geochemistry of tertiary igneous rocks of northern luzon philippines evidence for a back arc setting for alkalic porphyry copper gold deposits and a case for slab roll back oligocene to early miocene volcanic rocks are preserved in the central cordillera range and cagayan valley of northern luzon philippines basaltic and andesitic rocks of the pugo formation in the baguio district of the central cordillera were intruded by the 27 to 20 ma calc alkaline central cordillera diorite complex in the southern cagayan valley the subalkaline to alkaline late oligocene mamparang formation overlies the cretaceous caraballo formation and has been intruded by the didipio igneous complex the cordon syenite complex and the palali batholith the didipio complex comprises an early suite of diorites which were intruded by the strongly mineralized stocks of the dinkidi cu au porphyry deposit whole rock geochemical data for intrusive and extrusive rocks of the baguio district range from low k calc alkaline to shoshonitic basalts to dacites with rare earth element ree and high field strength element hfse characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt samples from didipio display higher alkali contents but similar trace element characteristics new age dates for the didipio area range from 25 7 to 24 8 ma the potassic magmas of the cagayan valley are interpreted to have formed in a back arc coeval to the mainarc sequence that is preserved in the baguio miocene rocks this contradicts earlier models which invoke an early miocene arc reversal in the northern luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes the lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back arc magmas in the east in northern luzon is best interpreted as the result of eastward subduction since the late oligocene the presence of 20 ma adakitic magmas in the baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back arc rifting

foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

role of subduction erosion in the generation of andean magmas subduction erosion or scraping off of the continental crust by subducting oceanic lithosphere is an important tectonic process along parts of the andean convergent plate boundary estimated rates of subduction erosion increase northward north of the southem andes due to decreasing subduction angle and decreasing sediment supply to the trench west of the central andes subduction erosion may remove as much as 535 km 3 of crust per 1 km of trench per 1 m y or 7 vol of the subducting oceanic lithosphere the higher 87 sr 86 sr ratios of mafic magmas erupted north of the southern andes may be explained by the higher rates of subduction erosion and increased amounts of contamination of the sub arc mantle magma source

multistage intrusion brecciation and veining at el teniente chile evolution of a nested porphyry system the el teniente copper molybdenum deposit is hosted by the late miocene teniente mafic complex a largely subvolcanic package of primarily basaltic andesite porphyry sills and stocks that were emplaced within the mid late miocene farellones formation in the late miocene pliocene a series of intermediate felsic plutons were intruded into the teniente mafic complex these are spatially associated with magmatic hydrothermal breccias and multiple vein types that form individual mineralized complexes here we present detailed observations on mineralogy textures and intrusion breccias and vein crosscutting relationships to constrain the nature and relative timing of magmatic and hydrothermal events our revised classification defines 13 vein types divided lilt three main stages 1 premineralization biotite and or k feldspar quartz anhydrite albite magnetite actinolite epidote veins that formed prior to emplacement of mineralized intrusions and breccias 2 main mineralization stage veins that grade from gangue dominated quartz anhydrite veins potassic alteration halos into sulfide dominated veins with phyllic alteration halos and 3 late mineralization veins containing sulfosalts five breccia types have been observed in the deposit typically forming individual vertically zoned complexes spatially associated with individual intrusions and overlapping in time i igneous cemented breccias 2 k feldspar cemented breccias 3 biotite cemented breccias 4 anhydrite cemented breccias and 5 tourmaline cemented breccias breccia cements display 1 similar paragenetic evolution to main mineralization stage veins indicating a close genetic link between them a distinction can be made between early premineralization vein types types 1 2 that represent veins formed possibly deposit wide prior to emplacement of intrusion breccia complexes late pre and main mineralization vent types types 3 8 which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex and late mineralization vein types types 9 10 that represent it single event linked to the emplacement of the braden breccia pipe the geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow felsic intermediate pipelike intrusions and the development of igneous mid mineralized magmatic hydrothermal breccias and vein halos the magmatic hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic hydrothermal activity which followed rather similar evolution paths single deposit wide models of fluid evolution and mineralization are therefore inappropriate we conclude that el teniente represents a nested but otherwise rather typical porphyry cu mo system unusual only in thia the teniente mafic complex pi is it particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic hydrothermal breccia complexes and its an effective chemical trap for deposition of sulfides the overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren intermineral porphyries resulted in its unusual size and high hypogene grades

contributions from mafic alkaline magmas to the bingham porphyry cu au mo deposit utah usa the bingham porphyry cu au mo deposit utah may only be world class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc alkaline system volcanic mafic alkaline rocks in the district are enriched in cr ni and ba as well as cu au platinum group elements pge and s the bulk of the volcanic section that is co magmatic with ore related porphyries is dacitic to trachytic in composition but has inherited the geochemical signature of high cr ni and ba from magma mixing with the mafic alkaline rocks the volcanic section that most closely correlates in time with ore related porphyries is very heterogeneous containing clasts of scoriaceous latite latitic and minette and flows of melanephelinite shoshonite and olivine latite in addition to volumetrically dominant dacite trachyte bingham ore related porphyries show ample evidence of prior mixing with mafic alkaline magmas intrusive porphyries that have not been previously well studied have several chemical and mineralogical indications of magma mixing these mixed lithologies include the hybrid quartz monzonite porphyry biotite porphyry and minette dikes even some of the more silicic latite and monzonite porphyries retain high cr and ba contents indicative of mixing and contain trace amounts of sapphire 1 mm the heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical mineralogical and isotopic data magma mixing calculations suggest about 10 of the monzonitic latitic ore related magma may have been derived from mafic alkaline magma similar to the melanephelinite if the original s content of the mafic magma was about 2 000 4 000 ppm comparable with similar magmas then the mafic magma may have been responsible for contributing more than half of the s and a significant portion of the cu au and pge in the bingham deposit

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

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

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

porphyry to epithermal transition in the altar cu au mo deposit argentina studied by cathodoluminescence la icp ms and fluid inclusion analysis the middle to late miocene altar porphyry cu au mo deposit located in the andean main cordillera of san juan province argentina is characterized by the superposition of multiple vein generations consisting of both porphyry type and high sulfidation epithermal style alteration and mineralization we constrain the physical and chemical evolution of the hydrothermal fluids that formed this deposit based on description and distribution of vein types scanning electron microscopy cathodoluminescence cl imaging trace elements in quartz veins and fluid inclusion microthermometry quartz cl textures and trace elements chiefly li al ti and ge differentiate among quartz generations precipitated during different mineralization and alteration events early quartz chalcopyrite pyrite veins and quartz molybdenite veins a and b veins show considerable complexity and were commonly reopened and some underwent quartz dissolution early quartz chalcopyrite pyrite veins a veins are dominated by equigranular bright cl quartz with homogeneous texture most of these veins contain higher ti concentrations than any other vein type average 100 ppm and have low to intermediate al concentrations 65 448 ppm quartz molybdenite b veins and chlorite rutile hematite c veins veins contain quartz of intermediate cl intensity that commonly shows growth zones with oscillatory cl intensity quartz from these veins has intermediate ti concentrations 20 ppm and al concentrations similar to those of a veins quartz from later quartz pyrite veins with quartz muscovite tourmaline halos d veins has significantly lower cl intensity low ti 15 ppm and elevated al concentrations up to 1 000 ppm and typically contains euhedral growth zones late veins rich in sulfides and sulfosalts show cl textures typical of epithermal deposits dark cl quartz crustiform banding and euhedral growth zones quartz from these veins typically contains less than 5 ppm ti and al li and ge concentrations are elevated relative to other vein types based on experimentally established relationships between ti concentration in quartz and temperature the decrease in ti content in successively later quartz generations indicates that the temperature of the hydrothermal fluids decreased through time during the evolution of the system vein formation at altar occurred at progressively lower pressure shallower paleodepth and lower temperature under lithostatic pressures the magma supplied low salinity aqueous fluids at depths of 6 to 6 8 km pressures of 1 6 1 8 kbar and temperatures of 670 to 730 c first quartz generation of early quartz chalcopyrite pyrite veins this parental fluid episodically depressurized and cooled at temperatures and pressures below the brine vapor solvus quartz molybdenite veins precipitated from fluids at temperatures of 510 to 540 c and pressures of 800 to 1 000 bars corresponding to depths of 3 to 3 7 km under lithostatic pressures further cooling of hydrothermal fluids to temperatures between 425 and 370 c under hydrostatic pressures of 200 to 350 bars produced pyrite quartz veins and pervasive quartz muscovite tourmaline and illite alteration that overprinted the early hydrothermal assemblages late veins rich in sulfides and sulfosalts that overlapped the deep and intermediate high temperature veins formed from fluids at temperatures of 250 to 280 c and pressures of 20 to 150 bars the epithermal siliceous ledges formed from low temperature fluids 230 c at hydrostatic pressures of 100 bars corresponding to depths of 1 km

magmatic evolution of the giant el teniente cu mo deposit central chile elteniente the world s largest cu deposit is hosted in late miocene and pliocene plutons that intrude the older teniente volcanic complex or farellones fm 14 2 6 5 ma the late miocene and pliocene plutonic host rocks of the deposit include sequentially the relatively large 450 km teniente mafic complex laccolith 8 9 1 4 ma the smaller 30 km sewell equigranular tonalite complex 7 05 0 14 ma and associated andesitic sills 8 2 0 5 to 6 6 0 4 ma small dacitic porphyry stocks 51km 6 09 0 18 ma the unusual cuand s rich porphyry a anhydrite bearing granitoid stock 51km 5 67 0 19 ma the teniente dacite porphyry dike 51km 5 28 0 10 ma minor latite dikes 4 82 0 09 ma and finally a small dacite intrusion 4 58 0 10 ma these plutonic rocks are all isotopically similar to each other sr sr1 4 0 7039 0 7042 end1 4 2 5 to 3 5 and also to the teniente volcanic complex extrusive rocks but distinct from both older late oligocene to early miocene volcanic rocks sr sr1 4 0 7033 0 7039 end1 4 3 8 to 6 2 and younger pliocene post mineralization mafic dikes and lavas sr sr1 4 0 7041 0 7049 end1 4 1 1 to 1 1 multiple cu mineralized magmatic hydrothermal breccia pipes were emplaced into these plutonic rocks during the same time period as the felsic porphyry intrusions between at least 6 31 0 03 and 4 42 0 02 ma these mineralized breccia pipes which formed by exsolution of magmatic fluids from cooling plutons have their roots below the deepest level of mining and exploration drilling and were derived from the same magma chamber as the felsic porphyries 44 km below the paleosurface to produce the 100 10 tonnes of cu in the deposit requires a batholith size 4600 km amount of magma with 100 ppm cu we suggest that both the mineralized magmatic hydrothermal breccias and the progressively smaller volumes of more fractionated but isotopically equivalent late miocene and pliocene felsic plutonic rocks that host the deposit were derived from the roof of a large long lived thermally and chemically stratified open system magma chamber or magmatic plumbing system recharged from below by mantle derived magmas only when this system fully solidified did post mineralization mafic olivine hornblende lamprophyre dikes 3 85 0 18 to 2 9 0 6 ma pass through the system from the mantle to the surface the significant progressive temporal isotopic evolution to higher sr sr from 0 7033 to 0 7049 and lower end from 6 2 to 1 1 that occurred between the late oligocene and pliocene in the vicinity of el teniente for mafic mantle derived magmas and by implication their sub arc mantle source region was due in part to increased mantle source region contamination by subducted crust tectonically eroded off the continental margin the post mineralization olivine hornblende lamprophyres also imply extensive hydration of the mantle below this portion of the andean arc by the pliocene which may have played a role in producing oxidized volatile rich magmas and mineralization at elteniente

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

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

regional geochemistry of tertiary igneous rocks in central chile implications for the geodynamic environment of giant porphyry copper and epithermal gold mineralization the giant porphyry copper molybdenum deposits of central chile formed within a thick sequence of cretaceous to pliocene volcanic rocks the predominantly calc alkaline basaltic andesites of the cretaceous las chilcas formation are characterized by la smn ratios of 1 8 to 2 5 and sm ybn of 1 8 to 2 8 the upper oligocene to lower miocene abanico formation variously defined as the los pelambres abanico or coya machali formations ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc alkaline to tholeiitic all samples from this formation are characterized by lree enrichment and moderately or locally strongly fractionated hree la smn 1 3 4 1 sm ybn 1 5 5 8 the basaltic andesites and andesites of the middle miocene salamanca formation have ree chemistry similar to that of the upper cretaceous strata la smn 1 5 2 7 sm ybn 1 6 2 7 the overlying middle miocene farellones formation ranges from tholeiitic to calc alkaline and from basalt to andesitic and has a similar lree enrichment and fractionated hree la smn 1 7 2 5 sm ybn 1 7 3 3 the pliocene la copa rhyolite complex however is strongly lree enriched and hree depleted la smn 3 8 3 9 sm ybn 4 2 4 7 overall the trace element geochemistry of the cretaceous to middle miocene volcanic rocks is characterized by enriched lree and negative nb anomalies consistent with an arc setting with only minor differences in the abundance of most elements crustal thickening during the miocene in central chile has been suggested to have been responsible for a transition from an amphibole to garnet dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits however the gradual increase in la yb through the early middle and late miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data instead a rapid change in the geochemical signature between the end of the eruption of the farellones formation and the eruption of the high la yb la copa rhyolite complex implies a more abrupt change in the tectonic environment isotopic data broadly support this although lower epsilon nd values in the farellones formation imply a greater role for crustal contamination in younger suites in the absence of gradual crustal thickening it is suggested that the subduction of the juan fernandez ridge may have been the key geodynamic process responsible for the genesis of the three middle miocene to lower pliocene giant porphyry copper deposits in central chile possibly by promoting crustal scale faulting and even acting as a source of metals

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

geochronology and geochemistry of porphyritic intrusions in the duolong porphyry and epithermal cu au district central tibet implications for the genesis and exploration of porphyry copper deposits abstract the duolong district in central tibet hosts a number of porphyry as well as high sulfidation epithermal copper gold deposits and prospects associated with voluminous calc alkaline volcanism and plutonism in this study we present new geochronological geochemical isotopic and mineralogical data for both economically mineralized and barren porphyritic intrusions from the duobuza and naruo porphyry cu au deposits zircon u pb analyses suggest the emplacement of economically mineralized granodiorite porphyry and barren granodiorite porphyry at naruo deposit took place at 119 8 1 4 ma and 117 2 0 5 ma respectively four molybdenite samples from the naruo deposit yield an isochron re os age of 119 5 3 2 ma indicating mineralization occurred synchronously with the emplacement of the early granodiorite porphyry at duobuza deposit the barren quartz diorite porphyry intruded at 119 5 0 7 ma and two economically mineralized intrusions intruded at 118 5 1 2 ma granodiorite porphyry and 117 5 1 2 ma quartz diorite porphyry respectively petrographic investigations and geochemical data indicate that all of the porphyritic intrusions were oxidized water rich and subduction related calc alkaline magmas zircons from the porphyritic intrusions have a wide range in the ehf 0 11 1 indicating that they were sourced from mixing of mantle derived mafic and crust derived felsic melts moreover the variation of trace element content of plagioclase phenocrysts indicates that the magma chambers were recharged by mafic magmas comparison of the composition of amphibole phenocrysts indicates the porphyry copper gold mineralization at duolong was generated in magma chambers at low crystallization temperatures and pressures 754 to 791 c 59 m to 73 mpa n 8 and under highly oxidizing conditions nno 2 2 to 2 7 n 8 in contrast barren intrusions were sourced from the magma chambers with higher crystallization temperatures and pressures 816 to 892 c 111 to 232 mpa n 22 that were less oxidizing nno 0 6 to 1 6 n 22 the requirement for a thermal contrast is supported by the declining of ti content in magnetite crystals in barren intrusions 12 550 to 34 200 ppm versus those from economically mineralized intrusions 600 to 3400 ppm moreover the v content in magnetite crystals from economically mineralized intrusions 990 to 2510 ppm is lower than those recorded from barren intrusions 2610 to 3510 ppm which might reflect the variation in oxidation state of the magma the calculated water solubility of the magma forming the economically mineralized intrusions 3 2 3 7 wt is lower than that of magma forming the barren intrusions 4 6 6 4 wt based on the chemical physical characteristics of economically mineralized magma our study suggests that the development of porphyry cu au mineralization at duolong was initiated by shallow level emplacement of a magma that crystallized at lower temperatures and pressures experimental studies show that copper and water solubilities in silicate melts decrease with falling temperatures and pressures indicating metals and ore forming fluids are more likely to be released from a magma reservoir emplaced at shallow crustal levels we propose the magnetite might be a convenient exploration tool in the search for porphyry copper mineralization because the variations in ti and v content of mineral concentrates and rock samples are indicative of barren versus mineralized intrusions

seasonal and interannual variability of atmospheric convergence zones in the tropical pacific observed with ers 1 scatterometer monthly images of atmospheric convergence zones in the tropical pacific were produced with 1992 1994 ers 1 scatterometer wind vectors from the images the position width and intensity of the convergence zones can be determined latitude time sections of ocean surface wind divergence show that the summer position of the itcz in the eastern pacific during the observed period exceeded the normal position northward by 3 6 latitude and the annual cycle in the western pacific for 1994 was quite weak implying a persistently unusual case

geology and geochronology of the tiegelongnan porphyry epithermal cu au deposit tibet china formation exhumation and preservation history abstract the tiegelongnan rongna deposit is a giant porphyry epithermal cu au deposit within the duolong porphyry district in the south qiangtang terrane central western tibet china the deposit is centered on multiphase granodiorite porphyry gp intrusions intruded in jurassic sandstone phase 1 and 2 gp are associated with biotite and sericite alteration whereas gp3 and 4 occur in middle and shallow parts of the deposit affected by sericite and advanced argillic alteration alunite kaolinite dickite pyrophyllite sericite alteration is pervasive in the deep part of the deposit replacing biotite alteration and hosting porphyry stage chalcopyrite bornite pyrite molybdenite mineralization advanced argillic alterationis associated with epithermal stage high sulfidation tennantite enargite bornite and digenite mineralization overprints the sericite and biotite alteration from the shallow part of the deposit the top of the tiegelongnan hydrothermal system was weathered with a layer of supergene oxidation and enrichment zone exhumated and covered by andesite and gravel muscovite sampled from deep sericite alteration yields a 40ar 39ar age of 120 9 0 8 ma which is consistent with previously reported hydrothermal biotite 40ar 39ar age of 121 1 0 6 ma molybdenite re os ages of 121 2 0 6 and 119 0 1 4 ma and gp 1 and 2 zircon u pb ages of 121 5 1 5 120 2 1 0 ma la icp ms results and 118 7 0 9 ma sims result phase 3 gp 3 yields a ca id tims zircon u pb age of 119 9 0 2 ma which constrains the age of porphyry magmatic hydrothermal events the porphyry system was then unroofed weathered and superimposed by discrete stages of advanced argillic alteration and high sulfidation mineralization two pulses of alunite events yield 40ar 39ar ages of 116 3 0 8 ma and 111 7 1 0 ma which represent the principal epithermal alteration and mineralization ages at tiegelongnan the first alunite pulse age is consistent with younger porphyry gp4 age of 116 2 0 4 ma zircon la icp ms age weathering and exhumation of the porphyry epithermal deposit were continuing until the cover of andesite at 110 ma and by post andesite gravel the andesite was affected by a younger weak hydrothermal alteration at 108 7 0 7 ma constrained by a muscovite 40ar 39ar age another barren alunite yields a 40ar 39ar age of 100 6 2 0 ma which may represent the youngest hydrothermal event the long life span discrete hydrothermal activities from 120 to 100 ma in the tiegelongnan deposit is consistent with the long lasting tectonic magmatic events of the bangong nujiang suture zone the discrete epithermal mineralization events at tiegelongan are younger than the porphyry mineralization events which is analogous to several other epithermal deposits worldwide the 10 m y exhumation history at tiegelongan is slower than the typical and rapid erosion history of 1 2 m y porphyry deposits in low altitude tropical climates regions and during a specific abrupt uplift pulse in the andean region the slow exhumation at tiegelongnan is interpreted to be an integrated result of an arid climate environment relatively flat topography and a slow uplift condition during cretaceous in central western tibet

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

contribution of mafic melt to porphyry copper mineralization evidence from mount pinatubo philippines and bingham canyon utah usa mount pinatubo in the philippines known for its cataclysmic eruption in 1991 hosts several porphyry copper deposits and active geothermal systems an underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption the eruption caused purging of the sulphur rich fluid from the dacite to the atmosphere and extensive fracturing similar events took place at bingham canyon utah site of the largest copper and gold deposit in north america at 38 ma the bingham canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma which consolidated to form the bingham stock and its underlying magma chamber injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock

geology mineralization alteration and structural evolution of the el teniente porphyry cu mo deposit sir the el teniente cu mo deposit in central chile is an extraordinary accumulation of cu mo ore fig 1 and we welcome the opportunity provided by skewes and stern 2007 to discuss and clarify several important issues regarding its classification and interpretation and to address several incorrect assertions that they make regarding our manuscript cannell et al 2005 we have interpreted el teniente to be a typical porphyry deposit in terms of its alteration assemblages vein and breccia styles and spatial and temporal relationships between cu mo mineralization and felsic porphyries skewes and stern 2007 dispute our claims that mineralization occurred primarily in veins and was related to felsic intrusive activity they also take issue with our interpretations of the various geochronological datasets from el teniente and our classification of el teniente as a porphyry cu mo deposit we address each of these in turn below in order to facilitate the following discussion table 1 correlates the geologic nomenclature of skewes and stern 2007 with that of cannell et al 2005 figures 1 and 2 illustrate the location of the main geologic units on underground mine level 6 at el teniente also shown on figure 1 are the approximate locations of the hypogene sulfide mineral zones figure 2 shows the block model of mo grades in the mine for more details of geologic relationships at el teniente refer to figure 4 of cannell et al 2005 and figures 4 and 5 of skewes et al 2002 although the dacite porphyry dike is shown as a single unit on figure 1 note that rojas 2002 2003 mapped multiple phases within this composite intrusive body skewes and stern 2007 criticized our use of what they assert to be an outdated map of hypogene mineralization

modeling the thermal evolution of fault controlled magma emplacement models implications for the solidification of granitoid plutons a two dimensional finite diaerence model is used to simulate the conductive thermal regime attend ing construction and maintenance of a continental magma chamber by intrusion of granite dikes into grano diorite host rocks displaced at various spreading rates final intrusion shapes include tabular square and vertical rectangular bodies emplaced in the shallow crust 5 15 km and tabular bodies emplaced in the middle crust 15 20 km fed by dikes with widths of between 20 and 100 m the formation of a steady state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling the solidifi cation of subsequently intruded material for spreading rates 10 mm year 1 construction of a steady state magma chamber in the shallow crust took 260 ka rectangular 360 ka square and 1 ma tabular whereas in the mid crust a steady state was reached in less than 30 ka tabular at faster spreading rates 25 and 50 mm year 1 ambient temperatures pass the solidus isotherm forming a steady state reservoir within 55 ka depending on intrusion depth and size for 10 25 mm year 1 spreading rates sheeted dikes make up from 10 to 100 of the intrusion the thermal modeling supports the following conclusions a episodic magma emplacement into a fault con trolled setting is a thermally viable means of constructing a steady state chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the intrusion solidus b isotherms will migrate outward during successive intrusion before converging back on the center of the intrusion after chamber construction c the margins of most intrusions formed by this scen ario should contain sheeted dikes d the solidus isotherm and thus the solidification front that it tracks will become progressively curviplanar during the construction of the magma chamber and will not represent the in itial shape of the intrusions i e sheets e the steady state chamber will be smaller than the total intrusion dimensions and f magmatic fabrics will form diachronously and not always parallel to sheet margins as they track the migrating solidification front because it is unlikely that most large intrusions formed instan taneously the eaects of continued addition of heat on the migration of solidification fronts may have signifi cant implications for magmatic processes in many emplacement scenarios 1998 elsevier science ltd all rights reserved

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

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

ore forming mechanism and physicochemical evolution of gutaishan au deposit south china perspective from quartz geochemistry and fluid inclusions abstract the gutaishan is an important gold deposit of the jiangnan orogen in south china the deposit is characterized by multiphase alteration mineralization including barren quartz pre ore stage i quartz dolomite pyrite arsenopyrite stage ii quartz dolomite native gold pyrite arsenopyrite stibnite stage iii and quartz dolomite pyrite stage iv cathodoluminescence cl observations show that each of stage ii and iii quartz comprises two sub stages stage ii qii1 and qii2 stage iii qiii1 and qiii2 and stage iv quartz qiv contains no sub stages qiii1 has distinct core rim texture with the cl intensity decreasing from core to rim qii1 qii2 qiii2 and qiv are texturally more homogeneous variations of aluminium concentration correlate with the cl intensity and reveal the determinated of ph in fluids during stage ii and iii with marked decline to stage iv fluid inclusions fis trapped in quartz are liquid rich and co2 rich suggesting a co2 h2o nacl fluid system fluid inclusion microthermometry data suggest that the ore forming fluids were under conditions of medium low salinity stage ii 1 02 11 46 wt nacl equiv stage iii 0 83 9 84 wt nacl equiv stage iv 4 39 8 10 wt nacl equiv and temperature stage ii 190 285 c stage ii 170 270 c stage ii and 163 240 c consistent with the low ti content

incremental growth of the patagonian torres del paine laccolith over 90 k y the miocene paine granite in the torres del paine intrusive complex southern chile is an extraordinary example of an upper crustal mafic and granitic intrusion the granite intruded as a series of three sheets each one underplating the previous sheet along the top of the basal paine mafic complex high precision u pb geochronology on single zircons using isotope dilution thermal ionization mass spectrometry yields distinct ages of 12 59 0 02 ma and 12 50 0 02 ma respectively for the first and last sheet of the laccolith this age relationship is consistent with field observations the zircon ages define a time frame of 90 40 k y for the emplacement of a 2000 m thick granite laccolith these precise u pb zircon ages permit identification of the pulses in a 20 k y range the data obtained for the paine granite fill the gap between 100 k y and 100 1000 yr pulses described in the literature for crustal magma chambers

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

refinement of the time space evolution of the giant mio pliocene rio blanco los bronces porphyry cu mo cluster central chile new u pb shrimp ii and re os geochronology and 40ar 39ar thermochronology data representing one of the largest known estimated 5 gt at 1 cu and 0 02 mo porphyry system the rio blanco los bronces deposit incorporates at least five hypabyssal intrusive and hydrothermal centres extending for about 5 km from the rio blanco and los bronces mines in the north through the don luis mine to the sur sur mine la americana and los sulfatos in the south the new geochronology data which now include data on different molybdenite vein types confirm the u pb ages of the pre mineralisation intrusions but slightly increase their age range from 8 8 to 8 2 ma the distinct magmatic pulses of the mineralisation associated porphyritic intrusives late porphyries indicate an age interval instead of the previously suggested individual ages the quartz monzonite porphyry ranges from 7 7 to 6 1 ma sur sur 5 74 0 13 ma the feldspar porphyry shows an interval from 5 8 to 5 2 ma and the don luis porphyry from 5 2 to 5 0 ma the new re os data on distinct molybdenite vein types confirm the protracted history of cu mo mineralisation inferred previously the vein development occurred at least from 5 94 to 4 50 ma indicating a time span of 1 5 ma for the hydrothermal activity hydrothermal minerals dated by the 40ar 39ar method are generally too young to record the age of early high temperature mineralisation the majority of the 40ar 39ar data in the rio blanco porphyry cluster record reheating by either the youngest member of the late porphyry suite or the post mineralisation dacite or rhyolite plug formations at around 4 9 4 7 ma

giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism cenozoic magmatic activity in northern chile led to the formation of two contrasting porphyry copper belts 1 a paleocene early eocene belt comprising small porphyry copper deposits e g lomas bayas of normal calc alkaline affinity and 2 a late eocene early oligocene belt hosting huge porphyry copper deposits e g chuquicamata of adakitic affinity although the first belt comprises both volcanic and plutonic rocks andesitic basaltic and rhyolitic lavas and tuffs and associated sub volcanic porphyries and felsic stocks the latter only includes intrusions mostly granodioritic types including porphyry copper deposits we suggest that the late eocene early oligocene belt formed when fast and oblique convergence between the south america and farallon plates led to flat subduction and direct melting of the subducting plate hence giving rise to plutonic rocks of adakitic affinity the absence of volcanism under prevailing compressional conditions prevented the escape of so2 from the adakitic sulfur rich highly oxidized magmas closed porphyry system which allowed formation of huge mineral deposits on the contrary coeval volcanic activity during formation of the paleocene early eocene calc alkaline porphyries allowed development of open systems hence to outgassing and therefore to small mineral deposits

geometry and brittle deformation of the subducting nazca plate central chile and argentina summary we use data from the chile argentina geophysical experiment charge broad band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the south american subduction zone between 30 s and 36 s this region contains a zone of flat slab subduction where the subducting nazca plate flattens at a depth of 100 km and extends 300 km eastward before continuing its descent into the mantle we use a grid search multiple event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the wadati benioff zone these contours reflect slab geometries from previous studies of intermediate depth seismicity in this region with some small but important deviations our hypocentres indicate that the shallowest portion of the flat slab is a

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

alteration mapping with short wavelength infrared swir spectroscopy on xiaokelehe porphyry cu mo deposit in the great xing an range ne china metallogenic and exploration implications abstract the newly discovered xiaokelehe porphyry cu mo deposit pcd is situated in northern part of the great xing an metallogenic belt northeastern china the syn mineralization granodiorite porphyry was intruded by multiple phases of post ore barren monzonite diorite porphyry and granite porphyry leading to complex and overprinting hydrothermal alteration patterns this paper present new data on the alteration mineralogy vein paragenesis sulfide assemblages and swir short wavelength infrared spectroscopy at xiaokelehe and discuss how swir spectral data can help to understand hydrothermal fluid evolution and assist pcd exploration at xiaokelehe oligoclase epidote alteration is distributed at the deeper levels of the granodiorite porphyry but played little role in mineralization the following potassic alteration hydrothermal biotite and k feldspar was developed in the granodiorite porphyry with the main cu mo mineralization drill core logging suggests that the subsequent chlorite illite alteration affected almost the entire granodiorite porphyry and is spatially and genetically associated with minor cu mineralization the youngest quartz muscovite pyrite alteration dominated in the upper level and outer parts of the pcd and overprints the earlier alteration phases with only very minor mineralization the short wavelength infrared swir analysis results indicate that chlorite and white micas illite and muscovite are the most abundant alteration minerals at xiaokelehe the wavelengths of fe oh band pos2250 of chlorite show a decreasing trend away from the granodiorite porphyry into the surrounding sedimentary wall rocks probably reflecting a drop in temperature and fe contents of the hydrothermal fluids the illite crystallinity ic values of white micas show a reverse trend different from many pcds which is likely caused by the distribution pattern of illite and muscovite formed in the ore proximal chlorite illite and ore distal quartz muscovite pyrite alteration respectively at xiaokelehe cu and mo mineralization is mainly distributed in quartz veins or disseminated within the potassic alteration zones resembling typical pcds meanwhile chlorite illite alteration is also associated with minor cu mineralization thus areas with intensive chlorite illite alteration overlapping on potassic alteration likely represent the best exploration targets for high grade cu mo mineralization these target areas coincide spatially with areas of lower ic 2245 nm values suggesting the latter two are effective vectoring tools for pcd exploration at xiaokelehe

the role of discontinuous magma inputs in felsic magma and ore generation for a long time granites have been considered as passive bodies ascending under intrinsic negative density and viscosity contrasts with their host rocks chemical variations within a granitic body resulted from in situ differentiation and crystal fractionation since the mid 1980s this global view has been significantly modified by i shifting melting from water saturated conditions to fluid absent reactions ii increasing the role played by the mantle during granite generation iii reassessing the rheology of partially molten rocks iv demonstrating stepwise segregation and ascent of magmas by analogue and numerical models v combining structural geophysical and geochemical studies to reveal the internal structures in granitic plutons it results that a granitic body is built up by a discontinuous accumulation of successive magma intrusions the discontinuous nature of magma emplacement has also significant consequences for its ability to generate ore the processes that lead to ore deposits are examined with a brief review of the magmatic and fluid phases that concentrate ore forming elements examples are taken from crustal derived granites and porphyry type deposits those are considered as the two end members of magmatic and hydrothermal ore deposits the source characteristics of the magma the emplacement mechanisms and magma mixing processes are the frame that controls the potential to carry base metals with the magma the distribution of elements is controlled by diffusion partition between minerals and melt solubility and redox conditions variations of those parameters are examined by considering their activation energy which controls the exponential dependence with temperature a characteristic length depending on the activation energy temperature variation and time is estimated for a characteristic time lag of 30 ka the intrusion of a magma into a magma chamber of similar composition hence temperature has few effects on diffusion partition coefficient and redox conditions because of a too low temperature contrast the intrusion of a mafic magma into a felsic one induces a variation of 300 c in both magmas the characteristic length of diffusion may vary by up to two orders of magnitude whereas the variation of partition coefficients is only one order of magnitude the redox conditions are about 2 5 log unit in the mafic magma but they can vary by 7 log units in the felsic magma hence a strong decrease in d values is observed in porphyry type deposits the effect is a removal of the elements with higher activation energy w sn zr from the mafic to the felsic magma deformation during the late stages of emplacement also controls ore formation

the altar porphyry cu au mo deposit argentina a complex magmatic hydrothermal system with evidence of recharge processes altar 31 29 s 70 28 w is a large porphyry cu au mo deposit with associated epithermal au ag cu veins located in the cordillera principal of southwest san juan province argentina altar is a complex magmatic hydrothermal system formed from several magmatic and hydrothermal pulses during the middle late miocene new la icpms u pb ages in zircons from the altar porphyries indicate four discrete events of intrusions over an extended magmatic life time of ca 3 m y it comprises a premineralization porphyry 11 75 0 24 ma three mineralized porphyries 11 62 0 21 and 11 68 0 27 ma 11 13 0 26 ma 10 35 0 32 ma related to hydrothermal breccias two postmineralization intrusions and a postmineralization breccia 8 9 0 4 ma the three mineralized porphyries porphyries 2 3 and 4 were emplaced within 0 7 to 1 3 m y amphibole phenocrysts from the porphyries crystallized from oxidized magmas f o2 nno 1 to 2 at temperatures of 780 to 850 c and pressures between 0 9 and 1 8 kbars corresponding to depths of 4 to 7 km anorthite and fe rich rims in the plagioclase phenocrysts suggest that the magmatic chambers were episodically recharged by a less evolved magma the middle late miocene intrusions are interpreted to have been derived from a deeper and relatively large magmatic reservoir that supplied magmas to smaller chambers located in the upper crust the focused magmatic output to shallow levels during a period of a few million years in the altar area has been a main requirement in the formation of this large porphyry copper deposit

apatite u th he thermochronology and re os ages in the altar region central andes 31 30 s main cordillera of san juan argentina implications of rapid exhumation in the porphyry cu au metal endowment and regional tectonics altar is a large porphyry cu au deposit located in the main cordillera of argentina 20 km to the north of the giant los pelambres el pachon porphyry copper cluster at the southern portion of the pampean flat slab segment of the andes although this region hosts telescoped porphyry epithermal deposits the precise temporal relationship between porphyry emplacement mineralization cooling and regional orogenic uplift are still poorly understood new re os molybdenite ages indicate that altar orebodies are associated with two magmatic hydrothermal centers altar east 11 16 0 06 ma and altar central 10 38 0 05 ma formed at temporally distinct periods new u th he ages from the early permian and late eocene plutons and the middle miocene subvolcanic stocks associated with cu au mineralization of the altar region reflect a rapid cooling pulse during the middle miocene 15 02 to 10 66 ma coeval with a major phase of tectonic shortening and regional uplift the main pulse of rapid cooling and related tectonic uplift in the altar region was synchronous with the formation of the hydrothermal systems and resulted in an increased focused metal endowment au cu grades due to the telescoping of epithermal mineralization over the rapidly uplifted porphyry system this 11 10 ma tectonically triggered exhumation event coincides with the collision of the e trending segment of the juan fernandez ridge with the peru chile trench at this latitude collision and ensuing ridge subduction may have driven a localized pulse of rapid cooling and exhumation of the main cordillera that has not been well documented to the north or south of the altar los pelambres region

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

the role of preexisting geologic architecture in the formation of giant porphyry related cu au deposits examples from new guinea and chile development of giant porphyry related copper and or gold deposits in new guinea and northern and central chile occurred during tertiary magmatic events that overprinted earlier extensional tectonic settings the earlier tectonic settings consisted of a mesozoic passive margin in new guinea a jurassic cretaceous back arc basin in northern chile and an oligocene intra arc basin in central chile the preexisting architecture of the basement rocks the fault systems and the stratigraphic packages associated with these settings played a strong role in controlling development of the giant deposits although the extensional tectonic settings in these terranes were different they share a number of elements that were used during formation of the tertiary ore deposits these were the presence of coupled systems of deeply detached listric synsedimentary faults and steep transverse faults and the presence of a relatively flat lying heterogeneous volcano sedimentary rock package during tertiary collision the deeply detached listric faults were inverted and strongly focused uplift exhumation and associated overpressuring induced failure and fluid flow in addition steep transverse faults were activated to form wrench systems with attendant steep deeply tapping pathways for magma and or fluid associated with dilatant jogs flexures or fault intersections ore deposits are commonly located in the hanging wall of the thrust faults under compression associated with collision the competent units of flat lying stratigraphic packages formed plates overlying the folded weaker units underneath and separated by a detachment or otherwise faulted contact examples of the competent upper units include the darai mendi limestone or equivalents in new guinea and the lavas of the farellones formation in central chile these plates appear to have impeded magma ascent and formed a cap on a magma and or fluid system pressurized by collision and orogenesis this geometry provided an ideal location for focusing of magmas and magmatic hydrothermal fluids from these three example terranes a set of exploration indicators have been constructed to highlight belts that are prospective for giant porphyry cu systems these indicators include 1 the presence of magmatic arcs that migrated into preexisting extensional architectures 2 coupled systems of deeply detached listric synsedimentary faults and steep transverse faults 3 rigid basement blocks that could form regional stress field perturbations and 4 fold belts with a major plate to 50 km wide of competent relatively undeformed or gently folded stratigraphy overlying more complexly folded and faulted parts of the sequence

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

sulphide magma as a source of metals in arc related magmatic hydrothermal ore fluids as magma ascends in subduction zones decreasing pressure and temperature cause it to separate into two or more phases in a process known as exsolution hydrothermal ore fluids and metal rich sulphide melts may be products of this exsolution conventionally the ore fluids are thought to acquire their metal content directly from the parent magma however it is probable that the ore fluids and sulphide melts interact after exsolution and therefore possible that the sulphide melts are an important source of metals for the ore fluids here we use petrographic analyses of erupted sulphide melts and compositional analyses of volcanic gases emitted immediately after an eruption at merapi volcano indonesia to show that magmatic hydrothermal fluids can derive their metals from dissolution of sulphide melts we find that the volcanic gases exhibit metal ratios that are comparable to those in the erupted sulphide melts furthermore we show that gases sampled immediately after the eruption have metal concentrations that are orders of magnitude higher than those of gases emitted during periods of quiescence we propose that the metal content of ores forming in volcanic arc environments is established by exsolution accompanying injections of primitive magma immediately before explosive eruptions the metal content of ore deposits formed during subduction zone volcanism was thought to be established when the ore fluid separates from the parent magma analyses of metal concentrations in erupted melts and the volcanic gases emitted after an eruption in indonesia reveal that metals can be added to the ore fluid later during mixing with separated melts

continental and oceanic crustal structure of the pampean flat slab region western argentina using receiver function analysis new high resolution results summary the pampean flat slab of central chile and argentina 30 32 s has strongly influenced cenozoic tectonics in western argentina which contains both the thick skinned basement cored uplifts of the sierras pampeanas and the thin skinned andean precordillera fold and thrust belt in this region of south america the nazca plate is subducting nearly horizontally beneath the south american plate at 100 km depth to gain a better understanding of the deeper structure of this region including the transition from flat to normal subduction to the south three iris passcal arrays of broad band seismic stations have been deployed in central argentina using the dense siembra array combined with the broader charge and esp arrays the flat slab is imaged for the first time in 3 d detail using receiver function rf analysis a distinct pair of rf arrivals consisting of a negative pulse that marks the top of the oceanic crust followed by a positive pulse which indicates the base of the oceanic crust can be used to map the slab s structure depths to moho and oceanic crustal thicknesses estimated from rf results provide new more detailed regional maps an improved depth to continental moho map shows depths of more than 70 km in the main cordillera and 50 km in the western sierras pampeanas that shallow to 35 km in the eastern sierras pampeanas depth to moho contours roughly follow terrane boundaries offshore the hotspot seamount chain of the juan fernandez ridge jfr is thought to create overthickened oceanic crust providing a mechanism for flat slab subduction by comparing synthetic rfs based on various structures to the observed rf signal we determine that the thickness of the oceanic crust at the top of the slab averages at least 13 19 km supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat the overthickened region is broader than the area directly aligned with the path of the jfr however and indicates along with the slab earthquake locations that the flat slab area is wider than the jfr volcanic chain observed in the offshore bathymetry further rfs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench parallel faults one explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted alternatively it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region

petrogenetic and metallogenetic responses to miocene slab flattening new constraints from the el indio pascua au ag cu belt chile argentina the eocene miocene volcanic and hypabyssal rocks of the el indio pascua au ag cu belt in the southern central andean flat slab region are medium high k calc alkaline arc suites ranging in composition from andesite to rhyolite a significant transition in magmatic trace element chemistry coinciding with a pronounced reduction in magma output occurred in the late middle miocene as documented by 40ar 39ar geochronology the upper eocene lower middle miocene rocks exhibit low sr y ratios 50 minor heavy ree fractionation with sm yb ratios not exceeding 3 5 and in some cases minor negative eu anomalies in contrast the largely dacitic rocks erupted after ca 13 ma are depleted in y 10 ppm have generally high but variable sr y ratios 30 200 exhibit moderate middle and heavy ree fractionation sm yb 3 7 5 9 and lack negative eu anomalies the latter features are characteristic of adakitic suites i e slab melts but the regional temporal and spatial distribution of arc magmatism precludes a major magma source in the downgoing slab this evolution is interpreted as reflecting a progressive increase in pressure and the availability of water in the lower crustal site of magma generation establishing both garnet and hornblende as major stable phases in the residuum the pressure in the lower crust increased in response to episodic crustal thickening related to the shallowing of the slab a process recorded by the incision of three regional pediplains over the period 17 6 ma elimination of the subarc asthenospheric mantle and much of the lithospheric mantle by ca 10 ma permitted direct incursion of slab derived highly oxidised metal and volatile rich supercritical fluids into the lower crust stimulating melting of mafic garnet amphibolitic and eclogitic assemblages

adakite like signature of late miocene intrusions at the los pelambres giant porphyry copper deposit in the andes of central chile metallogenic implications adakite like features are recognized in the late miocene 10 ma porphyritic intrusions of the los pelambres giant porphyry copper deposit central chile 32 s located within the southern portion of the flat slab segment 28 33 s of the chilean andes the al and na rich porphyries of los pelambres display distinctly higher sr y 100 300 and lan ybn 25 60 ratios than contemporaneous and barren magmatic units e g la gloria pluton cerro aconcagua volcanic rocks of the same andean magmatic belt strong fractionation of heavy rare earth elements hree absence of eu anomalies high sr y and zr sm and low nb ta ratios suggest melt extraction from a garnet amphibolite source the late miocene adakite like porphyritic intrusions at los pelambres formed closely related in time and space to the subduction of the juan fernandez ridge jfr hotspot chain along the chilean margin current tectonic reconstructions reveal that at the time of formation of the los pelambres rocks a w e segment of the jfr started to subduct beneath them producing a slow down of a previously rapid southward migration of a ne ridge trench collision these particular tectonic conditions are favorable for the origin of the los pelambres porphyry suite by melting of subducting young hotspot rocks under flat slab conditions the incorporation of crustal components into the oceanic lithopheric magma source by subduction erosion is evidenced by the sr nd isotope composition of the los pelambres rocks different from the morb signatures of true adakites a close relationship apparently exists between the origin of this adakite like magmatism and the source of the mineralization in the los pelambres porphyry copper deposit

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