rhenium behavior in molybdenite in hypogene and near surface environments implications for re os geochronometry abstract rhenium is concentrated mainly in molybdenite mos2 and occurs as a major cation in only a few rare minerals this affinity makes molybdenite an ideal mineral for re os geochronometry but research on the behavior of re in molybdenite is limited infrared microscope xrd back scattered electron bse and microprobe techniques have been used in this study to document re behavior in molybdenite affected by hypogene and near surface processes in the hypogene environment both 3r and 2h molybdenite can experience re loss during hydrothermal alteration which also causes increased infrared transparency ir alteration at temperatures as low as 150 c can cause re loss in the presence of advecting fluids and will affect re os dating if it occurs long after primary mineralization re loss in 3r and 2h molybdenite under supergene conditions does not increase ir transparency rhenium is not incorporated into supergene ferrimolybdite but is enriched in k al silicate intergrowths which may be illite these minute intergrowths are present in several samples and would not be detected in a simple optical examination in the supergene environment elemental os is stable whereas re is not rhenium in molybdenite may be removed by supergene fluids after some has decayed to 187os causing erroneously old ages or it may be adsorbed into the illite intergrowths creating ages which are too young in the weathering environment proterozoic molybdenites have altered to re enriched powellite which can be detected using back scattered electron imagery combined microprobe xrd bse and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating these techniques are non destructive and should be performed before any molybdenite is dated by the re os system

fluid chemistry structural setting and emplacement history of the rosario cu mo porphyry and cu ag au epithermal veins collahuasi district northern chile the rosario cu mo ag deposit is located in the collahuasi district of northern chile it comprises high grade cu ag au epithermal veins superimposed on the core of a porphyry cu mo orebody rosario has mining reserves of 1 094 million metric tons mt at 1 03 percent copper an additional 1 022 mt at 0 93 percent copper occurs in the district at the nearby ujina and quebrada blanca porphyry deposits the rosario reserve contains over 95 percent hypogene ore whereas supergene sulfide ores dominate at ujina and quebrada blanca mineralized veins are hosted within lower permian volcanic and sedimentary rocks lower triassic granodiorite and late eocene porphyritic quartz monzonite the rosario fault system a series of moderate southwest dipping faults has localized high grade cu ag au veins at cerro la grande similar high grade cu ag au veins are hosted in north northeast trending sinistral wrench faults normal movement in the rosario fault system is interpreted to have been synchronous with sinistral strike slip deformation at la grande hydrothermal alteration at rosario is characterized by a k feldspar core focused in the rosario porphyry that grades out to a secondary biotite albite magnetite assemblage paragenetic relationships indicate that magnetite was the earliest formed alteration product but has been replaced by biotite albite vein crosscutting relationships indicate that k feldspar formed during and after biotite albite alteration chalcopyrite and bornite were deposited in quartz veins associated with both k feldspar and biotite albite assemblages the early hydrothermal fluid was a hypersaline brine 40 45 wt nacl that coexisted with vapor between 400 degrees and 600 degrees c weakly mineralized illite chlorite intermediate argillic alteration of the early k and na silicate assemblages was caused by moderate temperature 250 degrees 350 degrees c moderate salinity brines 10 15 wt nacl molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration events these fluids were 350 degrees to 400 degrees c with salinities between 10 and 15 wt percent nacl porphyry style ore and alteration minerals were overprinted by structurally controlled quartz alunite pyrite pyrophyllite dickite and muscovite quartz phyllic alteration assemblages the quartz alunite pyrite alteration formed at 300 degrees to 400 degrees c from fluids with a salinity of 10 wt percent nacl the pyrophyllite dickite assemblage formed between 250 degrees and 320 degrees c from dilute 5 wt nacl fluids an upward flared zone of muscovite quartz pyrite altered rocks surrounds the fault controlled domain of advanced argillic alteration thick veins 0 5 2 m wide of fault hosted massive pyrite chalcopyrite and bornite precipitated brines with a salinity of 30 wt percent nacl at temperatures of 250 degrees to 300 degrees c pressure depth estimates indicate that at least 1 km of rock was eroded at rosario between formation of the k na silicate and advanced argillic assemblages this erosion was rapid occurring over a period of 1 8 m y the rosario porphyry intruded immediately after the incaic tectonic phase implying that it was emplaced as the domeyko cordillera underwent gravitational collapse expressed as normal faults in the upper crust gravitational sliding potentially accelerated exhumation and helped to promote telescoping of the high sulfidation environment onto the rosario porphyry the hydrothermal system responsible for porphyry cu mineralization at rosario was partially exhumed prior to the formation of high sulfidation ore and alteration assemblages this implies that emplacement of a second blind intrusion occurred somewhere beneath the rosario and cerro la grande high sulfidation vein systems and is supported by the fault geometry and zoning of precious metals and sulfosalts at the district scale

petrology geochemistry and u pb geochronology of magmatic rocks from the high sulfidation epithermal au cu chelopech deposit srednogorie zone bulgaria the chelopech deposit is one of the largest european gold deposits and is located 60 km east of sofia within the northern part of the panagyurishte mineral district it lies within the banat srednegorie metallogenic belt which extends from romania through serbia to bulgaria the magmatic rocks define a typical calc alkaline suite the magmatic rocks surrounding the chelopech deposit have been affected by propylitic quartz sericite and advanced argillic alteration but the igneous textures have been preserved alteration processes have resulted in leaching of na2o cao p2o5 and sr and enrichment in k2o and rb trace element variation diagrams are typical of subduction related volcanism with negative anomalies in high field strength elements hfse and light element lithophile elements hfse and rare earth elements were relatively immobile during the hydrothermal alteration related to ore formation based on immobile element classification diagrams the magmatic rocks are andesitic to dacitic in compositions single zircon grains from three different magmatic rocks spanning the time of the chelopech magmatism were dated by high precision u pb geochronology zircons of an altered andesitic body which has been thrust over the deposit yield a concordant 206pb 238u age of 92 21 0 21 ma this age is interpreted as the crystallization age and the maximum age for magmatism at chelopech zircon analyses of a dacitic dome like body which crops out to the north of the chelopech deposit give a mean 206pb 238u age of 91 95 0 28 ma zircons of the andesitic hypabyssal body hosting the high sulfidation mineralization and overprinted by hydrothermal alteration give a concordant 206pb 238u age of 91 45 0 15 ma this age is interpreted as the intrusion age of the andesite and as the maximum age of the chelopech epithermal high sulfidation deposit 176hf 177hf isotope ratios of zircons from the chelopech magmatic rocks together with published data on the chelopech area and the about 92 ma old elatsite porphyry cu deposit suggest two different magma sources in the chelopech elatsite magmatic area magmatic rocks associated with the elatsite porphyry cu deposit and the dacitic dome like body north of chelopech are characterized by zircons with hft90 values of 5 which suggest an important input of mantle derived magma some zircons display lower hft90 values as low as 6 and correlate with increasing 206pb 238u ages up to about 350 ma suggesting assimilation of basement rocks during magmatism in contrast zircon grains in andesitic rocks from chelopech are characterized by homogeneous 176hf 177hf isotope ratios with hft90 values of 1 and suggest a homogeneous mixed crust mantle magma source we conclude that the elatsite porphyry cu and the chelopech high sulfidation epithermal deposits were formed within a very short time span and could be partly contemporaneous however they are related to two distinct upper crustal magmatic reservoirs and they cannot be considered as a genetically paired porphyry cu and high sulfidation epithermal related to a single magmatic hydrothermal system centered on the same intrusion

re os geochronology and systematics in molybdenite from the endako porphyry molybdenum deposit british columbia canada precise re os molybdenite ages for the endako porphyry molybdenum deposit and the nithi mountain occurrence british columbia canada yield new information on the timing of molybdenite deposition three episodes of mineralization are indicated 1 154 ma 2 148 to 146 ma and 3 145 ma re os dates agree well with new u pb and 40 ar 39 ar data from granitoids of the nithi phase 155 2 1 5 ma the endako phase 148 4 1 5 ma and a preore dike 147 4 0 6 ma and the casey phase 145 1 0 2 ma plus hydrothermal biotite 144 5 0 75 ma villeneuve et al 2001 the excellent correlation between the re os dates from molybdenite and the ages of granitoids from the area indicates a direct genetic relationship between these plutons and molybdenite mineralization these re os data also provide further insight into the systematics and robustness of re os in molybdenite the endako molybdenum deposit is the product of two episodes of mineralization both associated with moderate salinity 5 15 wt nacl equiv and high temperature 440 c fluids a third low temperature 190 300 c hydrothermal event is associated with eocene faulting selby et al 2000 the clear distinction between two of the re os molybdenite dates in the deposit and the close agreement of these dates to the ages of granitoid intrusions suggest that the re os system in molybdenite is not reset by high temperature 440 c moderate salinity hydrothermal fluids in contrast magmatic biotite in a 147 ma preore dike has a 40 ar 39 ar age of 145 ma identical to the age of the hydrothermal biotite indicating that 440 c fluids reset the ar ar system in biotite hence we interpret that the re os isotope system in molybdenite is resistant to resetting by high temperature fluids 440 c in contrast to the 40 ar 39 ar system in biotite the re os system in molybdenite is also unaffected by postore hydrothermal fluids these data show that molybdenite can provide critical information in understanding sulfide mineral deposition in ore systems which is not recorded by other isotopic systems in alteration minerals

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

multiple mineralization events of the paleozoic tuwu porphyry copper deposit eastern tianshan evidence from geology fluid inclusions sulfur isotopes and geochronology the tuwu porphyry cu deposit located in eastern tianshan nw china is hosted by a plagiogranite porphyry and carboniferous qi eshan group volcanic rocks based on crosscutting relationships and mineral assemblages hydrothermal alteration and mineralization processes at tuwu can be divided into four stages early propylitic alteration stage i porphyry mineralization stage ii overprinting mineralization stage iii and post mineralization stage iv the porphyry mineralization stage stage ii contributed to the majority of the cu mo resource with cu mineralization occurring mainly as quartz chalcopyrite veins stage iii also produced minor cu mineralization characterized by chalcopyrite anhydrite chlorite calcite assemblages fluid inclusion fi study reveals that stage ii is characterized by a high temperature high salinity highly oxidized and k rich h2o nacl cacl2 fluid fluid boiling and mixing likely occurred during the porphyry mineralization stage leading to the precipitation of chalcopyrite and pyrite alteration and mineralization in stage iii were derived from a s rich h2o nacl cacl fluid with fluid boiling leading to the precipitation of chalcopyrite the 34s values of chalcopyrite from stages ii and iii are 0 8 0 6 and 1 1 1 3 respectively suggesting magmatic sources for the ore forming components of both stages 40ar 39ar dating indicates that stage ii likely occurred at 328 1 1 4 ma around the age emplacement of the causative plagiogranite porphyry ca 337 7 330 3 ma we suggest the overprinting mineralization event occurred at ca 323 ma spatially and genetically related to the emplacement of the quartz albite porphyry at 323 6 2 5 ma

hydrothermal alteration mineralization and structural geology of the zijinshan high sulfidation au cu deposit fujian province southeast china zijinshan is a world class au cu district located in southwest fujian province southeast china it contains a diverse array of ore deposits hosted by the zijinshan granite complex and surrounding volcano sedimentary rocks associated deposits include high sulfidation epithermal gold copper intermediate sulfidation epithermal polymetallic silver base metal and porphyry molybdenum copper deposits the zijinshan high sulfidation au cu deposit is located in the middle of the zijinshan district and ore zones are hosted in below and adjacent to the zijinshan lithocap the host rocks are part of a jurassic granite complex emplaced between 165 and 157 ma 40ar 39ar age spectra of hydrothermal alunite indicate that hydrothermal activity at zijinshan occurred at 102 86 0 61 to 101 19 0 60 ma and was associated with dacite porphyry dikes that intruded at 104 8 0 9 ma hydrothermal activity produced pervasive silicic and advanced argillic alteration assemblages that are zoned from a central massive quartz alteration domain outward to massive quartz dickite disseminated quartz alunite dickite and disseminated quartz muscovite dickite zones copper sulfides occur in hydrothermally cemented breccias veins and minor disseminations in granite copper minerals include covellite digenite and minor enargite oxide au ores are associated with strongly weathered silicic alteration in the upper parts of the deposit a set of subvertical ne striking faults were active prior to mineralization at zijinshan during mineralization a nw striking fault system controlled the emplacement of veins breccias and dacite dikes kinematic indicators show that most of the nw striking faults were active as normal oblique faults during mineralization postmineralization ene striking dextral strike slip faults are associated with minor conjugate nnw striking faults the premineralization structures at zijinshan likely formed during nw directed compression synmineralization structures formed under subvertical contraction and subhorizontal nne plunging extension postmineralization structures developed in a strike slip regime defined by a nw oriented axis of compression and ne trending axis of extension changes in the styles of pre and synmineralization structures are related to fluctuations in the regional stress regime the transition from compression to extension was fundamental to mineralization postmineralization faults are related to transient local stress regime variations at the deposit scale

porphyry epithermal transition maricunga belt northern chile the refugio aldebaran and la pepa districts in the maricunga belt of northern chile contain advanced argillic alteration zones that locally host high sulfidation epithermal gold deposits in proximity to porphyry gold copper deposits the spatial association suggests a genetic link mineralized zones are characterized by four main vein types that formed at different times and have specific zonal relationships a veinlets are the earliest and deepest vein type they are restricted to potassic alteration zones in intrusive rocks a veinlets contain variable amounts of quartz magnetite biotite and chalcopyrite and locally have k feldspar halos they have nonmatching irregular vein walls and lack internal symmetry hypersaline liquid rich inclusions coexisting with vapor rich inclusions in a veinlets indicate temperatures as high as nearly 700 c and pressures between 200 and 400 bars assuming a lithostatic load depths of 0 8 to 1 6 km are inferred zones of abundant a veinlets contain mostly 1 ppm gold and 0 1 to 0 4 percent hypogene copper banded quartz veinlets occur mostly above a veinlets and cut a veinlets where they overlap dark gray bands the color resulting from a high density of vapor rich fluid inclusions and micron sized grains of magnetite commonly occur as symmetric pairs near the vein walls vein walls are parallel and slightly wavy vuggy vein centers are common and alteration envelopes are absent data from rare liquid rich inclusions in banded quartz veinlets indicate temperatures 350 c at pressures between 20 and 150 bars assuming a hydrostatic load depths of 0 2 to 1 5 km are inferred zones of abundant banded quartz veinlets generally contain 0 5 to 2 ppm gold and 0 1 percent hypogene copper d veins are pyrite veins with quartz sericite pyrite halos they are widespread and crosscut a veinlets and banded quartz veinlets the brittle nature of d veins and limited fluid inclusion data suggest temperatures 400 c d veins serve as important time lines they are nowhere truncated or crosscut by intrusions a veinlets or banded quartz veinlets quartz alunite replacement veins referred to as ledges in this paper are typical of the high sulfidation epithermal environment they are mostly limited to overlying volcanic rocks they contain local core zones of vuggy residual quartz that can contain enargite or at higher elevations barite of the three districts studied only la pepa has mineable quartz alunite ledges which contain an average gold grade of about 20 ppm a spectrum of porphyry style deposits exists cerro casale at aldebaran shares many characteristics of porphyry copper deposits worldwide whereas verde at refugio is a true porphyry gold deposit potassic alteration zones and a veinlets are strongly developed at cerro casale whereas they are absent at verde banded quartz veinlets predominate at verde whereas they occur only at the upper levels of cerro casale the pancho deposit at refugio and the cavancha deposit at la pepa are telescoped systems in which banded quartz veinlets overprint potassic alteration zones and a veinlets a veinlets and banded quartz veinlets cut and are cut by intrusions indicating multiple cycles of intrusion potassic alteration a veinlets banded quartz veinlets during formation of porphyry style mineralization banded quartz veinlets are thought to have formed by flashing of magmatic fluids during episodic transitions from lithostatic to hydrostatic pressure loss of sulfur to the vapor phase during flashing inhibited formation of copper sulfides in banded quartz veinlets and therefore resulted in high gold copper ratios where rising magmatic vapors condensed into overlying meteoric water along faults barren quartz alunite ledges formed this conclusion is supported by equivalent 40ar 39ar dates on hydrothermal biotite associated with porphyry style ore and alunite from barren ledges at aldebaran 40ar 39ar dates at la pepa indicate alunite formed at least 140 000 years to as long as 900 000 years after hydrothermal biotite within the high sulfidation epithermal environment the development of ore depends on the ability of late moderate salinity magmatic fluids to reach the surface without condensing a brine upon ascent cooling and boiling of the moderate salinity fluid below its critical temperature results in the formation of sericite at depth and alunite near the surface that is essentially synchronous with high sulfidation ore formation the timing of the switch from lithostatic pressures to brittle hydrostatic conditions relative to the life of the hydrothermal system might determine how much porphyry style ore forms relative to high sulfidation epithermal ore

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

epidote trace element chemistry as an exploration tool in the collahuasi district northern chile the collahuasi district of northern chile hosts several late eocene early oligocene world class porphyry cu mo deposits including rosario ujina and quebrada blanca deposits and associated high sulfidation epithermal mineralization at la grande mineralization is hosted by intermediate to felsic intrusive and volcanic rocks of the upper paleozoic to lower triassic collahuasi group which experienced lower greenschist facies regional metamorphism prior to mineralization extensive hydrothermal alteration zones surround the porphyry and epithermal deposits associated with hypogene ore forming processes however outside of the observed sulfide halo the limits of geochemical anomalism associated with mineralization are difficult to define due to mineralogical similarities between weak distal propylitic alteration and regional metamorphism affecting the host rocks recent advancements in laser ablation inductively coupled plasma mass spectrometry analysis of epidote from hydrothermal alteration zones around porphyry and skarn deposits have shown that low level hypogene geochemical anomalies can be detected at distances farther from the center of mineralization than by conventional rock chip sampling selective analysis of propylitic epidote from the collahuasi district indicates that anomalous concentrations of distal pathfinder elements in epidote including as 50 ppm sb 25 ppm pb 100 ppm and mn 5 000 ppm were detectable 1 5 to 4 0 km from deposit centers significantly the concentrations of these trace elements in epidote were obtained from samples that contained whole rock concentrations of

timing and formation of porphyry cu mo mineralization in the chuquicamata district northern chile new constraints from the toki cluster the recently discovered toki cluster which includes the toki quetena genoveva miranda and opache porphyry cu mo prospects is located 15 km south southwest of the chuquicamata radomiro tomic mines in northern chile these prospects occur in an area of 5 6 km and are completely covered with neogene alluvial deposits inferred resources for the cluster are estimated at about 20 mt of fine copper with toki and quetena contributing 88 of these resources mineralization in these deposits is associated with tonalite porphyries that intruded andesites and dacites of the collahuasi group and intrusions of the fortuna los picos granodioritic complex hypogene mineralization in the toki cluster consists mainly of chalcopyrite bornite with minor molybdenite with mineralization grading outward to a chalcopyrite pyrite zone and ultimately to a pyrite halo alteration is dominantly of the potassic type with k feldspar and hydrothermal biotite sericitic alteration is relatively restricted to late quartz pyrite veins d type veins previous k ar geochronology for the cluster yielded ages within a range of 34 to 40 ma four new re os ages for toki indicate that molybdenite mineralization occurred in a single pulse at 38 ma re os ages for three different molybdenite samples from quetena are within error of the toki mineralization ages these ages are concordant with a new zircon u pb age of 38 6 0 7 ma from the tonalite porphyry in quetena two re os ages for genoveva 38 1 0 2 and 38 0 0 2 ma are also within error of the toki and quetena molybdenite ages four re os molybdenite ages for opache range between 36 4 and 37 6 ma the miranda prospect is the youngest with an age of 36 ma four new re os ages for the chuquicamata deposit range between 33 and 32 ma whereas nine new 40ar 39ar ages of biotite muscovite and k feldspar range between 32 and 31 ma analyzed molybdenites have re and os concentrations that vary between 21 3 099 ppm and 8 1 231 ppb respectively the highest re and os concentrations are found in the toki prospect three new 40ar 39ar ages for the toki cluster are younger than the re os mineralization ages the age spectra for these three samples show evidence of excess argon and have similar inverse isochron ages of 35 ma that probably reflect a late hydrothermal phyllic event the new geochronological data presented here for the toki cluster indicate that molybdenite mineralization occurred within a very short period probably within 2 ma and synchronously at 38 ma in three mineralization centers toki quetena and genoveva furthermore mineralization at the toki cluster preceded the emplacement of the chuquicamata deposit 35 31 ma and indicates that porphyry cu mo mineralization occurred episodically over a period of several million years in the chuquicamata district

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

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

re os geochronology of the el salvador porphyry cu mo deposit chile tracking analytical improvements in accuracy and precision over the past decade abstract sulfide minerals from the el salvador porphyry cu mo deposit chile were dated by re os geochronology to clarify the timing and duration of mineralization as these data are collected over the past 10 years they chronicle the evolution of re os analytical procedures and improvements in spike sample equilibration mass spectrometry and data reduction included in the data is the first tennantite pyrite re os isochron along with tennantite enargite pyrite os tracing of ore metals based on initial osmium systematics 187os 188osi porphyry associated mineralization follows the traditional early a transitional b and late d stage classification of relative timing of vein formation most cu and mo bearing sulfides were deposited during stages a and b field relationships clearly show the b stage veins are older than the d stage veins samples analyzed with carius tube digestion and mixed double spike indicate b stage molybdenite mineralization between 41 8 and 41 2 ma these ages best represent the timing of main stage mineralization at el salvador a five point 187re 187os isochron for pyrite tennantite from the latest d stage veins yields an age of 42 37 0 45 ma the isochron age while older is within error of early analyses of molybdenite by alkaline fusion digestion using single re and os spikes 42 4 42 0 ma a separate pyrite and tennantite enargite pair containing significant common os yields an initial 187os 188os ratio of 0 134 0 042 compatible with a predominantly mantle origin we appreciate the early analyses for their historical significance while emphasize the geological implications of the 41 8 41 2 ma molybdenite ages temperature estimates by sulfur thermometry and fluid inclusions are 390 510 c for the b stage molybdenites and 230 240 c for the latest d stage pyrite and tennantite analyses based on carius tube digestion and updated spike calibrations suggest a 0 6 m y duration 41 8 to 41 2 ma in line with published deposit geochronology the timing and duration of mineralization from re os dating of ore minerals is more precise than estimates from previously reported 40ar 39ar and k ar ages on alteration minerals the re os results suggest that the mineralization is temporally distinct from pre mineral rhyolite porphyry 42 63 0 28 ma and is immediately prior to or overlapping with post mineral latite dike emplacement 41 16 0 48 ma based on the re os and other geochronologic data the middle eocene intrusive activity in the el salvador district is divided into three pulses 1 44 42 5 ma for weakly mineralized porphyry intrusions 2 41 8 41 2 ma for intensely mineralized porphyry intrusions and 3 41 ma for small latite dike intrusions without major porphyry stocks the orientation of igneous dikes and porphyry stocks changed from nne ssw during the first pulse to wnw ese for the second and third pulses this implies that the wnw ese striking stress changed from 3 minimum principal compressive stress during the first pulse to hmax maximum principal compressional stress in a horizontal plane during the second and third pulses therefore the focus of intense porphyry cu mo mineralization occurred during a transient geodynamic reconfiguration just before extinction of major intrusive activity in the region

porphyry gold deposits of the refugio district maricunga belt northern chile the porphyry gold deposits of the refugio district and similar deposits in the maricunga belt contain the lowest known copper to gold ratios cu ppm au 0 03 of any porphyry type deposit the gold deposits are associated with subvolcanic andesitic to dacitic intrusions emplaced into coeval volcanic rocks both the verde and pancho deposits are zoned in space from a deeper zone of banded quartz veinlets associated with chlorite magnetite albite and or pyrite albite clay alteration to a shallow zone of pyrite albite clay and local quartz alunite ledges pancho contains an additional deepest porphyry copperlike zone with quartz veinlets a veinlets and potassic alteration relative to verde pancho is telescoped with all three zones present within a 400 m vertical interval the porphyry copperlike zone at pancho is characterized by a veinlets and pervasive potassic alteration both restricted to intrusive rocks a veinlets range from hairline streaks of magnetite biotite with minor quartz and chalcopyrite and k feldspar alteration envelopes to sugary quartz veinlets 1 cm in width with magnetite and chalcopyrite and no alteration envelopes hypersaline liquid inclusions coexisting with vapor rich inclusions indicate temperatures above 600 c and salinities as high as 84 wt percent nacl equiv a pressure estimate of 250 bars indicates a depth of 1 000 m assuming lithostatic pressure potassic alteration consists of a central zone of magnetite k feldspar oligoclase that changes outward to a biotite rich zone total sulfide content predominately as chalcopyrite is generally 1 vol percent whereas magnetite content is 2 to 5 percent where a veinlets and potassic alteration predominate grades are typically 0 1 wt percent hypogene copper and 0 5 to 1 ppm gold banded quartz veinlets are present at both verde and pancho where they occur mostly above a veinlets and cut a veinlets where they overlap they are less than 2 cm in thickness and lack alteration envelopes dark gray bands whose color is due to abundant vapor rich fluid inclusions and micrometer sized grains of magnetite commonly occur as symmetric pairs near the vein walls the bands are commonly botryoidal and are continuous through quartz grains suggesting that the quartz recrystallized from a silica gel rare liquid rich fluid inclusions in quartz indicate temperatures 350 c and salinities 35 wt percent nacl equiv estimated pressures are 200 bars suggesting depths of 190 to 1 500 m under hydrostatic pressure gold occurs both in the dark bands with magnetite and outside the dark bands with pyrite chlorite illite and k feldspar banded veinlets occupy steeply dipping radial and shallowly dipping concentric fractures zones of abundant banded veinlets without early a veinlets generally contain 0 5 to 2 ppm gold and 0 05 wt percent hypogene copper most of the differences between porphyry gold deposits at refugio and porphyry copper deposits can be attributed to shallower depths of formation less than 1 km compared to 1 5 to 4 km that is typical for porphyry copper deposits shallower depths resulted in lower sulfide concentrations local garnet veinlets widespread albite bearing alteration and most importantly banded quartz veinlets which are unique to porphyry gold deposits banded quartz veinlets are a direct result of episodic intrusion of magma to within 1 km of the surface and exposure of high temperature magmatic fluids to hydrostatic pressures episodic rupturing of a brittle ductile boundary surrounding the intrusive centers at verde and pancho led to flashing of magmatic fluids loss of sulfur to vapor and low sulfide gold ratios in ore

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

laser probe 40 ar 39 ar ages of early and late stage alteration assemblages rosario porphyry copper molybdenum deposit collahuasi district i region chile sensibly identical laser probe 40 ar 39 ar dates of 32 9 or 0 5 2sigma and 32 9 or 0 6 ma are determined for respectively early stage hydrothermal biotite and late stage hydrothermal muscovite from the giant rosario porphyry copper deposit in the collahuasi district of northern chile the age determinations were carried out on ca 1 cm 2 1 to 0 5 mm thick wafers of altered rock or on smaller chips broken therefrom obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure the age data although not unambiguous imply that this extremely large hydrothermal center was emplaced over a short interval hydrothermal activity in this central andean transect migrated eastward over a period of ca 3 to 4 m y from copaquire through quebrada blanca to rosario the intensity of mineralization increasing with time rosario chuquicamata and la escondida the largest of the west fissure domeyko fault system array were apparently emplaced coevally in the terminal stage of the late eocene early oligocene metallogenic episode

the late oligocene cevizlidere cu au mo deposit tunceli province eastern turkey the cevizlidere deposit located in the tunceli province of eastern anatolia is the largest porphyry cu au mo system in turkey the deposit is spatially related to a composite stock which was emplaced into paleozoic limestones and paleogene andesitic rocks to the southeast of the munzur mountains near the southwestern margin of the ovacik pull apart basin the host plutonic rocks at cevizlidere are porphyritic medium k calc alkaline diorites and granodiorites 40ar 39ar incremental step heating analysis of two igneous biotite separates obtained from syn mineral diorite porphyry yielded late oligocene cooling ages of 25 49 0 10 and 25 10 0 14 ma whereas hydrothermal biotite yielded an age of 24 73 0 08 ma re os ages obtained from two molybdenite separates 24 90 0 10 and 24 78 0 10 ma indicate that porphyry style alteration and mineralization developed shortly after magma emplacement the whole rock geochemical composition of the cevizlidere porphyry intrusions is consistent with derivation from partial melting of the metasomatized supra subduction zone mantle however based on regional tectonic reconstructions oligocene magmatic activity in this area appears to be related to a major kinematic reorganization that took place at around 25 ma during the switch from subduction to collisional tectonics in eastern anatolia this kinematic switch may be attributed to break off of the southern neo tethys oceanic slab prior to the arabia eurasia continent continent collision 12 10 ma following widespread middle eocene 50 43 ma arc back arc magmatism in this respect the subduction related tectonic setting of the late oligocene cevizlidere porphyry deposit is similar to that of the middle eocene copler epithermal au deposit the late timing of cevizlidere with respect to the southern neo tethys subduction may be comparable to some early to late miocene porphyry epithermal systems that lie within the contiguous urumieh dokhtar belt in central iran the later timing in iran reflects the diachronous nature of the eurasia afro arabia collision

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

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

genesis of the shangjinshan w mo polymetallic deposit in the eastern jiangnan tungsten belt evidences from geochemistry geochronology and zircon hf isotope analyses abstract the newly discovered shangjinshan w mo polymetallic deposit located at the southern anhui southeast china is a large scale skarn type tungsten polymetallic deposit according to mineral assemblages ore textures structures and superposition relationships of the mineralization three ore types have been recognized veinlet disseminated skarn the primary type quartz vein type and porphyry type the mineralized event of this deposit could be divided into four stages i the prograde skarn stage ii retrograde skarn stage iii quartz sulfide vein stage and iv quartz calcite vein stage the intrusion in the tungsten rich district of the shangjinshan is mainly composed of the granodiorite with zircon u pb ages of 138 4 1 2 ma to 136 7 1 3 ma meanwhile re os isotopic dating of five molybdenite samples yields an isochron age of 141 9 3 1 ma and a weighted average age of 139 3 1 7 ma respectively the ore forming ages are consistent with the rock forming ages within errors indicating that the w mo mineralization is genetically related to the shangjinshan granodiorite the granodiorite is i type granites characterized by metaluminous to slightly peraluminous and high k cal alkaline compositions they are also enriched in large ion lithophile elements liles and light rare earth elements lrees and depleted in high field strength elements hfses and heavy rare earth element hrees with obviously negative eu anomalies eu eu 0 50 0 84 the zircon ehf t values and two stage hf model ages tdm2 fall into the range 6 18 to 2 04 and 1 5 1 3 ga respectively the relatively low rhenium contents in molybdenite and high radioactive pb isotopes of the pyrites from stage ii indicate that the ore forming material was derived from shangjinshan granitic magmatism together with previously published data we propose that the w related granitoids in the jnb were likely generated by partial melting of the shangxi group with some additional mantle materials the granitic magmatism was caused by the asthenosphere upwelling triggered by the roll back of subduced paleo pacific plate during late jurassic to early cretaceous these granitic stocks predominately contributed to the production of skarn alteration and mineralization resulting in the giant tungsten polymetallic district in the jiangnan tungsten belt

physiographic and tectonic settings of high sulfidation epithermal gold silver deposits of the andes and their controls on mineralizing processes abstract gold and silver ores in the vast majority of andean high sulfidation epithermal au ag deposits occur at high present day elevations and typically 200 500 m below low relief landforms situated at 3500 to 5200 m a s l most deposits are middle miocene and younger and include el indio tambo pascua lama veladero el indio belt chile argentina cerro de pasco central peru pierina lagunas norte yanacocha northern peru quimsacocha ecuador and the california vetas mining district santander colombia jointly accounting for 130 moz au resources slightly older examples are only preserved in the atacama desert and include the middle eocene el guanaco and el hueso and the late oligocene early miocene la coipa deposits the absence of paleocene and older high sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift these conditions prevailed predominantly in segments of shallow angle subduction of the nazca or caribbean plate below the south american continent a tectonic setting also common for porphyry style cu au mo deposits stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent recording the terminal stages of local arc magmatism however dacitic domes are important at e g yanacocha and la coipa at lagunas norte a small stratovolcano largely pre dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high elevation low relief surfaces in the california vetas mining district and el indio belt hydrothermal alunite ages become generally younger upstream along the incising valleys indicating that hydrothermal activity and by inference ore deposition were facilitated by erosion the lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids ultimately enhancing ore deposition the host rock composition permeability and location of the water table control the distribution of alteration zones and ore intermediate volcanic rocks are the most common ore hosts but they typically pre date mineralization by several ma however high sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks california vetas mining district significantly older plutonic rocks pascua lama or quartzites lagunas norte large vuggy quartz alteration zones and commonly oxidized low grade large tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies whereas coherent volcanic plutonic or metamorphic rocks may host fault and breccia controlled ores the near surface steam heated zone can attain a thickness of several hundred meters in dry climates e g veladero pascua lama tambo but is typically poorly developed and less than 20 m thick in humid climatic zones the physiographic and tectonic settings of high sulfidation epithermal deposits are distinct from low sulfidation epithermal districts such as those of patagonia el penon chile or fruta del norte ecuador the latter range to significantly older ages jurassic to early eocene occur at mainly lower elevations and were emplaced in extensional settings a temporal coincidence between uplift erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low sulfidation districts

recognizing propylitic alteration associated with porphyry cu mo deposits in lower greenschist facies metamorphic terrain of the collahuasi district northern chile implications of petrographic and carbon isotope relationships the collahuasi district is a porphyry cu mo subprovince located on the andean high plateau of northern chile late eocene early oligocene mineralization in the district involved hydrothermal alteration of the upper paleozoic to lower triassic collahuasi group granodiorites diorites andesites dacites and rhyolites which had previously experienced regional lower greenschist facies metamorphism as well as uneconomic porphyry style mineralization and associated sericitic alteration alteration mapping represents an effective technique for prospecting for hydrothermal deposits but in low grade metamorphic terrains recognition of propylitic assemblages that represent the boundary of the mineralizing system is hampered by their resemblance to metamorphic mineral assemblages our results from collahuasi indicate that petrographic criteria can distinguish propylitic alteration from the effects of lower greenschist facies metamorphism but distinctions vary with rock type mafic intermediate and felsic rocks with varying degrees of propylitic alteration can be distinguished from background metamorphism mainly by the presence of calcite however propylitic alteration can also be distinguished by high whole rock 13c values and carbon contents 19 1 to 1 4 and up to 0 79 wt respectively that result from the incorporation of magmatic co2 during the mineralizing events lower 13c values 32 7 to 13 1 and carbon contents 0 09 wt were recorded in other types of hydrothermal alteration intermediate argillic sericitic and potassic alteration and associated with background metamorphism they reflect a dominantly 13c depleted carbon component that remains in the rocks after extensive co2 degassing during emplacement alteration by hydrothermal fluids or contamination by organic carbon carbon and oxygen isotope compositions of calcite from propylitic alteration which range from 12 to 0 3 and 6 6 to 25 9 respectively are consistent with a predominantly magmatic origin for the carbonates although subsequent resetting of the oxygen isotopes by local ground water and minor contributions of organic carbon are recorded in some samples spatial variation in 13c values and carbon contents of outcrop samples from the collahuasi district delineate zones of high magmatic co2 component that correspond exclusively to propylitic alteration surrounding economic deposits

re os ages for archean molybdenite and pyrite kuittila kivisuo finland and proterozoic molybdenite kabeliai lithuania testing the chronometer in a metamorphic and metasomatic setting seven 187re 187os ages were determined for molybdenite and pyrite samples from two well dated precambrian intrusions in fennoscandia to examine the sustainability of the re os chronometer in a metamorphic and metasomatic setting using a new 187re decay constant 1 666 10 11y 1 with a much improved uncertainty 0 31 we determined replicate re os ages for molybdenite and pyrite from the kuittila and kivisuo prospects in easternmost finland and for molybdenite from the kabeliai prospect in southernmost lithuania these two localities contain some of the oldest and youngest plutonic activity in fennoscandia and are associated with newly discovered economic au mineralization ilomantsi finland and a cu mo prospect kabeliai lithuania two re os ages for vein hosted kabeliai molybdenite average 1486 5 ma in excellent agreement with a 1505 11 ma u pb zircon age for the hosting kabeliai granite pluton the slightly younger age suggests the introduction of cu mo mineralization by a later phase of the kabeliai magmatic system mean re os ages of 2778 8 ma and 2781 8 ma for kuittila and kivisuo molybdenites respectively are in reasonable agreement with a 2753 5 ma weighted mean u pb zircon age for hosting kuittila tonalite these re os ages agree well with less precise ages of 2789 290 ma for a rb sr whole rock isochron and 2771 75 ma for the average of six sm nd tdm model ages for kuittila tonalite three re os analyses of a single pyrite mineral separate from the same sample of kuittila pluton that yielded a molybdenite separate provide individual model ages of 2710 27 2777 28 and 2830 28 ma re 17 4 12 1 and 8 4 ppb respectively with a mean value of 2770 120 ma in agreement with the kuittila molybdenite age the re and 187os abundances in these three pyrite splits are highly correlated r 0 9994 and provide a 187re 187os isochron age of 2607 47 ma with an intercept of 21 ppt 187os mswd 1 1 it appears that the re os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187os intercept reflects the in situ decay of 187re during the 160 to 170 m y interval from 2778 ma time of molybdenite pyrite deposition to 2607 ma time of pyrite resetting when the re os data for molybdenites from the nearby kivisuo prospect are plotted together with the kuittila molybdenite and pyrite data a well constrained five point isochron with an age of 2780 8 ma and a 187os intercept 2 4 3 8 ppt of essentially zero results mswd 1 5 we suggest that the pyrite isochron age records a regional metamorphic and or hydrothermal event possibly the time of au mineralization a proposed re os age of 2607 ma for au mineralization is in good agreement with radiometric ages by other methods that address the timing of archean au mineralization in deposits worldwide so called late au model molybdenite in contrast provides a robust re os chronometer retaining its original formation age of 2780 ma despite subsequent metamorphic disturbances in archean and proterozoic time

longevity of magmatic hydrothermal systems in the daye cu fe au district eastern china with implications for mineral exploration the daye district represents one of the largest concentrations of cu fe au skarn deposits in china with additional porphyry cu mo deposits these deposits are closely associated with late mesozoic intrusions of intermediate to felsic composition previous studies have yielded important age constraints for several deposits timing and history of metallogenesis for the entire district however remain not well understood in this paper we present twenty five new ages re os 40ar 39ar and u pb to better understand the temporal framework of the district wide polymetallic mineralization these ages when combined with existing data indicate a prolonged history of mineralization lasting 25 million years from late jurassic to early cretaceous 157 132 ma and that most ore deposits of the district are results of multistage hydrothermal activity discrete episodes of ore formation coincide well with emplacement ages of granitoid intrusions within the district as revealed by recent zircon u pb dating this age consistency substantiates a genetic relationship between ore formation and intrusive magmatism prolonged and episodic magmatic and hydrothermal activities must have played significant roles in the formation of polymetallic ore deposits in the district by repeatedly supplying heat fluids and metals previous studies have demonstrated that the daye district was dominated by lithospheric extension during late jurassic to early cretaceous when extensive magmatism and mineralization took place under the extensional setting the area could have experienced significant uplift or extensional exhumation that would have brought early stage intrusions and associated ore deposits to shallower levels with ore bodies formed by subsequent magmatic hydrothermal events tending to be localized at greater depth this in turn indicates potentials to find new ore bodies at depth of known deposits especially for those formed by episodic magmatic hydrothermal processes this view is partly confirmed by the recent success in deep exploration beneath the tieshan fe cu and tonglushan cu au fe deposits

the study of molybdenites through the187re 187os chronometer we determined the re and os concentrations in 11 molybdenites of various ages 30 2700 m y and from various areas no common osmium has been detected and os was found to be purely radiogenic therefore by comparing the concentrations we were able to determine 187 re 187 os ages for most samples these ages are in good agreement with those measured in surrounding rocks and probably correspond to the age of mineralization however two or three re os ages seem unreliable being probably related to some rhenium leaching through later metamorphic and or hydrothermal events this shows that the petrographic characteristics of ores and the type of crystallization of mos 2 are important in choosing samples suitable for dating re and os determinations combined with careful petrographic investigations may give important clues on the genesis and evolution of molybdenum ores

the chronology of early mesozoic arc magmatism in the yerington district of western nevada and its regional implications the yerington district west central nevada is underlain by arc volcanic sedimentary and plutonic rocks ranging in age from middle triassic to middle jurassic twenty three u pb radiometric dates from eight igneous units have been determined for zircon from both plutonic and volcanic rocks in the district which establish two distinct periods of voluminous intermediate to silicic high k calc alkaline magmatism the earlier period is constrained by zircon dates of 232 and 233 ma on a quartz porphyry and a metadiorite respectively which intrude and are possibly comagmatic with andesitic to rhyolitic volcanic rocks the second period of arc magmatism is constrained by 5 concordant zircon dates that define an approximately 4 m y period 169 165 ma in the middle jurassic during which as much as 4 km of volcanic and volcaniclastic strata accumulated and two major batholiths were emplaced middle jurassic magmatism began with eruption of the volcanics of artesia lake they were intruded by the probably comagmatic 250 km 2 yerington batholith which was differentiated emplaced and crystallized within approximately 1 m y 169 4 168 5 ma granite porphyry dikes 168 5 ma are the latest phase of the yerington batholith and are synchronous with porphyry and skarn copper mineralization disconformably overlying the artesia volcanics is a second volcanic suite consisting of as much as 2 km of latitic volcanics of fulstone spring 166 5 ma the volcanics of fulstone spring were subsequently intruded by quartz monzodiorite porphyry dikes 165 ma and the shamrock batholith 165 8 ma the triassic through lower jurassic stratigraphic column was contact metamorphosed and folded adjacent to and at least in part contemporaneously with emplacement of the yerington and shamrock batholiths 169 4 165 8 ma we interpret the early 232 233 ma period of magmatism to represent a minimum age for the establishment of a west facing triassic volcanic arc within west facing triassic volcanic arc within west central nevada following the permian triassic sonoma orogeny the timing of middle jurassic arc magmatism and deformation in the yerington district and surrounding regions is strikingly similar to the timing of magmatic and tectonic events documented in the klamath mountains region of northwestern california we suggest that the similar record of arc magmatism and deformation in these two regions is not fortuitous but reflects a common tectonic evolution within a single west facing arc constructed along the western edge of north america

a 53 year seasonally resolved oxygen and carbon isotope record from a modern gibraltar speleothem reconstructed drip water and relationship to local precipitation abstract the response of a climate proxy against measured temperature rainfall and atmospheric circulation patterns at sub annual resolution is the ultimate test of proxy fidelity but very few data exist showing the level of correspondence between speleothem climate proxies and the instrumental climate record cave sites on the gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with the longest known available precipitation isotopes and instrumental records an actively growing speleothem sampled from new st michaels cave in 2004 is composed of paired laminae consisting of light columnar calcite and a darker microsparitic calcite stable isotope analysis of samples micromilled in 100 m steps at the equivalent of bi monthly intervals reveals fabric correlated annual cycles in carbon isotopes oxygen isotopes and trace elements responding to seasonal changes in cave microclimate hydrology and ventilation patterns calcite 13 c values reach a minimum in the light columnar fabric and evidence from trace element behaviour and cave monitoring indicates that this grows under cave winter conditions of highest p co 2 whereas the dark microsparitic calcite characterised by elevated 13 c and 18 o values grows under low summer p co 2 conditions drip water 13 c dic reaches a minimum in march april at which time the attenuated 18 o signal becomes most representative of winter precipitation an age model based on cycle counting and the position of the 14 c bomb carbon spike yields a precisely dated winter oxygen isotope proxy of cave seepage water for comparison with the gnip and instrumental climate record for gibraltar the 18 o characteristics of calcite deposited from drip water representing winter precipitation for each year can be derived from the seasonally resolved record and allows reconstruction of the 18 o drip water representing winter precipitation for each year from 1951 2004 these data show an encouraging level of correspondence r 2 0 47 with the 18 o of rainfall falling each year between october and march and on a decadal scale the 18 o of reconstructed winter drip water mirrors secular change in mean winter temperatures

alunite 40ar 39ar and zircon u pb constraints on the magmatic hydrothermal history of the zijinshan high sulfidation epithermal cu au deposit and the adjacent luoboling porphyry cu mo deposit south china implications for their genetic association

carbon isotope evidence for microbial involvement in exotic copper silicate mineralization huinquintipa and mina sur northern chile exotic copper silicate and oxide mineralization a peripheral facies of several cenozoic porphyry cu mo systems in northern chile is traditionally ascribed to the lateral migration of cu rich initially highly acidic waters away from active zones of supergene leaching and sulfide enrichment copper precipitation as cu fe silicates and cu mn fe oxides in permeable accumulating pediment gravels or underlying fractured bedrock resulted in part from fluid neutralization attending hydrolytic alteration supergene activity was most intense in the late oligocene early miocene during a protracted middle cenozoic semiarid interval along the lower pacific slope of the central andean cordillera occidental samples from the huinquintipa and mina sur deposits contiguous with respectively the 32 7 to 34 4 ma rosario collahuasi and 31 1 to 34 6 ma chuquicamata porphyry centers were examined for carbon isotope evidence of the conditions of ore formation carbon was extracted as co2 from representative high grade chrysocolla rich samples using four different techniques combustion pyrolysis crushing and edta leaching and has a widely varying isotopic composition consistent with mixing of atmosphere derived carbon with a 13c value of 0 per mil plant derived carbon with 13c values between 25 and 15 per mil and carbon with exceptionally low 13c values of 47 per mil the extremely light carbon sampled primarily by leaching with a 10 percent edta solution is most plausibly ascribed to a microbial consortium including anaerobic methanogenic microbes the presence of mineralized carbon having such low 13c values provides evidence that microbial metabolism may have contributed directly to copper silicate precipitation in these deposits lower 13c values at huinquintipa than at mina sur could be attributed to higher water tables and higher toxic metal contents in the mineralizing fluids the new data reinforce the role played by microbes in the economically critical supergene modification of porphyry and epithermal ore deposits in the semiarid environment

small scale morphology across the surf zone small scale 5 m horizontal length nearshore morphologic height variations were measured by combining crab surveys with bed elevations acquired with a 1 mhz sonic altimeter mounted on the crab during the october phase of the duck94 experiment bedform plan views were recorded simultaneously using a 500 khz side scan sonar mounted on the crab waves and currents were measured at the same time significant temporal and spatial variations in the small scale morphology were measured in response to changing waves and currents during the 2 weeks examined three cases are examined in detail 1 mild waves and weak longshore currents resulting in wave ripples throughout the study area 2 storm waves with strong longshore currents resulting in lunate and straight crested mega ripples in the trough of the barred beach and 3 narrow band normally incident waves with a strong rip current resulting in a planar bed except in the throat of the rip where mega ripples were measured wavenumber spectra of the bed were generally broad indicating newly formed ripples coexisted with residual ripples from the past to form complex multi scaled ripple patterns

late paleozoic early triassic magmatism on the western margin of gondwana collahuasi area northern chile abstract the basement in the altiplano high plateau of the andes of northern chile mostly consists of late paleozoic to early triassic felsic igneous rocks collahuasi group that were emplaced and extruded along the western margin of the gondwana supercontinent this igneous suite crops out in the collahuasi area and forms the backbone of most of the high andes from latitude 20 to 22 s rocks of the collahuasi group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main andes of chile the frontal cordillera of argentina choiyoi group or choiyoi granite rhyolite province and the eastern cordillera of peru thirteen new shrimp u pb zircon ages from the collahuasi area document a bimodal timing for magmatism with a dominant peak at about 300 ma and a less significant one at 244 ma copper mo porphyry mineralization is related to the younger igneous event initial hf isotopic ratios for the 300 ma zircons range from about 2 to 6 indicating that the magmas incorporated components with a significant crustal residence time the 244 ma magmas were derived from a less enriched source with the initial hf values ranging from 2 to 6 suggestive of a mixture with a more depleted component limited whole rock 144nd 143nd and 87sr 86sr isotopic ratios further support the likelihood that the collahuasi group magmatism incorporated significant older crustal components or at least a mixture of crustal sources with more and less evolved isotopic signatures

a re os study of sulfide minerals from the bagdad porphyry cu mo deposit northern arizona usa rhenium and osmium isotopes in sulfide minerals from the bagdad porphyry cu mo deposit have been used to determine timing of mineralization and the source of osmium and by inference ore metals molybdenite chalcopyrite and pyrite were analyzed mainly from the quartz monzonite and porphyritic quartz monzonite units which are characterized by moderate to strong potassic alteration secondary biotite and k feldspar rhenium concentrations in molybdenite are between 330 and 642 ppm four re os analyses of two molybdenite samples from the quartz monzonite and porphyritic quartz monzonite yield a weighted average age of 71 8 0 2 ma 2 analyses of a third sample from a molybdenite vein in precambrian rocks outside of the main ore zone yield a weighted average age of 75 9 0 2 ma 2 and provide evidence of two separate mineralization episodes chalcopyrite samples contain 6 to 12 ppt os and 1 7 to 4 1 ppb re 187os 188os initial ratios are between 0 1 and 0 8 pyrite samples have osmium and rhenium concentrations varying in the range 8 17 ppt and 3 9 6 8 ppb respectively analyses from these pyrite samples yield an eight point isochron with an age of 77 15 ma 2 and an initial 187os 188os ratio of 2 1 0 8 mswd 0 90 the results presented here add to the growing body of work indicating that porphyry type mineralization is produced by long term multiple episodes of magmatism and associated mineralization the data also support the hypothesis that a significant part of the metals and magmas may have a crustal source as has been suggested for other copper deposits and districts in arizona

a revised late eocene age for porphyry cu magmatism in the escondida area northern chile new u pb zircon and 40 ar 39 ar single crystal incremental heating biotite dates are reported for porphyritic rocks from three mineralized centers in the escondida region of northern chile all dates are reported with 2sigma errors at the chimborazo porphyry cu prospect a weighted mean 40 ar 39 ar plateau age of 38 09 or 0 30 ma has been obtained for igneous biotite from a syn or postmineralization feldspar biotite quartz porphyry intrusion at the zaldivar porphyry cu mine zircon and biotite have been analyzed from a similar synmineralization feldspar biotite quartz porphyry the llamo porphyry the zircon u pb age is 38 7 or 1 3 ma whereas the weighted mean 40 ar 39 ar plateau age of igneous biotite is slightly younger at 37 40 or 0 18 ma u pb dating of two other rhyolitic quartz feldspar porphyry bodies that host much of the ore at zaldivar shows that these rocks are paleozoic in age antigua porphyry 290 or 4 ma zaldivar porphyry broadly coeval with antigua but age poorly defined at the escondida deposit the reportedly synmineralization escondida porphyry is dated by u pb at 37 9 or 1 1 ma whereas the reportedly postmineralization rhyolitic porphyry returned a u pb age of 34 7 or 1 7 ma these results indicate a single pulse of synmineralization magmatism in the escondida district at approximately 38 ma followed by minor largely nonmineralizing intrusive events the timing of main stage porphyry mineralization at escondida is apparently bracketed by the ages of the escondida 37 9 or 1 1 ma and rhyolitic porphyries 34 7 or 1 7 ma previously published k ar dates for intrusive rocks and alteration minerals at escondida which range from 39 to 31 ma are interpreted to reflect variable degrees of disturbance of the k ar system by later intrusive events and or by supergene weathering processes an age of approximately 38 ma for ore forming porphyry magmatism in the escondida region is significantly older than previously accepted best estimates based on k ar and u pb dating 32 34 ma but still falls within the age range of other large porphyry systems within the north south trending west fissure zone of northern chile 31 42 ma a common feature of the porphyry intrusions at zaldivar and escondida and also at chuquicamata and el salvador is the presence of zircons with inherited upper paleozoic cores these cores are interpreted to reflect interaction between arc magmas and crustal rocks during melting assimilation storage and homogenization mash and assimilation and fractional crystallization afc processes at various crustal levels

rhenium osmium evidence for regional mineralization in southwestern north america more than 40 base metal porphyry ore deposits in southwestern north america are associated with the laramide orogeny about 90 million to 50 million years ago rhenium osmium dates on molybdenite a rhenium enriched sulfide common in many of the deposits reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception magnitude or duration of magmatism deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization rhenium osmium ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago most deposits that formed in the oldest interval are within the older precambrian basement of northwestern arizona whereas the younger deposits are restricted to the younger precambrian basement in southern arizona and northern mexico synchronous widespread mineralization indicates that similar crust mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes

multiple rejuvenation episodes of a silicic magma reservoir at the origin of the large diatreme dome complex and porphyry type mineralization events at cerro de pasco peru abstract the cerro de pasco district in central peru hosts one of the world largest porphyry related epithermal polymetallic deposits the district is centered onto a large diatreme dome complex crosscut by numerous dacite to rhyodacite bodies showing domal structures and quartz monzonite dykes three temporally distinct high temperature porphyry type mineralization events have been recognized pm1 pm2 and pm3 dating of the latter by molybdenite re os and by zircon u pb la icp ms and ca id tims geochronology complements the already available dataset of zircon u pb ages of the subvolcanic and volcanic rocks mostly dacitic in composition occurring in the district and shows the existence of successive short lived episodes 50 at pressures between 0 9 and 3 4 kbar and at temperatures between 680 c and 725 c at such conditions magma is beyond the point of rheological lock up and is not eruptible the emplacement of the mostly dacitic subvolcanic and volcanic rocks has required a series of rejuvenation events of the upper crustal high crystallinity silicic magma reservoir field relationships and geochronology indicate that emplacement of the subvolcanic and volcanic rocks is preceded each time by high temperature 600 c quartz pyrite magnetite chalcopyrite and quartz molybdenite veining forming the three recognized porphyry type mineralization events hydrothermal quartz of these veins host silicate melt inclusions a rare feature in hydrothermal veins their composition determined by la icp ms analysis indicates that the mineralizing fluids potentially sourced from intermediate magma recharges have circulated in a magma reservoir in which the residual interstitial melt was more evolved than the melt trapped as inclusions in the magmatic quartz the combined geochemical and geochronological data obtained show that several episodes of rejuvenation of a highly crystallized upper crustal silicic magma reservoir probably triggered by intermediate magma recharges and by circulation of co2 and s rich fluids exsolved from them are at the origin of the large diatreme dome complex and porphyry type mineralization events at cerro de pasco

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

characteristics and 40ar 39ar geochronology of the erdenet cu mo deposit mongolia the early to middle triassic erdenet porphyry cu mo deposit in northern mongolia developed in a continent continent arc collision zone within the central asian orogenic belt the porphyry system is related to multiple intrusions of crystal crowded biotite granodiorite porphyry which formed a composite stock about 900 m in diameter with multiple porphyritic microgranodiorite dikes wall rocks are late permian to early triassic medium grained granodiorite with similar whole rock geochemistry mineralogy and composition to the granodiorite porphyry whole rock analysis of the granodiorite porphyry and wall rocks shows that these rocks cannot be discriminated but both have depleted middle heavy rare earth elements and y typical of fertile porphyry magmatic suites at the current pit level 1 250 m elev early porphyry style quartz veins a and b type are locally infilled by pyrite chalcopyrite with subordinate bornite but most of the chalcopyrite occurs in d veins that constitute more than 50 of the cu grade 0 5 wt cu the 0 3 wt cu shell resembles a molar tooth enveloping the granodiorite porphyry with deeper roots extending down the wal rock contacts molybdenite occurs in monomineralic veins and in finely laminated to massive quartz molybdenite veins the most important alteration is quartz muscovite which occurs as relatively coarse 100 500 m alteration selvages 1 5 cm that envelop d veins the d veins cut illite kaolinite smectite or intermediate argillic alteration intermediate argillic alteration together with abundant pink anhydrite commonly hydrated to gypsum extends from at least 1 300 to 900 m elevation in the deepest drill holes and has overprinted early potassic alteration or relatively unaltered red granodiorite meter wide zones of kaolinite cut the anhydrite gypsum at all levels there is an abrupt transition outward from the intermediate argillic alteration to chlorite epidote propylitic alteration at 50 to 200 m from the granodiorite porphyry contact although d veins and chalcopyrite extend outward to the propylitic zone the erdenet porphyry system was overprinted by advanced argillic alteration which outcrops 2 km northwest of the pit and forms a lithocap that extends over 10 2 5 km it is characterized by residual quartz andalusite na ca and k alunite diaspore pyrophyllite zunyite topaz dickite and kaolinite the upper part of the porphyry cu mo deposit removed by mining comprised a bornite chalcocite enriched zone up to 300 m thick with an average grade of 0 7 wt cu and up to 5 wt cu locally based on hypogene bornite chalcocite mineral textures and high sulfidation state mineralogy the enriched zone is inferred to be of hypogene origin but modified by supergene processes consequently it may be related to formation of the lithocap previous re os dates of 240 4 and 240 7 0 8 ma for molybdenite in quartz veins are comparable to new 40 ar 39 ar dates of 239 7 1 6 and 240 2 ma for muscovite that envelops d veins one 40 ar 39 ar date on k alunite from the lithocap of 223 5 1 9 ma suggests that it may be about 16 m y younger than erdenet but this result needs to be verified by further dating

thallium isotope variations in an ore bearing continental igneous setting collahuasi formation northern chile abstract thallium is a highly incompatible element and a large fraction of the bulk silicate earth tl budget is therefore expected to reside in the continental crust nonetheless the tl isotope systematics of continental rocks are essentially unexplored at present here we present new tl isotope composition and concentration data for a suite of 36 intrusive and extrusive igneous rocks from the vicinity of porphyry cu deposits in the collahuasi formation of the central andes in northern chile the igneous lithologies of the rocks are variably affected by the hydrothermal alteration that accompanied the formation of the cu deposits the samples display tl concentrations that vary by more than an order of magnitude from 0 1 to 3 2 g g whilst e 205 tl ranges between 5 1 and 0 1 e 205 tl is the deviation of the 205 tl 203 tl isotope ratio of a sample from a standard in parts per 10 4 these variations are primarily thought to be a consequence of hydrothermal alteration processes including metasomatic transport of tl and formation breakdown of tl bearing minerals which are associated with small but significant tl isotope effects the tl abundances show excellent correlations with both k and rb concentrations but no co variation with cu this demonstrates that tl displays only limited chalcophile affinity in the continental crust of the collahuasi formation but behaves as a lithophile element with a distribution that is primarily governed by partitioning of tl into k bearing phases collahuasi samples with propylitic alteration features which are derived from the marginal parts of the hydrothermal systems have on average slightly lighter tl isotope compositions than rocks from the more central sericitic and argillic alteration zones this small but statistically significant difference most likely reflects preferential retention of isotopically heavy tl in alteration phases such as white micas and clays which formed during sericitic and argillic alteration

variation of mo isotopes from molybdenite in high temperature hydrothermal ore deposits measurable molybdenum isotope fractionation in molybdenites from different ore deposits through time provides insights into ore genesis and a new technique to identify open system behavior of re os in molybdenites molybdenite samples from six porphyry copper deposits one epithermal polymetallic vein deposit four skarns and three fe oxide cu au deposits were analyzed the 97mo where open image in new window for all samples varied from 1 34 0 09 to 0 26 0 04 this is the largest molybdenum isotopic variation in molybdenite from high temperature ore deposits recorded to date 97mo of molybdenite varies as a function of the deposit type and the rhenium and osmium concentrations of the samples isotope values for mo also vary within the individual deposits in general molybdenites from porphyry copper deposits have the lightest values averaging 0 07 0 23 1 molybdenites from the other deposit types average 0 49 0 26 1 the variations could be related to the fractionation of mo into different mineral phases during the ore forming processes a comparison of the mo isotope ratios and the re os ages obtained from the same aliquot may possess a geochronological evaluation tool samples that yielded robust ages have different mo isotopic compositions in comparison to samples that yielded geologically unreasonable ages another observed relationship between the re os and mo isotope data reveals a weak correspondence between re concentration and mo isotope composition molybdenites with higher concentrations of re correspond to lighter mo isotope values

performance of a sonar altimeter in the nearshore abstract a 1 mhz sonar altimeter with automatic gain control is shown to provide accurate estimates of the distance between the instrument and the seafloor laboratory experiments indicate that distance estimates degrade slightly when the bottom is rough or sloped and when sediment is suspended in the water column results from field tests both within and seaward of the surf zone show some degradation owing to a combination of suspended sediment and bubbles bed undulations and perhaps the dynamic nature of the sand bottom under waves seaward of the surf zone the bottom can be located within 2 cm nearly continuously whereas inside the surf zone the bottom can be located only intermittently and the accuracy decreases to 3 cm a 300 m long cross shore transect of 16 altimeters was deployed from the shoreline to about 4 m depth for 3 months in summer fall 1994 near duck nc results show that the altimeters are robust and can usually provide estimates of the seafloor position every few minutes even in the surf zone during large storms

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