re os age of the pueblo viejo epithermal deposit dominican republic re os analyses of pyrite from the pueblo viejo high sulfidation epithermal deposit both monte negro and moore ore zones yield an isochron age of 111 9 3 7 ma and initial 187 os 188 os ratio of 0 22 0 18 mean square of weighted deviates 84 n 21 isochrons grouped by pyrite setting layered versus vein or locations yield ages within uncertainty of the overall isochron and with each other and as such do not yield temporally distinguishable ages sphalerite vein samples as well as shale source rocks show evidence of open system behavior and loss of re and therefore are not included in the above regression these results confirm that the pueblo viejo deposit is early cretaceous in age and coeval with the volcanic los ranchos formation which hosts the deposit recent paleontological age assignments for the hatillo formation which overlies the los ranchos formation show that it is only slightly younger opening the possibility that mineralization in the hatillo formation could also be related to late stages of los ranchos magmatism whether these revised los ranchos hatillo relations provide sufficient cover to account for the paragenetically late formation of pyrophyllite at pueblo viejo remains unclear recognition that pueblo viejo is coeval with the los ranchos formation shows that large scale high sulfidation epithermal mineralization can form from basal island arc tholeiitic magmatism a review of available experimental data confirms that these magmas can generate hydrothermal solutions of the necessary composition favorability of the los ranchos formation might have been enhanced by its bimodal composition these results suggest that exploration for epithermal precious metal deposits should include basal island arc tholeiite terranes

re os isotopic dating of molybdenite and pyrite in the baishan mo re deposit eastern tianshan nw china and its geological significance the baishan mo re deposit is located in the eastern section of the eastern tianshan orogenic belt nw china the deposit has a grade of 0 06 mo and a high content of rhenium of 1 4 g t rhenium and osmium isotopes in sulfide minerals from the baishan deposit are used to determine the age of mineralization rhenium concentrations in molybdenite samples are between 74 and 250 g g analysis of eight molybdenite samples yields an isochron age of 224 8 4 5 ma 2 pyrite samples have rhenium and osmium concentrations varying in the range 33 4 330 6 ng g and 0 08 0 81 ng g respectively isotope data on seven pyrite samples yield an isochron age of 225 12 ma 2 on the 187re 188os versus 187os 188os plot and an age of 233 14 ma 2 on the 187os versus 187re correlation diagram the ages of molybdenite and pyrite are consistent within the analytical errors combined with field observations the data indicate that mo re mineralization in the baishan deposit is produced by a magmatic hydrothermal event in an intracontinental extensional setting after late paleozoic orogeny the initial 187os 188os ratio of pyrite is 0 3 0 07 the 34s values of molybdenite vary from 0 5 to 3 6 both data indicate that mineralization is derived mainly from a mantle source

variations in os isotope ratios of pyrrhotite as a result of water rock and magma rock interaction constraints from virginia formation duluth complex contact zones abstract os isotope ratios in pyrrhotite bearing pelitic rocks of the 1 85 ga virginia formation are variable with perturbations linked to the emplacement of the 1 1 ga duluth complex pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1 1 ga involving a low 187 os 188 os fluid however because rocks with perturbed pyrrhotite os isotope ratios occur 1 km or more from the duluth complex the fluid is unlikely to have been of magmatic origin fluid inclusions in layer parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between 300 and 400 c analyses of fluid inclusions via la icp ms show that the fluids contain up to 1 7 wt na 1 1 wt k 4330 ppm fe 2275 ppm zn and 415 ppm mg the veins also contain pyrite or pyrrhotite plus minor amounts of chalcopyrite bornite pentlandite and sphalerite the re os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187 os 188 os fluids 18 o values of vein quartz range from 7 7 to 9 5 consistent with an origin involving fluid with a relatively low 18 o value between 2 and 5 meteoric water with such a low 18 o value could have interacted with the igneous rocks of the complex and would have acquired os with a low 187 os 188 os ratio strongly serpentinized olivine rich rocks of the complex are commonly characterized by such low 18 o values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the os isotopic system recorded by pyrrhotite in the virginia formation two important observations are that only pyrrhotite bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing os exchange are spatially restricted and not uniformly distributed os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite and where time integrated water rock ratios in the aureole were high enough to provide a supply of os troctolitic and gabbroic rocks of the partridge river intrusion duluth complex are characterized by os isotope ratios that are indicative of variable degrees of crustal contamination os values of 0 543 xenoliths of carbonaceous and sulfidic pelitic rocks of the virginia formation found in the igneous rocks provide evidence that os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle derived magma however residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187 os 188 os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole the os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of re os liberation during devolatilization kerogen maturation and pyrite to pyrrhotite conversion processes that contribute to magma contamination followed by os uptake by pyrrhotite during back reaction involving magma and or fluid characterized by a relatively low 187 os 188 os ratio the extreme os uptake recorded by pyrrhotite in the xenoliths as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole is in line with the high os diffusivity in pyrrhotite experimentally determined by brenan et al 2000 our data confirm that os isotope ratios in pyrrhotite bearing rocks may be readily perturbed for this reason caution should be exercised in the interpretation of os isotope ratios in rocks where pyrrhotite may be the primary host of os

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

an overview of the platinum group element nanoparticles in mantle hosted chromite deposits abstract this paper reviews the occurrence of platinum group elements nanoparticles pge nps in mantle hosted chromite deposits showing that pge nps are more frequently found in these deposits than previously thought a comparison of published experimental data with observations in natural samples reveal that pge nps might form at the pressure 1 0 5 gpa temperature 1200 c and f o 2 fmq 1 conditions in which chromite deposits form in the upper mantle the crystallization of pge nps may take place earlier or simultaneously to the segregation of chromite and or sulfide liquids from silicate melts if the pges are dissolved in the silicate melt the segregation of pge nps could be linked to local changes in f o 2 and f s 2 during the early crystallization of chromite and or olivine mobile crystals of chromite and droplets of sulfide melts entrained in the basaltic parent melt may also play an additional role as physical collectors of pge nps providing a complementary or even an alternative mechanism for the fractionation of pges in high temperature silicate melts furthermore hydrothermal alteration of the chromite deposits during seafloor metamorphism or exhumation i e retrograde metamorphism of upper mantle rocks has also a significant effect on the internal structure of the oxides and sulfides that host the pge nps frequently pge nps are found along the replacement contacts between primary and secondary minerals suggesting that pge nps can also form during low temperature alteration events 200 600 c finally heating events overimposed on chromite deposits previously affected by hydrous metamorphism may enhance fluid infiltration subsequently triggering chemical mineralogical or textural responses in the oxide or sulfide matrices hosting the pge nps promoting coarsening of metal nanoparticles the interplay between these processes can explain the wide spectrum of particle sizes of pge inclusions that are observed in many altered chromite deposits which can vary from a few nanometers to larger than a micron these studies provide evidence that pge nps can eventually form under a wide spectrum of thermal and pressure conditions and show that aqueous fluids may play a relevant role in producing pge nps during metamorphism and metasomatism of mantle hosted chromite deposits

subgrain scale decoupling of re and 187os and assessment of laser ablation icp ms spot dating in molybdenite abstract reproducibility of re os molybdenite ages depends on sample size and homogeneity suggesting that re and os are decoupled within individual molybdenite crystals and do not remain spatially linked over time in order to investigate the re os systematics of molybdenite at the subgrain micron scale we report la icp ms re os ages for an archean molybdenite crystal from aittojarvi finland analyzed in situ in a white aplite matrix a related aittojarvi molybdenite a996d in the form of a very fine grained mineral separate is used as one of our in house ntims standards and thus its age of 2760 9 ma is well established measurements of 187 re 187 os 185 re on micron scale spots along 200 m traverses across the crystal yield a wide range of ages demonstrating that in this case microsampling of molybdenite does not produce geologically meaningful ages experimentation with mineral separations and sample size over a 7 yr period predicted that this would be the outcome we suggest that 187 os is more likely to be the mobile species based on its charge and ionic radius and that 187 os becomes decoupled from parent 187 re with time on the micron and larger scale incompatible charge and ionic radius for os ions formed during reduction of molybdenite forming fluids may explain the widely observed absence of common initial os in molybdenite geologically accurate ages for molybdenite can only be obtained for fully homogenized crystals or crystal aggregates so that any post crystallization 187 re 187 os decoupling is overcome a growing number of geologically accurate id ntims 187 re 187 os ages for homogenized molybdenite suggest that postcrystallization mobility of radiogenic 187 os must be limited to within the molybdenite mineral phase we suggest that radiogenic 187 os may be stored in micron scale dislocations kink bands and delamination cracks produced by deformation and that the unusual structure and deformation response of molybdenite results in an increased chemical stability in this mineral migration of 187 os into adjacent silicate phases is highly unlikely but other contacting sulfides may take in os in an example from a proterozoic skarn deposit at pitkaranta western russia we demonstrate minor loss of radiogenic 187 os from molybdenite and a corresponding gain in adjacent chalcopyrite such that the molybdenite age is not perceptibly disturbed whereas the resulting chalcopyrite ages are impossibly old therefore it is unadvisable to perform re os analytical work on any sulfide in contact or intimate association with molybdenite in addition to large errors in the age if the isochron method is employed initial 187 os 188 os ratios could be erroneously high leading to seriously errant genetic interpretations

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

a study of strontium diffusion in k feldspar na k feldspar and anorthite using rutherford backscattering spectroscopy abstract sr chemical diffusion has been measured in orthoclase anorthoclase and anorthite under dry 1 atm conditions a strontium oxide aluminosilicate powder mixture was used as the source material with rutherford backscattering spectroscopy rbs used to measure diffusion profiles over the temperature range 725 1075 c the following arrhenius relations were obtained orthoclase normal to 001 d 5 97 10 0 0060 0 0030exp 67900 1600rtcm 2 s 1 measurements of diffusion perpendicular to 010 and 100 indicate little diffusional anisotropy in orthoclase anorthoclase normal to 010 d 4 51 10 1 358 5 6exp 89100 4800rt cm 2 s 1 normal to 001 d 2 25 10 2 1615 31exp 89300 4600rt cm 2 s 1 anorthite normal to 010 d 3 85 10 2 0 380 0 0039 exp 78800 5400rt cm 2 s 1 the correlation of sr uptake with a reduction of k and ca in orthoclase and anorthite respectively indicates that sr is exchanging with these species nuclear reaction analysis nra measurements of al in orthoclase suggests that the coupled exchange sr 2 al 3 k 1 si 4 is taking place as expected sr closure temperatures for feldspars calculated with the above parameters are relatively high compared to those of some other minerals e g biotite muscovite apatite however closure temperatures of feldspars may be significantly depressed if they are exsolved or twinned leading to small effective diffusion lengths feldspar crystals of millimeter size should retain 87 sr 86 sr ratios except when subjected to thermal events of extreme temperature or duration

tomographic inversion of pn travel times in china 1 pn travel times are useful for studying crustal and uppermost mantle structure and regional tectonics because they are affected by crustal velocity and thickness as well as uppermost mantle velocity and anisotropy we obtained 57 740 pn travel time picks from 5433 earthquakes and 307 stations from chinese national and provincial earthquake bulletins and the international seismological center bulletins to invert for pn velocity variation and anisotropy and station delays in china our inversion reveals significant features that correlate with surface geology the main results are as follows 1 the pn velocities show a mosaic of very fast and very slow anomalies mirroring the heterogeneous geology of china at the surface the pn velocities are high beneath the major basins in the west sichuan qaidam west tarim tulufan and junggar and low in areas of active volcanoes myanmar and western yunnan and quaternary volcanism in northern tibet in seismically active areas in north china and tien shan and in the southern part of south china the hainan plume 2 the pn anisotropy beneath the major basins in the west is generally weak strong anisotropy is found beneath high deformation regions the tibetan plateau southeastern margin of the tibetan plateau western tien shan and part of north china suggesting the anisotropy is likely related to recent large scale tectonic activity 3 a large area of north china shows prominent low pn velocity beneath archean basement with thin crust our observations are consistent with rifting lithospheric thinning and mantle upwelling in the region the pn anisotropy is consistent with a dextral simple shear in the nne direction in the lithosphere mantle during the last and ongoing major deformation period 4 the pn velocity in northern tibet is generally lower than that in the south southern tibet has significant e w structure low velocity anomalies can be traced from northern tibet across southwestern tibet and south central tibet to near the india plate anisotropy is absent beneath much of the himalaya block but consistent anisotropy with e w fast direction is present beneath the lhasa block and large anisotropy up to 4 is observed in low velocity regions of the northern and western tibet complex station delays in the eastern margin of the plateau suggest that the whole crust may be highly deformed the anisotropy pattern in the southeastern margin of the tibetan plateau suggests a mantle lithospheric deformation similar to the clockwise rotation of material observed at the surface 5 crustal thicknesses inferred from our station delays are consistent with previous models which correlate well with surface topography

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

mineralogy chalcopyrite reos geochronology and sulfur isotope of the hujiayu cu deposit in the zhongtiao mountains north china craton implications for a paleoproterozoic metamorphogenic copper mineralization abstract the hujiayu cu deposit located in the zhongtiao mountains district of southern north china craton is hosted by graphite schist and dolomitic marble with disseminated to veinlet stage i and thick vein stage ii mineralization stage i mineralization characterized by stratabound disseminated pyrite and chalcopyrite within the graphite schist host rock formed at the syn metamorphic stage graphite geothermometry showed that the host rock was subjected to an upper greenschist to lower amphibolite metamorphism at a temperature range of 486 to 596 c averaging of 546 35 c 1 n 19 stage ii mineralization consisting of brecciated dolomitic thick veins cemented by quartz sulfide assemblages was a product of metamorphic hydrothermal activity this thick vein was subdivided into an early hydrothermal dolomitic alteration composing of dolomitic breccia with some cobaltiferous pyrite stage iia and a late siliceous copper mineralization consisting of quartz sulfide assemblages stage iib a clausthalite siegenite sphalerite chalcopyrite mineral assemblage was observed in stage iib constraining the sulfur fugacity and selenium fugacity within 18 7 to 11 7 and 21 7 to 14 7 respectively it was inferred stage iib was marked by a drop in sulfur fugacity and a substantial increase in selenium fugacity after the major chalcopyrite precipitation calculations based on the compositions of cobaliterous pyrite and sphalerite reveal that the mineral assemblage at stage iia formed at an approximately temperature range of 400 to 300 c whereas the minerals in stage iib occurred at temperature of 256 9 c 1 n 7 sulfides from stage i have 34s value ranging from 10 1 to 22 2 with an average value of 16 9 3 4 1 n 27 supporting the model that sulfides precipitated through thermochemical reduction of sulfate at sedimentary stage followed by metamorphic homogenization of 34s isotopic signatures sulfides from the stage ii have 34s values in highly variable ranging from 3 4 to 19 2 indicating a rather complex source four chalcopyrite samples yielded a weighted model age of 1952 39 ma 1 mswd 1 5 suggesting that the copper mineralization formed synchronously with regional metamorphism 1970 1850 ma and hence a paleoproterozoic metamorphogenic copper mineralization is implicated therefore we envisaged disseminated veinlet mineralization formed during a metamorphic peak and the major hydrothermal copper mineralization occurred during the retrograde cooling

diffusion of osmium in pyrrhotite and pyrite implications for closure of the re os isotopic system abstract in order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite experiments consisted of either 1 isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered os bearing fe sulfide or 2 relaxation of initially high near surface osmium concentrations produced in the latter experiments pyrite only osmium penetration into samples was characterized by depth profiling using rutherford backscattering spectroscopy rbs pyrite or electron microprobe analyses across sectioned run products pyrrhotite results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces with the extent of penetration uncorrelated with run duration images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near surface occurred by solute uptake during step growth and not by volume diffusion prolonged 1000 h relaxation experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for rbs a maximum diffusion coefficient of 2 5 10 23 m2 s at 500 c was calculated experiments involving pyrrhotite over the temperature range of 950 1100 c showed extensive osmium uptake and osmium concentration gradients that conform with fickian diffusion behavior we found that pyrrhotite fe s could be varied by changes in the composition of the starting material and osmium source and over the range of fe s produced in experiments molar fe s 0 83 0 90 we observed no systematic variation in the osmium diffusion coefficient diffusion coefficients measured parallel to the a crystallographic axis were on average 1 4 higher than values measured parallel to c and regression of the c axis data yielded the arrhenius relation d t 1 31 0 16 10 5 m 2 s exp 211 5 14 7 kj mol rt the application of these diffusion data to simple models of diffusive exchange during static or polythermal time temperature histories is used to assess the conditions under which radiogenic osmium will be retained during isothermal annealing calculations indicate that the cores of millimeter sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in 0 5 ma at temperatures exceeding 400 c pyrite undergoing the same process at 500 c requires in excess of 10 ma before crystal cores are affected the relatively short core retention time scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events thus possibly accounting for the scatter that commonly occurs in re os isochrons generated from massive sulfide samples calculated closure temperatures tc for osmium exchange in pyrrhotite yielded values of 300 400 c for grain sizes ranging from 10 to 1000 m these values of tc are similar to those calculated for ar retention in biotite and considerably lower than for sr in apatite and plagioclase for example such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late stage hydrothermal fluids this latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage

two stage gold mineralization of the axi epithermal au deposit western tianshan nw china evidence from re os dating s isotope and trace elements of pyrite the axi gold deposit is one of the largest epithermal deposits in northwestern china and is characterized by multistage generations of pyrite associated with gold mineralization genetic models for superjacent epithermal au and porphyry cu systems involve either contemporaneous or episodic mineralization implying different metal sources and ore forming events nine pyrite separates from disseminated ores associated with pyrite sericite quartz alteration in the axi deposit yield a re os isochron age of 350 10 ma mswd 5 1 initial 187os 188os ios 0 20 0 11 this age likely reflects a mixed contribution of two subtypes of disseminated pyrite py1 based on zoned pyrite crystals the isochron age of 353 6 ma mswd 1 6 ios 0 107 0 021 n 6 excluding samples with pyrite cores overlaps previous ages for andesite from the dahalajunshan formation ca 356 ma indicating that the early mineralization event occurred shortly after the formation of host rocks ca 353 ma pyrite from disseminated ores shows little variation of sulfur isotopic compositions 2 9 to 4 0 and has high cu co ni and v contents which favors a magmatic hydrothermal origin pyrite grains from gray quartz veins yield a younger re os isochron age of 332 8 ma mswd 0 22 n 4 vein hosted pyrite shows characteristics of a mantle derived magma source ios 0 171 0 024 34s 0 10 3 10 which result from the intrusion driven convective hydrothermal circulation accompanying fluid exsolution and metal remobilization from basement and wallrocks our data demonstrate that the axi gold deposit was formed during two temporally separated hydrothermal events this model of ore formation may apply to other epithermal porphyry deposits within the axi tawuerbieke district disseminated au and porphyry style au cu mineralization occurred shortly after magmatic activity 356 353 ma whereas epithermal style au mineralization occurred at ca 332 ma

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

re os dating of chalcopyrite from selected mineral deposits in the kalatag district in the eastern tianshan orogen china the kalatag cu zn au district contains a number of economically important cu deposits in eastern tianshan in xinjiang nw china due to the lack of precise mineralization ages the metallogenesis of this area has long been a matter of debate in this study chalcopyrite re os isotope methods are used to date the south meiling cu zn and hongshi cu deposits in the eastern part of kalatag area the south meiling cu zn deposit is hosted in volcanic sedimentary rocks of the late ordovician to early silurian daliugou formation the deposit consists of two parts a concordant massive sulfide ores and discordant vein type ores located in the footwall strata the principal ore minerals are pyrite chalcopyrite sphalerite minor tetrahedrite galena and pyrrhotite gangue minerals include quartz sericite and barite and minor chlorite plagioclase and carbonate minerals the hongshi cu deposit represents a hydrothermal vein system hosted in the mafic volcanic rocks of daliugou formation the orebodies are associated with quartz veins and controlled by subsidiary faults of the kalatag fault the ore forming process can be divided into the early middle and late stages and is characterized by quartz pyrite quartz chalcopyrite pyrite and quartz carbonate gypsum veins respectively re os analyses of chalcopyrite from the south meiling cu zn deposit yield an isochron age of 434 2 3 9 ma and initial 187os 188os ratio of 0 647 0 098 mswd 0 59 re os analyses of chalcopyrite from the hongshi cu deposit yield an isochron age of 431 8 2 7 ma and initial 187os 188os ratio of 0 165 0 075 mswd 0 77 since chalcopyrite is the primary copper mineral we interpret these isochron ages as the timing of cu mineralization based on field geology and petrographic evidence these results suggest that the re os ages presented here provide for the first time a direct constraint on an early paleozoic cu mineralization event of the eastern tianshan orogen the high initial 187os 188os ratios 0 647 0 098 ratio of 434 ma chalcopyrite from the south meiling deposit suggest that the metal was sourced from a two end member mixing of crust and mantle materials moreover we propose that the vms mineral system and hydrothermal vein system of the kalatag district were related to the south dipping subduction of the kalamaili oceanic plate during the late ordovician silurian

rhenium osmium systematics of the mount isa copper orebody and the eastern creek volcanics queensland australia implications for ore genesis the syn tectonic breccia hosted mount isa cu deposit in northwest queensland is the largest sediment hosted cu deposit in australia whole rock samples of chalcopyrite rich cu ore form an isochron with a re os age of 1 372 41 ma this age is more than 100 ma younger than the previously accepted age of cu ore formation an ar ar mineral age for biotite separated from the host rocks within the alteration envelope to the cu orebody this discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for cu mineralisation or the deformation event associated with cu emplacement the 1 372 41 ma date may reflect a the time of cu deposition b the time of a hydrothermal event that reset the re os signature of the cu ore or c mixing of the re os isotope systematics between the host rocks and cu bearing fluids however a range of published ar ar and rb sr dates for potassic alteration associated with cu mineralisation also records an event between 1 350 and 1 400 ma and these are consistent with the 1 372 ma re os age the 1 8 ga eastern creek volcanics are a series of tholeiitic basalts with a primary magmatic cu enrichment which occur adjacent to the mount isa cu deposit the whole rock os isotopic signature of the eastern creek volcanics ranges from mantle like values for the upper pickwick member to more radiogenic crustal values for the lower cromwell member the re os isotope signature of the cu ores overlaps with those calculated for the two volcanic members at 1 372 ma hence the os isotope data are supportive of the concept that the os in the cu ores was sourced from the eastern creek volcanics by inference it is therefore postulated that the eastern creek volcanics are the source of cu in the mount isa deposit as both os and cu are readily transported by oxidised hydrothermal fluids such as those that are thought to have formed the cu orebody the pickwick member yields a re os isochron age of 1 833 51 ma which is within error of previously reported age constraints the initial 187os 188os isotopic ratio of 0 114 0 067 os 0 7 is slightly subchondritic and together with other trace element geochemical constraints is consistent with a subcontinental lithospheric mantle source the pickwick member records a minimum age of ca 1 95 ga for melt depletion in the subcontinental lithospheric mantle beneath the mount isa inlier prior to the extraction of the magmas which formed the eastern creek volcanics this corresponds with the end of subduction related magmatism along the eastern margin of the northern australian craton which included the mount isa inlier

heterogeneous os isotope compositions in the kalatongke sulfide deposit nw china the role of crustal contamination re os isotope compositions of mantle derived magmas are highly sensitive to crustal contamination because the crust and mantle have very different os isotope compositions crustal contamination may trigger s saturation and thus the formation of magmatic ni cu pge sulfide deposits the 287 ma kalatongke norite intrusion of nw china are hosted in carboniferous tuffaceous rocks and contain both disseminated and massive sulfide mineralization the re os isotope compositions in the intrusion are highly variable norite and massive sulfide ores have os values ranging from 59 to 160 and a re os isochron age of 239 51 ma whereas disseminated sulfide ores have os values from 117 to 198 and a re os isochron age of 349 34 ma the variability of os isotope compositions can be explained as the emplacement of two distinct magma pulses massive sulfide ores and barren norite in the intrusion formed from the same magma pulse whereas the disseminated sulfide ores with more radiogenic os isotopes formed from another magma pulse which underwent different degrees of crustal contamination re os isotopes may not be suitable for dating sulfide bearing intrusions that underwent variable degrees of crustal contamination to form magmatic sulfide deposits

origin of the inconsistent apparent re os ages of the jinchuan ni cu sulfide ore deposit china post segregation diffusion of os abstract apparent re os ages of some magmatic sulfide ore deposits are older than the zircon and baddeleyite u pb ages which are interpreted as the formation age of the host intrusions the jinchuan ni cu pge deposit of china the world s third largest is such a case we report apparent re os isochron ages of 1117 67 ma 1074 120 ma and 867 75 ma with initial 187os 188os ratios of 0 120 0 012 0 162 0 017 and 0 235 0 027 for disseminated ores sulfides from the disseminated ores and massive ores from jinchuan respectively using these data and re os ages from the literature we find that the oldest apparent re os age and lowest initial os isotope ratio are from disseminated ores which contain small amounts of sulfide minerals the highest initial os isotope ratios and youngest apparent re os ages consistent with the zircon and baddeleyite u pb ages are from massive ores containing 90 100 modal sulfide and net textured ores with about 25 modal sulfides yield apparent re os ages and initial os ratios intermediate between those of the disseminated and massive ores because os diffusion between sulfides is inhibited by the intervening silicates even at high temperatures re equilibration did not occur in the disseminated ore and the samples retained the os ratios of the contaminated magma leading to geologically meaningless ages that are older than the formation age of the rocks while os bearing sulfide minerals and magnetite show low closure temperatures of os diffusion and the sulfide minerals in the massive ore are closely connected with each other facilitating fast diffusion of os re equilibration of os was achieved during cooling of the ore from about 850 c after the segregation to about 400 c thus an age corresponding to the formation time and an elevated initial os ratio were yielded by the massive ore os isotopes in the net textured ore behave in the way intermediate between the disseminated and massive ores pb isotope data support the os results disseminated ores have heterogeneous pb isotope ratios whereas pb in the massive ores is more uniform consistent with pb isotopic equilibration in the massive ores but not in the disseminated ores

re os isotope and pge constraints on the timing and origin of gold mineralisation in the witwatersrand basin abstract analyses of the re os isotope system and pge abundances have been used to investigate the timing and origin of mineralisation in the vaal reefs section of the witwatersrand basin the world s largest gold deposit consistent interelement pge fractionation in both organic and sulphide phases intimately associated with gold mineralisation suggests that the re os isotope system was fractionated during gold mobilisation the re os isotope data on organic separates intimately associated with the gold in this part of the basin indicate open system behaviour until 2 26 0 19 ga 2 of the order of 450 myr younger than the minimum age of sedimentation the overlying ventersdorp volcanics preserve a re os isochron age of 2 43 0 21 ga 250 ma younger than their u pb zircon emplacement age these data indicate substantial post sedimentary noble metal mobility within portions of the witwatersrand basin re os model age calculations on os rich fractions suggest that noble metals were derived from a 3 0 3 1 ga terrain of granite greenstone affinities significantly younger than the barberton greenstone belt these data support a hybrid model involving post depositional mobilisation of ancient probably detrital noble metals for the origin of mineralisation in this part of the witwatersrand basin such a model is applicable to other areas of gold mineralisation in the basin that display similar textural characteristics and geochemistry

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

effects of frictional viscous oscillations and fluid flow events on the structural evolution and re os pyrite chalcopyrite systematics of cu rich carbonate veins in northern norway abstract mesothermal chalcopyrite pyrite magnetite bearing calcite dominated vein deposits in the repparfjord tectonic window northern norway have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation the veins cut across paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured coarse grained margins and the fine grained mylonitic cores they formed under overall viscous conditions although cyclically increased fluid pressures caused transient embrittlement as mineral precipitation sealed the fractures deformation was accommodated again viscously leading to mylonitization of the vein s core local brecciation of the calc mylonite demonstrates the cyclicity of this process re os chalcopyrite pyrite and k ar fault gouge dates define an almost continuous age range from 2540 ma to 460 ma regression of three re os analyses yields a 2069 14 ma age 187os 188os 0 18 0 04 interpreted as the age of vein emplacement sulfide precipitation and initial frictional viscous deformation k ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 ma hence indicating structural reactivation connected with silurian caledonian orogenic compression the spread in re os model ages reflects this reactivation wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing dynamic recrystallization and isotopic disturbance of the sulfides the study provides evidence for fluid flow during viscous deformation and demonstrates that strain and flow of oxidizing fluids can have a significant yet localized control on the integrity of the re os systematic in pyrite and chalcopyrite

re os isotopic ages of pyrite and chemical composition of magnetite from the cihai magmatic hydrothermal fe deposit nw china the eastern tianshan orogenic belt of the central asian orogenic belt and the beishan terrane of the tarim block nw china host numerous fe deposits the cihai fe deposit 90 mt at 45 6 fe in the beishan terrane is diabase hosted and consists of the cihai cinan and cixi ore clusters ore minerals are dominantly magnetite pyrite and pyrrhotite with minor chalcopyrite galena and sphalerite gangue minerals include pyroxene garnet hornblende and minor plagioclase biotite chlorite epidotite quartz and calcite pyrite from the cihai and cixi ore clusters has similar re os isotope compositions with 14 to 62 ppb re and 10 ppt common os pyrrhotite has 5 to 39 ppb re and 0 6 ppb common os pyrite has a mean re os model age of 262 3 5 6 ma n 13 in agreement with the isochron regression of 187os vs 187re the re os age 262 ma for the cihai fe deposit is within uncertainty in agreement with a previously reported rb sr age 268 25 ma of the hosting diabase indicating a genetic relationship between magmatism and mineralization magnetite from the cihai deposit has mg al ti v cr co ni mn zn ga and sn more elevated than that of typical skarn deposits but both v and ti contents lower than that of magmatic fe ti v deposits magnetite from these two ore clusters at cihai has slightly different trace element concentrations magnetite from the cihai ore cluster has relatively constant trace element compositions some magnetite grains from the cixi ore cluster have higher v ti and cr than those from the cihai ore cluster the compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures combined with regional tectonic evolution of the beishan terrane the re os age of pyrite and the composition of magnetite indicate that the cihai fe deposit may have derived from magmatic hydrothermal fluids related to mafic magmatism probably in an extensional rift environment

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

metal saturated sulfide assemblages in nwa 2737 evidence for impact related sulfur devolatilization in martian meteorites nwa 2737 a martian meteorite from the chassignite subclass contains minute amounts 0 010 0 005 vol of metal saturated fe ni sulfides these latter bear evidence of the strong shock effects documented by abundant fe nanoparticles and planar defects in northwest africa nwa 2737 olivine a ni poor troilite fe s 1 0 0 01 sometimes cr bearing up to 1 wt coexists with micrometer sized taenite tetrataenite type native ni fe alloys ni fe 1 and fe os ir ru alloys a few hundreds of nanometers across the troilite has exsolved flame like pentlandite fe fe ni 0 5 0 6 chalcopyrite is almost lacking and no pyrite has been found as a hot desert find nwa 2737 shows astonishingly fresh sulfides the composition of troilite coexisting with ni fe alloys is completely at odds with chassigny and nahkla sulfides pyrite metal deficient monoclinic type pyrrhotite it indicates strongly reducing crystallization conditions close to iw several log units below the fo2 conditions inferred from chromites compositions and accepted for chassignites fmq 1 log unit it is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures shock related s degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages with concomitant loss of magnetic properties as documented in some other martian meteorites

seismic anisotropy beneath the southern himalayas tibet collision zone shear wave phases scs sks and skks collected from the project international deep seismic sounding of tibet and himalayas ii indepth ii array in the eastern himalayas and tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern himalayas tibet collision zone no evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the indus tsangpo suture the lack of shear wave splitting beneath the stations in the vicinity of suture and the tethyan himalayas indicates that there is no appreciable large scale mantle polarization seismic anisotropy beneath this part of the collision zone one explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened indian lower continental lithosphere north of the gandese belt the fast directions of the split shear waves are oriented in a northeast southwest direction approximately parallel to the trend of a major left lateral strike slip fault zone that bounds the gulu rift the coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric wide coherent deformation

late paleozoic sedex deposits in south china formed in a carbonate platform at the northern margin of gondwana abstract sedex sulfide deposits hosted in black shale and carbonate are common in the south china block the dajiangping pyrite deposit is the largest of these deposits and is made up of stratiform orebodies hosted in black shales sandstone interlayered with stratiform orebodies contains detrital zircon grains with the youngest ages of 429 ma pyrite from the orebodies has a re os isochron age of 389 62 ma indicative of formation of the hosting strata and syngenetic pyrite ores in the mid late devonian the hosting strata is a transgression sequence in a passive margin and composed of carbonaceous limestone in the lower part and black shales in the upper part the ore hosting black shales have high toc total organic carbon mo as pb zn and cd indicating an anoxic euxinic deep basin origin the high redox proxies v v ni 0 6 and v cr 1 and the positive correlations of toc with mo and v in black shales are also consistent with an anoxic depositional environment the dajiangping deposit is located close to the ne trending wuchuan sihui fault which was active during the devonian the mid late devonian mineralization age and the anoxic euxinic deep basinal condition of this deposit thus imply that the formation of this deposit was causally linked to hydrothermal fluid exhalation in an anoxic fault bounded basin that developed in a carbonate platform of the south china block the regional distribution of many devonian stratiform carbonaceous sediment hosted sulfide deposits along the ne trending fault bounded basins in south china similar to the dajiangping deposit indicates that these deposits formed at a basin developed in the passive margin setting of the south china block during the devonian this environment was caused by the break up and northward migration of the south china block from gandwana

diagenetic origin of the stratiform cu co deposit at kamoto in the central african copperbelt sediment hosted stratiform ore deposits that underwent metamorphism and orogenesis are interpreted either to have a multiphase origin that might in part predate these events or to be exclusively related to fluid migration during the orogeny this controversy concerns the formation of many world class sediment hosted ore deposits such as in the central african copperbelt here we present re os dating results for disseminated and stratiform cu co sulfide pseudomorphs after anhydrite in nodules and layers from the copperbelt results demonstrate that at least some of the ores formed at around 800 ma during rifting and basin development and thus predate the pan african lufilian orogeny younger re os ages i e between 682 28 and 230 36 ma may be due to the replacement of the early cu co sulfides by younger sulfide phases alternatively these younger ages may result from the disturbance or resetting of the re os system during recrystallization metamorphism or the transition of chalcocite from its high to its low temperature polymorph

rhenium osmium isotope systematics of an early mesoproterozoic sedex polymetallic pyrite deposit in the north china craton implications for geological significance and the marine osmium isotopic record abstract the rhenium osmium re os isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the os composition of ancient seawater in particular the age of sedimentary exhalative sedex deposits can be determined by measuring re os isotopes in syn depositional pyrite a common constituent of sedex deposits here we report on the gaobanhe sediment hosted polymetallic pyrite deposit which is hosted within the gaoyuzhuang formation in the central part of the northern margin of the north china craton ncc re os isotopic data from syn depositional pyrite yield an isochron age of 1505 55 ma constraining the timing of primary mineralization as well as the minimum age of the host sediments to the mesoproterozoic analyzed pyrite grains indicate an initial 187os 188os osi value of 0 181 0 066 suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids with the os being primarily derived from seawater the osi value is slightly higher than that of paleoproterozoic seawater 0 13 suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased continental radiogenic os input to the oceans we propose that the re os isotopic system as recorded by these sedex deposits may provide key insights when examining changes in the os isotopic composition of seawater throughout the precambrian

petrography mineral chemistry fluid inclusion microthermometry and re os geochronology of the kure volcanogenic massive sulfide deposit central pontides northern turkey the re os isotopic system is applied for the first time to the sulfide ores and the overlying black shales at the kure volcanogenic massive sulfide deposit of the central pontides northern turkey the ore samples collected include predominantly pyrite accompanied by chalcopyrite sphalerite and other species massive ore is almost free of gangues whereas the stockwork ore includes quartz and calcite gangue the composition of sphalerite is similar to ancient and modern massive sulfide mineralizations globally microthermometric studies from quartz from the stockwork ore has shown two populations of two phase fluid inclusions with vapor liquid ratios between 4 to 28 low to intermediate th 161 5 317 0 c and low salinities 0 9 5 wt nacl equiv which are mostly in good agreement with the ranges for volcanogenic massive sulfide mineralizations these studies also suggest a h2o cacl2 kcl mgcl2 ore forming fluid system in a shallower subsurface near the seafloor vents the re os dating of the llhr sulfides yield a nominal depositional age of upper toarcian for the massive sulfide mineralization two largely different model ages obtained are attributed to other pyrite crystallization events prior to and postdating the main sulfide deposition some lower homogenization temperatures 200 c from the quartz of the stockwork may also similarly be related to the post vms events it is concluded that a submarine volcanic extrusion episode has continued until upper toarcian in the kure basin when it has entered a stagnation period that allowed the discharge of hydrothermally circulated sulfide laden fluids from the seafloor vents this age data promotes the palaeotectonic models interpreting the kure basin as a permian early jurassic marginal back arc basin of the devonian triassic karakaya ocean the re os data from the overlying black shale provide a glimpse to the initial os isotope ratio of the water column at the time of the sedimentation 0 45 0 46 for 180 ma the lack of common os from the sulfides does not let us to infer a source of os and initial 187os 188os ratios from the black shale are not statistically robust to make a significant deduction a further detailed study on the isotopic composition of the black shale strata may help us to make an apporach to the os source s in the deposition environment of the kure vms deposit

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

an overview of chalcophile element contents of pyrrhotite pentlandite chalcopyrite and pyrite from magmatic ni cu pge sulfide deposits we have compiled the trace element concentrations in pyrrhotite pentlandite chalcopyrite and pyrite from magmatic ni cu pge ore deposits with the aim of understanding their petrogenesis and whether these minerals can be used as indicator minerals among the samples there are some of the most studied world class ni cu aguablanca duluth jinchuan noril sk talnakh kharaelakh sudbury voisey s bay and others and pge dominated bushveld lac des iles stillwater great dyke and penikat deposits crustal assimilation may be constrained using as se and sb se ratios in pentlandite the degree of interaction between the silicate and sulfide liquids r factor can be estimated by the content of highly chalcophile elements dsulf liq sil liq above 1000 in sulfide minerals the fractional crystallization of the sulfide liquid can be traced using se te ratios of pentlandite pyrite formed by exsolution from mss has higher rh ru ir and os than co existing pyrrhotite whereas pyrite formed by hydrothermal alteration of pyrrhotite inherits the rh ru ir and os contents of the pyrrhotite it replaced sulfide minerals are preserved in transported glacial cover and their trace element chemistry can be used to discriminate their source pentlandite from ni cu deposits has much lower rh and pd concentrations than those from pge dominated deposits pyrite from magmatic deposits has higher co sb and se as ratios relative to pyrite from hydrothermal deposits and chalcopyrite from magmatic deposits has much higher ni and lower cd concentrations than those from hydrothermal deposits

moho offset across the northern margin of the tibetan plateau anomalous double pulse teleseismic p wave arrivals were observed at one station near the northern margin of the tibetan plateau the azimuthal dependence of the waveform distortion and its absence at nearby stations indicated that the distortion was produced by receiver side crustal heterogeneity modeling of the three component data revealed a 15 to 20 kilometer moho offset that occurs over a narrow lateral range of less than 5 kilometers this east west striking offset separates the thick tibetan plateau crust from the qaidam basin crust such a sharp crustal thickness change implies a weak tibetan plateau crust that thickens vertically in response to penetration by india from the south and to blockage caused by a strong qaidam basin crust to the north

diffusion in carbonates fluorite sulfide minerals and diamond this chapter focuses on diffusion in non silicate minerals that do not fall under any of the other chapter categories this includes minerals that are not oxides fluorite sulfide minerals diamond and those like the carbonates that are not among the oxide minerals discussed in chapter 17 of this volume van orman and crispin 2010 although there are numerous minerals in these categories limited diffusion data exist for many of them so the primary phases that will be considered are carbonates fluorite diamond and sulfide minerals carbonate minerals are major constituents of sediments and many sedimentary rocks fractionation of o and c isotopes between carbonate rocks and fluids can provide insight into paleoclimates and sedimentary histories the behavior of the isotopes of major elements in calcite and aragonite has become of increasing interest due to the important role of co2 and carbonate dissolution precipitation in the oceans and the connections to global climate e g arita and wada 1990 chacko et al 1991 dickson 1991 beck et al 1992 cole 1992 graham et al 1998 cole and chakraborty 2001 ca mg diffusion rates affect the process of dolomitization hardie 1987 fisler and cygan 1999 and characterization of diffusion of these species is important for refinements in application of the calcite dolomite geothermometer e g essene 1982 farver and yund 1996 and better understanding of deformation mechanisms for example calcium isotopic fractionation between seawater and calcite and aragonite has been observed in marine samples zhu and macdougall 1998 de la rocha and depaolo 2000 schmitt et al 2003 with possible applications as a thermometer ca isotopic fractionation also occurs in biologic systems as in the growth of foraminifera skulan et al 1997 zhu and macdougall 1998 diffusion of major elemental species plays an important rate limiting

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

variable initial pb isotopic compositions of rocks associated with the ug2 chromitite eastern bushveld complex abstract an investigation of the pb isotopic compositions of plagioclase and sulfide in a stratigraphic interval including the ug2 chromitite of the eastern bushveld complex has been conducted to determine the pb isotopic composition s of the magma s that crystallized to form this part of the intrusion gain a better understanding of why coexisting plagioclase and sulfide commonly exhibit widely different pb isotopic compositions and explore the use of pb isotopes in deducing post accumulation history analyses were obtained in situ with a nuplasma multicollector icp ms coupled with 193 nm excimer or 213 nm lasers most plagioclase compositions fall on the 207pb 204pb vs 206pb 204pb geochron of 2 06 ga which is the solidification age of the intrusion the measured ratios have not been affected by radiogenic ingrowth and plagioclase generally remained closed to pb exchange after initial cooling the array of plagioclase compositions on the geochron is significantly larger than that defined by analytical error this indicates that in terms of pb at least two different magma compositions were present the composition of the least radiogenic magma was approximated by that of the contemporaneous bse with 238u 204pb and 232th 204pb values of 9 0 and 35 respectively suggesting a mantle derivation with little or no involvement of the continental crust while the second magma possessed a pb isotopic composition similar to the upper crust with 9 6 compared to plagioclase sulfides generally possess slightly higher 206pb 204pb ratios for equivalent 207pb 204pb ratios such that their compositions fall between the 2 06 and 1 86 ga geochrons the latter age is much younger than the cooling age the data are interpreted to mean that the bushveld complex remained buried in the crust at temperatures of several hundred c for about 200 ma after solidification and that any sulfides accessible to fluid continued to re equilibrated during this time with more radiogenic pb the sulfide pb may have been transported into the bushveld complex by fluids from an external reservoir when the rocks were still partially molten and thus permeable alternatively the sulfide pb may have originated mainly from radiogenic decay of u and th present in minerals other than the sulfides in the immediately surrounding bushveld rocks followed by local redistribution of pb by whatever fluid was present indeed some sulfides are characterized by 208pb 204pb ratios sufficiently high that an external source is unlikely this observation and the fact that the sulfides display small scale heterogeneity suggest that most if not all of the radiogenic sulfide pb was locally derived it also implies that during the post solidification re equilibration period there was no large scale fluid flow through the microfracture network because otherwise the isotopic heterogeneities would not have been preserved the minerals in 2 of the 19 samples studied contain young pb presumably introduced by meteoric waters that permeated the macrofracture network

constraints on the timing of late eburnean metamorphism gold mineralisation and regional exhumation at damang mine ghana the damang gold deposit in southwest ghana is unique among known deposits in ghana comprising gold mineralisation in two distinct styles early gold hosted in a stratigraphically controlled auriferous quartz pebble conglomerate horizon is overprinted by later gold mineralisation contained in a sub horizontal fault fracture quartz vein array a multi system geochronological study is used to constrain the timing of igneous activity regional metamorphism gold mineralisation and the thermal history at damang u pb analysis of zircons from birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178 0 9 3 ma and 2164 6 8 0 ma respectively which is consistent with previous studies the age of formation of staurolite grade amphibolite facies peak metamorphic mineral assemblages at 2005 26 ma is provided by u th total pb epma analysis of metamorphic monazite grains in the tarkwa phyllite measured 40ar 39ar biotite ages range between 1980 9 ma and 1898 11 ma argon diffusion modelling with the programme diffarg suggests that this age range could be achieved by a period of rapid cooling at a rate of approximately 17 c ma followed by a prolonged period of much slower cooling at a rate of 0 15 c ma the period of rapid cooling is interpreted to represent localised exhumation of the damang host rocks during the latest stage of the eburnean orogeny at the time of hydrothermal gold mineralisation given these age constraints hydrothermal gold mineralisation is inferred to have occurred between approximately 2030 ma and 1980 ma these ages constrain metamorphism fluid flow and gold mineralisation at damang and are the youngest currently recognised in the birimian of sw ghana

re os geochronology of the iimori besshi type massive sulfide deposit in the sanbagawa metamorphic belt japan abstract besshi type cu deposits are strata bound volcanogenic massive sulfide deposits usually associated with mafic volcanic rocks or their metamorphic equivalents although there are numerous besshi type deposits in the sanbagawa metamorphic belt japan their tectonic settings and depositional environments remain controversial because of a lack of depositional age constraints we report re os data for the iimori deposit one of the largest besshi type deposits in western kii peninsula in order to examine the robustness of the re os isotope system for dating sulfide minerals in the high p t metamorphic belt and to elucidate the primary depositional environment of the iimori sulfide ores an 11 point re os isochron plot yields an age of 156 8 3 6 ma as this re os isochron age is significantly older than the timing of the sanbagawa peak metamorphism 110 120 or 90 ma and a well defined isochron was obtained the re os age determined here is most likely the primary depositional age despite high grade metamorphism at up to 520 25 c and 8 9 5 kbar the re os isotope system of the iimori sulfides was not disturbed hence we consider that the whole rock re os closure temperature for the iimori sulfide ores was probably higher than 500 c as the accretion age of the sanbagawa metamorphic belt is considered to be 120 130 or 65 90 ma on the basis of radiolarian and radiometric ages we estimated the time from the iimori sulfide deposition on the paleo seafloor to its accretion at the convergent plate boundary to be greater than 25 myr consequently the depositional environment of the iimori sulfide ores was not in the marginal sea but was truly pelagic

significance of late devonian lower carboniferous ages of hydrothermal sulphides and sericites from the urals volcanic hosted massive sulphide deposits abstract formation of the urals volcanic hosted massive sulphide vhms deposits is considered to be related with the intra oceanic stage of the island arc s development in late ordovician middle devonian time ca 460 385 ma based on the biostratigraphic record of ore hosting sedimentary rocks however the known radiometric ages of ore hosting volcanics are very limited here we present direct dating results of sulphide mineralisation from the yaman kasy and kul yurt tau vhms deposits using re os isotope systematics showing similar mineralisation ages of 362 9 ma and 363 1 ma these ages coincide with the previous re os dating of the alexandrinskoe 355 15 ma and dergamysh 366 2 ma vhms deposits this late devonian famennian age corresponds to the late stage of the magnitogorsk arc laurussia continent collision event and coincides with a beginning of large scale subduction related granitoid magmatism the younger mineralisation age relative to the biostratigraphic ages of host rocks is interpreted as one of the latest episodes of the multi stage history of vhms deposits development ar ar ages of sericites from metasomatic rocks of barsuchi log and babaryk deposits show even younger ages clustering around 345 ma and testify another late hydrothermal event in the history of the urals vhms deposits

chalcophile siderophile element systematics of hydrothermal pyrite from martian regolith breccia nwa 7533 unlike other martian meteorites studied so far martian regolith breccia nwa 7533 and 56 paired meteorites that have sampled 4 4 ga old impact lithologies show only sulfides of 57 hydrothermal origin mostly pyrite 1 the reasonably good positive correlation r 2 0 72 between se and 65 ni reflects a temperature control on the solubility of both elements apart from the chalcogens s se 66 and te pyrite appears to be a minor contributor 400 c temperature crystallization for nwa 7533 pyrite as deduced from 69 its se and ni contents ii magmatic sulfide depletion of brecciated early martian crust iii 70 precipitation from near neutral h 2 s hs h 2 o rich hydrothermal fluids that did not provide halogen 71 ligands for extensive transport of chalcophile siderophile metals it is suggested that the 1 4 ga 72 lithification event that precipitated hydrothermal pyrite left the chalcophile siderophile element 73 budget of the early martian crust nearly unmodified except for s se and te 74 75 76

pyrite re os isotope systematics at the zijinshan deposit of sw fujian china constraints on the timing and source of cu au mineralization abstract zijinshan is the largest high sulfidation epithermal cu au deposit on mainland china the primary mineralization at the deposit is characterized by pyrite digenite and covellite although some rb sr isochron ages of the fluid inclusions in quartz and the apparent k ar age of the cu bearing alunite alteration zone have been previously interpreted as the ore forming age the rb sr and k ar dating systems are usually readily reset due to their closure temperature and thus document the latest thermal event in order to precisely determine the age of the cu mineralization eight pyrite separates were re os dated and seven yielded an isochron age of 103 4 ma with an initial 187 os 188 os ratio of 0 45 0 14 this date is interpreted as the age of cu mineralization which is in agreement with geochronological data from molybdenite and the porphyritic granodiorite that hosts cu mo mineralization at luoboling located 2 km ne of zijinshan suggesting that these two deposits were formed during the same metallogenic event the relatively low initial 187 os 188 os ratio indicates that the source of the cu at zijinshan is predominantly the crust with input from the mantle

metamorphism disturbs the re os signatures of platinum group minerals in ophiolite chromitites the os isotope compositions of platinum group minerals pgms in ophiolite chromitites are commonly regarded as resistant to fluid related processes and have been used to track the evolution of earth s convecting mantle however we have found significant differences in 187os 188os between primary and secondary pgms from metamorphosed ophiolite chromitites of the dobromirtsi ultramafic massif in the central rhodope metamorphic core complex of southeastern bulgaria primary magmatic pgms hosted in unaltered chromite cores have 187os 188os from 0 1231 to 0 1270 and 187re 188os 0 002 t ma and t rd model ages calculated relative to the enstatite chondrite reservoir cluster around three main peaks ca 0 3 0 4 and 0 6 ga secondary pgms produced by alteration of magmatic pgms have a wider range of variation 187os 188os 0 1124 0 1398 187re 188os 0 024 these grains yield t ma and t rd model ages from 1 7 ga up to 2 2 ga the larger range in 187os 188os in the secondary pgms is interpreted as due to reactions between the primary pgms and infiltrating metamorphic hydrothermal fluids with a range of os isotope compositions this redistribution of os in pgms during metamorphism has significant implications for the interpretation of both whole rock and in situ os isotope data in mantle derived rocks

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

s process os isotope enrichment in ureilites by planetary processing abstract ubiquitous nucleosynthetic isotope anomalies relative to the terrestrial isotopic composition in mo ru and other elements are known from both bulk chondrites and differentiated meteorites but os isotope ratios reported from such meteorites have been found to be indistinguishable from the terrestrial value the carriers of s and r process os must thus have been homogeneously distributed in the solar nebula as large os isotope anomalies are known from acid leachates and residues of primitive chondrites the constant relative proportions of presolar s and r process carriers in such chondrites must have been maintained during nebular processes it has long been assumed that partial melting of primitive chondrites would homogenize the isotopic heterogeneity carried by presolar grains here ureilites carbon rich ultramafic achondrites dominantly composed of olivine and low ca pyroxene are shown to be the first differentiated bulk solar system materials for which nucleosynthetic os isotope anomalies have been identified these anomalies consist of enrichment in s process os heterogeneously distributed in different ureilites given the observed homogeneity of os isotopes in all types of primitive chondrites this os isotope variability among ureilites must have been caused by selective removal of s process poor os host phases probably metal during rapid localized melting on the ureilite parent body while mo and ru isotope anomalies for all meteorites measured so far exhibit s process deficits relative to the earth the opposite holds for the os isotope anomalies in ureilites reported here this might indicate that the earth preferentially accreted olivine rich restites and inherited a s process excess relative to smaller meteorite bodies consistent with earth s high mg si ratio and enrichment of s process nuclides in mo ru and nd isotopes our new os isotope results imply that caution must be used when applying nucleosynthetic isotope anomalies as provenance indicators between different classes of meteorites

constraints on the genesis of gold mineralization at the homestake gold deposit black hills south dakota from rhenium osmium sulfide geochronology the homestake gold deposit located in the black hills south dakota usa is one of the largest known hydrothermal gold deposits globally with total mining production exceeding 40 moz au rhenium osmium geochronology of ore associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern black hills arsenopyrite yields a rhenium osmium re os age of 1 736 8 ma mean squared weighted deviation 1 6 consistent with existing age constraints for gold mineralization whereas re os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age this is taken to indicate that the re os arsenopyrite chronometer is robust to at least 400 c whereas the re os pyrrhotite chronometer is likely disturbed by temperatures of 300 350 c the re os arsenopyrite age and initial os ratio 0 28 0 15 are interpreted to indicate that gold was introduced at ca 1 730 ma coincident with the onset of exhumation of crustal blocks and possibly the earliest intrusive phases of harney peak granite magmatism new in situ u pb monazite analyses from an aplite dike in the east central black hills indicate that granite magmatism was a protracted event persisting until at least ca 1 690 ma

the early formation of the iva iron meteorite parent body the iva iron meteorites are magmatic cumulates from the core of a small asteroid which broke apart similar to 400 ma ago as the depletion of this planetary body in volatile elements is expected to be reflected in high u pb ratios of its minerals we embarked on analyzing the isotope composition of pb in the cm sized troilite inclusions of muonionalusta and gibeon both iron meteorites belonging to the iva family the bulk of the data for nine troilite subsamples of muonionalusta scatter with an apparent age of 4 57 ga which is shown to reflect the presence of both primordial and common terrestrial pb components in addition to radiogenic pb the most radiogenic subsample however has fractions with pb 206 pb 204 ratios as high as over 1000 and gives a statistically significant 207 pb pb 206 age of 4565 3 0 1 ma mswd 0 08 consistent with the hf 182 w 182 metal silicate segregation age of 2 4 2 0 ma these data make the age of muonionalusta the oldest documented yet for all differentiated bodies in the solar system and constitute the first high precision pb pb age determined for crystallization of a phase contained within an iron meteorite group hence advancing our understanding of early solar system chronology using literature values for the cooling rate and assuming a closure temperature for pb of 300 degrees c it is further estimated that the iva parent body accreted within 1 ma of cai formation and had cooled to the pb closure temperature within an additional 1 2 ma the overlap between the high precision pb pb and hf w ages points to a small or rapidly fragmented planetary body the isotopic composition of pb in gibeon troilite yields a significantly younger age of 4544 7 ma mswd 1 5 consistent with evidence from the pd 107 ag 107 chronometer but we believe this age has been reset by melting upon shock one puzzling observation is that the apparent th 232 u 238 of muonionalusta troilite is particularly low similar to 0 32 requiring a mechanism capable of efficiently fractionating th from u presumably the reduction of u to its trivalent form or the crystallization of phosphate average pb concentrations of the order of 5 10 ppb and high u 238 pb 204 ratios of 1000 require u concentrations in troilite in the sub ppb to one ppb range this may indicate that troilite inclusions in iva meteorites do not represent metal sulfide unmixing but rather correspond to late stage s rich liquid residues from extreme crystallization of the interstitial melts

re os and u pb constraints on gold mineralisation events in the meso to neoarchaean storo greenstone belt storo southern west greenland the storo greenstone belt in godthabsfjord southern west greenland consists of juxtaposed rock units of different age and origin and hosts gold mineralisation that is associated with arsenopyrite along a contact between lithological units and along the axial plane of a large fold core the age and origin of the gold is debatable but in this paper we present new arsenopyrite re os and zircon u pb data to constrain the age of the storo gold deposit a 2 71 0 05 ga arsenopyrite isochron and 2 707 0 008 ga highly radiogenic arsenopyrite from a mineralisation along a rock contact together with a 2 64 0 02 ga arsenopyrite isochron along the axial plane of the fold core indicate a two stage mineralisation process while the 2 707 0 008 ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event a mixing origin cannot yet be excluded for the 2 71 0 05 ga isochron the 2 64 0 02 ga isochron is in perfect agreement with recent u pb zircon data nutman et al 2007 precambrian research 159 19 32 and these data are best explained by orogenic mineralisation during amphibolite facies metamorphism along structural weak planes the initial os 187 os 188 value of 0 56 0 16 for the 2 64 0 02 ga isochron indicates a crustal source for the metals whereas the initial os 187 os 188 0 1 0 6 for the 2 71 0 05 ga isochron remains unconstrained re os data are best explained by relatively short crustal residence times of less than 0 1 ga wherein the os and associated metals were extracted from the mantle at a time younger than 2 8 ga and in which the 2 64 0 02 ga stage formed by mobilisation of an earlier mineralisation detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at 2 84 ga and that these units were tectonically juxtaposed to similar to 3 05 ga rocks within the belt the units that were deposited 2 84 ga record metamorphic zircon u pb ages of similar to 2 63 ga but evidence for 2 72 ga metamorphism which is ubiquitous in adjacent terranes is lacking c 2012 elsevier b v all rights reserved

an early cretaceous carbonate replacement origin for the xinqiao stratabound massive sulfide deposit middle lower yangtze metallogenic belt china stratabound massive sulfide deposits are widespread along the middle lower yangtze metallogenic belt mlymb and serve as an important copper producer in china two contrasting genetic models have been proposed interpreting the stratabound massive sulfide deposits as a carboniferous sedex protore overprinted by cretaceous magmatic hydrothermal system or an early cretaceous carbonate replacement deposit these two contrasting models have been applied to the xinqiao stratabound cu au sulfide deposit which is dominated by massive sulfide ores hosted in marine carbonates of the carboniferous chuanshan and huanglong formations with minor cu au skarn ores localized in the contact zone between the cretaceous diorite jitou stock and the carboniferous carbonate rocks new sims zircon u pb dating suggests that the jitou stock formed at 138 5 1 1 ma 2 mswd 0 6 pyrite re os dating yields an imprecise date of 142 47 ma 2 mswd 7 8 the geochronological data thus constrain the mineralization of the xinqiao deposit at early cretaceous fluid inclusions in prograde skarn diopside have homogenization temperatures of 450 600 c and calculated salinities of 13 58 wt nacl equiv quartz from the stratabound ores and pyrite quartz vein networks beneath the stratabound ores have homogenization temperatures of 290 360 and 200 300 c with calculated salinities of 5 12 and 2 10 wt nacl equiv respectively quartz from the skarn ores and veins beneath the stratabound ores have 18o values of 12 32 0 55 2 sd n 22 and 15 57 1 92 2 sd n 60 respectively corresponding to calculated 18o values of 6 22 1 59 2 and 6 81 2 76 2 for the equilibrated ore forming fluids the fluid inclusion and oxygen isotope data thus support a magmatic hydrothermal origin rather than a sedex system for the stratabound ores with the hydrothermal fluids most likely being derived from the jitou stock or associated concealed intrusion results from this study have broad implications for the genesis and exploration of other stratabound massive sulfide deposits along the mlymb

late cretaceous metallogeny in the zhongdian area constraints from re os dating of molybdenite and pyrrhotite from the hongshan cu deposit yunnan china abstract the zhongdian area in yunnan southwestern china located at the southern end of the yidun volcano magmatic arc that was formed during the triassic westward subduction of the gaze litang ocean hosts numerous triassic large porphyry and skarn deposits the arc suffered jurassic to cretaceous arc continental orogenic collision and cenozoic intracontinental strike slip shearing the hongshan cu mo pb zn deposit is potentially a large deposit and contains two ore types 1 predominant layered skarn cu pb zn ores along marble hornfels contacts and 2 minor crosscutting vein type cu mo mineralization previous research forwards a two stage genetic model without sufficient dating evidence supposing the skarn mineralization is related to the triassic calc alkalic intrusions and the vein type mineralization related to cretaceous quartz monzonite porphyries re os dating of molybdenite from vein type ores and quartz monzonite porphyries and that of pyrrhotite from skarn ores are presented here to constrain the mineralization age and rebuild the genetic model analyses of eight molybdenite samples yield an isochron age of 79 7 3 1 ma mswd 9 2 for the vein type mineralization and a model age of 81 9 1 1 ma for the quartz monzonite porphyries isotope data on seven pyrrhotite samples from the skarn ores yield an isochron age of 79 16 ma z mswd 8 4 the re os ages for the two ore types are concordant within analytical errors indicating that the hongshan deposit was formed in the late cretaceous elevated re contents in molybdenite 13 65 to 63 91 g g and extremely radiogenic initial 187 os 188 os ratios in pyrrhotite 0 7673 to 0 8184 weighted average 0 796 0 038 together with elevated os values in pyrrhotite 507 to 547 average 528 imply a significant crustal component in the ore forming materials that was likely derived from a lower crustal reservoir combined with the tectonic evolution of the zhongdian area and geochemical characteristics of corresponding intrusions the ages of mineralization obtained in this study indicate that the hongshan deposit was formed in a post collision setting with a genetic relationship to the emplacement of the quartz monzonite porphyry these results provide significant new information for the study and exploration of the late cretaceous metallogeny in the zhongdian area

an experimental study on the geochemical behavior of highly siderophile elements hse and metalloids as se sb te bi in a mss iss pyrite system at 650 c a possible magmatic origin for co hse bearing pyrite and the role of metalloid rich phases in the fractionation of hse abstract pyrite the most abundant sulfide in the earth s crust is an accessory mineral in several magmatic sulfide deposits although most pyrite is hydrothermal previous experimental studies have shown that pyrite can also have a primary magmatic origin by exsolving from monosulfide solid solution mss during cooling of a sulfide melt if sulfur fugacity is sufficiently high pyrite from some localities has significant amounts of co and complex zonation in some low melting point chalcophile elements lmce such as as se sb te bi henceforth referred to as metalloids and some platinum group elements pge ru rh pd os ir pt however the origin of such pyrite and the causes of zonation are not clear because the distribution of some of these elements is heterogeneous and seems to be developed in concentric zones the zonation has been interpreted to represent growth stages some of them secondary and caused partly by hydrothermal fluids better constraints on the origin of co pge bearing pyrite could help unravel the geochemical processes affecting the sulfide assemblages in which it is found thus an experimental study was undertaken to characterize pyrite formation in magmatic sulfide environments and its relationship with metalloids and highly siderophile elements hse pge re au natural pyrrhotite chalcopyrite pentlandite and elemental s were mixed and doped with approximately 50 ppm of each hse a mixture of metalloids was added at 0 2 wt or 3 wt to aliquots of sulfide mixtures starting materials were sealed in evacuated silica tubes and fused at 1200 c the temperature was subsequently reduced to 750 c at 60 c h then to 650 c at 0 5 c h to produce relatively large euhedral pyrite crystals then quenched the experiments were analyzed using reflected light sem epma and la icp ms experimental products contained euhedral pyrite mss intermediate solid solution iss and metalloid rich phases interpreted as quench product of an immiscible metalloid rich liquid the results show that co ni hse bearing pyrite with complex zonation in ru rh os ir and pt can form by a subsolidus reaction involving both mss and iss and does not require secondary e g hydrothermal processes because such pyrite results from the cooling of a sulfide melt after mss and iss it can be described as magmatic among the hse ru rh os re and ir have identical zonation patterns in pyrite pt is also zoned but differently and au and pd are essentially excluded previously documented natural hse bearing pyrite also display identical zonation patterns in ru rh os and ir the complex zoning is likely preserved due to the extremely slow diffusion rates for those elements in pyrite thus the presence of pyrite with similar characteristic in natural sulfide assemblages is consistent with a magmatic origin and does not require hydrothermal processes the results also show that if a metalloid rich liquid is present it will significantly sequester au pd and pt but will not affect ru rh os re and ir in the absence of metalloid rich phases au partitions strongly into iss and pd partitions preferentially into mss

re os pyrite and u pb zircon geochronology from the taldybulak levoberezhny gold deposit insight for cambrian metallogeny of the kyrgyz northern tien shan abstract the taldybulak levoberezhny gold deposit located in the eastern part of the kyrgyz northern tien shan is hosted in highly deformed precambrian schist and gneisses that have undergone intense quartz carbonate fuchsite and tourmaline alterations gold mineralization is ultimately subdivided into two stages based on the observation of alteration assemblages orebody geometries and the occurrences of au bearing minerals negative thermal ionization mass spectrometry re os isotopic analyses of five au rich pyrite samples from the early stage yielded an isochron age of 511 18 ma zircon sensitive high resolution ion microprobe u pb dating of a diorite dike sample postdating the late stage mineralization yielded a wide range of ages from 3055 to 291 ma while a weighted mean 206 pb 238 u age of 414 6 6 8 ma is believed to represent the age of dike intrusion and the upper limit on the timing of the late stage quartz tourmaline gold formation the pyrite 187 os 188 os initial ratio of 0 132 0 011 together with os values varying from 0 to 14 indicate a major mantle component for the source of os and by inference ore metals which may be linked to the ophiolite suite of the kopurelisai complex in the taldybulak levoberezhny area considering the geodynamic setting of the kyrgyz northern tien shan during the early paleozoic we suggest that cambrian mineralization of the taldybulak levoberezhny deposit can be attributed to a subduction related setting probably associated with the earliest accretion of the northern tien shan

evidence for as contamination and the partitioning of pd into pentlandite and co platinum group elements into pyrite in the fazenda mirabela intrusion brazil the fazenda mirabela intrusion in brazil hosts two zones of mineralization the santa rita ni cu sulfide ore zone occurring predominantly in an orthopyroxenite layer and an underlying basal platinum group element pge anomaly hosted in s poor dunite we show that in the northern and southern marginal zones of the intrusion pt pd ni cu fe bi te 2 minerals are accompanied by as bearing platinum group minerals pgm sperrylite ptas2 and irarsite rhirpt ass these as bearing pgm are extremely rare in the central zone of the intrusion suggesting that as has been introduced into the margins of the intrusion from the country rocks via crustal assimilation or synmagmatic hydrothermal processes other pgm and precious metal minerals pmm including hessite ag2te pd ag tellurides electrum and native au are observed in the santa rita ore zone and do not show any lateral variation in the semimetal and s poor dunite hosting the basal pge anomaly pge are predominantly retained in the base metal sulfides and only minor pge alloys are present in this s poor dunite pt au and semimetals partitioned into a fractionated cu rich liquid which formed during the crystallization of monosulfide solid solution mss and was largely removed by high temperature magmatic fluids palladium was not affected by this process and significant concentrations of pd are identified in pentlandite suggesting that pd preferentially partitioned into mss as it crystallized and subsequently pentlandite as it exsolved pyrite in the santa rita ore zone is co rich and contains higher concentrations of ipge os ir ru and rh than pyrrhotite and pentlandite suggesting that pyrite has not replaced and inherited the pge concentrations of these sulfides the pd poor nature of pyrite also indicates that it has not replaced pd rich pentlandite it is proposed that the ipge and co preferentially partitioned into pyrite as it exsolved from mss

crustal thickness and upper mantle velocities in the tibetan plateau region from the inversion of regional pnl waveforms evidence for a thick upper mantle lid beneath southern tibet the crustal thickness and upper mantle velocity variation in tibet and surrounding regions was mapped through the inversion of 130 individual long period pnl waveforms the 130 paths crisscross the tibetan plateau hindu kush karakoram southwest china burma and northern india regions we regionalized these areas on the basis of topographic and geologic expression and determined the average upper mantle velocity and crustal thickness value for each block or region through a linear least squares inversion of the 130 parameter values crustal thicknesses in the tibetan plateau karakoram and hindu kush regions range from 63 to 72 km and pn velocities are 0 10 0 20 km s faster beneath these regions than those beneath northern india we observed impulsive first arrivals evidence for positive upper mantle gradients on seismograms recorded at distances between 1000 and 1400 km the paths of these waves crossed portions of tibet the hindu kush and east india regions simple modeling of the long period pn wave form and amplitude requires fairly high upper mantle gradients of 0 18 0 25 km s per 100 km beneath the pamir hindu kush region and requires an upper mantle lid in excess of 100 km thickness the southern half of tibet also requires a lid of about 100 km thickness a thinner upper mantle lid 50 km provided a poor match to the tibetan observations even with a very high gradient of 0 2 km s per 100 km the upper mantle structure of east india appears to be similar to the southern half of tibet 100 km thick positive gradient zone both the increase in pn velocity from india into tibet and the evidence for a thick upper mantle lid beneath tibet are consistent with the model in which indian lithosphere has underthrust southern tibet due to poor path coverage the structure beneath northern tibet cannot be resolved and thus underthrusting beyond central tibet cannot be constrained the rapid increase in upper mantle velocity from northern india into tibet can be explained by the pressure increase alone produced by the double thick crust and indicates that the indian lithosphere which underlies tibet is out of thermal equilibrium

geology geochronology and s pb os geochemistry of the alastuo gold deposit west tianshan nw china the alastuo gold deposit located in the narati region of the chinese west tianshan is a newly discovered gold deposit within the world class tianshan gold belt the gold mineralization is hosted by an early carboniferous granodiorite intrusion and controlled structurally by subsidiary faults of the transcrustal north narati fault the deposit consists of auriferous alteration assemblages and minor vein ores field relations and petrographic data suggest four stages of mineralization with gold precipitated coevally with quartz chlorite epidote pyrite and galena during the second and third stages that are characterized by tectonic transformation from ductile shearing to brittle deformation the temperature range of gold precipitation is estimated to be 280 330 c using chlorite geothermometry ten auriferous pyrite samples yielded a well defined re os isochron age of 325 3 ma and two sericite samples yielded plateau like 40ar 39ar ages of 321 7 3 0 ma and 321 2 2 8 ma these consistent ages suggest that the gold mineralization in the narati region was emplaced in a transitional regime from subduction to continental collision between the tarim block and middle tianshan terrane 325 310 ma rather than during the post collisional stage as in most part of the tianshan gold belt gold bearing pyrite has an initial osi value of 0 7 0 1 and lead isotopes ratios of 17 897 to 18 723 for 206pb 204pb 15 474 to 15 643 for 207pb 204pb and 37 522 to 38 299 for 208pb 204pb while samples of pyrite galena and sphalerite have 34sv cdt of 1 9 to 7 8 the isotope data suggest that the sulfur and metals are most likely derived from mixed mantle and crustal sources including metamorphic devolatilization of subducted oceanic slab and overlying sedimentary rocks the mineralization styles structural controls metal and sulfur sources and the timing of gold mineralization with respect to the orogenic event all suggest that the alastuo gold deposit represents an orogenic type deposit this newly recognized carboniferous orogenic gold mineralization event calls for a re evaluation of the gold metallogeny in the middle tianshan

evidence for mixing of re os isotopes at 2 7 ga and support of a remobilized placer model in witwatersrand sulfides and native au abstract this study analyzes re and os contents of au rich sulfides from the carbon leader tau lekoa and mponeng the vcr mponeng and tau lekoa and the black reef modder east as well as native au in the carbon leader to further investigate the detrital placer modified detrital placer and hydrothermal models for the source of au in mines from the witwatersrand an isochron plot of the results from the four locations yields an imprecise isochron age of 2381 140 ma mswd 17 and initial 187 os 188 os 0 24 0 21 isochron plots for the individual reefs also yielded ages that are younger than the deposition of the basin witwatersrand sediments samples that possess higher concentrations of os 100 ppb and low 187 re 188 os ratios to unify all of the data providing a large variation of chronologic information os mixing diagrams for both gold and sulfide minerals produce realistic mixing trends when solving os initial ratios for 2050 ma the two most probable end members for the mixing relationships are a high os concentration non radiogenic source and a low os concentration highly radiogenic source the high concentration non radiogenic os source has been measured in multiple studies and has been clearly derived from the subcontinental lithospheric mantle sclm the low concentration highly radiogenic source is debatable the measured concentrations of os in the witwatersrand ores are much greater than published os concentrations in other hydrothermal ores the data could indicate a redistribution of the detrital os signatures at 2050 ma thus a modified detrital model best explains the previously published data and this data

lateral variations in compressional velocities beneath the tibetan plateau from pn traveltime tomography summary a back projection algorithm is applied to 353 p traveltime measurements to image the lateral variations in compressional velocity in the uppermost mantle in and around tibet prior to the tomographic inversion we refined the apparent p slowness measurements based on a recently developed relocation procedure zhao helmberger 1991 and mathematical formulations of zhao 1993 these refinements are crucial in obtaining a high quality velocity image we obtained an average p velocity value for the uppermost mantle of 7 93 kms l and an average mantle p velocity gradient of 3 1 x lo s for tibet the final 2 d p velocity image in the uppermost mantle is characterized by a large 7 to 9 in dimension low velocity down to about 7 78 km s region in north central tibet and by large high velocity up to 8 11 km s i regions in the western and eastern flanks of tibet the crustal thickness is greater than 70 km inside the tibetan plateau it decreases toward the southern eastern and western boundaries of the plateau at and beyond these boundaries the crustal thickness is similar to those beneath the eurasian shield rigorous analyses of resolution and error suggest that 1 there is a pronounced difference between the tomographic velocity image obtained with and without the refinements to the p slowness measurements indicating a significant improvement in the quality of our inversion by these refinements 2 random errors in measuring earthquake origin times and locations and in the p arrival times should not cause errors of more than 0 1 km s in the velocity image 3 the resolution is generally within 6 to 10 the velocity image obtained in this study is consistent with previous s p traveltime studies and geothermal petrological studies and is consistent with a mantle convection model in which a convective upwelling is occurring in north central tibet and downwellings occurring in the surrounding regions molnar 1988 1990

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

lithospheric structure of the qiangtang terrane northern tibetan plateau from complete regional waveform modeling evidence for partial melt we report models of p and s wave velocity and attenuation for the the crust and uppermost mantle of the qiangtang terrane northern tibetan plateau inferred by fitting reflectivity synthetic seismograms to observed complete regional waveforms the data are three component broadband seismograms recorded by the 1991 1992 iris passcal incorporated research institutions for seismology program for array seismic studies of the continental lithosphere tibetan plateau experiment and global seismic network stations in the region the qiangtang terrane has thick crust 65 5 km with p and s wave velocities of 6 1 6 3 and 3 34 3 43 km s respectively yielding an anomalously high crustal poisson s ratio of 0 29 0 02 seismic velocities of the upper mantle of the qiangtang terrane are normal for p waves and slow for s waves 8 10 and 4 35 4 41 km s respectively with a high mantle poisson s ratio of 0 29 0 01 attenuation in the crust and upper mantle is high qp 100 200 and qs 44 89 modeling of the broadband p waveforms suggests that a decrease in mantle velocity occurs at about 160 km depth in the mantle however this is not unambiguously supported by the data and modeling the crust and uppermost mantle of the qiangtang terrane probably contains partial melt based on the high poisson s ratio low shear wave velocities and low q the absence of high frequency sn and the presence of volcanism of mantle lithospheric origin support the presence of partial melt crustal and uppermost mantle structure in the qiangtang terrane is different from that for the lhasa terrane immediately to the south based on results of our previous studies the average crustal p and s wave velocities are 4 faster and 2 slower respectively in the qiangtang terrane relative to the lhasa terrane this yields a significant difference in the crustal poisson s ratio with values of 0 29 for the qiangtang terrane and 0 25 for the lhasa terrane differences in the uppermost mantle p and s wave velocities and poisson s ratios of these two adjacent terranes cannot be explained by temperature differences alone using the mantle temperature estimates of mcnamara et al 1997 we suggest that partial melt of an ultramafic composition beneath the qiangtang terrane fits the velocity and poisson s ratio estimates

crustal structure of china and surrounding regions from p wave traveltime tomography 1 a three dimensional 3 d p wave velocity model is developed for the crust and uppermost mantle of china and the surrounding area by applying the tomography method of zhao et al using 500 000 high quality p wave first arrivals extracted from the annual bulletin of chinese earthquakes abce this tomographic method can accommodate velocity discontinuities such as the moho in addition to smooth velocity variations the spatial resolution is 1 1 in the horizontal direction and 10 km in depth the velocity images of the upper crust correspond well with the surface geologic features such as basins and the tibetan plateau high velocity anomalies are found in the lower crust beneath the precambrian regions tarim basin ordos basin sichuan basin and western half of songliao basin the highest velocity anomaly is beneath the sichuan basin high and low velocity anomalies imaged beneath the bohai gulf are associated with the presence of a major cenozoic rift system in the lower crust beneath the south china block p wave velocities are lower in the north than in the south the indochina block shows low velocities both in the crust and in the uppermost mantle due to volcanism the pn velocities in the tibet area are higher than those in other areas largely due to thicker crust tomographic model significantly reduces the traveltime residuals tests conducted by relocating large explosions and earthquakes validate the 3 d velocity model

preservation of re os isotope signatures in pyrite throughout low t high p eclogite facies metamorphism pyrite in lt hp eclogites from the western tianshan orogenic belt yields a re os age of 378 1 8 9 ma which is 30 70 ma older than ages previously obtained for the same rocks using the rb sr sm nd ar ar u pb and lu hf isotope systems the tianshan lt hp eclogite experienced temperatures of up to 570 c combined with pressures of up to 2 1 gpa during metamorphism these conditions are below the transition of pyrite to pyrrhotite which defines both pyrite stability and possibly its closure temperature for re os pyrite can preserve re os signatures through eclogite facies peak metamorphic conditions and thus allow determination of the formation age of pyrite in the protolith

re os isotopic systematics of the voisey s bay ni cu co magmatic ore system labrador canada abstract re and os concentrations and os isotopic compositions have been obtained for massive matrix and disseminated sulphide ores from three environments within the voisey s bay intrusion the ovoid eastern deeps and discovery hill zone in order to assess the role of crustal contamination in the genesis of this large cu ni co deposit these samples have high re concentrations 148 to 288 ppb in 100 sulphide for their common os concentrations 4 8 to 24 ppb in 100 sulphide yielding high re os ratios 12 to 33 these data confirm that the magma parental to the voisey s bay ore system was broadly basaltic in major element chemistry rather than picritic consistent with the low ni cu ratio of the ores 1 5 re os isotopic data exhibit a limited spread in 187 re 188 os 57 to 157 and define an imprecise 1323 135 ma model 3 isochron likely the result of small r factor variations within the ore system the re os isochron age is within error of 1334 ma u pb ages obtained for baddeleyite from the ore bearing troctolites demonstrating that whole rock re os isotopic systematics have remained closed since crystallisation the initial os isotopic composition of the isochron os 1040 200 implies significant magma interactions with radiogenic os that most likely resides in the nain churchill province crust these data are therefore consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components with further addition of base and precious metals as a function of r factor analyses of sulphide separates from the proterozoic tasiuyak churchill and archaean nain gneisses confirm that both units contained significant os 8 6 ppb and 0 38 ppb respectively that was very radiogenic at 1334 ma os 1908 and 5202 respectively yielding crustal residence tchur model ages of 2200 to 2400 ma however these model ages may have been affected by re and or os mobility during the 1 85 ga torngat and 1 1 ga grenville orogeny re os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the nain plutonic suite nps with either tasiuyak 2 contamination or nain 16 contamination gneiss followed by an r factor process 200 to 1000 which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active replenished voisey s bay intrusion magma chamber however the dominance of plagioclase rich high al2o3 magmas in the mafic members of the nps troctolites and anorthosites may indicate that other deeper forms of lithospheric interaction potentially involving mafic lower crust may be crucial to understanding this style of magmatic sulphide ore system

re os isotopic variations in carbonaceous pelites hosting the duluth complex implications for metamorphic and metasomatic processes associated with mafic magma chambers abstract previous mineralogic and isotopic studies of sulfide mineralization in the 1 1 ga duluth complex have suggested that sulfur and possibly metals have been derived from proterozoic sedimentary country rocks the re os isotopic system offers great potential in the evaluation of crustally derived metals in magmatic ore forming processes but an understanding of the mechanisms that could cause isotopic variations in country rock contaminants is a prerequisite a suite of sulfidic and carbonaceous pelitic rocks of the virginia formation country rock to the duluth complex ranging from unmetamorphosed through strongly recrystallized and partially melted xenoliths have been analyzed for their os isotopic composition sulfide mineral separates mainly pyrrhotite kerogen graphite isolates and whole rocks were analyzed whole rocks kerogen and graphite from metamorphosed samples plot between 1 85 ga and 1 1 ga chondritic reference isochrons sulfide separates from all metamorphosed samples plot nearer the chondritic 1 1 ga reference isochron and require exchange with a fluid or melt characterized by a chondritic os isotopic composition potentially derived from the duluth complex devolatilization and partial melting may perturb isotopic systematics when accompanied by the loss of re and os bearing fluids these processes could in part explain the scatter in isotopic values found in kerogen and metamorphosed whole rocks however metasomatic addition of os via interaction with a fluid or melt is required to explain the sulfide isotopic ratios and is consistent with sulfide minerals being more susceptible to os exchange with a fluid than kerogen the relative ease with which pyrrhotite may be reset during fluid interaction as well as the variable degrees of exchange shown by kerogen graphite suggest that a knowledge of the petrologic and isotopic evaluation of country rocks is necessary before a meaningful assessment of the isotopic variations found in igneous rocks or associated ore deposits can be undertaken

re os isotopic composition and pge contents of proterozoic carbonaceous argillites virginia formation northeastern minnesota abstract carbonaceous sulfidic argillites from the proterozoic virginia formation in minnesota have been investigated in terms of platinum group elements pge distribution and re os isotopic signatures kerogen from protoliths and slightly metamorphosed equivalents is characterized by re and os enrichment factors relative to whole rock concentrations that range from 4 to 272 re os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1 85 ga reference isochron this age is consistent with that estimated for the virginia formation and suggests that dating of organic rich precambrian sedimentary rocks and the retrieval of data pertinent to the os isotopic evolution of seawater may be possible using the re os isotopic system

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

contemporaneous east west extension and north south compression at 43 ma in the himalayan orogen abstract it was previously proposed that the onset of east west extension in the himalayan orogen occurred after ca 19 ma here we present a re os isochron age of ca 43 ma for sulfides from the zhaxikang zn pb sb ag polymetallic deposits which occur within n s striking normal faults that reflect e w extension in the tethyan himalayan orogen this age is consistent with previously reported ages for regional metamorphism ca 48 42 ma granitoids ca 47 40 ma and gabbros ca 45 ma occurring along the indus yarlung suture in addition this type of structure also broadly occurs in the lhasa terrane and plays a critical role in the eocene porphyry skarn copper polymetallic mineralization there by ca 45 43 ma the north south directed horizontal deviatoric stresses had peaked and subducted neo tethyan slab breakoff had occurred at great depth leading to abrupt slowdown of indo asia convergence ca 45 ma e w extension initially developed at shallow depths resulting in formation of the north south trending normal faults in the plateau this type of extension would lead to collapse of the tibetan plateau and decrease in altitude subsequently resulted in central asian and the global climate changes our study places new constraints on the timing and nature of early cenozoic tectonics and metallogenesis in the himalayan orogen and climate changes in central asia

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

re os isotopic constraints on the genesis and evolution of the dergamish and ivanovka cu co au massive sulphide deposits south urals russia abstract rhenium and osmium elemental and isotopic data have been obtained for the two mafic ultramafic hosted volcanogenic massive sulphide vms deposits of dergamish and ivanovka from the south urals the associated ophiolitic blocks belong to the main uralian fault muf melange zone considered to represent obducted early palaeozoic oceanic crust despite their close geographical proximity the two ore bodies are morphologically mineralogically and isotopically quite different sulphides from ivanovka possess higher ni and os and lower re and cu relative to those from dergamish the re and os isotope data for dergamish define a best fit line corresponding to a late devonian age of 366 2 ma 2 with an mswd of 4 6 this age is some 40 my younger than the inferred silurian crystallisation age of the associated mafic ultramafic rocks but in good agreement with the previously published rb sr and ar ar ages of 360 380 ma corresponding to the high pressure metamorphic age of the adjacent maksyutov metamorphic complex these data suggest that re os systematics of the dergamish sulphide deposit were reset either by diffusion or recrystallisation during high pressure metamorphism or subsequent cooling the preservation of unradiogenic os isotopic ratios in some of the ivanovka samples and the near chondritic initial os isotopic composition obtained for the dergamish samples indicates that most of the os in the massive sulphides was ultimately derived from the mantle the corresponding tectonic setting equates to an area with submarine high level mantle rocks in contrast sulphides from ivanovka have experienced continued re equilibration and have been modified by post depositional processes at least some of which occurred relatively recently

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics