geochronology stratigraphy and geochemistry of cindery tuff in pliocene hominid bearing sediments of the middle awash ethiopia cindery tuff is a subalkaline rhyolitic air fall deposit that was probably produced by a mixed magma eruption it is a distinctive datable regional isochronous marker bed within the pliocene sediments of the middle awash district and is stratigraphically situated between two new fossil hominid discoveries based on 40ar 39ar analyses of plagioclase rhyolitic glass and basaltic glass as well as fission track analyses of zircons we estimate its age to be 3 8 4 0 myr this implies that associated hominid skull fragments are at least 3 9 myr old

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

40 ar 39 ar isotopic dating of tin mineralization in furong deposit of hunan province and its geological significance the newly discovered furong ore deposit located in the southern part of the qitianling pluton emplaced at 151 162 ma in southern hunan is a superlarge tin deposit it consists of bailashui anyuan taoxiwo sanmenkou and maziping heishanli ore districts and its ore veins occur in qitianling pluton and the inner or outer contact zone between the granite pluton and the late palaeozoic sedimentary rocks in this study ar ar isotope dating of muscovite hornblende and phlogopite was carried out in order to constrain the tin mineralization timing of the furong deposit the plateau ages of three phlogopite samples from the bailashui ore district are 150 6 1 0 ma 157 3 1 0 ma and 154 7 1 1 ma respectively and the plateau age of hydrothermal hornblende from the same mining district is 156 9 1 1 ma the plateau ages of two muscovite samples from the taoxiwo ore district are 159 9 0 5 ma and 154 8 0 6 ma respectively these age data are coincident with the intrusion time of the qitianling granite 151 162 ma and also in accordance with the ages of other nonferrous metal deposits such as the shizhuyuan w sn mo bi f deposit the xintianling w deposit the yaogangxian w deposit and the huangshaping pb zn polymetallic deposit in this area the large scale mineralization in southern hunan is probably ascribed to the mesozoic lithospheric extension in south china

a unique association of scheelite and magnetite in the tiemuli w fe skarn deposit implications for early cretaceous metallogenesis in the nanling region south china abstract the tiemuli scheelite magnetite deposit in the central part of the nanling region south china is genetically related to a porphyritic biotite granite pluton the w fe orebodies are hosted within upper cambrian calcareous sandstone or impure limestone in the external zones la icp ms u pb dating of zircons from the granite yielded a weighted mean age of 136 6 1 1 ma the main ore minerals are magnetite scheelite and arsenopyrite and the gangue minerals are clinopyroxene grossularite vesuvianite amphibole and mica three paragenetic stages of skarn formation have been recognized i a skarn stage ii a retrograde stage and iii a quartz sulphide calcite stage massive magnetite and scheelite orebodies associated with hydrous minerals formed during stage ii molybdenite grains separated from tiemuli skarns yielded re os model ages of 136 3 133 8 ma with a weighted mean age of 135 1 0 8 ma biotite associated with fine grained magnetite from the ores has a 40 ar 39 ar plateau age of 132 67 0 43 ma these three independent dating techniques show that the granitic magmatism and tungsten mineralization were coeval within error whereas the late stage alteration recorded by biotite could be ca 2 m y younger than the early stage the granite is classified as an a type body on the basis of its geochemistry and mineralogy zircon grains from the granite show negative ehf t values of 7 7 to 2 3 and molybdenite grains from the skarn ores contain low re contents of 0 93 to 2 97 ppm these data indicate that the magma and ore forming components at tiemuli were derived mainly from crustal materials although mafic xenoliths in the granites are consistent with some mantle contribution the tiemuli deposit was produced by crust mantle interaction in an extensional regime possibly caused by rollback of the subducting paleo pacific plate during early cretaceous the skarn scheelite and magnetite association formed by interaction between magmatic fluids and calcite bearing sandstone or impure limestone of the host strata

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

the 1800 1610 ma stratigraphic and magmatic history of the eastern succession mount isa inlier and correlations with adjacent paleoproterozoic terranes the chronostratigraphic and magmatic framework for the proterozoic mount isa inlier northwest queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years this material and existing chronometric constraints is used to construct a new time space plot for the inlier and to compare eastern succession volcanosedimentary accumulations with temporally equivalent units in the western fold belt the accumulation of volcanosedimentary sequences in the eastern succession occurred in four main periods namely 1890 1850 ma 1800 1750 ma 1710 1690 ma and 1680 1610 ma temporally equivalent units can be identified in the western fold belt periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the inlier the locus of bimodal volcanism migrates from west to east with time intracontinental rifting is the most likely tectonic setting for the sedimentary basins the majority of the soldiers cap group was deposited between ca 1675 and 1650 ma with the exception of units in the boomarra horst now known to belong to older sequences deposited ca 1775 1750 ma and the llewellyn creek formation at snake creek which may be as old as ca 1690 ma upper amphibolite facies rock units in the southwest e g the gandry dam gneiss are likely to be higher metamorphic grade temporal equivalents of the soldiers cap group five rock units previously assigned to the cover sequence 2 mary kathleen group namely the answer slate kuridala formation staveley formation marimo slate and agate downs siltstone are now known to have been deposited between ca 1675 and 1610 ma these units bar the kuridala formation have been removed from the cover sequence 2 mary kathleen group and given the informal name young australia group the kuridala formation has been included in the soldiers cap group the rock units within the young australia group are likely to be lateral facies equivalents of the younger units within the soldiers cap group rock units within the mount albert group although undated are considered likely to be temporal equivalents of the soldiers cap group and the young australia group the roxmere quartzite is included here in the young australia group that it tectonically overlays the ballara quartzite and mitakoodi quartzite represent a single regionally extensive sequence deposited at ca 1755 ma the double crossing metamorphics previously mapped as basement are likely to correlate with the argylla formation and mitakoodi quartzite most of the eastern succession to the west of the pilgrim fault is likely to have been a topographic high between ca 1780 and 1755 ma prior to the ballara quartzite unconformably overlying the ca 1780 ma argylla formation detrital zircons of argylla and wonga ages indicate that the mary kathleen belt is likely to have been a topographic high at least periodically during accumulation of the soldiers cap and young australia groups soldiers cap group units in the southeast contain numerous detrital zircons giving ages between ca 1730 and 1710 indicating a possible siliciclastic source to the east the host rocks to the pb zn ag deposits are restricted to cover sequence 3 units deposited between ca 1680 and 1590 ma and with the exception of century young from the southeast towards the northwest cu au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation however many of the most significant deposits are found within the younger sequences

reinterpretation of the tectonic context of high temperature metamorphism in the broken hill block nsw and implications on the palaeo to meso proterozoic evolution abstract the origin of high temperatures during regional low pressure granulite facies metamorphism within the proterozoic broken hill block australia has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca 1 71 1 67 ga and then maintained during a ca 1 62 ga short lived mid crustal extensional event mid crustal extension at ca 1 62 ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients and occurred prior to activity along d2 high temperature shear zones peak low pressure granulite facies metamorphism ca 1 60 ga and crustal shortening during the olarian orogeny ca 1 60 1 59 ga this reinterpretation places the broken hill block within an environment in which multiple episodes of transient extension ca 1 71 1 62 ga were followed by a switch to shortening at ca 1 60 ga the interpreted tectonic environment is a continental back arc setting located in the over riding plate of a subduction zone along the southern margin of palaeoproterozoic australia

geometry of propagating continental rifts reconstruction of past plate configurations through palinspastic restoration of continental margins1 4 requires a detailed knowledge of the geometries of horizontal continental extension extension is often restricted to discrete linear zones termed rifts5 6 and rift models that invoke propagating vertical dykes and cracks7 10 are useful for assessing the relative significance and timing of uplift subsidence and extension in the tectonic evolution of rift basins however many models do not adequately account for the observed asymmetry of rifts i describe here the general three dimensional character of young and aborted continental rifts which can be used to derive a structural model for the propagation of rifts in continental lithosphere the rifts become asymmetric as a consequence of the role played by low angle normal faults in the overall rift geometry

radiogenic and stable isotope constraints on the genesis of the eloise cu au deposit cloncurry district northwest queensland new radiogenic 40ar 39ar and stable oxygen hydrogen and sulfur isotope analyses of metamorphic and metasomatic minerals constrain the age metasomatic evolution and genesis of the eloise cu au deposit 3 1 mt 5 5 cu 1 4 g t au and 16 g t ag biotite from a pre to syn d2 stage vein has an 40ar 39ar age of 1555 4 ma which is interpreted to coincide with a regional metamorphic event synchronous with d2 six later stages of alteration and mineralization are recognized all of which postdate peak metamorphism and d2 stages i to iii are volumetrically the most significant and comprise early albitization stage i quartz hornblende biotite veins and alteration stage ii and cu au mineralization stage iii stages iv v and vi are localized vein events and postdate the main cu au mineralization stage ii vein and alteration hornblendes have 40ar 39ar ages of 1530 3 ma biotite from the same stage has an 40ar 39ar age of 1521 3 ma muscovite from a postore shear zone has an 40ar 39ar age of 1514 3 ma these results provide a maximum ca 1530 ma and minimum ca 1514 ma age for the mineralizing event however the intimate relationship between stage ii mafic silicate veins and alteration and the stage iii cu au event combined with fluid inclusion results which indicate that a cooling evolving high salinity fluid was responsible for both stages suggests that the older age is likely to be closer to the age of mineralization the stage ii biotite age of ca 1521 ma is interpreted to record thermal resetting during postore ductile deformation the results suggest that micas in the cloncurry district are more susceptible to later thermal resetting than amphiboles which may have significant implications for deposits which have only been dated by micas quartz from stages ii iii and iv have 18oquartz values ranging between 10 1 and 11 9 per mil stage ii biotite has a lower 18o composition 5 6 than biotite from a pre d2vein 6 9 but both have identical d compositions 84 stage ii hornblende has lower 18o 6 8 and 7 4 and d 88 and 90 values than does stage iii actinolite 8 0 and 84 respectively the 34s values for chalcopyrite pyrrhotite and pyrite fall in a narrow range between 0 0 and 2 3 per mil a distinct trend can be recognized with 34s values becoming progressively greater from south to north thus reflecting the zoned alteration system magnetite pyrite rich in the south through to pyrrhotite rich in the north the change in mineralogy and sulfur isotope values may be due to cooling and sulfidation processes which resulted in changes in oxygen and sulfur fugacities the oxygen hydrogen and sulfur isotope data combined with high temperature and high salinity fluid inclusion data indicate a predominantly magmatic origin for the ore forming fluids the deposit is interpreted to have formed from magmatic hydrothermal fluids which were tapped by deep seated crustal structures

local to regional scale structural controls on mineralisation and the importance of a major lineament in the eastern mount isa inlier australia review and analysis with autocorrelation and weights of evidence although major crustal lineaments may play an important role in mineralisation the relationship between lineaments and mineral deposits can be quite cryptic and structural controls may vary as a function of scale along lineaments major lineaments alone may be of limited use for detailed target generation the cloncurry lineament in the eastern part of the mount isa inlier is a crustal scale structure defined by potential field derived worms weights of evidence quantifies the association between mineral occurrences and this lineament autocorrelation is used to recognise structural controls on mineralisation at different scales by progressively limiting the lengths of the vectors between mineral occurrence points in the autocorrelation plot the weights of evidence analysis shows that au au cu cu au and cu deposits have a positive spatial correlation to the cloncurry lineament which suggests it that acted as a primary crustal scale control on the localisation of cu and au through focussing mineralisation systems on a broad scale however autocorrelation defines a variety of local structural controls which can be interpreted as shear zones variably oriented fault sets en echelon fault arrays and potentially the orientation of bedding and or iron formations which localise fluid flow and mineral deposition at finer scales the results suggest that major lineaments defined by geophysical contrasts can be used in conjunction with techniques of spatial analysis for targeting structurally controlled mineralisation in areas under thin cover adjacent to mineralised terrains such as the mt isa inlier

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

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

depositional ages of the stratiform base metal deposits at mount isa and mcarthur river australia based on u pb zircon dating of concordant tuff horizons potassium rich tuff beds occur interbedded with major stratiform pb zn mineralization in shales and dolomitic shales of the mount isa group northwest queensland and the mcarthur group northern territory these deposits are some 600 km apart yet they exhibit many similarities which together with regional correlations suggest that the deposits formed by similar processes and that the successions developed more or less contemporaneously zircon is sparsely distributed as tiny crystal fragments or phenocrysts in the tuffs and its morphology and u pb systematics affirm that it is primarily of igneous origin u pb zircon data from a tuff in the mineralized h y c pyritic shale member of the mcarthur group give an age of the tuff of 1 690 super 29 sub 25 m y and from a tuff in the mineralized urquhart shale of the mount isa group an age of 1 670 super 20 sub 17 m y these results are consistent with all other geologic controls and are regarded as the best estimates of the depositional ages of the groups the ages of the mount isa and mcarthur groups are thus the same to within the present experimental uncertainties essentially confirming geologic synchrony of these important sedimentary successions pooled whole rock data from several of the mount isa tuff marker beds give an updated rb sr age of about 1 480 m y interpreted as indicating that isotopic exchange and regional metamorphism had essentially ceased by this time

re os dating of molybdenite from tonglushan ore district in southeastern hubei province middle lower yangtze river belt and its geological significance the porphyry skarn cu fe au polymetallic deposits in southeastern hubei constitute an important component part of the middle lower yangtze river metallogenic belt the tonglushan ore district is characterized by various types of mineralizations and large sized ore bodies and represents the most important economic mineral district in southeastern hubei province nevertheless little research work has been done on the timing of various skarn deposits and the genetic association of igneous rocks with mineralization as well as the tectonic framework of these deposit molybdenites from the tonglushan skarn cu fe au deposit and the jiguanzui skarn cu au deposits yielded re os model ages ranging from 136 8 1 9 to 138 1 1 8 ma and from 137 1 1 9 to 138 8 1 9 ma respectively with isochronal ages being 137 1 1 9 ma and 138 2 2 2 ma which are considered to be reliable estimates for the timing of the tonglushan and the jiguanzui deposit these data indicate that ages of various skarn deposits are almost identical to the zircon shrimp u pb age of 140 3 1 8 ma for the related granodiorite almost the same as ages of other porphyry skarn cu mo deposits and skarn fe deposits in southeastern hubei province and even those of the jiurui cu au mo the anqing cu fe mo au and the tongling cu au ore districts in the middle lower yangtze river metallogenic belt combined with geological geochemical and geophysical evidence it is proposed that the skarn cu fe au deposits in tonglushan constitute a part of extensive igneous rocks and corresponding skarn cu fe au mineralization that resulted from lithospheric thinning and basaltic magma underplating which in turn were triggered by the rollback and or window of the downgoing slab related to the subduction of pacific plate in east china

re os isotopic chronology of molybdenites in huangshaping lead zinc deposit southeast hunan and its geological implications the huangshaping lead zinc polymetallic deposit in southeast hunan is genetically related to the intrusion of huangshaping granitic rocks and occur in the contact zone between the granodioritic porphyry and the limestone of lower carboniferous shidengzi formation as well as in the neighboring bedding plane fault zone previously the metallogenic epoch of the deposit was indirectly deduced by the ages of such nearby granitic rocks as granite porphyry and quartz porphyry or altered minerals based on the re os isochron method of molybdenites this paper measured directly the ore forming age from skarn ore in the huangshaping deposit and got a re os age of 153 8 4 8 ma which is in accordance with the previous k ar age of 153 157 ma obtained from phlogopite in the ore bearing skarn of the deposit combined with related rare earth element and trace element geochemistry of huangshaping granites and ore bearing skarn it is concluded in this paper that the age of 154 ma should represent the metallogenic epoch of the deposit and that there existed a close relationship between the ore forming process of the deposit and the granite porphyry which were both formed during the peak stage of early yanshanian large scale rock forming and ore forming activities in south china

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

geological characteristics and re_os dating of molybdenites in hehuaping tin polymetallic deposit southern hunan province the hehuaping tin polymetallic deposit a large tin ore field newly discovered in southern hunan consists of skarn and cassiterite sulfide ores at present the controlled reserves amount to about 110 700 tons of tin 10 000 tons of bismuth 50 000 tons of lead and 10 000 tons of zinc with the advance of the exploration work four ore bodies have been explored which are located in the southeast portion of wangxianling indosinian gra nite pluton and hosted in granite granite porphyry and devonian limestone and sandstone respectively re os dating of molybdenites yields model ages between 223 3 2 9 and 225 2 2 7 ma and an isochron age of 224 0 1 9 ma for the skarn ore related to wangxianling granite indicating that w sn mineralization in southern hunan also took place in indosinian period the zircon shrimp u pb dating of the late granite porphyry is 142 3 ma the granite porphyry has been partly mineralized and altered and contains tin ore bodies all this implies that there existed late mineralization of yanshanian period in hehuaping area the early mineralization is supposed to correspond to underplating of the basic magma

timing of palaeoproterozoic au cu bi and w mineralization in the tennant creek region northern australia improved constraints via intercalibration of 40ar 39ar and u pb ages abstract the magnitude of systematic uncertainties inherent in comparisons of proterozoic 40ar 39ar and u pb based ages is illustrated via a consideration of age constraints on two contrasting styles of palaeoproterozoic mineralization in northern australia published 40ar 39ar ages for au cu bi mineralization in the tennant creek region range from 1825 to 1829 ma apparently younger than the timing of deformation bracketed by 207pb 206pb zircon ages of igneous rocks at 1850 ma despite this apparent age offset propagation of systematic uncertainties in the 40ar 39ar ages shows that they are indistinguishable from the 1850 ma zircon ages recalculation using revised estimates of the 40k decay constant and age of the 40ar 39ar standard shifts the 40ar 39ar ages to 1847 1851 ma in excellent agreement with 207pb 206pb zircon ages from tennant supersuite igneous rocks and with local geological constraints to the south of tennant creek in the davenport ranges numerous small w prospects have been interpreted to be coeval with post tectonic granites of the devils suite at 1710 1720 ma new 40ar 39ar ages from muscovite selvages on wolframite bearing veins range from 1697 to 1703 ma these ages shift to 1711 to 1717 ma when recalculated using revised estimates for 40k decay constant and age of standard material bringing them into agreement with existing 207pb 206pb zircon ages for devils suite granitoids and consistent with field and geochemical interpretations these examples highlight the importance of considering systematic sources of uncertainty when comparing radiometric ages determined via different isotopic methods particularly in the proterozoic and archean parts of the timescale ongoing work to calibrate the 40ar 39ar and u pb based timescales will further enhance the application of multiple isotopic methods to constrain geological processes

timing of superposed volcanism in the proterozoic mount isa inlier australia metamorphosed proterozoic successions in the mount isa inlier contain a variety of volcanic suites interspersed at several intervals throughout the 30 km thick stratigraphic pile the u pb isotopic systematics of zircons crystallised in superposed felsic volcanic units allow detailed definition of the chronology of this volcanism and therefore of the major depositional cycles this supracrustal development spanned a 270 ma interval in the early to mid proterozoic new u pb zircon data from the earliest recognised volcanic cycle leichhardt metamorphics yield a pooled age for this event at 1867 5 ma a far northern segment of the same basement succession contains a geochemically similar felsic sequence 1852 7 ma old these results confirm the coevality between this felsic volcanism and nearby intrusive bodies of the same age the second major cycle of volcanism is 60 80 ma younger than the first this interval representing the maximum duration of the intervening regional unconformity dacitic and mafic volcanic rocks bottletree formation which unconformably overlie the western flank of the 1870 ma basement are dated in two areas at 1790 810 and 1808 1722 rhyodacites argylla formation on the eastern side of the basement have a similar age 1783 5 ma so for purposes of correlation and crustal evolution these two sequences flanking either side of the basement can be considered to comprise a single volcanic cycle subsequent cycles of felsic volcanism are recognised in stratigraphically younger sequences which include rhyolites with well resolved u pb zircon discordance patterns indicating ages of 1720 7 ma doherty formation and 1678 3 ma carters bore rhyolite the youngest volcanic cycle 1603 6 ma is evidenced by rhyolitic lavas in a part of the sequence previously considered to be much older at least 1740 ma recognition of such a young volcanic episode has an important economic bearing for it implies that rocks equivalent in age to the prospective pb zn rich mount isa group in the west ca 1670 ma could well exist in the eastern succession of the inlier

crustal architecture of basin inversion during the proterozoic isan orogeny eastern mount isa inlier australia abstract the folded and metamorphosed supracrustal rocks of the eastern margin of the mount isa inlier nw queensland australia provide a record of basin inversion processes at high temperatures from the foreland toward the core of a developing orogen deep water turbiditic metasedimentary rocks of the lower maronan supergroup ca 1 71 1 66 ga occupy the hanging wall of a crustal scale thrust the cloncurry overthrust which separates them from calc silicate and meta evaporitic rocks of the doherty formation ca 1 74 1 72 ga in the footwall during the early stages of the isan orogeny ca 1 60 1 58 ga the rocks of the maronan supergroup were expelled from their basin and thrust to the north northwest shortening was restricted to the hanging wall and produced a 10 km scale north to northwest verging nappe with a crumpled frontal zone of upright east west trending folds this morphology is consistent with a push from the rear mechanism of nappe emplacement with the simple shear component of deformation increasingly accommodated within the hanging wall down dip toward the core of the orogen metamorphism associated with this folding reached upper amphibolite facies temperatures at moderate to low pressures 4 6 kbar metamorphic zones in the hanging wall of the cloncurry overthrust were antiformally folded with continuing deformation the relationship between faulting folding and metamorphic zoning across the cloncurry overthrust are consistent with a thrust system despite that fact that younger rocks are juxtaposed over older basin inversion provides the best explanation for this phenomenon within this context metamorphism during the early stages of orogenesis can be explained by the inheritance of elevated geothermal gradients produced by lithospheric thinning and sediment loading during formation of the basin the overall orogenic architecture was controlled by a combination of thermal softening which focussed early deformation within the basin and weak zones at the base of the basin and the base of the crust this resulted in crustal scale thrusts of opposite vergence that partitioned upper crustal and lower crustal deformation

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

the synchronism of crustal thickening and high t low p metamorphism in the mount isa inlier australia 1 an example the central soldiers cap belt abstract in the soldiers cap group of the central eastern part of the early to middle proterozoic mount isa inlier tracing of mesoscopic structures has led to the recognition of an early nw closing fold nappe with an e w trending coaxially shortened frontal zone d1 folded around a n s trending upright syncline d2 an andalusite sillimanite prograde metamorphic event indicating a paleo geothermal gradient of approximately 50 c km occurred during early d2 times the coexistence of such moderately high paleo geothermal gradients and crustal thickening is common in the early to middle proterozoic mobile belts of northern australia crust mantle delamination has been suggested by etheridge et al 1987 to explain this coexistence here the thermal implications of this model and the additional thermal effects of waning crustal extension magmatic events and of the radioactive selfheating of the thickened crust are qualitatively discussed

the synchronism of crustal thickening and low pressure facies metamorphism in the mount isa inlier australia 2 fast convective thinning of mantle lithosphere during crustal thickening the mount isa inlier is like some other northern australian early to middle proterozoic fold belts characterized by an anti clockwise p t t path i e regional prograde low p facies metamorphic assemblages grew during crustal thickening p t and were overprinted by a phase of essentially isobaric cooling p 0 t in this study two possible models for the synchronism of regional low p facies metamorphism and crustal thickening are quantitatively evaluated using a one dimensional finite difference technique 1 extreme elevation of isotherms immediately before the thickening event due to e g lithospheric extension magmatic activity or crust mantle delamination and 2 crustal thickening accompanied by convective thinning of the mantle lithosphere only the latter results in prograde low p facies metamorphism during crustal thickening it is the preferred model for the mount isa inlier

petrology geochronology and isotope geochemistry of the post 1820 ma granites of the mount isa inlier mechanisms for the generation of proterozoic anorogenic granites four distinct episodes of granitic intrusion occurred in the mount isa inlier at 1820 1800 ma 1760 1740 ma 1700 1670 ma and 1560 1480 ma combined these granites cover at least 4700 km2 and they contrast in composition and intrusive style with an earlier major felsic magmatic event which occurred between 1870 and 1840 ma post 1820 ma granite batholiths are of two types large major coarse grained anorogenic i type intrusives and small minor microgranites which are probably s type the i type granites are all pink to red colour and most contain fluorite rapakivi textures are common they have a high ratio of fe mg and are more enriched in tio2 zr nb y and u than the 1870 1840 ma felsic igneous rocks the post 1820 ma i types are interpreted to have formed by small degrees of partial melting of a mafic underplate the smaller microgranites are relatively k2o enriched and are sometimes muscovite bearing they are interpreted to be derived from proterozoic feldspathic metasediments the first three episodes of granite magmatism are coeval with major extensional sedimentary sequences but they are generally spatially displaced from the maximum area of surface rift development this separation is predicted by the asymmetric detachment model of continental extension the tectonic relationship of the youngest granite episode is not clearly defined the 143nd 144nd initial ratios of the post 1820 ma granites fall into two groups one having positive relatively primitive nd values of 1 8 to 3 5 the other having negative nd values ranging from 2 9 to 3 7 samples with positive nd values are interpreted as partial melts of mafic sources formed post 1870 ma whilst the group with negative nd values are derived from significantly older sources formed probably around 2300 2000 ma the sources of these granites are produced by significant underplating events that may occur during either extension or compression

turbulent mixing and hydraulic control of abyssal water in the samoan passage 1 we report the first direct turbulence observations in the samoan passage sp a 40 km wide notch in the south pacific bathymetry through which flows most of the water supplying the north pacific abyssal circulation the observed turbulence is 1000 to 10 000 times typical abyssal levels strong enough to completely mix away the densest water entering the passage confirming inferences from previous coarser temperature and salinity sections accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence specifically the flow accelerates substantially at the primary sill within the passage reaching speeds as great as 0 55 m s 1 a strong hydraulic response is seen with layers first rising to clear the sill and then plunging hundreds of meters downward turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill in addition to the primary sill other locations along the multiple interconnected channels through the samoan passage also have an effect on the mixing of the dense water in fact quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers suggesting that abyssal mixing depends sensitively on bathymetric details on small scales

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

fluid inclusion and isotopic c h o s and pb constraints on the origin of late mesozoic vein type w mineralization in northern guangdong south china abstract northern guangdong of south china occurs many large tungsten deposits including shirenzhang meiziwo and yaoling quartz vein type wolframite deposits in this study we carried out a detailed study on fluid inclusions and c h o s pb isotopic analyses of mineral separates for these three deposits in order to resolve the origin and evolution of ore forming fluids and the ore deposition mechanism the ore veins in these deposits present a similar mineral paragenesis that could be divided into three stages i pre ore stage ii syn ore stage and iii post ore stage the pre ore stage is the magmatic hydrothermal transition stage which is marked by fluid exsolution from the highly fractionated granites accompanied with potassic alteration and greisenization the syn ore stage can be further divided into two stages silicate oxide stage stage ii 1 that is characterized by significant tin tungsten mineral deposition and sulfide stage stage ii 2 that is characterized by abundant sulfide minerals deposition the post ore stage is dominated by quartz and fluorite the three deposits show similar temperature and salinity variations for syn ore and post ore stages in the shirenzhang deposit the fluid temperature and salinity range from 239 to 301 c and 1 4 to 8 7 wt nacl equiv in stage ii 1 from 206 to 256 c and 1 4 to 7 0 wt nacl equiv in stage ii 2 and from 186 to 236 c and 1 4 to 8 6 wt nacl equiv in stage iii in the meiziwo deposit the fluid temperature and salinity range from 242 to 310 c and 1 1 to 8 6 wt nacl equiv in stage ii 1 from 196 to 252 c and 1 4 to 8 1 wt nacl equiv in stage ii 2 and from 188 to 234 c and 1 2 to 7 3 wt nacl equiv in stage iii in the yaoling deposit the fluid temperature and salinity range from 230 to 304 c and 1 2 to 9 0 wt nacl equiv in stage ii 1 from 203 to 258 c and 1 4 to 6 5 wt nacl equiv in stage ii 2 and from 184 to 231 c and 1 6 to 8 3 wt nacl equiv in stage iii the ore forming fluids in the three deposits belong to a medium temperature low salinity h2o nacl system with trace amounts of volatile components including co2 ch4 and n2 the c h o isotope data 13cco2 18 9 to 6 7 18owater 1 1 to 6 3 dh2o 78 to 55 indicate that the ore forming fluids were mainly magmatic water which might be modified by fluid rock interaction and mixing with meteoric water sulfur 34s 7 0 to 0 3 and pb isotope data 206pb 204pb 18 531 18 693 207pb 204pb 15 728 15 753 208pb 204pb 38 902 39 081 suggest that the ore metal and sulfur are of magmatic origin the involvement of both organic matter in metasedimentary rocks during fluid rock interaction and oxidized meteoric water may have been responsible for the negative 34s and 13c values fluid boiling fluid rock interaction and minor input of meteoric water might have been effective factors for the precipitation of wolframite cassiterite and scheelite the sulfide precipitation may have resulted from cooling and dilution of magmatic fluids by mixing with meteoric water

the cloncurry lineament geophysical and geological evidence for a deep crustal structure in the eastern succession of the mount isa inlier the cloncurry lineament is a major feature evident from wavelet processed magnetic and gravity potential field data over the mount isa inlier eastern succession it is laterally continuous over 200 km of strike length and is inferred from the gravity data to extend to at least 30 km depth it delineates the contact between two major paleoproterozoic sedimentary sequences and is spatially associated with a number of mineral deposits including cannington ag pb zn ernest henry cu au and many smaller cu au deposits mapping and magnetic forward modelling along several profiles perpendicular to the cloncurry lineament suggests that it corresponds to the eastern margin of a 5 10 km wide deformation zone within the calc silicate dominated doherty formation the western boundary of this deformation zone is a distinct narrow fault zone the cloncurry fault the deformation zone consists of multiple generations of shearing and faulting accompanied by metasomatic alteration large variations in the magnetic response within the deformation zone can be modelled as sub vertical brecciation zones breccia types range from highly magnetic actinolite albite magnetite breccias through to late stage non magnetic quartz k feldspar hematite breccias the long lived history of multiple reactivation strike continuity width and vertical extent of the deformation zone suggest that it is a major crustal structure the structure may have originated during basin formation and later acted as a fluid pathway for alteration and base metal mineralization

chronology of magmatism skarn formation and uranium mineralization mary kathleen queensland australia uranium mineralization is hosted in a metasedimentary sequence intruded by felsic and mafic bodies in which the zircon u pb systems are 1 730 to 1 740 m y old felsic dikes characterized by exceptionally high uranium contents crystallized at essentially the same time 1 737 or 15 m y primary rb sr systematics in these igneous rocks are profoundly disturbed u pb discordia establish age of uranium mineralization as 1 550 or 15 m y derivation of uranium protore in the host rocks prior to regional metamorphism and 1 550 m y uraninite formation was probably by ca 1 740 m y hydrothermal activity modified journal abstract

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

the intra cratonic evolution of the central eastern mount isa inlier northwest queensland australia the central eastern part of the mount isa inlier can be subdivided into three major tectono stratigraphic domains from west to east i the mitakoodi anticlinorium a tight northeast plunging composite anticline which is developed within subaerial to shallow marine continental rift facies lithologies ii the marimo staveley block in which tight to isoclinal folds are developed within a marine shelf iii the central part of the soldiers cap group belt which is dominated by a series of metaturbidites and which hosts a fold nappe refolded about n s to ne sw trending axes the transition from subaerial sediments in the west through marine shelf sediments to turbidites in the east has been one of the main arguments used by advocates for a proterozoic continental margin at this locality here however it is argued that folding and differential uplift along late fault and shear zones led to the juxtaposition of different stratigraphic levels and to the apparent deepening of palaeobasins from subaerial in the west to deep water in the east the age of the soldiers cap group is poorly constrained but it is likely to be older than that of rocks cropping out in the marimo staveley block the marimo staveley block lithologies overlie those of the mitakoodi anticlinorium so here time transgression rather than eastward deepening may be deduced thus a palinspastic reconstruction based on an eastwards progressive deepening is not justified this adds to other evidence that refutes the continental margin model previously proposed for the eastern margin of the mount isa inlier instead the lithostratigraphic sequences reflect several stages of intra continental rifting all without formation of oceanic crust the pervasive post depositional deformational event is associated with regional low p facies metamorphism which indicates a further ensialic history

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

vein type tungsten deposits of china and adjoining regions abstract taking the south china tungsten province as representative this paper deals systematically with the geological setting geometry paragenesis wallrock alteration and geochemical characteristics of asian vein type tungsten deposits on the basis of data acquired since the 1970 s the regional metallogenic conditions and the genesis of the deposits are discussed the large scale distribution of vein type tungsten deposits in asia is inferred to be controlled by tungsten enriched formations multi period and multi stage granitic activity and favourable tectonic environment the deposits are not only controlled by magmatic differentiation but also by the overall evolution of the continental crust

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

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

genetic relationship between silver lead zinc mineralization in the wutong deposit guangxi province and mesozoic granitic magmatism in the nanling belt southeast china more than 50 of the world s total reserves of tungsten are in china and most tungsten deposits are located in the nanling range in southeast china this study explores the potential genetic relationship between tungsten tin w sn mineralization and shallower ag pb zn deposits in the nanling range based on data from the wutong deposit guangxi province the lead oxygen carbon sulfur and strontium isotopic compositions of minerals at wutong indicate that a single crustal derived fluid was responsible for mineralization wutong likely formed at relatively low temperatures 200 300 c and low pressures as indicated by the similarity between homogenization temperatures of fluid inclusions and those estimated from s isotopic compositions of minerals the hubnerite age 92 3 104 4 ma indicates that the wutong mineralization is likely related to nearby late yanshanian cretaceous s type granites derived from proterozoic crust this mineralization event coincides with the last w sn mineralization event and the cretaceous peak of mineralization in the nanling range

precise molybdenite re os and mica ar ar dating of the mesozoic yaogangxian tungsten deposit central nanling district south china the yaogangxian deposit in the central nanling region south china consists of vein type ore bodies hosted in cambrian to jurassic strata and mesozoic granitic intrusions wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz feldspar phlogopite and muscovite we have carried out molybdenite re os and phlogopite and muscovite 40ar 39ar dating to better understand the timing and genesis of mineralization re os dating of eight molybdenite samples yielded model ages ranging from 152 0 3 5 to 161 1 4 5 ma with an average of 156 0 ma the re os analyses give a well defined 187re 187os isochron with an age of 154 9 2 6 ma mswd 2 4 hydrothermal phlogopite and muscovite display extremely flat 40ar 39ar age spectra phlogopite yields a 40ar 39ar plateau age of 153 0 1 1 ma whereas muscovite yields a plateau age of 155 1 1 1 ma both 40ar 39ar ages are in good agreement with the re os ages placing the timing of tungsten mineralization at about 154 ma this age is consistent with the field relationships our new data when combined with published geochronological results from other major deposits in this region suggest that large scale w sn mineralization occurred throughout the central nanling region in the late jurassic

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

a metasomatic origin for the iron oxide au cu starra orebodies eastern fold belt mount isa inlier the starra ironstone hosted au cu deposit in northwest queensland australia displays evidence of a polyphase history of late post metamorphic structurally controlled metasomatism the mineralisation is hosted within several massive ironstone units which occur at a lithological contact of metamorphosed sandstone and siltstone sequences and foliated chlorite biotite magnetite schists the ironstones strike north south and occur as discontinuous lenses that cross cut the host rocks in places magnetite ironstone textures are generally massive in appearance but relict deformed breccia and foliation textures are locally preserved sulphide ore textures are more brittle and consist of fine pervasive brecciation of the ironstones and host rocks petrological and textural observations support a hydrothermal origin for both ironstone and mineralisation three dominant post peak metamorphic paragenetic stages of alteration and mineralisation are recognised 1 early widespread na ca metasomatism consisting of albite quartz actinolite scapolite 2 localised k fe metasomatism consisting of biotite magnetite hematite quartz pyrite and 3 the mineralising stage consisting of anhydrite calcite hematisation of magnetite pyrite chalcopyrite gold and extreme chloritisation of biotite the na dominant alteration occurs throughout the eastern fold belt and for at least 50 km to the east and 150 km to the north of starra the ironstones were products of localised fe metasomatism associated with shearing and brecciation of previously na metasomatised host rocks gold and copper mineralisation resulted from the interaction of magnetite a late oxidising fluid and an increase in ph this produced variable hematisation of magnetite and caused the solubility of chloride complexed gold to decrease dramatically high gold copper ratios can be explained in terms of the solubility differences of gold and copper in the thermochemical conditions that existed at starra

petrogenesis of ore forming granites with implications for w mineralization in the super large shimensi tungsten dominated polymetallic deposit in northern jiangxi province south china abstract the shimensi deposit is a newly discovered super large tungsten w polymetallic deposit in northwestern jiangxi province south china in this paper we reported new zircon u pb ages for the ore bearing porphyritic biotite granite 151 3 2 8 ma and fine grained granite 150 1 1 2 ma and 150 7 2 4 ma which represent the oldest mesozoic granitic magmatism reported at shimensi our ore bearing granite porphyry ages 142 7 2 3 ma and 143 3 1 5 ma have better constrained the timing of the jurassic early cretaceous ore forming magmatism geochemically the shimensi granitoids are strongly peraluminous with high contents of sio2 70 7 78 7 wt and k2o 4 0 wt p2o5 rb ta and u and low contents of ti sr ba and hrees the rocks also show positive p2o5 vs sio2 correlation suggesting a highly fractionated s type affinity different from most w bearing granites in the jiangxi region the calculated zircon ehf t 10 0 to 2 4 two stage model ages 1350 1824 ma and 18ozircon values 6 40 to 8 87 of the shimensi granites suggest that the latter were mainly derived from the partial melting of heterogeneous source rocks probably with combined contributions from the neoproterozoic shuangqiaoshan group metamorphic rocks and jiuling granodiorite the calculated zircon eu eu 0 05 0 58 mean 0 20 and ce4 ce3 3 83 96 49 mean 20 99 ratios of the granites indicate very low oxygen fugacity fo2 for the ore forming magmas a highly favorable factor for w mineralization potentially w and cu rich source rocks and highly fractionated and reducing granitic magmas were important factors that triggered super large w mineralization at shimensi

proterozoic extensional deformation in the mount isa inlier queensland australia the earliest of four distinct phases of deformation recognized in the central part of the proterozoic mount isa inlier involved brittle extensional faulting at shallow crustal levels extensional faulting produced stacks of imbricate fault slices listric normal faults and characteristic tourmalinerich breccias structures belonging to this phase occur over a large part of the inlier and indicate an important phase of basin forming crustal or lithospheric extension at 1750 1730 ma late intense ductile deformation and tight folding of the imbricate systems destroyed part of these older structures and obscures their existence in many parts of the inlier

multiple mesozoic mineralization events in south china an introduction to the thematic issue mesozoic mineral deposits in south china include world class deposits of w sn and sb and those that provide the major sources of ta cu hg as tl pb zn au and ag for the entire country these deposits can be classified into polymetallic hydrothermal systems closely related to felsic intrusive rocks sn w mo granites cu porphyries polymetallic and fe skarns and polymetallic vein deposits and low temperature hydrothermal systems with no direct connection to igneous activities mvt deposits epithermal au and sb deposits recent studies have shown that they formed in the triassic indosinian jurassic cretaceous early yanshanian and cretaceous late yanshanian stages indosinian deposits include major mvt pb zn ag deposits and granite related w sn deposits early yanshanian deposits are low temperature sb au and high temperature w sn and cu porphyry types many late yanshanian deposits are low temperature au as sb hg and u deposits and also include high temperature w sn polymetallic deposits the formation of these deposits is linked with a specific tectonothermal evolution and igneous activities this special issue brings together some of the latest information in eight papers that deal with the origins and tectonic environments of mineral deposits formed in these stages we anticipate that this issue will stimulate more interests in these ore deposits in south china

the crustal scale architecture of the eastern succession mount isa the influence of inversion the three dimensional crustal architecture of the eastern part of the mount isa inlier is investigated from serial cross sections constructed using geological map data revised chronostratigraphy gravity magnetics worms multiscale wavelet edges of potential field data and seismic data the top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 ma these rocks constitute the mount isa eastern succession and they were intruded by mafic felsic plutons dykes and sills of various ages before and during the isan orogeny ca 1 6 1 5 ga the eastern succession overlies a felsic metamorphic basement which in turn sits on a tonalitic gabbroic lower crust the depositional basin architecture for the eastern succession was controlled by major n s trending structures that penetrated the lower crust and accommodated e w extension these structures also underlie major upper crustal structures such as the mitakoodi culmination and snake creek anticline that were formed by contraction in the isan orogeny positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier and governs its architecture at the crustal scale inversion involved reactivation of basement penetrating structures which localised contractional structures in the cover sequences above as well as influencing pluton emplacement the felsic metamorphic basement may have been penetratively deformed during inversion the spatial association between the basin controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction or that much of the eastern succession has remained essentially parauthochthonous relative to the basement

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses

re os isotopic chronology of molybdenites from hongling tungsten deposit of guangdong province and its geological significance located in the middle part of reshui composite pluton the hongling tungsten deposit is a medium size tungsten ploymetallic deposit tungsten mineralizations in this deposit include quartz vein wolframite type and altered granitic scheelite type rhenium and osmium isotopic dating of 8 molybdenite samples from the hongling quartz vein type tungsten deposit has yielded a precise isochron age of 159 1 1 5 ma and a weighted average model age of 158 8 0 88 ma the intercept of isochron is 0 010 0 041 suggesting that there is no measurable normal os in these molybdenite samples statistics of precise ore forming ages of wsn deposits from eastern and central nanling region show that ore forming ages are concentrated mostly in the range of 150 160 ma corresponding to the main periods of large scale granitic emplacement in this region the results obtained by the authors show that there is no distinct age gap between tungsten mineralization and granite intrusion in nanling region

evidence for early extensional tectonics in the proterozoic mount isa inlier australia in the proterozoic mount isa inlier queensland australia structures belonging to an early brittle deformation event are overprinted and strongly disturbed by a phase of ductile east west shortening many of these early structures have been attributed to thrusting but data from the southern part of the inlier provide new evidence for extensional faulting analysis of the internal geometry of imbricates can provide a tool for the reconstruction of these strongly reworked precambrian fault belts