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

the combined effects of continents and the 660 km depth endothermic phase boundary on the thermal regime in the mantle abstract we explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km depth in simple two dimensional convection calculations phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection our study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary we show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering unlike the free slip case where the surface heat flow decreases substantially with increasing magnitude of the clapeyron slope surface heat flow is shown to be almost independent of the clapeyron slope for mixed boundary condition cases although very different when not layered models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the clapeyron slope we also calculate the critical boundary layer rayleigh number as a measure of the thermal resistance of the surface boundary layer our results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no slip cases respectively we find that even for purely basally heated models the mantle becomes significantly subadiabatic in the presence of partial continental coverage this is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells

textural control on gold distribution in as free pyrite from the dongping huangtuliang and hougou gold deposits north china craton hebei province china article i nfo pyrite from the dongping huangtuliang and hougou gold deposits hebei province china was investigated using a combination of ore microscopy including back scattered imaging and in situ laser ablation inductively coupled plasma mass spectroscopy la icp ms a range of elements including au te ag pb bi cu co ni and as was analyzed with the aim to constrain the textural controls and influence of superimposed deformation on the distribution of invisible gold in as free pyrite thedatasetshowsexcellentcorrespondence betweenmeasuredhighgoldvaluesandthreekeytexturalcriteria i areasofclusteredtellurideinclusions especiallyindongping ii microshearingandfracturing brecciation dongping and huangtuliang and iii pyrite recrystallization huangtuliang invisible gold is only at trace levels in the hougou and gold bearing telluride minerals are not observed this is considered to result from larger scale remobilization of gold during advanced brecciation and recrystallization of pyrite pyrite grains containing clustered inclusions have by far the highest gold concentrations up to 1 wt with consistent values of thousands of ppm au interpretation of textures and la icp ms data infers that in these areas the distribution of telluride inclusions extends pervasively from micron to nanoscale i e as fields of nanoparticles local employment of coupled dissolution reprecipitation reaction between pyrite and fluids driving the k metasomatic alteration dated at 150 ma is invoked to explain such patterns syn deformational grain scale mobilization of gold and other elements took place leading to a portion of the gold being recrystallized within fractures and microshears the deformational event leads to gold enrichment in the huangtuliang pyrite where brittle fracturing brecciation has been followed by a resorption and recrystallization in the latter case foam textured pyrites again without as can contain up to hundreds of ppm gold theinterpreted sequenceof pyritepost depositional eventsis inaccordancewith the protracted magmaticevolution of the area starting with variscan alkaline intrusions 390 ma and reactivated during the yanshanian orogeny 150 ma thestudyshows i thatas freepyritecanreadilycontainsignificantamountsofinvisiblegold bothasnanoparticles

convection under a lid of finite conductivity in wide aspect ratio models effect of continents on the wavelength of mantle flow 1 mantle convection in the earth and mars are dominated by large scale flow and this can be due at least in part to the effect of thermally insulating continent like heterogeneities indeed the presence of a finitely conducting lid on top of a convective isoviscous fluid at infinite prandtl number induces the formation of a zone of hot upwelling centered beneath the lid and a large horizontal cellular circulation on each side of the lid in a previous paper this lateral cellular circulation was limited in size by the dimensions of the model we now use very large aspect ratio models in order for this circulation to be unhampered the large scale circulation consists of the constructive superposition of hot and cold plumes attracted to and expelled from respectively the conductive lid the width of the cellular circulation increases with the rayleigh number of the fluid and we propose a scaling law to explain this expansion scaling laws for the velocity of the plumes and heat transfer by this large scale circulation are also developed although the model used is much simpler than actual planetary mantles the driving mechanism for the large scale circulation must operate in a similar fashion important implications in terms of heat transfer can therefore be predicted

modelling of the april 5 2003 stromboli italy paroxysmal eruption from the inversion of broadband seismic data abstract on april 5 2003 one of the largest eruptions in the last decades was observed at stromboli volcano italy the eruption occurred in a period of increased volcanic activity following a first explosion in december 2002 which interrupted the typical moderate strombolian behaviour we present an exhaustive analysis of the available broadband seismic data and relate them to the observed eruption phases prominent features of the seismic signals include an ultra long period signal starting a few tens of seconds prior to the explosive eruption as well as a strong energetic signal a few seconds after the onset of the eruption both signals are not exactly synchronized with the other geophysical observations we present a detailed study of those signals using spectral and particle motion techniques we estimate eruption parameters and seismic source characteristics by different inversion approaches results clearly indicate that the paroxysmal eruption was triggered by a shallow slow thrust faulting dislocation event with a moment magnitude of m w 3 0 and possibly associated with a crack that formed previously by dike extrusion at least one blow out phase during the paroxysmal explosion could be identified from seismic signals with an equivalent moment magnitude of m w 3 7 and is represented by a vertical linear vector dipole and two weaker horizontal linear dipoles in opposite direction plus a vertical force

convective heat transfer as a function of wavelength implications for the cooling of the earth 1 attempting to reconstruct the thermal history of the earth from a geophysical point of view has for a long time been in disagreement with geochemical data the geophysical approach uses parameterized models of mantle cooling the rate of cooling of the earth at the beginning of its history obtained in these models is generally too rapid to allow a sufficient present day secular cooling rate geochemical estimates of radioactive element concentrations in the mantle then appear too low to explain the observed present mantle heat loss cooling models use scaling laws for the mean heat flux out of the mantle as a function of its rayleigh number of the form q ra b recent studies have introduced very low values of the exponent b which can help reduce the cooling rate of the mantle the present study instead focuses on the coefficient c in the relation q cr a b and in particular on its variation with the wavelength of convection the heat transfer strongly depends on the wavelength of convection the length scale of convection in earth s mantle is that of plate tectonics implying convective cells of wide aspect ratio taking into account the long wavelength of convection in earth s mantle can significantly reduce the efficiency of heat transfer the likely variations of this wavelength with the wilson cycle thus imply important variations of the heat flow out of the earth on a intermediate timescale of 100 ma which renders parameterized models of thermal evolution inaccurate for quantitative predictions

mantle dynamics and the heat flow into the earth s continents reducedcontinental heat flow defined as the measured heat flow minus that due to radiogenic sources in the upper crust initially decreases with time after the last thermotectonic event in a region1 but then tends to stabilize at a roughly constant value which is higher than would be predicted by a conductive cooling model fig 1 here we argue that this behaviour results from the influence that continents have on mantle heat loss the finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale as is oceanic crust allows stable continental blocks to limit the local heat flux out of the mantle numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents causing lateral temperature variations at their base that parallel those in the mantle just below this tends to establish a constant flux of heat from the mantle into the continents which accounts for the trend in reduced heat flow

non adiabaticity in mantle convection seismic observations indicate that earth s lower mantle is homogeneous as revealed by smooth depth variations of the bulk sound velocity and bullen s inhomogeneity parameter being close to one here we show with 3d spherical convection simulations that it should also be non adiabatic because a significant proportion of mantle heat sources is internal the computer simulations predict non adiabaticity of 100 300 degrees for the lower mantle geotherm with 1 01 implying that the temperature drop across the core mantle boundary exceeds 1000 degrees

re os dating of molybdenites from au mo deposits in xiaoqinling gold ore district and its geological significance molybdenum mineralization with economic value was recently discovered in the dahu gold deposit and its nearby quanjiayu gold bearing quartz veins in the xiaoqinling gold ore district henan province for the purpose of studying the chronology of the molybdenum mineralization in this district the authors purified molybdenite from the dahu gold deposit and the quanjiayu gold bearing quartz veins re os dating of these molybdenite minerals yielded modal ages of 223 0 2 8 ma 223 7 2 6 ma and 232 9 2 7 ma for the three samples from dahu and 129 1 1 6 ma and 130 8 1 5 ma for the two samples from quanjiayu these results indicate that au mo mineralization in the xiaoqinling gold ore district took place not only at the yanshanian stage but also at the indo chinese stage different from things of jinduicheng huanglongpu and other mo deposits the molybdenites from the dahu and quanjiayu deposits have a very low content of re showing a crustal source for ore forming materials

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

phase transition modulated mixing in the mantle of the earth and discussion recently published a priori analyses of the mantle convection process demonstrate that the influence of the endothermic phase transformation at 660 km depth could be profound and such as to enforce a strongly intermittent style of circulation in this phase transition modulated regime mass transfer across the 660 km horizon is inhibited leading to the formation of an internal thermal boundary layer that straddles the phase change interface this thermal boundary layer is itself episodically disrupted by convective instability leading to the development of intense avalanches of cold material from the transition zone into the lower mantle the consequences of such a circulation regime to the understanding of trace element geochemistry and the supercontinent cycle may be extremely important equally important are the implications for mantle viscosity since a circulation of this type would appear to allow reconciliation of a wide range of geodynamic data with a single viscosity model the relevant data include convection related observables such as the aspherical geoid and postglacial rebound related observables such as relative sea level histories and certain anomalies in earth rotation including the so called non tidal acceleration of rotation and the ongoing wander of the rotation pole towards greenland at a rate near 0 95 degrees per million years

a hydrothermal origin for the large xinqiao cu s fe deposit eastern china evidence from sulfide geochemistry and sulfur isotopes abstract the xinqiao cu s fe deposit in the tongling ore district middle lower yangtze river valley metallogenic belt mlyb eastern china is located along the northern margin of the yangtze craton the stratiform and skarn type xinqiao mineralization comprises five stages namely the early skarn stage i garnet and diopside late skarn stage ii epidote dominated iron oxides stage iii hematite and magnetite colloform pyrite stage iv and quartz sulfides stage v there are three pyrite types at xinqiao i e colloform py1 stage iv fine grained py2 from py1 recrystallization stage v and coarse grained py3 stage v pyrites scanning electron microscope sem imagery for py1 reveals that they are cubic microcrystalline pyrite aggregates and the eds and xrd data indicate that some py1 contain minor siderite impurities electron microprobe analysis empa and la icp ms geochemical data demonstrate that the three pyrite types have relatively high fe s ratios and distinctly high mn cu and as concentrations compared to py2 and py3 py1 has higher pb bi and ag but lower co ni se cd te and au ratios of fe s 0 837 to 0 906 se te 2 39 to 14 50 and co ni 0 67 to 4 67 of the xinqiao pyrites resemble typical hydrothermal pyrites 34 s cdt of py1 0 6 to 2 7 average 0 58 py2 1 8 to 2 5 average 2 1 and py3 1 9 to 4 4 average 3 5 are close to those of the xinqiao skarn type orebodies 1 3 to 4 1 but distinct from those of the upper carboniferous huanglong formation limestone 9 5 to 15 4 suggesting that the three pyrite types especially py1 were genetically linked to the yanshanian jurassic cretaceous magmatic hydrothermal events with py1 probably reflecting rapid crystallization during fluid mixing we interpret that the xinqiao stratiform mineralization may have been associated with the jitou quartz diorite stock as may be the case also for the skarn type mineralization hosted in the contact between the yanshanian jitou stock and the lower permian qixia formation limestone overall the xinqiao cu s fe mineralization may have been generated by the jurassic cretaceous tectono thermal event in eastern china

pyrite textures and trace element compositions from the granodiorite hosted gold deposit at jonnagiri eastern dharwar craton india implications for gold mineralization processes

linking mesozoic lode gold deposits to metal fertilized lower continental crust in the north china craton evidence from pb isotope systematics abstract the source s of lode gold deposits formed in precambrian cratons related to accretion collision and cratonic reactivation formerly attributed to either supercrustal volcano sedimentary sequences or deep crustal sub crustal origin remain controversial largely because gold deposits are spatially related to metamorphosed rocks but geochemical data somewhat indicate a poorly understood deep source reconciling such conflicts is important to better understand the main factor controlling the formation of ore deposits and their genetic link with specific tectonic settings giant late mesozoic lode gold provinces in the north china craton ncc were formed ca 1 7 ga later than cratonization metamorphism and contemporaneous with intensively felsic to mafic magmatism related to cratonic reactivation in this study we conduct a comprehensive pb isotope study on major gold deposits from the eastern yanshan belt northern margin of the ncc in order to constrain the source s of gold we attempt to map regional pb isotope variations of lower continental crust lcc and develop a two stage quantitative model punctuated by three prominent geological events at 2 80 ga 1 85 ga and 0 16 ga to reproduce time integrated pb isotopic signatures of deep seated lithospheric reservoirs to make a comparison with pb isotopic signatures of the gold mineralization gold bearing pyrites within different types of host rocks have relatively uniform pb isotopic ratios which are significantly different to high grade metamorphosed host rocks but similar to those of spatially associated late mesozoic granitic rocks our data show that the pb isotopic signatures of gold deposits vary consistently with presumed regional pb isotopes of the lcc during the late mesozoic lead isotopic heterogeneity of the lcc was likely caused by underplating of mafic magmas derived from mantle sources during underplating highly chalcophile elements e g au ag and cu were concentrated at the base of the lcc due to sulfide saturation from mafic magmas integrating petrological geochemical geochronological and considering chalcophile element solubility we propose a new genetic model to describe the formation of late mesozoic gold deposits in the ncc 1 early formation of sulfide bearing cumulates with high au cu ratios during magma differentiation at the base of the lcc 2 subsequent fluid fluxed remelting of these cumulates at the onset of lithospheric extension and release and ascent of ore forming fluids to the site of precipitation considering the metallogenic characteristics of other au ore forming systems we suggest that metal fertilization in deep seated reservoirs and subsequent tectonic decompression are important factors controlling the development of au rich ore deposits worldwide this study demonstrates how pb isotope can be employed to trace source s of gold deposits

electron back scattered diffraction and la icp ms analysis of pyrite from the dahu lodegold deposit southern north china craton insights into geochemistry and distribution of trace element connection to microstructure of pyrite abstract the dahu lodegold deposit is situated in the xiaoqinling district hosted in the east west trending shear zones and characterized by auriferous quartz vein type based on the morphology and paragenetic assemblage three generations of pyrite termed as first generation pyrite py1 py2 and py3 can be classified the py2 can be further identified as brittle fractured pyrite py2 b and ductile deformed pyrite py2 d the different generations of pyrites feature distinct contents and distribution patterns of au and other trace elements the coarse grained euhedral py1 disseminating in the early formed pyrite poor quartz veins contained negligible to extremely low levels of au and associated trace elements probably indicating a slow growth rate from a hydrothermal fluid unsaturated with respect to fe s and au and other trace elements such as cu pb bi and te during the early orestage py2 occurring as laminations in the auriferous quartz veins consistently contained significant amounts of au bi and te with a strong positive correlation between these elements probably indicating a rapid growth rate and concentrated precipitation process of pyrite due to a hydrothermal fluid supersaturated with respect to fe s au and other trace elements during the pyrite rich auriferous quartz veining py3 which precipitated in the polymetallic sulfide orestage contained a small amounts of ag and a dramatically decreased content of au bi te and ratios of bi ag the results of electron back scattered diffraction analysis on the py2 revealed that py2 b was characterized by predominantly brittle deformation with a limited local plastic deformation by dislocation glide on a simple slide system whereas the py2 d displayed a significant crystal plastic deformation through a lattice rotation on two separate 1 0 0 axes and local dislocation creep microscopic petrography and laser ablation inductively coupled plasma mass spectrometry analyses revealed that the brittle fracturing dominated py2 b generally displayed homogenous distribution patterns of au and other trace elements and scarce visible gold grains whereas the plastically deformed py2 d generally exhibited an inhomogeneous distribution pattern of trace element and commonly contained fine grained irregular shaped visible native gold and other sulfide inclusions in the low angle boundaries and dislocation sites within py2 d possibly suggesting the significant influence of ductile deformation on remobilization and reprecipitation of au and associated trace elements once locked in py2 the corrosion of au te bi bearing py2 by the late polymetallic sulfide assemblage led to a liberation of invisible au and other trace elements and reprecipitation of visible native gold and other bi pb minerals the results indicate that brittle deformation exhibited no essential effects on the crystal structure and chemical component of pyrite whereas the plastic deformation can cause the readjustment of crystal lattices and liberation of impurities e g au te bi and cu from the crystal lattices of pyrite the plastic deformation and dissolution remobilization of pyrite are the important mechanisms responsible for the visible discrete gold grain formation in auriferous quartz veins macroscopically a preferred constant e w extending orientation and typical lamination structure of the auriferous quartz veins suggest a channeled fluid flowing along the closed and connected fractures under a syn mineralization s n oriented extensional tectonic regime related to the xiaoqinling metamorphic core complex here we report that the microstructures of ductile deformation and brittle fracturing of the auriferous quartz veins are probably indicative of the continuous shearing activation of the ore hosting normal fault

mantle convection models of continental collision and breakup incorporating finite thickness plates abstract a two dimensional numerical model has been developed to study mantle convection during continental collision and breakup the model incorporates rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness we systematically investigate the influences of continental width diffusivity thickness and internal heating on continental collision and breakup in addition we consider the influence of different degrees of internal heating in the mantle for each case considered we model a pair of identical continents being carried towards a site of plate convergence by underlying counter rotating mantle convection cells the continents collide at the mid plane of the model to form a motionless rigid conducting supercontinent whereas oceanic plate material continues to recycle through the mantle we find that changes in the mechanical boundary conditions at the upper surface are important factors in initiating flow reversals below the supercontinent more generally our findings based on 144 models are that the following factors favour the initiation of flow reversals below the supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and continental crustal heating furthermore lower percentages of internal heating in the mantle are necessary to sustain and promote the subcontinental flow reversals in our models

application of la icp ms analysis of arsenopyrite to gold metallogeny of the meguma terrane nova scotia canada abstract investigation of gold metallogeny in the paleozoic meguma terrane canada is conducted through la icp ms analysis of arsenopyrite collected from eight slate belt style vein gold deposits using a novel approach integrating elemental distribution maps and their derived elemental paragenesis with multi element binary plots the data reveal two distinct gold events 1 an early event characterized by a co ni mo sb se elemental association related to initial growth of arsenopyrite that reflects the presence of invisible gold 10 ppm and 2 a second event spatially associated with late fracture sets that is characterized by an al ti v mn element association and reflects either remobilization or upgrading of primary invisible gold and is manifest as visible gold the results of this study indicate a complex and protracted history of gold mineralization which has important ramifications for the meguma gold deposits as well as other orogenic gold districts globally in the case of the meguma terrane it involves an initial gold event that is followed by element mobilization and in the case of precious metals au ag an upgrading through a zone refining process in addition the variable coupling and decoupling of elements is only revealed using in situ derived la icp ms data

molybdenite re os dating and la icp ms trace element study of sulfide minerals from the zijinshan high sulfidation epithermal cu au deposit fujian province china abstract the zijinshan cu au deposit located in fujian province is the largest high sulfidation epithermal hse deposit in southeastern china and is usually regarded as a major part of the porphyry cu system in the zijinshan ore field molybdenite samples collected from the cu mineralization zone yield a first weighted mean re os age of 111 31 0 70 ma which is explained as the time of dickite alunite alteration combining the newly reported muscovite 40ar 39ar and zircon u pb ages 113 ma the mineralization of zijinshan is likely to initiate before ca 110 ma this result is obviously older than the re os age of the adjacent luoboling porphyry cu mo deposit 105 ma pyrite chalcopyrite bornite digenite and covellite collected from the deep potassic middle phyllic and upper epithermal zones are used to conduct la icp ms trace element analysis the spatial zonings of mineralization and alteration and the regular variations of trace elements in sulfides at vertical direction imply a potentially complete transition from porphyry to epithermal mineralization and the deep origin of ore forming fluids mineralogical and trace element characteristics indicate that the chalcopyrite formed in both stages whereas bornite is the product of epithermal mineralization rather than a porphyry stage residue the majority of digenite and covellite has hypogene genesis pyrite and digenite in the epithermal zone are major carriers of primary au au in pyrite is te bi related and exists as solid solutions or different sized telluride and bi sulfosalt inclusions compared to as te and bi played more important roles to scavenge au and ag and achieve the primary au enrichment differently au in digenite is independently locked in digenite lattice bornite and digenite are good carriers of primary ag which mainly exists as solid solutions the high sulfidation state stage is the major period for concentrations of primary au and ag the upward increase of au in primary sulfides of hse cu zone implies that the distribution pattern of upper au enrichment and lower cu enrichment is not only caused by supergene process but is also controlled by hypogene trend based on the mineralization and alteration zonings the spatial variation of trace elements and the presented re os age the ore forming fluids of the zijinshan cu au deposit most likely originate from deep region rather than from the adjacent luoboling porphyry deposit the zijinshan and the luoboling deposits should belong to two independent hydrothermal systems

gold telluride sulfide association in the jinqu au deposit xiaoqinling region central china implications for ore forming conditions and processes abstract the jinqu au deposit located in central xiaoqinling gold field along the north china craton s southern margin has total reserve of 21 t of au with 4 23 g t au on average it consists cccmainly of ore bearing auriferous quartz veins in amphibolite facies metamorphic rocks of the archean taihua group the deposit s mineral paragenesis and ore forming processes have never been studied this paper reports its mineral paragenesis which includes sulfides pyrite galena chalcopyrite pyrrhotite sphalerite molybdenite and bornite oxides rutile scheelite hematite magnetite tellurides sylvanite au ag 2te4 hessite ag2te petzite ag3aute2 krennerite au3agte8 calaverite aute2 stutzite ag5 xte3 empressite agte rucklidgeite pb bi 3te4 altaite pbte volynskite agbite2 tetradymite bi2te2s lingbaoite agte3 and melonite nite2 and native elements tellurium and gold based on our data the associated ore forming processes can be deduced as iron oxides co2 g and h2s g loss te2 s2 ratio increase sulfides au ag tellurides condensation of h2te vapor pb bi tellurides mixing with meteoric water carbonate it has been indicated by thermodynamic modelling that formation of au ag te bearing minerals occurred under physico chemical conditions of log te2 values from 7 28 to 3 21 log o2 values from 26 67 to 26 44 log h2s values from 1 34 to 0 34 log s2 values from 9 53 to 5 64 log au aq ag aq values from 4 72 to 4 55 and ph values from 3 33 to 4 20 at 365 and log te2 values from 10 53 to 6 62 log o2 values from 31 99 to 30 98 log h2s values from 0 65 to 0 36 log s2 values from 13 62 to 7 20 log au aq ag aq values from 6 42 to 6 21 at 335 the changes in physico chemical conditions and in precipitation of au ag te bearing minerals were due to boiling or to condensation of te rich magmatic gas into ore forming fluids these reflect evolution from an early oxidizing to a later relatively reducing mineralization environment therefore compared with the dahu au mo deposit and with the yangzhaiyu au deposit which exist in the southern and northern parts of the xiaoqinling region respectively the ore paragenesis and ore forming physico chemical conditions of the jinqu au deposit were different such that its characteristics are similar to low sulfidation au deposits

mineralogy of the sulphide deposits at sulitjelma northern norway abstract the pyritic vms deposits at sulitjelma northern norway have been regionally metamorphosed at amphibolite facies during the caledonide orogen the deposits contain a wide diversity of sulphide sulphosalt and related minerals which are systematically discussed with reference to their occurrence paragenesis and mineral chemistry most of these minerals are however rare to absent within the massive deposits which are dominantly composed of pyrite with subordinate chalcopyrite sphalerite and pyrrhotite galena arsenopyrite cubanite molybdenite stannite and tetrahedrite occur in minor amounts syn metamorphic mobilisation has however resulted in the redistribution of a range of elements including cu pb sb as ni and au these elements have been reconcentrated into mobilised segregations where they have crystallised under retrograde conditions into a range of discrete phases ore mobilisation which took place along tectonically controlled pathways is however of a highly localised nature with a considerable range in mineral textures and parageneses observed in different samples typical associations within these mobilisates include a range of cu pb sb and cu pb as sulphosalts including bournonite meneghinite seligmannite and jordanite breithauptite nisbite costibite arsenopyrite and gudmundite co existing galena is characterised by elevated concentrations of ag bi and se argentian tetrahedrite 9 17 wt ag the chief ag carrier in the deposits as well as other ag bearing phases including pyrargyrite hessite and empressite are strongly enriched in the mobilisates gold as electrum 50 75 wt au and more rarely as aurostibite is also relatively enriched in the mobilisate samples over those from more massive parts of the deposits a further range of minerals including native antimony native arsenic allargentum dyscrasite argentite mckinstryite jalpaite and native silver occur as low temperature breakdown products of earlier formed minerals preserved textures do not contradict a model involving more than a one phase of mobilisation during the metamorphic evolution

profiles of the bullen parameter from mantle convection modelling the state of adiabaticity in the mantle can be assessed by using the bullen parameter profile derived from seismology we have employed the extended boussinesq convection models in a cartesian 2 d domain to extract the bullen parameter as a function of depth in both base heated and internally heated configurations and for both constant and variable thermal conductivity we also studied models with depth dependent thermal expansivity our results show that the values of the bullen parameter for a rayleigh number of 10 6 lie close to one with sometimes excursions out to 0 9 and 1 1 they are sensitive to the mode of heating and the depth dependences of the thermal expansivity and changes of thermal conductivity we obtained a subadiabatic geotherm above the lower boundary layer in all the models with constant thermal expansivity but a decrease of thermal expansivity with depth can result in a subadiabatic geotherm below the upper boundary layer thus the profiles of the bullen parameter have a definite potential of being useful in constraining the physical parameters and flow structures associated with mantle convection fl 2000 elsevier science b v all rights reserved

assessment of pyrite composition by la icp ms techniques from massive sulfide deposits of the bathurst mining camp canada from textural and chemical evolution to its application as a vectoring tool for the exploration of vms deposits abstract pyrite is ubiquitous throughout the massive sulfide deposits of the bathurst mining camp canada pyrite locally preserves primary pseudo primary textures as well as secondary textures that formed in response to the hydrothermal metamorphic and deformation processes to which the host sequence was exposed in the bathurst mining camp most forms of pyrite are arsenian up to 3 13 wt as with considerable enrichment in tl sb sn bi ag se and to a lesser extent cd ga te hg ge in and au la icp ms analyses of primary pyrite show significant enrichment in fluid mobile elements in particular tl sb as and hg as well as bi pb and sn in contrast secondary pyrite syn to post deformation varieties is not enriched in these elements superimposed metamorphism typically causes loss of fluid mobile elements to different degrees except for se pyrite chemical variations have the potential to monitor ore petrogenesis at inter and intra deposit scales pyrite from the cu zone is enriched in se relative to pyrite from zn pb zone in which tl is elevated throughout the host sequence fluid mobile element concentrations of pyrite vary by several orders magnitude with the highest fluid mobile element abundances in footwall rocks immediately proximal to the deposit moreover elevated fluid mobile element concentrations of pyrite are recognized in footwall felsic volcanic and sedimentary rocks as much as several hundred metres away from the deposit hanging wall zones contain pyrite with fluid mobile element contents comparable to pyrite from sulfide horizon the fluid mobile element concentrations of pyrite are higher in exhalative sedimentary rocks than pyrite from the sulfide mineralization itself the spatial variation in fluid mobile element concentrations in pyrite as identified using la icp ms can be used to identify otherwise cryptic deposit related haloes ultimately this technique when combined with traditional exploration tools can be used to vector toward vms mineralization in the bathurst mining camp and possibly a useful indicator in a prospect scale base metal exploration

fluid inclusion characteristics of mesothermal gold deposits in the xiaoqinling district shaanxi and henan provinces people s republic of china fluid inclusions were studied in quartz samples from early stage i gold poor quartz veins and later stage ii gold and sulphide rich quartz veins from the wenyu dongchuang qiangma and guijiayu mesothermal gold deposits in the xiaoqinling district china fluid inclusion petrography microthermometry and bulk gas analyses show remarkably consistent fluid composition in all studied deposits primary inclusions in quartz samples are dominated by mixed co2 h2o inclusions which have a wide range in co2 content and coexist with lesser primary co2 rich and aqueous inclusions in addition a few secondary aqueous inclusions are found along late healed fractures microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15 30 mol co2 in stage i inclusions and 10 20 mol co2 in stage ii inclusions estimates of fluid salinity decrease from 7 4 9 2 equivalent wt nacl to 5 7 7 4 equivalent wt nacl between stage i and ii primary aqueous inclusions in both stages show consistent salinity with but slightly lower th total than their coexistent co2 h2o inclusions the coexisting co2 rich co2 h2o and primary aqueous inclusions in both stage i and ii quartz are interpreted to have been trapped during unmixing of a homogeneous co2 h2o parent fluid the homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids trapping conditions are typically 300 370 c and 2 2 kbar for stage i fluids and 250 320 c and 1 6 kbar for stage ii fluids the co2 h2o stage i and ii fluids are probably from a magmatic source most likely devolatilizing cretaceous yanshanian granitoids the study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage ii veins

pb sr nd isotope constraints on the fluid source of the dahu au mo deposit in qinling orogen central china and implication for triassic tectonic setting abstract the dahu au mo deposit is a structure controlled lode system occurring in the northern xiaoqinling terrane huaxiong block qinling orogen this paper reports a new sr nd pb isotope dataset obtained for ore sulfides and the hostrocks within the taihua supergroup in an attempt to constrain the source of the ore forming fluids from a new dimension 16 sulfide samples yield i sr ratios of 0 70470 0 71312 with an average of 0 70854 e nd t values between 13 5 and 18 1 with average of 15 1 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 033 17 285 15 358 15 438 and 37 307 37 582 with averages of 17 162 15 405 and 37 440 respectively 5 gneiss samples from the taihua supergroup yield i sr ratios of 0 70947 0 73201 averaging 0 72294 e nd t values of 20 0 to 31 1 averaging 25 1 and 206 pb 204 pb i 207 pb 204 pb i 208 pb 204 pb i ratios of 17 127 18 392 15 416 15 604 and 37 498 37 814 with averages of 17 547 15 470 and 37 616 respectively these data show that the ore sulfides have less radiogenic sr nd pb isotope systematics than the hostrocks and suggest that the ore forming fluids which interacted with the wallrocks to form ores must be sourced from a depleted mantle or a depleted subducted oceanic slab in combination with the spatial scenario and geochemical signatures of the triassic magmatites and mineral systems we argue that in the triassic the mianlue ocean was not completely closed and that the northward oceanic plate subduction still survived along the mian lue suture which caused the late triassic magmatism and associated mineralization in qinling orogen including the au mo dahu deposit

electron microprobe analyses of ore minerals and h o s isotope geochemistry of the yuerya gold deposit eastern hebei china implications for ore genesis and mineralization abstract the yuerya gold deposit in eastern hebei province china is located on the eastern margin of the north china craton and is hosted by mesozoic yanshanian granitoid rocks and adjacent mesoproterozoic gaoyuzhuang formation carbonates the auriferous quartz veins in this deposit are dominated by pyrite with subordinate sphalerite chalcopyrite and galena in a quartz dominated gangue that also contains calcite dolomite barite apatite and fluorite gold is present as native gold and electrum which are generally present as micron size infillings in microfissures within pyrite and less commonly as tiny inclusions within pyrite quartz and tellurobismuthite the pyrite in this deposit has high co ni ratios and contains elevated concentrations of both of these elements suggesting that the yuerya gold deposit has a magmato hydrothermal origin and that the ore forming fluids that formed the deposit leached trace elements such as co ni as and au during passage through archean metamorphic rocks mesoproterozoic carbonates and the yanshanian yuerya granitoid pyrite in the study area has s se ratios and s isotopic compositions that suggest that the sulfur and by inference the gold within the deposit was sourced from magmato hydrothermal fluids that were probably originally derived from archean metamorphic rocks and yanshanian granitoids tellurobismuthite in the study area is closely intergrown with gold and was the single telluride phase identified during this study the fineness of gold associated with tellurobismuthite is greater than the fineness of gold associated with pyrite although the fine particle size of the gold surrounded by tellurobismuthite means that the recovery of this gold is difficult in turn meaning that the tellurobismuthite has little significance to the economics of the yuerya gold deposit only trace amounts of sulfides are associated with the tellurobismuthite within the yuerya gold deposit suggesting that this mineral was deposited under conditions of low fs2 and or high fte2 in addition the presence of tellurides within the yuerya gold deposit reflects a genetic relationship between the deposit and magmatism quartz from mineralized veins in the study area has 18o values of 11 2 12 9 and the fluids that formed these veins have d values of 78 3 to 72 1 the 34s values of pyrite within the deposit are rather restricted 2 3 3 5 these data combined with the trace element geochemistry of sulfides within the deposit suggest that the formation of the yuerya gold deposit was closely related to both archean metamorphic rocks and the yanshanian yuerya granitoid

mantle plume interaction with an endothermic phase change high spatial resolution numerical simulations of mantle plumes impinging from below on the endothermic phase change at 660 km depth are used to investigate the effects of latent heat release on the plume phase change interaction both axisymmetric and planar upflows are considered and the strong temperature dependence of mantle viscosity is taken into account for plume strengths considered a clapeyron slope of 4 mpa k 1 prevents plume penetration of the phase change plumes readily penetrate the phase change for a clapeyron slope of 2 mpa k 1 and arrive in the upper mantle considerably hotter than if they had not traversed the phase change for the same amount of thermal drive i e the same excess basal temperature axisymmetric plumes are hotter upon reaching the upper mantle than are planar upwellings heating of plumes by their passage through the spinel perovskite endothermic phase change can have important consequences for the ability of the plume to thermally thin the lithosphere and cause melting and volcanism

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

microfabrics in situ trace element and sulfur isotope compositions of pyrite from the jinjiwo copper deposit in chengmenshan orefield northern yangtze block syngenetic stratabound mineralization and hydrothermal remobilization abstract the jinjiwo copper deposit is located in the jiurui ore district of the middle lower yangtze metallogenic belt the deposit has stratiform orebodies hosted by late carboniferous carbonate units pyrite is the dominant metallic mineral in the ores here we report on an integrated study on the microfabrics raman spectroscopy in situ la mc icp ms trace element and sulfur isotope of pyrite with a view to understand the geochemical variation genesis of copper mineralization and ore forming process textural observations suggested four types of pyrite corresponding to three metallogenic episodes syngenetic sedimentation pyi metamorphism or deformation pyii and skarn hydrothermal mineralization including the late skarn stage pyiii and hydrothermal stage pyiv raman spectra of pyrite reveal subtle differences in band position and widths fwhm and display the increasing formation temperature of from pyi through pyiv and pyiii to pyii colloform and laminated pyrite pyi share similarities in behavior of pairs of elements and have low co ni ratios and the former contains the highest concentrations of bi cu pb zn ag au mn that suggest rapid precipitation from highly saturated fluids pyii shows high co ni and as concentrations and co ni ratios of 0 03 to 6 19 pyiii and pyiv have greatly varying co ni ratios of 1 07 to 29 and typically low au concentrations suggesting a hydrothermal source pyiii is rich in co and se but is depleted in as cu pb zn ag and au in comparison with pyiv there are two kinds of sulfur one is that 34s values are near zero suggesting that the pyrite sulfur in the ores was derived from a mixed source in deep crust and the other is that 34s values are less than 39 1 indicating the sedimentary pyrite sulfur was originated from bacterial sulfate reduction the combined textural and compositional data of pyrite suggest that the formation of the jinjiwo copper deposit might have recorded the submarine exhalation sedimentation and the mesozoic tectonic transition deformation and hydrothermal overprint

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

estimates of heat flow in the deep mantle based on the power requirements for the geodynamo 1 regeneration of the magnetic field by convection in the core places demands on heat flow into the base of the mantle if the heat flow is too low thermal convection is shut off and the rate of generation of compositional buoyancy by the solidification of the core becomes too low to sustain the geodynamo conversely a large heat flow causes rapid growth of the inner core so that convection prior to the appearance of the inner core must be sustained by thermal buoyancy alone the attendant requirements on primordial heat become more severe as the age of the inner core decreases we show that estimates of the present day heat flow satisfy the power requirements for the geodynamo however the resulting thermal history is incompatible with estimates of mantle temperatures prior to 3 ga this discrepancy can be resolved by accumulating radioactive isotopes in d or adding heat sources to the core

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

early mesozoic mo mineralization in the qinling orogen an overview abstract as one of the most important mo provinces in the world the qinling orogen is host to mainly late mesozoic late jurassic early cretaceous porphyry mo systems recently additional early mesozoic mainly triassic plus minor early jurassic mo deposits have been identified which occur as fault controlled quartz veins or as carbonatite veins and as such they are fundamentally different from the commonly observed porphyry systems furthermore early mesozoic porphyry systems are also present but they are different from those of late mesozoic age thus early mesozoic mo systems not only offer a new and comprehensive view of the mo metallogenesis but also provide unique insight into the tectonic evolution of the qinling orogen taking into consideration the role of mineral deposits as ideal probes of the geodynamic evolution the known early mesozoic mo deposits in qinling can be classified into three types based on their geological and geochemical features porphyry carbonatite and orogenic systems porphyry mo systems including porphyry porphyry skarn and intrusion related veins are spatially and genetically associated with granitic porphyries they display typical porphyry type alterations and are formed by magmatic water carbonatites are considered as mantle derived igneous rocks and mo mineralization is associated with magmatic fluids orogenic mo systems occur as fault controlled quartz lodes and the ore forming fluids are medium high temperature co2 rich metamorphic fluids spatially early mesozoic porphyry mo systems are concentrated in the western and middle part of the qinling orogen whereas carbonatite veins and orogenic mo systems are located in the northeastern part temporally orogenic mo deposits were the earliest mainly between 220 and 250 ma followed by carbonatite veins mainly between 205 and 225 ma and porphyry mo systems mainly between 190 and 205 ma with some exceptions there is generally a trend of decreasing deposit ages toward the western part of the orogen detailed comparison reveals that the early mesozoic porphyry mo systems in the qinling orogen are different from the late mesozoic ones in terms of their economic importance spatial distribution ore causative intrusions hydrothermal alteration and the ore forming fluids to explain the above geological and geochemical features the triassic qinling is considered as an active continental margin caused by the northward subduction of the mian lue oceanic plate a transition from continental arc to back arc basin is suggested for the formation of triassic orogenic and carbonatite vein mo deposits whereas continental arc is host to the early mesozoic porphyry mo systems

geology geochemistry and ore genesis of the wenyu gold deposit xiaoqinling gold field qinling orogen southern margin of north china craton abstract the wenyu giant gold deposit is hosted in the precambrian taihua supergroup metamorphic rocks within the xiaoqinling terrane qinling orogen on the southern margin of the north china craton the mineralization can be divided into three stages quartz pyrite veins early quartz sulfide veins middle main and carbonate quartz veinlets late with gold being mainly introduced in main stage quartz formed in two earlier stages contains three compositional types of fluid inclusions i e pure co2 co2 h2o and nacl h2o but the late stage minerals only contain the nacl h2o inclusions the inclusions in quartz formed in the early main and late stages yield total homogenization temperatures of 262 417 c 236 407 c and 114 239 c respectively with salinities no higher than 13 wt nacl equiv trapping pressures estimated from co2 h2o inclusions are 139 399 mpa and 111 316 mpa in the early and main stages corresponding to mineralization depths of 14 km and 11 km respectively fluid boiling and mixing caused rapid precipitation of sulfides and native au through boiling and inflow of meteoric water the ore forming fluid system evolved from co2 rich to co2 poor in composition and from metamorphic to meteoric as indicated by decreasing 18owater values from early to late the carbon sulfur and lead isotope compositions suggest the hostrocks within the taihua supergroup to be a significant source of ore metals integrating the data obtained from the studies including regional geology ore geology fluid inclusion and c h o s pb isotope geochemistry we conclude that the wenyu gold deposit was an orogenic type system formed in the tectonic transition from compression to extension during the jurassic early cretaceous continental collision between the north china and yangtze cratons

phase boundary deflections at 660 km depth and episodically layered isochemical convection in the mantle numerical simulations have been performed using a multiphase anelastic axisymmetric spherical mantle convection model as part of an ongoing effort to explore the ability of the endothermic phase transition at 660 km depth to cause the circulation to assume a layered style in particular model solutions have been constructed for a rayleigh number of 107 internal heating corresponding to 50 heating from within and 50 heating from below and clapeyron slopes for the 410 km and 660 km phase boundaries set to 3 0 and 2 8 mpa k respectively in this regime the flow exhibits a substantial degree of radial layering wherein the radial mass flux is reduced significantly at 660 km depth this layered regime of flow is episodically disrupted by massive localized avalanches of fluid across the 660 km boundary uiat recur at intervals separated by hundreds of millions of years the degree of layering is related to the magnitude of the 660 km phase boundary deflection away from its average depth in these earth like simulations we find that the average magnitude of such phase boundary deflections is similar to the average magnitude of seismically observed deflections of this horizon

invisible gold distribution on pyrite and ore forming fluid process of the huangshan orogenic type gold deposit of zhejiang se china implications from mineralogy trace elements impurity and fluid inclusion studies the huangshan orogenic type gold deposit in zhejiang of se china occurred in quartz pyrite veins it is hosted by phyllonite that underwent greenschist facies metamorphism along a large jiangshan shaoxing tectonic belt with a ne sw direction trace elemental characteristics ore forming process and invisible gold on different forms of pyrite and quartz are studied the au associated pyrite can be classified into two categories recrystallized pyrite and euhedral pyrite the precipitation of invisible au on pyrite is mainly derived by co and ni with auhs2 complex in the mineralizing fluids in different events the xps results revealed that valence states of au3 replaced 2fe2 in the pyrite and au0 replaced si4 in the quartz structure the electron paramagnetic resonance and trace elemental results suggested that the element pairs of ge li al in quartz and mn co ni in pyrite have distinct impurities as identified a fluid inclusion study showed that the auriferous quartz is characterized by low saline and co2 rich fluids coexistence of the type i type iii inclusions and same range of homogenization temperature with different mode are evidences of immiscible fluid process the temperature pressure values of ca 250 c 1250 bar and ca 220 c 780 bar for gold precipitation have been calculated by intersection of coexisting fluids during the entrapment the huangshan orogenic type gold deposit may be associated with the wuyi yunkai orogeny during the early paleozoic including an upper mid greenschist facies metamorphism 450 420 ma all the features suggest that the huangshan gold deposit is probably a product linking with the early paleozoic orogeny in south china

mantle phase transitions and layered convection numerical simulations with an anelastic spherical axisymmetric mantle convection model have been conducted to address the question of the radial mixing length in the general circulation of the mantle continuing debate centers on the question as to whether or not the 670 km seismic discontinuity which we now understand to exist as a consequence of an endothermic phase change of mantle mineralogy from the spinel phase to a mixture of perovskite and periclase in combination with the 400 km discontinuity associated with the exothermic phase change from olivine to spinel will impose a sufficient barrier to the circulation so as to induce layering we argue herein that the mantle must currently be converting in a partially layered style but that the degree of layering is highly time dependent moreover in the perhaps not too distant past the propensity to layering was greater possibly to the extent that soon after planetary formation mantle mixing occurred in two distinct reservoirs as the planet cool

heterogeneity and time dependence in 3d spherical mantle convection models with continental drift abstract feedback between continents and large scale mantle flow through thermal blanketing has long been surmised as a mechanism for continental drift and wilson cycles paleomagnetism provides evidence for extensive continental displacements 10 000 km on time scales of 100 200 million years comparable to an intrinsic overturn in whole mantle convection here we model continental motions in vigorous 3d spherical convection models focusing on the effects of continent size mantle heating mode and a strong increase in lower mantle viscosity continents covering 30 10 and 3 of earth s surface representative of the former supercontinent pangea present day asia and antarctica respectively are introduced into simple end member mantle convection models characterized by pure core or internal heating and uniform or layered mantle viscosity supercontinents promote temperature anomalies on the largest scales spherical harmonic degrees 1 and 2 primarily through the organization of the long wavelength convective planform inherent in models with a high viscosity lower mantle bottom heating can promote long wavelength heterogeneity by clustering plumes beneath the continent however in isoviscous models small scale structure persists away from the continent regardless of the heating mode supercontinents respond to long wavelength heterogeneity by following great circle paths with variations in velocity on time scales of 1 billion years smaller continents are unable to promote long wavelength structure and the resulting motions are governed by bursts in velocity on time scales of the order of 100 million years continental velocities are roughly a factor of 3 smaller than those in oceanic regions an observation that may help explain the observed difference in the speed of predominantly continental or oceanic plates

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

dynamic feedback between a continentlike raft and thermal convection seismic observations of the mantle which include long wavelength structure a k 1 dependence of heterogeneity on harmonic k and a heterogeneous upper boundary layer and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection we have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry the azimuthal interconnectivity of this geometry is vital to resolve the two way dynamics between rafts and convection computations show that 1 raft motion is periodic 2 long wavelength thermal structure is significant within both thermal boundary layers and the fluid interior and 3 the large scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion these three results which are observed for a range of rayleigh numbers internal heating rates and raft sizes are a direct consequence of the dynamic interaction between the raft and convection the physical processes for a model with a rayleigh number of 10 5 are representative when the raft is stationary due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer heat accumulates beneath the raft and results in long wavelength thermal anomalies the long wavelength thermal anomalies enhance raft motion accompanying the enhanced raft movement the long wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest since convection models without rafts generate less long wavelength heterogeneity compared to the models with rafts or continental plates we suspect that continental plates may play a crucial role in mantle dynamics interestingly raft motion with a period of about 10 transit times is usually significant 10 transit times is about 600 m y if scaled to the earth this is close to the observed 300 500 m y period of supercontinent aggregation and dispersal

ore forming mechanism of quartz vein type w sn deposits of the xitian district in se china implications from the trace element analysis of wolframite and investigation of fluid inclusions abstract the xitian w sn district is located in eastern hunan province of se china and comprises the heshuxia goudalan and longshang w sn deposits our study on the occurrence mineral assemblage and geochemistry of the orebodies shows that quartz vein orebodies are dominant mode of mineralization and can be divided into three stages stage i quartz molybdenite wolframite i stage ii quartz wolframite ii polymetallic sulfides and stage iii quartz pyrite fluorite the elements nb ta sc sn zn cr v mo cu and zr are abundant in both wolframite i and wolframite ii wolframite i contains higher concentrations of nb 1408 9245 ppm ta 241 2407 ppm and sc 123 521 ppm than wolframite ii nb 144 1080 ppm ta 3 76 315 ppm and sc 1 86 11 44 ppm these results indicate that wolframite i was formed closer to the host rock and at a greater ore forming depth than wolframite ii the total ree concentrations of wolframite i and ii range from 34 96 to 133 36 ppm and 21 30 to 43 73 ppm respectively both wolframite i and wolframite ii have low lree concentrations 0 00 0 05 and 0 08 8 18 ppm respectively and high hree concentrations 34 96 133 31 and 21 23 41 87 ppm respectively the enrichment of hrees in wolframite may be due to the fact that the sizes of the hrees3 0 94 1 02 a in combination with nb5 or ta5 0 72 a are closer to that of w6 0 68 a in combination with ca2 1 08 a or mg2 0 80 a than that of the lrees3 1 03 1 13 a making the coupled substitution easier a study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage i to stage iii stage i 187 382 c average 275 c stage ii 122 278 c average 175 c stage iii 92 172 c average 139 c whereas the salinities of the fluid inclusions increase from stages i to ii and decrease from stages ii to iii stage i 1 8 18 2 wt nacleqv average 10 3 wt nacleqv stage ii 7 3 24 4 wt nacleqv average 14 9 wt nacleqv stage iii 0 2 5 3 wt nacleqv average 1 5 wt nacleqv raman spectroscopy reveals that the fluid inclusions mainly contain h2o co2 is minor and h2s ch4 and n2 are rare we conclude that the formation of wolframite was driven by post magmatic thermodynamic processes the ore forming fluid flowed through a multi fractured low pressure zone in the fault system in the district and a temperature pressure decrease led to fluid immiscibility characterized by co2 escaping in a low ph and high eh environment in stage i crust mantle mixed fluid and meteoric water mixing in stage ii and natural cooling of the fluid system in stage iii

multistage processes linked to tectonic transition in the genesis of orogenic gold deposit a case study from the shanggong lode deposit east qinling china abstract the shanggong lode deposit represents one of the largest mesozoic orogenic gold deposits in the east qinling metallogenic belt of central china here we investigate the texture trace element and sulfur isotopic composition of pyrite from this deposit to gain insights in the ore genesis three types of pyrite are recognized by its texture py1 is represented by coarse grained cubic and pentagonal dodecahedron belonging to the stage i quartz pyrite vein py2 comprises fine grained disseminated pentagonal dodecahedron belonging to stage ii quartz sulfides vein py2 can be further divided into core and rim porous cores py2c carrying abundant visible mineral inclusions and significantly higher arsenic concentration and clean rims py2r with low arsenic systematic la icpms trace elements and sims sulfur isotopic analyses reveal two distinct stages of mineralization with diverse source materials 1 the early stage is represented by py1 which contains 0 65 70 ppm au average 11 6 ppm with relatively low and variable concentrations of cu ag te pb zn and bi the 34 s values of py1 range from 2 7 to 7 2 suggesting that the sulfur source for the early stage fluid was mainly derived from the taihua and xiong er groups and that the depleted 34 s values reflect changes of ph f o2 and fluid rock interaction 2 the main stage is represented by py2 which contains au in the range of 0 14 189 ppm average 39 7 ppm the py2 carries significantly higher contents of compatible elements ni co v and cr and ore metals e g au ag pb zn as te than those in py1 with the ore metals occurring as invisible lattice bound or nanosized particles and visible micron sized inclusions the 34 s values of py2 show marked depletion with a range of 16 3 to 11 7 attributing to the fractionation between barite and sulfide minerals together with previous studies and trace element characters in py2 we identify a contribution of au from mantle source into the ore forming fluid system during the main stage we propose that the two stages of gold mineralization in the shanggong deposit in response to the collisional assembly of qinling orogenic belt preserve evidence for diverse metal sources with the early stage derived from partial melting of the lower crust taihua and xiong er groups in the compressional stage and the main stage characterized by mantle contribution in a post collisional extensional setting our findings may have important implications for the exploration of regional triassic gold and molybdenum deposits

trace element characteristics of different pyrite types in mesoarchaean to palaeoproterozoic placer deposits a comparative in situ la icp ms trace element study on pyrite from three different variably auriferous archaean to palaeoproterozoic palaeoplacer deposits in the ouro fino syncline quadrilatero ferrifero brazil the elliot lake area north of lake huron se canada and several deposits within the witwatersrand basin south africa revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits especially the ni and au concentrations as well as co ni and mo ni ratios were found to be systematically different in detrital compact detrital porous and post sedimentary hydrothermal pyrite grains from different source areas high co ni ratios and low au concentrations are typical of post sedimentary pyrite which is hydrothermal in origin in contrast relatively low co ni ratios and high au contents characterise detrital porous banded and concentric pyrite grains au 1 ppm which are syn sedimentary in origin in the elliot lake area and the witwatersrand basin detrital compact rounded pyrite is characterised by high co ni ratios which is in agreement with derivation from a hydrothermal source low au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks in contrast derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the ouro fino syncline because of low to intermediate co ni ratios high au contents may indicate a genetic relationship between pyrite and gold there systematic differences exist between the three areas with respect to au ni co mo and cu distributions in detrital pyrite which reflects differences in the provenance a predominantly mafic ultramafic source is indicated for the ouro fino a felsic source for the elliot lake and a mixed felsic mafic provenance for the witwatersrand pyrite populations independently of pyrite type the higher au endowment of the studied witwatersrand and ouro fino conglomerates are also reflected by an overall higher au concentration in the respective pyrite grains compared to the relatively au poor samples from elliot lake in general a strong positive correlation between au and pb levels in the various pyrite grains is noted analogous to pb which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete pbs phases au is considered to be present mainly in the form of discrete au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice

effects of mantle heat source distribution on supercontinent stability a two dimensional cartesian geometry numerical model is used to study the influence of internal heating on the ability of mantle convection to rift and disperse an assembled supercontinent our wide aspect ratio models incorporate migrating plate boundaries and finite thickness rigid mobile oceanic and continental plates 12 000 km across we examine models characterized by both whole mantle and upper mantle convection parameters specifying different degrees of internal heating in the mantle we compare the patterns of convection that develop below stationary continental plates that form as a result of the aggregation of smaller mobile continents following continental collisions we find that a reorganization of mantle flow may result in the development of deviatoric stresses at the base of the continental lithosphere which are capable of initiating continental breakup our results suggest that continental basal stresses are largely controlled by the wavelength of the subcontinental convection pattern and that the inclusion of internal heating in the mantle generally increases this wavelength for models with 80 internal heating in which active upwellings are intrinsically absent basal stresses arise primarily from subduction at the continental margins and result in a geologically reasonable duration for our model supercontinent assemblages 200 myr for models with 40 or less internal heating continental breakup is controlled by mantle upwellings and our supercontinents remain intact much longer 600 700 myr

geochemical and isotopic signature of pyrite as a proxy for fluid source and evolution in the candelaria punta del cobre iron oxide copper gold district chile

three dimensional numerical simulations of crustal deformation and subcontinental mantle convection abstract 3 d simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large scale mantle flow simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 1 1 cartesian domain convective stresses cause crust to thicken above a sheet like mantle downwelling for mild convective vigor an initial crustal thickness variation is required to induce 3 d lithospheric instability below the zone of crustal convergence the amplitude of the required variation decreases with increasing convective vigor morphologically instability is manifest in formation of drip like thermals that exist within the large scale roll associated with initial crustal thickening a strong surface signature of the drips is their ability to cause deviations from local airy compensation of topography after the initial thickening phase the crustal accumulation that forms serves as a model analog to a continent its presence leads to mantle flow patterns distinctly different from the steady state roll that results in its absence large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small scale thermal instabilities eventually these instabilities induce a restructuring of large scale mantle flow with the roll pattern being replaced by a square cell although preliminary and idealized the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2 d mountain chain by the formation of the chain itself the ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low order despite the seemingly trivial volume of crust relative to mantle

seismic heterogeneity in the mantle strong shear wave signature of slabs from joint tomography abstract the primary source of information on heterogeneity within the earth comes from seismic tomography a powerful tool for examining the character of heterogeneity comes from the comparison of images of bulk sound and shear wavespeed extracted in a single inversion since this isolates the dependencies on the elastic moduli however particularly in such multi parameter inversions there are many hidden facets which can have a strong influence on the results such as the weightings between parameters and in the misfit functions joint inversion with restricted data sets giving comparable cover for p and s waves provides useful checks on more inclusive studies and can provide relatively high resolution in some areas the relative behaviour of bulk sound and shear wavespeed can provide a useful guide to the definition of heterogeneity regimes for subduction zones a large part of the tomographic signal comes from s wavespeed variations in the upper mantle and transition there can be significant bulk sound speed contributions for younger slabs and in stagnant slabs associated with slab roll back for subducted oceanic lithosphere older than about 90 ma shear wavespeed variations nearly always are dominant and so the p wave images are controlled by shear modulus variations the narrow segments of fast wavespeeds in the depth range 900 1500 km in the lower mantle are dominated by s variations with very little bulk sound contribution deep in the mantle there are many fast features without obvious association with subduction in the last 100 ma which suggests long lived preservation of components of the geodynamic cycle

catastrophic overturn of the earth s mantle driven by multiple phase changes and internal heat generation the effects of phase changes and strong internal heat generation may combine to bring about brief but extremely intense episodes of rapid thermal convection in the earth s mantle numerical calculations using realistic thermodynamic properties for the exothermic olivine spinel and endothermic spinel perovskite magnesiowustite phase transitions suggest the transition region of the earth s mantle may act as a capacitor for subducting slabs slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached and is then rapidly discharged into the lower mantle in my calculations this occurs as a catastrophic burst of convection lasting 10 myr with elevated heat transfer rates lasting 100 myr such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra plate volcanism and stabilize the geodynamo against reversals the topography on the two phase change boundaries is found to be negatively correlated with the correlation becoming more negative during periods of rapid convection furthermore the results of this study suggest the transition region of the earth s mantle could be 250 k cooler on average and consequently more viscous than the surrounding mantle

isotope and fluid inclusion geochemistry and genesis of the qiangma gold deposit xiaoqinling gold field qinling orogen china abstract the qiangma gold deposit is hosted in the 1 9 ga taihua supergroup metamorphic rocks in the xiaoqinling terrane qinling orogen on the southern margin of the north china craton the mineralization can be divided as follows quartz pyrite veins early quartz polymetallic sulfide veinlets middle and carbonate quartz veinlets late stages with gold being mainly introduced in the middle stage three types of fluid inclusions were identified based on petrography and laser raman spectroscopy i e pure carbonic carbonic aqueous co 2 h 2 o and aqueous inclusions the early stage quartz contains pure carbonic and co 2 h 2 o inclusions with salinities up to 12 7 wt nacl equiv bulk densities of 0 67 to 0 86 g cm 3 and homogenization temperatures of 280 365 c the early stage is related to h 2 o co 2 n 2 ch 4 fluids with isotopic signatures consistent with a metamorphic origin 18 o water 3 1 to 5 2 d 37 to 73 the middle stage quartz contains all three types of fluid inclusions of which the co 2 h 2 o and aqueous inclusions yield homogenization temperatures of 249 346 c and 230 345 c respectively the co 2 h 2 o inclusions have salinities up to 10 9 wt nacl equiv and bulk densities of 0 70 to 0 98 g cm 3 with vapor bubbles composed of co 2 and n 2 the isotopic ratios 18 o water 2 2 to 3 6 d 47 to 79 suggest that the middle stage fluids were mixed by metamorphic and meteoric fluids in the late stage quartz only the aqueous inclusions are observed which have low salinities 0 9 9 9 wt nacl equiv and low homogenization temperatures 145 223 c the isotopic composition 18 o water 1 9 to 0 5 d 55 to 66 indicates the late stage fluids were mainly meteoric water trapping pressures estimated from co 2 h 2 o inclusions are 100 285 mpa for the middle stage suggesting that gold mineralization mainly occurred at depths of 10 km fluid boiling and mixing caused rapid precipitation of sulfides and native au through boiling and inflow of meteoric water the ore forming fluid system evolved from co 2 rich to co 2 poor in composition and from metamorphic to meteoric as indicated by decreasing 18 o water values from early to late the carbon sulfur and lead isotope compositions suggest the hostrocks within the taihua supergroup to be a significant source of ore metals integrating the data obtained from the studies including regional geology ore geology and fluid inclusion and c h o s pb isotope geochemistry we conclude that the qiangma gold deposit was an orogenic type system formed in the tectonic transition from compression to extension during the jurassic early cretaceous continental collision between the north china and yangtze cratons

gold deposits in the xiaoqinling xiong ershan region qinling mountains central china the gold rich xiaoqinling xiong ershan region in eastern shaanxi and western henan provinces central china lies about 30 50 km inland of the southern margin of the north china craton more than 100 gold deposits and occurrences are concentrated in the xiaoqinling west xiaoshan middle and xiong ershan east areas late archean gneiss of the taihua group and middle proterozoic metavolcanic rocks of the xiong er group are the main host rocks for the deposits mesozoic granitoids ca 178 104 ma are present in most gold districts but deposits are typically hosted in the precambrian basement rocks hundreds of meters to as far as 10 km from the intrusions and related hornfels zones deposits in the xiaoqinling and xiaoshan areas are best classified as orogenic gold deposits with ores occurring in a number of distinct belts both in quartz veins and disseminated in altered metamorphic rocks alteration assemblages are dominated by quartz sericite pyrite and carbonate minerals the ore forming fluids were low salinity co2 rich and characterized by isotopically heavy 18o four deposits dongchuang wenyu yangzhaiyu and dahu in the xiaoqinling area each contain resources of about 1 moz au some of the gold deposits in the xiong ershan area represent more shallowly emplaced tellurium enriched orogenic systems which include resources of approximately 1 1 5 moz au at shanggong and beiling or tantou others are epithermal deposits e g qiyugou and dianfang that are hosted in volcanic breccia pipes isotopic dates for all gold deposits although often contradictory generally cluster between 172 99 ma and are coeval with emplacement of the post kinematic granitoids the gold deposits formed during a period of relaxation of far field compressional stresses clearly subsequent to the extensive paleozoic early mesozoic accretion of arc terranes and the yangtze craton onto the southern margin of the north china craton hydrothermal and magmatic events occurred locally where extension related precambrian basement uplifting took place along the craton margin fluids for the orogenic gold deposits in the xiaoqinling xiaoshan and xiong ershan areas may have been released from evolving magmas or resulted from prograde metamorphic reactions within the uplift zones alternatively for the epithermal gold deposits at shallower levels in the xiong ershan area gold transporting fluids were mainly exsolved from coeval magmas although meteoric water was also involved in these hydrothermal systems

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

heat flow scaling for mantle convection below a conducting lid resolving seemingly inconsistent modeling results regarding continental heat flow numerical models of mantle convection below a conducting lid meant to mimic a continent have been used 1 to argue that heat flow variations in stable continental regions result principally from lithospheric thickness variations and 2 to argue that the relationship between lithospheric thickness and continental heat flow is too weak to allow for this we reconcile these results using a theoretical heat flow scaling which shows that the relationship between heat flow and lid thickness can take on two end member forms as the ratio of lid thickness to convecting layer depth increases the effects of lid thickness variations on heat flow move from weak to strong modeling studies that concluded that lithospheric thickness variations could lead to significant continental heat flow variations assumed a convecting layer depth appropriate to upper mantle convection and thus maximized the relative thickness of the continental lithosphere i e the conducting lid studies that concluded that the relationship is weak assumed whole mantle convection which placed them on the flatter end of the heat flow versus relative lid thickness curve

the multiple roles of sulfate reducing bacteria and fe ti oxides in the genesis of the bayinwula roll front type uranium deposit erlian basin ne china the bayinwula roll front type uranium deposit is hosted in sandstones of the early cretaceous saihan formation deposited during the postrift stage of the erlian basin in northeast china the present study aims to characterize 1 the roles of sulfate reducing bacteria and 2 the roles of fe ti oxides in the genesis of the bayinwula deposit drill cores of the host sandstone have been investigated with emphasis on petrographic observations whole rock geochemistry and geochemical and or mineralogical study of fe ti oxides iron disulfide uranium minerals emp la icpms and organic matter rock eval pyrolysis the 34 s value has been measured in situ by sims on the different generations of iron disulfide within regional sandstones preore uranium enrichment u mean 30 4 ppm in whole rock was identified on altered fe ti oxides 38 5 of the whole rock u content and constitutes a major source of uranium for the mineralization petrographic observations and rock eval data indicate that organic matter occurring in the host sandstone is mainly inherited from land plants and corresponds to type iii or type iv kerogens organic matter fragments disseminated in sandstones may also contain significant preore uranium concentration framboidal and collomorph ore stage iron disulfides have moderate to high concentrations of as ni and co and have a light sulfur isotope signature characterized by 34 s values from 30 5 to 7 5 suggesting that sulfur originated from bacterial sulfate reduction which was mainly responsible for 1 the liberation of u from fe ti oxides and organic matter 2 the generation of ore stage iron disulfides and 3 the production of a secondary h 2 s rich reducing barrier involved in the reduction of u vi and the precipitation of u iv uranyl and sulfate ions were transported through the host sandstone by low temperature oxygenated groundwater and u iv was precipitated at the redox front as p rich coffinite and ningyoite dominantly as replacement of ore stage iron disulfides which have partly to totally replaced organic matter and fe ti oxides therefore the combined mineralogical geochemical and isotopic characteristics of the bayinwula roll front uranium deposit support the theory that biogenic processes have widely contributed to the genesis of the uranium mineralization

trace element and fluid inclusion geochemistry constraint for genesis of gold polymetallic mineralization in the songhung deposit dpr korea abstract the songhung deposit is located in the southwestern part of the yangdok metamorphic core complex ymcc in the northern portion of the pyongnam basin the eastern part of the sino korean craton in this paper we present a detailed study on the magma sources of the granitoids yangdok and hoechang graintic intrusions genesis of pyrite and fluid inclusion characteristics for sufficient and additional understanding of tectonic events petrogenesis of the granitoids and mineralization in the study deposit the yangdok granite has lower al2o3 tio2 ratios mean value of 60 58 than the hoechang granite mean value of 994 46 indicating the higher temperature magma source of the former on the rb ba vs rb sr and cao tfeo mgo tio2 vs cao tfeo mgo tio2 discriminant diagrams the magma source of the yangdok granite belongs in the plagioclase rich and clay poor source rocks while those of the hoechang granite plots either in the plagioclase rich and clay poor source rocks for granite and fine grained granite or in the plagioclase poor and clay rich source rocks for granite porphyry and felsite on the nb vs y ta vs yb rb vs y nb and rb vs yb ta tectonomagmatic discrimination diagrams these two granites are connected with the volcanic arc related setting and plot in post collision setting moreover some data of the hoechang granite fall in wpg field suggesting that the volcanic arc setting may be changed to an intraplate setting from middle cretaceous the pyrite trace element geochemistry showing the high co ni ratio than one with an average of 8 2 and high co content and low ni content suggests that its formation was closely linked with i type granitic magma derived hydrothermal fluid mixed with meteoric water it could also be assumed by the co and as contents in pyrite indicating a certain degree of meteoric water in the ore forming fluid it is consonant to the pb s c and o isotope geochemistry in the previous study suggesting that ore forming fluids and metals could be derived from both the yangdok granite and the proterozoic and paleozoic meta sedimentary rocks exposed in the study area all of the pyrite grains have not compositional zoning and the similar distribution patterns of co and ni are appeared in single pyrite grain from core to rim suggesting the stable crystallization environment of pyrites and same ore forming fluid source in pyrite grains the positive correlation between gold and arsenic is appeared the au exists either as a micro and nano size inclusion in pyrite and other minerals or as a solid solution in pyrite lattice based on the petrological and microthermometric characteristics of the fluid inclusions in ore bearing quartz we suggest that the ore forming fluid is represented as nacl h2o or nacl kcl h2o systems with low salinity the absence of co2 enriched phase and vapor only or liquid only primary inclusions in quartz indicates the homogenous magma derived ore forming fluids and a lack of boiling during ore forming process the formation temperature and pressure of this deposit approximately vary from 317 to 326 c and from 105 46 to 111 69 105 pa respectively exhibiting the hydrothermal deposit origin in conclusion the songhung deposit is originated from i type granitic magma source derived hydrothermal fluid which was mixed with meteoric water

geology mineralogy and geochemistry of fault controlled hydrothermal cu au mineralization in the shanmen volcanic basin se china abstract the yukeng banling deposit is a typical fault controlled hydrothermal cu au deposit in the shanmen volcanic basin svb se china ore bodies commonly occur as lodes lenses and disconnected pods dipping sw with vertical zonation of ore minerals ore related hydrothermal alteration is well developed on both sides of the veins dominated by silicification sericitization chloritization and argillation with a banded alteration zonation the mineralization can be divided into three stages stages i ii and iii native gold is present as veinlets in fractures of fine grained pyrite from stage ii zircon u pb and rb sr isochron ages indicate that the cu au mineralization is coeval with the caomen alkaline granite and xiaokeng quartz diorite both emplaced at ca 102 ma microthermometric measurements of fluid inclusions in quartz and sphalerite from stage ii veins indicate that the yukeng banling deposit is an epithermal deposit six ore related quartz grains have d h2o values of 69 to 43 and 18 o h2o values calculated using total homogenization temperatures that range from 2 0 to 0 7 all samples plot in an area between the magmatic field and the meteoric line suggesting that the ore forming fluids are derived from a mixed source of magmatic and meteoric waters 34 s values for eight pyrite separates range from 2 1 to 4 1 with an average of 1 7 and 34 s values for galena and sphalerite are 2 3 and 2 2 similar to magmatic sulfur four alkaline granite samples have pb isotopic ratios 206 pb 204 pb t 18 175 18 411 207 pb 204 pb t 15 652 15 672 and 208 pb 204 pb t 38 343 38 800 three quartz diorite samples have ratios 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t of 18 277 18 451 15 654 15 693 and 38 673 38 846 respectively these age calculated lead isotopic data for alkaline granite are similar to those for the analyzed sulfides co ni ratios for stage ii pyrites range from 1 42 to 5 10 indicating that the yukeng banling deposit records the past involvement of magmatic hydrothermal fluids the isotope data together with geological mineralogical and geochronological evidence favor a primary magmatic source for sulfur and metals in the ore fluids mixing of the cu and au rich fluids with meteoric water led to precipitation of the cu au veins along nw trending faults the yukeng banling deposit the contemporaneous caomen alkaline granite and xiaokeng quartz diorite in the svb formed under an extensional setting due to high angle subduction of the paleo pacific plate the extensional setting facilitated the formation of cu and au rich magmas which was derived from enriched mantle and lower crust

gold and trace element zonation in pyrite using a laser imaging technique implications for the timing of gold in orogenic and carlin style sediment hosted deposits laser ablation icp ms imaging of gold and other trace elements in pyrite from four different sedimenthosted gold arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements in particular as ni pb zn ag mo te v and se and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and or hydrothermal pyrite or as narrow goldarsenic rims on the outermost parts of the overgrowth hydrothermal pyrite compared to the diagenetic pyrites the hydrothermal pyrites are commonly depleted in ni v zn pb and ag with cyclic zones of co ni and as concentration the outermost hydrothermal pyrite rims are either as au rich as in moderate to highgrade deposits such as carlin and bendigo or co ni rich and as au poor as in moderate to low grade deposits such as sukhoi log and spanish mountain the early enrichment of gold in arsenic bearing syngenetic to diagenetic pyrite within black shale facies of sedimentary basins is proposed as a critical requirement for the later development of carlin style and orogenic gold deposits in sedimentary environments the best grade sediment hosted deposits appear to have the gold climax event toward the final stages of deformation related hydrothermal pyrite growth and fluid flow

ore fluid evolution in the giant marcona fe cu deposit peru evidence from in situ sulfur isotope and trace element geochemistry of sulfides the marcona magnetite deposit located in the iron oxide copper gold iocg province of southern peru is an important fe deposit with sub economic cu and zn mineralization three generations of sulfides were identified at the marcona alteration mineralization stages iv v and vii magnetite pyrrhotite pyrite and minor chalcopyrite started to form at stage iv sulfides in stage v are dominated by chalcopyrite and pyrite whereas only pyrite was present in stage vii in situ sims sulfur isotope and la icp ms trace element analyses were conducted on the sulfides of these stages stage iv sulfides contain 34s largely in the range of 0 to 4 0 indicating a dominantly magmatic origin in particular 34s values of stage iv pyrite increase from core to rim and the corresponding trace elements concentrations vary along with 34s positive 34s vs se s correlation of stage iv pyrite indicates that the ore forming system was open the stage v pyrite are rich in mn cu ag sb te pb and bi and all stage v sulfides have also elevated 34s up to 7 8 indicative of possible seawater incursion during its formation the 34s values of stage vii pyrite are highly variable from 13 4 to 5 4 this together with ubiquitous disseminated voids low co ni ratios and enrichments of mn se sb te tl and pb suggests that stage vii pyrite may have formed by sulfate reduction associated with organic materials we propose that the ore forming fluids of stage iv are initially magmatic with involvement of external e g seawater at late where the fluids of stage v are likely mainly exotic the external fluids together with organic matter may have contributed to the ore forming fluids of stage vii indicating a complex fluid evolution history for this giant fe cu deposit

convection under a lid of finite conductivity heat flux scaling and application to continents 1 a scaling law for the heat flux out of a convective fluid covered totally or partially by a finitely conducting lid is proposed this scaling is constructed in order to quantify the heat transfer out of the earth s mantle taking into account the effect of the dichotomy between oceans and continents which imposes heterogeneous thermal boundary conditions at the surface of the mantle the effect of these heterogeneous boundary conditions is studied here using simple two dimensional models with the mantle represented by an isoviscous fluid heated from below and continents represented by nondeformable lids of finite thermal conductivity set above the surface of the model we use free slip boundary conditions under the oceanic and continental zones in order to study in an isolated way the possible thermal effect of continents independently of all mechanical effect a systematic study of the heat transfer as a function of the rayleigh number of the fluid of the width of the lid and of its thermal properties is carried out we show that estimates of continental lithosphere thickness imply a strong insulating effect from continents on mantle heat loss at least locally the heat flux below continents was low in the past and of the order of the present one if the continental thickness has remained broadly constant over the earth s history

multiple phase transitions and the style of mantle convection abstract multiple phase transitions based on experimental and theoretical results have been incorporated into an extended boussinesq model of mantle convection a formulation is used which facilitates the numerical computations of convection with multiple phase transitions and a triple point near the 670 km discontinuity the propensity for layered convection is enhanced by including the additional phase transitions the location of the triple point of the spinel to perovskite phase transitions plays an important role in controlling the fate of the ascending hot plumes a plume with a core temperature above this triple point temperature t c tends to pass easily through the phase boundary at 670 km plumes with a core temperature lower than t c are blocked by the multiple phase transitions both depth dependent thermal expansivity and internal heating increase the tendency of the convective system to layering the dynamical time scales in the transition zone are much shorter than those of the top and bottom boundary layers simulations with increasing rayleigh number ra from 10 6 to 5 10 7 show unambiguously the increased tendency toward layering with larger ra in the early earth when ra was higher and internal heating was several times stronger layered convection might have been the preferred mode of mantle convection with time as both ra and radioactive heating decreased the style of mantle convection would have become less layered

la icp ms trace element analysis of pyrite from the xiaoqinling gold district china implications for ore genesis abstract the xiaoqinling district the second largest gold producing district in china is located on the southern margin of the north china craton it consists of three ore belts namely the northern ore belt the middle ore belt and the southern ore belt pyrite from the dahu gold deposit in the northern ore belt and wenyu and yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in situ laser ablation inductively coupled plasma mass spectrometry la icp ms a range of trace elements was analyzed including au te ag pb bi cu co ni zn mo hg as and si the results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts in general au concentrations in pyrite are low from 2 rich ore fluids spatial association with large scale compressional structures of the qinling orogen 18 o and d data suggestive of mixing between metamorphic and meteoric waters 34 s and pb isotopic data that point to a mixed crustal mantle source all point to typical orogenic type gold deposits high ni concentrations up to 8425 ppm of pyrite possibly linked to deep seated mafic ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers generally low au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits therefore the gold mineralization in the xiaoqinling district is described as orogenic type and is probably related to indosinian collision between the north china craton and the yangtze craton

genesis of the wulong gold deposit northeastern north china craton constraints from fluid inclusions h o s pb isotopes and pyrite trace element concentrations abstract the wulong vein type gold deposit located along the northeastern margin of the north china craton ncc contains 80 tonnes of gold at an average grade of 5 35 g t a total of 380 auriferous quartz veins are hosted in late jurassic gneissic two mica granite and early cretaceous granodiorite a paragenetic sequence representing three hydrothermal stages has been identified 1 quartz pyrite 2 quartz polymetallic sulfide and 3 quartz calcite pyrite petrography fluid inclusion microthermometry and laser raman spectroscopy revealed three types of fluid inclusions co2 ch4 type i h2o co2 ch4 type ii and h2o nacl type iii fluid inclusion data indicate that during mineralization fluid temperatures evolved from 283 to 395 c to 219 328 c to 144 255 c and salinities varied from 0 70 to 8 95 wt to 0 18 7 86 wt to 0 18 4 96 wt nacl equivalent quartz yielded 18oh2o smow and dsmow values of 0 9 7 3 and 65 to 48 respectively indicating that the ore forming fluids were derived from magmatic water and mixed with meteoric water during the later stages of mineralization sulfides have 34scdt sulfur isotopic compositions ranging from 0 9 to 3 9 suggesting that the s was derived from a magmatic source pb isotopic compositions of sulfides 206pb 204pb 17 488 17 718 207pb 204pb 15 565 15 631 and 208pb 204pb 38 190 38 766 indicate that the pb was derived primarily from the upper crust with minor input from the mantle in situ laser ablation inductively coupled plasma mass spectrometry la icp ms trace element analyses of three generations of pyrite py1 py2 and py3 reveal that gold concentrations in py2 are higher than those in py1 and py3 which indicates that gold mineralization occurred primarily during the middle stage high co ni ratios of pyrite are consistent with a hydrothermal origin we suggest that the wulong gold deposit formed during lithospheric thinning associated with destruction of the ncc this process was triggered by subduction of the paleo pacific plate during the early cretaceous

a review of the geodynamic setting of large scale late mesozoic gold mineralization in the north china craton an association with lithospheric thinning abstract abundant gold deposits are distributed along the margins of the north china craton ncc occurring throughout the precambrian basement and located in or proximal to mesozoic granitoids these deposits show a consistent spatial temporal association with late jurassic early cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock their ages are mainly early cretaceous 130 110 ma and constrain a very short period of metallogenesis sr nd pb isotopic tracers of ores minerals and associated rocks indicate that gold and associated metals mainly were derived from multi sources i e the wall rocks precambrian basement and mesozoic granites and associated mafic rocks previous studies including high surface heat flow uplift and later basin development slow seismic wave speeds in the upper mantle and a change in the character of mantle xenoliths sampled by paleozoic to cenozoic magmas have been used to suggest that ancient cratonic mantle lithosphere was removed from the base of the ncc some time after the ordovician and replaced by younger less refractory lithospheric mantle the geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization 130 110 ma indicate that they were derived from an ancient enriched lithospheric mantle source whereas the mafic dikes and volcanic rocks younger than 110 ma were derived from a relatively depleted mantle source i e asthenospheric mantle according to their age and sources relation to magmatism and geodynamic framework the gold deposits were formed during lithospheric thinning the removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature the fluids derived from the lower crust were mixed with magmatic and meteoric waters and resulted in the deposition of gold and associated metals

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

geology c h o s pb isotope systematics and geochronology of the yindongpo gold deposit tongbai mountains central china implication for ore genesis abstract the yindongpo gold deposit is located in the weishancheng au ag dominated polymetallic ore belt in tongbai mountains central china the ore bodies are stratabound within carbonaceous quartz sericite schists of the neoproterozoic waitoushan group the ore forming process can be divided into three stages represented by early barren quartz veins middle polymetallic sulfide veinlets and late quartz carbonate stockworks with most ore minerals such as pyrite galena native gold and electrum being formed in the middle stage the average 18owater values changed from 9 7 in the early stage through 4 9 in the middle stage to 5 9 in the late stage with the d values ranging between 65 and 84 the 13cco2 values of ore fluids are between 3 7 and 6 7 with an average of 1 1 the h o c isotope systematics indicate that the ore fluids forming the yindongpo gold deposit were probably initially sourced from a process of metamorphic devolatilization and with time gradually mixed with meteoric water the 34s values range from 0 3 to 5 2 with peaks ranging from 1 to 4 fourteen sulfide samples yield 206pb 204pb values of 16 990 17 216 207pb 204pb of 15 419 15 612 and208pb 204pb of 38 251 38 861 both s and pb isotope ratios are similar to those of the main lithologies of the waitoushan group but differ from other lithologic units and granitic batholiths in the tongbai area which suggest that the ore metals and fluids originated from the waitoushan group the available k ar and 40ar 39ar ages indicate that the ore forming process mainly took place in the period of 176 140 ma during the transition from collisional compression to extension and after the closure of the oceanic seaway in the qinling orogen the yindongpo gold deposit is interpreted as a stratabound orogenic style gold system formed during the transition phase from collisional compression to extension the ore metals in the waitoushan group were extracted transported and then accumulated in the carbonaceous sericite schist layer the carbonaceous sericite schist layer especially at the junction of collapsed anticline axis and fault structures became the most favorable locus for the ore bodies

gold distribution and source of the j4 gold bearing breccia pipe in the qiyugou district north china craton constraints from ore mineralogy and in situ analysis of trace elements and s pb isotopes abstract the qiyugou district is one of the most important gold deposit distribution areas in the xiong ershan region north china craton these deposits are hosted in archean taihua supergroup rocks as gold bearing breccia pipes here we report a detailed in situ analysis of trace elements and s pb isotopes in sulfides from different ore stages in combination with electron probe microanalysis of ore minerals from the gold bearing j4 breccia pipe in the qiyugou district to unravel the gold distribution and their textural relationships to pyrite the metal sources and s pb isotopic composition variations during the ore forming processes the qiyugou breccia pipes hosted gold ore over 70 t au average grade of 2 5 g t that formed in four mineralization stages i e an early k feldspar biotite magnetite quartz pyrite py 1 stage i that can be further divided into two substages namely the ia k feldspar biotite magnetite stage and the ib quartz pyrite stage a gold pyrite py 2 quartz stage ii a gold quartz polymetallic sulfide py 3 stage iii and a late quartz carbonate stage iv the visible native gold and electrum grains occur as small inclusions and disseminations in the fractures within py 2 and py 3 tellurides and bi pb minerals are widespread in the stage ii stockworks and veins as is intergrowth with visible native gold and py 2 in situ la icp ms trace element analysis reveals the presence of invisible gold hosted in three types of pyrite with extremely low au content py 1 0 05 ppm py 2 0 03 ppm py 3 0 02 ppm concentrations of te bi and pb in pyrite are high and correlate well with au concentrations but as content is low therefore the te bi and pb played a significant role in gold mineralization in as deficient ore fluids in the j4 gold bearing breccia pipe from the qiyugou district high precision in situ laser ablation multiple collector inductively coupled mass spectrometry la mc icp ms lead isotope analyses indicate that the sulfides have 206pb 204pb 207pb 204pb and 208pb 204pb ratios from 17 250 to 17 629 from 15 401 to 15 621 and from 37 649 to 38 290 respectively which are similar to the granite porphyry in the district and indicative of their similar lead sources high precision in situ la mc icp ms sulfur isotope analyses of pyrite show a large 34s variation e g from 18 1 to 11 1 per mil for py 1 from 12 2 to 6 6 per mil for py 2 and from 4 9 to 1 6 per mil for py 3 with a remarkable increasing trend from stage i stage iito stage iii this trend may record a decreasing oxygen fugacity of the ore forming fluids during mineralization

ore genesis of the bake gold deposit southeastern guizhou province china constraints from mineralogy in situ trace element and sulfur isotope analysis of pyrite abstract the bake gold deposit is exceptional for its high native gold abundance relative to the other orogenic gold deposits in the southeastern guizhou district the grades of gold can be up to 2 to 3 wt in bonanza ore and native gold preferentially occurs within voids and fractures in pyrite arsenopyrite and quartz gold content can be up to 6 9 ppm in hydrothermal pyrite however the majority of the pyrite analyses have au contents less than 1 ppm mean 0 32 ppm based on la icpms analyses meanwhile empa reveals that arsenopyrite has a maximal au content of up to 0 14 wt but most analyses are below detection limit 0 01 wt moreover native gold grains in cataclastic pyrite and arsenopyrite are commonly high purity with fineness ranging from 949 to 977 given that paragenetically early hydrothermal pyrite and arsenopyrite contain variable but relatively high gold concentrations than later generations of sulfide it is suggested that early invisible gold may have been remobilized to form at least part of the microscopic visible gold post mineralization shear zones which cut the vein bearing anticline may have led to the deformation of the lode quartz vein system on the macro level and microscopically sulfide fragmentation and elemental remobilization although diagenetic pyrite from unaltered host rocks is enriched in invisible gold with mean values at 0 83 ppm evidence supporting the release of au from them is weak corresponding evidence comes from the distinct sulfur isotope signatures in different pyrite generations diagenetic pyrite has 34s values in the narrow range of 9 27 to 10 75 whereas hydrothermal pyrite shows a broad range 6 81 to 17 42 based on la mc icpms analyses the mesoproterozoic lengjiaxi group underlying the xiajiang group is most likely the source region on the basis of geological background and assuming the metamorphic devolatilization model

in situ trace element and sulfur isotope of pyrite constrain ore genesis in the shapoling molybdenum deposit east qinling orogen china abstract the early cretaceous ca 128 ma shapoling vein type molybdenum mo mineralization associated with the huashan intrusion is a newly discovered deposit in the xiong ershan district east qinling china where pyrite is a ubiquitous mineral phase here we present results from an integrated study on the texture in situ trace element chemistry from laser ablation inductively coupled plasma mass spectrometry la icp ms and sulfur isotopic composition using secondary ion mass spectrometry sims with a view to understand the geochemical variation genesis of mo mineralization and ore forming process the distinct textural patterns as revealed by petrographic observations suggest three types of pyrites py1 is composed of coarse grained euhedral pyrite in the wall rock and early barren quartz vein at stage i py2 is represented by subhedral anhedral grains coexisting with molybdenite in quartz veins at stage ii and py3 forms irregular interstitial grains in the quartz k feldspar molybdenite dominated veins at stage iii the mo enrichment mainly occurred in py3 with co ni ratios 1 and 34s values mainly