simulating the heterogeneity in braided channel belt deposits 2 examples of results and comparison to natural deposits in part 1 of this series we presented a methodology and a code for modeling the hierarchical sedimentary architecture in braided channel belt deposits here in part 2 the code was used to create a digital model of this architecture and the corresponding spatial distribution of permeability the simulated architecture was compared to the real stratal architecture observed in an abandoned channel belt of the sagavanirktok river alaska by lunt et al 2004 the comparisons included assessments of similarity which were both qualitative and quantitative from the qualitative comparisons we conclude that a synthetic deposit created by the code has unit types at each level with a geometry which is generally consistent with the geometry of unit types observed in the field the digital unit types would generally be recognized as representing their counterparts in nature including cross stratasets lobate and scroll bar deposits channel fills etc furthermore the synthetic deposit has a hierarchical spatial relationship among these units which represents how the unit types are observed in field exposures and in geophysical images in quantitative comparisons the proportions and the length width and height of unit types at different scales across all levels of the stratal hierarchy compare wellmore between the digital and the natural deposits a number of important attributes of the channel belt model were shown to be influenced by more than one level within the hierarchy of stratal architecture first the high permeability open framework gravels percolated at all levels and thus formed preferential flow pathways open framework gravels are indeed known to form preferential flow pathways in natural channel belt deposits the nature of a percolating cluster changed across different levels of the hierarchy of stratal architecture as a result of this geologic structure the percolation occurs at proportions of open framework gravels below the theoretical percolation threshold for random infinite media second when the channel belt model was populated with permeability distributions by lowest level unit type the composite permeability semivariogram contained structures that were identifiable at more than one scale and each of these structures could be directly linked to unit types of different scales existing at different levels within the hierarchy of strata these collective results are encouraging with respect to our goal that this model be relevant as a base case in future studies for testing ideas in research addressing the upscaling problem in aquifers and reservoirs with multi scale heterogeneity less

subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

gold mineralisation throughout about 45 ma of archaean orogenesis protracted flux of gold in the golden mile yilgarn craton western australia the golden mile deposit was discovered in 1893 and represents today the largest archaean orogenic lode gold system in the world 50 m oz produced gold the golden mile deposit comprises three major styles of gold mineralisation fimiston oroya and charlotte styles fimiston style lodes formed at 250 to 350 c and 100 to 200 mpa and are controlled by brittle ductile fault zones their subsidiary fault zone and vein networks including breccias and open cavity infill textures and hydrothermally altered wall rock fimiston lodes were formed late d1 prior to d2 regional upright folding hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite an increase in sericite and in fe content of carbonates lodes contain siderite pyrite native gold 17 different telluride minerals au ag tellurides contain 25 of total gold tourmaline haematite sericite and v rich muscovite oroya style lodes formed at similar p t conditions as the fimiston lodes and are controlled by brittle ductile shear zones associated dilational jogs that are particularly well developed at the contact between paringa basalt and black shale interflow sedimentary rocks and altered wall rock the orebodies are characterised by micro breccias and zones of intense shear zone foliation very high gold grades up to 100 000 g t au and the common association of tellurides and vanadian mica green leader oroya lodes crosscut fimiston lodes and are interpreted to have formed slightly later than fimiston lodes as part of one evolving hydrothermal system spanning d1 and d2 deformation ca 2 675 2 660 ma charlotte style lodes exemplified by the mt charlotte deposit are controlled by a sheeted vein stockwork complex of north dipping quartz veins and hydrothermally altered wall rock the mt charlotte orebody formed at 120 to 440 c and 150 to 250 mpa during movement along closely spaced d4 2 625 ma and reactivated d2 faults with the quartz granophyre in the golden mile dolerite exerting a strong lithological control on gold mineralisation veins consist of quartz carbonate minor scheelite and wall rock alteration comprises chlorite destruction and growth of ferroan carbonate sericite pyrite native gold pyrite pyrrhotite is zoned on the scale of vein haloes and of the entire mine giving a vertical temperature gradient of 50 100 c over 1 000 vertical metres the structural hydrothermal model proposed consists of four major stages 1 d1 thrusting and formation of fimiston style lodes 2 d2 reverse faulting and formation of oroya style lodes 3 d3 faulting and dissecting of fimiston and oroya style lodes and 4 d4 faulting and formation of mt charlotte style sheeted quartz vein system the giant accumulation of gold in the golden mile deposit was formed due to protracted gold mineralisation throughout episodes of an archaean orogeny that spanned about 45 ma fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing efficiency and duration in addition to the long lasting fluid plumbing system the wide variety of transient structural and geochemical traps multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world

the multiple sulfur isotope architecture of the golden mile and mount charlotte deposits western australia the golden mile and mount charlotte deposits in the kalgoorlie terrane western australia display three main mineralization styles fimiston comprised of interconnected shear zones associated with ankerite pyrite hematite magnetite gold telluride alteration oroya made up of breccia bodies with v muscovite ankerite pyrite pyrrhotite gold telluride alteration and mount charlotte which consists of vein arrays with symmetrical ankerite sericite albite pyrite pyrrhotite gold alteration pyrite is in equilibrium with gold in all three mineralization styles and has been selected as a proxy to record the sulfur source of the mineralizing fluids as well as the nature of the hydrothermal processes the 34s 33s and 36s analyses on pyrite grains from the different mineralization styles including oxidized and reduced sulfide oxide assemblages reveal 1 a large variation in 34s from 12 6 to 23 5 and 2 a previously unrecognized occurrence of anomalous 33s and 36s signatures from 1 0 to 1 1 and from 2 3 to 0 9 respectively it is argued that the mineralizing fluids that formed the golden mile and mount charlotte deposits record mixing among three components mantle sulfur oxidized seawater sulfur e g so4 and reduced elemental sulfur e g s8 petrographic evidence in conjunction with 33s and 36s data suggest that mif s was acquired during the deposition of shales and basalts present in the kalgoorlie terrane and later mixed with mantle derived sulfur during the mineralization events the negative 34s values that predominate in fimiston style mineralization are consistent with a prevalence of oxidized fluids during the ore forming process as reflected by the presence of hematite pyrite magnetite gold assemblages conversely the positive 34s values that dominate in the mt charlotte and oroya mineralization styles reflect a reducing environment as reflected by the presence of pyrite pyrrhotite gold assemblages

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

salt sources and water rock interaction on the yilgarn block australia isotopic and major element tracers internal drainage in the south east of western australia are underlain by predominantly granitoid rocks the regional shallow ground water are nacl brines ratios of 87sr 86sr in the brines are 0 7155 0 0015 which are close to the ratio of modern marine sr 0 7092 a two component isotopic mixing model with end members of marine sr and sr from basement granitoid rocks implies strongly that 5 of the sr is derived from rock weathering and 95 is marine the isotropic composition of s in gypsum crystallised from the brines is 19 8 0 3 cdt this value is close to that of s in recent evaporites 21 5 1cdt and confirms that weathering of archaean sulphides with 34sof0 4 cdt has contributed insignificant amounts of s to the brines the isotopic data are compatible with a late tertiary to recent age for the sr and s and by implication for the salts as a whole the formation of alunite has been a major control on brine composition while gypsum and calcrete formation have been minor influences none of the ground waters is saturated with halite which suggests the brines are immature and have evolved their present chemistry within the past few tens of thousands rather than millions of years marine aerosols are the main source of salt water sequestered in internal drainage basins by a post middle miocene regression possibly provided a subordinate fraction

geology fluid inclusions and isotope geochemistry of the herenping gold deposit in the southern margin of the yangtze craton china a sediment hosted reduced intrusion related gold deposit abstract the herenping gold deposit is a typical gold deposit in the xuefengshan arcuate tectonic zone in the southern margin of the yangtze craton and the major feature of the gold mineralisation in the zone is the varying degrees of tungsten and antimony mineralisation in the ore deposits the herenping gold deposit occurs in the reddish purple sandy slate in the middle section of the madiyi formation of the neoproterozic banxi group and is controlled by the fault fold zone associated with the liulincha anticlinorium and f8 fault zone however its sources of mineralisation and relationships between hydrothermal fluids and magmatic rocks are unclear which similar to those of many gold deposits in metasedimentary terranes in the worldwide the mineralisation can be divided into four stages an albite quartz stage i an albite quartz pyrite gold stage ii a quartz gold polymetallic sulphide stage iii and a quartz carbonate stage iv the gold mineralisation occurred in albite quartz veins in stage ii and in sulphide rich quartz veins in stage iii in the deposit a study of the fluid inclusions reveals that the homogenisation temperature and salinity were slightly higher in stage ii th 186 348 c w 5 2 17 43 wt nacl equiv than in stage iii th 145 249 c w 2 9 15 57 wt nacl equiv indicating that the temperature and salinity of the ore forming fluid are gradually decreasing the h o d 62 3 to 53 0 18o 4 2 11 8 isotope characteristics reveals that the ore forming fluid is a magmatic hydrothermal source the sulphur 34s 4 8 2 0 isotope characteristics and the alteration mineral assemblage sericite chlorite pyrite indicates that the ore forming fluids was reduced the lead 206pb 204pb 17 419 17 436 207pb 204pb 15 519 15 535 and 208pb 204pb 37 86 37 90 isotope characteristics reflect the ore forming materials were likely derived from a deep magmatic source in addition the geochemical characteristics of albitite veins show that they are silicon rich 64 02 66 26 wt and significantly reduced notably the herenping gold deposit is located in the western portion of the tungsten tin metallogenic provinces in south china is host in epimetamorphic detrital rocks and was formed at least two stages of mineralisation typical gold bearing albite quartz veins developed during the primary stage these characteristics are consistent with those of reduced intrusion related deposits such as the muruntau kumtor and telfer deposits thus the herenping gold deposit may be a sediment hosted intrusion related gold deposit based on the phase transition pattern from sulphide rich quartz veins to albite quartz veins to albitite veins with increasing depth in the deposit we propose that reduced intrusions deep in the herenping gold deposit could be discovered in the future which implies the possible existence of a reduced intrusion related gold system in the xuefengshan arcuate tectonic zone

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

dilution enhancement in hierarchical and multiscale heterogeneous sediments abstract we quantify the dilution of solute clouds in geological formations displaying hierarchical and multiscale sedimentary architecture through the use of a lagrangian framework this allows for a fundamental understanding of how dilution arises from the hierarchical architecture of sedimentary facies and enables for a quantitative decomposition of dilution into facies related contributions at different scales within the hierarchy the new developed semi analytical solution that quantify the dilution index is based on hierarchical expressions of the spatial covariance of the log conductivity the spatial correlation structure in the covariance expression is defined by the probability of transitioning within and across facies types of different scales which are parameterized by independent and physically quantifiable attributes of sediments including geostatistical properties of the conductivity field and the proportions and lengths of facies types we illustrate the applicability of the model in the well documented tracer test at the borden research site canada we show how different scales of sedimentary architecture contribute to the dilution enhancement of the solute cloud at borden aquifer by quantitatively decomposing the dilution index into facies related contributions we gain fundamental insights on the key factors controlling the time dependent rate of dilution the results also illustrate that the probability of transitioning from one facies to another i e cross transitions at different scales defines the temporal scaling for enhanced dilution additionally we perform sensitivity analyses to investigate the effects of mean difference in conductivity volume proportion the statistical anisotropy ratio and the scale of heterogeneity on the overall solute dilution

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

the elastic properties of the lithosphere beneath scotian basin abstract to assess the possibility that the north atlantic ocean may subduct at scotian basin east of canada we investigate the present compensation state of this deep basin a fourier domain analysis of the bathymetry depth to basement and observed gravity anomalies over the oceanic area east of nova scotia indicates that the basin is not isostatically compensated moreover the analysis emphasizes that in addition to the sediments density perturbations exist beneath the basin the load produced by the sediments and these density perturbations must have been supported by the lithosphere we simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior it is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere we relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature dependent rheology we suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin that were formed during the break up of africa and north america and were locked in the early stages after the break up the weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the north atlantic ocean in the future the reactivation of the faults would create a free boundary condition at the hinge zone allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent this may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces

sedimentary loading lithospheric flexure and subduction initiation at passive margins investigation of lithospheric flexure at continental margins by sedimentary loading has been carried out using elastic plate theory the geometry of mature marginal basins fits the deflection produced by loading of an infinite plate in which the flexural rigidity of the continental lithosphere is larger than that of oceanic lithosphere decoupling of a previously locked passive margin by reactivation of margin faults may cause the lithospheric bending behavior of the margin to change from that of an infinite plate to that of a semi infinite plate with a resultant increase in deflection of the marginal basin and possible subduction initiation

geologic factors controlling co2 storage capacity and permanence case studies based on experience with heterogeneity in oil and gas reservoirs applied to co2 storage a variety of structural and stratigraphic factors control geological heterogeneity inferred to influence both sequestration capacity and effectiveness as well as seal capacity structural heterogeneity factors include faults folds and fracture intensity stratigraphic heterogeneity is primarily controlled by the geometry of depositional facies and sandbody continuity which controls permeability structure the permeability structure in turn has implications for co2 injectivity and near term migration pathways whereas the long term sequestration capacity can be inferred from the production history examples of gulf coast oil and gas reservoirs with differing styles of stratigraphic heterogeneity demonstrate the impact of facies variability on fluid flow and co2 sequestration potential beach and barrier island deposits in west ranch field in southeast texas are homogeneous and continuous in contrast seeligson and stratton fields in south texas examples of major heterogeneity in fluvial systems are composed of discontinuous channel fill sandstones confined to narrow sinuous belts these heterogeneous deposits contain limited compartments for potential co2 storage although co2 sequestration effectiveness may be enhanced by the high number of intraformational shale beds these field examples demonstrate that areas for co2 storage can be optimized by assessing sites for enhanced oil and gas recovery in mature hydrocarbon provinces

the source of ore fluids and solutes in archaean lode gold deposits of western australia abstract the ore fluids which produced archaean lode gold deposits in sub amphibolite facies rocks of the yilgarn block western australia are characteristically low to moderate salinity h 2 o co 2 ch 4 fluids wall rock alteration and ore deposition were from a generally reducing near neutral fluid at 200 350 c and 1 3 kbar although more oxidising fluids produced some assemblages in the giant golden mile deposit gold was transported as reduced sulphur complexes the ore fluid composition and stable and radiogenic isotope data indicate multiple sources for the fluid and solute deep plumbing systems are invoked with the fluids being either metamorphic fluids that derived solutes from the lower and upper crusts or fluids exsolved from deep seated granitoids formed by anatexis of such crust

seasat observations of flexure evidence for a strong lithosphere abstract outer rises seaward of subduction zones arise due to flexure of the oceanic lithosphere a mechanical model of this flexure is used to predict the shape of the associated geoid signal these predicted geoid signals are compared with seasat altimeter profiles over outer rises adjacent to nine major subduction zones the nine zones include the aleutian mariana izu bonin puerto rico lesser antilles peru chile middle america south sandwich philippine and kuril the comparisons with seasat data provide estimates of effective elastic thickness he and maximum bending moment mm for each zone it is shown that 1 1 these thicknesses hc are approximately proportional to the square root of lithospheric age 2 2 the maximum bending moments mm increase roughly as the cube of thickness hc these relations 1 and 2 are both very similar to those predicted by a layered rheology model brittle elastic ductile of the lithosphere derived from mechanical experiments on rocks according to this model thermally activated creep is the mechanism of ductile flow in the lower lithosphere hence an isotherm 740 50 defines the bottom of the lithosphere thus old oceanic lithosphere may have significant strength to depths in excess of 60 km or about half that of the thermal lithosphere

contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world s oceans regional studies have shown that remanent magnetization of the cretaceous quiet zones cqzs created during the long period of geomagnetic normal polarity at 118 84 ma produce well defined magnetic anomalies at satellite altitudes we investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale we consider entire oceanic areas because oceanic lithosphere formed during periods other than cqz but with a predominant polarity also have appreciable contributions to the satellite anomalies the magnetic anomalies of the world s oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map plate relative motions and the apparent polar wander path of africa three magnetization models are examined thermoremanent magnetization confined to extrusive layer 2a and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km although all models lead to rather similar anomaly distributions the amount of magnetization required suggests the need for deeper sources all models produce the satellite anomalies associated with the cqz in the north atlantic and parts of the indian and pacific oceans with slight differences in location depending on the models low amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the indian and pacific oceans are successfully modeled a major difference between models and observation is the north south elongated model anomalies associated for instance with the cqzs in the south atlantic and the fast spreading east pacific rise these elongated anomalies are systematically absent on the observed map probably because they were removed by along track filtering in the early stages of processing the satellite data a careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4 0 1 and 0 6 a m for the extrusive basalts intrusive basalts gabbros and peridotites respectively

a quantitative three dimensional depositional model of gravelly braided rivers a quantitative three dimensional depositional model of gravelly braided rivers has been developed based largely on the deposits of the sagavanirktok river in northern alaska these deposits were described using cores wireline logs trenches and ground penetrating radar profiles the origin of the deposits was inferred from observations of 1 channel and bar formation and migration and channel filling interpreted from aerial photographs 2 water flow during floods and 3 the topography and texture of the river bed at low flow stage this depositional model quantitatively represents the geometry of the different scales of strataset the spatial relationships among them and their sediment texture distribution porosity and permeability in the model are related to sediment texture the geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type e g ripples dunes bedload sheets bars associated with deposition of the strataset in particular the length to thickness ratio of stratasets is similar to the wavelength to height ratio of associated bedforms furthermore the wavelength and height of bedforms such as dunes and bars are related to channel depth and width therefore the thickness of a particular scale of strataset i e medium scale cross sets and large scale sets of inclined strata will vary with river dimensions these relationships between the dimensions of stratasets bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits the depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

relative permeability and trapping of co2 and water in sandstone rocks at reservoir conditions 1 we report the results of an experimental investigation into the multiphase flow properties of co2 and water in four distinct sandstone rocks a berea sandstone and three reservoir rocks from formations into which co2 injection is either currently taking place or is planned drainage relative permeability and residual gas saturations were measured at 50 c and 9 mpa pore pressure using the steady state method in a horizontal core flooding apparatus with fluid distributions observed using x ray computed tomography absolute permeability capillary pressure curves and petrological studies were performed on each sample relative permeability in the four samples is consistent with general characteristics of drainage in strongly water wet rocks measurements in the berea sample are also consistent with past measurements in berea sandstones using both co2 brine and oil water fluid systems maximum observed saturations and permeabilities are limited by the capillary pressure that can be achieved in the experiment and do not represent endpoint values it is likely that maximum saturations observed in other studies are limited in the same way and there is no indication that low endpoint relative permeabilities are a characteristic of the co2 water system residual trapping in three of the rocks is consistent with trapping in strongly water wet systems and the results from the berea sample are again consistent with observations in past studies this confirms that residual trapping can play a major role in the immobilization of co2 injected into the subsurface in the mt simon sandstone a nonmonotonic relationship between initial and residual co2 saturations is indicative of a rock that is mixed or intermediate wet and further investigations should be performed to establish the wetting properties of illite rich rocks the combined results suggest that the petrophysical properties of the multiphase flow of co2 water through siliciclastic rocks is for the most part typical of a strongly water wet system and that analog fluids and conditions may be used to characterize these properties further investigation is required to identify the wetting properties of illite rich rocks during imbibition processes

broad synchroneity of three gold mineralization styles in the kalgoorlie gold field shrimp u pb and 40ar 39ar geochronological evidence there has been a long standing controversy regarding the timing and number of gold mineralization events at kalgoorlie uranium pb dating of zircons and hydrothermal monazite and xenotime as well as ar 40 ar 39 analysis of metasomatic fuchsite and white mica are used to date pre to synore dikes alteration and orebodies in order to resolve this issue the majority of gold mineralization at kalgoorlie including ductile brittle fimiston brittle ductile oroya and brittle mount charlotte style gold are different expressions of a complex mineralizing system that was active at broadly the same time at ca 2 64 ga gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time limier similar pt conditions this giant ore system formed after ea 2 69 ga basic magmatism intrusion of the golden mile dolerite sill at 2680 9 ma and intrusion or calc alkaline feldspar quartz porphyry dikes at 2670 5 ma gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 6 ma and overlapped the waning stages of hornblende and albite bearing porphyry dike emplacement at 2650 6 ma and regional metamorphism subsequent brittle deformation in the kalgoorlie gold held way accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the golden mile between about 2 61 and 2 60 ca this late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks it kalgoorlie and may be related to uplift lift and final cooling of the terrane despite this late event the geometry of the kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization

variations of elastic plate thickness at continental thrust belts after examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in europe asia and the americas we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere depending on the state of stress continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age we explain this observation as the result of two competing effects a thicker thermal plate beneath the continents which allows a deeper elastic ductile transition for plates more than 100 million years old and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses the 100 km values for elastic plate thickness of billion year old lithosphere require a thermal plate thickness of at least 250 km the magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust the low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following dip of the underthrust plate radius of curvature of the thrust front in map view and total length of the mountain belt using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones we arrive at the following sequence of causes and effects the primary variable is the dip of the downgoing plate presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle where this dip is steep the rigidity of the plate is weakened due to inelastic yielding the weakened plate can then buckle at small wavelengths into short arcuate mountain belts thus we believe that the mantle ultimately controls the pattern of surface tectonics but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed

streamline based simulation of carbon dioxide storage in a north sea aquifer 1 we model carbon dioxide co2 storage into a deep north sea aquifer using streamline based simulation we assume incompressible flow of liquid like co2 and aqueous phases we simulate dissolution of co2 and a rate limited precipitation reaction advective transport and reactions are solved along streamlines while dispersion and flow due to gravity segregation of the phases are solved on the underlying grid geological storage is modeled on a one million cell model the distribution of co2 after injection is dominated by advective transport due to multiphase flow and co2 moves preferentially through high permeability channels without reaction the regional groundwater flow causes the co2 to continue to migrate until it reaches residual saturation where it continues slowly to dissolve precipitation leads to a decrease in porosity and permeability while co2 is stored in the solid phase the storage efficiency is low around 2 because of aquifer heterogeneity

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

compensation of the ontong java plateau by surface and subsurface loading gravity data over ontong java plateau reveal deep crustal structure supporting the plateau s topography at long wavelengths 500 km new shipboard gravity and bathymetry profiles indicate local isostatic compensation at intermediate wavelengths 250 500 km however bouguer anomalies have large amplitudes too high to be explained by models of airy isostasy or volcanic loading on the surface of an elastic lithospheric plate this finding is also evident in admittance functions generated from maps of bathymetry and satellite derived bouguer anomalies which show high values at intermediate wavelengths in addition coherence between two dimensional bouguer gravity and bathymetry spectrums decreases to zero beginning at intermediate wavelengths two possible models can explain the high admittance and low coherence at intermediate wavelengths the first model explains the high admittance by large scale lithospheric folding and explains the low coherence by sedimentation and erosion that is uncorrelated with the igneous crustal structure lithospheric folding might result from tectonic compression imposed on ontong java by the solomon arc and trench system the second model involves a multiple stage accretion history with surface magmatism on a thin elastic plate and magmatic underplating beneath a thick plate the thick plate during underplating is required to explain the high admittance values at intermediate wavelengths while decorrelation between underplating and the surface loaded magmas can explain the low coherence the thick plate also suggests that the lithosphere cooled appreciably since the first magmatic stage and therefore magmatic underplating occurred at a significantly later time than the initial eruption calculations predict that the underplated material could comprise as much as 50 of the total crustal volume suggesting that the later eruptive stage s may have been as or more voluminous than the first

imaging and quantification of spreading and trapping of carbon dioxide in saline aquifers using meter scale laboratory experiments the role of capillary forces during buoyant migration of co2 is critical toward plume immobilization within the postinjection phase of a geological carbon sequestration operation however the inherent heterogeneity of the subsurface makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in situ plume evolution to overcome the lack of accurate and continuous observations at the field scale and to mimic vertical migration and entrapment of realistic co2 plumes in the presence of a background hydraulic gradient we conducted two unique long term experiments in a 2 44 m 0 5 m tank x ray attenuation allowed measuring the evolution of a co2 surrogate fluid saturation thus providing direct insight into capillarity dominated and buoyancy dominated flow processes occurring under successive drainage and imbibition conditions the comparison of saturation distributions between two experimental campaigns suggests that layered type heterogeneity plays an important role on nonwetting phase nwp migration and trapping because it leads to i longer displacement times 3 6 months versus 24 days to reach stable trapping conditions ii limited vertical migration of the plume with center of mass at 39 versus 55 of aquifer thickness and iii immobilization of a larger fraction of injected nwp mass 67 2 versus 51 5 of injected volume as compared to the homogenous scenario while these observations confirm once more the role of geological heterogeneity in controlling buoyant flows in the subsurface they also highlight the importance of characterizing it at scales that are below seismic resolution 1 10 m

geology and pyrite sulfur isotopes of the suoluogou gold deposit implication for crustal continuum model of orogenic gold deposit in northwestern margin of yangtze craton sw china abstract since the crustal continuum model was proposed mainly based on the variation of mineralization and alteration features of hypozonal mesozonal to epizonal deposits genetic relationship between the different types in one ore belt still requires research this issue was addressed by the research on the suoluogou mesozonal orogenic gold deposit and its comparison with the hypozonal orogenic gold deposit in northwestern margin of yangtze craton gold mineralization in suoluogou is controlled by the bends of roughly ew trending brittle fault developed in the late triassic strata and it is associated with quartz sericite chlorite carbonate and sulfide alteration pyrite and arsenopyrite are the predominant sulfide mineral with minor amounts of chalcopyrite sphalerite and galena four paragenetic stages are divided based on crosscutting relationship and mineral assemblage including the pre mineralization stage stage the hydrothermal quartz sulfide stage stage and carbonate quartz sulfide stage stage iii and the post mineralization carbonate pyrite stage stage the 34s values for pyrite in stage vary from 6 01 to 8 59 and those in stage iii are from 0 96 to 4 62 the decrease of 34s value from stage to iii is explained to the result of the isotope fractionation during separation of oxidised sulfur species in the brecciated space the sulfide 34s values in the initial gold mineralization stage of the suoluogou deposit are similar to those of regional hypozonal orogenic deposit with derivation from metasomatized mantle in the craton margin this similarity suggests that the mesozonal and hypozonal gold deposits formed in different depths of a crustal continuum model for the deposits sharing similar subcrustal source

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

two sulphur isotope provinces deduced from ores in the mount isa eastern succession australia the sulphur isotopic characteristics of ore deposits in the australian mount isa eastern succession are not well known unlike those of the western succession in this study new detailed analyses are provided for recently discovered eastern succession mineralisation such as the starra and osborne bif hosted cu au ores the dugald river sediment hosted pb zn prospect and four vein hosted cloncurry style cu au deposits hampden mt elliot swan mt cobalt and the answer mine all of the deposits of the eastern succession have 34ssulphide between 8 and 9 regardless of their genesis empirically a moderate 34s range averaging close to 0 characterises starra style cu au and pegmont pb zn bif ores whereas shear and vein style cu mineralisation populations are tighter and do not average close to 0 this is a particularly surprising result for dugald river where a larger isotopic variation more typical of stratiform sediment hosted pb zn ores in the region might have been expected by comparison western succession stratiform pb zn and vein style cu deposits span a huge range of 15 to 51 large sulphur isotope ranges typify sulphate evaporite or organic sulphur rich sedimentary successions the lack of such variation in the eastern succession in turn suggests that primary evaporite sequences there were halite dominated but sulphate poor and or contained only limited volumes of organic sulphur rich sediment eastern succession sequences were therefore less likely hosts for giant stratiform pb zn deposits because of their paucity of sulphur although local sulphur sources permitted small deposits such as dugald river to develop sedimentary conditions were more favourable for the development of sulphur poor synsedimentary hydrothermal systems such as starra osborne and pegmont although sulphur isotope evidence is equivocal about the origin of these epigenetic deposits close to the williams batholith mt dore hampden owe their clustering around 0 to their granitic fluid source

isotope geochemistry and genesis of the liyuan gold deposit shanxi north china abstract the liyuan lode gold deposit is located in the shanxi province central north china craton gold orebodies are mainly hosted in archean metamorphic rocks and structurally controlled by the nne trending faults gold occurs in disseminated and auriferous quartz sulfide veins and veinlets within hydrothermally altered rocks mineralization can be divided into three intervals 1 quartz pyrite stage 2 quartz polymetallic sulfides stage and 3 quartz carbonate stage gold formed mainly in the middle stage sericite sample associated with the middle stage pyrite from phyllic alteration zones yields an well defined 40ar 39ar plateau age of 133 3 1 2 ma which is remarkably consistent with zircon u pb age 133 4 1 1 ma of the quartz porphyry dikes in the liyuan mine indicating a close relationship between gold mineralization and granitic magmatism in the area the sulfur isotopic compositions of pyrite py1 2 from the early stage have a narrow range from 0 3 to 4 1 indicating a deep seated magmatic source however the sulfides py3 5 sphalerite galena from the middle stage have lower 34sv cdt values of 7 2 to 3 0 which is thought to be a result of fluid oxidation during gold mineralization the pb isotopic compositions of sulfides from liyuan ores have 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 16 671 16 860 15 236 15 255 and 41 452 44 159 respectively indicting a high thorium lower continental crust source of ore forming materials the 13cvpdb and 18ovsmow values of late stage calcite ranging from 7 1 to 5 8 and 12 2 to 13 7 respectively illustrating a deep seated magmatic source of carbon bearing portion given the absence of granite bodies at the present mining levels we speculate that the ore forming fluids may have been exsolved from a concealed granite pluton at greater depth based on the regional tectono magmatic events ore geology geochronologic and isotopic data obtained in this study together with the previous published fluid inclusion data we suggest that the liyuan deposit is an orogenic gold deposit that is probably related to the early cretaceous granite magmatism in the interior of the north china craton

simulating the heterogeneity in braided channel belt deposits 1 a geometric based methodology and code 1 a geometric based simulation methodology was developed and incorporated into a computer code to model the hierarchical stratal architecture and the corresponding spatial distribution of permeability in braided channel belt deposits the code creates digital models of these deposits as a three dimensional cubic lattice which can be used directly in numerical aquifer or reservoir models for fluid flow the digital models have stratal units defined from the kilometer scale to the centimeter scale these synthetic deposits are intended to be used as high resolution base cases in various areas of computational research on multiscale flow and transport processes including the testing of upscaling theories the input parameters are primarily univariate statistics these include the mean and variance for characteristic lengths of sedimentary unit types at each hierarchical level and the mean and variance of log permeability for unit types defined at only the lowest level smallest scale of the hierarchy the code has been written for both serial and parallel execution the methodology is described in part 1 of this paper in part 2 guin et al 2010 models generated by the code are presented and evaluated

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

ore forming processes of the daqiao epizonal orogenic gold deposit west qinling orogen china constraints from textures trace elements and sulfur isotopes of pyrite and marcasite and raman spectroscopy of carbonaceous material

the effective elastic lithosphere under the cook austral and society islands abstract we have determined the elastic thicknesste of the oceanic lithosphere along two volcanic chains of the south central pacific cook austral and society islands we used a three dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate the model was constrained by geoid height data from the seasat satellite along the cook austral chain the elastic thickness increases westward from 2 4 km at mcdonald hot spot to 14 km at rarotonga at mcdonald seamount however the data are better explained by a local compensation model the observed trend shows an increase ofte with age of plate at loading time however the elastic layer under the cook austral appears systematically thinner by several kilometers than expected for normal seafloor suggesting that substantial thermal thinning has taken place in this region considering the apparent thermal age of the plate instead of crustal age improves noticeably the results along the society chainte varies from 20 km under tahiti to 13 km under maupiti which is located 500 km westward when plotting together the society and cook australte results versus age of load we notice that within the first five million years after loading te decreases significantly while tending rapidly to an equilibrium value this may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young tahiti load 0 8 ma is not yet the equilibrium thickness

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

boron sulphur and copper isotope systematics in the orogenic gold deposits of the archaean hattu schist belt eastern finland abstract the hattu schist belt is located in the western part of the archaean karelian domain of the fennoscandian shield the orogenic gold deposits with au bi te geochemical signatures are hosted by ne sw n s and nw se oriented shear zones that deform 2 76 2 73 ga volcanic and sedimentary sequences as well as 2 75 2 72 ga tonalite granodiorite intrusions and diverse felsic porphyry dykes mo w mineralization is also present in some tonalite intrusions both separate from and associated with au mineralization somewhat younger unmineralized leucogranite intrusions 2 70 ga also intrude the belt lower amphibolite facies peak metamorphism at 3 5 kbar pressures and at 500 600 c temperatures affected the belt at around 2 70 ga and post date hydrothermal alteration and ore formation in this study we investigated the potential influence of magmatic hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope b s cu studies of tourmaline and sulphide minerals by application of in situ sims and la icp ms analytical techniques crystal chemistry of tourmaline from a mo w mineralization hosted by a tonalite intrusion in the hattu schist belt is characterized by fe3 al3 substitution indicating relatively oxidizing conditions of hydrothermal processes the range of 11b data for this kind of tourmaline is from 17 2 to 12 2 the hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry e g dravite povondraite compositional trend with fe3 al3 substitution and 11b values between 19 0 and 9 6 the uvite foitite compositional trend and 11b values between 24 1 and 13 6 characterize metasomatic hydrothermal tourmaline from the metasediment hosted gold deposits composition of hydrothermal vein filling and disseminated tourmaline from the gold bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment hosted tourmaline rock forming magmatic tourmaline from leucogranite has 11b values between 14 5 and 10 8 and the major element composition is similar to that of the metasediment hosted tourmaline the range of 34svcdt values measured in pyrite chalcopyrite and pyrrhotite is from 9 1 to 8 5 which falls within the typical range of sulphur isotope data for archaean orogenic gold deposits in the hattu schist belt positive 34svcdt values characterize metasediment hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite the 34svcdt values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite purely negative values were only recorded from the pyrite dominated gold mineralization within metavolcanic units therefore the shift of 34svcdt values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids the range of measured 65cunbs978 values from chalcopyrite is from 1 11 to 1 19 positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks positive and negative 65cunbs978 values occur in the ores hosted by metavolcanic rocks there is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains evaluation of sulphur and boron isotope data together and comparisons with other archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the hattu schist belt variations in major element and boron isotope compositions in tourmaline as well as in the 34svcdt values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes the results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes

state of stress in the oceanic lithosphere in response to loading summary flexure studies of the oceanic lithosphere constrained by bathymetry and gravity data suggest that the lithosphere behaves elastically over geological time scales for loads to be supported however large bending stresses approaching 10 kb in some cases are required at the top and bottom of the elastic plate these stress differences can be significantly reduced by introducing more complex rheologies we propose a model of layered lithosphere consisting of a purely elastic upper layer a transition zone with viscosity varying with depth and a perfectly plastic lower layer the transition layer is grossly centred at the bottom of the elastic plate such a model results in a noticeable reduction of stress differences reaching 60 per cent for flow laws representing creep mechanisms in olivine when applied to a number of seamount loads this model leads to maximum stress differences which do not exceed 1 2kb the approach used in this study allows us to follow stress relaxation over time taking account of the thermal cooling of the lithosphere we show that the elastic thickness of the lithosphere is stabilized after a given time while the time required for stabilization is found to be of the order 5 6 per cent of the age of the lithosphere at the date of loading

rigidity of the atlantic oceanic lithosphere beneath new england seamounts abstract investigation of the flexural response of the oceanic plate subject to loading of new england seamounts provides an insight into the rigidity of the lithosphere fourier domain analysis of the bathymetry and gravity anomalies over new england seamounts suggests that internal density perturbations exist in the crust and or uppermost mantle the loads due to the topography of the seamounts and density perturbations must have been supported by the lithosphere we group the seamounts into three representative ages and adopt a 3 d thin elastic plate model over an invicid mantle to assess the rigidity of the lithosphere under these loads the rigidity values are about 3 7 10 23 n m for lithospheric ages ranging from 30 to 60 my the base of the elastic part of the lithosphere follows an isotherm in the range of 750 850 c suggesting a strong lithosphere that makes the north atlantic ocean difficult to initiate subduction however the thermal blanketing by the thick sediments at scotian basin reduces the rigidity by one to two orders of magnitude and may facilitate initiation of subduction of the north atlantic oceanic lithosphere at scotian margin

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

formation and preservation of open framework gravel strata in unidirectional flows open framework gravel ofg in river deposits is important because of its exceptionally high permeability resulting from the lack of sediment in the pore spaces between the gravel grains fluvial ofg occurs as planar strata and cross strata of varying scale and is interbedded with sand and sandy gravel the origin of ofg has been related to 1 proportion of sand available relative to gravel 2 separation of sand from gravel during a specific flow stage and sediment transport rate either high falling or low 3 separation of sand from gravel in bedforms superimposed on the backs of larger bedforms 4 flow separation in the lee of dunes or unit bars laboratory flume experiments were undertaken to test and develop these theories for the origin of ofg bed sediment size distribution sandy gravel with a mean diameter of 1 5 mm was kept constant but flow depth flow velocity and aggradation rate were varied bedforms produced under these flow conditions were bedload sheets dunes and unit bars the fundamental cause of ofg is the sorting of sand from gravel associated with flow separation at the crest of bedforms and further segregation of grain sizes during avalanching on the steep lee side sand in transport near the bed is deposited in the trough of the bedform whereas bed load gravel avalanches down the leeside and overruns the sand in the trough the effectiveness of this sorting mechanism increases as the height of the bedform increases infiltration of sand into the gravel framework is of minor importance in these experiments and occurs mainly in bedform troughs the geometry and proportion of ofg in fluvial deposits are influenced by variation in height of bedforms as they migrate superposition of small bedforms on the backs of larger bedforms aggradation rate and changes in sediment supply if the height of a bedform increases as it migrates downstream so does the amount of ofg changes in the character of ofg on the lee side of unit bars depend on grain size sorting in the superimposed bedforms dunes and bedload sheets thick deposits of cross stratified ofg require high bedforms dunes unit bars and large amounts of aggradation these conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound bar tails and downstream of confluence scours increase in the amount of sand supplied relative to gravel reduces the development of ofg such increases in sand supply may be related to falling flow stage and or upstream erosion of sandy deposits

co2 wettability of seal and reservoir rocks and the implications for carbon geo sequestration we review the literature data published on the topic of co2 wettability of storage and seal rocks we first introduce the concept of wettability and explain why it is important in the context of carbon geo sequestration cgs projects and review how it is measured this is done to raise awareness of this parameter in the cgs community which as we show later on in this text may have a dramatic impact on structural and residual trapping of co2 these two trapping mechanisms would be severely and negatively affected in case of co2 wet storage and or seal rock overall at the current state of the art a substantial amount of work has been completed and we find that sandstone and limestone plus pure minerals such as quartz calcite feldspar and mica are strongly water wet in a co2 water system oil wet limestone oil wet quartz or coal is intermediate wet or co2 wet in a co2 water system the contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks the current contact angle data have a large uncertainty solid theoretical understanding on a molecular level of rock co2 brine interactions is currently limited in an ideal scenario all seal and storage rocks in cgs formations are tested for their co2 wettability achieving representative subsurface conditions especially in terms of the rock surface in the laboratory is of key importance but also very challenging

geology mineralogy and stable isotope geochemistry of the dzuunmod area in northern mongolia constraints for gold ore genesis and sources abstract the dzuunmod area located in the north khentii gold belt nkgb of northern mongolia includes lode gold deposits such as gatsuurt sujigtei and boroo with several minor gold deposits and occurrences they show similar hydrothermal alteration assemblages sericitic siliceous and potassic and ore mineral assemblages pyrite and arsenopyrite with minor amount of galena sphalerite and chalcopyrite gold occurs as native form and invisible gold in pyrite and arsenopyrite the major sulfide minerals are separated into earlier non auriferous stage and later auriferous grains containing invisible gold native gold postdates the major sulfide mineralization gold and arsenic content of pyrite grains indicates that gold exists mainly as solid solution form au 1 in the gatsuurt and boroo deposit whereas gold nanoparticle au0 is present in the sujigtei deposit high co ni and mo ni ratios of pyrite grain suggest a post sedimentary or hydrothermal origin and the ore forming fluid was significantly affected by fluid host rock interactions during mineralization processes large variation of 34s values of pyrite and arsenopyrite from 2 6 to 17 2 indicates that sulfur seems to be mainly derived from a source with heterogeneous sulfur isotope composition even though the role of magmatic sulfur as one of possible sulfur sources cannot be ruled out consistent with geological evidence relatively positive 34s values suggest that sulfidation plays an important role for gold and sulfide precipitation the calculated 18o values of hydrothermal fluid from the measured 18o values of quartz samples from 14 7 to 17 7 indicate a metamorphic derivation of ore forming fluid gold mineralization processes in the dzuunmod area seem to occur several times by multiple input of hydrothermal fluid and fluid host rock interactions the gold deposits in the dzuunmod area are considered to be orogenic gold type influenced by fluid host rock interactions in the deposit area

the classification of western australian greenstone hosted gold deposits according to wallrock alteration mineral assemblages abstract the epigenetic greenstone hosted gold deposits in the archaean yilgarn block of western australia can be classified according to their silicate carbonate gangue which forms the innermost wallrock alteration zone of the structurally controlled hydrothermal systems the proposed classification is descriptive and emphasizes associations of prominent macroscopically visible minerals the gold deposits are divided into two large groups high temperature deposits 400 700 c formed at pressures of 3 5 kbars in the lower parts of the greenstone belts 10 15 km depth and medium temperature deposits 250 400 c formed at pressures of 1 to 2 kbars in the upper parts of these belts 3 7 km depth the high temperature deposits are usually located in metamorphic terranes of medium grade amphibolite facies and are characterized by either microcline muscovite andalusite garnet pyroxene biotite or amphibole biotite calcite alteration in the ore zones deposits showing the latter two alteration styles may also be classified as skarns in the descriptive sense of einaudi and burt 1982 the classic medium temperature mesothermal deposits studies of which form the basis of all current models on archaean gold metallogenesis represent only about one half of the total population of deposits in the yilgarn block although they dominate by total gold production as they include the only giant deposit in western australia i e the golden mile at kalgoorlie the medium temperature deposits are usually located in metamorphic terranes of low grade greenschist facies and are characterized by biotite ankerite albite or sericite ankerite albite alteration in the ore zones it is possible that some of these deposits grade into amphibole biotite calcite skarns at depth

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

genesis of the zaozigou gold deposit west qinling orogen china constraints from sulfide trace element and stable isotope geochemistry abstract the west qinling orogen central china is endowed with dozens of world class sediment hosted gold deposits most of which have formed from metamorphic fluids released from paleozoic sedimentary rocks during the triassic collisional orogeny here we present trace element and sulfur isotope data of gold bearing pyrite and arsenopyrite along with oxygen and hydrogen isotopes of ore related sericite to show that the early triassic zaozigou gold deposit 142 t in the xiahe hezuo district may have contrasting origin gold mineralization at zaozigou is dominated by fine grained pyrite and arsenopyrite disseminations py1 and asp1 in hydrothermally altered triassic flysch and less significantly in early triassic granitoid dike swarm the disseminated ores are overprinted by quartz sulfide ankerite sulfide vein veinlets which contain less abundant coarser pyrite and arsenopyrite py2 and asp2 and are less economically significant gold occurs as invisible gold in fine grained arsenian pyrite and arsenopyrite of the disseminated ores with subordinate native gold grains locally observed in the quartz sulfide ankerite vein veinlets the two types of pyrite py1 vs py2 and arsenopyrite asp1 vs asp2 are consistently enriched in as au co ni mn cu zn ag sb w pb te and bi and have well comparable 34s values ranging from 12 0 to 5 5 grain scale variation has been observed for au and other trace elements both in py1 and py2 likely reflecting changes in physio chemical conditions such as fo2 fs2 and temperature due to fluid rock interactions and or fluid mixing the 34s values are broadly consistent with sulfur isotopic compositions of many early triassic ilmenite series intrusions in the xiahe hezuo district and thus suggest a magmatic sulfur derivation from the reduced magmas that have assimilated carbonaceous sedimentary rocks the d and 18o values of ore fluids calculated from isotopic compositions of ore related sericite range from 56 to 76 and 10 1 to 12 4 respectively indicating a magmatic fluids exsolved from the above mentioned ilmenite series magma or a mixture of such as fluid with a metamorphic derived component synthesis of geochronological and geochemical data throughout the xiahe hezuo district led us to conclude that the zaozigou gold deposits can be considered as a shallow seated distal product of a reduced intrusion related gold system

nature of ore forming fluid and formation conditions of bif hosted gold mineralization in the archean amalia greenstone belt south africa constraints from fluid inclusion and stable isotope studies abstract orogenic gold mineralization in the amalia greenstone belt is hosted by oxide facies banded iron formation bif hydrothermal alteration of the bif layers is characterized by chloritization carbonatization hematization and pyritization and quartz carbonate veins that cut across the layers the alteration mineral assemblages consist of ankerite ferroan dolomite minerals siderite chlorite hematite pyrite and subordinate amounts of arsenopyrite and chalcopyrite information on the physico chemical properties of the ore forming fluids and ambient conditions that promoted gold mineralization at amalia were deduced from sulfur oxygen and carbon isotopic ratios and fluid inclusions from quartz carbonate samples associated with the gold mineralization microthermometric and laser raman analyses indicated that the ore forming fluid was composed of low salinity h 2 o co 2 composition 3 wt nacl equiv the combination of microthermometric data and arsenopyrite pyrite geothermometry suggest that quartz carbonate vein formation gold mineralization and associated alteration of the proximal bif wall rock occurred at temperature pressure conditions of 300 30 c and 2 kbar thermodynamic calculations at 300 c suggest an increase in f o 2 10 32 10 30 bars and corresponding decrease in total sulfur concentration 0 002 0 001 m that overlapped the pyrite hematite magnetite boundary during gold mineralization although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site the calculated low f o 2 values are consistent with previously determined high fe fe mg ratios 0 7 in associated chlorite absence of sulfates and restricted positive 34 s values in associated pyrite based on the fluid composition metal association and physico chemical conditions reported in the current study it is confirmed that gold in the amalia fluid was transported as reduced bisulfide complexes e g au hs 2 at amalia gold deposition was most likely a combined effect of increase in f o 2 corresponding to the magnetite hematite buffer and reduction in total sulfur contents due to sulfide precipitation during progressive fluid rock interaction the epigenetic features coupled with the isotopic compositions of the ore forming fluid 34 s s 1 8 to 2 3 18 o h2o 6 6 to 7 9 and 13 c c 6 0 to 7 7 at 300 330 c are consistent with an externally deep sourced fluid of igneous signature or and prograde metamorphism of mantle derived rocks

anomalous elastic thickness of the oceanic lithosphere in the south central pacific to first order the elastic thickness te of the oceanic lithosphere under mid plate volcanoes increases linearly with the square root of plate age at the time of loading1 4 a large scatter around this simple relationship is observed however suggesting that factors other than age influence the elastic thickness and produce departure from the first order trend5 to look at second order effects we have conducted a worldwide analysis over the pacific atlantic and indian oceans and determined te under sixty volcanoes applying elastic flexure theory and using as observational constraints geoid height data from the seasat satellite we find that for the three main oceans the depth of the elastic layer under volcanoes increases with age of plate at loading time t according to the simple relationship te 2 70 0 15 t1 2 te in km t in myr except under all of the volcanic chains of the south central pacific where this relation does not apply the elastic layer is found to be much thinner there and indicates a broad regional anomaly presumably of thermal origin well correlated with several other geophysical anomalies reported in this region6 10

influence of small scale fluvial architecture on co 2 trapping processes in deep brine reservoirs a number of important candidate co2 reservoirs exhibit sedimentary architecture reflecting fluvial deposition recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to co2 injection and storage we used a geocellular modeling approach to represent this multiscaled and hierarchical sedimentary architecture with this model we investigated the dynamics of co2 plumes during and after injection in such reservoirs the physical mechanism of co2 trapping by capillary trapping incorporates a number of related processes i e residual trapping trapping due to hysteresis of the relative permeability and trapping due to hysteresis of the capillary pressure additionally co2 may be trapped due to differences in capillary entry pressure for different textural sedimentary facies e g coarser grained versus finer grained cross sets the amount of co2 trapped by these processes depends upon a complex system of nonlinear and hysteretic characteristic relationships including how relative permeability and capillary pressure vary with brine and co2 saturation the results strongly suggest that representing small scale features decimeter to meter including their organization within a hierarchy of larger scale features and representing their differences in characteristic relationships can all be critical to understanding trapping processes in some important candidate co2 reservoirs

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

factors in the formation of the giant kalgoorlie gold deposit abstract kalgoorlie golden mile plus mt charlotte accounts for more than half the gold won from the 2000 deposits that comprise the widespread gold mineralization of the archaean yilgarn block of western australia it is also one of the largest gold deposits globally 1800 t au production and reserves an analysis of the various stages of formation of the kalgoorlie deposits from source through solution of gold in hydrothermal fluids and their infiltration along channelways through to gold precipitation at depositional sites reveals several features that make kalgoorlie favourably disposed as a site for a major gold deposit however none alone account for its anomalously large size it is concluded that the giant kalgoorlie gold deposit owes its size and unique position in western australian gold production history to favourable host rock chemical composition favourable host rock thickness and mechanical properties and to the favourable geometry of its host units and hosting greenstone belt at favourable pt conditions within the greenschist facies collectively these factors together with proximity to the regional scale boulder lefroy fault allowed highly focussed fluid flow on both a deposit and district scale in this regard it is significant that the structural setting of gold mineralization and the greenstone architecture at the largest canadian archaean deposit timmins are remarkably similar to kalgoorlie orebody size distribution histograms for both the yilgarn block and the abitibi belt canada are highly skewed with extremely few giant deposits but numerous deposits of 10 tonnes gold existing this topology can be accounted for in terms of system efficiency the probability of all six factors in the ore forming system being optimized high heat flow large fluid reservoir proximity to major structure efficient fluid focussing and large and compositionally favourable host lithology is extremely small in contrast the probability of two processes coupling to give low efficiency is very large resulting in numerous small deposits kalgoorlie appears to have formed by the same basic processes that formed the plethora of smaller yilgarn gold deposits however it represents a mineralized domain in which there was a conjunction of a number of favourable parameters both on a regional and deposit scale that produced high system efficiency and allowed the effective fluid focussing and gold deposition that formed this giant deposit

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

gilt by association origins of pyritic gold ores in the victory mesothermal gold deposit western australia sulfur isotope data and calculated fluid rock reaction paths constrain the origin of pyritic gold ores in the victory mesothermal gold deposit of western australia pyrite 34s values range from 4 4 to 5 1 and are negatively correlated with gold tenor in mafic host rocks indicating a significant increase in fluid oxidation state accompanied mineralization geologic evidence argues strongly that mineralization occurred with wall rock sulfidation but this process alone cannot produce the degree of fluid oxidation necessary to explain the sulfur isotope range in contrast either carbonation of ferric iron bearing minerals or separation of immiscible fluid phases both of which are consistent with the available geologic data can drive fluid oxidation to the point of aqueous sulfate dominance coupled decreases in gold solubility and sulfide sulfur isotope values that are predicted by such a scenario provide an explanation for the trend between 34s and gold tenor observed at victory because both wall rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not require precipitation of iron sulfides these processes may explain the poor correlation between gold tenor and pyrite abundance in many mesothermal deposits and the occurrence of some gold ores in iron poor rocks

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise

fluid inclusion and stable isotope o h c and s constraints on the genesis of the serrinha gold deposit gurupi belt northern brazil the serrinha gold deposit of the gurupi belt northern brazil belongs to the class of orogenic gold deposits the deposit is hosted in highly strained graphitic schist belonging to a paleoproterozoic 2 160 ma metavolcano sedimentary sequence the ore zones are up to 11 m thick parallel to the regional nw se schistosity and characterized by quartz carbonate sulfide veinlets and minor disseminations textural and structural data indicate that mineralization was syn to late tectonic and post metamorphic fluid inclusion studies identified early co2 ch4 n2 and co2 ch4 n2 h2o nacl inclusions that show highly variable phase ratios co2 homogenization and total homogenization temperatures both to liquid and vapor interpreted as the product of fluid immiscibility under fluctuating pressure conditions more or less asso ciated with postentrapment modifications the ore bearing fluid typically has 18 33mol of co2 up to 4mol of n2 and less than 2mol of ch4 and displays moderate to high densities with salinity around 4 5wt nacl equiv mineralization occurred around 310 to 335 c and 1 3 to 3 0 kbar based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions stable isotope data on quartz carbonate and fluid inclusions suggest that veins formed from fluids with 18 oh2o and dh2o 310 335 c values of 6 2 to 8 4

relating lithofacies to hydrofacies outcrop based hydrogeological characterisation of quaternary gravel deposits abstract a considerable part of today s drinking water supplies in europe and north america rely on clean groundwater from gravelly valley aquifers of quaternary age the sedimentary architecture the distribution of lithofacies and of architectural elements in such heterogeneous deposits are of fundamental importance for the analysis of groundwater flow and contaminant transport as the aquifers are not directly accessible for observation representative outcrop analogues were used to study the sedimentology on a local scale conventional sedimentological classification schemes were adapted for the purpose of hydrogeological evaluations measurements of hydraulic properties were then used to define a set of 5 hydrofacies from 23 possible sediment lithofacies a digital photographic mapping procedure was developed to allow fast data acquisition in the field the sedimentologically interpreted outcrops were stored in a gis style database and thus allow the output for further sedimentological or hydrogeological analysis