u pb dating of prograde and retrograde titanite growth during the scandian orogeny u pb ages of metamorphic titanite from high grade gneisses of the nasafjallet window in the north central scandinavian caledonides record growth over an 20 m y interval during the late silurian to early devonian biotite amphibolite gneisses located near the basement cover boundary contain metamorphic titanite in two microtextural settings one as lenticular grains distributed throughout a recrystallized matrix of metamorphic phases and one as plate like grains enclosed within biotite grains these distinct morphological populations exhibit core overgrowth relationships with compositionally similar cores and overgrowths but the plate like titanite grains have a much higher proportion of overgrowth and give slightly younger ages cores and overgrowths in both types are inferred to have formed along the prograde ca 413 ma and retrograde ca 395 ma paths respectively of a single metamorphic cycle associated with the burial and exhumation of a slice of baltic basement during the caledonide orogeny baltica laurentia collision the existence of polygenic u pb systematics in metamorphic titanite demonstrates the potential usefulness of titanite to reveal multiple metamorphic and deformational episodes and to constrain the duration of metamorphic events

re os and pb pb geochronology of the archean salobo iron oxide copper gold deposit carajas mineral province northern brazil rhenium osmium ages were determined for two molybdenite samples and a pb pb age was derived from bornite chalcopyrite magnetite at the salobo iron oxide copper gold deposit to determine the timing of mineralization and its relation to the nearby old salobo granite rhenium osmium dating of molybdenite spatially associated with copper sulfide minerals yields ages with weighted means of 2576 8 and 2562 8 ma removing the error multiplier introduced by the decay constant uncertainty appropriate for comparing ages from the same isotopic system these data convincingly argue for two temporally separated pulses of molybdenite deposition at 2576 1 1 4 ma n 2 and 2561 7 3 1 ma n 3 the 2576 8 ma age coincides with a previously published u pb age of 2573 2 ma for the old salobo granite suggesting that main stage ore formation may have been contemporaneous with granite magmatism the slightly younger 2562 ma age most likely represents new molybdenite precipitation associated with the development or reactivation of local shear zones lead lead stepwise leaching of copper sulfide minerals yields a less precise isochron age of 2579 71 ma and supports an archean age for the salobo ores this is the first documentation of an archean iron oxide copper gold deposit and the re os and pb pb geochronology herein support 2580 2550 ma estimates for basement reactivation and regional granite magmatism associated with the development of brittle ductile shear zones

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 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

petrochronology and tims thermal ionization mass spectrometers or tims were developed by the pioneers of mass spectrometry in the mid 20th century and have since been workhorses for generating isotopic data for a wide range of elements later developed mass spectrometric techniques have many advantages over tims including higher spatial resolution with in situ techniques such as secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms and greater versatility in terms the elements that can be easily and well measured the reason tims persists as an important method for geochronology is that for some key parent daughter systems e g u pb sm nd it can produce isotopic data and resultant dates with 10 100 times higher precision and more quantifiable accuracy than in situ techniques even when sample sizes are very small such as those that might result from single crystals or even small portions of zoned crystals for many questions in the geosciences the highest achievable precision and accuracy are required to resolve the timescales of processes and or correlate events globally as an example modern tims u pb geochronology is capable of producing dates with precision and accuracy better than 0 1 of the age for single crystals with only a few picograms pg of pb therefore it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in mesozoic and younger rocks if these dates and rates can be connected with other igneous processes such as magma transfer emplacement and crystallization then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage as another example sm nd geochronology of garnet permits dates with precision better than 1 million years for garnets of any age including multiple concentric growth zones in single crystals such

u pb monazite ages in amphibolite to granulite facies orthogneiss reflect hydrous mineral breakdown reactions sveconorwegian province of sw norway in the rogaland vest agder terrain of the sveconorwegian province of sw norway two main sveconorwegian metamorphic phases are reported a phase of regional metamorphism linked to orogenic thickening m1 and a phase of low pressure thermal metamorphism associated with the intrusion of the 931 2 ma anorthosite charnockite rogaland igneous complex m2 phase m1 reached granulite facies to the west of the terrane and m2 culminated locally at 800 850 c with the formation of dry osumilite bearing mineral associations monazite and titanite u pb geochronology was conducted on 17 amphibolite to granulite facies orthogneiss samples mainly from a suite of 1050 2 8 ma calc alkaline augen gneisses the feda suite in these rocks prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012 1006 ma in the feda suite and other charnockitic gneisses concordant to slightly discordant monazite at 1024 997 ma probably reflects breakdown of biotite during granulite facies m1 metamorphism a spread of monazite ages down to 970 ma in biotite hornblende samples possibly corresponds to the waning stage of this first event in the feda suite a well defined monazite growth episode at 930 925 ma in the amphibolite facies domain corresponds to major clinopyroxene formation at the expense of hornblende during m2 growth or resetting of monazite was extremely limited during this phase in the granulite facies domain up to the direct vicinity of the anorthosite complex the m2 event was shortly followed by cooling through ca 610 c as indicated by tightly grouped u pb ages of accessory titanite and titanite relict inclusions at 918 2 ma over the entire region a last generation of u poor monazite formed during regional cooling below 610 c in hornblende rich samples at 912 904 ma this study suggests 1 that monazite formed during the prograde path of high grade metamorphism may be preserved 2 that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of rees and th and or fluid mobilisation 3 that the u pb system in monazite is not affected by thermal events up to 800 850 c provided that conditions were dry during metamorphism

re os ages for the erdenet and tsagaan suvarga porphyry cu mo deposits mongolia and tectonic implications the erdenet and tsagaan suvarga deposits are the two largest known porphyry cu mo deposits in mongolia replicate re os analyses of molybdenite from these deposits yield ages of 240 7 0 8 and 240 4 0 8 ma for erdenet and 370 1 1 2 and 370 6 1 2 ma for tsagaan suvarga 2 errors the re os age of erdenet is significantly older and that of tsagaan suvarga is slightly older than published 40ar 39ar ages for sericite associated with mineralization in these deposits 207 4 5 0 and 364 9 7 0 ma respectively these re os results suggest that in the hydrothermal environment 40ar 39ar ages may be partly to significantly reset whereas re os ages record and maintain the age of primary mineralization events these re os ages indicate that porphyry mineralization at erdenet and tsagaan suvarga are related to porphyritic intrusions representing the youngest event during regional magmatism in northern and southern mongolia respectively the re os age of erdenet marks the final stage of arc magmatism in northern mongolia followed by late permian collision between the mongolian north china and siberian blocks the re os age of tsagaan suvarga indicates that devonian arc magmatism in southern mongolia was active at the present latitude of 44 n at least until 370 ma

geochronology of the giant beiya gold polymetallic deposit in yunnan province southwest china and its relationship with the petrogenesis of alkaline porphyry abstract the beiya gold polymetallic deposit located at the junction of the sanjiang tethys himalaya orogenic belt and the yangtze craton in southwestern china is the largest gold deposit in the region mineralization mainly occurs in skarn which formed along the contacts between porphyry intrusions and limestone host rocks the ore formation process is suggested to be closely related to the himalayan age alkaline porphyries u pb dating of magmatic zircons from the beiya quartz syenite porphyry yields a weighted average 206 pb 238 u ages of 36 07 0 43 ma n 25 mswd 1 17 calculated isochron age of five molybdenite samples from beiya is 34 7 1 6 ma mswd 0 99 initial 187 os of 0 21 0 16 ppb which indicates that mineralization is coeval with the intrusion of the beiya alkaline porphyry this age is also in agreement with the results of apatite fission track analysis in the porphyry magmatic zircon e hf t values range from 7 4 to 0 1 which correspond to the two stage hf model with ages between 1112 ma and 1588 ma this finding suggests that the main source of porphyry was probably melting of thickened mesoproterozoic lower crust triggered by partial melting of the lithospheric mantle zircon ce iv ce iii ratios of ore related porphyry are high given that all eu n eu n ratios are above 0 4 which demonstrates that the magma source of the porphyry was highly oxidized which was favorable for cu au mineralization the medium re concentrations in molybdenite re 2 08 20 54 ppm suggest that the ore forming materials were characterized by mixed crustal mantle sources mineralization in the beiya deposit was one of the many metallogenic events related to the india eurasia collision that was responsible for the fertile deep sourced magmatism

silurian s type granite related w mo mineralization in the nanling range south china a case study of the pingtan w mo deposit abstract the pingtan deposit in southwestern hunan province is a newly discovered large sized w mo deposit this deposit is situated in the northwestern part of the miao ershan granite batholith western nanling range the tungsten mineralization is mainly hosted in biotite monzogranite and the alteration related to mineralization is controlled by a ne trending fault zone in this paper we present new molybdenite re os and zircon u pb ages zircon lu hf isotope and ree geochemical data aiming to constrain the w mo mineralization age and ore hosting granite petrogenesis in the pingtan w mo deposit molybdenite re os dating of eight ore samples yielded a weighted average model age of 427 0 5 4 ma mswd 0 24 the la icp ms zircon u pb analysis of two biotite monzogranite yielded 206pb 238u ages of 430 8 2 4 ma and 431 0 1 8 ma respectively suggesting that the w mo mineralization is spatial temporally associated with the biotite monzogranite the ore related granites have high contents of sio2 and k2o belong to weakly to strongly peraluminous a cnk 1 00 1 30 and geochemically classified as s type affinity molybdenite re contents 2 262 7 297 g g zircon ehf t 10 34 5 87 and 176hf 177hf 0 282241 0 282376 values of the biotite monzogranite suggest that the ore forming materials are predominantly derived from the crust this is the first identification of tungsten mineralization related to silurian s type granites caledonian granites in the miao ershan granite intrusion indicating caledonian is an important mineralization period of w in the western part of nanling range south china

a shear zone related greenstone belt hosted gold mineralization in the archean of west greenland a petrographic and combined pb pb and rb sr geochronological study abstract widespread gold showings have been found in an archean greenstone belt on storo in godthabsfjord west greenland the storo greenstone belt sgb comprises metasedimentary and metavolcanic rocks with minor magnetite rich bands gold mineralization is hosted by amphibolites mica schists and garnetite rocks gold occurs in various mineral associations but its presence in lollingite arsenopyrite pyrrhotite parageneses is prevalent to a lesser extent native gold occurs within seriticized plagioclase in sheeted quartz veins and as inclusions in garnet based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold bearing lollingite arsenopyrite pyrrhotite assemblage on storo in the first stage arsenopyrite likely formed with gold in solid solution in the second stage gold bearing arsenopyrite was replaced by lollingite and pyrrhotite this took place during regional prograde metamorphism gold accumulated within lollingite as discrete grains during subsequent retrograde metamorphic conditions lollingite and pyrrhotite reacted and formed arsenopyrite and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite lollingite grain boundaries during a later event a garnet overgrowth partly affected the gold bearing lollingite arsenopyrite pyrrhotite assemblage the isotopic composition of pb released from these garnets by pb stepwise leaching experiments pbsl and the bulk pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of pb in the fluid from which first the sulfides and subsequently garnet were formed bulk pb analyses and pbsl experiments of arsenopyrite and pbsl experiments of garnet yield ages of 2863 24 ma 2 mswd 0 2 for arsenopyrite and 2748 62 ma 2 mswd 220 for garnet the 2863 24 ma age is thus a minimum age for the mineralization rb sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal middle proterozoic ca 1 65 ga mineral ages well in accordance with a regional reheating event known to have affected the area and are thus interpreted as cooling ages thereof this regional reheating event has apparently not affected the pb pb isotopic system of arsenopyrite implying that the blocking temperature of the u pb system in arsenopyrite is likely to be above 300 c the late archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time the ore textures preserved today are interpreted as those formed under this regional metamorphic event

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

the remarkable re os chronometer in molybdenite how and why it works the re os rhenium osmium chronometer applied to molybdenite mos2 is now demonstrated to be remarkably robust surviving intense deformation and high grade thermal metamorphism successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite unique to each sample such that recognized spatial decoupling of 187re parent and 187os daughter within individual molybdenite crystals is overcome highly precise accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry id ntims spatial decoupling of parent daughter precludes use of the laser ablation icp ms microanalytical technique for re os dating of molybdenite the use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons the rb sr k ar and 40ar 39ar chronometers are susceptible to chemical and thermal disturbance particularly in terranes that have experienced subsequent episodes of hydrothermal magmatic activity and therefore should not be used as a basis for establishing accuracy in re os dating of molybdenite as has been done in the past re os ages for molybdenite are almost always in agreement with observed geological relationships and when available with zircon and titanite u pb ages for terranes experiencing multiple episodes of metamorphism and deformation molybdenite is not complicated by overgrowths as is common for some minerals used in u pb dating e g zircon monazite xenotime nor are re and os mobilized beyond the margins of individual crystals during solid state recrystallization moreover inheritance of older molybdenite cores incorporation of common os and radiogenic os loss are exceedingly rare whereas inheritance common pb and pb loss are common complications in u pb dating techniques therefore molybdenite ages may serve as point in time markers for age comparisons

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

rhenium variations in molybdenite mos2 evidence for progressive subsurface oxidation abstract temporal trends in earth s near surface mineralogy correlate with major events in earth s geochemical and tectonic history new and published analyses of 422 molybdenite mos2 specimens from 135 localities with known ages from 2 91 billion years ga to 6 3 million years ma reveal two statistically significant trends first systematic increases in average and maximum trace concentrations of re in molybdenite since 3 0 ga point to enhanced oxidative weathering by subsurface fluids in addition episodic molybdenum mineralization correlates with five intervals of supercontinent assembly from 2 7 ga kenorland to 300 ma pangaea

re os study of fe ti v oxide and fe cu ni sulfide deposits suwalki anorthosite massif northeast poland the suwalki anorthosite massif located in extreme northeast poland beneath more than a kilometer of phanerozoic cover hosts major fe ti v deposits these deposits discovered in 1962 are contained in fe and ti oxide minerals that coexist with subordinate quantities of fe cu ni and co sulfides in massif style anorthosites norites and gabbronorites accessibility and other considerations preclude development of this natural resource in the present economic climate detailed work by polish geologists during the last 35 years provides a sound geologic framework for this re os study of the age and origin of oxide and sulfide deposits associated with a major but lesser known anorthosite massif rhenium and osmium abundances and os isotopic compositions were measured for nine sulfides and four titanomagnetites from the suwalki anorthosite massif the titanomagnetites are over an order of magnitude lower in re 0 4 1 5 ppb and os 0 036 0 144 ppb concentrations than co precipitated pyrrhotite pyrite and chalcopyrite that yield consistent concentrations for re 30 55 ppb and os 1 6 ppb parallel lines connecting co existing titanomagnetite and sulfides have slopes of 1 on re versus common os concentration plots indicating that both re and os behave similarly during crystallization in their high preference for any sulfide phase over magnetite samples from three deposits within the anorthosite massif were analyzed an age of 1559 37 ma n 10 with an initial 187os 188os of 1 16 0 06 for the jezioro okr gle and krzemianka deposits is essentially identical to an age of 1556 94 ma n 3 for the udryn deposit udryn however yielded a marginally lower initial 187os 188os of 0 87 0 20 the high initial 187os 188os combined with the proterozoic re os age indicates that the source for suwalki oxides sulfides is older crust and hypothetically could involve archean rocks an average crustal value of 50 for 187re 188os yields a 2777 ma age for suwalki source rocks widespread phanerozoic cover severely limits knowledge of basement rocks in poland however and no archean rocks are known in the immediate region more likely 187re 188os ratios may be higher than average continental crust reflecting mafic crust in the source and may move the source age for suwalki anorthosite and mineral deposits toward younger values that easily include 2 0 ga proterozoic rocks this more favorable case also accommodates paleoproterozoic nd model ages regardless of archean or proterozoic source age the high initial 187os 188os ratios derived from the re os isochron indicate that the source for the oxide sulfide mineral deposits is more likely the crust and not the mantle given that these deposits are clearly magmatic the re os results add a new dimension to the long standing origin of anorthosite problem implying a crustal source for the anorthosite as well the 1559 ma suwalki age is compatible with a well exposed east west band of 1530 1660 ma rapakivi granite anorthosite magmatism to the immediate north transecting western russia southern finland estonia and latvia and central sweden in particular the age and isotopic character of suwalki are not unlike those of the well studied salmi rapakivi granite anorthosite batholith in western russia karelia

large scale vertical eddy diffusion in the main pycnocline of the central north pacific abstract indirect procedures are used to estimate the latitudinal distribution of the large scale vertical eddy diffusivity coefficient in the main pycnocline from the interannual change in t structure of the water column in the central midlatitude north pacific from 35 44 n 150 170 w using t s data from two identical hydrographic surveys one made in june 1976 and the other in may 1977 the interannual change in potential temperature is recorded on three potential density surfaces i e 26 6 27 0 and 27 2 that lie in the upper middle and lower part of the main pycnocline respectively by relating these interannual changes to the vertical gradients of the average potential temperature on each surface i e through the heat conservation equation the large scale vertical eddy diffusivity coefficient and its vertical gradient were determined by least squares estimate procedures the vertical eddy diffusivity coefficient k is found to have been positive over the entire region with magnit

temporal evolution of mineralization events in the bohemian massif inferred from the re os geochronology of molybdenite molybdenite is a common mineral accompanying sn w au and base metal mineralizations located in different geotectonic units of the bohemian massif but it is also widespread in granitoids and or related quartz veins pegmatites forming disseminated mo mineralization thirty re os ages were obtained for molybdenite samples from the bohemian massif to provide constraints on the timing and duration of mineralization event s within the framework of previously published geochronological data for the host and or associated rocks the obtained data for sn w li deposits in the erzgebirge metallogenetic province indicate the predominance of one and or multiple short time mineralization events taking place between 319 and 323 ma with the exception of the krupka deposit associated with the altenberg teplice caldera where the data may suggest prolonged activity until 315 ma the ages of the pb zn au mo h rky u rakovnika and fe cu as ob i d l mineralizations from the exocontacts of the cista pluton and krkonose jizera plutonic complex respectively provide evidence for synchronous emplacement of the ore and the associated granitic rocks in contrast the padr fe as mo mineralization postdates the age of the associated padr granite disseminated mo mineralization in cadomian and variscan granitoids and or related to quartz veins pegmatites provides re os ages that overlap with the previously published geochronological data for the host rocks suggesting coeval evolution molybdenite samples from the sazava suite granites of the central bohemian plutonic complex cbpc have resolvable younger ages than their host granites but similar to the age of spatially related au mineralization which is associated with the latest evolution of the cbpc

re os dating of low level highly radiogenic llhr sulfides the harnas gold deposit southwest sweden records continental scale tectonic events to obtain geologically meaningful and precise results on low level sulfides that are highly radiogenic blank corrections that account for the extreme difference in the os isotope composition of sample and blank are necessary and the resulting data is best plotted in 187 re 187 os space much like the approach used for molybdenites we here introduce the acronym llhr as a means of characterizing low level and highly radiogenic sulfides use of the traditional 187 re 188 os vs 187 os 188 os plot for llhr sulfides leads to large analytical uncertainties and highly correlated errors as 188 os is poorly determined and can generally be accounted for by the blank we now recognize that llhr sulfides such as those encountered at the harnas au deposit are a common occurrence and characteristic of many crustally affiliated ore systems the harnas au deposit in southwest sweden is located in the mjosa vanern mineral belt that transects from northwest to southeast the prominent geologic fabric of the sveconorwegian province including the mylonite zone the deposit is hosted in a strongly sheared and locally brecciated sulfide bearing quartz vein that crosscuts the foliation of orthogneisses belonging to the 1600 maa mal granitoid suite a 187 re 187 os isochron of 973 34 ma 2 for samples of pyrite and chalcopyrite from outcrops of the harnas alma and silvergruvan veins was obtained the harnas and subsidiary alma veins are au bearing whereas the silvergruvan 2 5 km to the southeast is not of the au bearing type four replicate analyses of an additional pyrite sample from the harnas vein yield a model age of 616 40 ma 95 c l in all samples re and 187 os concentrations are extremely low 0 35 to 7 1 and 0 005 to 0 074 ppb respectively and total common os 0 002 0 028 ppb is less than half the calculated radiogenic 187 os two samples of galena from the harnas vein yielded blank level re and os neither the 973 nor 616 ma time of mineralization is related to development of the mylonite zone at 920 ma or intrusion of the anorogenic bohus blomskog granites at 920 to 900 ma previously proposed sources for generating mjosa vanern mineral belt ore fluids an age of 973 ma for harnas alma silvergruvan suggests that sulfide deposition was associated with the waning stages 0 97 0 90 ga of the sveconorwegian grenvillian orogeny 1 15 0 90 ga postdating metamorphism and deformation that produced a regionally overthickened crust in the northern part of the sveconorwegian province the vein systems for these and numerous other small deposits in the mjosa vanern mineral belt could have resulted from regional exhumation in the late sveconorwegian with associated extension and the development of fracture systems at cooler middle to upper crustal levels in the southern part of the sveconorwegian province dominated by granulitic facies rocks no mineralization is present perhaps due only to increased exhumation and erosion that has exposed deeper crustal levels the late sveconorwegian age 973 ma records the main period of sulfide deposition for the harnas alma silvergruvan vein systems and indicates the robustness of the chronometer within tectonically complex terranes the late neoproterozoic age 616 ma records a later time of pyrite deposition probably associated with shearing and reopening of the harnas vein system because the au mineralization at harnas has been described as associated with shearing and occurring in fracture fillings in early pyrite we suggest that part or all of the au mineralization could have been introduced at 616 ma during reactivation of the vein system the 616 ma age for pyrite suggests a previously unrecognized and much younger mineralizing episode in the harnas region most likely reflecting continental scale extension and the late neoproterozoic opening of the iapetus ocean

geology re os age s and o isotopic composition of the lar porphyry cu mo deposit southeast iran abstract the lar porphyry cu mo deposit is located 20 km northeast of zahedan in the sistan suture zone ssz in the southeastern part of iran lar is a sub economic porphyry cu mo deposit with average grades of 0 16 cu 0 01 mo and 0 66 ppb au the cu mo mineralization developed in post collisional shoshonitic syenitic to monzonitic intrusions in an oligocene igneous complex known as the lar igneous complex lic the host and country rocks have undergone variable degrees of supergene and hypogene alteration including silicic potassic propylitic phyllic and argillic alteration the cu mo mineralization is mainly controlled by nnw striking faults and fractures and occurs as quartz veins and veinlets disseminated sulfides and stockworks the mineralization can be divided into two types hypogene and supergene hypogene mineralization is characterized by chalcopyrite pyrite bornite molybdenite and magnetite whereas supergene mineralization is characterized by enargite covellite chalcocite digenite malachite azurite and iron hydroxides sulfur isotope values for hypogene sulfides range from 0 28 to 1 76 and suggest an igneous source or mantle derived origin for sulfur the 18o values for quartz in mineralized quartz veins range from 9 6 to 11 5 these values would be in equilibrium with waters having calculated 18oh2o values of 9 3 11 2 suggesting magmatic water re os dating of two molybdenite samples from two different drill holes yields dates of 29 72 0 11 and 31 95 0 11 suggesting that the system was active for at least 2 0 m y

sulfide re os and rb sr isotope dating of the kangdian iocg metallogenic province southwest china implications for regional metallogenesis the kangdian iron oxide copper gold iocg metallogenic province contains a number of economically important fe cu deposits in south china and northern vietnam due to the lack of precise mineralization ages the metallogenesis of this province has long been a matter of debate in this study molybdenite re os pyrite re os and rb sr isotope methods are used to date the yinachang and e touchang fe cu deposits in the central part of this province fine grained 0 1 mm molybdenite occurs as veinlets along bandings or fractures of magnetite ores and hydrothermal veins in the yinachang deposit molybdenite separates from magnetite ores have highly reproducible re os model ages of 1654 7 ma whereas those from hydrothermal veins have a much younger re os model age of 1451 6 ma euhedral pyrite grains occur locally along foliations of hematite ores and their proximal alteration assemblages in the e touchang deposit pyrite separates have an re os isochron age of 1487 110 ma and a single grain rb sr isochron age of 1453 28 ma both of which are compatible with the molybdenite re os age of hydrothermal veins from the yinachang deposit the molybdenite age of magnetite ores is in good agreement with zircon u pb ages of dolerite intrusions in the region and is considered to represent the timing of a major ore formation event at 1655 ma the younger re os and rb sr ages likely record a secondary hydrothermal overprint event at 1450 ma which has not been recognized before the mineralization potential and implications of this event need to be investigated in future studies our new isotopic data set hence demonstrates that the kangdian iocg deposits were likely formed at 1655 ma synchronous with the late paleoproterozoic intracontinental rifting event of the western yangtze block

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

paired uraninite and molybdenite dating of the konigshain granite implications for the onset of late variscan magmatism in the lausitz block we present geochronological data for late variscan magmatism in the lausitz block of the saxo thuringian zone germany the th u total pb age of uraninite and the re os age of molybdenite from the composite biotite monzogranite pluton of konigshain overlap at the 2 confidence limit 328 6 1 9 ma uraninite and 327 0 1 3 ma and 327 6 1 3 ma molybdenite indicating that crystallization of magmatic uraninite and deposition of molybdenite were nearly contemporaneous these data imply that magmatic processes in this part of the variscan orogen already started in latest visean time about 10 ma earlier than previously assumed 315 320 ma the new ages correspond to ages for plutonic rocks in the elbe zone immediately west of the lausitz around 335 325 ma and the bulk of late variscan igneous rocks in the saxo thuringian zone 335 320 ma

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

inherited pb components in magmatic titanite and their consequence for the interpretation of upb ages abstract the occurrence of inherited radiogenic pb components in 35 ma old magmatic titanite is reported here for the first time the about 810 ma age of the inherited components is identical to that contained in zircon from the same amphibole bearing syenite sample in both minerals the precambrian components were not totally reset during magmatic new growth at 35 ma this age pattern substantiates that the u pb chronometer in titanite is resistant to temperatures in excess of 712 c as is already known for zircon and monazite in consequence u pb ages from crystalline titanites have to be interpreted as the age of mineral formation or re crystallization rather than the time of u pb system closure during cooling of the rock

u pb monazite xenotime and titanite geochronological constraints on the prograde to post peak metamorphic thermal history of paleoproterozoic migmatites from the grand canyon arizona the petrology and u pb geochronology of pelitic migmatite and calc silicate gneiss reveal a detailed prograde to post peak metamorphic thermal history for a single outcrop of paleoproterozoic supracrustal rocks in the eastern part of the grand canyon metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 ma and continued to grow until about 1697 ma the u pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting which involved the breakdown of muscovite to sillimanite commenced at about 1702 ma prior to the metamorphic peak partial melting continued until about 1690 ma the youngest u pb date from magmatic monazite in the leucosomes field and petrographic evidence as well as inheritance patterns in monazites from the leucosomes suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region between 1702 and 1690 ma the migmatite package heated to peak metamorphic conditions of about 720 c and 6 kbar cooled to about 675 c at a cooling rate 30 c million years and decompressed to about 4 kbar the u pb geochronological data for metamorphic titanite from a calc silicate gneiss exhibit a clear relationship between grain size and the 207pb 206pb date indicating that the titanite crystals record cooling ages these data combined with the titanite pb diffusion data of cherniak 1993 yield a cooling rate of 5 4 0 9 1 7 c million years integrated over the interval 1690 to 1676 ma and suggest that by 1675 ma the cooling rate slowed to less than 2 c million years the rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large scale tectonic processes associated with the accretion of juvenile crust to the margin of laurentia juvenile arc crust appears to have been assembled accreted and stabilized into laurentian lithosphere in less than 30 million years

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

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

inherited zircon and titanite upb systems in an archaean syenite from southwestern australia implications for upb stability of titanite abstract inherited zircon and titanite have been identified in a syenite from the archaean of southwestern australia conventional and shrimp analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 5 ma for the syenite in addition shrimp analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca 3250 ma inherited component conventional u pb ages on titanite also fall between ca 3250 ma and ca 2650 ma demonstrating that inherited titanite as well as zircon is present in the syenite the syenite has been affected by regional upper amphibolite facies metamorphism at an estimated temperature of 625 650 c retention of the inherited radiogenic pb in the titanite is evidence that the closure temperature for titanite is greater than 650 c the presence of inherited titanite and zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of crustal rocks by a potassic magma at

geochronological constraints on the timing of granitoid magmatism metamorphism and post metamorphic cooling in the hercynian crustal cross section of calabria exposed cross sections of the continental crust are a unique geological situation for crustal evolution studies providing the possibility of deciphering the time relationships between magmatic and metamor phic events at all levels of the crust in the cross section of southern and northern calabria u pb rb sr and k ar mineral ages of granulite facies metapelitic migmatites peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late hercynian peak metamorphism and granitoid magmatism as well as on the post metamorphic cooling monazite from upper crustal amphibolite facies paragneisses from southern calabria yields similar u pb ages 295 293 4 ma to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust 300 10 ma monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303 302 0 6 ma zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290 2m a which may be interpreted as the minimum age for metamorphism or intrusion u pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern calabria sila also point to a near synchronism of peak metamorphism and intrusion at 304 300 0 4 ma at the end of the granulite facies metamorphism the lower crustal rocks were uplifted into mid crustal levels 10 15 km followed by nearly isobaric slow cooling c 3 cm a 1 as indicated by muscovite and biotite k ar and rb sr data between 210 4 and 123 1 ma the thermal history is therefore similar to that of the lower crust of southern calabria in combination with previous petrological studies addressing metamorphic textures and p t conditions of rocks from all crustal levels the new geochronological results are used to suggest that the thermal evolution and heat distribution in the calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late hercynian orogeny

domainal variations of u pb monazite ages and rb sr whole rock dates in polymetamorphic paragneisses ktb drill core germany influence of strain and deformation mechanisms on isotope systems polyphase metamorphic paragneisses from the drill core of the continental deep drilling project ktb nw bohemian massif are characterized by peak pressures of about 8 kbar medium p metamorphism followed by strain accumulation at t 650 c initially by dislocation creep and subsequently by diffusion creep u pb monazite ages and rb sr whole rock data vary in the dm scale indicating ordovician and mid devonian metamorphic events such age variations are closely interconnected with dm scale domainal variations of microfabrics that indicate different predominant deformation mechanisms u pb monazite age variations dependent on microfabric domains exceed grain size dependent age variations in mylonitic domains recording high magnitudes of plastic strain dislocation creep and minor static annealing monazite yields concordant and near concordant lower ordovician u pb ages and the rb sr whole rock system shows isotopic disequilibrium at an mm scale in mineral growth mobilisate domains in which diffusive mass transfer was a major strain producing mechanism promoting diffusion creep of quartz and feldspar and in which static recrystallization annealing reduced the internal free energy of the strained mineral aggregates concordant u pb ages are mid devonian locally in such domains rb sr dates among mm3 sized whole rock slabs reflect post ordovician resetting in transitional domains the u pb ages are discordant we conclude that medium p metamorphism occurred at 484 2 ma and a second metamorphic event at 380 370 ma mid devonian caused progressive strain in the rocks dislocation creep at high rates even at high temperatures does not reset the rb sr whole rock system while diffusion creep at low rates and stresses i e low e deff ratios static annealing and the presence of intergranular fluids locally assist resetting at temperatures above 650 c diffusive pb loss did not reset ordovician u pb monazite ages and in domains of overall high imposed strain rates and stresses resetting was not assisted by dynamic recrystallization crystal plasticity however during diffusion creep at low rates pb loss by dissolution and precipitation recrystallization of monazite produces discordance and devonian concordant u pb monazite ages hence resetting of these isotope systems reflects neither changes of temperature nor directly the presence or absence of strain

molybdenite re os dating constrains gravitational collapse of the sveconorwegian orogen sw scandinavia abstract re os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the sveconorwegian orogen in sw scandinavia a 1019 3 ma planar quartz vein defines a minimum age for brittle behaviour in central telemark in rogaland vest agder molybdenite associated with deformed quartz and pegmatite veins formed between 982 3 and 947 3 ma in the amphibolite facies domain three deposits and between 953 3 and 931 3 ma west of the clinopyroxene in isograd two deposits in the vicinity of the 0 93 0 92 ga rogaland anorthosite complex the data constrain the last increment of ductile deformation to be younger than 0 95 and 0 93 ga in these two metamorphic zones respectively molybdenite is the product of an equilibrium between biotite oxide and sulfide minerals and a fluid or hydrated melt phase after the peak of 1 03 0 97 ga regional metamorphism molybdenite precipitation is locally episodic a model for gravitational collapse of the sveconorwegian orogen controlled by lithospheric extension after 0 97 ga is proposed in the west of the orogen the rogaland vest agder sector is interpreted as a large shallow gneiss dome formed slowly in two stages in a warm and structurally weak crust the first stage at 0 96 0 93 ga was associated with intrusion of the post collisional hornblende biotite granite suite the second stage at 0 93 0 92 ga restricted to the southwesternmost area was associated with intrusion of the anorthosite mangerite charnockite suite most of the central part of the orogen was already situated in the brittle upper crust well before 0 97 ga and did not undergo significant exhumation during collapse in the east of the orogen situated against the colder cratonic foreland exhumation of high grade rocks of the eastern segment occurred between 0 97 and 0 95 ga and included preservation of high pressure rocks but no plutonism

u pb geochronology of grossular andradite garnet abstract this study presents a new laser ablation inductively coupled plasma mass spectrometry la icpms based u pb geochronometric method for dating grossular andradite grandite garnet grandite is a primary skarn mineral therefore dating its growth directly dates hydrothermal activity as zircon u pb geochronometry provides a high resolution record of magmatic processes grandite u pb dating has the potential to provide a complementary record for hydrothermal systems this study characterizes four garnets of variable grossular andradite content and age as potential reference materials for la icpms u pb geochronology willsboro andradite 1020 ma adirondacks usa red and yellow mali grandite 200 ma southern mali and lake jaco grossular 35 ma coahuila mexico isotope dilution thermal ionization mass spectrometry id tims u pb analyses of willsboro andradite yield a mean 206pb 238u date of 1022 16 ma we use willsboro andradite as a primary reference material for la icpms u pb characterization of the two other garnets the non radiogenic pb pbc concentration in grandite is variable between specimens willsboro andradite and mali grandites contain almost purely radiogenic pb pb however lake jaco grossular has a higher pbc concentration comparatively and yields discordant u pb ratios for specimens with highly variable pb pbc linear regression is employed to derive a lower intercept age la icpms u pb dates for yellow mali grandite 202 2 ma 206pb 238u weighted mean and lake jaco grossular 35 2 ma lower intercept agree with independent id tims u pb 202 0 1 2 ma and u th he dates 35 5 ma derived for each specimen the precision of la icpms u pb dates varies between 1 10 2 for the analyzed garnets considering the short estimated lifespans of pluton related hydrothermal systems tens of thousands to a few million years neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels as a performance test for the method we present new u pb andradite data from a late miocene skarn system on serifos island greece this garnet yields a lower intercept age of 9 15 0 36 ma in agreement with biotite rb sr and zircon u pb dates from the causative pluton

re os geochemistry and geochronology of the ransko gabbro peridotite massif bohemian massif the ransko gabbro peridotite massif in eastern bohemia is a strongly differentiated intrusive complex which hosts low grade ni cu ores mainly developed close to the contact of olivine rich rocks with gabbros in troctolites and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase rich peridotites gabbro troctolite peridotite and ni cu ores from the jezirka ni cu pge deposit considered to be a typical example of the liquid segregation style of mineralization were analyzed for re os concentrations and isotopic ratios seven barren and mineralized samples from the jezirka deposit yielded a re os regression of 341 5 7 9 ma mswd 69 strongly mineralized peridotite with mantle like initial 187os 188os ratio of 0 125 suggests that os as well as other pge present in the ni cu mineralization are predominantly of mantle origin on the other hand barren and low mineralized samples have radiogenic initial 187os 188os ratios of 0 14 0 16 suggesting some import of re and or radiogenic 187os most likely through contamination by continental crust during magma emplacement the re os age of the ransko massif is significantly younger than the previously suggested lower cambrian age but it is similar to and or younger than the age of metamorphism of the adjacent kutna hora crystalline complex and the moldanubian unit therefore it is likely that the emplacement of the ransko massif and its ni cu mineralization was closely connected with the late stage evolution of the kutna hora crystalline complex

petrogenesis and geodynamic setting of granitoids at machangqing cu mo au deposit western yangtze craton southwestern china constraints from zircon u pb and molybdenite re os geochronology lu hf isotopes and geochemistry the machangqing cu mo au deposit is located in the central part of the jinshajiang red river belt in the sanjiang orogen which lies across the qiangtang terrane and western yangtze craton sou

the formation of the suyunhe large porphyry mo deposit in the west junggar terrain nw china zircon u pb age geochemistry and sr nd hf isotopic results abstract newly discovered suyunhe mo deposit 0 57 mt mo metal located in the barluk mountains of the west junggar terrain xinjiang nw china is the largest porphyry mo deposit discovered in xinjiang so far the suyunhe mo deposit is associated with the suyunhe intrusive complex which was emplaced into the volcano sedimentary sequences of the middle devonian barluk group the suyunhe complex consists of the deep granite intrusion and the shallow granite porphyry granodiorite porphyry monzonitic granite porphyry and tonalite porphyry stocks which are cut by the felsic dikes dominant mo mineralization is hosted in the barluk group volcano sedimentary rocks but is minor in the granite granite porphyry granodiorite porphyry and monzonitic granite porphyry of the suyunhe complex we report in situ zircon u pb ages whole rock geochemistry sr nd isotopic and zircon lu hf isotopic data for the ore bearing intrusive rocks from the suyunhe deposit secondary ion mass spectrometry sims zircon u pb dating indicates that the deep granite emplaced at 298 4 1 9 ma 295 3 3 ma and the shallow granodiorite porphyry intruded during 294 7 2 1 ma 293 7 2 3 ma the suyunhe granite intrusion including fine and medium grained granite belongs to the high k calc alkaline series most of them have low zr nb y ce 2 7 values showing a geochemical affinity to i type granite these granites are enriched in light rare earth elements lree and depleted in heavy rare earth elements hree and eu ba sr nb p and ti indicating that they have experienced advanced fractional crystallization they have low initial 87sr 86sr ratios 0 70318 0 70380 high whole rock end t values 4 4 to 6 0 high zircon ehf t values 10 8 to 14 6 and young whole rock nd and zircon hf model ages they also show low mg 0 35 these features demonstrate that the ore bearing granite formed by partial melting of the juvenile lower crust and experienced advanced fractional crystallization the suyunhe granite porphyry and monzonitic granite porphyry are compositionally similar to the granite indicating that they share a common magma source and process with the granite the suyunhe granodiorite porphyries belong to slightly high k calc alkaline series and have lree enrichment and hree nb and ti depletion they have low 87sr 86sr i ratios 0 70421 0 70444 high end t values 4 9 to 6 2 high zircon ehf t values 9 6 to 15 2 and young whole rock nd and zircon hf model ages they also show low mg

re os dating and sulfur isotope composition of molybdenite from tungsten deposits in western namaqualand south africa implications for ore genesis and the timing of metamorphism both stratiform stratabound and granite related models have been used to explain the genesis of w mo deposits in the okiep copper district in western namaqualand south africa apparently stratabound mineralization fe rich wolframite with accessory molybdenite occurs in foliation parallel quartz veins in high grade 750 c 5 6 kbar metapelites of the wolfram formation and less commonly in small bodies of silicified leucogranites and pegmatites six re os ages for molybdenites from four deposits nababeep tungsten far west kliphoog narrap tweedam range between 1000 4 and 1026 5 ma these molybdenites define a well constrained 187re 187os isochron with an age of 1019 6 ma which is interpreted as the age of w mo mineralization this age is significantly younger than proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites but geologic evidence requires overlap with a period of high grade metamorphism we suggest that w mo mineralization is genetically linked to intra crustal magmatic processes at 1020 ma thereby precluding the 1060 ma concordia granite as the source for mineralizing fluids a narrow range of positive 34s compositions 3 6 to 4 5 for eight molybdenites from five w mo mines is consistent with a so2 rich fluid and a granite related genetic model post peak metamorphic deformation and metamorphism of w mo ores is most likely related to the retrograde stage of the namaquan orogeny which overlaps emplacement of late orogenic evolved granites and pegmatites and the formation of w mo deposits in western namaqualand therefore the effects of retrograde namaquan metamorphism extend at least to 1020 ma or alternatively these w mo veins were affected by a poorly constrained later event e g early pan african

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

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

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

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 dating of sulfides associated with gold mineralization in central victoria australia we report re os isotope ages that are the first for sulfides closely associated with gold mineralization at the bendigo and maldon goldfields in central victoria two arsenopyrite samples from bendigo contain 11 ppb re and 0 08 ppb highly radiogenic os llhr samples stein et al 2000 and have model ages of 444 7 and 447 7 ma with a mean model age of 446 5 ma a five point isochron based on these arsenopyrite analyses plus three pyrite analyses including one replicate gives an age of 438 6 ma molybdenite associated with au te cu and sb mineralization at maldon gives an re os age of 376 2 ma similar to the age of the nearby har court granite these results support previous 40 ar 39 ar data indicating that initial emplacement of gold mineralization in central victoria occurred in the late ordovician during the earliest stages of accretionary development of the southern lachlan fold belt the molybdenite data are also consistent with the mobilization and or introduction of gold during late devonian magmatism

geology geochronology and geochemistry of the dabate cu mo deposit northern chinese tien shan implications for spatial separation of copper and molybdenum mineralization abstract the dabate deposit is a porphyry cu mo deposit located in the sailimu block northern chinese tien shan the deposit shows a clear separation of cu and mo mineralization with cu mineralization occurring along the edge of the rhyolitic porphyry and mo mineralization in the inner part of the rhyolitic porphyry sm nd isotopic analyses of six fluorite samples from the cu sulfides fluorite veins yield an isochron age of 298 1 5 0 ma and re os isotopic analyses of four molybdenite samples from the molybdenite quartz veins yield an isochron age of 299 6 3 2 ma these geochronological data and accompanying geologic evidence indicate that the cu and mo mineralization belongs to the same hydrothermal system and are closely related to the rhyolitic porphyry which has a youngest zircon 206pb 238u age of 297 3 8 0 ma rare earth element ree and fluid inclusion studies were carried out for fluorite in the cu ores the fluorite related to cu mineralization have high ree 63 4 216 9 ppm and show generally similar ree patterns with the rhyolitic porphyry suggesting that the fluid of cu mineralization is closely related to the rhyolitic porphyry the slight lree depletion and ce depletion of fluorite relative to the rhyolitic porphyry may be due to preferential partitioning of hree into the fluid in neutral to weak alkaline conditions and to mixing with meteoric water which is characterized by ce depletion three types of fluid inclusions were recognized in the fluorite from the cu sulfides fluorite veins liquid vapor lv type co2 bearing c type and solid mineral bearing s type the lv type inclusions have homogenization vapor to liquid temperatures from 175 to 328 c and salinities of 0 4 14 9 wt nacl eqv mode at 0 8 wt nacl eqv and is of h2o nacl composition the c type inclusions have homogenization vapor to liquid temperatures from 206 to 279 c and salinities of 1 6 8 5 wt nacl eqv and is of h2o co2 nacl ch4 composition the s type inclusions have partial homogenization vapor to liquid temperatures of 173 233 c and salinities of 1 2 7 9 wt nacl eqv and is of h2o nacl composition the solid particle in s type inclusions did not change during heating up to 550 c indicating that the solid is not daughter mineral and the s type inclusions are actually the same as the lv type inclusions the lv type inclusions have microthermometric attributes similar to those typically found in the late stages of porphyry deposits whereas the c type inclusions have similar compositions as the inclusions in the mo ores as documented in previous studies at dabate the presence of both lv type and c type inclusions suggests that the cu mineralization may be related to mixing between meteoric water and magmatic fluids as is also indicated by the ree of fluorite comparing with data from mo ores reported in previous studies it is suggested that the mo stage fluids were more reduced and acidic and mo mineralization may be mainly related to fluid pressure decrease and immiscibility whereas the cu stage fluids were more oxidized neutral to weakly alkaline and cu mineralization may be mainly controlled by mixing and cooling the vapor phase produced by fluid immiscibility in the deep area may have carried cu as ag hf co2 and migrated to the outer part of the porphyry precipitated cu sulfides arsenopyrite and fluorite by mixing with meteoric water from the country rocks whereas the liquid phase produced by fluid immiscibility at depth may have carried mo nacl and migrated to the middle west part of the rhyolitic porphyry become immiscible and precipitated molybdenite

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

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

monazite xenotime thermochronometry methodology and an example from the nepalese himalaya monazite xenotime thermochronometry involves the integration of petrographic geochronological and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals the method is illustrated in this paper by application to an orthogneiss sample from the everest region of the nepalese himalaya that contains leucogranitic segregations produced by in situ anatexis observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of himalayan tertiary and pre himalayan age nearly concordant u pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28 37 to 17 598 ma making determination of the timing of anatexis difficult without additional information presuming that monazite and xenotime were in equilibrium over that entire interval temperatures estimated from the yttrium contents of dated monazites range from 677 535 c only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition implying that the 25 4 24 8 ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis two monazite crystals yielded 207pb 235u dates that are statistically indistinguishable from the 207pb 235u dates of coexisting xenotime crystals permitting the application of both quantitative y partitioning and semi quantitative nd partitioning thermometers as a cross check for internal consistency one of these sub populations of accessory minerals with a mean 207pb 235u date of 22 364 0 097 ma provides inconsistent y partitioning 641 39 c and nd partitioning 515 560 c temperatures we suspect the discrepancy may be caused by the high th concentration 6 12 wt tho2 in this subpopulation s monazite the y partitioning thermometer was derived from experimental data for the ce y po4 binary and may be inappropriate for application to high th monazites for the other subpopulation mean 207pb 235u date 22 11 0 22 ma the y and nd partitioning temperatures are indistinguishable 535 49 and 525 550 c respectively this consistency strongly suggests that the sample experienced a temperature of 535 c at 22 11 ma this finding is tectonically important because temperatures at higher structural levels were much higher by 100 c at the same time lending support to earlier suggestions of a major structural discontinuity within the upper part of the himalayan metamorphic core at this longitude an additional finding of uncertain importance is that inherited monazite and xenotime yielded u pb discordia with indistinguishable upper intercept ages 465 5 8 7 and 470 11 ma respectively and application of the y partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 c whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes but it appears that monazite xenotime thermochronometry may be useful for seeing through high grade metamorphism to extract temperature time information about inherited mineral suites

re os isochron ages for arsenopyrite from carlin like gold deposits in the yunnan guizhou guangxi golden triangle southwestern china abstract the yunnan guizhou guangxi golden triangle is considered to be one of the regions hosting carlin like gold deposits in china gold deposits in this region can be grouped into lode type that are controlled by faults and layer like type controlled by stratigraphy arsenopyrite is one of the major gold bearing minerals in these deposits rhenium os isotopic dating of arsenopyrite from the lode type lannigou and jinya and the layer like type shuiyindong gold deposits yields isochron ages of 204 19 ma 206 22 ma and 235 33 ma respectively the data suggest that the carlin like gold deposits formed in late triassic to early jurassic which is clearly earlier than the ca 100 80 ma acid to ultra basic magmatism in this part of southwestern china the ages are consistent with ore formation during a period of post collisional lateral transpression which is similar to that of the carlin like gold deposits in western qinling of china but quite different from carlin type gold deposits in nevada u s a

187os enriched domain in an archean mantle plume evidence from 2 8 ga komatiites of the kostomuksha greenstone belt nw baltic shield the re os data on archean komatiites from the kostomuksha greenstone belt in the baltic shield are presented this greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacement of an ancient plume head puchtel et al earth planet sci lett 155 1998 57 74 samples of flowtop breccia spinifextextured and cumulate komatiites and a chromite separate all collected from the core of a 300 m deep diamond drill hole yielded a re os isochron with an age of 2795 40 ma and an initial 187 os 188 os of 0 1117 0 0011 q 187 os 3 6 1 0 the high positive q 187 os t implies that the komatiites were derived from a mantle source with a time integrated suprachondritic re os ratio recycling of oceanic lithosphere to produce the enriched 187 os isotope signature is considered unlikely as 15 25 crustal component is required to be incorporated into the plume source as early as 3 5 4 3 ga such a substantial proportion of mafic material in the source would likely destroy the major and trace element characteristics of the komatiites our tentative interpretation is that the 187 os enrichment in the kostomuksha plume represents an outer core signature if confirmed by the ongoing pt os isotope studies the results would provide evidence for the existence of whole mantle convection in the late archean and might place constraints on the timing of core differentiation in the early earth fl 2001 elsevier science b v all rights reserved

the geology and re os geochronology of the palaeoproterozoic vaikijaur cu au mo porphyry style deposit in the jokkmokk granitoid northern sweden the vaikijaur cu au mo deposit is located in the ca 1 88 ga calc alkaline jokkmokk granitoid near the archaean proterozoic palaeoboundary within the fennoscandian shield of northern sweden the skellefte vms district lies immediately to the south and the northern norrbotten fe oxide cu au deposits to the north the vaikijaur deposit occupies an area of 2 3 km within the jokkmokk granitoid and includes stockwork quartz sulphide veinlets and disseminated chalcopyrite pyrite gold molybdenite magnetite and pyrrhotite porphyritic mafic dykes were emplaced along fractures in a ring dyke pattern the jokkmokk granitoid dykes and the mineralized area are foliated indicating that mineralization predated the main regional deformation the mineralized area is characterized by strong potassic alteration phyllic and propylitic alteration zones are also present a pyrite rich inner core is surrounded by a concentric zone with pyrite chalcopyrite and gold molybdenite is distributed irregularly throughout the chalcopyrite zone geophysical data indicate a strongly conductive central zone in the mineralized area bordered by conductive and high magnetic zones five high precision re os age determinations for three molybdenite occurrences from outcrop and drill core samples constrain the age of porphyry style cu au mo mineralization to between 1889 10 and 1868 6 ma a younger molybdenite is associated with a much later metamorphic event at about 1750 ma these data suggest that primary porphyry style mineralization was associated with calc alkaline magmatism within the archaean proterozoic boundary zone at ca 1 89 1 87 ga