is the transition zone an empty water reservoir inferences from numerical model of mantle dynamics abstract water is probably present everywhere in the earth s mantle today with abundances ranging between scales of percent to the parts per million ppm mantle total water content is estimated to be between 10 to several times that of the present day hydrosphere numerous studies have been devoted to the determination of water solubility in mantle material d r bell g r rossmann science 255 1992 1391 1397 j ingrin h skogby eur j mineral 12 2000 543 570 they all show strong solubility variations from one mineral phase to another principally water partitioning has made the transition zone a probable trap for water from the earth s mantle n bolfan casanova et al earth planet sci lett 182 2000 209 221 d l kohlstedt et al contrib mineral petrol 123 1996 345 357 nevertheless water distribution within the mantle is still debated we have studied the role of mantle dynamics in water distribution by modeling water transport and mantle convection in a two dimensional 2 d cartesian geometry the model takes into account water partitioning between the mantle s transition zone and the upper mantle of 10 1 and between the lower mantle and the transition zone of 1 100 i e respectively between olivine and spinel and spinel and post spinel we have modeled the mantle temperature field using depth dependent viscosity and plate like surface conditions water injection at the trench has also been simulated our numerical experiments suggest that diffusivity of water has to be very high at least two orders of magnitude higher than the one experimentally determined d r bell g r rossmann science 255 1992 1391 1397 j ingrin h skogby eur j mineral 12 2000 543 570 to significantly influence water distribution in earth s mantle in fact the diffusion process is not efficient enough to balance the mixing due to mantle dynamics and to force water into the transition zone we show that the distribution of water should be quite homogeneous throughout the mantle if advection and diffusion are the only processes involved in water transport in the mantle this homogeneity implies that water below the transition zone could be in excess according to the lower mantle rocks solubility this addresses the question of stability of free water in the lower mantle and its mobility by percolation process which could be a very efficient transport process previously unconsidered in this field of research

incorporating self consistently calculated mineral physics into thermochemical mantle convection simulations in a 3 d spherical shell and its influence on seismic anomalies in earth s mantle 1 phase assemblages of mantle rocks calculated from the ratios of five oxides cao feo mgo al2o3 sio2 by free energy minimization were used to calculate the material properties density thermal expansivity specific heat capacity and seismic velocity as a function of temperature pressure and composition which were incorporated into a numerical thermochemical mantle convection model in a 3 d spherical shell the advantage of using such an approach is that thermodynamic parameters are included implicitly and self consistently obviating the need for ad hoc parameterizations of phase transitions which can be complex in regions such as the transition zone particularly if compositional variations are taken into account convective planforms for isochemical and thermochemical cases are however not much different from those computed using our previous simple parameterized reference state which means that our previous results are robust in this respect the spectrum and amplitude of seismic velocity anomalies obtained using the self consistently calculated material properties are more realistic than those obtained when seismic velocity is linearly dependent on temperature and composition because elastic properties are dependent on phase relationship of mantle minerals in other words pressure and temperature in all cases the spectra are dominated by long wavelengths spherical harmonic degree 1 to 2 similar or even longer wavelength than seismic tomographic models of earth which is probably due to self consistent plate tectonics and depth dependent viscosity in conclusion this combined approach of mantle convection and self consistently calculated mineral physics is a powerful and useful technique for predicting thermal chemical phase structures in earth s mantle however because of uncertainties in various parameters there are still some shortcomings in the treatment of the postperovskite phase transition additionally transport properties such as thermal conductivity and viscosity are not calculated by this treatment and are thus subject to the usual uncertainties

a geophysically constrained litho structural analysis of the eburnean greenstone belts and associated granitoid domains burkina faso west africa abstract interpretation of airborne geophysical data integrated with field structural and lithological observation was successfully employed in the creation of the litho structural framework in a poorly exposed paleoproterozoic granite greenstone terrain of the west african craton the geophysical data portray with sufficient detail all of the lithological units and structural features present the results suggest that the granitoid domains are formed by numerous small to medium sized plutons the existence of several generations of magmatic episodes has a significant impact on the development of a regional tectonic model the magnetic data provided a better definition of the actual pluton shapes and several highly magnetic late orogenic plutons were reliably identified relic f1 fold hinges were recognized in the hounde and boromo greenstone belts the d1 penetrative structures are generally affected by the d2 transcurrent shear zones which often form as well the granite greenstone belt boundaries previously unreported s2 shear zones including the bossie shear zone transecting the sideradougou granitoid domain and abundant small scale shear zones affecting all of the lithologies represent prospective areas for gold exploration airborne gamma ray spectrometry data aided in the mapping process in areas with less regolith cover including erosional ridges and parts of the banfora greenstone belt the magnetic data revealed at least three generations of doleritic dykes crosscutting the western burkina faso

the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

four pleistocene formations in southwest british columbia their implications for patterns of sedimentation of possible sangamonian to early wisconsinan age semiahmoo and dashwood drifts were deposited during the penultimate glaciation and highbury sediments and muir point formation during the last interglacial these sediments are defined and described and stratotypes are established for them based on arboreal pollen assemblages lithologic similarity and relative stratigraphic position semiahmoo drift of the fraser lowland is tentatively correlated with dashwood drift of vancouver island stone provenance till fabrics glaciotectonic structures cross bedding and stone imbrication indicate that regional ice and meltwater flowed out of major fiords in the coast mountains and into the ancestral strait of georgia during the penultimate glaciation ice flowed southeastward against the east coast of vancouver island and down the axis of the strait pollen and molluscs in semiahmoo and dashwood fossiliferous muds give evidence of transition from glaciomarine to marine conditions at the close of the penultimate glaciation highbury sediments of the fraser lo

stratigraphy structure absolute age and paleontology of the upper pleistocene deposits at sankaty head nantucket island massachusetts the sankaty head cliff of nantucket island massachusetts exposes drift of at least two glaciations and interglacial marine deposits radiocarbon amino acid racemization and uranium thorium analyses were used to determine the absolute ages of the beds the results indicate that 1 the sankaty sand correlates with oxygen isotope stage 5 sangamonian 2 the underlying drift is older than stage 5 illinoian or older and 3 the overlying drift is wisconsinan in age ostracodes and molluscs within the sankaty sand indicate that the marine climate during deposition of the lower part was somewhat warmer than the present climate off sankaty head and that the marine climate during the deposition of the upper part was as cold as or somewhat colder than the present climate the paleoenvironmental data support a stage 5 sangamonian age for the marine deposits

crustal scale transcurrent shearing in the paleoproterozoic sefwi sunyani comoe region west africa abstract the paleoproterozoic sefwi sunyani comoe region that straddles the ghana ivory coast border in west africa has been characterised as resulting from a combination of compression and simple shear with leucogranite intrusion either being prior to the compression or synchronous with late shearing the analysis of regional magnetic datasets combined with field observations allows us to better define the geometry of the major lithostratigraphic packages and their structural contacts in this region this analysis reveals a series of elongate rounded leucogranite intrusions enveloped by deformed metasediments recent finite element modelling of two phase aggregates has shown that we can analyse the geometry of these systems both in terms of their finite deformation and their mechanical contrast we interpret the geometries we see in the sefwi sunyani comoe region as reflecting the activity of a major crustal deformation zone which was dominated by simple shear the comparison with our modelling suggests a finite shear strain of approximately 5 gamma which in turn implies a lateral displacement of 400 km parallel the sefwi greenstone belt which places southern ghana eoghana near eastern burkina faso prior to deformation our analysis also suggests that the leucogranites were already acting as more rigid bodies during the shearing suggesting that their emplacement was predominantly pre kinematic

characterization of sand lenses embedded in tills abstract tills dominate large parts of the superficial sediments on the northern hemisphere these glacial diamictons are extremely heterogeneous and riddled with fractures and lenses of sand or gravel the frequency and geometry of sand lenses within tills are strongly linked to glaciodynamic processes occurring in various glacial environments this study specifically focuses on the appearance and spatial distribution of sand lenses in tills it introduces a methodology on how to measure and characterize sand lenses in the field with regard to size shape and degree of deformation a set of geometric parameters is defined to allow characterization of sand lenses the proposed classification scheme uses a stringent terminology to distinguish several types of sand lenses based on the geometry it includes sand layers sand sheets sand bodies sand pockets and sand stringers the methodology has been applied at the kallerup field site in the eastern part of denmark the site offers exposures in a number of till types that underwent different levels of glaciotectonic deformation sand lenses show high spatial variability and only weak uniformity in terms of extent and shape secondly the genesis of the various types of sand lenses is discussed primarily in relation to the depositional and glaciotectonic processes they underwent detailed characterization of sand lenses facilitates such interpretations finally the observations are linked to a more general overview of the distribution of sand lenses in various glacial environments due to the complex and mutable appearance of sand lenses geometric descriptions can reveal the deformation history and even give indications on the palaeo glaciological conditions during the deposition of the surrounding tills this information can support the understanding of till genesis and further inform till classifications in this regard structural heterogeneity such as sand lenses can supplement traditional directional element analysis to identify till types and may be used as a novel tool in till investigations

lobal interactions rheologic superposition and implications for a pleistocene ice stream on the continental shelf of british columbia abstract data on structures stone striae clast pavements fabrics morphology provenance and matrix grain size of four pre late wisconsinan diamictons are presented from the lower accessible part of an 855 m long sea cliff at cape ball east coast of graham island queen charlotte archipelago british columbia the data indicate that an ice stream crossed the continental shelf from the british columbian mainland to the queen charlotte islands local queen charlotte piedmont ice coalesced with the western edge of the cordilleran ice sheet resulting in northward flow deflection along the east coast of graham island generally consistent structural and stone data indicate that diamictons were deposited as primary subglacial tills and that subglacial dynamics and rheologic conditions were complex two of the tills can be traced continuously over the cape ball section which provides a north south transect oblique to ice flow each of the two tills reveals five major changes in rheology and genesis ranging from a strong lodgement component to dominantly deformation mainly by subglacial squeeze flow true end member tills are rare and in almost all places the tills are hybrids that formed within a continuum of till forming processes commonly involving stone rotation and superposition of structures and rheologic states this could have occurred mainly in response to fluctuating till pore water content along the transect two dimensional stretches of contrasting till types probably represent a three dimensional patchwork of contrasting till rheologies and pore waters if patches of ductily deforming till were common and or widespread they could have accounted for much of the movement of cordilleran ice over glaciomarine mud on the continental shelf when sea level was much lower than present deforming probably saturated muddy till reconstituted glaciomarine would have possessed negligible shear strength resulting in a thin rapidly moving glacier most likely an ice stream that reached cape ball but failed to remove the record of previous glacial events the ice stream probably issued out of the mainland s skeena valley which is aligned with cape ball and is approximately the size of valleys in antarctica that today conduct small outlet glaciers and ice streams evidence for this and other ice streams on the northwestern continental shelf of north america imply that destabilization and rapid decay of the western edge of the cordilleran ice sheet may have been accomplished by a combination of ice streams sea level rise and extensive calving

the pampe gold deposit ghana constraints on sulfide evolution during gold mineralization abstract the ashanti belt a world renowned gold producing region in southwest ghana has received renewed attention in recent years most studies however focused on the major deposits situated along the ashanti shear zone and in the adjacent tarkwa basin to the east neglecting smaller yet important occurrences such as the pampe deposit which occurs few kilometers to the west of this shear zone on the akropong trend nevertheless investigating such simpler smaller scale mineralizing systems is attractive in that this can help shedding light on the processes that control gold deposition at the regional scale at pampe gold endowment has been estimated at approximately half a million ounces with an average gold grade of 2 8 g t the mineralization is related to two sets of quartz veins a first set v 1 which has a ne trend and is sub parallel to the main foliation s 1 and a second set v 2 which crosscuts this foliation the v 2 veins have a nnw sse trend with local conjugate geometries indicating that they formed during nnw sse shortening which regionally is linked to major orogenic gold deposition gold mineralization is systematically associated with sulfides which occur disseminated in the vein walls and in the surrounding host rocks the ore sulfide paragenesis consists of 1 a first generation of pyrite which is associated with v 1 veins 2 a second generation of sulfides consisting of an intergrowth of arsenopyrite and pyrite that crystallized contemporaneously with the formation of the second vein set 3 a late phase of pyrite growth which occurs as overgrowths on phase 2 sulfides and formed during the waning stages of v 2 emplacement invisible sub microscopic gold was detected in all sulfide generations by la icp ms the analytical profiles for the au signal are mimicked by those for pb cu as ag te and bi invisible gold is thus interpreted to have precipitated within sulfides in the form of nanoparticles colloidal gold alloys conversely visible native gold grains were recognized exclusively in association with arsenopyrite from late v 2 veins either as inclusions or more commonly at the boundary with other sulfides as well as in micro fractures that crosscut the sulfides gold precipitation was likely induced by sulfidation of the wall rock during fluid rock interaction the pampe deposit exemplifies the mineralization processes that took place at larger scale in neighboring world class deposits such as obuasi bogoso and prestea

formation and deformation of pyrite and implications for gold mineralization in the el callao district venezuela the el callao mining district is the most important gold producing region in venezuela it is hosted in the paleoproterozoic guasipati el callao greenstone belt which forms part of the guayana craton the venezuelan extension of the guiana shield of south america it consists of volcanic and volcanosedimentary sequences that were affected by several deformation events particularly localized shear zones the colombia mine the largest active mine in the district produces 4 tonnes t 128 600 oz of gold annually with reserves estimated at 740 t 24 moz and grades of up to 60 g t gold mineralization is concentrated within a vein network in the colombia corridor a shear fracture hosted mesh of interconnected quartz ankerite albite veins enclosing fragments of altered metabasaltic host rocks gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite in which it occurs as invisible gold micron sized native gold inclusions and filling fractures based on optical and scanning electron microscopy backscattered electron observations two types of pyrite are recognized a simple zoned pyrite and a less common oscillatory zoned pyrite both types consist of a mineral inclusion rich core and a clearer rim however in oscillatory zoned pyrite the latter is composed of complex rhythmic overgrowths of alternating as rich and as poor bands laser ablation inductively coupled plasma mass spectrometry la icp ms analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite the highest concentrations 5 23 ppm au are found in oscillatory zoned pyrite rims which correlate with the highest as concentrations 16 000 23 000 ppm in as poor bands au up to 1 5 ppm and as 300 6 000 ppm concentrations decrease by about an order of magnitude copper bi te sb pb and ag always occur with invisible gold particularly in pyrite cores suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids visible native gold grains occur as small inclusions throughout core and rim of both pyrite types as well as in fractures within it in both occurrences chalcopyrite sphalerite tellurobismuthite ankerite albite and chlorite accompany native gold and gold fineness ranges between 900 and 930 at an early stage of vein mesh formation pyrite formed in the metabasaltic fragments at the expense of ankerite which in turn resulted from alteration of fe ti oxides gold together with other chalcophile elements was incorporated within the structure of pyrite most likely by destabilization of metal sulfide complexes during ankerite replacement subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite generating local and repeated decreases in pressure which triggered local boiling of the hydrothermal fluid as evidenced by the presence of primary fluid inclusions containing immiscible liquid rich and vapor rich aqueous carbonic fluids this process was responsible for a number of physical chemical changes in the liquid all of which contributed to the formation of the as and au rich overgrowths in pyrite 1 removal of h 2 o into the vapor phase inducing saturation of dissolved metals in the remaining liquid 2 an increase in ph due to partition of h 2 s and co 2 into the vapor thus decreasing the solubility of sulfide minerals and 3 an adiabatic decrease in temperature lowering the solubility of as and au in the liquid waning of this process restored precipitation of as poor pyrite until the onset of a new cycle because pressure drops are more significant adjacent to open spaces oscillatory zoned pyrite probably crystallized near newly formed veins whereas simple zoned pyrite formed away from them previously formed pyrite underwent fracturing during reactivation of the deformation especially through the brittle deformation events that postdated shearing resulting in local pulverization of pyrite this newly created porosity facilitated fluid circulation and remobilization of structurally bound gold as well as of other chalcophile elements ag cu bi te pb and sb which reprecipitated together with pyrite in the form of native gold sulfides and tellurides either as small inclusions or as larger grains within fractures this remobilization process facilitates the exceptionally high gold tenor found in the deposit where the colombia corridor is intersected by the santa maria fault

structural and gold mineralizing evolution of the world class orogenic mana district burkina faso multiple mineralizing events over 150 million years abstract the mana district located in the northern part of the birimian hounde greenstone belt in western burkina faso is a world class paleoproterozoic orogenic gold district 8 moz including five gold deposits fofina nyafe siou wona kona and yaho these deposits are located in specific lithostratigraphic domains and gold is controlled by various structural features deposit and regional scale mapping intrusion age and geochemistry as well as airborne aeromagnetic and electrical resistivity geophysical data were used to decipher the tectonic evolution of each gold deposit and the district five deformational and four gold mineralizing events were recognized the first deformation event d1md e w oriented shortening affected the metamorphosed volcanic and sedimentary rocks of the lower birimian group this early deformation episode was correlated with the formation of gently n plunging folds f1md and n s striking thrusts faults coeval with emplacement of the pre to synkinematic wona kona and siou plutons dated at 2172 ma under greenschist facies metamorphism the quartz carbonate veins v1md at fofina and siou formed during d1md at eoeburnean time manifesting the first gold event at approximately 2172 ma the following deformation event d2md e w oriented extension is associated with the deposition of the upper birimian group mana basin overlying the lower birimian group the geometry of the mana basin is controlled by the mana and maoula shear zones the tarkwaian type rock formation overlying the upper birimian group controlled by the wona kona and siou shear zones is constrained at the end of d2md or at the beginning of the d3md event with a maximum deposition age at 2113 ma the third deformation event d3md e w to wnw ese transpression affected the entire supracrustal rock such event is correlated with the formation of map scale f3md folds and dextral shear zones during the eburnean orogeny 2113 2090 ma a second gold mineralizing event occurred during d3md and is manifested by quartz carbonate veins v3md and disseminated sulfides at the yaho fofina and nyafe and possibly wona kona deposits the fourth deformation event d4md nnw sse transpression is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric nne striking folds f4md associated with vertical fold axes syn d4md mineralization is characterized mainly by a strong silicification si4md with disseminated pyrite and arsenopyrite along the wona kona shear zone and by tiny quartz carbonate veinlets v4md this event is considered the main gold bearing event in the western margin of the mana district the fifth and last deformation event d5md is brittle in character and was responsible for the formation of e w subvertical crenulation cleavages and reverse faults under overall n s shortening this late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the wona kona and siou deposits this event could be as young as 2022 ma the age obtained from ar ar datation of muscovite schists at the wona kona deposit our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit and regional scale mapping it is interpreted that gold was introduced as early as 2172 ma and possibly as late as 2022 ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the birimian belts

petrological and geochronological constraints on lower crust exhumation during paleoproterozoic eburnean orogeny nw ghana west african craton we present new petrological and geochronological data on high grade ortho and paragneisses from northwestern ghana forming part of the paleoproterozoic 2 25 2 00 ga west african craton the study area is located in the interference zone between n s and ne sw trending craton scale shear zones formed during the eburnean orogeny 2 15 2 00 ga high grade metamorphic domains are separated from low grade greenstone belts by high strain zones including early thrusts extensional detachments and late stage strike slip shear zones paragneisses sporadically preserve high pressure low temperature hp lt relics formed at the transition between the blueschist facies and the epidote amphibolite sub facies 10 0 14 0 kbar 520 600 c and represent a low 15 c km 1 apparent geothermal gradient migmatites record metamorphic conditions at the amphibolite granulite facies transition they reveal a clockwise pressure temperature time p t t path characterised by melting at pressures over 10 0 kbar followed by decompression and heating to peak temperatures of 750 c at 5 0 8 0 kbar which fit a 30 c km 1 apparent geotherm a regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise p t t path characterised by peak metamorphic conditions of 7 0 10 0 kbar at 550 680 c which match a 20 25 c km 1 apparent geotherm these p t conditions were reached after prograde burial and heating for some rock units and after decompression and heating for others the

the high level marine shell bearing deposits of clava inverness shire and their origin as glacial rafts abstract the enigmatic high level till covered cold water marine shell bearing deposits at clava inverness shire are described systematically in the light of new observations made at sites documented in the literature the marine deposits named here as the clava shelly formation include three members the unfossiliferous clava sand the underlying clava shelly clay and a shelly diamicton known as the clava shelly till the first two members form a conformable coarsening upwards sequence containing a shallow water high boreal to low arctic fauna and flora the clava shelly till is essentially glacially re sedimented glaciomarine clay containing a sparse fauna but its stratigraphic relationship and age are not absolutely clear the shelly clay is ascribed to a mid devensian interstadial episode on the basis of amino acid dating it is concluded that the clava shelly clay and several discrete masses of clava sand and shelly till are glacially transported allochthons derived from the great glen the rafts were probably detached as a result of high pore water pressure building up in laterally restricted aquifers beneath a confined glacier that flowed north eastwards across the loch ness basin this glacier was deflected eastwards and upwards towards clava by ice flowing from the northern highlands along the beauly firth during the build up of the last scottish ice sheet the rafts were stacked at the ice margin when the glacier entered the nairn valley before being overriden by the expanding ice sheet

the bepkong gold deposit northwestern ghana abstract the bepkong gold deposit is located in the wa lawra belt of the paleoproterozoic baoule mossi domain of the west african craton in nw ghana it occurs in pelitic and volcano sedimentary rocks metamorphosed to greenschist facies in genetic association with zones of shear interpreted to form during the regional d 3 deformational event denominated d b1 at the deposit scale the ore zone forms a corridor like body composed of multiple quartz carbonate veins surrounded by an alteration envelope characterized by the presence of chlorite calcite sericite quartz and disseminated pyrite arsenopyrite plus subordinate pyrrhotite and chalcopyrite the veins contain only small proportions of pyrite whereas most of the sulphides particularly arsenopyrite occur in the altered host rock next to the veins pyrite is also common outside of the ore zone gold is found in arsenopyrite where it occurs as invisible gold and as visible albeit micron size grains in its rims and as free gold within fractures cross cutting this sulphide more rarely free gold also occurs in the veins in fractured quartz in the ore zone pyrite forms euhedral crystals surrounding arsenopyrite but does not contain gold suggesting that it formed at a late stage from a gold free hydrothermal fluid

a preliminary numerical study on water circulation in convecting mantle with magmatism and tectonic plates abstract we present a preliminary self consistent numerical model of solid state convection in a wet mantle with magmatism and tectonic plates to understand how magmatism and tectonic plates affect circulation of water injected into deep mantle the solidus temperature of mantle materials depends on water content but their water saturation limit is assumed to be constant and water content dependence of viscosity is neglected when the mantle is mildly heated by heat producing elements plate tectonics steadily occurs and subducted basaltic crusts accumulate on the core mantle boundary water injected into deep mantle is mostly transported to basaltic accumulation areas which serve as a major storage of injected water the characteristic time for the water circulation is 2 6 gyr water significantly enhances magmatism when the mantle is more strongly heated and the internal heating rate exceeds a threshold however basaltic accumulations become convectively unstable cause vigorous magmatism and split the lithosphere into smaller fragments water transported into deep mantle by subducting lithospheric fragments distributes more uniformly and the water storage capacity of the mantle becomes much smaller than that under the mild internal heating below the threshold the characteristic time for water circulation is 1 2 gyr the characteristic time of water circulation in the mantle is comparable to the age of the earth under mild internal heating and the water circulation depends on the regime of magmatism and mantle convection which in turn depends on the internal heating rate a full understanding of dynamics of magmatism and mantle convection is crucial for understanding water circulation in the mantle and the effect of water on mantle evolution

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

wet plume atop of the flattening slab insight into intraplate volcanism in east asia abstract geophysical observations imply the intraplate volcanism in east asia is related to dehydration of slab stagnating in the transition zone to better understand the dynamics of such process a thermochemical mantle convection model is constructed to simulate numerically the thermal evolution of slab and the transportation of water in the process of slab downgoing flattening and stagnation equation of water transfer is included and water effects on density and viscosity are considered model results indicate the warming of slab by surrounding mantle is rather slow water could be successfully dragged into the transition zone if the reference viscosity of the hydrous layer with initial water of 2 wt is higher than 1017 pa s and that of mantle is 1021 pa s wet plumes could then originate in the flat lying part of the slab relatively far from the trench generally the viscosity of the hydrous layer governs the initiation of wet plume whereas the viscosity of the overlying mantle wedge controls the activity of the ascending wet plumes they are more active in the weaker wedge the complex fluid flow superposed by corner flow and free thermal convection influences greatly the water transport pattern in the upper mantle modeling results together with previous modeling infer three stages of water circulation in the big mantle wedge 1 water is brought into the mantle transition zone by downward subducting slab under some specific thermo rheological conditions otherwise water is released at shallow depth near wedge tip 2 wet plume generates from surface of the flattening slab warmed by surrounding mantle and 3 water spreads over the big mantle wedge wet plume from the flattening pacific plate arrives at the lithospheric base and induces melting which can explain the intraplate cenozoic volcanoes in east asia

the nassara gold prospect gaoua district southwestern burkina faso abstract the nassara au prospect is located in the birimian boromo greenstone belt in southwestern burkina faso it is part of a larger mineralized field that includes the cu au porphyry system of gaoua to the north at nassara mineralization occurs within the west batie shear zone that follows the contact between volcanic rocks basalt and andesite and volcano sediments pyroclastics and black shales at the southern termination of the boromo belt gold is associated with pyrite and other fe bearing minerals that occur disseminated within the sheared volcanic and volcano sedimentary rocks in particular highest grades are distinguished in alteration halos of small quartz albite ankerite veins that form networks along the shear zone here pyrites are marked by as poor and as rich growth zones the latter containing gold inclusions gold mineralization formed during d2na subsequent shear fractures related to d3na related are devoid of gold nassara is a classical orogenic gold occurrence where gold is associated to disseminated pyrite along quartz veins

gold mineralization in as rich mesothermal gold ores of the bogosu prestea mining district of the ashanti gold belt ghana remobilization of invisible gold the bogosu prestea mining district of southwestern ghana is a 33 km section of the early proterozoic ashanti gold belt greenschist facies carbonaceous and carbonate bearing turbidites and greywackes and mafic dikes host numerous economic mesothermal gold deposits structurally higher ores in the bogosu concession have brittle deformation and consist of disseminated sulphide lodes in tectonically disrupted sedimentary rocks and carbonate altered mafic dikes most gold occurs as micrometre size particles in arsenian pyrite and as invisible gold in arsenian pyrite and arsenopyrite the structurally deeper ores of the adjoining prestea concession are associated with brittle ductile deformation and consist of extensive crack seal quartz veins and graphitic shear zones only minor amounts of invisible gold were detected in these deeper lodes gold occurs dominantly as abundant microscopic and larger particles in sulphide arsenide minerals and in gangue the gold distribution patterns revealed by sims microprobe analysis and ion maps emp and colour staining suggest that most of the primary gold in the bogosu prestea system precipitated in solid solution with sulphide arsenide minerals however post depositional concentration and redistribution occurred in increasing degree with 1 increase in metamorphic hydrothermal gradients in the gold system depth 2 decrease in the refractory properties of the host mineral and 3 increase in the amount of post depositional host mineral recrystallization and deformation gold evolved from primary solid solution within sulphide arsenide minerals to colloidal and micrometre size particles concentrated in voids fractures and internal grain boundaries and finally to microscopic and larger particles at sulphide arsenide grain margins and in the gangue assemblage the general conclusions presented here are applicable to as rich gold deposits of all ages worldwide the presence of gold in late fractures is insufficient evidence for late stage introduction of gold in mesothermal gold systems

till as a glacial tectomict its internal architecture and the development of a typing method for till differentiation abstract till has been studied for over a century yet still a clear understanding of the mechanics of deposition and or emplacement of till eludes us tills in the past have been generally subdivided into sediments formed in various sub environments of the glacial system as such it has been generally agreed that tills can be separated in terms of these types based on various attributes such as clast fabric increasingly such separation into various sub types of till has become however highly debatable and tenuous based upon the use of micromorphology and a greater in depth appreciation of the internal architecture of till it has become apparent that previously established classifications of till are erroneous micromorphology reveals that deposition and or emplacement of tills is a complex tectonic process such that lodgement melt out and flow tills as previously defined do not appear to exist rather a new term better describes tills as glacial tectomicts by using micromorphology in establishing the presence of sets of microstructures a statistical method is introduced that permits differentiation of tills from other diamictons the implications for this new understanding of tills allied to statistical typing is considerable in terms of till formation glacial stratigraphy and understanding glacial environments in terms of stress histories porewater transmission subglacial deformation mechanics till rheological phase changes and the timeline of till formation

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

controls on density stratification in the early mantle previous numerical models of convection in the hot early mantle incorporating simulated subducted oceanic crust and some viscosity stratification developed thermal and compositional stratification with cooler strongly depleted mantle above hotter slightly enriched mantle the robustness of these results is explored through variation of some important parameters the stratification is minor at present mantle temperatures but stronger at higher mantle temperatures because mantle viscosities are reduced the results are not much affected by variations in plate velocity nor by whether internal heat sources are directly associated with the mafic component nor by 20 of heat entering from below however stratification was reduced somewhat at higher temperatures by increasing the number of tracers indicating that the effect was moderately overestimated in a previous paper the present results still indicate that oceanic crust would be no more than 6 8 km thick at high mantle temperatures because of the strong depletion of the upper mantle when the effect of a phase transformation in the transition zone is included the stratification is moderately enhanced and crustal thicknesses of 3 5 km result at 1650 c oceanic crustal thickness is highly variable in space and time with some plates having essentially zero crust for significant periods the new results support the previous conclusions that the stratification may explain the strong depletion of incompatible elements documented for the earliest mantle and that early plate tectonics may have been more viable that hitherto thought

mineralogical fluid inclusion and stable isotope characteristics of birimian orogenic gold mineralization at the morila mine mali west africa abstract gold mineralization at morila is hosted in flat lying meta graywacke associated with a birimian aged massigui schist belt in mali three major events of deformation metamorphism fluid flow d 1 d 2 and d 3 are recognized at morila with mineralization associated with various increments of deformation and fluid flow during d 2 three distinct vein sets v 1 v 3 associated with d 1 d 3 respectively have also been recognized v 1 vein sets are thin poorly defined quartz feldspar metamorphic segregations broadly parallel to the regional fabric v 2 veins are generally boudinaged foliation parallel quartz feldspar veins or occur as felsic melt segregations in the form of quartz k feldspar muscovite biotite blebs located essentially within the ore zone v 3 veins occur as sub parallel to high angle biotite arsenopyrite gold veins that appear to be extensional in nature the ore zone is characterized by compositionally distinct biotite higher mgo and lower feo and more calcic plagioclase than otherwise texturally identical minerals in the footwall and hanging wall of the mineralized zone suggesting that prograde metamorphic minerals were modified by post metamorphic mg and ca rich mineralizing fluids gold forms discrete grains in the v 2 veins or in the adjacent host rock and is closely associated with pyrrhotite and arsenopyrite gold also occurs with sb bi ore minerals in rare microfractures numerous thin biotite arsenopyrite gold veinlets cut d 1 and d 2 fabrics and often carry significant gold associated with kink bands in the d 2 fabric fluid inclusion and oxygen isotope study provide evidence that morila fluids are intermediate salinity 12 16 wt nacl equivalent nacl cacl 2 mgcl 2 h 2 o ch 4 n 2 fluids of magmatic origin 18 o water 10 9 0 3 the ore forming fluid subsequently unmixed to form a compositional range of vapor rich ch 4 n 2 dilute nacl ch 4 n 2 aqueous and liquid rich brine associated with the opening of a micro fracture network and vein development quartz biotite isotope geothermometry indicates that mineralization occurred between 350 to 450 c gold mineralization at morila was an integral part of coeval plutonism deformation and regional metamorphism while the modification of the ore fluid by unmixing was the most important factor associated with the concentration of economically viable gold grades at morila

dynamic glacigenic deformation of near surface metamorphic bedrock heath steele mines new brunswick near surface metamorphic bedrock at heath steele mines new brunswick was fractured to depths of at least 35 m and possibly as deep as 120 m during two stages of glacier overriding fracturing was accompanied by crack filling of sediment laden melt waters increased pore pressures freezing and joint expansion the upper few meters of the bedrock surface were extensively brecciated and folded brecciated bedrock fragments were reoriented as passive strain markers of interstitial ice deformed by subglacial shear dynamic glacigenic deformation is distinguished from older structures by 1 structural associations with direction of glacial movement 2 fracture infillings of distally derived and striated material 3 iron staining along open fractures 4 near surface strain partitioning of deformation and 5 absence of axial plane foliations in fold structures

the effects of deep water cycling on planetary thermal evolution 1 we use a parameterized convection model to investigate the effects of deep water cycling on the thermal evolution of an earth like planet the model incorporates two water reservoirs a surface and an interior mantle reservoir exchange between the two is calculated using a mantle convection parameterization that allows for temperature and water dependent mantle viscosity together with internally self consistent degassing and regassing parameterizations the balance between degassing and regassing depends on the average spreading rate of tectonic plates the amount of water partitioned into melt the thickness of a mantle melt zone and of a hydrated layer at the top of subducting plates degassing scales with melt zone thickness such that an early period of extensive melting would create a drier and more viscous mantle shifting the solidus line in a direction that would reduce the melt zone thickness and the rate of mantle heat loss coupling a hydrated zone thickness dependent regassing factor to the model to mimic water delivery to the mantle via a serpentinized layer allows for the potential of a reversing point where the overall water flow direction switches from degassing to regassing as the mantle cools the water effect on viscosity creates a negative feedback that tends to regulate the final amount of water in the mantle so it is not strongly dependent on the initial amount of planetary water the final amount of water in the surface reservoir is then determined by this feedback effect together with the initial water budget of the entire planet this implies that if the initial water budget of a planet can be estimated from planetary formation models then the volume of surface water can be used to estimate the volume of water in the mantle of an earth like planet applying this methodology to the earth leads to predictions for water concentration in the earth s mantle that are in line with geochemical and petrological constraints

water circulation and global mantle dynamics insight from numerical modeling we investigate water circulation and its dynamical effects on global scale mantle dynamics in numerical thermochemical mantle convection simulations both dehydration hydration processes and dehydration melting are included we also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed due to effective slab dehydration at shallow depths water still affects significantly the global dynamics by weakening the near surface oceanic crust and lithosphere enhancing the activity of surface plate motion compared to dry mantle case as a result including hydrous minerals the more viscous mantle is expected with several orders of magnitude compared to the dry mantle the average water content in the whole mantle is regulated by the dehydration hydration process the large scale thermochemical anomalies as is observed in the deep mantle is found when a large density contrast between basaltic material and ambient mantle is assumed 4 5 comparable to mineral physics measurements through this study the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection

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

progressive gold mineralization along the syama corridor southern mali west africa abstract this contribution reports on three significant gold occurrences in southern mali the syama and tabakoroni deposits and the tellem prospect located in what is locally known as the syama corridor they are part of the syama and tabakoroni goldfields which are the object of a twin paper in this issue ballo et al this volume they occur in the northern part of the bagoe greenstone belt which in the study area consists of a suite of paleoproterozoic metamorphosed volcanic and sedimentary rocks the sediments include conglomerate black shale sandstone and greywacke the volcanic rocks have basaltic andesitic and dacitic compositions the basalts have flat rare earth element ree patterns no nb anomaly and very low zr y whereas the latter two rock types are enriched in light ree depleted in nb ta and zr y 7 all rock units are crosscut by spessartite type lamprophyre sills even though the three gold occurrences are aligned along the same major structure each one has distinct geological features including i the nature of the dominant host rock in the mineralized zone a basaltic volcanic sequence at syama a diorite pluton at tellem and a felsic volcanic sequence at tabakoroni ii the structural characteristics of the zone highly mineralized breccias for syama and vein stockwork for tabakoroni and tellem and iii the main sulfides present pyrite in association with arsenopyrite at tabakoroni and tellem and pyrite alone at syama at tabakoroni and tellem gold mineralization occurred mostly during d4sy whereas at syama a large part of the visible gold occurs as infill in fractures in breccia zones likely related to a brittle late e w phase of shortening d5sy

seismic evidence for deep water transportation in the mantle we report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern japan our data indicate that water is released from the hydrated oceanic crust at shallow depths approximately 100 kilometers and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment

plate like regime of a numerically modeled thermal convection in a fluid with temperature pressure and stress history dependent viscosity 1 a series of numerical models are presented for two dimensional thermal convection in a fluid with viscosity that nonlinearly depends on degree of damage as well as temperature and pressure increases and viscosity decreases with time when the convection induces strong viscous dissipation the convecting fluid recovers from the damage with a characteristic time that depends on temperature the dependence of viscosity induces a stress viscosity relationship that consists of two branches the intact branch at low stress and the damaged branch at high stress with a hysteresis between the two branches is small large and viscosity is high low on the intact damaged branch the hysteresis lets the viscosity depend on stress history the dependence of viscosity on stress history due to the hysteresis induces a regime where the numerically modeled lithosphere along the surface boundary is fragmented into smaller pieces that are on the intact branch and rigidly move the pieces of lithosphere are separated by narrow mechanically weak zones that are on the damaged branch each piece of lithosphere is further fragmented and mechanically weak zones are newly formed only when the piece is tapped by particularly strong hot uprising plumes this plate like regime is the only regime among the convective regimes searched here where each piece of lithosphere rigidly moves and still is not ruptured by the forces that drive the piece the earth s mantle is suggested to be on the plate like regime

glaciotectonic origin of the massachusetts coastal end moraines and a fluctuating late wisconsinan ice margin late wisconsinan end moraines on cape cod and islands south and west of cape cod are believed to be glaciotectonic features formed by advancing ice fronts evidence of a glaciotectonic origin includes the stratified drift composition of the moraines dislocated preglacial and glaciolacustrine deposits tilted preglacial and glacial beds folding and thrust faulting and morphologic similarity to ancient and modern glaciotectonic moraines evidence for major ice readvances during over all general recession includes the moraines themselves till atop stratified drift and the numerous basal tills that are inferred to exist beneath cape cod bay the thompson glacier end moraine in the canadian arctic archipelago is considered to be a modern example of how late wisconsinan end moraines on cape cod and the islands were formed the thompson glacier is overriding its outwash plain displacing outwash deposits forward and upward beyond the ice front new sheets are added to the base of the moraine and till is deposited atop the moraine as the ice overrides it evidence from cape cod and the nearby islands indicates that over all recession of late wisconsinan ice was characterized by alternating episodes of ice front retreat during which the outwash plains were formed and of ice front advance during which the moraines were built retreat of the ice from cape cod and the islands may have been similar to the retreat of the lake michigan lobe deposits of which contain evidence of at least 12 moraine building episodes caused by readvancing ice during the over all retreat of the late wisconsinan ice sheet

early proterozoic ore deposits and tectonics of the birimian orogenic belt west africa abstract the early proterozoic of the west african craton comprises a series of volcanic troughs and sedimentary basins the birimian with granitic terranes accreted on archean nuclei in the man and reguibat shields studies of the birimian of the man shield indicate a model of polycyclic evolution with a major collision event d 1 2 1 ga thrusting part of the proterozoic terrane over the archean before individualization of numerous volcanic troughs and clastic infill basins the proposed evolution for the birimian orogenic belt comprises 1 deposition of the sedimentary lower birimian b1 with minor tholeiitic volcano sedimentary intercalations containing chert and or mn formations and with most of the detritus being derived from early proterozoic sources apart from contamination near the proterozoic archean contact 2 pre b2 crustal thickening related to d 1 thrusting 3 formation over about 40 ma of the upper birimian b2 with numerous volcanic troughs of different composition tholeiitic and rare komatiitic bimodal tholeiitic to calc alkaline volcano plutonic and tarkwaian clastic infill basins and 4 major transcurrent tectonics with sinistral d 2 and dextral d 3 strike slip faults such a tectonic evolution from a collision d 1 phase to a transcurrent d 2 d 3 phase is typical of collision belts and in the present case the evolution of the birimian orogenic belt can be extended into guyana the metallogenic history of the birimian shows a three phase evolution coinciding with the orogenic evolution and extends over almost 150 ma from the perkoa massive zn ag sulfides 2 12 ga with a clear mantle affinity to the latemesothermal au quartz veins 2 ga with according to lead isotopes a high crustal participation the economic mineralization of belt thus consists of 1 1 pre orogenic pre d 1 deposits related to early extension zones this was diverse with stratiform au tourmalinite type 1 au loulo in mali dorlin in guyana stratiform fe cu faleme in senegal and mn nsuta in ghana tambao in burkina faso and a single massive zn ag sulfide deposit perkoa in burkina faso associated with regional volcanosedimentary variably tholeiitic stratigraphic marker beds 2 2 syn orogenic post d 1 to syn d 2 d 3 deposits with disseminated au sulfides type 2 au yaoure in the ivory coast in extensional zones of the b2 followed by auriferous paleoplacers type 3 au in b2 extensional zones tarkwaian banket conglomerate or syn d 2 transtensional zones debris flow of orapu in guyana 3 3 late orogenic post peak d 2 d 3 deposits with mesothermal au mineralization evolving from a disseminated gold bearing arsenopyrite and au quartz lode type type 4 au ashanti in ghana to a quartz vein type with free gold and cu pb zn ag bi paragenesis most of the gold in west africa comes from this phase finally the metallogeny of the birimian appears as rich in au and mn formations and poor in volcanogenic mineralization and bif it differs from the archean metallogeny of west africa and other regions through the presence of certain deposit types such as au stratiform tourmalinite gold bearing conglomerate with au and fe ti oxides but no uranium or sulfides and au arsenopyrite rich shear zone deposits that are very common in but not exclusive to the proterozoic and paleozoic

u pb dating of the end of the pan african orogeny in the tuareg shield the post collisional syn shear tioueine pluton western hoggar algeria abstract the tioueine pluton intrudes the neoproterozoic series of the iskel terrane located in the tuareg shield western hoggar the consistency of the internal structures as well as the nature and organization of the associated microstructures demonstrate that the tioueine pluton was emplaced syn kinematically while n s strike slip shear zones were active the syn tectonic emplacement of the tioueine massif implies that this pluton although belatedly crystallized entirely belongs to the concept of post collisional magmatism in order to date precisely the late pan african tectono metamorphic event in the studied area an u pb age of 523 1 ma was obtained from abraded zircons of a late quartz syenite from the tioueine pluton this early cambrian age is younger than the other plutons of the tuareg shield which were mainly emplaced between 630 ma and 580 ma this dating also shows that the tuareg shield was not a single coherent block at 525 ma but rather an amalgam of active terranes moving each other along major shear zones finally the tioueine massif represents probably the final welding of the tuareg shield assembly of terranes and consequently the end of the post collisional orogenic episode in the whole pan african belt

superimposition of ribbed moraines on a palaeo ice stream bed implications for ice stream dynamics and shutdown the sediments and landforms preserved on palaeo ice stream beds can provide important information about their subglacial conditions and flow mechanisms and the processes accompanying their shutdown in this paper detailed observations of an intriguing subglacial landform assemblage of ribbed moraines superimposed on glacial lineations on the dubawnt lake ice stream bed north west canadian shield are presented including their morphometry internal structure from ground penetrating radar gpr surveys and from glaciogeological analysis and sedimentological characteristics from sediment architecture and lithofacies analysis the observations suggest an abrupt change in ice dynamics that correlates with two phases of glacial landform development this hypothesis is based on evidence from a deformed lodgement till which subsequently underwent brittle deformation and developed prominent thrust shear structures and tension fractures tension fractures are observed in a sediment exposure and thrust structures are observed in gpr surveys where they occur most prominently in the ribbed moraine ridge crests the presence of the fractures and their association with a population of clasts in the till that are orientated with their a axes transverse to the inferred ice flow direction suggests a compressional flow regime it is therefore inferred that the glacial lineations were formed under an extensional flow regime during ice stream activity but that at some point patches of till under the ice stream stiffened through dewatering the subsequent increase in basal shear stress resulted in compressional flow and the development of subglacial thrusting and the building of ribbed moraines we therefore suggest that ribbed moraines may form in areas of compressional flow under ice streams i e sticky spots and or at the transition between slow and fast ice flow along parts of an ice stream the general absence of ribbed moraines on most other palaeo ice stream beds suggests that either these ice streams continued operating during deglaciation or processes other than the development of localized compressional flow sticky spots led to their shutdown e g ice depletion

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

styles of sedimentation beneath svalbard valley glaciers under changing dynamic and thermal regimes glacier beds provide crucial information concerning past and present ice dynamics and thermal regime in this paper we present structural and sedimentological data from four valley glaciers austre lovenbreen midtre lovenbreen vestre lovenbreen and austre broggerbreen on broggerhalvoya nw spitsbergen the main focus of this paper is midtre and austre lovenbreen two typical high arctic land based polythermal valley glaciers the former has a long record of documentation regarding its response to twentieth century climatic warming structural mapping on the glacier surface and analysis of sediments in the proglacial area of midtre lovenbreen indicate that the dynamic regime and thermal structure of the glacier have changed through time dynamically midtre lovenbreen was once heavily crevassed and relatively fast moving but now is virtually crevasse free and slow moving the sedimentary record indicates that extensive areas of this glacier were wet based when it was in a more advanced state probably at its neoglacial maximum late nineteenth early twentieth centuries during this advance a thin deforming layer of diamicton commonly fluted was draped over the existing morphology in the presently exposed proglacial area this morphology consisted of large streamlined ridges aligned parallel to ice flow erosion of the underlying bedrock appears to have been limited radio echo soundings of the glacier show that at present it is characterized by a polythermal basal thermal regime with wet based subglacial conditions only beneath its thicker parts modification of the bed is limited under this thermal regime and as a result the supraglacial environment dominates modern sedimentation comparative studies on austre lovenbreen which also is probably polythermal indicate similar sedimentary characteristics and facies associations although here there are much more extensive areas of striated bedrock indicative of former basal sliding in contrast both austre broggerbreen and by analogy vestre lovenbreen are known to be predominantly cold based collectively these four glaciers suggest a trend of glacier recession and thinning accompanied by a change in thermal regime from predominantly wet based through partly frozen to completely frozen this study suggests that svalbard valley glaciers have several dynamic modes and that glaciers switch between these modes largely as a reaction to changes in mass balance

water partitioning in the earth s mantle abstract we have conducted h2o partitioning experiments between wadsleyite and ringwoodite and between ringwoodite and perovskite at 1673 k and 1873 k respectively these experiments were performed in order to constrain the relative distribution of h2o in the upper mantle the mantle transition zone and the lower mantle we successfully synthesized coexisting mineral assemblages of wadsleyite ringwoodite and ringwoodite perovskite that were large enough to measure the h2o contents by secondary ion mass spectrometry sims combining our previous h2o partitioning data chen et al 2002 with the present results the determined water partitioning between olivine wadsleyite ringwoodite and perovskite under h2o rich fluid saturated conditions are 6 30 15 1 respectively because the maximum h2o storage capacity in wadsleyite is 3 3 wt e g inoue et al 1995 the possible maximum h2o storage capacity in the olivine high pressure polymorphs are as follows 0 7 wt in olivine upper mantle just above 410 km depth 3 3 wt in wadsleyite 410 520 km depth 1 7 wt in ringwoodite 520 660 km depth and 0 1 wt in perovskite lower mantle if we assume 0 2 wt of the h2o content in wadsleyite in the mantle transition zone estimated by recent electrical conductivity measurements e g dai and karato 2009 the estimated h2o contents throughout the mantle are as follows 0 04 wt in olivine upper mantle just above 410 km depth 0 2 wt in wadsleyite 410 520 km depth 0 1 wt in ringwoodite 520 660 km depth and 0 007 wt in perovskite lower mantle thus the mantle transition zone should contain a large water reservoir in the earth s mantle compared to the upper mantle and the lower mantle

bimodal distribution of gold in pyrite and arsenopyrite examples from the archean boorara and bardoc shear systems yilgarn craton western australia this study investigates the microstructures geochemistry and hydrothermal evolution of gold bearing pyrite and arsenopyrite from six orogenic gold deposits in the archean eastern goldfields province western australia scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectroscopy la icp ms analyses show that the gold bearing minerals possess a number of similar textural features including the occurrence of invisible gold within initial phases of growth and later stage visible gold associated with alteration rims the alteration rims are characterized by a higher than average atomic mass mainly owing to arsenic enrichment and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite these observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite textural observations and la icp ms data suggest that some invisible gold was remobilized from early formed pyrite and arsenopyrite to form visible gold during development of these alteration rims gold may also have been added by hydrothermal fluids during a later stage of mineralization in situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures based on sulfide stability relations our data indicate that development of arsenopyrite alteration rims associated with late stage visible gold formation was related to an increase in temperature maximum increase from 310 to 415 c and up to of sic orders of magnitude increase in sulfur fugacity whereas changes in oxygen fugacity were less important la icp ms analyses show that the relative and absolute variations in selected trace element au ag sb bi ba te pb co and mo concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite in general trace elements within pyrite and arsenopyrite have a relatively uniform distribution whereas later stage alteration rims have more variable trace element distributions although the observed textures are typical of prograde metamorphic coronae we suggest that they are the consequence of variations in fluid conditions and chemistry and that mineralization occurred in response to syn and or postpeak metamorphic fluid infiltration

basaltic accumulation instability and chaotic plate motion in the earliest mantle inferred from numerical experiments 1 a series of self consistent numerical models of mantle convection with magmatism and moving plates are presented to clarify the dynamics of the strongly heated mantle of the earliest earth broad hot accumulations of subducted basaltic crusts develop on the core mantle boundary and remain there for geologic time only when the internal heating rate is below a threshold above the threshold the basaltic accumulations become unstable instead the thermal buoyancy induced by the strong internal heating repeatedly causes a massive upwelling or burst of hot materials from deep mantle the mantle burst induces vigorous magmatic activity on the surface the magmatic activity in turn damages the lithosphere and disintegrates it into smaller fragments in contrast to the well ordered plate motion observed below the threshold the lithospheric fragments chaotically move and new subduction zones sporadically develop the mantle burst and chaotic motion of lithospheric fragments effectively stir the mantle and the mantle remains chemically rather homogeneous in spite of the chemical differentiation due to the vigorous magmatism the earth s mantle convection has probably changed its regime from one dominated by frequent mantle bursts and chaotic motion of lithospheric fragments to one dominated by stable basaltic accumulations and well ordered plate motion as the internal heat source decays the suggested regime transition fits in with the earth s tectonic evolution inferred from geologic observations of the continents of various ages

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

mineralogical siting and distribution of gold in quartz veins and sulfide ores of the ashanti mine and other deposits in the ashanti belt of ghana genetic implications the ashanti belt of ghana constitutes a gold province which has produced a total of about 1500 t of gold historically gold mineralization is found in steep nne ssw to ne sw trending shear zones predominantly transecting metasediments of the palaeoproterozoic birimian supergroup 2 2 2 1 ga disseminated in ca 2 1 ga granitoids in paleo conglomerates of the tarkwaian group 2135 ma and in recent placers the distribution of gold its chemistry paragenesis and mineralogical siting in the mesothermal ores of the major mines in the ashanti belt namely konongo ashanti bogosu and prestea mine are the subject of this study at the localities studied gold is present in two main types of ores 1 quartz veins with free milling gold the gold is relatively silver rich true fineness values from 730 to 954 and is accompanied by a distinct suite of cu pb sb sulfides 2 sulfide ores consisting of arsenopyrite pyrite and rarer pyrrhotite and marcasite with refractory gold the ores have apparent fineness values larger than 910 arsenopyrite and locally at bogosu pyrite were identified as the hosts of submicroscopic gold mean concentrations of gold in arsenopyrite in various samples from the different mines obtained by secondary ion mass spectrometry sims range from 67 to 314 ppm au gold concentration mapping in individual arsenopyrite crystals from the different deposits revealed similar patterns of gold distribution the grains possess a gold poor core and elevated gold contents are present along distinct crystal growth zones towards their rims the outermost crystal layer is usually gold poor the well preserved distribution patterns also indicate that remobilization of gold from the sulfides played an insignificant role in the ores of the ashanti belt multiple quartz veining and growth zoning of the sulfides are interpreted as manifestations of multiple episodes of fluid infiltration fluid flow and mineral deposition the bimodal occurrence of gold in spatially closely associated quartz vein and sulfide ores indicates a genetic link between these ore types a model implying a grossly coeval formation of the ores from mesothermal fluids is proposed

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

superplumes plates and mantle magmatism in two dimensional numerical models 1 two dimensional numerical models are systematically presented for magmatism in a convecting mantle internally heated by incompatible radioactive elements plate tectonics is self consistently modeled as a part of convection of newtonian fluid with temperature and stress history dependent viscosity driven by thermal and compositional buoyancy while magmatism is modeled as an upward migration of basaltic magma generated by decompression melting ridge magmatism actively takes place to chemically differentiate the mantle and a superplume is well developed in the lowermost mantle as a broad hot region enriched in basaltic component provided that the lithosphere is mechanically strong enough to inhibit spontaneous initiation of subduction by its own weight and that the internal heating rate is sufficiently high hot narrow plumes frequently rise up from the superplume to induce deep magma generation and hot spot magmatism which in turn frequently induces new plate margins the frequent plate margin formation keeps the activity of plate tectonics rather steady the thermal and chemical state of the mantle remains rather steady too as the internal heating rate becomes lower however the narrow plumes from superplume become sporadic and significantly colder deep magma generation becomes rare and new plate margin formation becomes less frequent as a consequence the activity of plate tectonics and the thermal and chemical state of the mantle become fluctuating much with time the mantle structure and plume behavior modeled here match with many observations for those of the earth from the archean to the present

glaciotectonic structures as useful ice movement indicators in glacial deposits four canadian case studies glaciotectonic deformation structures are useful directional indicators of ice movement but are rarely reported by north american authors being three dimensional they are commonly conspicuous in cleaned sections measuring just a few of them can provide accurate ice movement directions at a site and they assist in deciphering the interaction of past glaciers and their substrata this paper concentrates on structures generally less than 10 m long in unconsolidated sediments that were induced by overriding glacial ice examples and interpretations of various types of structures are presented in four new canadian case studies from british columbia and southern ontario that illustrate different deformational circumstances in sediments of various ages

chemical stratification in a two dimensional convecting mantle with magmatism and moving plates 1 a self consistent numerical model is presented for magmatism in a convecting mantle with moving plates mantle convection is modeled as a thermal chemical convection of binary eutectic material with newtonian rheology in a two dimensional rectangular box internally heated by radioactive elements viscosity is assumed to depend on stress history as well as temperature and pressure to self consistently reproduce moving plates the barrier effect of the solid solid phase transitions at depths around 660 km to convective flow across the 660 km phase boundary is also taken into account magmatism is modeled as a permeable flow of basaltic melt produced upon decompression melting of the convecting material through the coexisting matrix magmatism makes mantle chemically stratified with the shallower part occupied by chemically buoyant residue of magma and the deeper part occupied by hot but chemically dense materials enriched in basaltic component even under the influence of moving plates and subducting slabs the magmatism is induced by moving plates ridge volcanism mantle overturn and hot plumes uprising from the 660 km phase boundary or the lower mantle depending upon the internal heating rate and the strength of the 660 km barrier a weaker 660 km barrier leads to more continuous plate tectonics when plates move subducting slabs penetrate deep into the lower mantle and induce broad thermal and chemical heterogeneity at depth in the lower mantle the chemically stratified mantle with moving plates is compared to the earth s mantle

drumlins subglacial meltwater floods and ocean responses drumlins and erosional marks in bedrock give evidence for broad subglacial meltwater floods that have discharge rate estimates of about 10 6 m 3 s similar discharge rates are obtained for other late glacial catastrophic floods the total volume of meltwater that is thought to have formed the livingstone lake saskatchewan drumlin field is estimated at 8 4 x 10 4 km 3 this volume is equivalent to a eustatic rise of 0 23 m in global sea level meltwater release and roughly contemporaneous formation of drumlin fields in north america and europe could have involved several metres of sea level rise in a few years the implications of such floods for the generation of myths and the interpretation of the oxygen isotopic record of the oceans are discussed high meltwater discharges are of potential importance to the generation of a lid of cold fresh water over the north atlantic and its effects on late glacial climate

shear related gold mineralization in northwest ghana the julie deposit abstract the julie deposit is currently the largest gold prospect in nw ghana it is hosted in sheared granitoids of ttg composition of the paleoproterozoic julie greenstone belt the main mineralization consists of a corridor of gold bearing quartz veins forming a network of a few tens of metres in thickness trending e w and dipping 30 60 n contained within the main shear zone that affects these rocks the core of this vein corridor is altered by sericite quartz ankerite calcite tourmaline and pyrite and is surrounded by an outer halo consisting of albite sericite calcite chlorite pyrite and rutile a second set of veins conjugate to the first set occurs in the area these veins have alteration halos with a similar mineralogy as the main corridor however their extent as well as the size of the mineralization is less important in the main corridor gold forms micron sized grains that occur in pyrite as inclusions on its edges and in fractures crosscutting it silver tellurium bismuth copper and lead commonly accompany the gold pyrite occurs disseminated in the veins and in the surrounding rocks up to several ppm au occurs in the structure of pyrite from the main mineralization

water flow to the mantle transition zone inferred from a receiver function image of the pacific slab variations in seismic velocity near the subducting slab provide constraints on the thermal structure and the distribution of hydrous minerals and water in a subduction zone we investigated the seismological structure in the upper mantle beneath japan using both radial and transverse receiver functions rfs radial rf can image horizontal layers of velocity contrast while transverse rf can image dipping layers the combination allows us to simultaneously image both horizontal and dipping velocity discontinuities we investigated the records of 45 teleseismic events observed with tiltmeters at 678 hi net stations the frequency band we used is from 0 02 to 0 16 hz the resultant rf image shows an elevated 410 km discontinuity and more importantly the top surface of the pacific slab down to below the 410 km discontinuity with forward modeling we determined that the mantle wedge is about 8 slower in shear wave speed than the subducting slab at depths deeper than 200 km the seismic velocity contrast is presumably caused by a sequence of hydrous minerals at the base of the mantle wedge which receives water released by dehydration reactions in the oceanic crust we employed numerical simulations to determine the distribution of water in and around the subducting slab the result suggests that hydrous minerals are continuously stable above the subducting slab in relatively cool conditions and carry water to the mantle transition zone

submarine flow tills at victoria british columbia reply during winter to maintain the same average position over a year perhaps these cones originated by slumps on the foreslope of subaquatic outwash fans similar to those postulated by aario 1972 and rust and romanelli 1975 for a glacimarine setting the small proportion of traction current deposits may be due to subglacial streams not having a sufficiently high sediment concentration to maintain an underflow away from the efflux occasionally the streams were sufficiently dense to deposit outwash sand and gravel in the foreslope area the gravity flows may have incorporated the stream gravel sand and mud as well as melt out debris from englacial and basal debris layers mixing of these sources during liquefaction perhaps induced by iceberg calving powell 1980 p 70 may produce poorly sorted sediment flows thick accumulation by supraglacial melt out from debris layers in a tidewater glacier may best occur where the glacier has almost retreated from the sea and ablation occurs primarily by surface melting rather than iceberg calving the situation in facies association 111 of powell 1981 a similar situation of ice in shallow water under mid latitude temperature conditions was envisaged by evenson et al 1977 to explain stratified quaternary tills although their interpretation has been questioned gibbard 1980 here the glacial regime and processes are similar to those of the terrestrial environment mudflows and debris flows may occur from the ice surface and retain their integrity whilst flowing down a shallower though still relatively steep slope into shallow water if such deposits are not modified by wave action and remain preserved then they may indicate proximity to sea level in a similar way to deltaic plains

the geology of the taparko gold deposit birimian greenstone belt burkina faso west africa the taparko gold deposit located in the eastern branch of the proterozoic birimian bouroum yalogo greenstone belt burkina faso consists of a network of quartz veins developed in a n 170 trending shear zone 250 m wide 4 km long superimposed on the regional birimian structural pattern the quartz vein network is composed of a a dominant array of quartz veins type 1 parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation layering ribbon tension gashes etc b oblique and subparallel related veins type 2 of gray to white weakly deformed quartz crosscutting the dominant quartz veins resulting in breccia structures and c shallow dipping veins type 3 cross cutting veins types 1 and 2 and filled by undeformed white buck structure quartz cross cutting relationships and different quartz types in different veins and within individual veins imply a concomitant filling of the veins during the progressive deformation initial sinistral transcurrent shearing evolved with time to sinistral reverse shearing metallic minerals occur only in type 1 and 2 veins and were deposited in two stages with native gold being related to second stage sulfides gold and chalcopyrite precipitated preferentially upon the surfaces of fractured pyrite grains in low pressure sites pressure shadow zones around and or within the sulfide grains along subsequently annealed fractures the formation of the south taparko deposit can be divided into a succession of events a during the first event n 170 directed sinistral transcurrent shearing resulted in a n 20 mylonitic foliation and fractured rock which allowed h2o co2 and sio2 rich fluids to circulate and deposit quartz with buck texture b during the second event type 1 quartz was strongly deformed and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane and c during the third event the sinistral transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile to brittle ductile during the last phase of deformation gold nucleated and deposited in low pressure zones

the kalana vein hosted gold deposit southern mali abstract the kalana gold deposit occurs within metamorphosed paleoproterozoic volcano sedimentary series of the birimian baoule mossi domain of the west african craton in the south of mali these formations are intruded successively by small dioritic bodies as well as andesitic and tonalitic dykes which exhibit volcanic arc setting signatures mineralization is hosted by two sets of quartz veins that intersect the regional schistosity the first set of veins is the most important in terms of grades and size and consists of thick veins up to meter size that range in trend from n s to e w with shallow plunges the second set consists of much thinner centimeter size sub vertical quartz veins oriented ne sw the two sets of veins are interpreted to have formed during the evolution of late gently dipping thrust faulting two episodes of gold precipitation are recognized a first episode during early stages of vein growth formed micron size native gold inclusions in arsenopyrite a later episode during vein shearing and fracturing of the quartz lodes precipitated native gold in free form in quartz in fractured arsenopyrite and associated with chalcopyrite galena sphalerite bi sulfides and native bismuth all evidence suggests that the kalana deposit represents orogenic gold mineralization formed during a relatively long lived hydrothermal system at a late stage of the tectonic history of the greenstone belt

phase relations and melt compositions in cmas pyrolite h2o system up to 25 gpa abstract phase relations and melt compositions in the cao mgo al 2 o 3 sio 2 pyrolite under hydrous 2 of h 2 o and anhydrous conditions have been determined at 10 25 gpa and temperatures from 1400 to 2400 c at the pressure of 16 and 25 gpa the apparent solidus temperatures for the hydrous system are about 200 and 100 c lower than the solidus temperatures for the dry system respectively the apparent solidus temperature changes drastically from 1600 c at 13 5 gpa to about 1850 c at 16 gpa majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 gpa the liquidus phases in the dry pyrolite are olivine at 10 14 gpa majorite at 14 20 gpa and periclase at pressures above 20 gpa we observed expansion of the stability field of anhydrous phase b and its complex relations with the other phases in the cmas pyrolite system we suppose it is formed not only by incongruent melting of wadsleyite but also by the subsolidus reaction of wadsleyite and majorite in the dry condition compositions of partial melts formed by low degree of melting 10 20 have al 2 o 3 depleted and cao rich compositions up to 22 gpa and enriched in al 2 o 3 5 7 wt and cao at 25 gpa at 10 gpa dry and hydrous melts formed by low degree of melting have high sio 2 48 51 wt and relatively high al 2 o 3 3 5 5 2 wt their compositions are generally consistent with those of aluminum depleted komatiite at 13 22 gpa dry and hydrous melts have low sio 2 44 49 wt and al 2 o 3 1 7 3 0 wt and high cao contents 9 15 wt an essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume the present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle

geological and geochemical controls on the silver content fineness of gold in gold silver deposits abstract a compilation of microprobe fineness data and a re evaluation of bullion production fineness data suggest that the gold fineness model of fisher 1945 can be updated and applied to a wide range of gold mineralised environments the major deposit classes are characterised by the overall average or range of deposits averages and total range of gold fineness values as follows archaean 940 780 1000 slate belt 920 800 1000 plutonic 825 650 970 porphyry 700 1000 650 1000 volcanogenic 650 850 520 870 epithermal 440 1000 0 1000 the archaean including witwatersrand slate belt and plutonic classes are characterised by a high and consistent average fineness and a narrow fineness range there is no clear subdivision within these classes according to deposit style or inferred genesis the porphyry volcanogenic and epithermal classes have a variable average deposit fineness and a wide total range of fineness values within these three classes higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings deposits with cu au rather than cu mo or pb zn element associations and deposits with telluride or selenide minerals solubility calculations indicate that in all these environments gold and silver form the same aqueous species with agcl 2 dominating for silver and au hs 2 dominating for gold in all the environments the fluids transport an equal amount of gold and silver in the form of their bisulphide complexes therefore the observed gold fineness values can not be explained by the transporting mechanism only low silver in the ores and higher gold fineness values observed in the archaean plutonic and slate belt environments can be attributed to sulphidation as the dominant mechanism of ore deposition sulphidation of wall rocks destabilises only the bisulphide complexes of gold and silver in contrast epithermal volcanogenic and porphyry environments are characterised by a diversity of ore depositing mechanisms cooling boiling mixing which act alone or in combination and destabilise chloro and bisulphide complexes of gold and silver the high silver concentration in the ores low fineness values and large variation in the fineness values in these environments can be attributed to the complexities in the ore depositing mechanisms

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

timing and structural controls on gold mineralization at the bogoso gold mine ghana west africa the bogoso gold deposit comprises 20 open pits scattered along 18 km of a major fault zone which separates birimian supergroup turbiditic sedimentary rocks to the northwest from tarkwaian group rocks to the southeast individual pits are centered on 200 to 400 m long ore shoots which are located in three broad structural settings steeply pitching relatively high grade orebodies are commonly localized in left hand dilational jogs in the strike of the controlling faults these jogs often coincide with intersecting minor faults gold is largely confined to the graphitic mylonite which defines the fault and imbricated slices of carbonate altered rock gently dipping mineralized quartz vein stockworks and disseminated auriferous pyrite and arsenopyrite form broader zones between more steeply dipping bounding faults in the vicinity of right hand bends and splays in these faults smaller zones of disseminated sulfide mineralization coincide with areas where bedding and or foliation in the adjacent birimian sedimentary rocks is strongly discordant with the adjacent mineralized fault generally where the faults cut the short limbs of tight upright east trending folds approximately 75 percent of the gold extracted from the bogoso district has been mined from a cluster of pits that coincides with the densest and most highly connected concentration of larger faults in the bogoso concession the majority of ore shoots are located along the largest of these faults the narrower faults generally host smaller and less frequent ore shoots the fault network originated during an early episode of regional scale shortening involving thrusting and isoclinal folding the structural setting and geometry of the ore shoots implies that mineralization occurred during a later minor episode of sinistral strike slip movement on the controlling faults after development of the fault network the undeformed nature of the auriferous sulfides implies that this was the last phase of ductile brittle deformation on the host faults this conclusion is consistent with earlier work indicating that the giant ashanti deposit 150 km along strike from bogoso also developed during a period of small displacement sinistral offset late in the history of the host fault network

spatially varying hydraulic and solute transport characteristics of a fractured till determined by field tracer tests funen denmark a natural gradient tracer test that simulated one dimensional flow was conducted in a large 4 m 4 8 m surface area isolated block of fractured till in funen denmark to determine both bulk and spatially distributed hydraulic properties hydraulic conductivity based on free flow tests was higher in the upper 2 5 m 5 81 10 5 m s 1 than in the upper 4 m 8 00 10 6 m s 1 of till indicating the more highly fractured and bioturbated nature of the upper part of the till chloride was introduced at the surface of the block as nacl solution 490 mg l 1 cl and breakthrough of cl was monitored in 13 horizontal screens at depths of 2 5 and 4 m arrival times of half of the input cl concentrations c c0 0 5 ranged about sevenfold among various screens at each sampling depth 5 35 hours at 2 5 m and 11 68 hours at 4 m chloride breakthrough data for the two depths were fitted to both an equivalent porous media epm model based on the convective dispersive equation and a discrete fracture model these results indicate differences in transport characteristics of the till with depth at the 2 5 m depth the epm model fit the initial part of the breakthrough curves quite well for most samplers but did not predict the long tails adequately the converse was generally true for the discrete fracture model thus it appears that transport in the upper 2 5 m of till is controlled by a complex interconnected system of fractures and macropores field measurements of fractures and macropores at this site substantiate these results at the 4 m depth the discrete fracture model described solute transport in all samplers better than the epm model indicating that vertical fracture flow dominates in the lower portion of the block fracture apertures and spacings were estimated based on the cubic law and fitting the breakthrough data to the discrete fracture model fitted fracture spacings at the 4 m depth 0 025 0 08 m were slightly smaller than the minimum spacing 0 09 m mapped in the field calculated apertures at the 4 m depth were large compared with other studies and ranged from 65 to 127 m average value of 94 m

the hemlo gold deposit ontario an example of melting and mobilization of a precious metal sulfosalt assemblage during amphibolite facies metamorphism and deformation the timing of mineralization at the hemlo gold deposit has long been a controversial issue we present arguments that use partial melting of the ore assemblage to explain the mechanism of ore mobilization and resolve problems with the model of premetamorphic mineralization the results have implications for mine exploitation and exploration strategies we show that the ore mineral assemblage underwent partial melting during middle amphibolite facies metamorphism 600 650 c 6 7 kbars primarily through breakdown of stibnite and arsenopyrite in a high s2 environment concurrent deformation led to segregation of the resulting sb and as rich sulfosalt melt interaction between this melt and a range of unmelted sulfides led to further melting and incorporation of other elements into the melt the gold bearing melt was mobilized from compressional high strain sites into dilational domains such as boudin necks and extensional fractures developed in competent lithologies ore minerals that did not melt significantly pyrite molybdenite pyrrhotite and sphalerite were not extensively mobilized and largely remained within high strain compressional domains this segregation of melt from residue thus resulted in the observed heterogeneous distribution of ore minerals within the deposit crystallization of the sulfosalt melt produced a diverse suite of ore minerals dominated by stibnite and realgar and containing an array of rare sulfosalts native elements intermetallic compounds and tellurides some sulfosalt melt persisted to low temperature 300 c allowing continued small scale localized mobilization during late deformation although gold occurs at moderate concentrations within the compressional high strain domains it is particularly concentrated in the dilational domains a consequence of its mobilization within a sulfosalt melt our model of partial melting of the ore mineral assemblage with consequent mobilization explains how ore minerals that are unstable even at greenschist facies conditions came to be hosted in structures that formed at or near the peak of amphibolite facies metamorphism

the inata deposit belahouro district northern burkina faso the inata gold deposit is hosted in the bouroum greenstone belt of northern burkina faso and contains ca 5 moz of gold resource the greenstone belt is divided into 4 distinct domains the pali west pali minfo and fete kole domains comprised of variable proportions of mafic to intermediated volcanic volcaniclastic and sedimentary rocks and the sona basin comprised of feldspathic sandstones and turbidites potential tarkwaian like conglomerates are rarely observed on the eastern margin of the basin the stratigraphy is crosscut by a series of intrusions between 2172 15 ma and 2122 4 ma a complex deformation sequence is recorded in the rocks and has been interpreted in a five stage scheme early syn depositional basin margin faults reactivated through time and partitioning all subsequent regional deformation deb n s compression d1b 2172 ma e w compression d2b ca 2122 ma nw se compression d3b and a late n s compression d4b d2b d4b overprint all rocks including those of the sona basin and tarkwaian like conglomerates peak metamorphism is mid to upper greenschist facies mineralisation at inata is hosted in black shales and volcaniclastic rocks of the pali minfo domain and comprises shear zone hosted quartz tourmaline ankerite veins with associated sulphides dominated by pyrite and arsenopyrite three generations of pyrite py1 py2 py3 and one generation of arsenopyrite apy2 have been identified py1 is parallel to bedding and early d1b foliation and not associated with gold py2 and apy2 are coeval contain up to 1 ppm gold and are spatially associated with auriferous quartz veins py3 locally overprints previous assemblages and is also associated with au fluid inclusions in quartz indicate h2o to h2o co2 nacl fluids in auriferous quartz veins microscopic to macroscopic observation of fabric mineral vein crosscutting relationships indicate that mineralisation is syn d2b disrupted and remobilised during d3b all observations and data are consistent with inata representing an orogenic style of gold mineralisation formed relatively late in the evolution of the host terrane

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

glacigenic rafting of weathered granite charlie lake new brunswick glaciation in the area of charlie lake new brunswick has produced a granitic till very similar in appearance to weathered bedrock portions of weathered granite were moved intact and redeposited as imbricated rafts under basal till the rafts vary from 0 3 to 2 m in thickness over a 10 m wide exposure and show deep weathering zones and brittle deformation despite the fact that they are highly friable the rafts are separated by glacigenic fault planes defined by lenses of diamict and rare clasts of distant origin they are recognizable only in excavations where fault planes can be seen to delineate slabs of rock imbricated mainly upglacier surface weathering features including core stones were preserved during movement over short distances

laminated sediments in glacimarine environments diagnostic criteria for their interpretation abstract fine grained laminated deposits are often reported from contemporary and ancient glacimarine sedimentary sequences laminated glacimarine sediments form by a variety of processes including suspension settling from turbid overflow plumes turbidity current deposition and contour current activity subglacial deformation of pre existing sediments may also produce laminated deposits in the form of glacitectonite and deformation till the development of diagnostic criteria for genetic discrimination of laminated glacimarine sediments is important both for their correct interpretation in sedimentary sequences and also for the accurate reconstruction of former glacier extent and geometry as laminated sediments may form in subglacial ice proximal and ice distal environments this paper reviews the processes by which laminated glacimarine sediments form and their resulting sedimentology the aim is to provide a synthesis of diagnostic sedimentological criteria for laminated glacimarine deposits that will assist in the interpretation of such sediments in the geological record laminated sediments are genetically differentiated on the basis of sedimentary structures bedding contacts stratigraphic relationships and macrofaunal content bioturbation these attributes also facilitate discrimination between glacimarine and glacilacustrine rhythmites the paper concludes by discussing the glaciological significance of laminated glacimarine sediments in terms of glacier hydrology reconstructions of former ice dynamics and basal thermal regime