circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

late pliocene margin development and mega debris flow deposits on the antarctic continental margins evidence of the onset of the modern antarctic ice sheet abstract major global climatic changes in the cenozoic coincided with major changes in volume of polar ice sheets which are reflected in the sedimentary regime of polar continental margins we compile and review the most recent evidences of the neogene architectural change in the sedimentary units of the antarctic continental margins to provide a comprehensive interpretation of timing and circumstances of the last of these changes we review and interpret the seismic stratigraphy and the borehole data where available from 6 key sectors of east and west antarctica the continental margins of the antarctic peninsula prydz bay wilkes land weddell sea eastern and western ross sea in these margins we identify the following change in the geometry of the sedimentary deposits margin wide erosion and subsequent progradation on the continental shelf stratal downlap on the continental slope major mass wasting deposits on the continental rise the change is apparently synchronous and dated about 3 ma in the antarctic peninsula and prydz bay margins and broadly concomitant in the others sectors this suggests a common driving force which we suggest to be the late pliocene climatic change that induced the transition of the antarctic ice sheet regime from polythermal to present polar cold dry based conditions we infer that in consequence of the late pliocene global cooling meltwater decreased in quantity and focussed within the main ice troughs where large ice streams began to flow the regional erosion was followed by focussing of the sedimentary transport at the base of the large ice streams and faster progradation of sedimentary wedges at the mouth of the ice troughs this induced large failures of the continental slope while the continental rise was progressively deprived of terrigenous sediments

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

the present and past bottom current flow regime around the sediment drifts on the continental rise west of the antarctic peninsula throughout the last decade large sediment drifts located on the upper continental rise west of the antarctic peninsula were the target of oceanographic measurements bathymetric mapping seismic investigations shallow sediment coring and deep sea drilling by ocean drilling program odp leg 178 these studies concluded that for most of the late neogene and quaternary a generally sw ward directed bottom current affected the deposition on the drifts in particular during glacial periods the deposition was additionally influenced by nw ward directed transport of terrigenous detritus supplied by turbidity currents from the antarctic peninsula continental slope in a recent study however the palaeomagnetic signal of the drift sediments recovered at two odp leg 178 sites 1095 and 1101 was interpreted to provide spatial and directional information on the physical record of the ne ward flowing antarctic circumpolar current acc here we investigate the link between the clockwise flowing acc and the generally sw wards flowing near bottom contour current we show that at the odp leg 178 sites on the western antarctic peninsula continental margin the acc only affects the ocean circulation above ca 1000 m water depth therefore the signal of acc flow might only be archived in the ice rafted debris ird content of the drift sediments however the ird from the leg 178 sites accounts for only a small proportion of the drift sediments and is dominantly of local origin past variability of bottom water flow around the sediment drifts was reconstructed on the basis of seismic studies and clay mineralogical and grain size analyses these reconstructions provide useful information about both downstream direction and velocity changes of the bottom current and point to its sw ward flow along the upper rise during at least the last 9 4 ma

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

paleoproterozoic gold deposits hosted by albite and carbonate altered tonalite in the chirano district ghana west africa paleoproterozoic birimian supergroup rocks of southwest ghana host numerous mesothermal lode gold deposits including the 38 million ounce moz ashanti deposit at obuasi and other multimillion ounce deposits at bogoso konongo prestea bibiani and in the yamfo sefwi region gold deposits of the chirano district are localized along the chirano lode horizon a 9 km segment of the chirano and related shear zones on the southeast margin of the sefwi bibiani belt economically significant mineralization at chirano is largely hosted by altered tonalite intrusions weaker alteration and subparts per million gold grades extend into adjacent birimian mafic igneous rocks and tarkwaian group sedimentary rocks pink albite alteration albite quartz pyrite hematite rutile leucoxene chalcopyrite gold and later pale carbonate alteration ankerite sericite quartz pyrite hematite gold fineness 910 940 ag au telluride characterize the gold deposits of the chirano district alteration assemblages and the bi te and mo contents of the mineralized rocks at chirano are more typical of lode gold deposits than orthomagmatic gold deposits alteration assemblages at chirano contrast with the graphite carbonate pyrite arsenopyrite pyrrhotite assemblages that dominate the lode gold deposits in southwest ghana which are hosted by graphitic shear zones and birimian sedimentary rocks differences between alteration assemblages at chirano and those in graphitic shear zones may reflect the different wall rock assemblages with which the hydrothermal fluid equilibrated rather than different fluid sources mineralization in the chirano district formed during a relatively late episode of small displacement brittle cataclasis on the chirano lode horizon after reverse ductile shearing associated with regional scale shortening this structural history is similar to that documented at the larger ashanti and bogoso deposits on the opposite side of the kumasi basin timing relationships are compatible with all three deposits forming during a single mineralizing event late in the eburnean 2 orogeny at chirano limited fault vein geometric relationships imply a component of reverse movement during mineralization whereas sinistral strike slip movement accompanied mineralization at ashanti and bogoso no consistent correlation was observed between the asymmetry of bends and splays in the strike of the chirano lode horizon and the location or style of individual gold deposits unlike at ashanti and bogoso the structural settings of deposits in the chirano district indicate that an exploration strategy which assumes mineralization occurred during sinistral offset in the host structures similar to relationships at ashanti and bogoso may result in significant targets being ignored

the early proterozoic birimian supergroup of ghana and some aspects of its associated gold mineralization abstract rocks of the birimian supergroup of early proterozoic age sm nd isochron age 2166 66 ma form part of the west african craton ghana contains the eastern portion of the birimian terrane five parallel evenly spaced several hundred kilometre long volcanic belts consisting mainly of low grade metamorphic tholeiitic lavas morb chemistry characterize the birimian supergroup of ghana the belts are separated by basins containing isoclinally folded dacitic volcaniclastics wackes and argillitic sediments as well as granitoids lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents within the sedimentary basins various lithofacies are distinguished in the transition zone between belt volcanics and basin sediments a chemical facies occurs this is defined by cherts manganiferous and carbon rich sediments fe ca mg carbonates and sulphide mineral disseminations after their emplacement birimian lavas and sediments were folded in the course of the eburnean tectono thermal event intruded by various types of granitoids uplifted and eroded the erosion products were deposited as sediments of the tarkwaian group in long narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five birimian volcanic belts tarkwaian sediments were deformed by gravity tectonics two major types of eburnean granitoids in ghana are characterized as follows basin granitoids cape coast type occur as synorogenic foliated batholiths chiefly in the central portions of birimian sedimentary basins they are peraluminous and generally granodioritic in composition belt granitoids dixcove type mostly form late orogenic unfoliated intrusions in volcanic belts they possess a metaluminous character and commonly are of tonalitic composition bongo granitoids are of post tarkwaian age and k rich in contrast to these granitoid types which have strong mantle affinities the winneba type had an archean sialic precursor a model involving small scale equidimensional parallel and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed recent sm nd pb pb and rb sr data of volcanic and plutonic rocks are incorporated most ghanaian birimian gold occurrences and mines are concentrated in narrow corridors of 10 15 km width in the transition zone between volcanic belts and sedimentary basins as are the chemical facies and regionally extensive shear zones at the volcanics sediment interface birimian gold in ghana is present as two major types 1 the disseminated sulphide type which is generally lithofacies controlled i e controlled by chemical sediments and to a lesser extent by selvages of gold quartz veins and 2 the quartz vein type which is exclusively structure controlled data on geochemical underground traverses across both types of gold mineralization are briefly summarized

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

chemical and boron isotope composition of tourmaline from the kiaka orogenic gold deposit burkina faso west african craton as a proxy for ore forming processes the kiaka orogenic gold deposit burkina faso located in the paleoproterozoic domain of the west african craton is characterized by a two stage gold mineralization hosted in volcano sedimentary metamorphic rocks that was formed during the eoeburnean 2 20 2 13 ga and eburnean 2 13 2 05 ga orogenic cycles these two stages include an early disseminated low grade gold mineralization and a late vein hosted high grade gold mineralization paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline the aim of this paper is twofold i to determine the processes responsible for deposition of the economic disseminated gold mineralization and ii to identify the source of the mineralizing fluids for this purpose we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different 11b values falling within the two intervals of 25 1 to 22 0 and 19 6 to 15 1 respectively our results suggest that tourmaline formed from a distal high temperature ca 400 c reduced and low salinity hydrothermal fluid that interacted with the local host rocks based on the modeling of tourmaline fluid boron isotope fractionation we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca 2 13 ga this metamorphic fluid rock interaction model may possibly extend to other orogenic gold deposits in the west african craton

were west antarctic ice sheet grounding events in the ross sea a consequence of east antarctic ice sheet expansion during the middle miocene abstract seismic correlation of glacial unconformities from the ross sea outer continental shelf to chronostratigraphic control at dsdp sites 272 and 273 indicates that at least two west antarctic ice sheet wais expansions occurred during the early part of the middle miocene i e well before completion of continental scale expansion of the east antarctic ice sheet eais inferred from 18o and eustatic shifts therefore if the volume of the eais was indeed relatively low and if the ross sea age model is valid then these wais expansions contractions were not a direct consequence of eais expansion over the transantarctic mountains onto west antarctica an in situ development of the wais during the middle miocene suggests that either west antarctic land elevations were above sea level and or that air and water temperatures were sufficiently cold to support a marine based ice sheet additional chronostratigraphic and lithologic data are needed from antarctic margins to test these speculations

intrusion related affinity and orogenic gold overprint at the paleoproterozoic bonikro au mo deposit cote d ivoire west african craton the bonikro gold deposit and satellite hire open pits are located at the southern tip of the oume fettekro granite greenstone belt in cote d ivoire country rocks in the region have undergone polyphase deformation and prolonged arc magmatism during the paleoproterozoic eburnean orogeny intrusive host rocks at hire and bonikro have been dated at 2180 6 and 2086 4 ma u pb on zircon respectively these plutonic bodies acted as favourable sites for fluid flow due to their brittle rheological characteristics gold mineralisation at bonikro is hosted by i a sheeted quartz vein array characterised by an au w bi te ag metal association in the cupola of a porphyritic granodiorite and ii an overprinting swarm of fault fill smoky quartz molybdenite veins gold mineralisation at hire is shear hosted and shares a similar relative timing with the latter set of auriferous veins this second phase of gold mineralisation occurred during a period of transcurrent tectonics late in the eburnean tectono magmatic history soon after the cessation of compressional deformation it is responsible for the bulk of gold resources in the region e g bonikro hire agbahou and has been dated at 2074 16 ma re os on molybdenite the integration of structural paragenetic relationships with high precision dating of magmatic and hydrothermal events highlights the superposition of late eburnean orogenic gold mineralisation on an earlier intrusion related gold system linked to the local emplacement of a porphyritic granodiorite at bonikro and illustrates the genetic diversity of paleoproterozoic granitoid hosted gold mineralisation in the west african craton it also further supports that gold mineralisations in the west african craton occurred diachronously throughout the eburnean orogeny through a variety of deposit types including au skarn intrusion related au orogenic au and porphyry cu au

the hydrothermal fluid of archaean lode gold deposits at different metamorphic grades compositional constraints from ore and wallrock alteration assemblages a low salinity mixed aqueous carbonic fluid is common to all archaean lode gold deposits throughout the range of mineralising conditions from sub greenschist to lower granulite facies temperatures alteration assemblages and fluid inclusion data give constraints on the fluid composition fluid xco2 is 0 1 0 3 in typical greenschist facies mesothermal deposits at higher temperatures the assemblages are consistent with formation from a fluid of similar composition but slightly higher or lower xco2 cannot be ruled out and fluid inclusion data indicate that ch4 may be an important component in ore fluids at these temperatures fluid ph is neutral or weakly alkaline at all conditions a range of relative oxidation states of four orders of magnitude fo2 is indicated at any temperature with deposits more oxidising relative to qfm at lower temperature sulphur contents of the fluids vary from 10 to 10 3 5m s with a trend towards lower sulphur contents at lower temperatures the relative concentrations of major cations in solution are similar at all conditions with na k ca although ca may be less abundant at low temperatures the broad similarities in ore fluid composition at all temperatures give support to crustal continuum models in which archaean lode gold mineralisation involved either a single fluid moving through the middle and upper crust or derivation of ore fluids by similar processes at different crustal levels many of the compositional differences between high and lowtemperature ore fluids may be attributed to evolution of deep sourced hydrothermal solutions as they rise along structurally controlled conduits the constancy of major ore fluid component concentration e g co2 cl k suggests fluid buffering and high fluid rock ratios along fluid pathways fluid buffered conditions can also explain the ore fluid fo2 temperature relations with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state in contrast the concentrations of components present in lesser abundance in archaean gold ore fluids e g s ca h were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature or were rock buffered through fluid rock reactions extrapolation to high temperatures of the k na and ca contents of the gold bearing fluids indicates that their composition is consistent with derivation from or final equilibration with rocks of intermediate granitic composition thus giving support to isotopic and geological arguments for ore fluid source regions external to the greenstone belts the fluid oxidation states are characteristic of a wide range of potential source rocks including mantle derived igneous rocks calc alkaline granitoids and magmas and seaflooraltered metabasalts strongly oxidised magmatic sources or unusually oxidising source processes e g co2 streaming during granulitisation of the lower crust are therefore not required in the genesis of arch an lode gold deposits

the influence of dikes on auriferous shear zone development within granitoid intrusions the bourlamaque pluton val d or district abitibi greenstone belt shear zone related gold quartz veins in granitoid intrusions are commonly intimately associated with mafic dikes which may have a profound influence on the localization orientation and kinematics of auriferous shear zones the bourlamaque pluton of the val d or district contains several economic auriferous shear zones most of which follow and overprint diorite dikes mineralization in all deposits consists of quartz tourmaline pyrite veins in reverse oblique orientation with a significant range of strike dip and slip direction the geometry and kinematics of shear zone and vein array within the pluton is more complex than the simple conjugate pattern predicted for a deforming homogeneous intrusion the stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly directed compression recorded by similar shear zones outside the pluton this indicates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes which

flow of the west antarctic ice sheet on the continental margin of the bellingshausen sea at the last glacial maximum geophysical data show that during the last glaciation the west antarctic ice sheet wais drained to the continental shelf edge of the bellingshausen sea through a cross shelf bathymetric trough belgica trough as a grounded fast flowing ice stream the drainage basin feeding this ice stream probably encompassed southwestern palmer land parts of southern alexander island and the bryan coast of ellsworth land with an area exceeding 200 000 km2 on the inner continental shelf streamlined bedrock and drumlins mapped by swath bathymetry show that the ice stream was fed by convergent ice flow draining from eltanin bay and bays to the east as well as by ice draining the southern part of the antarctic peninsula ice sheet through the ronne entrance the presence of a paleoice stream in belgica trough is indicated by megascale glacial lineations formed in soft till and a trough mouth fan on the continental margin grounding zone wedges on the inner and midshelf record ice marginal stillstands during deglaciation and imply a staggered pattern of ice sheet retreat these new data indicate an extensive wais at the last glacial maximum lgm on the bellingshausen sea continental margin which advanced to the shelf edge in conjunction with ice sheet reconstructions from the antarctic peninsula and pine island bay this implies a regionally extensive ice sheet configuration during the lgm along the antarctic peninsula bellingshausen sea and amundsen sea margins with fast flowing ice streams draining the wais and antarctic peninsula ice sheet to the continental shelf edge

chemistry origin and evolution of mineralized granites in the lachlan fold belt australia the metallogeny of i and s type granites granites crop out over 20 percent of the total exposed area of the lachlan fold belt and comprise subequal proportions of i and s types despite the volume of granite present economic mineralization is sporadic major sn mineralization is related to s and i type granites that are both reduced and have undergone extended feldspar dominated crystal fractionation mo showings are common but as for cu large deposits are absent scheelite skarn mineralization is represented by the major king island deposits however granite related w mineralization of a similar scale is not found elsewhere within the belt the i type granites of the belt represent the products of partial melting of older tonalitic granodioritic source rocks in the deep crust which were themselves ultimately derived through fusion of mantle materials m types this evolution is also mirrored in the nature of related mineralization which changes from cu dominated at the chemically primitive end of this series through to lithophile dominated in the more evolved i types most of the lachlan fold belt i types have not undergone significant crystal fractionation and are neither strongly oxidized nor reduced two prominent mineralized i type suites the boggy plain supersuite of the central lachlan fold belt and the grassy suite of king island are anomalously mineralized relative to other lachlan i types both are chemically distinct from other i types in the belt by being enriched in the incompatible elements and ni and cr mafic s types are universally restite rich and unmineralized s type magmas are in general more evolved chemically than 1 types are all relatively reduced and when highly fractionated are related to sn or w mineralization the ore element ratios sn w cu mo etc of granite related mineralization in the lachlan fold belt is a straightforward function of the relative oxidation state and degree of fractionation within the associated granite

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

the silidor deposit rouyn noranda district abitibi belt geology structural evolution and paleostress modeling of an au quartz vein type deposit in an archean trondhjemite late orogenic gold bearing quartz vein deposits within the rouyn noranda mining district occur mainly within archean tonalitic plutons of the blake river group southern abitibi belt the silidor deposit is a representative example of a pluton hosted lode gold deposit the silidor mine contained 2 95 million tons mt mined and estimated reserves grading 5 1 g t au 15 t au the mineralized zone is 900 m in length and has a vertical extent of 900 m an average thickness of 3 5 m and trends northwest southeast with a dip of 50 to 70 ne its alteration envelope consists of a red hematite altered trondhjemite measured 18 o values for quartz veins 7 7 10 9 and 34 s values for pyrite 6 1 to 9 4 imply oxidizing conditions during gold deposition the mineralized zone comprises vein quartz white gray and smoky varieties beige mineralized trondhjemite and green carbonate sericite fuchsite breccia which resulted from shearing and metasomatism of an early northwest southeast dioritic dike a quantitative microtectonic study 400 measurements was carried out on the powell tonalitic sill in the silidor mine area to reconstruct the deformation before during and after the mineralizing events the reduced stress tensors display large variations in 1 orientation trending successively northwest southeast northeast southwest and finally north south the silidor deposit is the result of several vein filling events which occurred during evolution from strike slip faulting to reverse faulting regimes with 1 remaining northeast southwest but with exchange of orientation of 2 and 3 such variations may reflect an oblique collision and processes of stress permutation paleostress mapping of archean terranes can be used as a targeting tool for mesothermal lode gold deposits reduced stress tensors were used for the computation of paleostress maps using a distinct element model the silidor au quartz mineralization appears on these paleostress maps in an area characterized by low mean stress throughout the deformation history of the area near the horne creek fault this study emphasizes the role of second order faulting in the location of low and high pressure zones in the archean crust and the possible role of a tectonic indentor in the location of au mineralization

fluid pressure dominated orogenic gold mineralization under low differential stress case of the yaoure gold camp cote d ivoire west africa the 1 5 moz yaoure gold deposit which is located within the birimian greenstone belts of central cote d ivoire west africa is associated with two stages of economic gold mineralization both ore forming stages occurred during protracted e w directed shortening associated with the eburnean orogeny the first mineralization stage is hosted by fault fill veins along conjugate ene striking dextral and wnw striking sinistral strike slip fault arrays this ore stage is associated with a polymetallic assemblage including mo bi pb w te ag and au and cryptic alteration in wall rocks the second and main gold mineralization stage is associated with a low displacement fault fracture vein mesh formed during a reverse faulting episode this second gold phase is associated with a polymetallic assemblage including w and au pb and is characterized by pronounced albite ankerite pyrite hydrothermal alteration in wall rocks shear veins associated with this second mineralization episode formed during episodic fluid pressure variations under low differential stress within pre existing low angle reverse fault planes permeability destruction by albite ankerite pyrite hydrothermal sealing of these low angle reverse faults resulted in fluid pressure build up and ultimately in the development of extension veins under low differential stress in the late stages of the structural evolution of the deposit this study presents the structural evolution of a polyphase orogenic gold deposit and highlights that the interplay between fault zone activity hydrothermal alteration and fluid pressure fluctuations at low differential and deviatoric stresses were key in controlling fluid flow and the formation of ore within optimally oriented faults

eocene circulation of the southern ocean was antarctica kept warm by subtropical waters 1 near the eocene s close 34 million years ago the climate system underwent one of the largest shifts in earth s history antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed previous studies conjectured that poleward penetration of warm subtropical currents the east australian current eac in particular caused eocene antarctic warmth late eocene opening of an ocean gateway between australia and antarctica was conjectured to have disrupted the eac cooled antarctica and allowed ice sheets to develop here we reconstruct eocene paleoceanographic circulation in the tasmanian region using 1 biogeographical distributions of phytoplankton including data from recently drilled ocean drilling program leg 189 sites and 2 fully coupled climate model simulations we find that the eac did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern our results do not support changes in thermal isolation as the primary driver of the eocene oligocene climatic transition

extent and dynamics of the west antarctic ice sheet on the outer continental shelf of pine island bay during the last glaciation abstract swath bathymetry and topas sub bottom profiler acoustic data reveal the presence of a major cross shelf bathymetric trough containing streamlined subglacial bedforms formed in soft till that extends to the continental shelf edge of outer pine island bay amundsen sea west antarctica the new data indicate that the west antarctic ice sheet wais extended to the shelf edge of pine island bay during the last glaciation and was drained by a grounded fast flowing palaeo ice stream through this outer shelf trough topography and subglacial geology are likely to have exerted a strong control on the development and location of palaeo ice streams in pine island bay topas records show that mega scale glacial lineations are formed in an acoustically transparent sediment layer interpreted to be soft till and are inferred to be the product of subglacial sediment deformation beneath the palaeo ice stream the flat to irregular nature of the basal reflector of the acoustically transparent layer of soft till implies that a combination of groove ploughing of the substrate and subglacial sediment deformation were important processes beneath the palaeo ice stream in the outer trough of pine island bay as well as palaeo ice streams elsewhere in antarctica therefore the formation of the soft till layer and possibly associated subglacial bedforms are likely to be a function of these palaeo ice stream processes in a regional context the wais extended to the continental shelf edge of the bellingshausen amundsen and ross seas and was drained by a number of palaeo ice streams during the last glaciation

a recount of ross sea waters abstract oceanographic observations within the ross sea have grown dramatically in recent years both in number and quality this has prompted a parallel recount of the circulation and structure of all water masses in the southwestern continental margins of the pacific ocean a high resolution set of horizontal property distributions was combined into a new climatology which in turn is the basis of a fine volumetric s census of all ross sea water masses inshore of the shelf break 700 m isobath the ross sea volume 25 10 4 km 3 partitioning into layers of neutral density n is 25 in the top layer n 3 of antarctic surface water aasw 22 in the middle layer 28 00 kg m 3 n 3 of oceanic thermocline water 12 and modified circumpolar deep water mcdw 10 and 53 in the bottom layer n 28 27 kg m 3 of shelf water sw 1 85 c 22 precursor of the antarctic bottom water aabw offshore aasw flows westward along the outer shelf and southward along the eastern coast past cape colbeck oceanic thermocline waters 28 00 kg m 3 n 3 cross the shelf break west of 170 w and follow southward paths along banks shoaling progressively to near the sea surface of the central and western inner shelves winter buoyancy loss converts aasw and oceanic thermocline water into denser types of mcdw and sw which move cyclonically toward the sills of major troughs the census shows a continuous mode of relatively dense mcdw 28 10 kg m 3 n 3 directly linked to the circumpolar deep water cdw offshore effectively ventilating and freshening the deep ocean at levels below the salinity maximum and above aabw mcdw outflows from the ross sea shelf are traced to the antarctic circumpolar current near 155 e into the australian antarctic basin along the northern flank of the southwest pacific ridge and to near 135 w past the eastern end of the ross gyre two sources of salty sw are inferred near the coastal polynyas in the terra nova bay and western ross ice shelf areas the latter contributes to a major outflow of ice sw that reaches the shelf break along 180 vertical mixing of mcdw and sw produce msw over the shelf continuing over the sills as new aabw export of low salinity aabw s

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

gold mineralization in the ashanti belt of ghana genetic constraints of the stable isotope geochemistry the ashanti belt of ghana is the key district of gold mineralization in the paleoproterozoic terrane of west africa the area considered in southwest ghana is covered by lithologies of the volcanic sedimentary birimian supergroup and the overlying elastic sedimentary tarkwaian group which were jointly folded and metamor phosed under greenschist facies conditions during the eburnean teetonothermal event at about 9 1 ga regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism four major tsoes of primary gold mineralization are present in the ashanti belt 1 mesothermal generally steeply dipping quartz veins in shear zones mainly in birimian sedimentary rocks 9 sulfide ores with auriferous arsenopyrite and pyrite spatialty dosely associated with the quartz veins 3 sulfide disseminations and stockworks in granitoids and 4 paleoplaeers of the tarkwaian group this study concentrates on types 1 and 9 of the hydrotherlnal gold mineralization stable isotope analyses of host rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization carbonaeeous matter in the birimian metasediments displays 93c values ranging from 11 4 to 9 8 3 per mil relative to pdb indicating an organogenie origin carbonates display a unimodal distribution of 9ac values ranging from 9 9 to 17 0 per rail relative to pdb coz extracted from fluid inclusions in the auriferous quartz veins has 93c values ranging from 9 5 to 15 7 per mil relative to pdb it is proposed that these carbon isotope compositions of carbonates and coz reflect extensive interaction of the co 2 rieh hydrothermal fluids with reduced carbon in birimian sediments in the deeper parts of the hydrothermal systems carbonates and auriferous vein quartz have 6 so values ranging from 19 9 to 9 9 9 and 19 8 to 15 6 per mil relative to smow respectively carbonates and quartz were deposited in near isotopic equilibrium with respect to 6 so indicating fluid dominated conditions during ore formation from fluids of metamorphic or magmatie origin such an origin is corroborated bysd values of water extracted from fluid inclusions in vein quartz 37 to 53 relative to smoxv pyrite of synsedimentary diagenetie origin in birimian schists displays sulfur isotope compositions ranging from 7 3 to 9 0 9 per rail median ca 10 relative to cdt similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534s values in the range 5 3 to 10 9 per rail relative to cdt the tight unimodal distribution of 534s values indicates a large homogeneous fluid reservoir the low 534s values are interpreted as source inherited not related to unusual ph eh temperature or depositional conditions sulfides in birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems the c o h and s isotope compositions of ore related hydrothermal minerals and fluid inclusion compo nents indicate that the mineralizing fluids interacted extensively with the paleoproterozoie rocks especially birimian sediments at deeper crustal levels and at high temperatures the isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invomng birimian strata during pro grade metamorphism at depth metamorphic fluids

structural controls on granitoid hosted gold mineralization and paleostress history of the edikan gold deposits kumasi basin southwestern ghana the 9 moz total aggregate gold endowment at the edikan mine kumasi basin ghana is contained within a cluster of orogenic gold deposits located along the akropong fault zone the granitoid hosted orebodies at edikan e g ag2 ag3 fobinso esuajah essentially an interconnected mesh of gold bearing quartz veins formed during deformation event d3edk which postdates the penetrative regional d2edk deformation the gold bearing quartz veins developed in and adjacent to n s and nw se trending low angle thrust faults that crosscut lithological contacts and earlier formed steeply dipping d2edk faults our paleostress analysis shows that the d3edk deformation during which the mineralized fault system developed was characterized by a wnw ese hybrid compression that evolved to a strike slip regime this progressive deformation is best described with the following stress regimes wnw ese transpression pure compression t1 associated with low angle thrusting subsequent transpression strike slip t2 and later strike slip transtension t3 associated with steeply dipping strike slip faulting the bulk of the granitoid hosted gold mineralization at edikan is associated with two principal sets of gold bearing quartz veins including low angle fault fill veins controlled by thrusts and shallow dipping oblique extension veins that developed during t1 the activation of the reverse and sinistral strike slip faults led to the development of restraining jogs characterized by abundant shallow and steeply dipping gold quartz veins with moderately ne plunging ore shoots the geometry of the mineralized fault fracture meshes is consistent with fault valve behavior in a horizontal compressive stress regime under sustained conditions of supralithostatic fluid pressures at low differential stress

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

late archean granitoid hosted lode gold deposits yilgarn craton western australia deposit characteristics crustal architecture and implications for ore genesis granitoid hosted lode gold deposits constitute a sub group of late archean lode gold deposits in the yilgarn craton of western australia they share a number of common characteristics including 1 a strong structural control on a variety of scales 2 in most instances the presence of mineralisation in adjacent supracrustal sequence 3 wallrock alteration and vein assemblages consistent with the addition of sio2 k2o co2 s na2o but dependent on the p t conditions of the host environment and ore fluid 4 a metal association comprising au ag as bi te w mo and low concentrations of cu pb and zn and 5 deposition from a low moderate salinity near neutral h2o co2 ch4 fluid over a temperature and pressure range from 250 to 600 c and 1 to 4 kbar which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies these characteristics suggest that the granitoid hosted lode gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone hosted lode gold deposits of the craton in combination lode gold deposits irrespective of their host rocks and host terrane formed over a range of crustal levels and developed late in the tectonic sequence of the craton granitoids which host lode gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and in combination with thermodynamic and isotope studies indicate that the host granitoids are not the dominant source of the ore fluid however the host granitoids do provide a structural and to a lesser extent chemical trap for gold bearing fluids in combination trace element and isotope studies of greenstone and granitoid hosted deposits suggest that the ore fluid originated from a deep mid to lower crustal source and at least interacted with granitic rocks at some stage the ore fluid composition is quite constant over the range of crustal levels from deep to shallow implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial

amundsen sea sediment drifts archives of modifications in oceanographic and climatic conditions abstract drift deposits document stages of particular dynamic bottom currents and associated sedimentary transport activities the analysis of seismic reflection data from the amundsen sea southern pacific ocean reveals sediment drift formation already in eocene oligocene times this observation indicates bottom current activity and hence a cold climate for the late palaeogene in an area which today lies under the influence of antarctic bottom water aabw originating in the ross sea the generation of sediment drifts is accompanied by the occurrence of mass transport deposits leading to the identification of a phase of strong ice sheet expansion 15 4 ma which due to a change in ice regime from wet to dry based was followed by less material input during the last 4 ma

the effects of layer anisotropy on auriferous shear zones the norbeau mine quebec the norbeau gold deposit comprises quartz veins within four differently oriented sets of shear zones which cut the bourbeau sill a metamorphosed layered mafic intrusion despite their differing orientations all shear zones display evidence of oblique reverse shear which three dimensional analysis shows to result from a nearly uniaxial downdip extension of the sill with a concurrent component of shortening along the strike of the sill this guiding of strain axes can be attributed to the response of a stiff layer the sill in a matrix of softer rocks during a regional compressive deformation this study demonstrates that veins in such rocks are strongly controlled by the geometry of the bodies and less by external stress conditions thus applications of regional exploration models that rely on the prediction of vein patterns based solely on a consideration of paleostress axes will fail to provide an accurate assessment of the specific vein distribution in such layered rocks which host a number of important archean lode gold deposits

visions of ice sheets in a greenhouse world abstract backstripped eustatic estimates from new jersey and the russian platform show large 25 m and rapid 18 o record highlights the enigma this was the warmest interval of the past 200 my yet it was bracketed by two inferred eustatic falls of 25 m that were associated with two large 0 75 deep sea 18 o increases 92 93 ma mid turonian and 96 ma mid cenomanian we reconcile records of warm high latitudes with glacioeustasy by proposing that late cretaceous early eocene ice sheets generally reached maximum volumes of 8 12 10 6 km 3 20 30 m glacioeustatic equivalent but did not reach the antarctic coast hence coastal antarctica hence deep water remained relatively warm even though there were significant changes in sea level as the result of glaciation unlike the oligocene and younger icehouse world these ice sheets only existed during short intervals of peak milankovitch insolation leaving antarctica ice free during much of the greenhouse late cretaceous to middle eocene these results highlight the need to re evaluate the paradigm that polar ice sheets did not exist during times of warm high latitude climates

depositional patterns at drift 7 antarctic peninsula along slope versus down slope sediment transport as indicators for oceanic currents and climatic conditions abstract understanding the processes and phases of deep sea sediment drift formation is essential for a reconstruction of their evolution this leads to a better understanding of the properties of oceanographic currents active during drift formation and in turn to information on the climatic conditions a system of sediment drifts at the antarctic peninsula pacific rise has been chosen to learn more about the neogene evolution of both current systems and palaeo climate in that area drift 7 was extensively surveyed seismic and sampling and two odp leg 178 sites were drilled there using this information maps of reflectors depth and seismic unit thickness were compiled and interpreted regarding the controlling depositional processes the depositional model shows an initially major along slope sediment transport by a sw setting bottom current 25 15 ma which deflected sediment supplied from the continental shelf between 15 ma and 9 5 ma down slope transport took over as a result of the growth of the antarctic peninsula ice sheet the sw setting bottom current appears to have broken down down slope transport has decreased since 9 5 ma but a re onset of the bottom current can only be observed since 5 3 ma the analysis has further shown that the nucleus of the drift is connected to a basement ridge it is hence inferred that basement topography played a major role in the formation of this sediment drift

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

the geology and geochemistry of jinchangyu gold deposit north china craton implications for metallogenesis and geodynamic setting abstract the jinchangyu au deposits contained an original resource of 50 t of gold and is located close to the northeastern margin of the north china craton ncc the orebodies are controlled by structures in the amphibolite units of the archaean zunhua group mineralization is generally associated with albite and hematite which is indicative of na al si fe alteration the most common styles of mineralization are subdivided based on the cross cutting relationships of mineral assemblages as follows i quartz albite hematite ii quartz albite polymetallic sulfides with gold and molybdenite iii quartz pyrite and iv quartz carbonate quartz samples from the second stage ii and third stage iii assemblages contain two phase fluid inclusion types these are i co2 h2o fluid inclusions and ii daughter mineral bearing inclusions which have homogenization temperatures of 270 350 c for assemblage stage ii and 180 240 c for stage iii and salinities of 13 wt nacl equiv a few s type fluid inclusions suggesting salinities of 27 6 28 3 wt nacl equiv the s isotope composition of pyrite and molybdenite from stage ii 34s 4 4 to 1 9 indicates that the mineralizing fluid originated predominantly from a magmatic source the h o isotope signatures from stage ii indicate a magmatic dominated source for the mineralizing fluid with some addition of meteoric fluids previously published pb and c isotope data indicate that ore and magma in the eastern part of the hebei province of china interacted with precambrian country rocks and so the fluids moved from the lower crust to shallower level during mineralization the red quartz albite hematite veins from jinchangyu were generated under strongly oxidizing alkaline conditions that were favorable for the incorporation of gold into the fluid and the crystallization of albite during the early stage of gold deposition seven molybdenite samples from jinchangyu yield re os model ages of ca 233 to 219 ma with a weighted mean age of 225 4 ma and an isochron age of 223 5 ma this indicates that at least some of the gold associated with molybdenite is late triassic in age and could be associated with buried intrusions that are part of the late triassic dushan granites in the eastern portion of the ncc this mineralization was formed after the closure of the paleo asian ocean these results indicate that the intrusion related jinchangyu deposit is the far field structural product of the collision of the ncc with the siberian craton to the north along the northern margin of the ncc

deposit scale structural architecture of the sigma lamaque gold deposit canada insights from a newly proposed 3d method for assessing structural controls from drill hole data deposit scale structural analysis is a practical method of structurally analyzing 3d drilling data this interpretation method uses readily available distributed 3d data at the most relevant scale for mining and exploration this method does not depend on collecting traditional structural data from core such as orientation of planar and linear data but relies on the premise that grade distributions sampled from drilling provide insight into the structural control of the deposit with the interpretation of structural geometry done using a computer 3d viewer this method is applied to the well studied archean sigma lamaque gold deposit which features late orogenic vein mineralization and which is controlled by three orders of structural features the first order control is the easterly plunging pipe like conduit parallel to f2 fold plunge defined by the intersection of bedding s0 and foliation s2 1 5 km radius 3 km down plunge the second order controls nested within the first order are the younger subvertical late diorite plugs that intruded vertically into the host rocks along the established s2 foliation anisotropy 1250 400 150 m dimension for main plug the third order control laterally limited by first and second order structural controls is the well documented gold bearing vein sets 1 10 m thick and up to 1 5 km in extent that developed in the late diorites and in the older folded volcanogenic stratigraphy and diorites it is proposed that the first order control defined by the moderately plunging f2 fold is the main fluid pathway resulting from structural permeability formed during earlier folding the second order late diorite plugs and other competent lithological units which intersected and fractured during late stage d2 compression served as chemically reactive sites causing gold to precipitate along this first order pathway it is predicted that intrusions that do not intersect the first order structural conduits are less likely to be mineralized greenfield orogenic gold exploration requires that the first order controls are identified and the geometry of hydrothermal alteration is best understood in the context of the structural architecture of the host rocks

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

gold mineralisation at the lady bountiful mine western australia an example of a granitoid hosted archaean lode gold deposit the lady bountiful granitoid hosted lode gold deposit located in the mid greenschist facies metamorphosed ora banda greenstone sequence is hosted predominantly by the late tectonic liberty granodiorite gold mineralisation is localised along quartz veined sinistral brittle fault zone s that transect the boundary between the liberty granodiorite and mt pleasant sill quartz vein textures indicate two stages of a single gold related vein development event with high grade gold mineralisation restricted to the second stage ore minerals include pyrite chalcopyrite pyrrhotite galena sphalerite au ag bi pb tellurides and native gold fluid infitration has resulted in narrow 1 m bleached wallrock alteration envelopes to the fault zones comprising albite k mica chlorite calcite rutile assemblages temperature pressure conditions varied from stage i 300 50 c 2 kbar to stage ii 250 50 c 0 5 to 2 kbar with the hydrothermal fluid in both stages characterised by x co2 0 15 and moderate salinity 1 28 m nacl intermittent phase separation of stage ii mineralising fluids initiated by pressure fluctuations in dilational sites and or fluid dominated fluid wallrock interaction are invoked as the dominant depositional mechanisms the granitoid hosted lady bountiful lode gold deposit shares many features with other granitoid hosted lode gold deposits in the yilgarn craton and the superior province granitoid hosted lode gold deposits such as the lady bountiful deposit provide additional evidence that the dominant control on the localisation of gold mineralisation within a granitoid host is structure with competency contrasts playing a significant role furthermore the hydrothermal wallrock alteraction and orefluid chemistry characteristics of the granitoid hosted lode gold deposits are comparable to those established for greenstone hosted lode gold mineralisation

record of a mid pleistocene depositional anomaly in west antarctic continental margin sediments an indicator for ice sheet collapse abstract modern global warming is likely to cause future melting of earth s polar ice sheets that may result in dramatic sea level rise a possible collapse of the west antarctic ice sheet wais alone which is considered highly vulnerable as it is mainly based below sea level may raise global sea level by up to 5 6 m despite the importance of the wais for changes in global sea level its response to the glacial interglacial cycles of the quaternary is poorly constrained moreover the geological evidence for the disintegration of the wais at some time within the last ca 750 kyr possibly during marine isotope stage mis 11 424 374 ka is ambiguous here we present physical properties palaeomagnetic geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the west antarctic continental margin in the amundsen sea and spans more than the last 1 myr within the sedimentary sequence proxies for biological productivity such as biogenic opal and the barium aluminum ratio and the supply of lithogenic detritus from the west antarctic hinterland such as ice rafted debris and clay minerals exhibit cyclic fluctuations in accordance with the glacial interglacial cycles of the quaternary a prominent depositional anomaly spans mis 15 mis 13 621 478 ka the proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single prolonged interglacial period even though no proxy suggests environmental conditions much different from today we conclude that if the wais collapsed during the last 800 kyr then mis 15 mis 13 was the most likely time period apparently the duration rather than the strength of interglacial conditions was the crucial factor for the wais drawdown a comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout mis 15 mis 13 some of these anomalies are observed in the pelagic southern ocean and the south atlantic and might originate in major ice sheet drawdown in antarctica but further research is required to test this hypothesis

geometry and development of glacial continental margin depositional systems in the bellingshausen sea we present multi channel seismic mcs reflection profiles and bathymetry data acquired across a remote and poorly surveyed part of the antarctic continental margin in the bellingshausen and amundsen seas this new information has been combined with published data and used to interpret the style of sedimentation on the continental shelf slope and rise and to describe sedimentation processes which have been active in this region most seismic reflection profiles crossing the continental margin show prograded sequences beneath the outer shelf and upper slope and we infer that the stratal characteristics of these sequences indicate that grounded ice sheets reached the shelf edge during previous glacial times although there are general similarities in stratal geometry on these profiles in detail they reveal significant longitudinal variations in sediment input from the shelf to the upper slope on several profiles we found evidence of mass wasting of the continental slope in the form of slump and debris flow deposits at greater depth turbidity flows bottom currents and coriolis force have controlled the further transportation and deposition of sediment which has resulted in the development of mounds channels and sediment wave fields the distribution and variations in the size and geometry of the mounds reflect sediment input and the relative contribution of these other factors which control sedimentation on the continental rise

evidence for abundant subglacial meltwater beneath the paleo ice sheet in pine island bay antarctica marine geological and geophysical data collected from the continental shelf in pine island bay antarctica reveal a complex paleo subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge significant amounts of freely flowing meltwater existed beneath former ice sheets in pine island bay subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores bedrock geology is characterized using seismic data large scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock there is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf on the outer shelf a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock no evidence for flowing meltwater exists on sedimentary substrates instead meltwater formed a the ice sediment contact was incorporated into the sediments contributing to development of a deforming lied which was sampled in piston cores characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the west antarctic ice sheet today

the effects of interannual climate variability on the moraine record valley glacier moraines are commonly used to infer past mean annual precipitation and mean melt season temperature however recent research has demonstrated that even in steady climates multi decadal kilometer scale fluctuations in glacier length occur in response to stochastic year to year variability in mass balance when interpreting moraine sequences it is important to include the effect of interannual weather variability on glacier length moraines record advances that are forced either by interannual variability or by a combination of climate change and interannual variability we address this issue for the last glacial maximum lgm glaciers of the colorado front range united states using a linear glacier model that allows thorough exploration of parameter uncertainties supplemented by a shallow ice flowline model our analyses suggest that 1 glacial standstills longer than 50 years were unlikely 2 mean glacier lengths are 10 15 up valley from maximum glacier lengths and 3 individual lgm terminal moraines were formed by a combination of a climate change and interannual variability forced advances

paleoproterozoic turbidite hosted gold deposits of the ashanti gold belt ghana west africa comparative analysis of turbidite hosted gold deposits and an updated genetic model abstract the paleoproterozoic gold deposits of the ashanti gold belt ghana occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics alternately block faulted tarkwaian sediments they are suggested in this article to belong to that class of gold deposits recognized as turbidite hosted hence evidence from previous investigations of turbidite hosted and turbidite associated gold deposits occurring in circumstances roughly similar to the ashanti deposits are used where relevant to produce an updated genetic model for the latter given the relatively significant amounts of graphite associated with the major ashanti deposits the role of non carbonate carbon is assessed in the light of conclusions reached by work on other turbidite hosted and turbidite associated gold belts conclusions as to the origins of mineralizing fluids and temporary repositories of gold auriferous interflow sediments reached by work on these other belts are also analysed a concept of the tectonic development of the ashanti belt kumasi basin area introduced in this article is deemed to be critical to the origin of the ashanti gold deposits it encompasses the role of non carbonate carbon turbidites and the primary sources and secondary repositories of au and as in the genesis of the major ashanti gold deposits it examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments the occurrences of felsic metasandstones in transition zone sediments and ttg granitic material and fragments of chemical sediments in turbidites all of which are incompatible with one or the other of existing concepts of tectonic development in the area

north atlantic deep water export to the southern ocean over the past 14 myr evidence from nd and pb isotopes in ferromanganese crusts the intensity of north atlantic deep water nadw production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate here we present a new approach to reconstruct the overall strength of nadw export from the north atlantic to the southern ocean over the past 14 myr applying the deep water nd and pb isotope composition as recorded by ferromanganese crusts and nodules we present the first long term nd and pb isotope time series for deep southern ocean water masses which are compared with previously published time series for nadw from the nw atlantic ocean these data suggest a continuous and strong export of nadw or a precursor of it into the southern ocean between 14 and 3 ma an increasing difference in nd and pb isotope compositions between the nw atlantic and the southern ocean over the past 3 myr gives evidence for a progressive overall reduction of nadw export since the onset of northern hemisphere glaciation nhg the nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37 depending on location

reconstruction of the west antarctic ice sheet in pine island bay during the last glacial maximum and its subsequent retreat history abstract marine geological and geophysical studies were conducted to determine the late quaternary glacial history of the west antarctic ice sheet in pine island bay antarctica the data set includes sea floor imagery in the form of multibeam swath bathymetry and deep tow side scan sonar surveys distribution of sedimentary deposits and basal surface conditions are revealed through intermediate resolution seismic data and chirp 3 5 khz sub bottom profiles sedimentary deposits were sampled with piston cores and calcareous material was dated using ams radiocarbon techniques results from the study indicate the ice sheet was grounded on the outer shelf during its maximum extent sometime in the past thick ice flowing slowly and resting on crystalline bedrock was influenced by significant amounts of subglacial meltwater on the inner shelf thinner but more rapidly flowing ice characterized the seaward extent of the ice sheet which was grounded on a sedimentary substrate deglaciation occurred in two phases beginning soon after the last glacial maximum the outer most extension of the ice sheet retreated gradually pausing on a bathymetric high on the middle shelf by 16 000 14c years ago between 16 000 and 12 000 14c years ago ice sheet retreat resumed and may have proceeded rapidly to reach a present day position by 10 000 14c years ago

oceanography recycling bottom waters the formation and flow of deep water around antarctica contributes to the global ocean circulation new measurements report the strongest flow speeds recorded so far in a current below 3 000 m depth and help to document a pathway for antarctic bottom water

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

geology geodynamics and orogenic gold prospectivity modelling of the paleoproterozoic kumasi basin ghana west africa abstract this paper describes the geology and tectonics of the paleoproterozoic kumasi basin ghana west africa as applied to predictive mapping of prospectivity for orogenic gold mineral systems within the basin the main objective of the study was to identify the most prospective ground for orogenic gold deposits within the paleoproterozoic kumasi basin a knowledge driven two stage fuzzy inference system fis was used for prospectivity modelling the spatial proxies that served as input to the fis were derived based on a conceptual model of gold mineral systems in the kumasi basin as a first step key components of the mineral system were predictively modelled using a mamdani type fis the second step involved combining the individual fis outputs using a conjunction product operator to produce a continuous scale prospectivity map using a cumulative area fuzzy favourability caff curve approach this map was reclassified into a ternary prospectivity map divided into high prospectivity moderate prospectivity and low prospectivity areas respectively the spatial distribution of the known gold deposits within the study area relative to that of the prospective and non prospective areas served as a means for evaluating the capture efficiency of our model approximately 99 of the known gold deposits and occurrences fall within high and moderate prospectivity areas that occupy 31 of the total study area the high and moderate prospectivity areas illustrated by the prospectivity map are elongate features that are spatially coincident with areas of structural complexity along and reactivation during d4 of ne sw striking d2 thrust faults and subsidiary structures implying a strong structural control on gold mineralization in the kumasi basin in conclusion our fis approach to mapping gold prospectivity which was based entirely on the conceptual reasoning of expert geologists and ignored the spatial distribution of known gold deposits for prospectivity estimation effectively captured the main mineralized trends as such this study also demonstrates the effectiveness of fis in capturing the linguistic reasoning of expert knowledge by exploration geologists in spite of using a large number of variables the curse of dimensionality was precluded because no training data are required for parameter estimation

early proterozoic crustal evolution in the birimian of ghana constraints from geochronology and isotope geochemistry abstract representative suites of ghanaian granitoids metavolcanics and metasediments were selected from birimian rock units for rb sr pb pb and sm nd analyses isotopic data on the igneous rocks provide evidence for a major early proterozoic crust forming episode which took place over a maximum time interval from 2 3 to 2 0 ga by a differention from a depleted mantle source there is little evidence for the involvement of significantly older e g archaean crust in the genesis of the igneous rock units the only exception so far found is the winneba granitoid which appears to contain a magmatic contribution from presumably underlying archaean basement sm nd model ages indicates that birimian sediments were derived from adjacent penecontemporaneous volcanic belts the birimian of ghana forms part of a major proterozoic eburnean episode of juvenile crustal accretion which has been recognised in the surrounding areas of mauretania senegal ivory coast burkina faso mali and niger where it has been dated at 2 1 2 2 ga abouchami et al the birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents

western ross sea continental slope gravity currents antarctic bottom water of the world ocean is derived from dense shelf water that is carried downslope by gravity currents at specific sites along the antarctic margins data gathered by the anslope and clima programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the ross sea when high salinity shelf water exits the shelf through drygalski trough joides trough immediately to the east offers an additional escape route for less saline shelf water while the glomar challenger trough still farther east is a major pathway for export of the once supercooled low salinity ice shelf water that forms under the ross ice shelf the drygalski trough gravity currents increase in thickness from 100 to 400 m on proceeding downslope from 600 m the shelf break to 1200 m upper slope sea floor depth while turning sharply to the west in response to the coriolis force during their descent the mean current pathway trends 35 downslope from isobaths benthic layer current and thickness are correlated with the bottom water salinity which exerts the primary control over the benthic layer density a 1 year time series of bottom water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of shelf water export through drygalski trough these cold 34 75 pulses correlate with strong downslope bottom flow extreme examples occurred during austral summer fall 2003 comprising concentrated high salinity shelf water 1 9 c 34 79 and approaching 1 5 m s 1 at descent angles as large as 60 relative to the isobaths such events were most common during november may consistent with a northward shift in position of the dense shelf water during austral summer the coldest saltiest bottom water was measured from mid april to mid may 2003 the summer fall export of high salinity shelf water observed in 2004 was less than that seen in 2003 this difference if real may reflect the influence of the large iceberg c 19 over drygalski trough until its departure in mid may 2003 when there was a marked decrease in the coldest saltiest gravity current adjacent to drygalski trough northward transport of cold saline recently ventilated antarctic bottom water observed in march 2004 off cape adare was 1 7 sv including 0 4 sv of high salinity shelf water

evolution of hydrothermal fluids in the ashanti gold belt ghana stable isotope geochemistry of carbonates graphite and quartz the oxygen and carbon isotope systematics of ankerite siderite calcite quartz and graphite from the bogosu and prestea mining districts of the ashanti gold belt ghana are used to speculate on the processes leading to hydrothermal alteration and gold deposition the geologic environment of this turbidite and graywacke hosted gold system was a sediment dominated accretionary complex the gold ores are localized within the ashanti structural belt which we suggest became a conduit for deep seated fluids during uplift and dilatancy ankerite and siderite in mineralized sedimentary rocks are broadly similar in oxygen and carbon isotope compositions to corresponding minerals in the metasedimentary country rocks evidence of alteration that preceded gold mineralization is best preserved in spatially associated altered mafic dikes the compositions of fluids responsible for the gold mineralization and associated alteration have been reconstructed using temperatures from carbonate mineral geothermometry alteration of country rocks occurred under rock dominant greenschist facies conditions the isotopic composition of the least altered mafic dikes was influenced by interaction with fluids generated from birimian greenschist facies metasedimentary rocks later extensive carbonate alteration of mafic dikes adjacent to the structural conduit occurred during slow upward migration of fluids lateral diffusion of hydrogen into the structural conduit permitted partial conversion of co 2 to methane resulting in 13 c enrichment of both the residual co 2 and the deposited carbonate minerals rapid even explosive expulsion of deep seated fluids and co 2 h 2 o phase separation occurred episodically throughout the evolution of the hydrothermal system this caused precipitation of sulfides arsenides and gold at relatively high crustal levels and low ambient pressure and temperature during mineralization fluid ascent was too rapid for isotopic and chemical reequilibration between the deep seated ore fluid and adjacent country rocks the ore fluid had chemical and isotopic compositions consistent with deep seated metamorphic fluids that were subsequently modified by preferential partitioning of 18 o and 13 c into a gas phase the composition of the deep seated metamorphic fluid determined using chemical equilibria co 2 h 2 o rich 7 25 mole co 2 is in approximate agreement with that required to produce the isotopic depletion of the residual fluid via phase separation

bottom current controlled sedimentation a synthesis of the contourite problem abstract an overview of the main items concerning deep bottom current controlled deposits is presented these include the definition of contourites their processes of deposition and facies characteristics contourite drift types and seismic patterns the interplay of turbidity and bottom current processes and the correlation between bottom current processes and global changes in climate sea level from this discussion several of the more significant problems are highlighted which we believe should be priority targets for future research 1 identification of contourites especially in ancient series where they are interbedded with turbidites and other facies 2 refinement of the numerous parameters used as tracers of palaeocirculation patterns 3 understanding the interaction between the different types of bottom currents and also between bottom current and other deep water processes 4 distinction in the sedimentary record between the effects of short time scale bottom current variations due to regional causes like benthic storms and geological scale current fluctuations related to climatic or astronomic control 5 understanding the control of glacial interglacial climatic cycles on the nature and rate of contourite deposition