tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

localization of the gravity field and the signature of glacial rebound the negative free air gravity anomaly centred on hudson bay canada shows a remarkable correlation with the location of the laurentide ice sheet suggesting that this gravity anomaly is the result of incomplete post glacial rebound this region however is also underlain by higher than average mantle seismic velocities suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings here we analyse the global gravity field as a simultaneous function of geographic location and spectral content we find that the hudson bay gravity low is unique with anomalously high amplitude in the spectral band where the power from the laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest we estimate that about half of the hudson bay gravity anomaly is the result of incomplete post glacial rebound and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central laurentide and fennoscandian regions this model has a jump in viscosity at 670 km depth comparable to that in dynamic models of the geoid highs over subducted slabs but lacks a low viscosity asthenosphere consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions

re os isotope systematics of sulfides from felsic igneous rocks application to base metal porphyry mineralization in chile dating sulfides and determining the source of ore forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits here we analyze sulfides from base metal porphyry deposits from chile to demonstrate the potential of the re os system to determine both the age and the source of metals for hydrothermal mineralization cogenetic chalcopyrite bornite pyrite and sphalerite from el teniente ca 5 ma have 187 re 188 os ratios from 0 3 to 21 8 and initial 187 os 188 os ratios from 0 17 to 0 22 a paragenetically late pyrite that has an initial 187 os 188 os of 0 88 indicates that the ore deposit is not isotopically homogeneous in os throughout the formation of the deposit pyrite samples from andacollo ca 100 ma have 187 re 188 os ratios from 15 to 3600 and different isochrons yield ages between 87 and 103 ma consistent with different sericite k ar ages initial 187 os 188 os ratios of 0 2 to 1 1 are similar to those of el teniente the relatively homogeneous 187 os 188 os ratios 0 19 for cogenetic sulfides from el teniente suggest that most of the os in the ore was from the causal intrusive the latestage pyrite probably incorporated os leached from the country rock by meteoric fluids which mixed with magmatic fluids at the periphery of the hydrothermal system we demonstrate that the re os isotope system can be a powerful geochronological tool in hydrothermal ore deposits the large range in re os ratios of sulfides permits the age of mineralization to be well constrained despite isotopic heterogeneities of the hydrothermal fluids

non chondritic platinum group element ratios in the earth s mantle the earth s upper mantle has an overabundance of highly siderophile elements platinum group elements gold and rhenium relative to what would be expected from equilibrium with the metallic core1 3 this excess is now widely believed to have been introduced as a late veneer by meteorite bombardment during early earth history but after separation of the core4 here we report high precision analyses of platinum group elements pges from fertile upper mantle iherzolites which show variations in relative abundances that exceed those in the chondritic meteorites that are thought to have furnished the late veneer5 in particular the pd ir ratio at about 1 76 is significantly greater than that of known chondritic meteorites6 our results combined with previous indications of both non chondritic and nearly chondritic pge ratios in the few other fertile mantle iherzolites that have been analysed2 3 7 11 strongly suggest that the mantle is heterogeneous in its pge content on scales of 100 kilometres if the observed pge ratios can be reconciled with a plausible meteoritic source the observed heterogeneity may reflect an original spatial variation in the late veneer otherwise an additional fractionation mechanism seems to be required

the os isotopic composition of himalayan river bedloads and bedrocks importance of black shales re os data are presented for bedrocks and bedloads of selected rivers from the himalayan range in central nepal and bangladesh these data demonstrate that the os isotopic composition of most of the bedrocks is similar to that of the average continental crust notable exceptions to this are the black shales from the lesser himalaya lh which contain large quantities of radiogenic osmium in the black shales the os is not concentrated in the sulfides and may instead be associated with organic matter bedloads collected from rivers which include lh black shales in their catchment basins have notably high os ratios 187os 188os 3 this radiogenic signature persists into the ganga river in contrast the brahmaputra river is less radiogenic suggesting an ophiolite input in the headwater region simple physical mixtures of lh black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high os ratios observed at the outflow of the range assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture respectively this observation suggests that erosion may not be occurring at steady state in addition mass balance calculations indicate that about 20 of the os and 60 of the re are carried in dissolved form implying that chemical erosion plays an important role in the osmium budget finally the contrast in os isotope ratio between ganga bedloads and bulk sediments in the bengal fan suggests that radiogenic os carried by particulate matter and later remobilized may significantly influence the os composition of seawater

five decades of strong temporal variability in the flow of brunt ice shelf antarctica data showing velocity changes on the brunt ice shelf bis antarctica over the last 55 years are presented and analysed during this period no large scale calving events took place and the ice shelf gradually grew in size ice flow velocities however fluctuated greatly increasing twofold between 1970 and 2000 then decreasing again to previous levels by 2012 after which velocities started to increase yet again in the observational period velocity changes in the order of 10 a 1 have commonly been observed and currently velocities are increasing at this rate by modelling the ice flow numerically we explore potential causes for the observed changes in velocity we find that a loss of mechanical contact between the bis and the mcdonald ice rumples following a local calving event in 1971 would explain both the increase and the subsequent decrease in ice velocities other explanations involving enlargement of observed rift structures are discounted as the effects on ice flow are found to be too small and the spatial pattern of velocity change inconsistent with data the most recent phase of acceleration remains unexplained but may potentially be related to a recent re activation of a known rift structure within the bis

evolution of the hydrothermal system at los azufres mexico based on petrologic fluid inclusion and isotopic data abstract the los azufres geothermal reservoir formed in a fractured upper miocene to pliocene andesite and basalt base complex is sealed to the surface by a silicic mainly rhyolitic sequence of pleistocene age almost the entire sequence is affected by hydrothermal alteration to varying extent petrological and fluid inclusion studies confirmed vertical zonation of the reservoir by secondary processes hydrothermal alteration processes under low temperature conditions at a depth of 1200 1500 m the maximum ice melting temperatures tmi values of 0 7 to 4 c and salinities of 6 4 wt nacl eq indicate boiling conditions of the geothermal brine in a vapor rich zone chlorite calcite quartz zeolite anhydrite albite sphene pyrite hematite and illite form the hydrothermal mineral paragenesis with maximum temperatures of 250 c and pressure conditions of 150 bar below 1500 m tmi reach maximum values of 0 1 c and low salinities of 0 2 wt nacl eq the minerals epidote amphibole prehnite and garnet indicate temperatures above 250 c and pressure conditions between 150 and 200 bar the measured homogenization temperatures th of fluid inclusions fi are consistently higher than the in situ measured temperatures which indicates retrograde cooling of the los azufres geothermal reservoir since the time of the hydrothermal mineral formation updoming temperature isotherms at the center of the geothermal field below well az 9 az 23 and az 25 indicate the heating up of the fluids by a shallow magma chamber stable isotope data 18o d of the geothermal brine indicates mixing processes between meteoric water and a minor magmatic component secondary reactions forming sericite may have caused a further positive shift in 18o in the shallow part of the geothermal reservoir

the early cretaceous yangzhaiyu lode gold deposit north china craton a link between craton reactivation and gold veining the yangzhaiyu gold deposit is one of numerous lode gold deposits in the xiaoqinling district southern margin of the north china craton gold mineralization is hosted in neoarchean to early paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein proximal alteration zone ore related hydrothermal alteration is dominated by sericite quartz sulfide assemblages close to gold veins and biotite quartz pyrite chlorite epidote alteration generally distal from mineralization pyrite is the predominant sulfide mineral locally coexisting with minor amounts of chalcopyrite sphalerite and galena gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with au bearing tellurides mainly petzite and calaverite coexisting with hessite tellurobismuthite and altaite fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures 175 313 c from aqueous or aqueous carbonic fluids with moderate salinity 5 14 wt nacl equiv 34s values of sulfide minerals range mainly from 2 0 to 4 4 whereas auriferous quartz vein samples have 18o values of 12 4 to 9 6 with calculated 18oh2o values of 6 0 to 3 2 gold related pyrite grains yield elevated 3he 4he ratios 1 51 0 32 ra relative to crustal reservoirs and mantle like 20ne 22ne and 21ne 22ne ratios 9 90 9 68 and 0 029 respectively the stable and noble gas isotopes thus suggest deep seated most likely magmatic and mantle derived sources for the ore fluids sulfur and by inference other components in the ore system 40ar 39ar dating of ore related sericite and biotite separates indicates two episodes of gold genesis at 134 5 to 132 3 and 124 3 to 123 7 ma the mineralization ages overlap zircon u pb ages of 141 0 1 6 to 125 8 1 4 ma 2 for the wenyu and niangniangshan monzogranite plutons and a number of mafic to intermediate dikes intruding these plutons all being proximal to the yangzhaiyu gold deposit the synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur the geochronologic data also suggest that gold veining took place billions of years after the stabilization of the north china craton and associated metamorphism in the late archean to early paleoproterozoic this contrasts sharply to lode gold deposits in other precambrian cratons that formed predominantly in late archean to paleoproterozoic temporarily and genetically related to regional high grade metamorphism and compressional or transpressional tectonism available data have demonstrated that the north china craton was reactivated in the late mesozoic as marked by voluminous igneous rocks faulted basin formation high crustal heat flow and widespread metamorphic core complexes in the eastern part of the craton it is thus suggested that the yangzhaiyu gold deposit together with other deposits of similar ages in the xiaoqinling district were products of this craton reactivation event lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy fluid and sulfur required for the formation of the gold deposits

mantle derived helium in two peruvian hydrothermal ore deposits strong differences exist between the helium isotopic compositions of mantle derived magmas1 8 axial vent hotspring fluids9 14 subduction related magmas15 20 and many associated hot springs21 24 and the atmospheric and crustal values fig 1 here we report helium isotope ratios for two late tertiary fossil hydrothermal systems responsible for precious metal base metal and tungsten mineralization and compare them to those of circum pacific hot spring emanations the 3he 4he ratios of inclusion fluids hosted by gangue and sulphide minerals from casapalca and pasto bueno peru all lie between two and three times the atmospheric value indicating the presence of a dilute component of mantle helium consistent with previously reported stable isotope evidence for the presence of magmatic fluids during mineralization25 27 although post trapping modification of the helium isotope composition is possible we suggest that these apparently low values represent the composition of helium inherited from contributing magmas as a tracer of magmatic fluids in hydrothermal systems helium isotopes are decoupled from the factors that govern other stable isotope signatures

osmium sulphur and helium isotopic results from the giant neoproterozoic epithermal imiter silver deposit morocco evidence for a mantle source the giant epithermal ag hg deposit of imiter anti atlas morocco was investigated to study the transfer of chalcophile elements from the mantle to superficial crustal levels the origin of metals and ligands sulphides was deduced from s fluid inclusions he and re os data obtained on sulphide phases and gangue minerals sulphur isotope analyses show the existence of two distinct isotopic reservoirs one found in pyrite from the surrounding black shale country rocks 34scdt 38 and the other in pyrites associated with a synchronous rhyolitic dome 34scdt 7 to 2 the 34scdt values of the silver mineralization event range from 28 to 2 and are interpreted as resulting from preferential degassing of so2 in ascending fluids as well as mixing between the magmatic isotopic reservoir and a country rock reservoir helium isotope analyses of sulphides and gangue minerals yield similar results with 3he 4he ratios ranging from 0 76 to 2 64ra these data and the absence of 20ne in the analyzed fluid inclusions suggest a mantle origin for the fluids associated with the epithermal silver event os isotopic ratios have been measured for the first time in ag and ag sulphosalts these data and those obtained on other sulphide phases directly associated with the ag mineralization show measured 187os 188os ratios of 0 142 0 197 indicating a dominantly mantle source for the associated os assuming that os and ag are derived from the same source the black shale country rock provided less than 10 of the silver metal stock this study highlights the conditions required to transfer 8000 metric tons of ag and other elements such as hg co ni pb zn from the mantle i e i an efficient vector such as the sulphide ligand ii a major and sudden break in the geodynamic environment similar to the one which occurred near the precambrian cambrian transition to provide pathways from the mantle wedge and the lithospheric base to the surface and iii a strongly extensional tectonic setting to provide traps for the concentration of metals in ore deposits

age and source constraints for the giant muruntau gold deposit uzbekistan from coupled re os he isotopes in arsenopyrite the muruntau gold deposit uzbekistan is one of the largest gold deposits known worldwide but its origin remains controversial we used re os arsenopyrite geochronology to precisely determine the age of main stage gold mineralization at muruntau to be 287 5 1 7 ma which overlaps the emplacement of proximal post tectonic granitoid magmatism additionally we suggest that arsenopyrite growth may have occurred over an interval of at least 2 m y os initial ratios derived from arsenopyrite were coupled with he isotopic data from fluid inclusions within arsenopyrite to constrain the source of ore metals and fluids muruntau arsenopyrite yields relatively unradiogenic initial os 0 37 0 27 and elevated 3he 4he ratios 0 23 0 33 r a relative to purely crustal os he reservoirs these data suggest the presence of a mantle derived component in the ore system that was probably introduced during the generation of the granitoid magmas these new timing and source constraints provide important new insight into the generation of this giant gold deposit and they necessitate reexamination of genetic models for muruntau and potentially other giant orogenic gold deposits worldwide

hercynian age of the cobalt nickel arsenide gold ores bou azzer anti atlas morocco re os sm nd and u pb age determinations the age of the hydrothermal co ni as au mineralization in the bou azzer el graara inlier was determined using different isotopic systems on ore and mineralization related gangue minerals namely re os on molybdenite sm nd on carbonates and u pb on brannerite re os dating of molybdenite was difficult owing to the intimate intergrowth of molybdenite with co arsenides and the fact that sampling by drilling complex sulfide mixtures resulted in poor concentration of molybdenite accordingly re os dating of the impure molybdenite sulfide mixtures yielded inconsistent ages in the range 400 to 350 ma this scatter is attributed to 1 inadequate concentration of pure molybdenite 2 complex re os systematics possibly involving excess radiogenic os derived from older generation s of sulfide and or 3 mixture of at least two re os systems consisting of a component with pre hercynian history and later disturbance by hercynian metamorphism carbonates and brannerite that coexist with molybdenite gave more consistent ages of 308 31 ma sm nd and 310 5 ma u pb respectively because both ages agree within their errors we regard the brannerite u pb age of 310 5 2 ma as the best and most precise estimate for the age of the co ni as au mineralization although an earlier onset of mineralization cannot be ignored totally the new age data underline that the principal co ni as au mineralization at bou azzer occupying the main ore bearing structures was driven by and formed during the end of the hercynian orogeny

andean cenozoic volcanic centers reflect basement isotopic domains isotopic data for quaternary volcanic centers from lat 17 5 to 22 s in northern chile provide evidence for crustal contributions to arc magma genesis in the central andes and delineate basement domains crustal thickness distance from trench height of volcanic edifices above the seismically active subduction zone and sediment supply to the trench are all constant along this segment of the arc the only significant variable is crustal age paleozoic in the south proterozoic in the north pb isotopic compositions of quaternary lavas in the northern sector are lower 206 pb 204 pb 17 89 18 28 and sr and o isotope ratios are less variable 87 sr 86 sr 0 7058 0 7077 18 o 6 5 8 4 than in the southern sector 206 pb 204 pb 18 59 18 79 87 sr 86 sr 0 7053 0 7083 18 o 6 4 9 3 for rocks similar in bulk composition from 53 to 74 wt sio 2 together these features are interpreted to reflect interaction of magmas with crustal contaminants of different isotopic character rather than changes in the mantle source of central andean are magmas resulting from either subduction erosion or involvement of old enriched lithosphere

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

bermuda and appalachian labrador rises common non hotspot processes other than the corner rise new england seamounts and associated white mountains most postbreakup intraplate igneous activity and topographic uplift in the western north atlantic and eastern north america do not readily conform to simple hotspot models for example the bermuda rise trends normal to its predicted hotspot trace on continental crust cretaceous eocene igneous activity is scattered along a northeast trending belt 500 1000 km west of and paralleling the continent ocean boundary corresponding activity in the western atlantic generated seamounts preferentially clustered in a belt 1000 km east of the boundary the eocene volcanism on bermuda is paired with coeval magmatism of the shenandoah igneous province and both magmatic belts are associated with northeast trending topographic bulges the appalachian labrador rise to the west and the bermuda rise eocene to the east the above observations suggest the existence of paired asthenosphere upwelling paralleling and controlled by the deep thermal contrast across the northeast trending continental margin such convection geometry apparently fixed to the north american plate rather than to hotspots is consistent with recent convection models by b hager the additional importance of plate kinematic reorganizations causing midplate stress enhancement is suggested by episodic igneous activity ca 90 100 ma and 40 45 ma

age and source of the kure odemis arsenopyrite gold mineralization menderes massif turkey determined by re os he isotopes abstract the menderes massif mm is considered to be one of the principal regions hosting vein type quartz arsenopyrite orogenic gold deposits and occurrences in turkey gold mineralizations in the mm can be grouped into i schistosity conformable e trending veins and ii shear controlled nw trending veins arsenopyrite is one of the major gold bearing minerals in these deposits or occurrences rhenium osmium isotopic dating of two types of arsenopyrite from the schistosity conformable and vein type kure odemis gold deposit has yielded two groups of model ages maximum ages at 557 574 ma and 246 250 ma respectively the data suggested that the quartz arsenopyrite gold deposit formed in the neoproterozoic and early triassic with the former and the latter corresponding to pan african compressional orogenesis and palotethys rifting respectively fluid inclusions in kure arsenopyrite have 3he 4he ratios of 0 08 0 09 ra ra 1 39x10 6 the 3he 4he ratio of air being within the range of middle to upper crustal values a minor helium contribution of mantle fluids to the kure gold deposit is possible because the 4he concentrations in arsenopyrite are enriched 170 to 9520 times relative to argon and typical atmospheric values indicating that contribution of atmospheric he to the mineralizing fluids is negligible and 0 08 0 09 ra is slightly above typical crustal 3he 4he ratios arsenopyrite yields relatively non radiogenic initial os isotopic compositions also indicating possiblecontribution from mantle fluids carbonic fluid inclusion type 2 h2o co2 nacl ch4 in quartz of nw trending veins at kure are gas liquid hydrate crystal rich and their homogenization temperatures th range from 244 to 387 c salinity values range from 0 2 to 5 7 wt nacl equiv the homogenization temperatures th of aqueous fluid inclusions type 1 h2o nacl in quartz from nw trending veins at kure vary between 237 and 358 c whereas salinity values range from 1 7 to 7 7 wt nacl equiv quantitative epma spot analyses on arsenopyrite minerals have shown that as concentrations in ew trending and nw trending veins range from 32 5 to 33 8 and 32 3 33 8 respectively based on a phase diagram for the fe as s system along with the arsenopyrite geothermometer the ranges of temperature and corresponding logf s2 at which arsenopyrite is in equilibrium with pyrite are between 380 and 480 c and 7 4 and 4 3 for apy 1 respectively these values are between 390 and 475 c and 7 0 and 4 5 for apy 2 respectively

geology geochemistry and genesis of axi a paleozoic low sulfidation type epithermal gold deposit in xinjiang china abstract axi is a low sulfidation type epithermal gold deposit hosted in paleozoic subaerial volcanic rocks in the western tianshan orogenic belt xinjiang china the resource is more than 50 t gold at an average grade of 4 4 ppm the deposit occurs in the tulasu volcanic fault basin in the paleozoic active continental margin on the northern side of the yili central tianshan plate the host rocks are andesitic volcaniclastic rocks of the paleozoic dahalajunshan formation and the orebodies occur as veins in annular faults of a paleocaldera mineralization at axi can be subdivided into five stages quartz and or chalcedony vein quartz vein quartz carbonate vein sulfide vein and carbonate vein there are two types of ore host quartz vein and altered rocks ore minerals are native gold electrum pyrite marcasite arsenopyrite hematite limonite and trace amounts of pyrargyrite polybasite naumannite cerargyrite sphalerite chalcopyrite tetrahedrite galena pyrrhotite and clausthalite gangue minerals are mainly quartz chalcedony illite calcite siderite dolomite adularia and laumontite the main wall rock alteration is silicification and phyllic alteration carbonatization and propylitization the deposit is characterized by an enrichment relative to crustal abundance of au ag as sb bi hg se te and mo depletion in base metals cu pb and zn and a low ag au ratio 0 5 3 7 three types of fluid inclusions were recognized in quartz from the major mineralization stages liquid aqueous inclusions liquid rich two phase inclusions and small amounts of vapor rich two phase inclusions microthermometric measurements indicate that the final ice melting temperatures are 0 3 to 4 4 c corresponding to salinities of 0 5 6 9 wt nacl equiv 2 2 wt nacl equiv in average the peak temperatures of ice melting varies from 0 4 to 1 9 c corresponding to salinities of 0 7 3 1 wt nacl equiv homogenization temperatures range mainly between 120 and 240 c with an average of 190 c and a maximum of 335 c the fluid density is 0 73 to 0 95 g cm 3 and thus the estimated maximum mineralization depth is about 700 m hydrogen and oxygen isotopic compositions of the ore fluids lie within a narrow range d h2o is 98 to 116 and 18 o h2o 1 8 to 0 4 3 he 4 he ranges from 0 0218 to 0 138 ra with an average of 0 044 ra indicating that he derived predominantly from crust with negligible mantle he in the ore fluids by contrast the 40 ar 36 ar ranges from 317 7 to 866 0 suggesting that crust derived radioactive 40 ar accounts for 7 0 to 66 and atmospheric 40 ar about 43 to 93 in the ore fluids hydrogen oxygen carbon sulfur and noble gas isotopes indicate that the ore forming fluids of the axi gold deposit consisted predominantly of circulating meteoric water ore forming metals may have derived mainly from the host volcaniclastic rocks of the dahalajunshan formation and basement rocks the occurrence of adularia platy calcite and quartz or sulfide aggregates as pseudomorphs after bladed calcite in ore veins and occurrence of aqueous liquid and liquid rich and vapor rich two phase inclusions indicates that boiling of the ore forming fluid have occurred leading to supersaturation of the hydrothermal solution and deposition of ore metals this is the main mineralization mechanism for quartz vein type ores in axi the ore forming fluid was buffered to a near neutral ph in a reduced environment during mineralization the preservation of this paleozoic axi deposit and its discovery required a rapid accumulation of sediments in the basin after formation of the deposit and minimal amount of erosion after late cenozoic uplift

re os isotope systematics in black shales from the lesser himalaya their chronology and role in the 187os 188os evolution of seawater abstract re and os abundances and os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines maldeota and durmala in the lesser himalaya the re and os concentrations in these samples exhibit wide range 0 2 to 264 ng g and 0 02 to 13 5 ng g respectively with a mean re os of 25 the 187 os 188 os of these samples also show a broad range 1 02 to 11 6 with many of them far more radiogenic than the currently eroding continental crust the black shales from the maldeota and durmala mines collected a few meters above the krol tal pc c boundary and occurring in the same stratigraphic horizon yield 187 re 187 os isochron ages of 554 16 and 552 22 ma respectively consistent with fossil evidences and those assigned for the pc c boundary at various other locations these results indicate closed system behaviour of re and os in these mine samples and reaffirm the validity of 187 re 187 os pair to date organic rich sediments the age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary re os age of 839 138 ma for black shales from theog collected from the inner belt these initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt however more analyses of black shales are needed to confirm this the role of weathering black shales from the himalaya in contributing to the os isotope evolution of seawater over the past 25 ma was assessed using a simple budget model for seawater os and assuming values for os fluxes and 187 os 188 os based on the available limited data on os concentration and its 187 os 188 os in rivers and oceans these calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187 os 188 os over the past 16 ma is difficult to satisfy by varying only the os flux from htp rivers 187 os 188 os kept constant at 1 324 or their 187 os 188 os 188 os flux constant at today s value 131 moles yr the requirements to balance os isotope budget in ocean through weathering of black shales however is easier to accommodate if both the os flux from htp himalayan tibetan plateau rivers and their 187 os 188 os are allowed to vary measurements of os abundance in htp rivers and its isotopic composition are needed to test the validity of the model results

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

basinal brines at the origin of the imiter ag hg deposit anti atlas morocco evidence from la icp ms data on fluid inclusions halogen signatures and stable isotopes h c o the imiter ag hg deposit is located in the precambrian volcano sedimentary formations of the saghro inlier eastern part of the anti atlas mountains morocco the orebodies consist of northeast southwest to east west veins and lenses hosted by cryogenian black shales and gray wackes and neoproterozoic conglomerates and are controlled by an east west fault network the so called imiter fault mineralogical and paleo fluid geochemistry investigations microthermometry raman spectroscopy la icp ms on individual inclusions bulk crush leach analyses and stable isotope data o h show that the main ag ore stage is related to circulation of deep basinal sedimentary brines na k mg salinity 16 7 to 26 wt nacl equiv molar cl br 330 18 o 2 15 2 35 smow and d 53 8 to 65 5 smow at temperatures of about 180 to 220 c and hydrostatic pressures the main driving mechanism for silver ore deposition is the dilution of ore bearing brines by a low salinity meteoric fluid containing a low density volatile component n 2 ch 4 co 2 t h 180 to 220 c 18 o 1 4 smow and d of about 28 2 smow silver content of the brines ranges from 2 to 30 mmol kg solution up to 3 200 ppm ag avg ag concentration about 900 ppm whereas the maximum ag content found in dilute waters is about 0 4 mmol kg solution 40 ppm the ore forming model proposed for the imiter deposit is 1 ag extraction from the basement by the penetration of deep basinal brines and 2 deposition in a structural trap through fluid mixing with recharge fluids this model is comparable to that described worldwide for the origin of pb zn f ba and u deposits near unconformities between basement and sedimentary basins similarities among the major ag deposits from the anti atlas imiter zgounder bou azzer strongly suggest that they resulted from a unique event likely related to the opening of the atlantic ocean the silver ores are superimposed on the same lineament as a preexisting uneconomic pan african co ni as system linked to magmatic intrusions but ag ores have no genetic relationship with them

basinal brines at the origin of the imiter ag hg deposit anti atlas morocco evidence from la icp ms data on fluid inclusions halogen signatures and stable isotopes h c o a discussion we congratulate essarraj et al 2016 for their paper on the polymetallic ag hg imiter deposit anti atlas morocco which is one of the most intensely investigated yet still discussed mineral deposits in term of ore genesis in their contribution essarraj et al have employed fluid inclusion halogen and isotopic analyses in an attempt to constrain the genesis of the imiter ag hg deposit it is through detailed geochemical studies such as this that the origin s of this world class orebody will be deciphered their contribution represents a significant advance in the investigation of polymetallic hydrothermal deposits of the anti atlas however we feel compelled to stress certain problems with their interpretation important geologic attributes of the imiter deposit have been documented by numerous workers ouguir et al 1994 pasava 1994 baroudi et al 1999 cheilletz et al 2002 essarraj et al 2005 barral et al 2011 levresse et al 2005 2016 however there is a lack of direct age information to constrain the timing of hydrothermal activity and duration of vein filling during the silver mineralizing event s this age uncertainty has resulted in the proposal of several conflicting models involving different fluid sources and genetic processes that have been ignored by essarraj et al in their contribution essarraj et al relate the genesis of silver ore at imiter to a single regional scale hydrothermal circulation of basinal brines but do not mention preexisting models that emphasize the polyphase character of the mineralization the authors point out the role of triassic evaporitic brines and development of the central atlantic magmatic province as the main ore forming processes that triggered formation of the imiter ag hg ores key evidence for their proposed model is based on the late permian and triassic ages of borisenko et al 2014 that were determined on a

helium isotopic evidence for a lower mantle component in depleted archean komatiite archean magnesium rich komatiites require hot and presumably deep mantle sources but their trace element composition and radiogenic isotope composition are similar to those of modern mid ocean ridge basalts which originate in the upper mantle the isotopic composition of helium extracted by sequential crushing of fresh olivines separated from two archean and one mid proterozoic komatiites varies over three orders of magnitude between a radiogenic end member rich in helium 4 and a component rich in helium 3 such helium 3 enrichment suggests the presence of a lower mantle component in archean komatiites

comments on a late triassic 40 ar 39 ar age for the el hammam high ree fluorite deposit morocco mineralization related to the central atlantic magmatic province by cheilletz et al mineralium deposita 45 323 329 2010

open system degassing of sulfur from krakatau 1883 magma we present the first sulfur and oxygen isotopic data for tephra from the catastrophic 1883 eruption of krakatau sulfur isotopic ratios in unaltered krakatau tephra erupted august 26 27 1883 are markedly enriched in 34s relative to mantle sulfur high 34s values of 6 3 to 16 4 can best be explained by open system or multi stage degassing of so2 from the oxidized rhyodacitic and gray dacitic magmas with 34s enrichment of so2 4 remaining in the melt lower whole rock 34s values of 2 6 and 4 0 in two oxidized gray dacitic samples indicate more primitive subarc mantle sulfur in the 1883 magma chamber initial 34s of the rhyodacitic magma was probably in the 1 5 to 4 0 range and similar to 34s values measured in arc volcanic rocks from the mariana arc

effects of vertical boundaries on infinite prandtl number thermal convection summary the physics of thermomechanical coupling of the continental lithosphere and its surrounding mantle is studied using a simple hydrodynamic model in which a fluid is heated from below and is bounded by rigid side walls rayleigh numbers up to 106 are considered a series of finite element stability analyses is employed to characterize systematically the nature of convective instability in such a system the marginal stability of a bounded fluid is increased not only by the rigid boundary condition on side walls but also by heat conduction across side walls the planform of convection at the marginal state is 3 d having convective rolls aligned perpendicular to side walls the characteristics of non linear convection at supercritical rayleigh numbers are investigated by first calculating 2 d steady state solutions and then applying 2 d and 3 d stability analyses the 2 d stability analysis shows that the presence of side walls significantly reduces the transition rayleigh number for time dependent convection and the 3 d stability analysis shows that heat conduction through side walls strongly prefers 3 d convective motion finally the characteristics of 3 d convection are analysed through the 3 d single mode approximation both 2 d and 3 d single mode solutions demonstrate that conducting side walls reduce the strength of convection at low rayleigh numbers but have a negligible influence at higher rayleigh numbers we therefore propose that the basal topography of the continental lithosphere can modulate the convective planform efficiently in the upper mantle

application of the 187re 187os system to black shale geochronometry the decay of 187re to 187os provides a tool for determining depositional ages of black shales re and os concentrations and os isotopic compositions of whole rock samples of the bakken shale a mississippian devonian boundary black shale yield a whole rock isochron with an age of 354 49 ma this age is in agreement with the accepted age of the bakken shale 360 ma the initial 187os186os ratio of the isochron is 6 2 3 3 this value is indistinguishable from the 187os186os ratios observed in modern black sea sediments the concentration of common os in bakken shale samples is strongly correlated with total nitrogen indicating that a large fraction of the os in these samples is associated with a hydrogenous component which overwhelms any os supplied to the sediment in association with detrital material or cosmic dust the dominance of the hydrogenous component imparts a relatively homogeneous initial 187os186os ratio to the sediment at the time of deposition the degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity the pattern of scatter is consistent with postdepositional mobilization of re and or os on a small spatial scale we suggest that this mobilization may be a consequence of petroleum formation and migration in the bakken shale

the moroccan anti atlas the west african craton passive margin with limited pan african activity implications for the northern limit of the craton abstract the moroccan anti atlas region located south of the south atlas fault has been viewed traditionally as containing two segments separated by the anti atlas major fault these two segments are said to consist of a 600 700 ma pan african segment located in the northeast and b 2 ga eburnian segment situated to the southwest on the basis of observations in the zenaga and saghro inliers and of a recent literature review we suggest that this subdivision is inappropriate in that eburnian and pan african materials occur throughout the anti atlas region the entire anti atlas is underlain by eburnian crust unconformably overlain by a lower neoproterozoic passive margin allochthonous pan african ocean crustal slices were thrust onto the west african craton wac passive margin sequence 685 ma ago as a result of pan african accretion tectonics high level high k calc alkaline and alkaline granitoids locally intruded the anti atlas sequence as a whole at the end of the pan african orogeny at 585 560 ma the intervening 100 m y interval was marked by quiescence this succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the pan african events that occurred to the east in the tuareg shield and to the north in avalonian terranes this model implies that the actual northern limit of the wac is located at the south atlas fault saf and not at the anti atlas major fault aamf we propose that the aamf corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the wac during early neoproterozoic times this is consistent with the development of the gourma aulacogen on the eastern side of the wac this model further suggests that the northeastern boundary of the wac occurs north and not south of ougarta algeria support for this model is provided by geological and geophysical evidence

superplumes and ridge interactions between ontong java and manihiki plateaus and the nova canton trough the initial pulse of volcanism on the ontong java and manihiki plateaus began prior to 123 124 ma and had largely ceased by about 122 ma whereas the intervening pacific phoenix spreading ridge probably was disrupted between 120 and 115 ma by formation of the nova canton trough rift system plateau formation may have resulted from a large plume head that rose beneath each of the two plateau sites during the plateau eruption phase these two plumes spread beyond the plateaus and within the asthenosphere they eventually coalesced beneath the intervening spreading ridge where the nova canton trough rift system is now located at about the time volcanism ceased on the plateaus the two plumes combined to form the nova canton trough rift system by reheating the nascent lithosphere near the ridge crest and widening the plate boundary from the usual 1 10 km to a much broader region the present rift system is a maximum of 500 km wide and much of this area may have formed by normal faulting and crustal accretion across the broadened plate boundary as plate divergence continued eventually coherent spreading resumed somewhere south of the nova canton trough rift system

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources