key trends in the resource sustainability of platinum group elements abstract platinum group elements pges are increasingly used in a variety of environmentally related technologies such as chemical process catalysts catalytic converters for vehicle exhaust control hydrogen fuel cells electronic components and a variety of specialty medical uses amongst others almost all of which have strong expected growth to meet environmental and technological challenges this century economic geologists have been arguing on the case of abundant geologic resources of pges for some time while others still raise concerns about long term supply yet there remains no detailed analysis of formally reported mineral resources and key trends in the pges sector this paper presents such a detailed review of the pges sector including detailed mine production statistics and mineral resources by principal ore types providing an authoritative case study on the resource sustainability for a group of elements which are uniquely concentrated in a select few regions of the earth the methodology compiled data sets and trends provide strong assurance on the contribution that pges can make to the key sustainability and technology challenges of the 21st century such as energy and pollution control

sources sinks and storage of river sediment in the atlantic drainage of the united states the history of sediment and its movement in the atlantic drainage demonstrate some of the difficulties of modeling sediment on a river basin scale soil erosion was accelerated by a factor of at least 10 when european settlers cleared forests and planted crops although increasing soil conservation practice and decreasing crop farming have since reduced the rates of erosion large quantities of eroded material are still stored on hillslopes and in stream valleys where they continue to augment the sediment loads of the rivers the sediment from this episode of erosion that is largely past can be expected to emerge from storage for many decades and perhaps even several centuries to come the reservoirs that have been built on many of the major rivers trap significant portions of the moving sediment which in some places may be remobilized by large floods essentially all the river sediment that reaches the atlantic coastal zone is trapped in estuaries and coastal marshlands probably less than 5 is deposi

precious and base metal geochemistry and mineralogy of the grasvally norite pyroxenite anorthosite gnpa member northern bushveld complex south africa implications for a multistage emplacement the grasvally norite pyroxenite anorthosite gnpa member within the northern limb of the bushveld complex is a mineralized layered package of mafic cumulates developed to the south of the town of mokopane at a similar stratigraphic position to the platreef the concentration of platinum group elements pge in base metal sulfides bms has been determined by laser ablation inductively coupled plasma mass spectrometry these data coupled with whole rock pge concentrations and a detailed account of the platinum group mineralogy pgm provide an insight into the distribution of pge and chalcophile elements within the gnpa member during both primary magmatic and secondary hydrothermal alteration processes within the most unaltered sulfides containing pyrrhotite pentlandite and chalcopyrite only the majority of ipge rh and some pd occur in solid solution within pyrrhotite and pentlandite with an associated pt as and pd bi te dominated pgm assemblage these observations in conjunction with the presence of good correlations between all bulk pge and base metals throughout the gnpa member indicate the presence and subsequent fractionation of a single pge rich sulfide liquid which has not been significantly altered in places the primary sulfides have been replaced to varying degrees by a low temperature assemblage of pyrite millerite and chalcopyrite these sulfides are associated with a pgm assemblage characterized by the presence of pd antimonides and pd arsenides which are indicative of hydrothermal assemblages the presence of appreciable quantities of ipge pd and rh within pyrite and to a lesser extent millerite suggests these phases directly inherited pge contents from the pyrrhotite and pentlandite that they replaced the replacement of both the sulfides and pgm occurred in situ thus preserving the originally strong spatial association between pgm and bms but altering the mineralogy precious metal geochemistry indicates that fluid redistribution of pge is minimal with only pd au and cu being partially remobilized and decoupled from bms this is also indicated by the lower concentrations of pd evident in both pyrite and millerite compared with the pentlandite being replaced the observations that the gnpa member was mineralized prior to intrusion of the main zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal s we therefore believe that our observations are more compatible with a multistage emplacement model where preformed pge rich sulfides were emplaced into the gnpa member such a model explains the development and distribution of a single sulfide liquid throughout the entire 400 800 m thick succession it is therefore envisaged that the gnpa member formed in a similar manner to its nearest analogue the platreef notable differences however in pge tenors indicate that the ore forming process may have differed slightly within the staging chambers that supplied the platreef and gnpa member

strength behavior of methane hydrate bearing sand in undrained triaxial testing abstract gas hydrates represent a potential future energy source as well as a considerable geohazard in order to assess both the benefits and risks that gas hydrate bearing sediments pose fundamental information about their physical properties is required in this study the undrained shear strength of methane hydrate bearing sand was investigated the experimental program required modifications to an existing triaxial apparatus and accurate determination of the hydrate saturation lead to the use of two methods for comparison of the saturation calculations strength results indicated that the presence of gas hydrate will increase the sediment s undrained shear strength and corresponding stiffness the relative contribution of cohesion and friction angle was observed to be a function of the hydrate saturation for this particular hydrate formation methodology

the influence of vertical cylindrical tetrahydrofuran hydrate veins on fine grained soil behaviour over the last 10 years marine gas hydrate drilling expeditions utilizing advances in pressure coring techniques and imaging have routinely encountered gas hydrates residing in fine grained sediments the hydrate typically occurs as fracture filling near vertical veins that displace the sediment potentially leading to increased sediment strength that may prevent normal consolidation of the sediment thus leading to underconsolidation destabilization of this hydrate through climate change or human activity on the seafloor may cause dramatic loss of strength of the sediment and pose a significant geohazard to assess the impact of hydrate veins on sediment behaviour this paper reports on a series of consolidated cu and unconsolidated undrained uu triaxial tests carried out on fine grained soil specimens hosting simplified vertical cylindrical tetrahydrofuran thf hydrate veins of varying diameters the results show that increasing hydrate vein diameter significantly increases strength and stiff

the upper critical zone of the eastern bushveld complex precursor of the merensky reef the upper critical zone in the central sector of the eastern bushveld complex consists predominantly of norites although bronzitites anorthosites chromitites and other chromitic rocks appear at intervals in the sequence gabbronorites appear near the middle of the zone the rock sequence is divided into 14 principal units plus the series from the base of the merensky reef to the top of the bastard reef mineralogical compositions of successive units have been determined from modal analyses and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis the results indicate that although parts of the zone show normal fractionation the zone as a whole is not the product of progressive fractionation of a magma crystal settling appears to have played a relatively minor role in evolution of the zone successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations but features of certain thick norite units suggest stratification of the magma in the chamber the study suggests that the merensky reef registers a sudden event in the bushveld magma chamber an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals the alternative hypothesis that the reef is the culmination of a long period of magma fractionation is not supported by the evidence presented chromite occurs in a variety of associations with plagioclase and bronzite the associations are consistent with phase relations in the system mg 2 sio 4 iron oxide caal 2 si 2 o 8 sio 2 at elevated pressure

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

on the critical state characteristics of methane hydrate bearing sediments abstract reasonable prediction of marine ground stability is vital for the safe exploitation of methane gas from the seabed and development of relevant technologies for gas production constitutive models based on a critical state soil mechanics framework for methane hydrate bearing sediments have been proposed in recent decades they required careful calibration of laboratory results to obtain accurate information on model parameters this study presents a comprehensive analysis of triaxial shear test data to examine the effects of hydrate saturation on the geomechanical characteristics of methane hydrate bearing sediments particularly at the critical state multiple critical state lines are identified for methane hydrate bearing sediments with different levels of hydrate saturation on the specific volume and logarithm of mean stress plane a rise in hydrate saturation shifts the critical state line of methane hydrate bearing sediments upward and slightly increases the gradient of critical state line in the v ln p plane non unique critical state lines are observed for methane hydrate bearing sediments under plane strain loading conditions considering methane hydrate saturation as an extra dimension of critical states the critical state line in the plane of specific volume and logarithm of mean stress becomes a three dimensional critical state surface

geochronological evidence for an extension of the northern lobe of the bushveld complex limpopo province south africa abstract we have conducted the first geochronological study of the ultramafic mafic succession the waterberg project located in the southern marginal zone of the limpopo belt north of the northern lobe of the bushveld complex using u pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the waterberg group unconformably overlying the mafics this contribution is the first in a series of publications describing the well mineralized succession and will therefore give an overview of the local geology u pb baddeleyite ages for the main mafic sill intruding the area supplement the data the succession is extremely well mineralized and therefore of interest to the wider geological community the intrusion ages for the lobe are 2059 3 and 2053 5 ma and within error coeval with the intrusion ages of the bushveld complex detrital zircon age probability density distributions indicate a maximum age of deposition of c 2045 ma for the waterberg group and show abundant peaks of archean age paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe while the archean zircons are most likely sourced from the limpopo complex to the northeast the major dolerite sill intruding the project area has been confirmed to belong the umkondo igneous province with a baddeleyite age of 1125 5 ma the combination of these new age dates local geology and existing literature is used to argue that i the ultramafic mafic succession is related to the bushveld complex ii a significant unconformity is present between igneous rocks and the overlying sediments and iii the area was exposed to a level of paleoproterozoic tectonism not previously observed in the bushveld complex

sediment transport along lower fraser river 1 measurements and hydraulic computations a comprehensive program of sediment transport measurements was conducted along lower fraser river british columbia between 1966 and 1986 the data yield a detailed sediment budget annual total suspended loads at three stations are virtually identical averaging 17 106 tonnes year t yr 1 the suspended sand load is about 5 5 106 t yr 1 about one third of the total in the gravel bed reach of the river the sand behaves as wash load significant transport of gravel begins at agassiz at a discharge of about 5000 m3 s 1 the annual gravel transport was estimated to be about 0 23 106 t yr 1 only 1 of the total load all of this material is deposited in the reach upstream of mission at mission sands finer than 0 177 mm make up more than 50 of the suspended sand load but are virtually absent from the sand bed therefore a portion of the sand load at mission is wash load the total bed material load here was estimated to be 3 0 106 t yr 1 about 18 of the total sediment load virtually all of the bed material load was transported in intermittent suspension near the bed less than 5 occurring as bed load in the long term the suspended sand load upstream is approximately equal to the total sand load at mission however within individual years some of the sand is stored within the reach temporarily and then reentrained later

prediction of the mechanical response of hydrate bearing sands in recent years there has been an increasing interest in production of methane gas from hydrate bearing sediments located below the permafrost in arctic regions and offshore within the continental margins in order to simulate the geomechanical response of the hydrate accumulation during gas production comprehensive evaluation of the sediments properties is imperative this paper presents an analysis of the mechanical properties of methane hydrate bearing sediments determined through numerical simulation of drained triaxial compression tests on three different sand types the adjustment of the numerical to the experimental results was performed for the entire stress strain curves and therefore enables a good understanding of the material constitutive relations new constitutive relations are suggested for the hydrate related properties an optimization process was used finding separately the soil skeleton related coefficients and the hydrate related coefficients the values of the obtained coefficients associated with the hydrate were found to have minor deviations from each other for the three examined sand types by separating the soil skeleton and the hydrate related response this paper suggests a prediction method for the mechanical response of hydrate bearing sands

platinum group elements in the boulder bed western bushveld complex south africa concentrations of platinum group elements in samples from the boulder bed at five localities in the western bushveld complex range between 50 ppb and 70 ppm boulders thus have much more variable and sometimes highly enriched pge contents relative to the other lithologies in the immediate foot wall sequence of the merensky reef the pge enrichment can largely be modelled as a result of primary magmatic processes including collection of pge by segregating sulphide melt and fractionation of mss other features of the boulder bed such as the selvages of pure anorthosite and the chromitite stringers surrounding some of the boulders bear evidence of recrystallisation a model is proposed by which the boulder bed formed as a result of a combination of early and late magmatic processes the pges were collected by magmatic sulphide melt which accumulated in a pyroxenite layer the host rock to the pyroxenite was a thick package of norites which recrystallised in response to upward migrating magmatic fluids the fluids caused partial hydration melting of the norites adjacent to the pyroxenite producing anorthosite the boulders represent the broken up remnants of the pyroxenite layer the selvages of chromite and pure anorthosite around some of the boulders remain poorly understood but may represent the latest recrystallisation event in response to localised late magmatic fluid overpressure upon cooling

petrology and mineralisation of the southern platreef northern limb of the bushveld complex south africa the platreef the putative local analogue of the merensky reef forms the floor to the mafic succession in the northern limb of the bushveld complex we define the platreef as the lithologically variable unit dominated by pyroxenite which is irregularly mineralised with pge cu and ni between the transvaal metasedimentary footwall or archaean basement and the overlying main zone gabbronorite we define the mineralisation around calcsilicate xenoliths within the main zone in the far north of the limb as a platreef style mineralisation the platreef ss has a strike extent of 30 km whereas platreef style mineralisation occurs over a strike length of 110 km the platreef varies from 400 m thick in the s to 30 modal in some intersections these are dominated by pyrrhotite with lesser pentlandite and chalcopyrite minor pyrite and traces of a wide compositional range of sulphides in the southern sector mineralised zones have cu grades of 0 1 0 25 and ni 0 15 0 36 massive sulphides are localised commonly but not exclusively towards the contact with footwall metasedimentary rocks magmatic sulphides are disseminated or net textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite much of the sulphide is associated with intergranular plagioclase or quartz feldspar symplectites along the margins of rounded cumulus orthopyroxenes the pges in the southern sector occur as tellurides bismuthides arsenides antimonides bismuthoantimonides and complex bismuthotellurides pgm are rarely included in the sulphides but occur as micron sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones the pt pd ratio 1 and pge grade may be decoupled from s and base metal abundance

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

testing a thermo chemo hydro geomechanical model for gas hydrate bearing sediments using triaxial compression laboratory experiments natural gas hydrates are considered a potential resource for gas production on industrial scales gas hydrates contribute to the strength and stiffness of the hydrate bearing sediments during gas production the geomechanical stability of the sediment is compromised due to the potential geotechnical risks and process management issues the mechanical behavior of the gas hydrate bearing sediments needs to be carefully considered in this study we describe a coupling concept that simplifies the mathematical description of the complex interactions occurring during gas production by isolating the effects of sediment deformation and hydrate phase changes central to this coupling concept is the assumption that the soil grains form the load bearing solid skeleton while the gas hydrate enhances the mechanical properties of this skeleton we focus on testing this coupling concept in capturing the overall impact of geomechanics on gas production behavior though numerical simulation of a high pressure isotropic compression experiment combined with methane hydrate formation and dissociation we consider a linear elastic stress strain relationship because it is uniquely defined and easy to calibrate since in reality the geomechanical response of the hydrate bearing sediment is typically inelastic and is characterized by a significant shear volumetric coupling we control the experiment very carefully in order to keep the sample deformations small and well within the assumptions of poroelasticity the closely coordinated experimental and numerical procedures enable us to validate the proposed simplified geomechanics to flow coupling and set an important precursor toward enhancing our coupled hydro geomechanical hydrate reservoir simulator with more suitable elastoplastic constitutive models

contemporary squamish river sediment flux to howe sound british columbia squamish river drains 3600 km2 in the southern coast mountains of british columbia to howe sound at squamish some 50 km north of vancouver this study uses differencing of digitized bathymetric surfaces based on the canadian hydrographic services surveys of 1930 1973 and 1984 at the head of howe sound to yield a long term sedimentation rate for squamish river delta the sediment flux from squamish river to howe sound is determined to be 1 29 106 m3 a 1 or 1 81 109 kg a 1 rates consistent with loads calculated from flow and sediment concentration regimes measured in the estuary in 1973 1975 and 1987 1988 the latter data indicate that the modal discharge effectiveness class is 600 700 m3 s 1 moving 13 of the annual suspended sediment load discharges up to 1400 m3 s 1 constitute 99 8 of all flows and are responsible for transporting 81 5 of the annual load a very significant 18 5 of the load is moved by large magnitude floods 1400 m3 s 1 which occur less than 0 2 of the time squam

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

mechanical behavior of hydrate bearing pressure core sediments visualized under triaxial compression abstract geomechanical and geotechnical properties are essential for evaluating the stability of deep seabed and subsea production systems for gas hydrate extraction from marine sediments in this study natural gas hydrate bearing sediment was subjected to triaxial compression tests shearing using a newly developed triaxial testing system tactt to investigate the geomechanical behavior of sediments recovered from below the seafloor in the eastern nankai trough where the first japanese offshore production test was conducted in 2013 the sediments were recovered using a hybrid pressure coring system with pressure cores cut using onboard pressure core analysis tools the pressure cores were subsequently transferred to our shore based laboratory and subsampled using pressure core non destructive analysis tools pnats for the tactt system pressure and temperature conditions were maintained within the hydrate stability boundary during coring and laboratory testing an image processing technique was used to capture deformation of the sediment sample within the transparent acrylic test cell and digital photographs were obtained for each 0 1 strain level experienced by the sample during the triaxial compression test analysis of the digitized images showed that sediments with 63 hydrate saturation exhibited brittle failure whereas hydrate free sediments exhibited ductile failure the increase in shear strength with increasing hydrate saturation in natural gas hydrates is in agreement with previous data from sediments containing synthetic gas hydrates

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

distribution of mafic sills in the transvaal supergroup northeastern south africa systematic measured profiles on a regional scale are used to document the distribution of noritic and amphibolitic sills 787 in total in the largely sedimentary latest archaean early proterozoic succession of the transvaal supergroup in northeastern south africa an aggregate thickness of over 2 2 km of sills intruded this 12 km thick succession the proportion of sill to sill plus sedimentary rock increases upward average 16 towards the overlying mafic phase of the bushveld complex the average thickness of sills is 34 m but increases in the same sense the emplacement level of the bushveld complex transgresses across some 5 km of the stratigraphy of the upper formations of the transvaal supergroup a node of maximum intensity of noritic sill intrusion is developed in these uppermost strata adjacent to the location where the transgressive relationship is most pronounced the structural styles of the transvaal supergroup strata influence intensity of sill intrusion other factors being equal maximum intrusion intensity is associated with areas of shallow homoclinal dip supplementary material a spreadsheet including all 440 formation traverses including grid references formation thicknesses and number and thicknesses of every sill therein is available at http www geolsoc org uk sup18807

the mineralogy and mineral associations of platinum group elements and precious metals in the aurora cu ni au pge deposit northern limb bushveld complex abstract aurora is a platinum group element pge prospect hosted in the northern limb of the bushveld complex south africa it is one of only three deposits discovered in the northern limb so far to be hosted in the melanocratic leucocratic gabbroic cumulates of the main zone of the rustenberg layered suite aurora moorddrift and waterberg t zone deposits rather than in predominantly ultramafic rocks e g platreef the host cumulates at aurora have been divided into three principal units and they intrude the dolomites of the lower transvaal supergroup base metal sulphide bms mineralisation with pge is present in the leucogabbronorites and gabbronorites of unit 2 and in coarse grained gabbronorite veins which intrude the peridotites of unit 1 these veins contain up to 50 interstitial pyrrhotite pentlandite chalcopyrite pyrite unit 2 contains 1 3 pentlandite pyrrhotite chalcopyrite assemblages and 1 5 chalcopyrite pyrite pyrrhotite associated with hydrothermal alteration the pge content of aurora however is predominantly controlled by the presence of platinum group minerals pgm not bms la icp ms analysis of sulphides shows the bms in aurora have lower pge concentrations than other bushveld magmatic sulphides with pentlandite carrying much lower concentrations of pd average 23 ppm than the platreef or the merensky reef sem eds analysis of 26 sections characterised 995 platinum group minerals pgm and precious metal bearing minerals pmm with a total area of 27850 m2 and an average size of 28 2 m2 of the pgm and pmm identified in aurora 85 by area are pd te bi minerals with 6 pd te minerals 4 electrum and 3 ag te minerals along with minor pd bi pd as pt te bi pt as and pt s minerals that collectively comprise 2 of total area only 25 of the pgm and pmm in aurora are bms hosted with the rest hosted in silicates of the total pgm and pmm area 22 are hosted in alteration silicates quartz chlorite or actinolite in an alteration halo around sulphides unusually 52 of the pgm and pmm are spatially removed from bms instead hosted in alteration silicates and within cracks in primary silicates away from any bms this indicates a multi stage ore genesis model with hydrothermal remobilisation of pge important for ore formation the style and host rocks for mineralisation in the aurora deposit are fundamentally different from other deposits in the northern limb of the bushveld hosted in ultramafic rocks such as the platreef gnpa member deposits and the f zone of the waterberg deposit all of which contain a greater diversity of pgm and bms with higher precious metal contents the mineralisation most similar to aurora is the t zone of the waterberg deposit located to the north of aurora which been suggested to be an along strike equivalent of the aurora main zone mineralisation however despite strong similarities in pgm mineralogy and s isotope signatures there are significant differences in bms mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically at present it seems more likely that aurora and the waterberg t zone reflect similar fluid influenced processes operating in different parts of the main zone perhaps at different times and in different structural basins rather than a continuous mineralised zone along strike

strength characteristics of sediments from a gas hydrate deposit in the krishna godavari basin on the eastern margin of india abstract knowledge of the strength of sediments overlying sub seafloor gas hydrate deposits is crucial for predicting borehole and seafloor stability during hydrate extraction ideally sediment strength should be determined along a continuous downhole profile from the seafloor to the hydrate reservoir but few such profiles have been obtained in this study we used cores retrieved at site nghp 02 23 in the krishna godavari basin in unconfined penetration tests on split cores and in triaxial deformation experiments on hydrate free sediment samples although penetrometer tests identified relatively low strength 70 250 kpa likely due to hydrate dissociation in the hydrate bearing interval 90 300 m below seafloor mbsf sediment strength exceeded 350 kpa in the intervals 140 170 and 250 270 mbsf each of which lies just above a zone of high gas hydrate concentration the average stress ratio of triaxial strength at 4 axial strain to effective mean stress qe 4 p of hydrate free silts was about 0 67 throughout the hole an exception to this trend was in fine sands from 280 mbsf in the deeper gas hydrate zone where the stress ratios were greater than 1 0 the stress ratios of hydrate bearing sediments in the deeper gas hydrate zone that were reported from the pressure core measurements were far greater than those of any hydrate free sediments for a given effective mean stress the high strength intervals in silty sediments we identified by the penetration tests could be associated with zones of high hydrate concentration because high strength layers in fine grained silty sediments commonly exhibit lower permeability than sandy layers potential hydrate host they may act as seals that assist the precipitation of hydrate below them

source conveyance and fate of suspended sediments following hurricane irene new england usa hurricane irene passed directly over the connecticut river valley in late august 2011 intense precipitation and high antecedent soil moisture resulted in record flooding mass wasting and fluvial erosion allowing for observations of how these rare but significant extreme events affect a landscape still responding to pleistocene glaciation and associated sediment emplacement clays and silts from upland glacial deposits once suspended in the stream network were routed directly to the mouth of the connecticut river resulting in record breaking sediment loads fifteen times greater than predicted from the pre existing rating curve denudation was particularly extensive in mountainous areas we calculate that sediment yield during the event from the deerfield river a steep tributary comprising 5 of the entire connecticut river watershed exceeded at minimum 10 40 years of routine sediment discharge and accounted for approximately 40 of the total event sediment discharge from the connecticut river a seriesof surface sediment corestaken in floodplainponds adjacent to the tidalsec

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

regional clastic sediment yield in british columbia the data archive of the water survey of canada supplemented by data of the british columbia hydro and power authority is used to investigate the areal pattern of fluvial sediment yield in british columbia the data represent suspended sediment loads at 63 stations derived from observations within the period 1966 1985 in most of the province there is a single annual peak in sediment transport in spring corresponding to the dominant snowmelt freshet however on the coast winter rainfall induces the sediment peak and in the coast and cascade mountains there is a transitional regime with two peaks from 65 to 90 of annual sediment yield occurs in the spring three months sediment yield per unit area specific sediment yield increases with drainage area at all scales from 10 up to 30 000 km2 the pattern persists seasonally throughout the year this contradicts the conventional model in which sediment yield declines downstream because of deposition along channels of a portion of the load derived from e

seafloor subsidence induced by gas recovery from a hydrate bearing sediment using multiple well system abstract the response behavior of the methane exploitation from natural gas hydrate ngh using multiple well system is complex and needs to be investigated as gas production from a single vertical well generally cannot meet commercial demand this study numerically investigates the production performance and geomechanical response of an unconfined hydrate deposit in shenhu area south china sea under single and multiple vertical well conditions for a single vertical well with a mild constant bottom hole pressure gas production is relatively stable however seafloor subsidence exhibits an initial rapid drop and a subsequent mild drop stage the vertical displacement is highly developed at the top and bottom of the production zone the results from doublet and triplet vertical wells indicate that both the gas and water production and seafloor subsidence increase with the increase in number of production wells the superimposition of subsidence leads to a deterioration in subsidence and the change in location for the largest subsidence which may affect the risk location of well instability the interference of pore pressure and subsidence increases with the decrease in well spacing however gas production decreases and water production changes insignificantly furthermore a same subsidence at seafloor cannot indicate the same evolution of subsidence in the vertical and lateral direction the results presented in this study help in balancing the production and subsidence of the ngh exploitation in complex well configurations

textural and compositional features of chromite in the lower and critical zones of the bushveld complex south of potgietersrus chromite is developed as an accessory mineral in a 1 700 m thick sequence of lower and lower critical zone cumulates in the area south of potgietersrus and as a major constituent in a number of chromitite layers within this sequence the lower zone consists of a 1 500 m thick sequence of ultramafic rocks characterized by varying proportions of olivine orthopyroxene and chromite which allows the pile of cumulates to be subdivided into 37 cyclic units the critical zone on the other hand is only 350 m thick of which only the lower 150 m contains chromite bearing lithologies electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the mg mg fe super 2 and cr fe super 2 fe super 3 ratios of chromite and the mg mg fe super 2 ratio of the mafic silicates the composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks the analytical data have also shown the tio 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma a detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f sub o 2 and a rise in the cr 2 o 3 content of the liquid thereby causing an increase in the cr 2 o 3 content of the chromite within the chromitite layers postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering sintering took place in the presence of a reactive mg rich liquid which resulted in an increase in the mg mg fe super 2 ratio of the chromite this process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent

strengthening mechanism of cemented hydrate bearing sand at microscales on the basis of hypothetical particle level mechanisms several constitutive models of hydrate bearing sediments have been proposed previously for gas production however to the best of our knowledge the microstructural large strain behaviors of hydrate bearing sediments have not been reported to date because of the experimental challenges posed by the high pressure and low temperature testing conditions herein a novel microtriaxial testing apparatus was developed and the mechanical large strain behavior of hydrate bearing sediments with various hydrate saturation values sh 0 39 and 62 was analyzed using microfocus x ray computed tomography patchy hydrates were observed in the sediments at sh 39 the obtained stress strain relationships indicated strengthening with increasing hydrate saturation and a brittle failure mode of the hydrate bearing sand localized deformations were quantified via image processing at the submillimeter and micrometer scale shear planes and particle deformation and or rotation were detected and the shear band thickness decreased with increasing hydrate saturation

simulated geomechanical responses to marine methane hydrate recovery using horizontal wells in the shenhu area south china sea abstract the recovery of natural gas hydrate ngh from marine sediments faces challenging due to not only the gas productivity itself but also the possible geohazards such as seafloor subsidence submarine landslide wellbore instability and possible sand production the coupled thermal hydrodynamic mechanical thm processes during ngh recovery are generally complex and numerical simulation tools are needed to assess related geomechanical responses we have extended the biot consolidation model in our previous simulator tough2biot and incorporated into the existing tough hydrate code resulting in a thm simulator for the ngh recovery the thm simulator is used to assess the geomechanical responses to gas recovery from an unconfined hydrate bearing sediment hbs in the shenhu area south china sea we investigated depressurization using constant bottom hole pressure through a horizontal well results show that methane production quickly reaches a stabilized state and the water rate increases linearly the drawdown of pore pressure around the well controls the increase in effective stress subsidence becomes significant after depressurization due to the quickly propagation of pore pressure depressurization in early stage could contribute to more than half of the total subsidence the decreasing production pressure leads to an increase in the methane production rate but deterioration in subsidence a decrease in intrinsic permeability of overlying and underlying layer is undesirable due to its decrease in methane production rate and the worse of subsidence a balance between gas productivity and related geomechanical response must be achieved the methods and preliminary results presented in this study could help us to understand the geomechanical behaviors during ngh recovery and to design trial production schemes under similar conditions

a comprehensive review on the geomechanical properties of gas hydrate bearing sediments abstract natural gas hydrates ngh that form in sediments beneath the ocean beds are a potential source to meet growing global energy demand gas hydrate bearing sediments ghbs exhibit improved geomechanical properties viz shear strength stiffness compressibility and poisson s ratio that are responsible for their greater stability as compared to the native sediments however dissociation of gas hydrates during extraction of gas instability in the ghbs could trigger submarine slope failures seabed subsidence and failure of the foundations of seafloor installations in order to reduce the risk of occurrence of such geohazards it is imperative to ascertain the stability of ghbs by understanding the variations in their geomechanical properties through field and laboratory investigations which are time consuming and prohibitively expensive for many researchers with this in view the present study attempts to decipher the variation of geomechanical properties with respect to various parameters that affect them viz hydrate saturation sh hydrate morphology and confining pressure temperature conditions by critically synthesizing the existing data in the literature it has been noticed that the shear strength and stiffness are the major parameters influencing the stability of ghbs and both share power law relationship with sh further it is noticed that the cohesion is the predominant component of the shear strength of ghbs and shows significant improvement for sh values greater than 15 50 the variability of cohesion and stiffness with respect to sh have been analysed in tandem with the volumetric deformation during the shearing process to propose particle level mechanism hypothesis that provides an insight in to the stress strain behavior of ghbs the proposed relationships and mechanisms would aid in understanding the behavior and development of improved modeling of geomechanical properties of ghbs

the geologic setting of the platiniferous ug 2 chromitite layer on the farm maandagshoek eastern bushveld complex the platiniferous ug 2 chromitite layer forms part of the upper chromitite group of the critical zone on the farm maandagshoek 254kt in the eastern bushveld complex the upper chromitite group is made up of four cyclic units namely ug 1 ug 2 ug 3 and ug 3a each of which consists of a basal chromitite layer with or without olivine overlain successively by melanorite and norite and terminated with anorthosite the ug 2 chromitite layer and its associated basal pegmatoid is found close to the base of the ug 2 cyclic unit the cyclic units are separated by sharp contacts and have a remarkable persistence along strike and downdip whole rock cu total ni pt and pd show an upward enrichment from the base of the ug 2 unit reaching a maximum in the main chromitite layer they are depleted in the hanging wall sequence of melanorite bulk analyses of the ug 2 chromitite and its enclosing lithologies for pt pd ru rh ir au s sulfide ni cu and co show that the platinum group elements and base metals are concentrated 1 close to the top and base of the main chromitite layer 2 in the leader layer and 3 in minor chromitite lenses and layers both in the footwall pegmatoid and the hanging wall melanorite the pt pt pd and cu ni ratios are low in the footwall pegmatoid and base of the main chromitite layer values of these ratios however increase upward with pt pt pd decreasing at the upper contact of the ug 2 layer the calculated concentration of platinum group metals in the sulfide phase in the ug 2 chromitite layer ranges from 3 000 to just under 12 000 ppm the above determination is primarily controlled by the concentration of s in the layer with the average precious metal distribution remaining relatively constant the calculated d super sulfide liquid sub silicate liquid with the parental liquid values in parentheses are pt 52 575 40 ppb pd 132 940 16 ppb rh 22 080 8 ppb ir 730 000 0 4 ppb and au 2 190 21 ppb evidence of an influx of magma into the eastern lobe of the bushveld complex at a level between the upper and middle chromitite groups is indicated by discontinuous and disturbed layers in borehole drilling on maandagshoek it is suggested that this magma batch was slightly more enriched in the platinum group elements in order to produce the increased value of these elements in the ug 2 chromitite layer it is proposed that preexisting sulfides in the ug 2 chromitite layer collected platinum group elements from upward migrating highly fractionated intercumulus fluids under highly reducing conditions during the early stages of formation of the layer the relatively large volume of sulfide initially present in the layer was later reduced by volatiles enriched in o 2 passing through the layer resorbing the sulfides and blowing off so 2 the partial resorption of the sulfides accounts for the presence of minor pt fe alloys and the abnormally high partition coefficients of the platinum group elements in the sulfide liquid of the ug 2 chromitite layer

diapirism initiated by the bushveld complex south africa the bushveld complex induced the development of a unique type of diapir during its intrusion these diapirs are preserved as large domal structures exposed on the margin and within the northeastern mafic layered sequence they were produced by gravitational loading and heating of argillaceous and arenaceous floor rocks gravity induced deformation is restricted to rocks above a detachment level that corresponds approximately with the 550 c peak metamorphic isotherm and with lithological contacts of contrasting competency strongly foliated and boudinaged contact metamorphosed sedimentary rocks between domes were deformed by lateral extension along flow lines directed toward dome culminations at the highest structural levels domes have bulbous geometries and the enclosing layered igneous rocks are deformed into series of outward verging folds that define a broad rim syncline deformation within the domal cores is represented by constriction that produced radial curtain type folds that have steeply plunging lineations and concentrically oriented folds in the outer shell diapirism is closely linked to magma emplacement mechanisms floor folds in the country rocks were initiated in the interfinger areas of a fingered intrusion with the coalescence of intrusion fingers into a single sheet interfinger folds matured into large diapiric domes that rose to the upper levels of the magma chamber strain rates estimated from strain analyses dome geometry and model cooling calculations are on the order of 10 14 s 1 corresponding to diapiric uplift rates of 0 6 cm yr

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

effects of dissociation on the shear strength and deformation behavior of methane hydrate bearing sediments abstract a series of tests were conducted in order to investigate the shear strength and deformation behavior of methane hydrate bearing sediments during dissociation using the thermal recovery method or depressurization method an innovative temperature controlled high pressure triaxial apparatus which can reproduce the in situ conditions of hydrate reservoirs was used the results indicate that 1 the failure strength of isotropically consolidated methane hydrate bearing sediments which dissociated completely using the thermal recovery method is less than that of pure toyoura sand however the initial stiffness and volumetric strain are higher than that of pure toyoura sand 2 the thermal recovery method will cause the failure of methane hydrate bearing sediments when the axial load is higher than the strength of methane hydrate bearing sediments after dissociation 3 the depressurization method will not cause collapse of methane hydrate bearing sediments during depressurization however water pressure recovery will lead to failure when the axial load is larger than the strength of the methane hydrate bearing sediments after dissociation 4 the depressurization rate shows little effect on the ultimate deformation of methane hydrate bearing sediments while the initial deformation rate increases with increasing depressurization rate 5 the larger the reduction of pore pressure the larger axial strain and volumetric strain

cyclicity in the main and upper zones of the bushveld complex south africa crystallization from a zoned magma sheet the major element composition of plagioclase pyroxene olivine and magnetite and whole rock sr sr data are presented for the uppermost 2 1 km of the layered mafic rocks upper main zone and upper zone at bierkraal in the western bushveld complex initial sr sr ratios are near constant 0 7073 0 0001 for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation the 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite magnetite ilmenite apatite layers and changes up section from gabbronorite an72 plagioclase mg 74 clinopyroxene to magnetite ilmenite apatite fayalite ferrodiorite an43 mg 5 clinopyroxene fo1 olivine the overall fractionation trend is however interrupted by reversals characterized by higher an of plagioclase higher mg of pyroxene and olivine and higher v2o5 of magnetite in the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite these reversals in normal fractionation trends define the bases of at least nine major cycles we have calculated a plausible composition for the magma from which this entire succession formed forward fractional crystallization modeling of this composition predicts an initial increase in total iron near constant sio2 and an increasing density of the residual magma before magnetite crystallizes after magnetite begins to crystallize the residual magma shows a nearconstant total iron an increase in sio2 and decrease in density we explain the observed cyclicity by bottom crystallization initially magma stratification developed during crystallization of the basal gabbronorites once magnetite began to crystallize periodic density inversion led to mixing with the overlying magma layer producing mineralogical breaks between fractionation cycles the magnetitite and nelsonite layers mainly occur within fractionation cycles not at their bases in at least two cases crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically and triggered the proposed density inversion resulting in close but not perfect coincidence of mineralogical breaks and packages of magnetitite layers

downstream channel adjustment in a low relief glacially conditioned watershed abstract river management practices are often informed by theoretical expectations of downstream channel adjustment which may not be valid in low relief glacially conditioned watersheds such as those in the lower north american great lakes region downstream trends in channel morphology and bed material size within a low relief glacially conditioned watershed are explored here and compared to a theoretical watershed model where slope and grain size are expected to decline exponentially the observed channel morphology is then tested against a theoretical concept of reach scale channel grade the downstream hydraulic geometry relations w bf q bf 0 51 s 0 02 and d bf q bf 0 32 s 0 21 were found to best describe downstream changes in channel morphology and are consistent with some prior studies bed material size varies irregularly down the channel slope controlled downstream fining trends are evident where inputs from glacial materials and tributaries are negligible but are masked by cobble boulder lag deposits where the channel is cut into these glacial deposits the asynchronous variability in slope and grain size produces downstream variations in graded and nongraded understeepened conditions separated at ex 0 graded reaches exist where ex 0 but an upper boundary with nongraded oversteepened reaches is less clear the results emphasize the geomorphic legacy of inherited slopes and sediment sources in dictating the modern downstream patterns of fluvial characteristics and morphologies in glacially conditioned and similarly complex watersheds

evaluation of failure modes and undrained shear strength by cone penetrometer for natural gas hydrate bearing pressure core sediment samples recovered from the krishna godavari basin offshore india abstract compressional wave velocity measurements and cone penetration tests were carried out for pressure cores recovered from the krishna godavari basin off the eastern coast of india using an instrumented pressure testing chamber constructed by the japanese national institute of advanced industrial science and technology for pressure cores with high gas hydrate saturations sh and exhibited high compressional wave velocities also display a high loading force that resulted in tensile failure a pressure core sample with low compressional wave velocities and low gas hydrate saturations led to a low loading force which resulted in radial shear the cone factor connecting the cone resistance and the undrained shear strength nk was estimated using cone penetration testing and the uniaxial compressional results we estimated its value as nk 45 by using a uniaxial compression loading speed of 0 01 min and a cone insertion loading speed of 0 25 mm s undrained shear strengths estimated from cone penetration results using nk 45 were assumed to be equal to the tensile strengths between hydrate hydrate and hydrate particle interface when sh is less than 50 and increases when sh is more than 50 assuming that gas hydrate morphology was matrix supporting

pge tenor and metal ratios within and below the merensky reef bushveld complex implications for its genesis the merensky reef is the consequence of several major influxes of magma into the bushveld magma chamber that gave rise to at least two cyclic units including the pre merensky and merensky cyclic units analysis of the s ni cu platinum group element pge and au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the reef to close to the ug 2 horizon of the western bushveld complex provide important petrogenetic constraints on the origin of the reef particular attention is paid to variations in the calculated pge tenors and cu pd ratios of sulfides both vertically and laterally all of the profiles show a decrease in pt tenor and concomitant increase in cu pd ratio upward across the pyroxenite of the merensky unit and laterally away from certain centres that are interpreted as feeder zones the data also document that more mafic horizons within the merensky to ug 2 interval of the southwestern bushveld are characterized by much lower cu pd 100 400 than the ambient ratio 4000 this is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there the favoured interpretation for the merensky reef is that the bushveld chamber s was occupied by a resident magma crystallizing orthopyroxene and plagioclase pre merensky and subsequently merensky magmas entered the chamber s intermittently from localized feeder structures mixing or mingling with the resident magma and giving rise to the more mafic cumulates at specific horizons including the merensky reef the mixing or mingling caused sulfide immiscibility in these cases as the new magma pulses flowed laterally away from the entry zones they deposited sulfide becoming depleted in pge on coming to rest the magma continued to segregate sulfide and crystallize pyroxene so that the sulfides became progressively depleted in pge and acquired a higher cu pd upward through the pyroxenite this model requires that the merensky magmas could dissolve up to two orders of magnitude more pge than current measurements on dry diopside anorthite melts have indicated it is suggested that very pge rich magmas can develop as a result of sulfide unsaturated magma reacting with early formed sulfide in a staging chamber dissolving fes and enriching the remaining sulfide in pge later magmas equilibrating with this enriched sulfide will themselves become highly enriched in pge

colluvial sediment dynamics in mountain drainage basins abstract colluvial sediment dynamics are examined using a 70 year landslide inventory in formerly glaciated mountain drainage basins of coastal british columbia canada landslide sediment transfer is documented by identifying landslide types and by characterizing preferential sites of landslide initiation delivery and storage across spatial scales data analysis reveals that open slope landslides delivering material to seasonal or perennial channels and fluvial terraces are the dominant mechanisms of sediment transfer this finding suggests high instability of the low order channel network and its ongoing re organization degradation after generalized sediment recharge occurred in the last glacial maximum in the study period landslide activity across the landscape has generated net degradation on planar slopes and first order colluvial channels whereas unchannelled valleys higher order colluvial channels and alluvial channels have accumulated material the scaling relation of the landslide sediment yield appears to be controlled by the spatial arrangement of the relict glacial macro forms landslide yield is highest in unchannelled topography decreases at the scale of channel initiation ad 0 002 km2 and remains constant for drainage areas where length scales of cirque valley walls and hanging valley floors overlap 0 002

a new occurrence of merensky reef on the flanks of the zaaikloof dome northeastern bushveld complex relationship between diapirism and magma replenishment exploration for platinum group elements has resulted in the discovery of an occurrence of merensky reef in the eastern limb of the bushveld complex south africa the new discovery was made on the dwarsrand property in the poorly known boundary area between the intensively explored western and central sectors of the eastern limb this boundary area is largely composed of domes and basins that may in part be aligned along a regional lineament the wonderkop fault transvaal supergroup metasediments in the core parts of the domes form prominent hills whereas the bushveld complex in the intervening basins is restricted to areas of poor outcrop the formation of domes is attributed to diapirism triggered by gravitational loading and heating of the transvaal supergroup in response to intrusion of the bushveld complex merensky reef was located beneath cover rocks in an area where the critical and main zones are uplifted and attenuated against the southern flank of the zaaikloof dome drill holes although collared in upper zone intersected the critical and main zones at relatively shallow depths as a result of the uplift the critical and main zones including the merensky reef are steeply dipping the base of the upper zone forms an intrusive unconformity and the layering in this zone has a much shallower dip economically significant grades were obtained on the dwarsrand property but exploration was terminated because of poor strike continuity the new findings suggest that uplift and attenuation of the bushveld complex on the flanks of domes in the northeastern part of the complex are both more extensive and more severe than previously realized a complex history of intrusion and uplift is implied some domes even penetrate into the overlying bushveld granite the occurrence of markedly angular unconformities may be contrasted with the layercake sequences typically associated with the bushveld complex diapirism in response to crustal loading and heating by ultramafic sequences occurred in the early stages of magma emplacement episodic intrusion and uplift are consistent with multiple replenishment models in which each of the many dozens of cycles that comprise the critical and main zones corresponds to a new magma pulse with the oldest layers being dragged to higher structural levels in response to the uplift periodic reactivation of doming is indicated by the lowermost layers within the critical and main zones abutting against the southern flank of the zaaikloof dome at progressively higher levels the transgressive nature of the upper zone and bushveld granite relative to the critical and main zones may indicate that the upper zone formed by new intrusions of magma rather than the generally accepted model of differentiation within the chamber the discovery of hidden merensky reef at relatively shallow depths beneath transgressive upper zone and granite could provide the impetus for further exploration possibilities within the bushveld complex particularly on the flanks of regionally extensive domal structures

bushveld symplectic and sieve textured chromite is a result of coupled dissolution reprecipitation a comparison with xenocrystic chromite reactions in arc basalt textures of bushveld chromite from thin seams and accessory disseminations in the platreef and the northernmost waterberg project area were compared with textures of xenocrystic chromite from mantle xenoliths found in neogene basalt in the kurile island arc the sieve textured to symplectic rims around the resorbed chromite in the kurile samples resulted from the reaction between chromite and chromite undersaturated basaltic melt with the inclusions in chromite being entrapped during episodes of chromite primary growth chemical dissolution and reprecipitation or secondary growth the relics of the lattice oriented etch tunnels suggest that the dissolution preferentially developed along the crystallographic planes and defects the bushveld chromites exhibiting similar textures are interpreted as reaction textured chromites by analogy with the kurile samples the bushveld sieve textured fish hook to symplectic and amoeboidal to atoll like chromites are believed to have been formed due to coupled dissolution reprecipitation of the earlier cumulus or xenocrystic chromite during interaction with chromite undersaturated evolved melt the electron backscattered diffraction data confirm the same single crystal crystallographic orientation of all domains of the reaction textured chromites as well as their clustered semi dissolved relics therefore bushveld inclusion rich chromite might have captured different populations of melt inclusions during its discontinuous out of equilibrium growth with fast episodic resorption and regeneration the occurrence of reaction textured chromites indicates a zone of interaction between dynamic magmatic influxes where chemical equilibrium was not achieved whereas a complete re equilibration between chromite and the stagnant and sequestered interstitial liquid was attained during the formation of the massive chromitites

the chronology of magma influxes to the eastern compartment of the bushveld complex as exemplified by its marginal border groups the floor contact of the eastern compartment of the bushveld complex has been mapped in detail for 130 km field relations petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated these represent the 4 main magma influxes which formed the malic phase of the bushveld complex and are in chronological order 1 a contaminated magnesian basalt b1 which gave rise to a saucer shaped laminated marginal zone at the base of the complex with diverse related sills 2 a harrisitic peridotite representing quenched lower zone magma 3 a border group to the more evolved probably basaltic critical zone magma which was intruded in two phases an early quenched norite facies n being followed by a chilled bulk critical zone facies b2 which forms a skin to the whole critical zone finally 4 a border group to the main and upper zone b3 which represents an early magnesian pulse of this last immense volume of tholeiitic liquid there is also evidence for minor influxes of magma within the critical zone and main zone

nested temporal suspended sediment yields green lake basin british columbia canada suspended sediment yield was determined for a mountain watershed by calculating the volume of sediment stored in the watershed s outlet lake basin and monitoring sediment transport for the lake s primary inflow lake based yields for the 1931 2001 period 0 21 0 03 mg km 2 day 1 are 10 higher than estimates for the last 3000 years 0 19 0 05 mg km 2 day 1 fluvial 0 24 0 08 mg km 2 day 1 and lake based yields ca 0 21 0 07 mg km 2 day 1 are comparable over the period of overlap 1999 2001 persistence in the long term yield record is dominated by low frequency century to decadal periodicity this periodicity is shown to inflate the standard error of the sediment yield estimate for the basin elevated transport of suspended sediments following years of extreme yield is uncommon in the green lake record and indicates a sediment transport regime that is supply limited for this watershed approximately 50 years of suspended sediment monitoring would be required to reduce the uncertainty of the yield estimate to 37 of its average value a summer rainstorm in 1991 delivered more suspended sediment to the green lake basin than any other event in the last 3000 years

triaxial compressive properties of artificial methane hydrate bearing sediment 1 knowledge of the mechanical properties of gas hydrate bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas hydrate reservoir in this study drained triaxial compression tests were conducted on artificial methane hydrate bearing sediment samples under hydrate stable temperature pressure conditions toyoura sand average particle size d50 0 230 mm number 7 silica sand d50 0 205 mm and number 8 silica sand d50 0 130 mm were used as the skeleton of each specimen axial loading was conducted at an axial strain rate of 0 1 min 1 at a constant temperature of 278 k the cell and pore pressures were kept constant during axial loading we found that the strength and stiffness of the hydrate sand specimens increased with methane hydrate saturation and with the effective confining pressure and the secant poisson s ratio decreased with the effective confining pressure the stiffness depends on the type of sand forming the skeleton of the specimens although the strength has little dependence on the type of sand according to an earlier work hydrate sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect as is the case for many other geological materials the test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work

an experimental investigation on methane hydrate morphologies and pore habits in sandy sediment using synchrotron x ray computed tomography abstract as pore scale morphologies and spatial distribution pore habits of natural gas hydrates in marine sediments considerably affect their physical mechanical properties they have extensively been investigated by x ray computed tomography xrct and especially synchrotron x ray computed tomography sxrct while both image spatial and scan temporal resolutions are being improved over time it is still challenging to distinguish water from methane hydrate in an image due to their low absorption contrast in this study methane hydrate formation and growth in wet sand were observed at submicron micron scale using sxrct saline water potassium iodide ki was used in order to improve the image contrast evolutions of methane hydrate morphologies and distribution at both the pore and sample scales were observed results are discussed based on various mechanisms related to material behaviors and experimental conditions e g suction variation during methane hydrate formation local temperature gradient in the sample and particularly the interaction of x rays with the sample methane hydrate formation is interpreted as a dynamic process favoring the ostwald ripening furthermore morphologies and pore habits of methane hydrates under excess gas and excess water conditions are discussed some recommendations are finally given for further studies on methane hydrates bearing sediments via xrct or sxrct

study on the mechanical properties of hydrate bearing silty clay the mechanical properties are important for the evaluation of stratum deformation and instability in hydrate exploitation by using an integrated test apparatus for synthesis of hydrate sediment and triaxial tests a series of compression tests is conducted on the silty clay containing tetrahydrofuran hydrate scth similar to that of south china sea the stress strain curves and mohr circles are obtained at different degree of hydrate saturations and confining pressures the silty clay containing tetrahydrofuran hydrate shows typical ductile behavior the shear strength increases linearly with the increase of hydrate saturation and confining pressure the secant modulus increases with the increase of hydrate saturation the shear strength is analyzed based on the drucker prager criterion mohr coulomb criterion lade duncan criterion while the secant modulus is analyzed using the mixed law of composite materials it is shown that the strength can be well described by the drucker prager criterion and mohr coulomb criterion and the secant modulus is close to the harmonic average of the modulus of soil skeleton and hydrate in scth c 2015 elsevier ltd all rights reserved

strain rate dependent hardening softening characteristics of gas hydrate bearing sediments the presence of gas hydrates ghs increases the stiffness and strength of marine sediments in elasto plastic constitutive models it is common to consider gh saturation sh as key internal variable for defining the contribution of ghs to composite soil mechanical behavior however the stress strain behavior of gh bearing sediments ghbs also depends on the microscale distribution of gh and on gh sediment fabrics a thorough analysis of ghbs is difficult because there is no unique relation between sh and gh morphology to improve the understanding of stress strain behavior of ghbs in terms of established soil models this study summarizes results from triaxial compression tests with different sh pore fluids effective confining stresses and strain histories our data indicate that the mechanical behavior of ghbs strongly depends on sh and gh morphology and also on the strain induced alteration of gh sediment fabrics hardening softening characteristics of ghbs are strain rate dependent which suggests that gh sediment fabrics dynamically rearrange during plastic yielding events we hypothesize that rearrangement of gh sediment fabrics through viscous deformation or transient dissociation and reformation of ghs results in kinematic hardening suppressed softening and secondary strength recovery which could potentially mitigate or counteract large strain failure events for constitutive modeling approaches we suggest that strain rate dependent micromechanical effects from alterations of the gh sediment fabrics can be lumped into a nonconstant residual friction parameter we propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data plain language summary gas hydrates ghs are crystalline like solids which are formed from natural gas molecules and water at high pressure and low temperature ghs and particularly methane hydrates are naturally abundant in marine sediments it is known that the presence of gh increases the mechanical stiffness and strength of sediments and there is strong effort in analyzing and quantifying these effects in order to understand potential risks of sediment destabilization or slope failure based on our experimental results from high pressure geotechnical studies we show that not only the initial amount and distribution of gh are important for the increased strength of gh bearing sediments but also the dynamic rearrangement of gh sediment fabrics during deformation characterizes the stress strain response and enables strength recovery after failure we propose that different microstructural mechanisms contribute to this rearrangement and strength recovery of gh sediment however we consider these complicated processes in a simplified manner in an improved numerical model which can be applied for geotechnical risk assessment on larger scales

influence of fines content on the mechanical behavior of methane hydrate bearing sediments methane hydrate bearing sediments with different amounts of fines content and at three densities were artificially prepared under controlled temperature and pressure conditions the void ratios of specimens after isotropic consolidation tend to decrease with a rise in fines content the fines particles enter into the pore space between sand grains and densify the specimens a series of triaxial compression tests were performed to systematically investigate the influences of fines content and density on the shear properties of hydrate free sediments and methane hydrate bearing sediments the test results demonstrate that a rise in fines content within methane hydrate bearing sediments significantly enhances peak shear strength and promotes dilation behavior these influences are particularly prominent for specimens at loose packing state a decrease in void ratio increases the shear strength and stiffness of hydrate free sediments and methane hydrate bearing sediments containing fines content of 0 and 8 9 it is noted that the formation of methane hydrate in samples with varying amounts of fines content increases the stress ratios at the critical state the addition of fines particles into coarse grained sand grains alters the internal microstructure of sand matrix and the hydrate formation pattern in the pore space between sand grains and fines particles

role of critical state framework in understanding geomechanical behavior of methane hydrate bearing sediments a proper understanding of geomechanical behavior of methane hydrate bearing sediments is crucial for sustainable future gas production there are a number of triaxial experiments conducted over synthetic and natural methane hydrate mh bearing sediments and several soil constitutive models have been proposed to describe their behavior however the generality of a sophisticated model is questioned if it is tested only for a limited number of cases furthermore it is difficult to experimentally determine the associated parameters if their physical meanings and significance are not elucidated the objective of this paper is to demonstrate that a simple extension of the critical state framework is sufficient to capture the geomechanical behavior of mh bearing soils from various sources around the world while the significance of each parameter is quantified through variance based global sensitivity analyses our results show that the influence of hydrates can be largely represented by one hydrate dependent parameter pcd which controls the expansion of the initial yield surface this is validated through comparisons with shearing and volumetric response of mh bearing soils tested at various institutes under different confining stresses and with varying degrees of hydrate saturation our study suggests that the behavior of mh bearing soils can be reasonably predicted based on pcd and the conventional critical state parameters of the host sediments that can be obtained through typical geotechnical testing procedures

the merensky cyclic unit and its impact on footwall cumulates below normal and regional pothole reef types in the western bushveld complex the merensky reef of the bushveld complex occurs in its highest stratigraphic position as a heterogeneous pegmatitic feldspathic melanorite bounded by two narrow chromitite stringers at the base of the merensky cyclic unit mcu in the swartklip facies of the rustenburg layered suite the occurrence of widespread thermal and mechanical erosion termed potholing has led to the subdivision of the merensky reef into normal reef and regional pothole reef sub facies the transition between the two sub facies occurs where the mcu transgresses the lower chromitite stringer of the normal merensky reef and cuts down into the underlying cumulate lithologies in the regional pothole reef at the northam platinum mine several economic reef types are identified where the merensky reef becomes conformable to cumulate layering in particular to the footwall marker np2 reef type and the upper pseudoreef p2 reef type the normal merensky reef as well as the p2 and np2 reefs contains economic platinum group element pge grades and includes the lower portion of the mcu melanorite and the merensky chromitite whole rock geochemistry indicates that this package is compositionally identical in normal p2 and np2 reefs suggesting that the base of the mcu is a relatively homogeneous drape over both normal and regional pothole reef regions however the lower sections of the three reefs are variables depending on the depth of transgression of the mcu in the normal and p2 reef types transgression by the mcu was arrested within harzburgites melanorites and norites resulting in coarse pegmatitic textures in the immediate footwall units for the np2 reef transgression by the mcu was arrested within leucocratic rocks and resulted in the formation of troctolites below the merensky chromitite these troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and pge contents relative to equivalent units in the footwall of the normal reef along with micro textural relationships these features suggest that troctolization of leucocratic cumulates in the np2 reef beneath the merensky chromitite was a result of a reactive infiltration of a chromite saturated melt and an immiscible sulphide liquid from the overlying mcu rather than a significant fluid flux from below in all reef types the concentration of s defines symmetrical peaks centred on the merensky chromitite and chromitites from pre existing cyclic units in normal and p2 reefs whereas pge concentrations define asymmetrical peaks with higher pge contents in reconstituted footwall rocks relative to the mcu melanorite this signature is attributable to a magmatic model of pge collection followed by deposition towards the base of the mcu and within reconstituted footwall rocks the continuity of the asymmetrical magmatic pge signature between the normal reef and regional pothole reef sub facies indicates that pge mineralization inherent to the merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall

the role of contamination in the evolution of the platreef of the bushveld complex the platreef is a thick greater than 100 m coarse grained feldspathic orthopyroxenite which transgresses northward across metasediments of the transvaal sequence and onto a granitic floor it is overlain by main zone gabbronorites vertical profiles for rb and sr isotope ratios through the platreef and overlying main zone show differences depending upon footwall type against a granitic floor the platreef has initial 87 sr 86 sr ratios ranging from 0 7107 to 0 7226 calculated at 2 05 b y suggesting heterogeneous contamination by a melt derived from the granite overlying a dolomitic footwall the platreef shows lower initial ratios 0 7054 to 0 7147 but high rb contents again suggest addition of a granitic component the extent of contamination is least at the base the role of dolomite assimilation is difficult to quantify the lowermost rocks of the main zone show high rb contents due to contamination but their initial sr isotope ratios have not been changed significantly due to the high proportion of cumulus plagioclase orthopyroxene has a much lower initial ratio than its whole rock indicating that the main cumulus phase formed prior to contamination and that the granitic liquid reacted with the interstitial basic magma to produce the contaminated intercumulus phases the tenor of platinum group elements in the platreef precludes the formation of immiscible sulfide liquid from an interstitial liquid rather it must be an early formed phase hence the separation of sulfide liquid may predate the main siliceous contamination process

interaction of floor rocks with the platreef on overysel potgietersrus northern transvaal the platreef on the basal contact of the rustenburg layered suite has been studied on the farm overysel 30 km north of potgietersrus the major element trace element and sr isotope geochemistry of the platreef feldspathic pyroxenite and the adjacent floor rocks is presented the immediate floor rocks consist of a suite of highly metamorphosed banded tonalitic gneisses with leucotonalitic veins to the south the floor rocks are metamorphosed sediments of the transvaal sequence these were intruded by granites which show signs of metamorphism close to the layered suite one facies of the granite yielded an age of 2 504 or 0 109 b y with an initial 87 sr 86 sr ratio of 0 7035 or 6 the cumulate textured pyroxenites of the platreef are geochemically different from other units of the rustenburg layered suite in having a high and variable initial 87 sr 86 sr ratio ranging from 0 7104 to 0 7227 with an average of 0 7160 calculated at 2 05 b y whereas overlying gabbro has a ratio of 0 7079 other ratios such as rb sr and ba zr are anomalously high in the platreef ranging from 0 05 to 0 26 and 16 to 44 respectively these data and the presence of relatively sodic plagioclase biotite and quartz suggest substantial assimilation of siliceous material the sedimentary transvaal sequence forming the footwall to the platreef to the south of overysel and consisting of dolomites and banded iron formation because of their low 87 sr 86 sr ratios and low k 2 o rb and sio 2 cannot produce the observed anomalies the banded tonalitic gneisses and associated veins are an unusual suite of rocks pyroxene geothermometry indicates that they have been metamorphosed to or 900 degrees c rb sr ratios range from 0 02 to 0 21 and 0 to 1 8 percent k 2 o 87 sr 86 sr ratios at 2 05 b y range from 0 7024 to 0 7258 total assimilation of these rocks cannot have produced the anomalous features of the platreef however their low k 2 o contents and high 87 sr 86 sr ratio in relation to their present low rb sr suggest that they have undergone a partial melting or loss of fluid component as a result of contact metamorphism however there are isotopic grounds for rejecting this as the sole source of contamination the granite close to the platreef has a high rb sr ratio initial 87 sr 86 sr ratios of 0 73 to 0 79 at 2 05 b y ago and 3 9 percent k 2 o total assimilation of these rocks could produce the trace element characteristics of the platreef but only if over 60 percent assimilation has occurred major element considerations preclude this a partial melt or fluid phase generated from these granites could have been the contaminant a composition calculated to produce the observed trace element and isotopic features of the platreef with less than 25 percent contamination has 150 ppm sr 87 sr 86 sr of 0 8 9 000 ppm ba 600 ppm rb and 10 ppm zr

the geological setting of the platinoid and base metal sulfide mineralization in the platreef of the bushveld complex in drenthe north of potgietersrus on the farm drenthe the platreef consists of a complex assemblage of norites melanorites pyroxenites serpentinites and xenoliths of dolomite it is a zone of up to 250 m thick between the basal contact of the complex and the overlying main zone norites the composition of the orthopyroxene shows a rapid increase from en 60 to en 70 in the lower 30 m of the sequence and a sharp drop from en 70 to en 60 at the top of the platreef this compositional break at the upper contact is due to a change from primary bronzite in the platreef to inverted pigeonitc in the main zone high temperature metamorphic assemblages are preserved in the larger xenoliths whereas their outer margins and the smaller xenoliths contain retrograde metamorphic mineral assemblages the igneous rocks display irregularly distributed zones of alteration to bronzite bastitites and serpentinites the alteration process was probably dominated by degassing of the xenoliths the distribution and composition of the major sulfide minerals indicate a change from a sulfur rich to a metal rich environment during the crystallization of the platreef pyrite monoclinic pyrrhotite pentlandite and chalcopyrite dominate the lower part of the platreef and hexagonal pyrrhotite pentlandite and chalcopyrite or cubanite prevail in the upper portion whole rock analyses show an upward decrease in the ni cu ratio from 1 95 to 1 70 large variations occur in the distribution and grade of ni cu and platinum group elements although highest values are associated with a zone of serpentinization the ni and cu values are unreliable indicators in predicting pt and pd concentrations in zones of major metal enrichment ni and cu correlate strongly while pt and pd have more significant correlations with ni than cu although the minerals of the platinum group elements are primarily associated with chalcopyrite the pt pd ratio decreases upward in the sequence the mineralization is considered to have been caused by the breakup of dolomite xenoliths that added co 2 h sub 2 o and s to the magma thus simultaneously lowering the solubility of sulfur in the magma and adding sulfur from an external source which resulted in the precipitation of an immiscible sulfide liquid

production potential and stability of hydrate bearing sediments at the site gmgs3 w19 in the south china sea a preliminary feasibility study abstract according to the preliminary geological data of gas hydrate bearing sediments ghbs at site gmgs3 w19 in the third chinese expedition to drill gas hydrates in 2015 a production model using three different recovery pressures was established to assess the production feasibility from both production potential and geomechanical response the simulation results show that for this special class 1 deposit it is a little hard for gas production rate to reach the commercial extraction rate because the degree of hydrate dissociation is limited due to the low reservoir permeability and the permeable burdens however the free gas accumulating in the lower part of the ghbs can significantly increase gas to water ratio it also generates many secondary hydrates in the ghbs at the same time decreasing the well pressure can be beneficial to gas recovery but the recovery increase is not obvious in term of geomechanical response of the reservoir during the gas recovery the permeable burdens are conducive to reduction of the sediment deformation though they don t facilitate the gas recovery rate in addition significant stress concentration is observed in the upper and lower edges of ghbs around the borehole during depressurization because of high pressure gradient and the greater the well pressure drop the more obvious the phenomenon yield failures and sand production easily take place in the edges therefore in order to achieve the purpose of safe efficient and long term gas production a balance between the production pressure and reservoir stability should be reached at the hydrate site the production pressure difference and sand production must be carefully controlled and the high stress concentration zones need strengthening or sand control treatment during gas production besides the sensitivity analyses show that the hydrate saturation heterogeneity can affect the production potential and geomechanical response to some extent especially the water extraction rate and the effective stress distribution and evolution increasing ghbs and its underlying free gas formation permeabilities can enhance the gas production potential but it probably introduces geomechanical risks to gas recovery operations

non sequential injection of pge rich ultramafic sills in the platreef unit at akanani northern limb of the bushveld complex evidence from sr and nd isotopic systematics

assessing the geomechanical stability of interbedded hydrate bearing sediments under gas production by depressurization at nghp 02 site 16 abstract establishing the geomechanical stability of marine sediments in the vicinity of a production well is one of the key design considerations in planning offshore gas production from marine hydrate reservoirs this paper presents an assessment of the sediment stability at india s national gas hydrate program expedition 2 nghp 02 site 16 area b offshore eastern india for which gas production is to be carried out by depressurization one important feature of the study is that extensive calibration of constitutive model parameters has been conducted based on laboratory test data from pressured core samples from analysis perspective the site is challenging because the hydrate reservoir consists of thin layers of hydrate bearing sands interbedded with mud moreover depressurization at the depth of a reservoir more than 2750 m below sea surface will lead to a pore pressure drop and accordingly an effective confining stress increase as high as 25 mpa in dealing with thin interbedded hydrate bearing strata meshing requirements for flow and geomechanical analysis are quite different from those for reservoirs with thicker massive layers an axisymmetric model and one way coupling simulations were thus adopted for this study in which the geomechanical study utilizes pore pressure and hydrate saturation output from the flow study but the flow study does not takes the porosity changes from the geomechanical analysis instead the reduction of porosity due to sediment deformation in the flow study is based on a pressure dependent pore compressibility relationship derived from geomechanical modeling the rationality is validated through back computing the pore compressibility from the geomechanical deformation results the study shows that large compression in the reservoir will result in movement of the sediments from above and below as well as laterally in smaller magnitudes and the sediment is deemed stable during the gas production period

the genesis of sulfide mineralization in a portion of the potgietersrus limb of the bushveld complex in the potgietersrus limb of the bushveld complex south africa sulfide mineralization is associated with the intrusive contact between bushveld pyroxenitic gabbros and floor rocks consisting of malmani dolomite the overlying penge banded ironstone and sedimentary rocks of the pretoria series whole rock values of up to 0 5 and 1 8 weight percent copper and nickel respectively were obtained in borehole intersections through the contact zone and values of up to 28 ppm platinum group metals have been reported from the area petrographic and isotope geochemical studies were undertaken on a representative suite of rocks it was shown that the pyroxenitic gabbro in contact with the floor rocks was equivalent in composition to the merensky reef xenoliths of malmani dolomite incorporated into the bushveld rocks reacted with the magma and were metamorphosed to calc silicate and carbonate silicate hornfelses increased sulfide concentrations are found in association with the xenoliths these rocks together with the bushveld pyroxenitic gabbros and recrystallized argillaceous sediments of the pretoria series and the pre bushveld noritic sills constitute a 120 m thick mineralized zone known as the platreef the results of carbon and oxygen isotope determinations on unaltered malmani dolomite are consistent with values expected for an evaporitic environment of deposition during the precambrian anhydrite normally associated with evaporitic sequences is likely to have been preserved in the malmani dolomite when it was incorporated as xenoliths into the bushveld magma sulfur isotope studies on sulfides from the platreef zone give a range of delta 34 s values of from 6 8 to 9 2 per mil as the delta 34 s values for primary magmatic bushveld sulfides fall in the range 0 6 to 3 5 per mil the isotope geochemistry clearly indicates a contribution of sedimentary derived sulfur to the original bushveld magma sulfide mineralization in the platreef is considered therefore to have been derived from the combined influence of a sulfur rich merensky reef type magma and the incorporation of additional sulfur from the malmani dolomite either through direct diffusion or in association with a carbonate melt between the banded ironstone and the platreef zone there is a 50 m thick zone of coarsegrained iron and sulfide rich pegmatoidal pyroxenite pyrrhotite from these rocks gave delta 34 s values of 2 7 and 8 7 per mil which are within the possible range of magmatic sulfur these rocks may be related to the intrusive pipes found elsewhere in the bushveld complex

evaluation of bed load transport formulae using field evidence from the vedder river british columbia abstract bed load transport formulae have not been tested extensively over medium spatial and temporal scales the lack of such testing is undoubtedly due to a paucity of field data available for testing at these scales extensive transport morphologic and discharge data available for a 10 year period along an 8 km study reach of the vedder river british columbia are used to test the predictive capabilities of bed load transport formulae at medium scales the transport data were obtained using the morphologic approach which is based on the premise that changes in channel morphology reflect the transfer of sediment such transport estimates can be made over larger spatial and temporal scales than is possible using traditional measurement techniques this study focuses on evaluating the original and revised versions of the bagnold stream power formula the meyer peter and muller formula and a stream power correlation overall the formulae are found to underpredict gravel transport rates for the vedder river the discrepancy is particularly pronounced in 1982 1983 and 1987 1990 which are periods during which significant dredging occurred dredging activity may have loosened the bed structure or resulted in a disequilibrium morphology thereby increasing sediment movement analysis is undertaken to assess how the use of mean daily discharge vs 15 min increment discharge series affects transport predictions results show that only the meyer peter and muller formula is particularly sensitive to which discharge series is used in calculations modification of the shields parameter is found to significantly affect transport predictions although it cannot explain alone the discrepancies between field data and calculated results predictions of total deposition in the study reach are well within an order of magnitude for all equations the bagnold type formulae and the meyer peter and muller formula do not capture the relatively even distribution of deposition along the study reach surprisingly the simple stream power correlation captured this downstream pattern of deposition cumulative distributions of predicted bed load transport are found to be most realistic for the original and revised bagnold formulae results of this study do not suggest that one particular formula is consistently preferred

pcats triaxial a new geotechnical apparatus for characterizing pressure cores from the nankai trough japan abstract understanding the physical nature and mechanical behaviour of hydrate bearing sediments is of fundamental importance in assessing the resource potential of methane gas hydrates advances in pressure coring techniques and associated processing equipment have enabled intact samples to be recovered under in situ pressures however testing of these samples under the in situ stress conditions has not been possible to help address this issue the pcats triaxial apparatus was developed to enable the physical properties of such samples to be measured the apparatus was deployed for the first time during the jogmec funded site investigation of the eastern nankai trough in the summer of 2012 prior to a planned hydrate production test in 2013 a number of pressurized core were recovered and sub samples successfully tested in pcats triaxial to determine a range of geomechanical properties including small strain stiffness from resonance testing stress strain properties triaxial shear tests and permeability samples tested included fine grained soils with no appreciable hydrate sands with hydrate saturation greater than 20 and one sample that had a combination of both materials testing showed an increase in stiffness and undrained shear strength with increasing grain size hydrate saturation and applied effective stress permeability was significantly reduced for hydrate bearing sands compared to clayey samples with no hydrate present

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

gas hydrates in permafrost distinctive effect of gas hydrates and ice on the geomechanical properties of simulated hydrate bearing permafrost sediments

the ug2 merensky reef interval of the bushveld complex northwest of pretoria northwest of pretoria the ug2 merensky reef interval overlies a critical zone lower zone sequence that contains numerous large blocks of floor material nevertheless individual layers can be correlated with equivalent units at crocodile river mine the rustenburg impala union and amandelbult sections concentrations of platinum group elements in two borehole intersections of the ug2 chromitite are 4 ppm over 1 2 m and 2 4 ppm over 2 2 m therefore bulk pge levels appear to be only moderately lower than those at western platinum mine this renders models explaining pge enrichment by upward percolating melt or fluids problematic the merensky reef although containing sulphides is only weakly mineralized with pge 0 6 ppm the ug2 pyroxenite is separated from the ug2 chromitite by a 15 m noritic layer the introduction of feldspathic cumulates between two units that elsewhere directly overly each other may be explained by the more evolved composition of resident magma in those parts of the chamber distally located with regard to a major feeder zone at union section it also suggests that the ug2 unit is a multiple rather than a single cyclic unit

geomechanical issues in the exploitation of natural gas hydrate abstract as a clean fossil fuel with great reserves natural gas hydrate ngh is widely regarded as an important future alternative energy source ngh is widely distributed in onshore tundra and shallow sedimentary layers in the deep sea these sedimentary layers typically exit shallow burial depth poor diagenesis and low strength characteristics moreover the decomposition of ngh can also greatly reduce reservoir strength therefore ngh development can easily causes many geomechanical problems including reservoir instability sand production and seabed landslides etc which may further trigger a series of environmental disasters such as tsunamis natural gas leakage and the acceleration of global warming this study mainly reviews the research progress regarding geomechanical issues in ngh development including mechanical properties of ngh bearing sediments borehole stability hydraulic fracturing sand production reservoir settlement and seabed landslides in addition to previous research achievements regarding geomechanical problems in ngh exploitation the limitations and challenges are also discussed and several questions and insightful suggestions are put forward for future research from our point of view

litho and chemostratigraphy of the flatreef pge deposit northern bushveld complex the flatreef is a world class platinum group element pge deposit recently discovered down dip from existing mining and exploration operations on the northern limb of the bushveld complex current indicated resources stand at 42 moz pge 346 mt with 3 8 g t pt pd rh au 0 32 ni and 0 16 cu which in the case of pt is equivalent to 10 years global annual production making it one of the largest pge deposits on earth the grade and thickness of the flatreef mineralised interval is highly unusual with some drill core intersections containing up to 4 5 g t pt pd rh au over 90 m in drill core here we document the down dip and along strike litho and chemostratigraphy of the flatreef and its footwall and hanging wall rocks based on a diamond drill core database totalling 720 km at the base of the sequence intersected in the drill cores are up to 700 m thick sills of ultramafic rocks dunite harzburgite pyroxenite emplaced into pelitic dolomitic and locally quartzitic and evaporitic rocks belonging to the duitschland formation of the transvaal supergroup next is an approximately 100 200 m sequence of low grade sulphide mineralised layered mafic ultramafic rocks containing abundant sedimentary xenoliths and in places several chromite seams or stringers this is overlain by a 100 m thick sequence of well mineralised mafic ultramafic rocks the flatreef sensu strictu overlain by a laterally persistent mottled anorthosite layer at the base of 1 km of homogenous main zone gabbronorite based on stratigraphic lithological and compositional analogies to the layered rocks in the eastern and western bushveld complex we correlate the flatreef and its chromite bearing footwall rocks with the upper critical zone notably the interval between the ug2 chromitite and the bastard reef as found elsewhere in the bushveld complex this includes recognition of a merensky reef correlative the ultramafic rocks below the main chromitite seam ug2 correlative in the flatreef footwall are correlated with the lower critical and lower zones however compared to the western and eastern bushveld limbs the studied sequence is strongly enriched in sulphide and pge many of the rocks show elevated cao k2o rb and zr contents and lateral continuity of layers between drill cores is less pronounced than elsewhere in the bushveld whereas ultramafic units are locally considerably thickened these compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels intrusion of granitic magmas and the influence of multiple structural events such as pre to syn emplacement regional scale open folding and growth faults evidence for the existence of potholes also exists in the shallow up dip portions of the project area the entire magmatic sequence below the main zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores this package represents the platreef as exposed in outcrop and shallow bore holes across much of the northern limb of the bushveld complex the new data presented here thus indicate that the platreef is a relatively more contaminated up dip extension of parts of the critical and lower zones

zonation of merensky style platinum group element mineralization in turfspruit thick reef facies northern limb of the bushveld complex the stratigraphy whole rock composition and mineralogy from a thick undisturbed section of the platreef at depth on turfspruit in the northern limb of the bushveld complex have been examined in detail this section has much more in common with the critical zone than with the platreef structure updip although the high grade platinum group element pge reefs located close to the boundary with the main zone are much thicker than any known merensky reef facies a key feature that marks the conversion of the platreefstyle mineralization into the merensky style is the regular appearance of chromite seams which become more persistent with decreasing contamination westward and downdip the similar zonal distribution of platinum group minerals pgm in two consecutive thick pge reefs correlates with changes in silicate compositions and vertical zonation of pge concentrations in whole rocks and base metal sulfides this supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes the typical merensky style ore assemblages of predominant pt sulfides and alloys are developed within 1 m thick intervals which correspond to the highest temperature mixing zones between two magmas whereas the rest of the reefs host pt pd bismuthotellurides and arsenides pge content and tenor as well as cu pd in major base metal sulfides in the turfspruit reefs are within the range of the typical high grade merensky values suggesting the same enrichment process