u pb dating of zircon and 40ar 39ar dating of biotite at bingham utah uranium lead dating of zircon and 40ar 39ar dating of biotite provide a new estimate of the age of the bingham hydrothermal system the u pb age of zircon from the last chance monzonite establishes the age of crystallization of this intrusion at 38 55 0 19 ma 2 mswd 2 08 the oldest hydrothermal event in the monzonite is dated at 38 40 0 16 ma and is based on a 40ar 39ar determination plateau age of primary igneous biotite this age is indistinguishable from the u pb zircon age of emplacement of the monzonite petrographic and fluid inclusion data show that the biotite bearing sample was propylitically altered at 400 c which would reset the 40ar 39ar age within 115 ka thus the concordance of the biotite age with the intrusion age of the monzonite suggests that propylitic alteration was essentially contemporaneous with monzonite emplacement the 40ar 39ar ages from three hydrothermal biotites sampled within the 0 35 percent cu isopleth in the monzonite overlap within uncertainty and indicate that potassic alteration in monzonite occurred at 37 57 0 11 ma the youngest hydrothermal activity is identified from a biotite sample 37 07 0 21 ma correlation age in potassically altered quartz monzonite porphyry within the central mos2 zone hydrothermal biotite within the bingham deposit was produced over a time period of approximately 0 50 0 32 ma

early cretaceous bimodal volcanism in the duolong cu mining district western tibet record of slab breakoff that triggered ca 108 113 ma magmatism in the western qiangtang terrane abstract we report new zircon u pb ages and hf isotope compositions and whole rock major and trace element and sr nd isotope data for the meiriqiecuo formation mf bimodal volcanic rocks collected from the duolong cu mining district dcmd in the western qiangtang terrane qt western tibet these data provide important constraints on the petrogenetic evolution and geodynamic setting of early cretaceous magmatism in the dcmd the mf bimodal volcanic rocks are mainly basaltic andesite and andesite with subordinate rhyolite four mafic samples yielded zircon u pb ages of ca 108 2 113 0 ma and one silicic sample has an age of 109 3 2 2 ma indicating that the mafic and silicic eruptions were contemporaneous the mf bimodal volcanic rocks belong to the medium k calc alkaline to shoshonite series the rocks show arc type affinities characterized by significant enrichment in light rare earth lan ybn 7 74 12 60 and large ion lithophile elements rb cs k and pb but depletions in the high field strength elements nb ta and ti which geochemically resemble andean arc basalts therefore the mf bimodal volcanic rocks were likely emplaced at an andean type active continental margin and represent an early cretaceous magmatic arc that was located at the western qt margin moreover the mafic volcanic rocks have high initial sr isotopic ratios 0 705269 0 705413 and negative end t values of 1 5 to 0 6 compared with the silicic volcanic rocks 87sr 86sr i 0 704770 0 704903 end t 1 2 to 1 3 zircons from silicic samples have significantly higher ehf t values 11 6 to 15 5 and predominantly lower paleoproterozoic hf crustal model ages tdmc 180 428 ma than the mafic samples which have variable ehf t values of 3 4 to 13 0 and tdmc ages of 346 952 ma these results indicate that the mafic and silicic end members of the mf bimodal suite were generated from mantle and crustal sources respectively the basaltic andesite and andesite may have been derived from mantle enriched by the metasomatism of subducted fluids whereas the rhyolite could have been derived by partial melting of mafic juvenile crust that originated from an older and more depleted mantle in light of the geochemical characteristics and field relationships we propose that breakoff of the bangong nujiang oceanic lithosphere was responsible for the generation and emplacement of the mf bimodal volcanic rocks the fact that the mf bimodal volcanic arc magmatism was active at ca 108 113 ma indicates that it was associated with closure of the bangong nujiang ocean via an arc arc soft collision during the early cretaceous

the timing of magmatism and ore formation in the el abra porphyry copper deposit northern chile implications for long lived multiple event magmatic hydrothermal porphyry systems zircon la icp ms and ca tims and molybdenite re os geochronology for the el abra porphyry copper deposit chile document a 8 6 ma protracted magmatic hydrothermal history initial magmatic events occurred at 45 ma and continued to 36 ma an 1 8 m y magmatic lull is recorded in the early stages of magmatism which is characterized by equigranular rocks from the pajonal suite magmatism from 41 to 36 ma corresponding to the el abra granodiorite complex is more recurrent ending in a 1 4 m y period of porphyritic intrusions which are coeval with cu mo mineralization late porphyritic units reveal subtle zircon inheritance likely associated with early magmatic pulses the young stages of magmatism indicate system rejuvenation based on plagioclase phenocryst compositions suggesting that the porphyry system is ultimately linked to a less evolved magma although there are multiple porphyry and veining events molybdenite re os ages define a focused mineralization episode from 36 34 to 36 18 ma molybdenite re os ages from nearby related prospects belonging to the el abra cluster expand the mineralization period in the district up to 1 0 m y suggesting a multistage long lived hydrothermal system integration of the molybdenite re os dates and those from previous works at the el abra and toki clusters accompanied by available tectonic reconstruction along the domeyko fault system suggest the development by late eocene times of a single large 30 km across porphyry copper cluster which was subsequently separated by an offset of 35 km along the west fault

exhumation and preservation of porphyry cu au deposits at oyu tolgoi south gobi region mongolia field relationships and u pb data zircon are used to constrain the timing of tectonomagmatic events that occurred shortly after the formation of late devonian 372 ma porphyry cu au deposits at oyu tolgoi mongolia in the two northernmost of the several discrete porphyry centers hugo dummett deposits upper devonian rocks positioned stratigraphically above copper gold mineralization preserve a record of uplift exhumation and subsequent burial shortly after mineralization strongly altered and mineralized quartz monzodiorite and volcanic rocks at hugo dummett are overlain by an unmineralized dacite sequence 369 2 ma the basal breccia and conglomerate of the dacite sequence contain mineralized porphyry clasts derived from erosion of the porphyry deposits two zircon grains recovered from one of these mineralized porphyry clasts yielded chemical abrasion thermal ionization mass spectrometry ca tims 206 pb 238 u dates of 374 0 1 9 and 371 4 1 6 ma which are consistent with previously published ages for mineralized intrusions this implies that oyu tolgoi or other nearby porphyry deposits were exhumed shortly after the main mineralization event weakly mineralized granodiorite dikes intruded the postmineralization dacite sequence as well as devonian sedimentary and volcaniclastic rocks that structurally overlie the dacite a weighted mean 206 pb 238 u age of 370 6 0 5 ma for this rock is based on the three youngest grains analyzed by the ca tims technique mswd 1 14 burial of the cu au deposits at oyu tolgoi soon after their exhumation was critical to their preservation exhumation and subsequent burial occurred at 371 ma during a short time frame within the overlapping u pb zircon ages for the quartz monzodiorite mineralized porphyry clast unmineralized dacite sequence and granodiorite unit

petrogenesis of ore bearing porphyries from the duolong porphyry cu au deposit central tibet evidence from u pb geochronology petrochemistry and sr nd hf o isotope characteristics abstract the duolong porphyry cu au deposit 5 4 mt at 0 72 cu 41 t at 0 23 g t au which is related to the granodiorite porphyry and the quartz diorite porphyry from the bangongco copper belt in central tibet formed in a continental arc setting here we present the zircon u pb ages geochemical whole rock sr nd whole rock and zircon in situ hf o isotopic data for the duolong porphyries secondary ion mass spectrometry sims zircon u pb analyses for six samples yielded consistent ages of 118 ma indicating a cretaceous formation age the duolong porphyries sio2 of 58 81 68 81 wt k2o of 2 90 5 17 wt belong to the high k calc alkaline series they show light rare earth element lree enriched distribution patterns with la yb n 6 1 11 7 enrichment in large ion lithophile elements e g cs rb and ba and depletion of high field strength elements e g nb with negative ti anomalies all zircons from the duolong porphyries share relatively similar hf o isotopic compositions 18o 5 88 7 27 ehf t 3 6 7 3 indicating that they crystallized from a series of cogenetic melts with various degrees of fractional crystallization this along with the general absence of older inherited zircons rules out significant crustal contamination during zircon growth the zircons are mostly enriched in 18o relative to mantle values indicating the involvement of an 18o enriched crustal source in the generation of the duolong porphyries together with the presence of syn mineralization basaltic andesite the mixing between silicic melts derived from the lower crust and evolved h2o rich mafic melts derived from the metasomatized mantle wedge followed by subsequent fractional crystallization fc and minor crustal contamination in the shallow crust could well explain the petrogenesis of the duolong porphyries significantly the hybrid melts possibly inherited the arc magma characteristics of abundant f cl cu and au elements and high oxidation state which contributed to the formation of the duolong porphyry cu au deposit

porphyry cu mo au deposits related to melting of thickened mafic lower crust examples from the eastern tethyan metallogenic domain abstract most porphyry cu deposits in the world occur in magmatic arc settings and are formed in association with calc alkaline arc magmas related to subduction of oceanic lithosphere this contribution reviews a number of significant porphyry cu deposits in the eastern tethyan metallogenic domain they widely occur in a variety of non arc settings varying from post late collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes their spatial temporal localization is controlled by strike slip faults orogen transverse normal faults lineaments and their intersections in these non arc settings these deposits are dominated by porphyry cu mo deposits with minor porphyry cu au and epithermal au deposits and exhibit a broad similarity with those in magmatic arcs the associated magmas are generally hydrous relatively high fo2 high k calc alkaline and shoshonitic and show geochemical affinity with adakites they are distinguished from arc magmas and or oceanic slab derived adakites by their occurrence as isolated complexes high k2o contents 1 2 8 5 and much wider range of end t values 10 to 3 and positive ehf t values 4 6 to 6 9 these potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower crust or delaminated lower crust but also involving various amounts of asthenospheric mantle components key factors that generate hydrous fertile magmas are most likely crust mantle interaction processes at the base of thickened lower crust in non arc settings rather than oceanic slab dehydration as in arc settings breakdown of amphibole in thickened lower crust e g amphibole eclogite and garnet amphibolite during melting is considered to release fluids into the fertile magmas leading to an elevated oxidation state and higher h2o content necessary for development of porphyry cu mo au systems copper and au in hydrous magmas are likely derived from mantle derived components and or melts which either previously underplated and infiltrated at the base of the thickened lower crust or were input into the primitive magmas by melt mantle interaction in contrast mo and part of the s in the fertile magmas are probably supplied by old crust during melting and subsequent ascent

geology and geochronology of naruo large porphyry breccia cu deposit in the duolong district tibet abstract the duolong district is located in the south qiangtang terrane of tibet and is the most significant ore cluster within the bangongco nujiang metallogenic belt duolong contains one giant three large and two medium to small sized porphyry epithermal breccia copper deposits and several other mineralized porphyry bodies all deposits are closely associated with early cretaceous 123 115 ma intermediate felsic intrusions naruo is a poorly studied porphyry breccia copper deposit in the north of the duolong district hydrothermal alteration surrounding the ore bearing granodiorite at naruo is characterized by an inner potassic zone and an outer propylitic zone overlapped locally by minor phyllic and argillic alteration assemblages a detailed paragenetic study has identified five distinct hydrothermal veins m a b c d within the porphyry system hydrothermal b veins are strongly related to copper mineralization strong propylitic alteration is also observed throughout the hydrothermal breccias identified at naruo sandstone breccia diorite bearing breccia and granodiorite bearing breccia were identified according to the distribution and composition of clasts u pb zircon dating has determined the ages of the ore bearing granodiorite 121 6 1 3 ma and a barren intrusion 115 5 1 1 ma within the porphyry system diorite clasts 122 3 0 9 ma and later diorite matrix 120 5 1 0 ma in the hydrothermal breccia system suggesting that with the exception of the late barren intrusion they all belong to the same mineralizing event at duolong the geological and geochemical evidence presented in this study suggest that the porphyry and breccia systems may have originated from the same magma source but are now spatially independent

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

cretaceous magmatism and metallogeny in the bangong nujiang metallogenic belt central tibet evidence from petrogeochemistry zircon u pb ages and hf o isotopic compositions abstract porphyry cu au and porphyry skarn cu au mo deposits are widely distributed in the bangong nujiang metallogenic belt central tibet zircon u pb dating has revealed that cretaceous ore bearing intrusions related to cu au mo mineralization formed in two periods 118 115 ma and 90 88 ma these primarily high k calc alkaline series intrusions show light rare earth element enrichment lan ybn 6 06 20 2 and negative to no eu anomaly eu eu 0 39 1 02 on primitive mantle normalized diagrams all the intrusions show strong enrichments in large ion lithophile elements e g cs rb and k depletions in nb ta and ti and negative ba anomalies characteristic of arc magma these intrusions show a wide range of zircon ehf t values from 6 3 to 10 9 and 18o values from 5 82 to 10 82 suggesting variable contributions from mantle and crustal sources considering the 100 ma lhasa qiangtang collision the 118 115 ma magmas and related deposits were likely formed by melting of subduction metasomatized mantle wedge in a continental arc setting during northward subduction of the bangong nujiang ocean and then further evolved in the upper crust as a result of mash processes the younger 90 88 ma ore bearing magmas were potentially derived from melting of previously metasomatized lithospheric mantle during slab tear and break off after the lhasa qiangtang collision

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

contemporaneous formation of adjacent porphyry and epithermal cu au deposits over 300 ka in northern luzon philippines there is commonly a close spatial relation between porphyry cu au and high sulfidation epithermal cu au deposits throughout the world although a genetic association has not been proven nowhere is this spatial association better seen than in northern luzon philippines where the lepanto epithermal cu au deposit overlies the far southeast fse porphyry cu au deposit both world class orebodies fresh rock and hydrothermal mineral separates yield k ar ages indicating that premineralization and postmineralization volcanism occurred at 2 2 1 8 ma and 1 2 0 9 ma respectively and that the hydrothermal system was active from 1 5 to 1 2 ma k ar ages of alunite from lepanto have the same range as those of hydrothermal biotite and illite from the fse deposit indicating that both epithermal and porphyry mineralization formed from an evolving magmatic hydrothermal system that was active for about 300 ka this temporal relation strengthens the argument for a genetic link between these two styles of ore deposit and has implications for exploration where one style of mineralization is found there is potential for the other nearby

origin and tectonic setting of the giant duolong cu au deposit south qiangtang terrane tibet evidence from geochronology and geochemistry of early cretaceous intrusive rocks abstract we constrain the origin and tectonic setting of the giant duolong porphyry epithermal cu au deposit in the south qiangtang terrane of northern tibet based on new zircon u pb ages and hf isotopic data as well as whole rock major and trace element data from poorly studied ore associated intrusions in the duolong area the la icp ms zircon u pb dating indicates that the ore associated rocks formed between 121 and 126 ma these ore associated rocks are geochemically similar to low k tholeiitic m type granitoids and to mid to high k calc alkaline i type granitoids they have variable and predominantly positive zircon e hf t values 1 4 to 15 6 and variable crustal model ages t c dm hf 176 1122 ma taking into account previous data and the regional geology of the study area we propose that the ore associated rocks originated from fractional crystallization of mantle derived mafic melts and magma mixing of mantle derived mafic and hybrid lower crust derived felsic melts and the hybrid lower crust included a mix of juvenile and older continental material the duolong porphyry epithermal cu au deposit formed within an ensialic forearc of an active continental margin as a result of the northwards subduction of the bangong nujiang ocean crust beneath the south qiangtang terrane

geology and genesis of the cerro la mina porphyry high sulfidation au cu mo prospect mexico the cerro la mina au cu mo porphyry high sulfidation prospect is located in chiapas state southeastern mexico outside of the major metallogenic provinces of mexico the prospect is hosted by pleistocene alkaline volcanic rocks of the chiapanecan volcanic arc that formed in a complex triple junction tectonic setting cerro la mina s stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1 04 0 04 ma u pb zircon and that were overlain by debris flows and synvolcanic trachyandesite domes the volcanic stratigraphy of cerro la mina is dominated by pyroclastic flows and rare basalts that are cut by the cerro la mina breccia pipe a matrix rich granular vertically oriented downward tapering polymict lithic rock unit that is host to all of the significant alteration and mineralization a nw trending sinistral wrench fault which was active throughout the history of cerro la mina is responsible for dismembering the prospect after mineralization the magmatic hydrothermal system was composed of early porphyry style potassic veins quartz k feldspar biotite and stage 1 pyrite that are preserved in clasts within the breccia pipe suggesting that brecciation disrupted an embryonic porphyry system late potassic alteration occurred after the formation of the breccia pipe as its matrix is strongly k feldspar altered hydrothermal fluids then produced phyllic alteration composed of quartz muscovite illite illite smectite and chlorite that is associated with stage 2 pyrite chalcopyrite molybdenite quartz veins an unusual zoned pattern of advanced argillic argillic alteration overprinted potassic and phyllic alteration this zoning included a low temperature 120 c quartz dickite kaolinite pyrophyllite alunite that occurs from 250 m to the present day surface the advanced argillic argillic altered rocks host the most significant au cu mineralization which is associated with stage 3 marcasite sphalerite galena and barite and stage 4 arsenian pyrite enargite covellite the magmatic hydrothermal system at cerro la mina began sometime between monzodiorite emplacement 1 04 0 04 ma zircon u pb and the precipitation of porphyry stage 2 molybdenite at 780 10 ka re os 40ar 39ar dating of biotite 689 13 ka records the age at which the hydrothermal system cooled below the biotite closure temperature of 300 c and provides a maximum estimate for the onset of advanced argillic argillic alteration sulfur isotope results of sulfides 2 5 to 4 9 mean 0 7 n 20 and a sulfate barite 10 5 n 1 suggest a magmatic source of sulfur for all four stages of mineralization the lack of residual quartz rare alunite and anomalous halloysite kaolinite alteration may be explained by the high acid buffering capacity of alkaline volcanic host rocks high co2 contents of the alkaline magma and or potentially by a highly reduced magmatic hydrothermal fluid at the regional metallogenic scale the cerro la mina prospect along with the nearby santa fe mine and campamento deposit represent parts of a porphyry copper system specifically a porphyry high sulfidation proximal skarn and intermediate sulfidation deposit respectively the characteristics of cerro la mina i e anomalous halloysite kaolinite alteration broaden the window for additional discoveries to be made in the porphyry epithermal environment

mesozoic cenozoic evolution of the xining minhe and dangchang basins northeastern tibetan plateau magnetostratigraphic and biostratigraphic results 1 accurate stratigraphic ages are crucial to understanding the deformation history of the tibetan plateau prior to and during the indo asian collision efforts to quantify mesozoiccenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata magnetostratigraphic and biostratigraphic results from the xining minhe longzhong basin complex and dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern tibetan plateau 34 37 n 101 105 e analyses of 171 magnetostratigraphic levels and 24 palynological assemblages 120 species indicate late jurassic early cretaceous to mid tertiary deposition although magnetic polarity zonation is incomplete independent palynological age control partially restricts possible correlations to the geomagnetic polarity timescale the sediment accumulation record basin provenance structural geology and published thermochronological data support a history of jurassic exhumation late jurassic early cretaceous fault related basin initiation and cretaceous paleogene reduced accumulation these patterns which are compatible with late jurassic early cretaceous extension and cretaceous paleogene postrift thermal subsidence were disrupted at about 40 30 ma when shortening related to the indo asian collision induced localized range uplift vertical axis rotation and amplified subsidence indexterms 1520 geomagnetism and paleomagnetism magnetostratigraphy 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic 9609 information related to geologic time mesozoic keywords tectonics magnetostratigraphy tibetan plateau cenozoic mesozoic sedimentary basins

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

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

time scales of porphyry cu deposit formation insights from titanium diffusion in quartz porphyry dikes and hydrothermal veins from the porphyry cu mo deposit at butte montana contain multiple generations of quartz that are distinct in scanning electron microscope cathodoluminescence sem cl images and in ti concentrations a comparison of microprobe trace element profiles and maps to sem cl images shows that the concentration of ti in quartz correlates positively with cl brightness but al k and fe do not after calibrating cl brightness in relation to ti concentration we use the brightness gradient between different quartz generations as a proxy for ti gradients that we model to determine time scales of quartz formation and cooling model results indicate that time scales of porphyry magma residence are 1 000s of years and time scales from porphyry quartz phenocryst rim formation to porphyry dike injection and cooling are 10s of years time scales for the formation and cooling of various generations of hydrothermal vein quartz range from 10s to 10 000s of years these time scales are considerably shorter than the 0 6 m y overall time frame for each porphyry style mineralization pulse determined from isotopic studies at butte montana simple heat conduction models provide a temporal reference point to compare chemical diffusion time scales and we find that they support short dike and vein formation time scales we interpret these relatively short time scales to indicate that the butte porphyry deposit formed by short lived episodes of hydrofracturing dike injection and vein formation each with discrete thermal pulses which repeated over the 3 m y generation of the deposit

the role of exhumation in the temporal distribution of ore deposits we show here that the temporal age frequency distribution of ore deposits is controlled largely by exhumation the combination of burial uplift and erosion that exposes subsurface rocks and ore deposits exhumation rates calculated from modal ages and emplacement depths for epithermal porphyry copper and orogenic gold deposits of phanerozoic age are 167 158 and 63 m m y respectively these rates agree with rates estimated for continent scale terranes by numerous independent methods thus the scarcity of precambrian epithermal and porphyry copper deposits is probably due to their removal by exhumation archean and pale oproterozoic orogenic gold deposits are more abundant than indicated by the exhumation record of their phanerozoic counterparts suggesting that the precambrian deposits formed in greater numbers recognition of the pivotal role of exhumation requires that the temporal distribution of ore deposits be corrected for this effect before it can be used as an indicator of large scale changes in earth history

magmatic hydrothermal evolution of the cretaceous duolong gold rich porphyry copper deposit in the bangongco metallogenic belt tibet evidence from u pb and 40ar 39ar geochronology abstract the duolong gold rich porphyry copper deposit was recently discovered and represents a giant prospect inferred resources of 4 5 mt fine cu with a grade of 0 72 cu 30 50 t fine gold with a grade of 0 23 g t au in the bangongco metallogenic belt tibet zircon shrimp and la icp ms u pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes the first at about 121 ma bolong mineralized granodiorite porphyry bmgp and barren granodiorite porphyry bgp and the second about 116 ma duobuza mineralized granodiorite porphyry dmgp moreover the basaltic andesites also have two episodes at about 118 ma and 106 ma respectively one andesite yields an u pb zircon age of 111 9 1 9 ma indicating it formed after the multiple granodiorite porphyries by contrast the 40ar 39ar age of 115 2 1 1 ma hydrothermal k feldspar vein hosted in dmgp reveals the close temporal relationship of ore bearing potassic alteration to the emplacement of the dmgp the sericite from quartz sericite vein hosted in dmgp yields a 40ar 39ar age of 115 2 1 2 ma therefore the ore forming magmatic hydrothermal evolution probably persisted for 6 m y additionally the zircon u pb ages 106 121 ma of the volcanic rocks and the porphyries suggest that the neo tethys ocean was still subducting northward during the early cretaceous

patterns of tectonic stress in sicily from borehole breakout observations and finite element modeling the orientation of in situ tectonic stress was deduced from borehole breakout analysis of 22 wells from onshore sicily the results allow us to distinguish the stress field of different geological units 1 a nearly nnw 148 orientation is detected in the hyblean plateau 2 a nne shmax direction characterizes the gela area to the north within the thrust belt around mount judica the shmax direction swings to ne in the northeastern segment of the foredeep the catania plain the direction of shmax is roughly parallel to the ne trending grabens that mark the northern margin of the hyblean plateau 3 the southwestern segment of the foredeep has no preferential shmax orientation and may act as a transition zone with isotropic horizontal stresses 4 in western sicily a shmax orientation from n s to nw se is observed this fits well the kinematics of the se migrating egadi and adventure thrust belts and the direction of shortening inferred from the modern seismicity of the area a three dimensional finite element modeling including the most important tectonic features was performed modeling results of the stress field indicate that the nnw trending shmax is locally influenced by the variation of crustal thickness and local zones of weakness in addition the pantelleria rift causes a rotation of the regional nw se stress orientation to nne along the northern rim of the rift system including large on shore areas in the adjacent central south sicily

late pliocene high sulfidation epithermal gold mineralization at the la bodega and la mascota deposits northeastern cordillera of colombia la bodega and la mascota are plio pleistocene high to intermediate sulfidation epithermal gold deposits hosted in proterozoic gneisses and mesozoic granites of the santander massif eastern cordillera of colombia and are part of the california vetas mining district coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10 1 ma are the only igneous rocks younger than jurassic known from the vicinity of the epithermal deposits as of 2010 la bodega and la mascota contained an inferred resource of 3 47 moz au 19 2 moz ag and 84 4 mlbs cu at a 2 g t au cut off mineralization exhibits ne trending nw dipping structural control associated with the right lateral strike slip la baja fault mineralization at la bodega is composed of vein networks and tectonic hydrothermal breccias while at la mascota it is largely contained in tabular fault controlled tectonic hydrothermal breccia bodies alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry style phases and a late epithermal style phase with up to four distinct stages porphyry phases comprise stages 1 and 2 stage 1 is characterized by propylitic alteration with epidote chlorite calcite specularite veins minor pyrite and chalcopyrite molybdenite from veins collected 0 7 km southwest of la mascota yielded an re os age of 10 14 ma which constrains the age of stage 1 stage 2 is related to a separate younger magmatic hydrothermal system and characterized by phyllic alteration sericite illite quartz pyrite associated with quartz pyrite veins and its age is constrained by 40 ar 39 ar geochronology on hydrothermal muscovite i e sericite to 3 4 ma the epithermal phase comprises stages 3 through 6 and is related to multiphase hydrothermal breccia development and quartz alunite alteration stage 3 is defined by presence of copper sulfides such as bornite covellite and chalcopyrite stage 4 by wolframite stage 5 by enargite and stage 6 by sphalerite pyrite quartz and alunite are common to all epithermal stages residual vuggy quartz is scarce gold silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2 alunite 40 ar 39 ar ages constrain epithermal mineralization to between 2 6 and 1 6 ma primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vapor rich inclusions and together with quartz vein textures suggest that boiling was an important ore precipitating mechanism homogenization temperatures of these range from 143 and 238 c and salinities from 0 5 to 5 6 wt nacl equiv pyrite 34 s ranges from 16 9 to 11 3 at la mascota and 8 3 to 6 1 at la bodega the strongly negative sulfur isotope signature can be explained by fluid boiling or alternatively by biogenic sulfur source from the proterozoic paragneisses based on 18 o and d data alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water

a genetic linkage between subduction and collision related porphyry cu deposits in continental collision zones the genesis of continental collision related porphyry cu deposits pcds remains controversial the most common hypothesis links their genesis with magmas derived from subduction modified arc lithosphere however it is unclear whether a genetic linkage exists between collision and subduction related pcds here we studied jurassic subduction related cu au and miocene collision related cu mo porphyry deposits in south tibet the jurassic pcds occur only in the western segment of the jurassic arc which has depleted mantle like isotopic compositions e g 87 sr 86 sr i 0 7041 0 7048 e nd t as high as 7 5 and e hf t as high as 18 by contrast no jurassic pcds have been found in the eastern arc segment which is isotopically less juvenile e g 87 sr 86 sr i 0 7041 0 7063 e nd t 4 5 and e hf t 12 these results imply that incorporation of crustal components during underplating of jurassic magma induced copper accumulation as sulfides at the base of the eastern jurassic arc inhibiting pcd formation at this time miocene pcds are spatially confined to the jurassic arc and the giant miocene pcds cluster in its eastern segment where no jurassic pcds occur this suggests that the arc segment barren for subduction related pcds could be fertile for collision related pcds miocene ore forming porphyries have young hf model ages and sr nd hf isotopic compositions overlapping with those of the jurassic rocks in the eastern segment whereas contemporaneous barren porphyries outside the jurassic arc have abundant zircon inheritance and crustlike sr nd hf isotopic compositions these data suggest that remelting of the lower crustal sulfide bearing cu rich jurassic cumulates triggered by cenozoic crustal thickening and or subsequent slab break off led to the formation of giant miocene pcds the spatial overlap and complementary metal endowment between subduction and collision related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts

how accurately can we date the duration of magmatic hydrothermal events in porphyry systems an invited paper determining the absolute duration of magmatic hydrothermal events leading to the formation of porphyry systems i e including porphyry copper skarn and epithermal deposits is one of the key questions in ore geology this is so because the duration of magmatic hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy the problem of determining the absolute duration of magmatic hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo annum ka isotopic dating in contrast is the only possible way to determine the life span of magmatic hydrothermal events in fossil porphyry systems analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating u pb 40ar 39ar re os allow us to date minerals with internal precisions 0 2 2 for a 10 ma old mineral this corresponds to a 20 ka uncertainty which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo annum scale however many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events which extend the overall life of the porphyry systems to a few 0 x and up to 2 ma in some cases internal precisions of the above mentioned dating methods allow us in theory to comfortably discriminate events at the 0 x scale and the combination of u pb 40ar 39ar and reos geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic hydrothermal events in porphyry systems in this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems we show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated we also provide basic guidelines to follow when trying to resolve the duration of magmatic hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

stress and the direction of slip on fault planes the shear stress direction on a fault plane depends only on four of the six components of the stress tensor assuming only that the slip direction marks the shear stress direction on any fault plane and that stress is homogeneous it is possible to estimate these four stress parameters from populations of fault planes with known slip directions as several workers have observed different formulations of the problem may yield varying best fitting stresses and estimates of uncertainty in the simplest case no assumptions are made regarding the orientations of fault planes relative to the stress tensor thus the technique allows for the possibility that the fault planes may be very weak here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four parameter stress model which can be used to illustrate the significance of various inverse strategies in particular this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model 1 the pole rotation the angle between the observed and predicted slip direction on the observed fault plane and 2 the minimum rotation the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model by allowing for variation of the fault plane as well as the slip vector the minimum rotation procedure generally achieves a more stable and presumably realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure in a test case using 17 earthquake focal mechanisms from the yuii region of eastern taiwan separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates additional constraints on the stress tensor such as the effect of friction can be superimposed on the ones used here

stress directions and shear wave anisotropy observations from local earthquakes in southeastern sicily italy the spatial distribution of 414 earthquakes 1 0 m l 4 6 recorded from 1994 to 2002 in southeastern sicily italy has been analyzed the seismicity generally coincides with mapped plio quaternary faults including the north north west south southeast striking offshore fault system which is the most important tectonic structure of the area for the best located events we computed 70 focal mechanisms by combining p wave polarities with s wave polarizations a predom inance of strike slip and normal faults was observed focal mechanisms were then inverted for stress tensor parameters by using the algorithm of gephart and forsyth the results highlighted a region governed mainly by a north northwest south south east to northwest southeast compressional stress regime moreover anisotropy anal ysis of shear waves showed a polarization of fast s waves coherently aligned with this stress direction a finer scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis r1 the eastern sector displays a nearly horizontal r1 trending northwest southeast the central sector is affected by a low dip north northwest south southeast r1 whereas in the western sector a north northwest south southeast oriented r1 with a higher dip angle was detected finally the comparison of the spatial distribution of seismicity occurring during 1994 2002 with locations of previous instrumental earthquakes and larger m 5 0 historical events showed that the seismicity patterns are persistent

genesis of acid sulfate alteration and au cu ag mineralization at summitville colorado the summitville au cu ag deposit occurs within a porphyritic quartz latite primarily in a zone of vuggy silica alteration characterized by leaching of all major elements except silica and iron vuggy silica is best developed between an elevation of 3 700 and 3 500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz alunite alteration the vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3 400 m these deep vuggy silica zones are enclosed by quartz kaolinite alteration with little or no alunite sulfide mineralization occurs primarily within the vuggy silica the sulfide assemblage changes with decreasing elevation from covellite luzonite or native gold to chalcopyrite tennantite sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite gold also occurs in a near surface barite goethite jarosite assemblage that crosscuts the vuggy silica consideration of alunite kaolinitc and pyrite stability relations and of aluminum speciation at 250 degrees c indicates that a ph of 2 or below and a log f sub o 2 of 31 or 1 at 250 degrees c were necessary to produce vuggy silica alteration the chalcopyrite tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions the covellite luzonite or gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization barite gold mineralization is interpreted as a near surface assemblage produced under highly acidic oxidizing conditions these textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid leaching event the change from extreme acid leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic vapor dominated to a liquid dominated system leading to more reducing and less acidic conditions during sulfide deposition

comparison of orientations of stress and strain tensors based on fault plane solutions in kaoiki hawaii the stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly m 3 5 0 6 located in the 10 km radius kaoiki crustal volume separate inversions for subvolumes containing 20 50 events yielded the same results in several adjacent volumes suggesting that the stress tensor is homogeneous in those parts of the kaoiki area and that the inversion results are stable and meaningful five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 20 80 and at confidence levels exceeding 95 the volcano summits of kilauea and mauna loa and their rift systems are identified as the source of stress in the kaoiki crust because the greatest principal stress points to kilauea and mauna loa in addition the strain tensor due to energy released by these 238 earthquakes was computed for the kaoiki area and several subvolumes of it by summing the moment tensors the moment tensor of each earthquake was constructed from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment magnitude relationship a comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike slip faulting in these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes these observations are interpreted to show that in the strike slip regime of the upper part of the crust neither of the nodal planes is preferred for faulting rupture probably occurs along the nw and along the nh striking nodal planes in separate earthquakes subvolumes with more decollement faulting showed significant differences of 30 40 between the principal strain and stress directions in these volumes the near horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions these facts are interpreted to indicate that the decollement plane is weak allowing slip on it even if the principal stresses are inclined at a large angle to it it is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength no well developed fault plane and regimes in which a fault with low frictional strength dominates as a function of time significant rotations of the strain tensor by approximately 45 can be observed which seem to be related to the occurrences of kaoiki mainshocks during three background periods of about 7 years each the average strain tensor showed an orientation typical for predominant decollement faulting while two premainshock periods of 2 5 years each showed an orientation closer to strike slip faulting it is proposed that this pattern may be repeated before the next kaoiki mainshock the strain released seismically is less than the geodetically observed strain by approximately an order of magnitude stress strain fault plane solutions hawaii

the subsidence history of the northern hyblean plateau margin southeastern sicily abstract the southeastern corner of sicily is dominated by the hyblean plateau which is composed of carbonate rocks of triassic to miocene age the plateau forms part of the african foreland which collided during tertiary times with the calabrian arc giving rise to the maghrebian thrust belt the present study attempts to reconstruct the subsidence history of the northern hyblean plateau during the neogene climax of the plate collision this area has been a centre for widespread basaltic volcanic activity 8 1 ma and uplift at the close of the collisional events pliocene early pleistocene the area under consideration has been affected by subsidence and the progressive consumption of the northern margin of the hyblean plateau led to the development of the modern foredeep a quantitative analysis was done by one dimensional airy backstripping of seven deep wells located on this crucial part of the hyblean plateau the results of the analysis show that the subsidence history of the northern hyblean plateau area is characterized by four main phases similar to other domains in the central and eastern mediterranean basins these phases are a neotethyan rifting during late triassic early jurassic time b a slow thermal subsidence from early jurassic until late cretaceous time leading to the formation of a widespread mesozoic carbonate platform c compressional phase during late cretaceous palaeogene time which resulted in the formation of fold systems and d uplift and subsidence events during the middle tertiary which are probably the result of the continued collision of the european african plates after the early pleistocene unlike other terranes within the mediterranean an uplift occurred in the northern hyblean plateau following the upper miocene lower pleistocene massive volcanic activity this uplift is probably associated with the latest stages of thrusting along the frontal part of the maghrebian thrust belt

birth and demise of the bangong nujiang tethyan ocean a review from the gerze area of central tibet abstract reconstructing the evolution of the bangong nujiang tethyan ocean bnto plays an indispensable role in understanding the linkage between gondwana dispersion and tibetan plateau accretion the existence of the bnto is inferred from presence of dismembered ophiolite bodies along the bangong nujiang suture zone which separates lhasa from qiangtang in this paper we present a detailed synthesis of ophiolite metamorphism magmatism lithostratigraphy and tectonism from the western segment gerze area of the lhasa qiangtang collision zone in order to reconstruct the wilson cycle of the bnto our results suggest that the bnto initially opened in the late permian 260 ma as indicated by the earliest record of oceanic crust regionally strong evidence supports a northward subduction of the bnto beneath qiangtang likely initiating in the early jurassic 195 ma as suggested by detrital zircon age spectra from mesozoic arc sourced strata northward oceanic subduction generated three episodes of arc magmatism i e 186 170 ma 169 146 ma and 129 100 ma along the southern qiangtang margin in which the early jurassic magmatism is only sporadically preserved in modern outcrops amdo area meanwhile the recognition of an incomplete arc trench system on the northern lhasa margin supports a southward subduction beneath lhasa as well our new provenance data from lower cretaceous strata on the northern lhasa margin together with available multi disciplinary constraints suggest that final closure of the bnto did not occur until the latest jurassic early cretaceous 150 130 ma therefore the wilson cycle of the bnto is hereby reconstructed providing significant insights into the amalgamation of the tibetan plateau

timing of porphyry cu mo and base metal zn pb ag cu mineralisation in a magmatic hydrothermal system morococha district peru the morococha district in central peru is characterised by economically important cordilleran polymetallic zn pb ag cu vein and replacement bodies and the large toromocho porphyry cu mo deposit in its centre u pb re os and 40ar 39ar geochronology data for various porphyry related hydrothermal mineralisation styles record a 3 5 ma multi stage history of magmatic hydrothermal activity in the district in the late miocene three individual magmatic hydrothermal centres were active the codiciada toromocho and ticlio centres each separated in time and space the codiciada centre is the oldest magmatic hydrothermal system in the district and consists of a composite porphyry stock associated with anhydrous skarn and quartz molybdenite veins the hydrothermal events are recorded by a titanite u pb age at 9 3 0 2 ma and a molybdenite re os age at 9 26 0 03 ma these ages are indistinguishable from zircon u pb ages for porphyry intrusions of the composite stock and indicate a time span of 0 2 ma for magmatic hydrothermal activity the small ticlio magmatic hydrothermal centre in the west of the district has a maximum duration of 0 3 ma ranging from porphyry emplacement to porphyry mineralisation at 8 04 0 14 ma 40ar 39ar muscovite cooling age the toromocho magmatic hydrothermal centre has a minimum of five recorded porphyry intrusions that span a total of 1 3 ma and is responsible for the formation of the giant toromocho cu mo deposit at least two hydrothermal pulses are identified post dating a first pulse of molybdenite mineralisation wide spread hydrous skarn covers an area of over 6 km2 and is recorded by five 40ar 39ar cooling ages at 7 2 6 8 ma these ages mark the end of the slowly cooling and long lived toromocho magmatic hydrothermal centre soon after last magmatic activity at 7 26 0 02 ma district wide 50 km2 cordilleran base metal vein and replacement bodies post date the youngest recorded porphyry mineralisation event at toromocho by more than 0 5 ma polymetallic veins 5 78 0 10 and 5 72 0 18 ma 40ar 39ar ages and the manto italia polymetallic replacement bodies 6 23 0 12 and 6 0 0 2 ma 40ar 39ar ages are interpreted to have been formed by a single hydrothermal pulse hydrothermal activity ceased after the formation of the base metal vein and replacement bodies overlapping monazite u pb 8 26 0 18 ma and muscovite 40ar 39ar ages 8 1 0 5 ma from the early base metal stage of one cordilleran vein sample in the sulfurosa area provide evidence that a discrete hydrothermal pulse was responsible for polymetallic vein formation 2 6 ma prior to the district wide polymetallic veins these ages pre date those of toromocho porphyry cu mo formation and show that zn pb ag cu mineralisation formed during several discrete magmatic hydrothermal pulses in the same district

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

roots of etna volcano in faults of great earthquakes abstract results from several seismic methods allow us to sketch the deep structure of etna and its ionian margin under etna a volume of high velocity material is found in a structurally high position the emplacement of this suggests spreading of the surrounding medium just offshore down to the east normal faults penetrate through the upper crust the deeper crustal structure beneath appears upwarped from the basin towards etna juxtaposed with the crust of sicily a thinner crust reaches from the ionian basin under etna and the mantle is upwarped in such a structure magma can then be viewed as a melted lens capping a mantle upwarp at shallow depth rather than in an intracrustal chamber this reduces the conflict between estimates of its volume from excess output of volatiles and short residence times a link in time is indicated between volcanic and seismic activity at a large scale over the millennium the reported ends of episodes of high output rates of magma are followed by the reported occurrences of magnitude 7 earthquakes which caused destruction in southeastern sicily several steep active normal fault have been imaged to a depth of 10 km the crust up to 30 km offshore of the cities of catania and augusta which may be fault planes for such large earthquakes they expand and prolongate the system of the timpe faults on the eastern flank of etna thus linking large scale tectonics offshore with the volcano etna developed together with normal faulting upwarp and spreading during the recent evolution of the former ionian subduction activation of the material at depth at the lateral edge of the slab by vertical motion with extension above could produce the peculiar type of etna magmatism

the nadun cu au mineralization central tibet root of a high sulfidation epithermal deposit abstract a new high sulfidation epithermal cu au occurrence nadun has been discovered adjacent to the cretaceous duolong porphyry cu au deposit within the bangong nujiang metallogenic belt central tibet the nadun cu au mineralization is hosted in a tectonic hydrothermal breccia with advanced argillic alteration which occurs above sandstone associated with quartz pyrite veins the granodiorite porphyry with strong argillic alteration yields a zircon u pb age of 119 1 1 3 ma whereas the weakly argillic granodiorite porphyry intruded into the breccia has a younger age of 116 1 1 3 ma this indicates that cu au epithermal mineralization likely occurred between 116 ma and 119 ma consistent with the duration of magmatic hydrothermal activity at duolong 115 118 ma and providing evidence that nadun and duolong were formed during the same event moreover the nadun and duolong porphyries have similar hf isotopic compositions ehf t values ranging from 8 8 to 8 1 mean 5 0 1 1 n 32 likely indicating that the deposits are comagmatic in addition boiling assemblages in vapor rich inclusions coexisting with brines occur in early stage quartz pyrite veins and likely record phase separation at a temperature of 550 300 c and pressure of 700 110 bars most liquid rich fluid inclusions formed at the breccia stage show similar salinity 1 7 19 3 wt nacl equiv to vapor rich inclusions from the underlying quartz pyrite veins likely indicating vapor contraction during cooling at elevated presssure this suggests that quartz pyrite veins may act as conduits for ore forming fluid traveling from the porphyry to the epithermal hydrothermal system o and h isotopic compositions 18ofluid 0 42 9 71 and d 102 to 66 suggest that ore forming fluids are dominantly from a magmatic source with a minor addition of meteoric water at a later stage the s and fe isotope compositions of sulfides 34s 5 9 to 0 5 and 57fe 2 15 to 0 17 decrease from the quartz pyrite vein to breccia ore indicating that ore forming fluids gradually become so42 enriched and relatively oxidized this body of evidence suggests that the nadun cu au mineralization may represent the root of a high sulfidation epithermal deposit

active fault kinematics and crustal stresses along the ionian margin of southeastern sicily since the late cretaceous onset of plate convergence between africa and europe the malta escarpment has been converted from a mesozoic passive margin into a mega hinge fault system with an additional sinistral strike slip component the modern tectonic stress regime with nw se directed maximum horizontal stresses has been established since late messinian times since the pleistocene sinistral strike slip deformation and contemporaneous normal faulting along the malta escarpment have induced the opening of oblique trending onshore grabens at the eastern margin of the hyblean plateau in this study we focus on the kinematics the controlling state of stress and the temporal variation of the neotectonic to active strike slip and normal fault structures the stress tensor calculations reveals that the widespread map scaled to meso scaled normal fault structures are governed by the long term extensional state of stress during the quaternary this long term stress tensor is predominantly controlled by gravitational induced stresses due to vertical load 1 sv and lateral extension due to the topographic gradient of the malta escarpment 3 sh ne sw in this case the average tectonic stresses 2 sh nw se transmitted by the regional to plate tectonic stress field are significantly smaller than the gravitational induced stresses in contrast the clear localization of conjugate sets of meso scaled strike slip fault structures and shear zones without accompanying normal fault structures give strong indications for episodic seismotectonic strike slip faulting under critical stress conditions in this state tectonically induced maximum horizontal stresses are successively increased by ongoing plate convergence from low level stress magnitudes 1 sv 2 sh nw se up to critical stress magnitudes 1 sh nw se 2 sv which are significantly larger than gravitational stresses at the critical state seismotectonic stress release occurs by active strike slip faulting as recently indicated in mid crustal levels by the moderate 1990 augusta earthquake and re establishes the long term extensional state of stress because the strike slip faults and shear zones in the study area were formed as surface ruptures they additionally give indications of episodic large earthquakes in this region with magnitudes greater than 6 0 as reported by several large earthquakes devastating e and se sicily in historical times

new gps constraints on the africa eurasia plate boundary zone in southern italy 1 southern italy is a key area for understanding the tectonic processes in the africa eurasia collision zone we analyze new gps measurements carried out between 1994 and 2001 the results are presented in terms of time series trajectories and velocities of crustal motion as well as a geodetic strain rate field while central italy corsica sardinia and the tyrrhenian sea move like the eurasian plate the overall motion of the sicily rift zone region matches african plate motion unexpected are the north northwest directed motions of northeastern sicily and the eolian islands most striking on the deformation field are a north south oriented compression along the northern sicilian coast compression between apulia and northwestern greece as well as extension in the sicily rift zone and the interior of sicily

40ar 39ar and re os geochronology of porphyry copper molybdenum deposits and related copper silver veins in the collahuasi district northern chile the rosario and ujina porphyry cu mo deposits together with porphyry copper mineralization at the nearby quebrada blanca deposit constitute the third largest concentration of copper mineralization associated with the domeyko fault system in northern chile at rosario fault hosted cu ag rich massive sulfide veins are associated with pyrophyllite alunite quartz altered rocks the rosario massive sulfide veins cut biotitealbite magnetite k feldspar and illite chlorite altered rocks associated with porphyry style copper molybdenum mineralization similar massive sulfide veins occur in the la grande area 1 to 2 km south of rosario copper ore at ujina is associated with a k feldspar biotite altered quartz monzonite intrusion the k feldspar biotite assemblage has been overprinted by white mica chlorite alteration new geochronologic data presented in this paper constrain the ages of hydrothermal activity in the rosario and ujina deposits the 40ar 39ar biotite illite and alunite dates at 2 sigma error and a re os molybdenite date at 0 5 error are reported for porphyritic rocks and hydrothermal alteration from the rosario and ujina mineralized centers a weighted mean 40ar 39ar plateau age of 34 4 plus or minus 0 3 ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the rosario deposit illite and hypogene alunite from separate overprinting alteration events yielded 40ar 39ar ages of 34 5 plus or minus 0 5 ma plateau age and 32 6 plus or minus 0 3 ma plateau age respectively an re os age of 33 3 plus or minus 0 2 ma for molybdenite at rosario is slightly younger than the 40ar 39ar age of illite but older than the alunite a weighted mean plateau age of 32 7 plus or minus 1 6 ma for hypogene alunite from the la grande cu ag au vein south of rosario is indistinguishable from the age of rosario alunite at ujina the weighted mean 40ar 39ar plateau age of igneous biotite for a monzonite intrusion that hosts copper mineralization is 35 2 plus or minus 0 3 ma the monzonite is intruded by postmineralization porphyry dikes of similar composition from which igneous biotite yielded a 40ar 39ar age of 34 7 plus or minus 0 3 ma igneous biotite in the rosario porphyry cooled through its closure temperature during and after the formation of illite implying that porphyry style ore and alteration minerals in the rosario deposit had formed by 34 3 ma the age of alunite at rosario and la grande indicate that a second discrete episode of hydrothermal activity was superimposed 1 8 plus or minus 0 4 m y later onto the earlier formed porphyry cu system hydrothermal activity at ujina is constrained by the 40ar 39ar ages of igneous biotite in the premineralization and postmineralization intrusions and occurred during a minimum interval of 0 5 plus or minus 0 4 m y the biotite granite at la profunda 1 5 km east of ujina has a igneous biotite age of 81 2 plus or minus 2 9 ma indicating that this intrusion is unrelated to the mineralized eocene oligocene porphyry intrusions at ujina rosario and quebrada blanca the 40ar 39ar data for igneous biotite in the rosario and ujina porphyries are cooling ages after multiple hydrothermal events in most cases these are minimum hydrothermal ages i e ages of the last high temperature hydrothermal event to have affected the samples not magmatic ages however the biotite age for the inca porphyry is possibly only slightly younger than the intrusion age because biotite has not been affected by later hydrothermal alteration

seismic strain and seismogenic stress regimes in the crust of the southern tyrrhenian region abstract an investigation has been performed to identify and characterize the seismic deformation zones active over the last decades in the region of italy that has experienced the strongest seismicity during the last centuries the study is based on the estimate of hypocenter locations fault plane solutions seismogenic stress and seismic strain tensor orientations carried out using the entire dataset of the national and local seismic networks and the recently improved three dimensional 3d crustal velocity model of the study area a series of simulation tests have been performed to evaluate the significance of the earthquake space distribution obtained and whether it was influenced by network geometry problems related to the sea and the lack of ocean bottom seismometers in the sectors where hypocentral location was synthetically proved to be reliable space distributions of earthquakes located with epicenter and focal depth errors less than 3 and 4 km respectively have been compared with local geology in order to identify seismogenic faults the dataset of 32 fault plane solutions estimated with fault parameter errors less than 20 has been used to investigate space variations of seismogenic stress and seismic strain orientations over the study area stress was found to be uniform in the messina strait and southern calabria where inversion of the available set of 11 fault plane solutions showed clear evidence of an extensional regime the different orientations of the minimum compressive stress and strain found in this sector together with the information available on local geology and tectonics lead us to propose that the seismicity occurring over the last decades in the messina strait and southern calabria was not in general produced at the main faults but at minor faults activated by the main tectonic stress field acting in the area to the west in the sector including western etna the nebrodi chain and the western aeolian islands analysis of the available set of 16 fault plane solutions revealed a certain degree of stress heterogeneity with an apparent prevalence of north south compression this east to west change of stress strain regimes is evaluated in the light of current hypotheses regarding the geodynamics of the study region

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

a probability method for local earthquake focal mechanisms we propose a method for obtaining focal mechanisms of earthquakes recorded on a local network it is based on the non linear estimation of the probability function for strike dip and slip of the focal mechanism from p polarities s polarizations and s p amplitude ratios the method is shown to be effective for the analysis of small earthquakes recorded by local networks in particular the s p amplitude ratios bring strong constraints on fault parameters even if large errors are present an application to a small earthquake recorded at the volcanic zone of campi flegrei naples italy during a seismic outburst is presented

geology of the boyongan and bayugo porphyry cu au deposits an emerging porphyry district in northeast mindanao philippines the boyongan and bayugo porphyry cu au mineral deposits discovered under postmineralization cover during the previous decade are part of an emerging belt of intrusion centered au rich cu mineral deposits and prospects in the surigao district of northeast mindanao philippines since their formation in the early pleistocene exhumation and weathering of these deposits have led to the development of a 600 m thick oxidation profile at boyongan and a modest 30 70 m oxidation profile at bayugo debris flows volcanic material and fluviolacustrine sediments accumulating in the actively extending mainit graben subsequently covered the weathered deposits preserving their supergene profiles the mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages three premineralization diorite porphyry stocks and a silt sand matrix breccia complex represent early stages of magmatism and brecciation significant cu and au introduction followed these events and occurred in association with small early mineralization diorite porphyry stocks at boyongan and bayugo within the diorite complex the two mineral deposits are spatially distinct separated by approximately 1 km of premineralization diorites inter and late mineralization intrusions were emplaced as the magmatic hydrothermal system waned a characteristic progression of vein and k silicate alteration styles affected each of the synmineralization intrusions rare comb quartz unidirectional solidification textures stage 0 mark the transition from magmatic to hydrothermal conditions quartz poor wispy magnetite biotite k feldspar veinlets characterize stage 1 stage 2 veins consist of quartz with selvage and or disseminated magnetite or biotite and k feldspar halos stage 3 quartz veins have k feldspar halos but generally lack magnetite and biotite stage 4 veins consist of massive bornite chalcopyrite and chalcopyrite pyrite with k feldspar halos stage 3 quartz veins and stage 4 sulfide veins host cu au mineralization of the greatest volumetric significance reflecting the general paucity of sulfide in the earlier vein stages despite the simplicity of this sequence detailed paragenetic reconstructions reveal that this characteristic progression of veining and k silicate alteration was repeated with the emplacement of each synmineralization intrusive event revealing multiple magmatic hydrothermal cycles of alteration and mineralization at boyongan and bayugo intense and pervasive illite alteration in association with pyrite chalcopyrite and tetrahedrite tennantite developed in narrow structures crosscutting quartz k feldspar veins debris flows in the burial sequence above and adjacent to the boyongan bayugo complex also contain abundant clasts with intense pervasive illite and alunite pyrophyllite dickite kaolinite alteration assemblages that have overprinted k silicate style quartz veins in both mineral deposits cu and au are associated with intense quartz vein stockworks composed primarily of k silicate stage 3 veins despite this association not all of these quartz vein stockworks contain cu and au to the same tenor quartz vein stockworks in the eastern high grade of boyongan have been intersected over a vertical interval of 800 m having affected much of the early mineralization stock however hypogene cu grades exceed 0 5 by weight only in the upper 300 m of the stockwork in the cupola of the early mineralization stock superior grade development in high grade zones at boyongan locally exceeding 2 cu and 3 g t au developed where fertile vein stages from two or more magmatic hydrothermal cycles affected the same wall rock the documented paragenetic relationships demonstrate multiple discrete cycles of k silicate stage veining and alteration associated with each synmineralization intrusive event all such events predate formation of feldspar destructive illite smectite chlorite illite pyrite and quartz alunite clay assemblages existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly in a period of less than 200 000 years 2 3 2 1 ma geologic and geochronological constraints on the life cycle of boyongan and bayugo describe an extremely dynamic history of emplacement exhumation weathering and burial over a period of 2 3 m y the study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting

alunite and the role of magmatic fluids in the tambo high sulfidation deposit el indio pascua belt chile the tambo high sulfidation deposit located within the el indio pascua belt in chile produced almost 25 t 0 8 m oz of gold from altered tertiary rhyodacitic volcanic rocks episodic magmatic hydrothermal activity in the district occurred over at least 4 my and is characterized by several stages of acid sulfate alteration including magmatic hydrothermal magmatic steam steam heated and apparent supergene assemblages two stages of aufag mineralization are recognized and are hosted in barite and alunite within hydrothermal breccias and veins isotopic compositions of fluid in alunite show a dominant magmatic signature with only a variable 18 o enriched meteoric water component throughout the entire hydrothermal process alunite 40 ar 39 ar ages constrain the timing of alteration and the duration of the hydrothermal system pre ore alteration occurred at about 10 to 11 ma and was contemporaneous with the volcanism of the tambo formation alunite from this stage of alteration occurs in the matrix of barren breccias and as fine intergrowths of alunite quartzfclays that selectively replaced feldspars and pumice fragments the textural relationships combined with stable isotope systematics suggest a magmatic hydrothermal origin for the alunite with a local magmatic steam overprint early ore stage alunite 8 7f0 2 ma occurs with barite and goldfwalthierite within open spaces of the breccia matrix and has d 34 s values 24 27x typical of magmatic hydrothermal alunite reflecting equilibrium between aqueous h2s and so4 fluid inclusion ratios of h2s so2 approximately 6 are consistent with ratios determined from stable isotope data and indicate reduced fluid conditions during ore deposition vaporphase transport of au and deposition from condensed magmatic vapor rising from the brittle ductile transition is inferred late gold coprecipitated with a third stage of alunite 8 2f0 2 ma that is characterized by nearly uniform chemical compositions and d 34 s values 1x similar to those for associated enargite reflecting disequilibrium between h2s and so4 this third stage of alunite is isotopically and chemically similar to that of post ore coarse banded alunitefhematite quartz veins that crosscut the breccias in the tambo area analyses of fluid inclusion gas from alunite in these veins indicate high so2 and disequilibrium co2 co ch4 h2 species consistent with a magmatic steam origin the gases are also depleted in he and the late goldbearing alunite was probably precipitated from rapidly ascending so2 rich vapors that were flashed from condensed magmatic

alunite in the pascua lama high sulfidation deposit constraints on alteration and ore deposition using stable isotope geochemistry the pascua lama high sulfidation system located in the el indio pascua belt of chile and argentina contains over 16 million ounces moz au and 585 moz ag the deposit is hosted primarily in granite rocks of triassic age with mineralization occurring in several discrete miocene age phreatomagmatic breccias and related fracture networks the largest of these areas is brecha central which is dominated by a mineralizing assemblage of alunitepyrite enargite and precious metals several stages of hydrothermal alteration related to mineralization are recognized including all types of alunite bearing advanced argillic assemblages magmatic hydrothermal steam heated magmatic steam and supergene the occurrence of alunite throughout the paragenesis of this epithermal system is unusual and detailed radiometric mineralogical and stable isotope studies provide constraints on the timing and nature of alteration and mineralization of the alunite pyrite enargite assemblage in the deposit early preore alteration occurred prior to ca 9 ma and consists of intense silicic and advanced argillic assemblages with peripheral argillic and widespread propylitic zones alunite of this stage occurs as fine intergrowths of alunite quartz kaolinite dickite and pyrophyllite that selectively replaced feldspars in the host rock stable isotope systematics suggest a magmatic hydrothermal origin with a dominantly magmatic fluid source alunite is coeval with the main stage of au ag cu mineralization alunite pyrite enargite assemblage ore which has been dated at approximately 8 8 ma ore stage alunite has an isotopic signature similar to preore alunite and 34salun py data indicate depositional temperatures of 245 to 305 c the d and 18o data exclude significant involvement of meteoric water during mineralization and indicate that the assemblage formed from h2sdominated magmatic fluids thick steam heated alteration zones are preserved at the highest elevations in the deposit and probably formed from oxidation of h2s during boiling of the magmatic ore fluids coarsely crystalline magmatic steam alunite 8 4 ma is restricted to the near surface portion of brecha central postmineral alunite jarosite were previously interpreted to be supergene crosscutting veins and overgrowths although stable isotope data suggest a mixed magmatic meteoric origin for this late stage alteration only late jarosite veinlets 8 0 ma associated with fine grained pseudocubic alunite have a supergene isotopic signature the predominance of magmatic fluids recorded throughout the paragenesis of the pascua system is atypical for high sulfidation deposits which typically involve significant meteoric water in near surface and peripheral alteration and in some systems even ore deposition at pascua the strong magmatic signature of both alteration and main stage alunite pyrite enargite assemblage ore is attributed to limited availability of meteoric fluids this is in agreement with published data for the el indio pascua belt indicating an event of uplift and subsequent pediment incision as well as a transition from semiarid to arid climatic conditions during the formation of the deposit in the mid to late miocene

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

structure related and stress induced shear wave velocity anisotropy observations from microearthquakes near the calaveras fault in central california we attempt to distinguish between structure related and stress induced shear wave velocity anisotropy in the upper crust using microearthquakes m 2 6 that occurred before and after the tres pinos california earthquake m 5 5 of 26 january 1986 the tres pinos earthquake occurred on the quien sabe fault a northwest southeast trending right lateral strike slip fault approximately 10 km east of the calaveras fault in central california digital three component seismograms are available from 1984 to 1992 from the quien sabe ranch station hqr located only 7 3 km north northeast of the epicenter of the main event we studied polarization directions and delay times using a method that combines cross correlation techniques and particle motion plots to enhance the significance of our interpretations we reduced uncertainties related to the hypocenter location of the earthquakes by source relocation based on joint hypocenter determination and cross correlation in the study area we found two zones with different but stable polarization directions around the quien sabe fault system shear wave polarizations of over 30 microearthquakes clearly indicate that the slow shear wave direction is normal to the northwest southeast trend of this vertical right lateral strike slip fault thus velocity anisotropy appears to be caused by the fault system itself however anisotropy associated with a cluster of microearthquakes located about 4 km to the east is quite different in this zone the fast and slow shear polarization directions are correlative with the maximum and minimum horizontal stress directions respectively thus in this case it appears the crustal velocity anisotropy is controlled by the state of stress in the crust the observation of these two polarization directions at one station suggests that the shallow crust below the station hqr does not influence shear wave polarization directions

longevity of porphyry copper formation at quellaveco peru metal introduction at the late paleocene to early eocene quellaveco porphyry copper molybdenum deposit in southern peru spans several phases of quartz monzonite porphyry emplacement and is bracketed by a precursor granodiorite pluton and a late mineral porphyry body that postdates essentially all copper introduction together the u pb ages of zircons from these intrusive rocks show that 1 08 0 58 m y elapsed between the precursor pluton and initiation of stock emplacement the porphyry system was active intermittently for at least 3 25 m y 4 07 0 82 m y and at least three quarters of the copper inventory was deposited in a maximum of 3 12 m y 2 51 0 61 m y recent u pb zircon dating of several other major central andean porphyry copper deposits in combination with other isotopic techniques suggests that 2 5 to 4 m y life spans are commonplace the longevity of porphyry copper systems implied by these studies appears to reflect the protracted time gaps between the multiple intrusions that intermittently replenished porphyry stocks other precise isotopic methods re os 40 ar 39 ar typically document shorter life spans because it is more difficult if not impossible to date the full sequence of events involved in porphyry copper formation

results from a new model of river basin evolution this paper briefly describes a model of the erosional development of catchments and their channel networks the model differentiates between the dominant transport processes in hillslope and channels the development of channels and hillslopes occurs in an integrated manner as a function of physically observable mechanisms the growth of a river basin is qualitatively described the model concepts are used to study the basin during periods of growth transient periods as well as during dynamic equilibrium this leads to hypotheses about the relationship between slopes relief tectonic uplift erodability runoff and catchment area it is shown that the model leads to very reasonable and desirable behaviour of hillslopes during retreat and degradation

hydrothermal fluid evolution at the tiegelongnan porphyry epithermal cu au deposit tibet china constraints from h and o stable isotope and in situ s isotope abstract the tiegelongnan porphyry epithermal cu au deposit is located in the duolong porphyry district north of the bangong nujiang suture zone tibet china mineralization is hosted by jurassic sedimentary sandstone and several phases of diorite and granodiorite porphyry dikes intruded between 123 and 116 ma the hydrothermal alteration is characterized by alunite kaolinite dickite overprinting quartz muscovite pyrite and biotite alteration zones porphyry chalcopyrite pyrite molybdenite stage 1 mineralization is associated with biotite alteration porphyry chalcopyrite bornite stage 2 and covellite stage 3 mineralization is associated with quartz muscovite pyrite alteration formed at 121 ma epithermal mineralization consisting of pyrite alunite stage 4 chalcopyrite bornite digenite stage 5 and tennantite enargite stage 6 is hosted by two pulses of alunite kaolinite breccia and veins at 116 ma and 112 ma the fluid composition related to muscovite with average 18o of 8 9 and d of 56 indicates a magmatic water origin fluid 18o composition in equilibrium with quartz veins decreases from 6 7 to 2 3 which are likely the results of the water rock isotopic exchange quartz fluid inclusions d values between 50 to 84 are partly lower than that obtained from muscovite alteration fluids which may result from h fractionation during fluid inclusions decrepitation epithermal stage fluid composition equilibrium with alunite yield 18o from 1 2 to 2 7 and d from 71 to 51 n 11 which is comparable to the fluid composition equilibrium with type kaolinite hosting ores with 18o between 2 5 and 2 9 and d between 72 and 51 it suggests that alunite and type kaolinite formed with mixing between magmatic and high altitude cretaceous meteoric water late types and iii kaolinite filling alunite and quartz veins fluid 18o and d values plot along a mixing line between magmatic and low altitude cretaceous meteoric water probably following the erosion and plateau subsidence porphyry mineralization sulfide stage 1 chalcopyrite and pyrite yield 34s values between 5 8 and 0 9 with an average fluid 34sh2s 2 5 n 10 whereas stage 2 chalcopyrite returns 34s values from 8 7 to 3 with an average 34sh2s 5 6 n 5 the lower fluid 34sh2s values during sulfides stage 2 compared to that of stage 1 suggest that the chalcopyrite bornite mineralization formed under higher oxidation conditions than that of the chalcopyrite pyrite mineralization alunite yields 34s values from 11 to 18 3 n 8 and the associated sulfide stage 4 pyrite have varying 34s values from 32 2 to 5 4 disequilibrium s isotope in alunite pyrite pairs was likely because of rapid cooling and retrograde s isotope exchange during later sulfides emplacement epithermal mineralization sulfide stage 4 s equilibrated pyrite 14 9 to 9 5 stage 5 chalcopyrite 11 6 to 8 2 and stage 6 enargite 5 4 to 2 6 display increasing 34s values suggesting epithermal fluid compositions evolve towards more reducing conditions

distribution of porphyry deposits in the eurasian continent and their corresponding tectonic settings in the eurasian continent there are three huge metallogenic belts of cu and mo porphyry deposits comprising the paleozoic central asian ore belt in the north the tethyan eurasian ore belt of jurassic to cenozoic age in the southwest and the east margin ore belt of the eurasian continent of jurassic to cretaceous age in the east the latter is considered to be part of the vast circum pacific ore belt some of the main features of the spatial temporal distribution of cu and mo porphyry systems and related geodynamic processes of the three metallogenic belts are described in particular the key role of postsubduction related porphyry ore systems is emphasized comprising collisional and post collisional cu mo porphyry deposits during the geological history of the eurasian continent the recurrent feature of these ore systems and related felsic rocks is their derivation from partial melting of stagnant or residual oceanic slabs and mixing with a variable amount of crustal material during magma ascent to shallower levels 2013 published by elsevier ltd

a climate signal in exhumation patterns revealed by porphyry copper deposits porphyry copper deposits are emplaced at a fixed crustal depth in convergent tectonic settings the age and distribution of deposits exposed at the surface suggest that mountain exhumation is influenced by precipitation rates

cretaceous structures in the duolong region of central tibet evidence for an accretionary wedge and closure of the bangong nujiang neo tethys ocean abstract this article focuses on the cretaceous structures in the duolong region northern tibet our aim was to constrain the metallogenic setting of the duolong giant porphyry cu au deposit and to discuss the implications for the closure of the bangong nujiang neo tethys ocean there are two tectonic stages in the duolong region during the early to late cretaceous the first stage is characterised by numerous large scale folds and faults with ne sw compression the 40ar 39ar plateau age of sericite from mylonite is 127 8 1 1 ma which represents the timing of tectonic emplacement of the jurassic mugagangri formation jm and ophiolite this stage corresponds to the northward subduction of the bangong nujiang neo tethys ocean the second stage is characterised by the duolong thrust nappe with a series of sinter deposits slip surfaces fault fracture zones klippes and tectonic windows basalt from the hanging wall yields a zircon u pb age of 110 4 1 7 ma which is older than the upper cretaceous abushan formation ka of the footwall a red layer obtained from drillcores is younger than the overlying jurassic strata on the basis of the youngest detrital zircon u pb age of 116 2 ma this stage is correlated with the closure of the bangong nujiang neo tethys ocean we suggest the upper triassic riganpeicuo formation jm and ophiolite successively accreted to the southern margin of the south qiangtang terrane during the late jurassic to early cretaceous these constitute the main body of the epicontinental accretionary wedge this accretionary wedge includes many early cretaceous epicontinental arc volcanic rocks and granites and the duolong cu au deposits were formed in an andean type active continental margin setting we also conclude that the bangong nujiang neo tethys ocean closed during the late early cretaceous to early late cretaceous and that continental collision continued until the end of the late cretaceous

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

oyu tolgoi mongolia siluro devonian porphyry cu au mo and high sulfidation cu mineralization with a cretaceous chalcocite blanket the oyu tolgoi porphyry cu au mo deposit is located in the gobi desert of southern mongolia the deposit consists of three main mineralized zones north central and south oyu interpreted to constitute at least two separate porphyry copper centers oyu tolgoi is associated with a series of small structurally controlled stocks and dikes of monzonitic and dioritic composition which intrude a sequence of siluro devonian volcanic and sedimentary rocks country rocks consist of pillow basalt and basaltic andesite interbedded with fine grained clastic sedimentary horizons the sequence is interpreted to have formed in a subaqueous island arc volcanic environment which formed part of the extensive paleozoic tuva mongol arc these island arc rocks are covered by carboniferous terrigenous sequences and are intruded by carboniferous syenite and granite and permian alkaline granite much of the area is covered by poorly consolidated cretaceous sedimentary sequences central oyu consists of a multiple phase hydrothermal breccia crosscutting an altered fine grained feldspar porphyry containing porphyry type alteration and mineralization the breccia underwent texture destructive advanced argillic alteration characterized by several associations of quartz alunite dickite pyrophyllite sericite and zunyite plus lesser svanbergite and fluorite these associations overprint earlier formed k silicate and quartz sericite illite assemblages copper mineralization at central oyu is present in a supergene chalcocite blanket that formed at the expense of a pyrite rich hypogene chalcocite covellite tennantite arsenosul vanite sulvanite chalcopyrite bornite sulfide suite that accompanied the advanced argillic alteration event at south oyu a feldspar hornblende porphyry of monzonitic composition intrudes a sequence of fine grained andesite and basaltic andesite hypogene copper gold mineralization occurs as early magnetite rich pyrite poor quartz chalcopyrite bornite stockworks and sheeted veinlets hydrothermal alteration is composed of biotite and k feldspar with apatite and minor albite magnetite averages 7 to 10 vol percent and copper and gold grades vary sympathetically these early assemblages are overprinted by a sericite chlorite event with ore grade copper and erratic gold and molybdenum values and also by structurally controlled pyrite rich advanced argillic alteration k ar ages show that the hypogene copper gold mineralization at south oyu is latest silurian earliest devonian 411 3 ma and that the entire district was intruded by postmineralization syenite during the carboniferous 307 4 ma the supergene chalcocite blanket at central oyu formed in the early cretaceous earliest late cretaceous based on ages for supergene alunite of 117 1 and 93 1 ma which makes central oyu one of the oldest preserved chalcocite blankets in the world chalcocite blanket preservation was favored by climatic desiccation into the late cretaceous and the tertiary it is inferred that significant regional uplift took place at the silurian devonian boundary this synmineral ization uplift and erosion may have been in part responsible for the telescoping of the hydrothermal systems at oyu tolgoi including the superposition of epithermal style associations over higher temperature deeper seated assemblages typical of the porphyry environment

geochronology geochemistry and zircon hf isotopic compositions of mesozoic intermediate felsic intrusions in central tibet petrogenetic and tectonic implications abstract mesozoic intermediate felsic intrusions are widely distributed in the southern qiangtang terrane central tibet zircon u pb dating shows that these intrusions formed in two periods in the jurassic 169 150 ma and cretaceous 127 113 ma they mostly belong to the high k calc alkaline series and show strong enrichments in large ion lithophile elements e g cs rb and k depletions in nb ta and ti and negative ba anomalies on primitive mantle normalized diagrams p 2 o 5 contents decrease with increasing sio 2 content and th contents increase with increasing rb content consistent with the evolution trend of i type magmas these intrusions show a wide range of zircon ehf t values from 19 4 to 11 2 including in rocks with similar sio 2 contents suggesting variable contributions from mantle and qiangtang crustal sources fine grained mafic to intermediate igneous enclaves in jurassic intrusions have similar zircon u pb ages and similar or slightly higher zircon ehf t values to the host rocks suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers magma mixing is also supported by the wide range of zircon hf isotopic compositions ehf t 19 4 to 2 5 from within individual jurassic and cretaceous intrusions and jurassic enclaves the jurassic cretaceous magmas likely formed in a continental arc setting during subduction of the bangong nujiang ocean between 170 and 110 ma and evolved in the upper plate crust by mash processes