 with fmq 3 0 at depths of 12 km fmq is the deviation of logfo2 from the fayalite magnetite quartz fmq buffer the fo2 of evolved parental magma subsequently decreased with fmq 1 9 due to injection of relatively reduced dioritic magmas fmq 1 4 to 2 3 from a deeper chamber 17 21 km depth into the primary magma chamber at 10 12 km depth magma mixing had largely ceased at 6 10 km depth the parental magma then ponded within the reduced tumugou formation at a depth of 3 7 km where magmatic fo2 decreased to a moderately oxidized state fmq 1 6 and finally to a moderately reduced state reflected by log fe2o3 feo ratios of 0 5 for p2 porphyry results of this study of magmatic fo2 indicate that porphyry magmas associated with reduced pulang ore assemblages were initially generated as highly oxidized magma which was subsequently reduced through magma mixing and contamination by reduced sedimentary rocks of the tumugou formation the sharp fo2 decrease at very shallow depth prevented the early loss of cu and au because the magma remained oxidized until it was emplaced at 3 7 km depth moderately reduced magmas may thus have a genetic association with porphyry cu au mineralization

anhydrite bearing andesite and dacite as a source for sulfur in magmatic hydrothermal mineral deposits magmaticanhydritefromandesitesanddacitesoccursasinclusionsinhigh andlow aluminum amphibole and pyroxene and indicates that sulfate saturated magmas spanned a period of six million years at yanacocha peru magmatic anhydrite from yanacocha and other sites is characterized by light rare earth element enriched patterns and elevated strontium contents distinct from magmatic hydrothermal anhydrite petrologic arguments suggest that the hydrous and oxidized yanacocha magmas contained more than 1000 ppm sulfur both dissolved in the melt and as a separate sulfate phase which is sufficient to provide all the sulfur for the genetically related giant sulfur rich yanacocha epithermal gold deposits high aluminum amphiboles contain unusual anhydrite with wormy and amoeboidal textures which are tentatively interpreted to represent trapping of an immiscible caso4 water melt together with sulfur rich apatite at a temperature of 950 c and a water pressure 3 kbar such unusually sulfate rich magmas may be required to produce sulfur rich magmatic hydrothermal mineral deposits

double plume integral models for near field mixing in multiphase plumes we present a generalized integral model for multiphase plumes in stratified ambient conditions based on the double plume approach where the plume is composed of a rising multiphase core plume surrounded by a counterflowing outer ring plume of dense fluid the generalized model captures as limiting cases the current approaches in the literature including two fluid and mixed fluid equations continuous and discrete detrainment dispersed phase mass transfer and two models for entrainment in the counterflow region these modeling approaches are compared and validated against both laboratory and field scale data in unstratified conditions all model formulations perform equally well in stratification entrainment in the counterflow region is best represented by correlation to the inner plume velocity instead of the difference between the inner and outer plume velocities the vertical distribution of the exchange between the inner and outer plumes in the models differs from that measured in the prototype due to enhanced entrainment at the detrainment zone and forced entrainment from the collapsing intrusion layer nonetheless the models predict well the length scales and volume fluxes at the detrainment zone and intrusion layer applications are demonstrated for reservoir air bubble plumes the mass transfer and near field mixing in the double plume integral model prove sufficiently accurate to predict the depth of maximum plume rise both the locations of total dissolution of the bubbles and the maximum height of the decelerating plume and the volume flux dissolved constituent mass flux and trap height of the intrusion

fate of rising methane bubbles in stratified waters how much methane reaches the atmosphere there is growing concern about the transfer of methane originating from water bodies to the atmosphere methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport this work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere the bubble model predicts the evolving bubble size gas composition and rise distance and is suitable for almost all aquatic environments the model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep oxic saline water with excellent results methane bubbles from within the hydrate stability zone typically below 500 m water depth in the ocean are believed to form an outer hydrate rim to explain the subsequent slow dissolution a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone the calibrated model explains the impressively tall flares 1300 m observed in the hydrate stability zone of the black sea this study suggests that only a small amount of methane reaches the surface at active seep sites in the black sea and this only from very shallow water areas 100 m clearly the black sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere although substantial amounts of methane may reach the surface in shallow lakes and reservoirs

the evolution of aqueous vapor from silicate melts effect on oxygen fugacity abstract the simplest possible aqueous vapor evolved from an iron bearing magma under normal conditions is an h 2 o h 2 mixture hydrogen is produced by the oxidation of fe ii to fe iii by water in the magma yielding an increase in f o 2 equations are derived which model this mass transfer process the resulting differential equation expresses the change in magmatic f o 2 as a function of a change in n 1 h 2 o the vapor evolution progress variable the calculations demonstrate that in this simple scheme very iron poor magmas h 2 o feo are required to produce significant increases in magmatic f o 2 the incremental increase in o 2 during vapor evolution becomes progressively smaller as o 2 increases with this process becoming highly inefficient as the hematite stability field is approached other more complete processes involving meteoric water assimilation of altered rocks and subsolidus processes may produce the observed increase in f o 2 inferred to have occurred in systems of normal iron contents in low iron systems such as the high silica rhyolitic magmas associated with climax type porphyry molybdenum systems high oxygen fugacities may be obtained by this mechanism

amphibole geochemistry of the yanacocha volcanics peru evidence for diverse sources of magmatic volatiles related to gold ores the yanacocha mining district is located in the andes of northern peru in an area of relatively thick continental crust 35 km and long lived cenozoic subduction related volcanism volcanic activity in the district began at 20 ma and gold deposits total resource of 1500 tonnes of gold are spatially and temporally associated with eruption of the 80 km 3 miocene yanacocha volcanics from 14 5t o 8 4 ma the yanacocha volcanics consist of five successive eruptive groups the atazaico andesite lavas the colorado pyroclastics andesite dacite the azufre andesite and dacite lavas the san jose ignimbrite and related domes dacite and small volumes of coriwachay dacite dikes domes and rhyolite ignimbrite most dacite magmas probably did not erupt but rather are inferred to have episodically crystallized to granite at depth to produce ore fluids two distinct populations of amphiboles distinguished by their aluminum content are found in the dacites on the basis of phase equilibrium the low aluminum low al amphiboles were formed at 750 8408c and 110 240 mpa whereas the high aluminum high al amphiboles are estimated to have formed at 900 9508 ca ndph2o4250 mpa the trace element contents of amphibole and whole rocks are consistent with crystallization of the high al amphibole at near liquidus temperatures from a basaltic andesite to andesite magma whereas the low al amphibole crystallized at lower temperatures in equilibrium with a rhyolitic melt derived from a crystal rich dacite magma hydrogen isotopic compositions of both high and low al amphibole exhibit a large range from 40 to 120o for the 12 5 11 0m a andesite dacite and have a restricted range from 100 to 112o for the younger coriwachay dacite 10 8 8 4 ma the high dd values of some high al amphiboles 41o probably represent subduction derived water dissolved in a water and fluorine rich chlorine poor and sulfate saturated basaltic andesite magma this magma was injected into an upper crustal chlorine rich silicic magma chamber characterized by low dd of low al amphibole 60o or lower short residence times 5 1 year of high al amphibole in the upper crustal chamber are estimated from dehydration rims and hydrogen diffusion lengths following the eruption of the lower san jose ignimbrite at 11 5 ma a new shallow dacite magma chamber was established and minor amounts of mafic magma input continued as shown by the high al amphiboles present in the san jose domes and in the middle and upper sanjose ignimbrite these high al amphiboles ddamph 81 to 102o had sufficiently long residence time 4 1 year in the shallow chambers prior to eruption to equilibrate isotopically with the predominant low dd dacite the young coriwachay dacite magmas probably assimilated meteoric hydrothermally altered low dd rocks to generate the low d do f the low al amphibole 100 to 120o these dacites are related to the main gold ore stages and contain low al amphibole that is zinc and chlorine rich but copper and fluorine poor compared with the associated high al amphibole these results imply that deep mafic magmas may have supplied much of the sulfur fluorine copper and by inference gold whereas upper crustal recycling may have supplied a significant proportion of the water and chlorine to the late dacite magmas and ore fluids

post keweenawan compressional faults in the eastern lake superior region and their tectonic significance glimpce aeromagnetic data in eastern lake superior are characterized by a series of strong easterly and northeasterly oriented gradients that relate to mapped post keweenawan faults occurring along the eastern shore the reversed nature of three of the faults is established through field observations and potential field modelling middle keweenawan volcanic rocks at mamainse point are in fault contact on their south side with upper keweenawan sandstone of bayfield jacobsville type gravity modelling suggests that the fault is a low angle thrust dipping to the north field observations and high resolution aeromagnetic data show that it extends inland along the southern margin of the batchawana greenstone belt for at least 17 km to the west the mamainse point fault may extend across eastern lake superior to the keweenaw peninsula linking several offsets in the seismic data that are consistent with the same attitude and sense of displacement along the south side of batchawana bay at havilland sandstones of bayfield jacobsville type are isoclinally folded against a package of upthrust older rocks that include drag folded middle keweenawan volcanics at grindstone point north of cape gargantua a reverse fault separating isoclinally folded upper keweenawan sandstones from archean basement may on aeromagnetic evidence be an eastward extension of the michipicoten island fault these faults mark a significant change in the style of late compressional tectonism observed within the midcontinent rift all cut keweenawan rocks across strike the inference is that broad north south or northwest southeast compression consistent in timing and orientation with the grenville orogeny led to a reversal of movement along the major graben faults in western lake superior and was taken up in the eastern region by reverse faults oriented normal to the extensional axis of the rift

composition and activity of sulfurous species in quenched magmatic gases associated with pyrrhotite bearing silicic systems study of pyrrhotite bearing volcanic units such as the el chichon st helens and fish canyon tuffs demonstrates that the activity of sulfurous gases in calc alkaline systems is often quite high oxidizing systems have especially high activities of so 2 in early volatile phases upon cooling oxidized compositions react to form sulfate rich solutions through the reaction 4so 2 4h 2 o 3 h 2 so 4 h 2 s whereas reduced compositions form hydrogen sulfide rich solutions by the reaction so 2 3h 2 h 2 s 2h 2 o in both cases the quenched magmatic component is high in total sulfur content and posseses high oxygen and sulfur fugacities these conditions fall near the sulfide sulfate dominant boundary which defines a maximum in sulfur fugacity for solutions in the oxygen sulfur system such solutions fall within the stability fields of native sulfur pyrite pyrite bornite iron free sphalerite galena alunite and other highly oxidized and sulfidized minerals upon mixing with lower sulfur content meteoric waters these solutions will trace a path through f sub o 2 f sub s 2 space which duplicates much of the mineral zoning seen with depth in porphyry systems in addition this magmatic component can account for much of the chemical and mineral data from fumaroles associated with recent calderas the high sulfate content either inherited directly from the reaction of magmatic gases or derived from the oxidation of magmatic h 2 s by meteoric waters may be an important component for the transport of certain ore forming elements in addition the changing conditions during mixing of these sulfur rich magmatic waters with meteoric systems may cause deposition of ore minerals the conditions derived from these calculations duplicate those determined from natural magmatic dominated systems such as summitville colorado

mesoproterozoic porphyry copper mineralization at mamainse point ontario canada in the context of midcontinent rift metallogeny abstract the jogran copper prospect in southern ontario canada is located on the northeastern shoulder of the north american midcontinent rift system where it is centered on two small closely spaced quartz monzonite porphyry intrusions emplaced into neoarchean mafic metavolcanic rocks notwithstanding an atypical alteration zoning pattern the distinctive alteration styles and veinlet types confirm its porphyry copper affiliation two samples of molybdenite from chalcopyrite bearing quartz veinlets in the two centers returned re os ages of 1101 5 and 1094 5 ma showing that the mineralization took place during the main stage of extension and magmatic activity that gave rise to the midcontinent rift and associated large igneous province lip the nearby copper bearing tribag breccia pipes and coppercorp veins have been shown previously to also be intrusion related and likely formed penecontemporaneously with jogran the jogran and nearby copper mineralization overlapped temporally with generation of low grade magmatic copper nickel pge deposits in the duluth intrusive complex but predated the strata bound native copper and chalcocite deposits in the keweenaw peninsula which are believed to have formed from basinal fluids during rift inversion although such magmatic and strata bound copper deposits typify intracontinental rift metallogeny porphyry copper deposits are normally confined to convergent margin magmatic arcs thereby making the rift lip setting of jogran exceptional and possibly unique

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

sulfur evolution of the 1991 pinatubo magmas based on apatite abstract using electron microprobe emp and laser ablation inductively coupled plasma mass spectrometry la icp ms to collect major and rare earth elements ree respectively from apatites from the 1991 mt pinatubo juvenile eruption products we have determined that two statistically distinct populations of apatite exist one population crystallized from the juvenile basaltic melt basalt apatites and the other population crystallized from the main dacitic magma body silicic apatites both populations contain high s apatites 0 7 wt so3 apatite has previously been shown to be a potential monitor for magmatic sulfur contents via numerous proposed coupled substitutions of p5 for s6 however simple apatite melt partitioning cannot account for high s silicic apatites which grew from a silicic melt with an apparent maximum s concentration of 80 ppm disparate apatite morphology i e skeletal and acicular for basalt apatites and euhedral for silicic apatites as well as compositional evidence reveal that high s silicic apatites were not inherited from the juvenile basalt during mingling mixing prior to eruption sulfur gain from neighboring anhydrite phenocrysts can also be ruled out as a source of high sulfur emp sulfur mapping of silicic apatites shows highly irregular patterns of sulfur enrichment that do not correspond with adjacent anhydrite and can be found within apatites hosted by other minerals e g hornblende and fe ti oxides with these data in mind we propose high s silicic apatites from pinatubo and other sulfur rich systems achieved elevated sulfur concentrations during high sulfur fluxing events that originated from underplated basalt during degassing of a so2 rich fluid phase that basalts were indeed sulfur rich and oxidized is here indicated by high s contents of apatites growing in basalt the predominant location of s rich areas of silicic apatite is crystal interiors of apatite inclusions in other mineral phases while large apatite microphenocrysts surrounded by groundmass glass have low sulfur contents this is evidence for placing sulfur fluxing periods long before events that ultimately led to the 1991 eruption

oxygen isotope and trace element geochemistry of zircons from porphyry copper system implications for late triassic metallogenesis within the yidun terrane southeastern tibetan plateau abstract it is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore bearing ones from barren ones especially by examining the oxygen isotopic and trace element compositions of zircon the oxygen isotopic composition and trace element abundances of zircons from the mineralized pulang porphyries and the coeval disuga and lannitang andesites of the yidun terrane eastern tibetan plateau have been determined by secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icp ms in this study respectively analyzed zircons have relatively high but invariant 18 o values 5 8 6 9 that are indicative of derivation from homogeneous mantle or mantle derived magmas together with contribution of mature crustal materials we propose a model for the late triassic magmatism in the yidun terrane involving magmas derived from partial melting of subduction metasomatized mantle peridotites that subsequently experienced melting assimilation storage homogenization mash processes the low calculated ti in zircon temperatures 644 c 863 c generally between 640 c and 690 c for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas and partly controlled by the crystallization of amphibole zircon grains from porphyry copper system display small negative europium anomalies mostly eu eu 0 5 contrasting with pronounced negative eu anomalies in zircons mostly eu eu magmato hydrothermal quartz vein cu mo w deposit magmato hydrothermal quartz vein mo w deposit this study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits

zircon compositional evidence for sulfur degassing from ore forming arc magmas porphyry cu mo au and epithermal au ag deposits are major sources of mined metals and are commonly formed by magmatic hydrothermal fluids derived from hydrous magmas in phanerozoic convergent margin settings the igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings but while many have produced acidic magmatic fluids very few past or present have produced sufficient metal chlorine and sulfur enrichments necessary to engender an ore deposit the reasons for this remain uncertain we report shrimp rg ion microprobe analyses of hafnium titanium and rare earth element ree abundances in zircon a nearly ubiquitous and robust trace mineral in crustal magmas comparison of the compositions of zircons in ore forming and barren granitic plutons indicate that ore forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies mostly eu n eu n 0 4 we interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to so 2 degassing from magmas with a relatively low fe s ratio oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate s 6 to so 2 s 4 upon degassing this interpretation reinforces the important role of oxidized sulfur rich fluids in porphyry and epithermal mineral deposit formation zircon compositions thus may be used to identify ancient magmas that released significant amounts of so 2 rich gases and regional surveys of zircon composition are potentially a valuable tool for mineral exploration

the geochemistry geochronology and paleomagnetism of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract the midcontinent rift mcr of north america comprises a series of basaltic sheets flows and intrusive rocks emplaced in the lake superior region during the mesoproterozoic the mafic rocks preserved on the northern flank of lake superior represent the older portions of the rift sequence and offer insights into the early development of the rift new geochronological geochemical and paleomagnetic data are presented for the dikes and sills located in and south of thunder bay ontario three sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill the predominant logan sills and nipigon sills in the north of the study area in addition three dike sets are recognized the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike the geochemical data demonstrate that the majority of sills south of thunder bay are of logan affinity and distinct from those of broadly similar age in the nipigon embayment to the north the pigeon river dikes that intrude the sills are geochemically coherent but distinct from the logan sills and could not be feeders to the sills the new age of 1109 2 4 2 ma for the cloud river dike and its r polarity are consistent with published magnetostratigraphy the mt mollie dike age 1109 3 6 3 ma indicates that it is not coeval with the spatially associated crystal lake gabbro as previously thought the complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south the geochemistry and geochronology of the intrusions suggest a long lived and complex magmatic history for the midcontinent rift

sulfur evolution of oxidized arc magmas as recorded in apatite from a porphyry copper batholith uniformly sulfur rich cores abruptly zoned to sulfur poor rims 1 to 0 2 wt so3 in apatite from the yerington batholith nevada indicate that early magma that is crystal poor oxidizing and sulfate rich evolved to sulfate poor magma via crystallization of anhydrite a mineral observed in magmas from pinatubo and el chichon we predict that the characteristic zonation to sulfur poor rims of apatite in the yerington batholith is common in other oxidized hydrous calc alkaline magmas and can be used to track cryptic anhydrite saturation as well as to monitor sulfur evolution sulfate rich arc magmas such as yerington magmas may crystallize to produce hydrothermal fluids rich in chlorine copper and sulfur and porphyry copper ores

high water contents of magmas and extensive fluid exsolution during the formation of the yulong porphyry cu mo deposit eastern tibet abstract the yulong porphyry cu mo ore district in eastern tibet features a series of eocene felsic porphyry intrusions only one of which is intensively mineralized in this study zircon u pb ages and major and trace element compositions of whole rock samples and in situ accessory minerals zircon apatite and titanite from these various intrusions were analyzed to determine the factors controlling cu mineralization new zircon u pb dating together with published zircon u pb ages suggest that these porphyry intrusions were emplaced over a relatively short period of time 43 9 0 6 ma to 40 9 0 3 ma with the mineralized yulong intrusion emplaced at a late stage of the magmatic activity 41 1 0 3 ma to 40 9 0 3 ma these intrusions have indistinguishable whole rock major and trace element compositions but the mineralized yulong intrusion has higher apatite and titanite la yb ratios average apatite la yb 115 1 28 5 n 52 average titanite la yb 22 4 4 9 n 38 than the earlier subeconomic intrusions average apatite la yb 50 4 12 3 n 72 average titanite la yb 12 4 3 5 n 58 suggesting more fractionation of amphibole from the magmas that sourced it additionally a negative relationship between la yb and cen cen ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution indistinguishable zircon eun eun values and apatite core so3 contents from the mineralized zircon eun eun 0 69 0 06 n 25 apatite so3 0 65 0 24 wt n 21 and subeconomic intrusions zircon eun eun 0 62 0 05 n 64 apatite so3 0 61 0 27 wt n 15 suggest that the entire magmatic suite was relatively oxidized hydrous and sulfur rich and was therefore fertile for ore formation however apatite crystals from the mineralized yulong intrusion have significantly lower cl contents 0 08 0 03 wt n 72 and higher f cl ratios 47 80 21 97 n 72 than those from the subeconomic intrusions cl 0 29 0 29 wt f cl 12 87 5 16 n 78 these data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing yulong magmas sudden and voluminous release of volatiles coeval with emplacement of the yulong intrusion was triggered by an as yet unidentified process but possibly including magmatic recharge

self reversed magnetization held by martite in basalt flows from the 1 1 billion year old keweenawan rift canada in some basalt flows of the 1 1 billion year old keweenawan rift exposed at mamainse point ontario there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession the paleomagnetic and geological context of this component demonstrates that it is not a chemicaloverprint whoseremanence is attributableto a subsequently reversed geomagnetic field but that the component is a self reversal of the primary magnetization here we use rock magnetic experiments and mossbauer spectroscopy to show that this phase occurs in the most oxidized flowsandisheldbya fine grainedpopulationofhematitethatacquiredits self reversedremanencethrough interactions with a phase of lower blocking temperature we propose that this self reversed hematite is a manifestation of the self reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and thathas been attributed to negative exchange coupling across the crystal lattices we suggest that hematite formed in association with iron silicates in the basalt flows carries a remanence reflective of the field in which it formed but that martite hematite pseudomorphed after magnetite which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self reversed remanence this study marks the second time that a naturally occurring self reversed magnetization has been attributed to this mechanism and marks the oldestreportedinstanceof anytypeof self reversedremanence themartitethatformsthrough thisprocess has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years

calcite twinning strains from syn faulting calcite gouge small offset strike slip normal and thrust faults we have evaluated the stress strain behavior of calcite precipitated and mechanically twinned in small offset strike slip normal and thrust faults of a variety of ages and from a variety of tectonic settings n 3001 twin measurements 63 strain analyses from 18 field sites five strike slip faults with syn faulting horizontally striated calcite rake 0 were studied and we report the orientations of the contemporaneous stress strain field associated with each fault intrusion of the marathon large igneous province mafic dikes 2 1 ga in archean crust minnesota usa post keweenaw rift 1 1 ga faulting island lake fault central ontario canada subduction associated with metamorphic core complex formation cretaceous china subduction cretaceous to miocene italy and continental extension recently active furnace creek fault death valley california usa seven normal faults with synfaulting dip slip striated calcite were studied and are from the following tectonic settings a normal fault slip surface in an ordovician piedmont fold appalachian s paleo subduction associated with cretaceous metamorphic core complex formation china 3 sites the paleo extensional atlantic margin 55 ma ireland 1 site with a u pb calcite age continental extension 1 active site mojave desert a transcurrent margin jamaica 1 active fault site and subduction 2 active faults along the eur african margin in italy with calcite u th disequilibria ages and crete respectively six thrust fault examples are all from convergent orogenic settings the basal thrust of the penokean 1850 ma fold and thrust belt the penokean orogen foreland in mesabi range banded iron formation folds an offset breccia body in the permian gondwanide belt ellsworth mountains antarctica the frontal thrust of the gondwanide cape belt south africa the paleocene frontal prospect thrust sevier belt wyoming and an alpine foreland back thrust lulworth cove u k for each strike slip fault system the twinning shortening strain is horizontal and at an angle of 0 60 to the respective fault plane dextral or sinistral although in the majority of cases the shortening axis is parallel to fault strike 13 of 23 results in each normal fault example dip slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress 1 and shortening strain axis e1 are not 45 to the fault plane as predicted but are sub horizontal and either strike parallel 25 of 35 results or strike normal 10 of 35 results thrust faults preserve shortening strain axes parallel to the dip slip kinematic direction within the fault plane plane strain and not at 45 to the principal plane 5 of 5 results none of the fault stress strain field results reported here support the andersonian or mohr coulomb criteria for stress strain relations predicted along faults

experimental evidence for the alteration of the fe3 σfe of silicate melt caused by the degassing of chlorine bearing aqueous volatiles the temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria we present new experimental data that demonstrate that a cl bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of fe 2 into the volatile phase the data are consistent with a model wherein the exsolution of a cl bearing volatile phase from the melt modifies the mgo feo ratio of the residual degassed melt increasing the fe 3 fe ratio of the melt degassing induced alteration of the melt may cryptically affect the phases that constitute the hypersolidus crystallizing assemblage the new results indicate that a low to moderate salinity magmatic volatile phase may contain several weight percent iron present as fecl 2 as such passive degassing over geologically plausible protracted periods has the potential to extract significant quantities of fe 2 resulting in a residual melt depleted in feo relative to fe 2 o 3 notably preferential mass transfer of fe 2 from the melt to the fluid results in self oxidation of the melt driving the oxygen fugacity of the melt across the phase boundary that demarcates the stability fields of sulfide and sulfate phases stabilizing sulfate at the expense of sulfide such a process has significant implications for the remobilization of chalcophile metals during the evolution of magmatic hydrothermal systems

trace element composition of igneous zircon a thermal and compositional record of the accumulation and evolution of a large silicic batholith spirit mountain nevada hafnium u th and ree content of zircons from the spirit mountain batholith in southern nevada correlate with calculated temperatures from the ti in zircon thermometer to support field and petrologic evidence of rejuvenation of crystal mush and melt extraction events during the 2 million year accumulation of the granitoid batholith marked variation in zircon composition from sample to sample from grain to grain within individual samples and from zone to zone within individual grains documents in detail a history of fluctuating conditions with repeated episodes of replenishment reheating crystal mush rejuvenation fractional crystallization and melt segregation the zircons exhibit compositional and thermal variability indicative of variations in host melt composition due to 1 melt rejuvenation mixing and fractionation 2 coeval growth of other ree rich accessory minerals and possibly 3 fluctuation in fo2

can economic porphyry copper mineralization be generated by a typical calc alkaline melt numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that typical calc alkaline magmas contain sufficient copper chlorine and water to produce economic porphyry copper mineralization neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles the most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper the ratio of initial water in the melt to the water saturation level and the initial chlorine water ratio of the melt the absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume the bulk salinity of the exsolved fluid may vary from 2 0 wt nacl to saturation levels 84 wt nacl at 700 c indicating that boiling is not necessary to produce high salinity brines at appropriate p t xnacl conditions the magmatic aqueous fluid separates into a saline liquid which transports most of the copper and a low salinity vapor the salinities of the two immiscible phases are governed by the p t conditions while the bulk fluid salinity determines the mass fractions of liquid and vapor formed pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid this results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt as copper is transported from the melt by the earliest exsolving fluids in deep 2 0 kbar systems and by late exsolving fluids in shallow 0 5 kbar systems the relative timing of pressure quenching aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures model results incorporating petrologic constraints determined for yerington nevada are in good agreement with observed mineralization

structural geologic evolution of the butte district montana the butte district contains two porphyry cu mo deposits and the younger zoned main stage base metal lode deposits that formed in the period from 66 to 62 ma synchronous with laramide compressional thrust faulting and folding in southwest montana new geologic mapping of the butte district reveals three distinct episodes of deformation 1 gentle 20 10 ne tilting produced during laramide compressional deformation uplift and exhumation during and after ore formation in the late cretaceous to paleocene 66 53 ma 2 moderate 10 50 nw tilting of eocene age produced by ne striking normal faults that are limited to the northwest part of the district and synchronous with the eruption of the lowland creek volcanics and 3 miocene to present gentle 15 ese tilting in the hanging walls of the continental east ridge and rocker normal faults stratigraphic relationships isotopic ages hornblende barometry and fluid inclusion data indicate that exposures of the butte granite which hosts mineralization crystallized at 6 to 9 km depth and that early porphyry cu mo deposits formed at similar depths of 5 to 9 km a period of rapid uplift and erosion removed about 7 km of rock from above the ore deposit between 66 to 64 ma and 53 ma but was mostly completed by 59 ma estimated exhumation rates range from 0 5 to 2 mm yr and uplift was likely most rapid between 63 5 and 62 ma during main stage mineralization which occurred at depths of 2 to 6 km the butte granite and boulder batholith were emplaced at 79 to 76 ma into thrust fault ramps which later became inactive near butte as laramide crustal shortening moved eastward in the helena salient the accelerated period of uplift and unroofing is attributed to renewed deformation along the montana disturbed belt south of butte that uplifted the south end of the boulder batholith and produced 20 of gentle ne tilting episode i in the district the exhumation history of the deposit is therefore broadly contemporaneous with ore formation and is consistent with the general transition from lithostatic to hydrostatic conditions that is evident from the telescoping of early porphyry cu mo ores cut by the main stage lode veins within a 5 m y period the laramide compressional regime near butte evolved into an extensional environment characterized by normal faulting the normal faults were synchronous with eruption of the 53 to 48 ma lowland creek volcanics which directly overlie the main stage veins in the northwest part of the district structural reconstruction of the district at the time of mineralization produces orientations of quartz porphyry dikes porphyry cu mo veinlets main stage veins and alteration zonal patterns that are steeply dipping and symmetrical similar to observations from other upright or restored porphyry copper deposits early porphyry cu mo veins and dikes are oriented e w and likely originated as tensile hydrofractures above a ductile magma carapace during laramide shortening additionally during the laramide compressional phase two conjugate fault systems occupied by main stage veins are normal oblique slip faults that record minor e w and vertical shortening and n s extension these normal faults with minor offset likely resulted from brittle failure caused by doming above a volatile rich and buoyant granitic magma chamber and cupola at depth the structural evolution of the butte district therefore reflects the interaction of a far field compressive stress regime with local buoyant magmatic forces

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

high sr y arc magmas and porphyry cu mo au deposits just add water porphyry cu mo au deposits are commonly associated with intermediate composition arc related igneous rocks with high sr y and la yb ratios and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism adakites however similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge derived magmas by fractionation of amphibole and or garnet and or by interaction with crustal materials during ascent through the upper plate lithosphere while there is widespread evidence for such fractionation and contamination processes in porphyry magmas there is little direct evidence for an origin by slab melting the enhanced fertility of arc magmas relates primarily to their high water content which is required for the formation of magmatic hydrothermal systems upon shallow crustal emplacement magmatic water contents 4 wt percent h 2 o cause abundant amphibole garnet fractionation but suppression of plagioclase crystallization at deep crustal levels resulting in increasing sr y and la yb ratios with differentiation commonly into adakitic ranges but not reflecting slab melting additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content but these are secondary to the requirement for sufficient water thus arc magmatic suites with high sr y ratios and evidence for the presence of hydrous phenocryst phases hornblende and or biotite are indeed prospective for porphyry cu mo au deposits but only because these parameters indicate high magmatic water content