molybdenite re os and muscovite 40 ar 39 ar dating of quartz vein type w sn polymetallic deposits in northern guangdong south china northern guangdong is an important part of nanling tungsten tin metallogenic belt south china the tungsten mineralization in this area consists of mainly quartz wolframite vein type mineralization with w sn polymetallic deposits mostly distributed at the outer contact zone between concealed late jurassic granitic stocks and cambrian ordovician low metamorphosed sandstones and shales molybdenite re os and muscovite 40ar 39ar isotopic dating of three typical tungsten vein type deposits yaoling meiziwo and jubankeng in northern guangdong show that two episodes of late jurassic w sn polymetallic mineralization occurred in this area an early episode during the late jurassic 158 159 ma represented by the yaoling hongling and meiziwo tungsten deposits and a younger event during the early cretaceous 138 ma represented by the jubankeng deposit analysis of available radiometric ages of several w sn deposits in the nanling region indicate that these deposits formed at several intervals during the mesozoic at 90 100 134 140 144 162 and 210 235 ma and that large scale w sn mineralization in this region occurred mainly between 150 and 160 ma

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

re os geochronology of the hitachi volcanogenic massive sulfide deposit the oldest ore deposit in japan the hitachi volcanogenic massive sulfide vms deposit in the hitachi belt is one of the largest cu zn sulfide deposits in japan however its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems e g k ar ages on wall rocks further metamorphism has greatly reduced the fossil record which has limited the use of biostratigraphy to establish the ore deposit age we applied re os isotope dating to samples from two representative sulfide deposits of the greater hitachi vms deposit the fudotaki and fujimi deposits re os isotope data from the fudotaki deposit yields a model 3 age of 533 13 ma with an initial 187 os 188 os ratio of 0 35 0 20 in contrast re os isotope data from the fujimi sulfide ore did not exhibit a linear correlation suggesting that the re os isotope system there has been disturbed by regional metamorphism of higher grade than the fudotaki deposit as well as contact metamorphism during intrusion of cretaceous granitic rocks although the fudotaki sulfide ores have been metamorphosed to lower epidote amphibolite facies the well defined re os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor some orebodies of the greater hitachi vms deposit have been estimated to be early carboniferous based on the scarce fossil records and considered to be distributed in the daioin formation but our results revealed that the fudotaki deposit were formed during the cambrian and occurs in the upper akazawa formation which strongly supports the idea that the cambrian beds are widely distributed in the hitachi belt and that a significant unconformity exists between the cambrian and early carboniferous above the fudotaki deposit the cambrian re os date classifies the hitachi vms deposit as the oldest dated ore deposit in japan and in turn provides an insight to understand the geologic framework and genesis of the proto japanese islands and the east asian region

marine os isotopic fluctuations in the early eocene greenhouse interval as recorded by metalliferous umbers from a tertiary ophiolite in japan abstract the mineoka umbers occur within the tertiary accretionary complex of central japan immediately overlying 53 7 ma tholeiitic basalts the associated tholeiitic basalts exhibit typical normal mid ocean ridge basalt n morb type geochemical signatures and are interpreted to be a remnant of tertiary oceanic crust centimeter resolution bulk rock chemical analyses were performed to cover the entire section of the 35 cm thick umber deposit there are striking similarities in major element trace element re and os contents between the umbers and modern metalliferous sediments along the east pacific rise these results demonstrate that the mineoka umbers are ancient counterparts of mor metalliferous sediments that are plume fall out ferromanganese precipitates scavenging various seawater derived elements including p v y rees and os and confirm that the umbers subaerially exposed in the accretionary complex generally preserve their primary geochemical signatures at the time of deposition the marine 187 os 188 os ratios inferred from the mineoka umbers gradually increase up section with moderate fluctuations the 187 os 188 os composition ranging from 0 47 to 0 54 is significantly higher than that of the prior period post paleocene eocene thermal maximum petm 187 os 188 os 0 38 this was probably the result of an increase in the chemical weathering rate of continental crust caused by the long duration of noticeably warm climate during the early eocene greenhouse interval several fluctuations observed in the marine 187 os 188 os record during this interval are most likely related to petm like brief hyperthermals supporting the suggestion that the intensity of continental crustal weathering controlled by climatic changes can fluctuate on timescales of 10 5 yr

precise zircon u pb and molybdenite re os dating of the shuikoushan granodiorite related pb zn mineralization southern hunan south china abstract the shuikoushan ore district located in southern hunan province south china contains pb zn au mineralization hosted in the devonian to triassic strata and mesozoic granodiorite intrusions ore minerals are mainly pyrite sphalerite galena and minor molybdenite molybdenite usually intergrown with pyrite formed during pb zn hydrothermal mineralization in order to determine the precise age of pb zn mineralization and further understand the relationship between magma emplacement and hydrothermal mineralization in the ore district molybdenite re os dating and zircon sims u pb dating were undertaken the zircon u pb dating reveals that the granodiorite intrusion was emplaced at 158 8 1 8 ma mswd 0 40 re os isotopic age of seven molybdenite samples yields model ages ranging from 157 5 2 5 ma to 161 0 2 4 ma and gives a well defined 187re 187os isochron age of 157 8 1 4 ma 2 mswd 1 3 indicating the timing of pb zn mineralization in the shuikoushan ore district at about 158 ma this date coincides well with the zircon sims u pb age of the granodiorite revealing a genetic association between the pb zn mineralization and the granitic magmatism combined with geochronological data published for other pb zn cu deposits in southern hunan it can be concluded that the granodiorite related pb zn mineralization throughout southern hunan mainly occurred at 160 156 ma rather than 180 170 ma or 170 160 ma as considered previously the pb zn mineralization and major w sn mineralization in southern hunan are coeval and may be related to the same geological event

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

the geomorphology of the mississippi river chenier plain abstract the chenier plain of the mississippi river is a shore parallel zone of alternating transgressive clastic ridges separated by progradational mudflats the term chenier is derived from the cajun term chene for oak the tree species that colonizes the crests of the higher ridges the mississippi river chenier plain stretches 200 km from sabine pass texas to southwest point louisiana and ranges between 20 and 30 km wide with elevations of 2 6 m the timing and the process of formation could be re evaluated in the light of new chronostratigraphic findings in the mississippi river delta plain the stratigraphic relationship between the teche and lafourche delta complexes and ship shoal offshore indicates that these delta complexes belong to different delta plains that developed at different sealevels it appears that the teche delta complex is associated with the late holocene delta plain which developed 7000 to 3000 yrs b p when sealevel stood 5 6 m lower than present a regional transgression occurred between approximately 3000 bp and 2500 yrs b p leading to the transgressive submergence of the late holocene delta plain producing the regional teche shoreline the timing of this transgression conforms to the age of the most landward ridge in the chenier plain the little chenier little pecan island trend which dates at about 2500 yrs b p this ridge trend was originally interpreted as representing the teche delta complex switching event with the landward holocene pleistocene contact representing the high stand shoreline the implication of this new interpretation is that the little chenier little pecan island trend represents the high stand shoreline a continuation of the teche shoreline separating the late holocene and recent delta plains and that the holocene pleistocene contact represents the leading edge of the marshes transgressing onto the prairie terrace significant mudflat progradation seems to require a westerly position of the mississippi river but the numerous different forms and ages of cheniers do not correspond well to the timing of major delta complex switching progradation of the chenier plain appears to be associated with building of the recent delta plain and not the teche complex of the late holocene delta plain the occurrence of individual ridges appears to be primarily tied to delta lobe switching within the lafourche complex and variations in sediment supply from local rivers the recent development of the atchafalaya delta complex to the west is the closest position of an active distributary to the chenier plain since sealevel stabilization a new episode of rapid mudflat progradation is thus taking place

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

an isotopic study of the ni cu pge rich wellgreen intrusion of the wrangellia terrane evidence for hydrothermal mobilization of rhenium and osmium abstract re os sm nd rb sr and oxygen isotope systematics of the wellgreen intrusion in the wrangellia terrane were investigated in an effort to deduce the origin of this mafic ultramafic sill and its attendant ni cu pge deposit radiogenic initial os ratios 1 06 1 82 and sr ratios 0 7044 0 7062 and heavy 18 o 7 3 7 9 suggest alteration of the intrusion by hydrothermal fluids that carried radiogenic 187 os and 87 sr from the surrounding country rocks the great majority 99 of the os however and by inference the other pges derived from a mantle derived magma that suffered little or no interaction with the crust prior to crystallization initial nd isotope ratios are not as variable nd t ranges from 2 02 4 49 and suggest that the rocks were derived from a light rare earth element depleted mantle source the nd results together with os data from relatively undisturbed wehrlites are compatible with either a plume richards et al 1991 or island arc samson et al 1990 model for wrangellia this study contributes to a growing body of evidence that documents the ready mobilization of os in hydrothermal fluids the potential effects of this mobility must be carefully evaluated prior to invoking crustal assimilation to explain variable and radiogenic os initial ratios in layered intrusions

multistage exhumation of the anjiayingzi gold deposit northern north china block geodynamic settings and exploration implications abstract post mineralisation exhumation is pivotal because of its strong effects on the preservation of ore deposit the jiguanzi quartz monzonite hosts the anjiayingzi au deposit at the centre of the kalaqin antiform or kalaqin metamorphic core complex in the northern north china block ncb we can link the exhumation of the deposit with regionally geodynamic events and evaluate its exploration implications using low temperature thermochronology from the quartz monzonite host sericite 40ar 39ar dating of auriferous quartz sulphide veins indicates that gold mineralisation timing is 130 1 ma zircon fission track ages of fresh samples from the host quartz monzonite vary from 118 6 to 99 4 ma and apatite fission track ages vary from 99 6 to 66 5 ma a four stage post mineralisation exhumation history is revealed by combining the binomial peak fitting of single grain ages and low temperature thermal history inverse modelling the first exhumation event is dated at ca 130 100 ma and characterised by the most rapid exhumation rate of 167 m ma and ca 5 km rise this movement is related to the top to nw and a se ductile brittle extensional deformation of the kalaqin antiform regionally it is also contemporaneous with the formation of extensional metamorphic core complexes development of basins and increasing heat flow and decreasing thermal lithospheric thickness in the eastern ncb all of these synchronous events can be linked to the back arc setting of the subducting izanagi oceanic plate and resulted geodynamic changes beneath the ncb the relatively slow stage 2 exhumation dated at ca 100 65 ma with an exhumation rate of 20 m ma was triggered by the collision between the izanagi oceanic plate and eastern china at ca 100 ma and following change in the drift direction of the izanagi oceanic plate sedimentation of basins in the ncb was continuous during this period resumed stage 3 exhumation ca 65 45 ma 27 m ma and synchronous sedimentary inversion are products of regional compression event related to the collision between india and the eurasian plates change of the pacific oceanic plate drifting direction at ca 50 ma finally induced the monotonic cooling and exhumation of stage 4 19 m ma the estimated post mineralisation exhumation is 7 1 km and the ore forming depth of the anjiayingzi au deposit is between 5 1 and 10 1 km in combination with previously published data the differential exhumation history exhumation degree and preservation potential of major gold provinces in ncb are confirmed which further indicate that their gold prospectivity must be assessed individually

estimating shallow subsidence in microtidal salt marshes of the southeastern united states kaye and barghoorn revisited abstract simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow 3 5 m subsurface datum were made in selected coastal salt marshes of louisiana florida and north carolina to quantitatively test kaye and barghoorn s contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate rates of subsidence of the upper 3 5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation erosion table surface elevation change was significantly lower than vertical accretion at each site after 2 years indicating a significant amount of shallow subsidence had occurred ranging from 0 45 to 4 90 cm the highest rate of shallow subsidence occurred in the mississippi delta results confirm kaye and barghoorn s contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate including not only compaction but also apparently shrink swell from water storage and or plant production decomposition at some sites indeed surface elevation change was completely decoupled from vertical accretion at the florida site the assumption of a 1 1 relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes consequently the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits these findings also indicate the need for greater understanding of the influence of subsurface and small scale hydrologic processes on marsh surface elevation

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

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

rb sr dating of sphalerites from tennessee and the genesis of mississippi valley type ore deposits mississippi valley type mvt ore deposits commonly consist of some combination of lead zinc and iron sulphides accompanied by barite fluorite dolomite and calcite they are thought to form by fluid expulsion from sedimentary successions1 3 but their exact origin has remained controversial because of the scarcity of reliable geochronological data in a recent and widely quoted model the mvt deposits of the central and eastern united states formed as a result of large scale lateral fluid flow driven by the tectonic squeezing of sedimentary successions during the late palaeozoic alleghenian orogeny 330 250 myr 4 here we report the first direct rb sr dating of sphalerites from an mvt deposit and propose this as a useful geochronological technique for sulphide mineralization with poor age control rb sr data for sphalerites from coy mine east tennessee define an isochron age of 377 29 myr ruling out any genetic connection with the alleghenian orogeny instead mvt mineralization in the eastern united states was apparently generated from fluids expelled during thrusting associated with the older acadian orogeny 380 350 myr

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

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

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

abstract geologic framework processes and rates of subsidence in the mississippi river delta plain abstract since the 1930 s louisiana has lost an estimated 3 950 km2 of coastal wetlands and barrier islands the loss of these coastal lands resulted primarily from subsidence and erosion rather than the draining and filling of wetlands more than 40 of the coastal wetlands in the u s are found in louisiana and 80 of our nation s total wetland loss occurs here at alarming rates beach erosion rates exceed 10 m yr and the rate of wetland loss is currently measured at 75 km2 yr the causes of louisiana s coastal land loss include delta switching storm impacts man s impacts on this deltaic system and high rates of subsidence recent studies indicate that the highest subsidence and coastal land loss rates occur where the underlying holocene sediments are the thickest this relationship suggests that subsidence through the consolidation and settlement of these young deposits is of primary importance to the coastal land loss problem found in louisiana to understand the role of subsidence in louisiana s coastal land loss problem requires a knowledge of the late quaternary history of the lower mississippi river of the infilling of its incised valley during the holocene transgression and of the processes of delta switching end of record last page 644

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

metallogenic age and hydrothermal evolution of the jidetun mo deposit in central jilin province northeast china evidence from fluid inclusions isotope systematics and geochronology abstract the jidetun deposit is a large porphyry mo deposit that is located in central jilin province northeast china the mo mineralization occurs mainly at the edge of porphyritic granodiorite as well as the adjacent monzogranite field investigations cross cutting relationships and mineral paragenetic associations indicate four stages of hydrothermal activity to determine the relationships between mineralization and associated magmatism and better understand the metallogenic processes in ore district we have undertaken a series of studies incluiding molybdenite re os and zircon u pb geochronology fluid inclusions microthermometry and c h o s pb isotope compositions the molybdenite re os dating yielded a well defined isochron age of 168 9 1 9 ma mswd 0 34 that is similar to the weighted mean 206 pb 238 u age of 173 5 1 5 ma mswd 1 8 obtained from zircons from the porphyritic granodiorite the results lead to the conclusion that mo mineralization occurred in the middle jurassic 168 9 1 9 ma was spatially temporally and genetically related to the porphyritic granodiorite 173 5 1 5 ma rather than the older monzogranite 180 1 0 6 ma fluid inclusion and stable c h o isotope data indicate that the initial h 2 o nacl fluids of mineralization stage i were of high temperature and high salinity affinity and exsolved from the granodiorite magma as a result of cooling and fractional crystallization the fluids then evolved during mineralization stage ii into immiscible h 2 o co 2 nacl fluids that facilitated the transport of metals mo cu and fe and their separation from the ore bearing magmas due to the influx of abundant external co 2 and heated meteoric water subsequently during mineralization stage iii and iv increase of ph in residual ore forming fluids on account of co 2 escape and continuous decrease of ore forming temperatures caused by the large accession of the meteoric water into the fluid system reduced solubility and stability of metal clathrates thus facilitating the deposition of polymetallic sulfides

winter storm impacts on the chenier plain coast of southwestern louisiana abstract stormy conditions associated with periodic winter cold front passages are closely related to transport of suspended sediment to the continental shelf coastal erosion and coastal progradation along shoreline sectors where abundant fine grained sediments are stored on the inner shelf cold front passages occur between october and april on three to five day cycles their typical nw to se direction of approach large spatial scales and numerous yearly occurrences 20 30 events year drive physical processes that cause significant coastal change both remotely sensed multispectral and high quality photographic data have been collected from altitudes of 0 9 mi 1500 m 5 6 mi 9000 m and 13 mi 21 000 m before and after cold front passages to form a data base for evaluating coastal change and suspended sediment transport pathways satellite imagery provide a longer term perspective on coastal change remotely sensed data sets are augmented with ground truth measurements of coastal configuration sedimentology and water quality physical processes active in the prefrontal phases of a winter cold front passage are considerably different from those of the post frontal phase along the chenier plain coast prefrontal stages are characterized by prolonged periods on the order of several days of high wave action from the southerly quadrants water level setup along the coast and strong alongshore as well as onshore sediment transport at these times mud from the atchafalaya river stored on the nearshore shelf is transported onshore and alongshore to slightly prograde the chenier coast post frontal conditions bring dry cold winds from the northerly quadrants causing water level setdown along the coast a significant reduction in nearshore wave energy and drying of the newly deposited mudflats fronting the chenier plain rapid drying causes an important increase in sediment strength mud crack formation and effective armoring of the mud flats by dried mud clasts that resist erosion in subsequent cycles

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

reos dating of molybdenites from ore deposits in japan implication for the closure temperature of the reos system for molybdenite and the cooling history of molybdenum ore deposits abstract rhenium and osmium abundances were determined on molybdenites from eighteen ore deposits in japan fifteen vein type a skarn type a greisen type and a pegmatite type deposit re os ages obtained for molybdenites from northeastern central and southwestern japan are 100 130 ma 50 75 ma and ca 10 ma respectively reflecting the ages of regional igneous activities in individual areas for vein type ore deposits re os ages for molybdenites agree with rb sr whole rock ages of host granitoids while they are 3 12 ma older than k ar mineral ages of the host rocks on the other hand re os ages of molybdenites from skarn pegmatite and greisen type ore deposits agree with k ar ages of the host rocks the comparison of re os ages for molybdenites with rb sr and k ar ages of host rocks in vein type deposits suggests that re os closure temperature for vein molybdenite is close to whole rock rb sr closure temperature for host granitic rocks i e roughly estimated to be around 500 c and higher than k ar closure temperature for host granodiorites i e 230 370 c if the thermal history of vein molybdenites is essentially equivalent to that of host rocks one possible explanation for age discrepancies of 3 12 ma between re os ages for molybdenites and k ar ages for the wall rocks observed in vein type molybdenum deposits is that later thermal events after formation of molybdenum ores may reset k ar ages hydrothermal alteration might have occurred in such a temperature condition as to disturb k ar mineral systems in wall rocks and made them younger but not the re os system in molybdenite the other explanation might be that the age discrepancies correspond to a cooling interval from ca 500 to 300 c of host granitoid on the other hand the agreement of re os ages of molybdenites and k ar ages of host rocks in skarn pegmatite and greisen type deposits in this study indicates no later disturbance for k ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits

the molybdenite re os isotope chronology in situ scheelite and wolframite trace elements and sr isotope characteristics of the chuankou tungsten ore field south china abstract the chuankou tungsten ore field situated in the central area of the qing hang ancient suture zone in south china is characterized by predominantly quartz vein type tungsten mineralization that formed near the contact between the chuankou granite and slightly metamorphosed silt bearing sericite slate based on field work cathodoluminescence molybdenite re os dating and in situ laser ablation inductively coupled plasma mass spectrometry la icp ms analysis the chuankou tungsten ore was formed at 230 6 2 9 ma to 229 4 2 6 ma in a typical indosinian metallogenic event in addition three distinct generations of scheelite and two types of wolframite can be clearly distinguished type 1 wolframite has a semiautomorphic grain or brecciated vein structure with a high heavy rare earth element hree content and an obvious negative eu anomaly type 2 wolframite grains are strongly altered as residual pseudomorphic structures by ore forming fluids and have relatively low ree contents sch 1 scheelite is present in the interior of the type 2 wolframite grains the sch 1 scheelite grains have an elevated ree content and low 87sr 86sr ratios with a strongly negative eu anomaly which suggests an inheritance signal of the host rocks comparatively sch 2 and the altered sch 1 scheelite have characteristic extremely low ree contents and remarkably strong positive eu anomalies these scheelites also have increased 87sr 86sr ratios and low 88sr signals sch 3 is idiomorphic with perfect oscillating zoning the ree content of sch 3 which typically has the lowest 87sr 86sr ratios is higher than those in sch 1 and sch 2 and the extremely high 88sr signals indicate a distinct thermal fluid event that was directly derived from the host rock the correlations between the ree and nb of scheelite imply a type b substitution of sch 1 and sch 2 but a type c substitution for sch 3 the correlation of eun and eun shows an eu3 substitution process for stages sch 1 and sch 2 and an eu2 substitution process for stage sch 3 thus we confirm that ore bearing hydrothermal rocks must undergo early stage metasomatism and filling middle stage fluid mixing between wall rocks and magmatic hydrothermal activity and refilling of late stage magmatic hydrothermal activity during the metallogenic process during the indosinian period the mineralization stages of polymetallic deposits in the indosinian were composed of stage 1 240 to 220 ma and stage 2 220 to 200 ma the chuankou tungsten ore field can be considered as the typical representation of the stage 1 metallogenic event in south china this ore field is derived from remelting of the ancient basement and was reduced by regional extension under the dominant compression regime in the south china block

reliable re os age for molybdenite abstract re os ages for four molybdenites were determined by our newly developed method with isotope dilution mass spectrometry special caution was given to achieve osmium isotopic equilibration between sample and spike during chemical treatment re os data for molybdenites from mt mulgine australia matasvaara finland preissac canada and climax usa define ages of 2790 ma 2600 ma 2680 ma and 26 6 ma respectively the reproducibilities of our re os age determination are shown to be within 2 by the replicate analyses for mt mulgine matasvaara and preissac samples the ages obtained are in good agreement with reference ages estimated for wall rocks of molybdenite deposits concerned thus with careful experimental procedure it appears that the re os geochronometer can be used to reliably determine the age of molybdenite

evolution of the danube deep sea fan since the last glacial maximum new insights into black sea water level fluctuations abstract the danube deep sea fan nw black sea is one of the most developed deep sea sediment depositional systems in europe although the morphology and the architecture have been widely described in the past years little is known about the stratigraphy of this depositional system for the late quaternary this results from the lack of significant stratigraphic markers the scarcity of radiocarbon ages and the difficulty in constraining reservoir ages recent robust quantification of reservoir ages has allowed the construction of a new stratigraphic framework for the black sea from the end of the last glacial period to the holocene thus giving the opportunity to correlate sedimentological and geochemical features previously described on the nw black sea margin with climatic events identified in the northern hemisphere based on this approach we propose an improved chrono lithostratigraphic framework for the danube deep sea fan channel levees since the last glacial maximum we show that the danube deep sea fan was active during the last glacial maximum until the younger dryas early holocene transition ca 11 700 cal a bp when the turbidite activity abruptly terminated in the whole system throughout this period the danube river was the main source of the deep depositional system except between ca 17 200 cal a bp and 15 700 300 cal a bp at that time the deposition of red turbidites in the deep basin concomitant with the deposition of the so called red layers onto the continental shelf and the upper slope emphasises the direct impact of the increased meltwater runoff of the dnieper river as far as the danube deep sea fan some significant changes in the location of the depocentre of the danube deep sea fan occurred through time the main change in depocentre location occurred at ca 28 000 cal a bp with the northward avulsion leading to the formation of the middle channel although the middle and northern channel became the preferential turbidite depocentres after this event the deposition of turbidites has persisted in the southern channel levee until 14 700 cal a bp indicating that the southern channel was definitely not abandoned after the upstream northward avulsion unsurprisingly water level fluctuations and river sediment flux acted as the main forcings on the evolution of the danube deep sea fan based on these results and on the morphology of the danube canyon we propose that i lowstand conditions 110 m prevailed during the last glacial maximum and possibly between 15 700 300 cal a bp and 14 700 cal a bp ii water level ranged from 110 and 70 m between 14 700 and 11 700 cal a bp and iii water level ranged from 70 and 30 m from 11 700 cal a bp and until the reconnection of the black sea lake with the global ocean at ca 9000 cal a bp

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

disequilibrium induced initial os isotopic heterogeneity in gram aliquots of single basaltic rock powders implications for dating and source tracing abstract the re os isotopic heterogeneity of mantle derived rocks has been well documented however it is unclear whether and how the os isotopic heterogeneity of a mantle source can influence re os isotopic dating and source tracing of mantle derived melts here we report that replicate analyses of gram aliquots of single basaltic powders one of reference material bhvo 2 and three of the hatu basalts from the western junggar region china show large variations in both os concentrations and isotopic ratios more importantly these replicate data define good apparent re os isochron correlations yielding ages significantly older than their formation ages and display remarkable correlations between the 187 os 188 os ratio and the reciprocal of the common os concentration i e 1 192 os this indicates that the initial os isotopic composition in a basaltic magmatic system may be heterogeneous and cannot be homogenized by pulverizing theoretical and mathematical deduction demonstrates that the observed apparent re os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation primarily due to limited diffusional exchange of os isotopes between refractory os bearing inclusions and host minerals under mantle conditions the regressed initial os isotopic composition may bias the true value of the mantle source pulsed mantle melting and magma mixing during ascent are potential processes responsible for such initial os isotopic heterogeneity in basaltic rock systems thus when using the re os isotopic system to date and to trace the source of basaltic rocks or other rocks with relatively low os concentrations it is necessary to consider whether the initial os isotopic composition was heterogeneous or whether it had reached complete isotopic equilibrium

decratonic gold mineralization evidence from the shangzhuang gold deposit eastern north china craton abstract the jiaodong peninsula in the eastern north china craton ncc hosts some of the world class gold deposits among these the shangzhuang gold deposit represents a typical fault zone hosted disseminated and stockwork style gold mineralization the mineralization is characterized by intense hydrothermal alteration halos in the wallrocks with the mineralized veins intruding into altered wallrocks zircon u pb isotope dating constrains the timing of emplacement of the guojialing granodiorite as 130 1 ma five molybdenite samples from the main mineralization stage veins yielded a well fitted isochron age of 126 2 ma mswd 1 3 marking the timing of gold mineralization four types of fluid inclusions are identified at shangzhuang and their petrographic and microthermometric features suggest fluid immiscibility followed by fluid mixing this inference is also supported by oxygen and hydrogen isotopic data of quartz veins from stage iii the measured 18 o values of quartz are 12 3 to 13 7 for stage ii and 11 7 to 12 0 for stage iii with calculated 18 o fluid values for stage ii ranging from 5 9 to 7 3 and for stage iii from 3 4 to 3 7 d values for stage ii and iii quartz range from 65 3 to 75 2 and 70 6 to 74 7 respectively the data suggest that the ore forming fluids were initially derived from magmatic sources followed by mixing with meteoric water sims in situ sulfur isotope analyses of two types of pyrite py i and py ii from different mineralization stages show different sulfur isotope features the 34 s values for py i 10 2 to 11 3 mean 10 8 n 25 are higher than those of the later mostly gold related py ii 4 1 to 6 8 mean 5 4 n 27 the higher 34 s values of py i are correlated to degassing of mantle wedge metasomatized by the slab fluids the relatively lower 34 s values of py ii might suggest increasing of f o2 through fluid mixing between ore forming fluids and meteoric water the temporal spatial and tectonic relations between the giant gold mineralization and destruction of the ncc suggest a genetic linkage the gold mineralization between 126 and 117 ma at jiaodong was probably initiated at the peak of decratonization ca 125 ma of the eastern ncc phanerozoic craton margin orogeny also played a critical role in the generation of the early cretaceous giant gold deposits in the eastern ncc integrated geological characteristics mineralization features metallogenic age sources of ore forming fluids and sulfur and tectonic settings favor a decratonic gold mineralization model for the shangzhuang gold deposit the geodynamic scenario proposed for the early cretaceous giant gold mineralization event in the eastern ncc includes the paleo west pacific plate subduction asthenospheric upwelling multiple subduction collision processes along the margins

time temperature relation of mineral isochrons a thermodynamic model and illustrative examples for the rbsr system abstract we have developed a thermodynamic model for the determination of the closure temperature t c at which the minerals defining an internal isochron in rb sr or similar geochronological system were set with a geochronological clock it is shown that the equilibrium fractionation of 87 rb and 87 sr between a pair of minerals at t c k d 87 rb 87 sr c is given by the ratio of the quantity 87 rb 86 sr in the two minerals as measured at the present time k d 87 rb 87 sr which equals the element distribution coefficient k d rb sr under equilibrium condition can be calibrated as a function of temperature and compared with the retrieved value of k d 87 rb 87 sr c in a natural pair to obtain t c the various mineral pairs defining an internal isochron will yield concordant or discordant values of t c depending on whether or not they closed simultaneously with respect to the diffusion of rb and sr both types of results are expected and are important in the analyses of the evolutionary history of the host rocks preliminary analyses of the published data in the rb sr system suggest a fairly wide range of t c even for the same mineral pair reflecting differences in the cooling rates and physico chemical environments of the host rocks

u pb and re os geochronology and geodynamic setting of the dabaoshan polymetallic deposit northern guangdong province south china abstract the dabaoshan polymetallic deposit in northern guangdong province contains iron copper lead zinc molybdenum tungsten and sulfur mineral resources porphyry type mo w and skarn type mo w mineralization occurs along the internal and external contact zones of the granodioritic porphyry respectively la icp ms u pb dating of zircons from two granodioritic porphyry samples yielded a weighted mean 206 pb 238 u age of 175 8 1 5 ma mswd 0 037 and 175 0 1 7 ma mswd 0 41 they can be pooled together to yield a combined weighted age of 175 4 1 6 ma mswd 0 26 which is interpreted as the emplacement age of the granodioritic porphyry re os dating of three molybdenite samples from porphyry and skarn ores yielded consistent model ages of 163 2 2 3 ma to 165 2 2 4 ma with a weighted mean of 163 9 1 3 ma mswd 0 81 which is the age of mo w mineralization these ages are consistent with the molybdenite re os model age 164 7 3 ma measured by mao et al 2004a for the stratiform cu pb zn orebody and they can yield a weighted mean of 164 0 2 5 ma mswd 0 16 this implies that mo w and cu pb zn mineralization in the dabaoshan polymetallic deposit are the products of one mineralization event the mineralization in the deposit coincides closely with that of mo polymetallic mineralization 164 149 ma elsewhere in the nanling region comprising an important polymetallic metallogenic belt of south china and corresponds to the second episode of mesozoic metallogenesis in south china combined with previous studies we suggest that the dabaoshan polymetallic deposit is related to post collisional lithosphere extension in the nanling region of south china geological data and pb isotopic evolution diagrams together with stable isotopic data of fluid inclusions 18 o 3 75 7 0 d 50 7 to 56 1 and ore sulfides 34 s 2 3 suggest a genetic relationship between the dabaoshan polymetallic deposit the granodioritic porphyry and the dacitic porphyry these data combined with the re content 64 7 to 102 4 ppm of molybdenite indicate that the ore forming components were derived from mixed crustal and mantle sources

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

rhenium and osmium isotopes in black shales and ni mo pge rich sulfide layers yukon territory canada and hunan and guizhou provinces china rhenium and osmium abundances and osmium isotopic compositions were determined by negative thermal ionization mass spectrometry for samples of devonian black shale and an associated ni enriched sulfide layer from the yukon territory canada the same composition information was also obtained for samples of early cambrian ni mo rich sulfide layers hosted in black shale in guizhou and hunan provinces china this study was undertaken to constrain the origin of the pge enrichment in the sulfide layers samples of the ni sulfide layer from the yukon territory are highly enriched in re os and other pge with distinctly higher re 192os but similar pt re compared to the black shale host re os isotopic data of the black shale and the sulfide layer are approximately isochronous and the data plot close to reference isochrons which bracket the depositional age of the enclosing shales samples of the chinese sulfide layers are also highly enriched in re os and the other pge re 192os are lower than in the yukon sulfide layer re os isotopic data for the sulfide layers lie near a reference isochron with an age of 560 ma similar to the depositional age of the black shale host the osmium isotopic data suggest that re and pge enrichment of the brecciated sulfide layers in both the yukon territory and in southern china may have occurred near the time of sediment deposition or during early diagenesis during the middle to late devonian and early cambrian respectively

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

fission track thermochronology of the tuwu yandong porphyry cu deposits nw china constraints on preservation and exhumation abstract the carboniferous tuwu yandong porphyry cu deposits located in the eastern tianshan are the largest porphyry cu deposits in the xinjiang province china this work presents new zircon and apatite fission track zft and aft data from this porphyry cu deposit combined with inverse thermal modeling and previously published ages to reveal their preservation condition and exhumation history specifically four cooling episodes are recognized rapid cooling 24 44 c ma during the early carboniferous 340 332 ma thick burial 4 km and reheating from the late carboniferous to late triassic slow cooling 1 45 2 21 c ma from the late triassic to the paleocene and final protracted slow cooling 0 47 0 28 c ma from paleocene to the present day the first phase of cooling can be mainly attributed to magmatic heat transfer and thermal relaxation whereas the second reheating phase was likely related to post deposition burial the latter two phases of cooling were predominantly controlled by uplift and denudation processes overall our results indicate that the tuwu yandong porphyry cu deposits experienced two stages of slow post burial exhumation at rates of 48 64 m ma and 9 16 m ma respectively we estimate that at least 8 km thickness of cover has been removed from above the tuwu yandong porphyry cu deposits we conclude that early thick burial and later extremely slow exhumation were two key factors that led to preservation of this ancient porphyry system i e tuwu yandong porphyry cu deposit

thermal history of the giant qulong cu mo deposit gangdese metallogenic belt tibet constraints on magmatic hydrothermal evolution and exhumation abstract a complete thermal history for the qulong porphyry cu mo deposit tibet is presented zircon u pb geochronology indicates that the mineralization at qulong resulted from brecciation veining events associated with the emplacement of a series of intermediate felsic intrusions combined with previously published ages our results reveal a whole intrusive history of the qulong composite pluton causative porphyries were emplaced at 16 0 ma as revealed by 40ar 39ar dating of hydrothermal biotite 15 7 0 2 ma and sericite 15 7 0 2 ma zircon and apatite u th he zhe and ahe dating of qulong revealed that both followed similar monotonic thermal trajectories from 900 c u pb ages 17 5 15 9 ma to 200 c zhe 15 7 14 0 ma and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200 c myr the qulong deposit was exhumed between 13 6 ma and 12 4 ma ahe at an estimated rate of 0 16 0 24 mm y which is consistent with previous estimates for other gangdese miocene porphyry deposits our ahe thermochronology results suggest that neither the gangdese thrust system nor the yadong gulu graben affected or accelerated exhumation at the qulong deposit