multiscale organisation of organic matter associated with gold and uranium minerals in the witwatersrand basin south africa abstract organic matter from the northern part of the early proterozoic witwatersrand basin carbon leader reef was investigated using optical om and transmission electron tem microscopes completed by xrd analysis the multiscale organization texture microtexture structure of the organic matter was observed in order to gain information about the processes which affected organic material after its deposition in sediments in the micrometre scale optical microscope the shape and size of the reflectance indicating surface ris of the organic matter were determined the organic matter reveals a prevailing biaxial symmetry the size of ris is generally dependent on uranium and increases with increasing uranium concentration furthermore it appears that more than one ris is present within the scale of a single sample each with a different symmetry and size the presence of domains differing in organisation of the aromatic framework was confirmed by tem observation in the df mode the aromatic skeleton of organic matter is composed of short often crumpled mostly isolated non stacked polyaromatic layers whose fringe length corresponds to 3 16 aromatic rings the data indicate reorganization of the polyaromatic organic matter structure under stress in high pressure and relatively low temperature conditions the organic matter was in a solid state within the rocks before the pressure event

storm impacts on hydrodynamics and suspended sediment fluxes in a microtidal back barrier estuary abstract recent major storms have piqued interest in understanding the responses of estuarine hydrodynamics and sediment transport to these events to that end flow velocity wave characteristics and suspended sediment concentration ssc were measured for 11 months at eight locations in chincoteague bay md va usa a shallow back barrier estuary daily breezes and episodic storms generated sediment resuspending waves and modified the flow velocity at all sites which occupied channel shoal and sheltered bay environments with different bed sediment characteristics despite comparable ssc during calm periods ssc at the channel locations was considerably greater than at the shoal sites during windy periods because of relatively more erodible bed sediment in the channels sediment fluxes were strongly wind modulated within the bay s main channel depth integrated unit width sediment flux increased nonlinearly with increasing wind speed when averaged over all sites about 35 of the flux occurred during windy periods wind speed greater than 6 m s 1 which represented just 15 of the deployment time at channel sites the net water and sediment fluxes were opposite to the direction of the wind forcing while at shoal sites the fluxes generally were aligned with the wind implying complex channel shoal dynamics yearly sediment fluxes exceed previous estimates of sediment delivery to the entirety of chincoteague bay these observations illustrate the dynamic sedimentary processes occurring within microtidal back barrier lagoons and highlight the importance of storm events in the hydrodynamics and overall sediment budgets of these systems

metamorphism of the witwatersrand gold fields conditions during peak metamorphism pelitic assemblages from all major witwatersrand gold fields record metamorphic conditions of the greenschist facies with minimal regional grade changes over at least 200 km strike length diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites the actively exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons bulk rock composition has been a major control on assemblage development key metapelitic assemblages include pyrophyllite chloritoid chlorite and muscovite in each gold field with less common metamorphic biotite accessory minerals are pyrite tourmaline rutile and zircon the abundance of chloritoid and pyrophyllite in thin shaly units together with their minor but widespread distribution in quartzites and conglomerates indicate that metamorphic temperatures reached 350 c 50 c in all the gold fields pressures are less well constrained 1 2 kbar being inferred outside the gold fields higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions the temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to re mobilize gold and may explain its secondary textural features

physical structural and isotopic characteristics and growth processes of fast sea ice in lutzow holm bay antarctica a sea ice ocean study was conducted off queen maud land and enderby land antarctica from 1990 to 1991 by the japanese antarctic research expedition observations of multiyear land fast sea ice were made in lutzow holm bay over a period of 2 years to determine the snow and ice characteristics and ice growth processes the snow depth in the bay reached large values of 1 0 to 1 5 m during the winter season at offshore locations from the analysis of ice thickness measurements it is confirmed that the fast ice with deep snow cover grew little in winter but substantially thickened during the summer months on the basis of ice core structure salinity and stable isotopic composition we conclude that the summer growth was caused by upward growth at the top of the ice to which snow ice and superimposed ice formation contribute these processes were the primary contributors to sea ice growth and characteristics only where the snow accumulation was large in areas of low snow accumulation there was no surface growth superimposed ice formation on sea ice in antarctica has not been reported previously evidence for snow cover melting which is a prerequisite for superimposed ice formation was also found

hydrothermal alteration in the witwatersrand goldfields abstract alteration is recorded throughout much of the 3 km thick upper witwatersrand succession in south africa and extends for 300 km around the basin and 50 km into the basin the evidence for this alteration comes from petrography mineralogy geochemistry rock fabrics and stratigraphy the alteration conceived as a giant halo enveloping each major goldfield requires a major hydrothermal event focused on the auriferous upper wits central rand group the scale and nature of the alteration and its relationship to gold distribution on various scales strongly support a hydrothermal model for gold deposition such a model can account for the 50 000 t of gold mined from the witwatersrand goldfields the model works within the context of the major advances in understanding of the geological framework of the witwatersrand basin including the recognition of the importance of post burial processes it is highly significant that the age of witwatersrand sedimentation is much older than 2700 my and thus the host rocks for gold were deposited at least 100 my before the major global archean greenstone gold event the shale units from 40 major mines covering the seven goldfields are examined to establish the nature and distribution of the alteration and to make the connection between the fluid infiltration that caused the alteration and the introduction of gold into the witwatersrand basin in the hydrothermal model described here an auriferous low salinity fluid is produced at about 500 c during the metamorphism of mafic material that was below and adjacent to the witwatersrand basin large shear zones in the footwall of goldfields are the inferred fluid pathways into the upper wits succession and then unconformities smaller shear zones veins and the reef packages guide local fluid flow in this model gold precipitation occurs in response to the reaction of the fluid with carbon and iron bearing rocks that are abundant in the reef packages immediately overlying unconformity surfaces gold precipitation occurred at about 350 c and 10 km depth with the implication of a high geothermal gradient approaching 35 c per km the alteration of the shales but lack of abundant gold in them is predicted from the paucity of appropriate rocks or minerals in the shale units to precipitate gold but fluid infiltration still changed their composition fundamentally the source of the witwatersrand gold its present distribution and the alteration are explained within this hydrothermal model with similarities to the models invoked for other major gold provinces it is worth noting that quartz pebble conglomerate is a poor term for this gold type and has almost certainly limited exploration opportunities globally

widespread fluid infiltration during metamorphism of the witwatersrand goldfields generation of chloritoid and pyrophyllite chloritoid and pyrophyllite occur together in all major goldfields of the witwatersrand basin and are widespread in virtually all rock types of the upper witwatersrand supergroup including metaconglomeratic reefs and altered mafic rocks both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages but they are also developed locally in the regionally persistent metapelites that have basin wide extent pyrophyllite is particularly common in foliated zones adjacent to quartz veins and near unconformably overlying auriferous conglomerates the wide distribution of chloritoid and pyrophyllite in metapelites of the witwatersrand basin is attributed to alteration of chlorite rich shales rather than to unusual premetamorphic starting materials this alteration event involved the redistribution of many elements with up to 40 volume loss mainly due to removal of silica removal of most of the mg and some fe accounts for the stabilization of chloritoid and pyrophyllite relatively immobile elements included al ti nb cr v p la and ce whereas si fe mn zn co ni cu mg and ca were lost and k rb and ba were introduced by an infiltrating fluid the alteration event is inferred to have been within the chloritoid and pyrophyllite stability field and thus syn metamorphic as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite rather than to lower grade minerals such as kaolinite muscovite chlorite chloritoid and muscovite chloritoid pyrophyllite assemblages are attributed to fluid buffering along appropriate curves as their production by metamorphism of lower grade mineral mixes is considered unlikely based on the present bulk rock compositional data a metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration the transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite quartz veining and textures fluid rock ratios calculated from a silica budget in one metapelitic horizon exceed 100 1 over many square kilometres these values need not imply multi pass fluid flow as much of the silica migration may be redistribution on a scale of a few metres from source rocks into veins although infiltration during metamorphism may have affected much of the upper witwatersrand succession channelized fluid flow within reef packages along faults and unconformities and in certain metaconglomerates and metapelites is inferred

on the surface energy budget of sea ice the surface energy budget was investigated during a cruise through the pack ice in the southern ocean the time of observation was close to mid summer some of the more important findings were the mean albedo varied from 11 for open water to 59 for 10 10 ice cover hourly values span the range from 6 open water to 76 total ice cover the net heat flux into the ocean b was on average 109 w m 2 if this energy were used solely for melting of sea ice 30 mm could be melted each day for low surface albedos ice concentration below 7 10 the net radiation increased with decreasing cloudiness however the opposite was the case for a high surface albedo the last point shows the importance of clouds on the surface energy budget not only should their presence or absence be known but also the reflectivity of the underlying surface as it might change the net radiation in opposite ways

sedimentation in the albemarle pamlico lagoonal system synthesis and hypotheses abstract relative to its size and in terms of its sedimentary processes the albemarle pamlico lagoonal system is one of the least studied coastal lagoons in the united states a synthesis of the current knowledge about sedimentation in the albemarle pamlico system is presented based on archived data published over the last 30 years and new unpublished data collected over the past two years the literature reflects considerable past research effort on sediment grain size and mineralogy but very little on sediment dynamics or flux of suspended particulate material the physiographic complexity of the system which includes barrier islands a deep central basin four river systems and extensive fringing embayments does not appear to be reflected in the form of sedimentologic complexity in general the surficial cover of sediments ranges from medium sands in the inlets and on the shoals of the outer banks to fine silts and organic rich clays in the central basin and embayed river mouths and channels the transition zones from sands to muds are typically sharp and are usually related to bathymetry it is concluded that 1 parts of the albemarle pamlico lagoonal system may have high sedimentation rates relative to rates of sediment input and that rapid vertical flux by large aggregates offers an explanation for the fluid mud deposits that would otherwise be absent 2 fine grained sediments that escape the estuarine sediment trap are confined to pamlico sound by the outer banks barrier islands and are simply recycled until finally coming to rest in the deep basin 3 short term advective processes are secondary to longer term processes such as sea level rise and barrier island migration in the net advection of albemarle pamlico sediments and 4 at the present rate of terrigenous sediment input and relative sea level rise the albemarle pamlico lagoonal system will never reach a sediment filled state

the origin of no3 and n2 in deep subsurface fracture water of south africa abstract deep 0 8 km depth fracture water with residence time estimates on the order of several ma from the witwatersrand basin south africa contains up to 40 m of no3 up to 50 mm n2 90 times air saturation at surface and 1 to 400 m nh3 nh4 to determine whether the oxidized n species were introduced by mining activity by recharge of paleometeoric water or by subsurface geochemical processes we undertook n and o isotopic analyses of n species from fracture water mining water pore water fluid inclusion leachate and whole rock cores the no2 no3 and nh3 nh4 concentrations of the pore water and fluid inclusion leachate recovered from the low porosity quartzite shale and metavolcanic units were 104 times that of the fracture water the 15n no3 and 18o no3 of the pore water and fluid inclusion leachate however overlapped that of the fracture water with the 15n no3 ranging from 2 to 7 and the 18o no3 ranging from 20 to 50 the 15n no3 of the mining water ranged from 0 to 16 and its 18o no3 from 0 to 14 making the mining water no3 isotopically distinct from that of the fracture pore and fluid inclusion water the 15n n2 of the fracture water and the 15n n from the cores ranged from 5 to 10 and overlapped the 15n no3 the 15n nh4 of the fracture water and pore water nh3 nh4 ranged from 15 to 4 although the no3 concentrations in the pore water and fluid inclusions were high mass balance calculations indicate that no3 accounts for 10 of the total rock n whereas nh3 nh4 trapped in fluid inclusions or nh4 present in phyllosilicates account for 90 of the total n based on these findings the fluid inclusion no3 appears to be the source of the pore water and fracture water no3 rather than paleometeoric recharge or mining contamination irradiation experiments indicate that radiolytic oxidation of nh3 to no3 can explain the fluid inclusion no3 concentrations and perhaps its isotopic composition but only if the no3 did not attain isotopic equilibrium with the hydrothermal fluid 2 billion years ago the 15n n 15n n2 and 15n nh4 suggest that the reduction of n2 to nh4 also must have occurred in the witwatersrand basin in order to explain the abundance of nh4 throughout the strata although the depleted no3 concentrations in the fracture water relative to the pore water are consistent with microbial no3 reduction further analyses will be required to determine the relative importance of biological processes in the subsurface n cycle and whether a complete subsurface n cycle exists

stratigraphic control of chemistry and mineralogy in metamorphosed witwatersrand quartzites within metamorphosed quartzites of the witwatersrand supergroup bulk chemistry is the major control on the distribution of metamorphic mineral phases and on their chemistry bulk chemistry is in turn controlled by stratigraphic position pyrophyllite distribution varies directly with whereas feldspar distribution varies inversely with bulk al chloritoid distribution is more complex and varies with both bulk al and bulk fe mg despite the overprint of low grade metamorphism and possibly of other alteration processes both detrital and metamorphic phyllosilicates can be recognized and characterized al in metamorphic white mica and metamorphic chlorite varies with bulk al whereas the composition of detrital white mica is unrelated to bulk chemistry similarly mg in metamorphic chlorite varies with bulk mg bulk chemical variation particularly in al mg and na and thus metamorphic mineral chemistry is controlled by stratigraphic position both within individual stratigraphie units and in the strat

drag coefficients for winter antarctic pack ice air ice and ice water drag coefficients referenced to 10 m height winds for winter antarctic pack ice based on measurements made from r v polarstern during the winter weddell sea project 1986 wwsp 86 and from r v akademik fedorov during the winter weddell gyre study 1989 wwgs 89 are presented the optimal values of the air ice drag coefficients made from turbulent flux measurements are 1 79 0 06 x 10 exp 3 for wwsp 86 and 1 45 0 09 x 10 exp 3 for wwgs 89 a single ice water drag coefficient for both wwsp 86 and wwgs 89 estimated from periods of ice drift throught to represent free drift conditions is 1 13 0 26 x 10 exp 3 and the ice water turning angle is 18 18 deg it is suggested that for a typical antarctic winter pack ice cover the ice cover reduces the momentum flux from the atmosphere to the ocean by about 33 percent

trace element and upb isotope compositions of pyrite types in the proterozoic black reef transvaal sequence south africa implications on genesis and age abstract the proterozoic black reef is in most cases indistinguishable from the gold uranium and pyrite bearing archaean witwatersrand reefs however the prevailing opinion that the black reef gold mineralization was largely derived from reworking of the underlying kimberley reef is not supported by the morphology trace element geochemistry and u pb isotope data on detrital pyrite in these reefs the grade of metamorphism recorded in the black reef quartzite formation is significantly lower than the 300 400 c that has been proposed for the sediments of the witwatersrand supergroup the u pb isotope and trace element geochemical signatures in pyrite have therefore been preserved in the black reef the morphological and compositional differences retained by the detrital and synsedimentary pyrite types both occurring together in the black reef imply that they were formed by different processes and in contrasting geochemical environments in the black reef the data for the detrital pyrite are consistent with their derivation from a 3000 2800 ma granitic probably hydrothermally altered source region by contrast the geochemical and pb isotopic composition of the concretionary pyrite is interpreted to reflect its synsedimentary formation in a lagoonal or tidal flat environment prior to 2500 ma the morphology primary mineral inclusions and the unradiogenic pb isotopic compositions preserved in the black reef detrital pyrite are irreconcilable with models proposed for the formation of witwatersrand pyrite by a process of pyritization of fe bearing detrital minerals fluids present in the black reef depositional diagenetic environment were anomalously enriched in radiogenic pb and any transformation process would not be able to escape acquiring this enriched pb isotopic signature

water level response in back barrier bays unchanged following hurricane sandy on 28 30 october 2012 hurricane sandy caused severe flooding along portions of the northeast coast of the united states and cut new inlets across barrier islands in new jersey and new york about 30 of the 20 highest daily maximum water levels observed between 2007 and 2013 in barnegat and great south bay occurred in 5 months following hurricane sandy hurricane sandy provided a rare opportunity to determine whether extreme events alter systems protected by barrier islands leaving the mainland more vulnerable to flooding comparisons between water levels before and after hurricane sandy at bay stations and an offshore station show no significant differences in the transfer of sea level fluctuations from offshore to either bay following sandy the post hurricane sandy bay high water levels reflected offshore sea levels caused by winter storms not by barrier island breaching or geomorphic changes within the bays

structure variability and salt flux in a strongly forced salt wedge estuary this researchwas funded by national science foundation grant oce 0452054 ralston also received partial support from the penzance endowed fund in support of assistant scientists and the john f and dorothy h magee fund in support of scientific staff at woods hole oceanographic institution

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

18o concentrations in sea ice of the weddell sea antarctica we present data on ice texture salinity and delta o 18 obtained from identical sections of ice cores during the winter weddell sea project 1986 on rv polarstern from july through august 1986 in the longitude range between 5 degrees w and 7 degrees e we find no uniquely definable relationship between delta o 18 values and ice texture in a particular section however most of the snow ice as well as some sections of frazil ice ar found to have negative delta o 18 concentrations this is due to varying degrees of admixtures of meteoric ice snow and sea water during formation of snow ice in contrast to common assumptions our results seem to indicate that a snow cover contributes positively to sea ice growth rather than slowing down the overall growth rate based on a simple model we have estimated the contributions of meteoric ice mean of 3 3 and the combined meteoric ice sea water fraction a minimum of 7 6 to the total ice thickness for the majority of the sampled floes although this is only a moderate contribution to the overall mass balance in the absence of congelation growth it nevertheless enhances ice growth in general this hypothesis is dependently supported by our snow and ice thickness data wadhams and others 1987 which demonstrate that the depression of the snow ice interface below the water line i e a negative freeboard and the formation of snow ice is a common occurrence in the weddell sea therefore we hypothesize that the major part of the observed apparent increase in ice thickness between our inbound and outbound tracks of wwsp 86 may not be derived from regular thermodynamically driven congelation growth but rather from the snow ice component in floes of the weddell sea

sudoite in the archaean witwatersrand basin sudoite with almost ideal composition of mg2al4si3o10 oh 8 has been found in the khaki shale of the witwatersrand basin south africa the sudoite occurs in aggregates of elongate bundles alternating with muscovite and rarely pyrophyllite in an assemblage of sudoite pyrophyllite muscovite quartz rutile pyrite the stability of sudoite is attributed to a particularly mg rich bulk rock composition elevated sulphur activity and metamorphic conditions just above the kaolinite to pyrophyllite transition nearby low variance chlorite chloritoid and pyrophyllite bearing assemblages represent differing bulk rock compositions at potentially very similar metamorphic conditions as the first find of sudoite in the archaean this occurrence in the witwatersrand basin extends the metamorphic setting and age in which sudoite is found to well beyond phanerozoic belts of crustal thickening

the barrier island system abstract barrier islands are part of a major coastal system composed of six interactive sedimentary environments the six environments are also elements needed to impose the designation barrier island on littoral sand bodies the elements are 1 mainland 2 backbarrier lagoon 3 inlet and inlet deltas 4 barrier island 5 barrier platform and 6 shoreface the morphodynamic and sedimentary evolution of each element affects adjacent environments as well as the entire system the interactive elements are defined and described by their morphologic sedimentologic and stratigraphic relationships to the barrier island and the barrier island system the stratigraphic relationships of the various elements of the system may be a valuable tool for paleogeographic reconstruction and sand body mapping

seasonal variations of the antarctic coastal ocean in the vicinity of lutzow holm bay during the period of 1990 1992 year round oceanographic observations were conducted in the vicinity of lutzow holm bay east antarctica it was found that the thickness of the winter water ww layer characterized by a cold fresh oxygen rich water exhibits its maximum in the austral fall typically 500 m and its minimum in the austral summer typically 350 400 m the associated density variation of the water column explains only about one third of the seasonal variations in sea level at the coast which suggests a large seasonal variation in barotropic coastal flow prominent freshening occurs in the ww layer during fall this appears to be caused by the accumulation of ww whose upper portion is freshened in the preceding summer these seasonal variations appear to occur every year we propose that the seasonal variation in the ww layer is mainly caused by the seasonal variation in the wind over the coastal ocean in fall the prevailing easterly wind intensifies which increases the ekman convergence of ww in the coastal ocean while in summer the opposite occurs

observations of low frequency variability in great south bay and relations to atmospheric forcing abstract sea level and current data collected around great south bay new york during december 1979 are examined in conjunction with atmospheric data for evidence of wind forced low frequency variability in the bay and on the adjacent shelf the subtidal sea level along the coast was found to be highly coherent from sandy hook to montauk point with a single empirical mode accounted for more than 97 of the total variance these coherent fluctuations were forced primarily by longshore winds along 250 070 t through the coastal ekman effect the sea level within the bay exhibited large and spatially coherent subtidal fluctuations as a result of a strong coupling with the adjacent shelf the characteristic volume exchange associated with this bay shelf coupling was an active simultaneous inflow or outflow through both ends of the bay with fluctuations in excess of 20 cm s 1 in response to the rise or fall of coastal sea level induced by longshore winds

the origin of the gold and uranium ores of the black reef formation transvaal supergroup south africa abstract the black reef formation is a laterally extensive sedimentary succession at the base of the transvaal supergroup the quartz pebble conglomerates host erratic concentrations of gold and uranium however in areas that are located spatially above the gold and uranium bearing reefs of the witwatersrand supergroup the concentrations can reach ore grade although the depositional environment was similar to that of the witwatersrand supergroup the processes that lead to the high gold and uranium concentrations in the black reef were quite different the black reef experienced intense post depositional hydrothermal alteration by circulating aqueous and hydrocarbon fluids oils that deposited large volumes of native gold uranium minerals and pyrobitumen the gold which is filamentous is concentrated along as ni rich surfaces of pyrite and with uraninite in pyrobitumen in both environments the gold is interpreted to be the product of chemically triggered precipitation from hydrothermal fluid s the uraninite occurs either as precipitates in pyrobitumen or is invisible in the pyrite occurring as nanoparticles in this mineral this indicates that conditions were sufficiently oxidizing for the hydrothermal transport of uranium and contrasts with the case for the witwatersrand reefs where much of the uranium is detrital luminescent radiation damage associated with vein quartz and pyrobitumen as well as the diffuse distribution pattern of uranium in the pyrobitumen may indicate that some uranium was transported by liquid hydrocarbons pyrite in the black reef sedimentary detrital and hydrothermal pyrite is highly enriched in gold uranium and other redox sensitive trace elements provenance analyses using the trace element compositions of the pyrite show that it was not the result of erosional reworking of the underlying witwatersrand reefs however the close spatial association of high gold and uranium concentrations in the black reef with ore bearing witwatersrand reefs suggests strongly that gold and uranium whether in pyrite or occurring as native gold and uraninite were hydrothermally recycled uranium may have been recycled by hydrocarbon fluids from the witwatersrand reefs

bathymetric controls on sediment transport in the hudson river estuary lateral asymmetry and frontal trapping 1 analyses of field observations and numerical model results have identified that sediment transport in the hudson river estuary is laterally segregated between channel and shoals features frontal trapping at multiple locations along the estuary and varies significantly over the spring neap tidal cycle lateral gradients in depth and therefore baroclinic pressure gradient and stratification control the lateral distribution of sediment transport within the saline estuary sediment fluxes are strongly landward in the channel and seaward on the shoals at multiple locations bottom salinity fronts form at bathymetric transitions in width or depth sediment convergences near the fronts create local maxima in suspended sediment concentration and deposition providing a general mechanism for creation of secondary estuarine turbidity maxima at bathymetric transitions the lateral bathymetry also affects the spring neap cycle of sediment suspension and deposition in regions with broad shallow shoals the shoals are erosional and the channel is depositional during neap tides with the opposite pattern during spring tides narrower deeper shoals are depositional during neaps and erosional during springs in each case the lateral transfer is from regions of higher to lower bed stress and depends on the elevation of the pycnocline relative to the bed collectively the results indicate that lateral and along channel gradients in bathymetry and thus stratification bed stress and sediment flux lead to an unsteady heterogeneous distribution of sediment transport and trapping along the estuary rather than trapping solely at a turbidity maximum at the limit of the salinity intrusion

gold mineralization within the witwatersrand basin south africa evidence for a modified placer origin and the role of the vredefort impact event the chemical composition of gold within the archaean metasedimentary rocks of the witwatersrand supergroup displays significant heterogeneity at the micro meso and regional scales a detailed electron microbeam analytical and petrological study of the main auriferous horizons in the central rand group throughout the witwatersrand basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres contemporaneous chlorite formation was strongly rock buffered gold mobilization occurred under fluid poor conditions at temperatures that did not exceed 350 c widespread circulation of mineralizing fluids within the central rand group is not supported by the gold and chlorite chemical data brittle deformation that affects most of the paragenetic sequence of the central rand group late in its post depositional history is followed by sequences of mineral growth and dissolution that appear throughout the central rand group and have consistent textural relationships with gold the consistent location within the paragenetic sequence the wide regional and stratigraphic extent of the brittle deformation together with mineral chemical and petrological data suggest that the vredefort impact event 2 02 ga was the cause of this late deformation and that post impact fluid poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event

the dependence of sea surface roughness on the height and steepness of the waves it is proposed that the sea surface roughness zo can be predicted from the height and steepness of the waves zo hs a hs lp b where hs and lp are the significant wave height and peak wavelength for the combined sea and swell spectrum best estimates for the coefficients are a 1200 b 4 5 the proposed formula is shown to predict well the magnitude and behavior of the drag coefficient as observed in wave tanks lakes and the open ocean thus reconciling observations that previously had appeared disparate indeed the formula suggests that changes in roughness due to limited duration or fetch are of order 10 or less thus all deep water pure windseas regardless of fetch or duration extract momentum from the air at a rate similar to that predicted for a fully developed sea this is confirmed using published field data for a wide range of conditions over lakes and coastal seas only for field data corresponding to extremely young waves u10 cp 3 were there appreciable differences between the predicted and observed roughness values the latter being larger on average significant changes in roughness may be caused by shoaling or by swell a large increase in roughness is predicted for shoaling waves if the depth is less than about 0 2lp the presence of swell in the open ocean acts on average to significantly decrease the effective wave steepness and hence the mean roughness compared to that for a pure windsea thus the predicted open ocean roughness is at most wind speeds significantly less than is observed for pure wind waves on lakes only at high wind speeds such that the windsea dominates the swell do the mean open ocean values reach those for a fully developed sea

sea ice and oceanic processes on the ross sea continental shelf we have investigated the spatial and temporal variability of antarctic sea ice concentrations on the ross sea continental shelf in relation to oceanic and atmospheric forcing sea ice data were derived from nimbus 7 scanning multichannel microwave radiometer smmr brightness temperatures from 1979 1986 ice cover over the shelf was persistently lower than above the adjacent deep ocean averaging 86 during winter with little month to month or interannual variability the large spring ross sea polynya on the western shelf results in a longer period of summer insolation greater surface layer heat storage and later ice formation in that region the following autumn newly identified pennell and ross passage polynyas near the continental shelf break appear to be maintained in part by divergence above a submarine bank and by upwelling of warmer water near the slope front warmer subsurface water enters the shelf region year round and will retard ice growth and enhance heat flux to the atmosphere when entrained in the strong winter vertical circulation temperatures at 125 m depth on a mooring near the ross ice shelf during july 1984 averaged 0 15 c above freezing sufficient to support a vertical heat flux above 100 w m2 monthly average subsurface ocean temperatures along the ross ice shelf lag the air temperature cycle and begin to rise several weeks before spring ice breakout the coarse smmr resolution and dynamic ice shelf coastlines can compromise the use of microwave sea ice data near continental boundaries

evidence for major mass transfer and volume strain during regional metamorphism of pelites systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism statistical analysis of the data shows that barrovian zone metamorphism of pelitic schist generally causes increases in the whole rock concentrations of the low solubility elements ti and all the observed increases in ti and al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species application of mass balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30 of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions the bulk of the lost mass is silica not volatiles in addition other elements particularly ca na and k appear to be highly mobile in deep crustal metamorphism contrary to conventional interpretations it is concluded that the regional metamorphism of pelites is not an isochemical process

diagenetic alteration of a mesozoic fluvial gold placer deposit southern new zealand abstract gold paleoplacers become progressively more affected by diagenetic processes with age and burial mesozoic paleoplacer deposits in southern new zealand display intermediate stages of diagenetic transformation compared to little affected late cenozoic paleoplacers and strongly affected paleozoic and precambrian paleoplacers the mesozoic cretaceous diagenesis resulted in near pervasive alteration cementation and lithification of the paleoplacer lithic clasts and matrix have been extensively altered to illite ferrous iron bearing smectite vermiculite and kaolinite and the cement consists mainly of clays and calcite diagenetic pyrite marcasite vivianite and mn oxide also contributed to cementation alteration occurred under near surface

on meteorologically induced flushing in three u s east coast estuaries abstract meteorologically induced exchange with the coastal ocean is examined for three u s east coast estuaries using time series of subtidal volume flux over a common five month period a volume of water equal to that of the estuary was discharged every 86 4 135 3 and 108 8 days for narragansett delaware and chesapeake bays respectively however a large fraction of the water discharged to the shelf is recirculated ocean water for chesapeake bay outflowing estuarine water can be considered as a mixture of 2 4 3 0 water of mean estuarine salinity and the remainder water of oceanic salinity the degree of meteorological flushing is strongly influenced by mean depth as a drop in sea level of a given amplitude will force a greater fraction of water from a shallow estuary than from a deep system the relative importance of the meteorologically forced circulation and the gravitational circulation is frequency dependent as illustrated by salinity variance at the entrance to chesapeake bay seasonal salinity variance must ultimately be controlled by seasonal changes in runoff through the mechanism of gravitational circulation however the high coherence of wind induced changes in bay volume and bay entrance salinity at periods of 2 20 days suggests that meteorologically forced advection is primarily responsible for salinity variance at these time scales

u pb and pb pb isotopic studies relating to the origin of gold mineralization in the evander goldfield witwatersrand basin south africa abstract this study presents new u pb and pb pb isotopic data for the central rand group in the evander goldfield located in the eastern extremity of the witwatersrand basin single grain detrital zircon dating exhibits a more restricted range of u pb ages than has previously been published from elsewhere in the witwatersrand basin and a shift towards younger ages the oldest grain measured is c 3180 ma while the majority of detritus falls in the range 3050 2850 ma several zircon grains from the sheared and altered portions of the kimberley reef yield ages that are younger or similar to that of the overlying ventersdorp lavas these ages are attributed to partial or complete isotopic resetting of older detritus at some stages during the post depositional history of the basin the existence of both detrital and authigenic pyrites is also supported by pb pb isotopic data of the present study a small proportion of the pyrite mainly extracted from unaltered sediments in the kimberley reef footwall yield ages that are in excess of the minimum depositional age of the witwatersrand basin authigenic pyrite as well as detrital grains from highly altered portions of the kimberley reef define secondary isochrons at about 2370 ma and 2020 ma suggesting fluid circulation in the basin at approximately these times

geochemistry of fluid inclusions from the mcintyre hollinger gold deposit timmins ontario canada type 1 h 2 o co 2 type 2 h 2 o co 2 type 3 ch 4 co 2 h 2 o and type 4 ch 4 fluid inclusions type 1 type 3 and some type 2 inclusions are primary or pseudosecondary in origin whereas type 4 and most type 2 inclusions are secondary the primary or pseudosecondary type 1 and 2 inclusions homogenized at temperatures of 220 degrees to 385 degrees c secondary type 2 inclusions homogenized at 160 degrees to 215 degrees c crushing tests confirm the presence of co 2 in samples with type 1 and 2 inclusions visually estimated x co2 values in type 1 inclusions range from 3 to 24 mole percent with bulk molar volumes of 19 to 28 cm 3 mole mineralizing fluids were at equilibrium with graphite at temperatures of 380 degrees to 480 degrees c and pressures of 1 to 3 kb calculated oxygen fugacities lie near the quartz fayalite magnetite buffer the mineralizing fluids differ compositionally from fluids in other types of ore deposits but resemble fluids derived from metamorphic rocks and deep level felsic intrusions modified journal abstract

evidence for submarine weathering from metamorphosed weathering profiles on basaltic rocks tananao metamorphic complex taiwan the metamorphosed paleo weathering horizon s developed on basaltic rocks in the tananao metamorphic complex forms distinct al fe rich rocks and occurs as two parallel belts demonstrating its potential as a stratigraphic correlation unit in this structurally complicated metamorphic terrain these al fe rich rocks have been metamorphosed under amphibolite greenschist facies conditions leading to the formation of garnet chloritoid rocks chloritoid rich rocks the chemical changes along three profiles from metabasite to garnet chloritoid rock show prominent k rb cs and ba enrichment yet also a large depletion of other elements if it is assumed that ti zr nb ta and hf remained relatively immobile the ree s however behaved non coherently these chemical characteristics are attributed to pre metamorphic processes and are shown to be similar to those observed with submarine weathering of basaltic rocks in contrast the chloritoid rich rocks reveal distinct k rb cs and ba depletion it is suggested that this resulted from subaerial weathering processes k enrichment is not an uncommon feature in paleozoic precambrian paleosols the present case study demonstrates that submarine weathering should be considered as a probable cause

chemographic analysis of assemblages involving pyrophyllite chloritoid chlorite kaolinite kyanite quartz application to metapelites in the witwatersrand goldfields south africa the witwatersrand goldfields contain abundant assemblages that include pyrophyllite chloritoid chlorite kaolinite and or kyanite with quartz a chemographic analysis of the system fe mg al si o h involving these minerals yields 22 potential phase diagrams using orientation criteria and thermodynamic calculations as further constraints this list has been reduced to three possible diagrams new thermodynamic data favour one of these in particular this chemographic analysis demonstrates that formation of chloritoid is not restricted to the breakdown reaction of kaolinite plus chlorite in the f m ash system as stated by previous studies but could be from pyrophyllite chlorite chloritoid quartz h2o the metamorphic temperature variation between witwatersrand goldfields exceeded 65d c based on chlorite and chloritoid compositions the lower and upper pressure limits are constrained by the andalusite to kyanite and the sudoite chlorite to carpholite boundaries i e 1 5 2 8 and 7 kbar respectively the widespread pyrophyllite chlorite and fe chloritoid in all the witwatersrand goldfields and the local occurrence of sudoite indicate a consistent low pressure environment in which mg chloritoid would not be stable

roughness of weddell sea ice and estimates of the air ice drag coefficient the roughness of a sheet of sea ice encodes its deformational history and determines its aerodynamic coupling with the overlying air and underlying water here we report snow surface ice surface and ice underside roughness computed from 47 surface elevation profiles collected during a transect of the weddell sea the roughness for each surface parameterized as the standard deviation of the surface elevation segregates according to whether or not a floe has been deformed deformed ice has greater roughness than undeformed ice regardless of deformational history the underside roughness is almost always greater than the snow surface and ice surface roughnesses which are nearly equal roughness spectra for all three surfaces and for both deformed and undeformed ice roll off roughly as k 1 when the wavenumber k is between 0 1 and 3 rad m 1 the snow surface and underside spectra roll off somewhat faster than k 1 and the ice surface spectra roll off somewhat slower than k 1 both top and underside arctic ice roughness spectra on the other hand have been reported to roll off faster than k 2 we speculate that the excess spectral intensity at high wavenumbers in the antarctic ice surface spectra results from the small scale roughness that the ice sheet had on consolidation this excess high wavenumber spectral intensity persists in the ice surface spectra of second year ice evidently once formed the ice surface remains unchanged on the microscale until the entire ice sheet melts with a remote measurement of roughness we should be able to decide whether an ice floe is deformed or undeformed our spectral analysis hints that remote sensing may also be able to differentiate between first year and second year ice from the snow surface spectra we compute a roughness scale that parameterizes the air ice momentum coupling and lets us estimate the neutral stability drag coefficient referenced to a height of 10 m cdn10 typical cdn10 values are 1 1 1 4 10 3 over undeformed ice and 1 3 1 8 10 3 over deformed ice

turbulent mixing in a strongly forced salt wedge estuary up to about 400 m 3 s 1 above which a majority of the mixing occurs offshore observed buoyancy fluxes were more consistent with the k turbulence closure than the mellor yamada closure and more total mixing occurred in the estuary with k calculated buoyancy fluxes were sensitive to horizontal grid resolution as a lower resolution grid yielded less integrated buoyancy flux in the estuary and exported lower salinity water but likely had greater numerical mixing

hydrothermally altered peraluminous archaean granites as a provenance model for witwatersrand sediments hydrothermal alteration of peraluminous granitic rocks occurring at varkenskraal northwest of the witwatersrand basin consists of two major styles the first style pervasive in nature is represented by two stages i a propylitic alteration and ii a disseminated sericitization both alteration stages were accompanied by a deposition of disseminated sulphides the second style is represented by a vein related alteration in four stages including potassium feldspar minor carbonate and minor sulphide mineralization nodules of uraniferous carbonaceous matter and small amounts of particulate gold could possibly be associated with either one or both styles hydrothermally altered rocks of similar nature have been observed in a number of other localities close to the witwatersrand basin their abundance large volumes and low grade disseminated pyrite gold mineralization indicate that rocks of this type could have been the provenance of the witwatersrand gold bearing sediments

evidence for an intrabasinal source and multiple concentration processes in the formation of the carbon leader reef witwatersrand supergroup south africa laser ablation inductively coupled plasma mass spectrometer la icp ms trace element maps of pyrite and gold from the carbon leader reef in the witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal derived from the west rand group or equivalent shales stratigraphically below this reef rounded detrital pyrite grains in the carbon leader reef vary from compact to porous to sooty with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the west rand group detrital porous and sooty pyrites contain 0 4 to 11 3 ppm solid solution gold sulfur isotope 34s values from 17 to 16 and high levels of as and te plus a wide range of other trace elements in decreasing order ni co sb cr cu u pb bi mo zn and ag the sooty pyrite is the most au te and mo rich and is intergrown with alumino silicates and organic matter the detrital pyrite textures s isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions the laicp ms maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro inclusions of gold brannerite and ni as sulfides the pyrite overgrowths are also enriched in solid solution gold tellurium and other trace elements including in decreasing order as co ni cr cu mn pb bi and ag fractured and brecciated pyrite associated with brittle bedding parallel fracture zones in the carbon leader reef are also enriched in these elements due to alteration of pyrite surrounding the fractures laser icp ms pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 ma with hydrothermal overgrowths originating between 2100 and 2020 ma incorporating highly radiogenic pb this study demonstrates that both of the two competing theories for the origin of the witwatersrand gold reefs are likely to be correct we suggest that the hydrothermal event was widespread kilometer scale and involved basinal fluids that scavenged gold tellurium arsenic and trace elements co ni cr cu mn pb bi ag from gold bearing sedimentary units in the central rand group 2013 society of economic geologists inc

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

petrographic evidence for emplacement of carbon into witwatersrand conglomerates under high fluid pressure abstract much gold in the witwatersrand conglomerates is directly associated with seams of carbon that are generally parallel with bedding some seams locally crosscut bedding and form anastomosing fracture patterns showing that emplacement of liquid hydrocarbons in those seams was postdepositional and infilled fractures that are generally parallel to bedding seams that are not observed to crosscut bedding are nonetheless petrographically identical and are believed to have a common postdepositional origin carbon also occurs along sets of bedding parallel microfractures through individual quartz pebbles in mineralized conglomerates planes of fluid inclusions parallel to the microfractures in pebbles show further evidence for the importance of bedding parallel fluid flow the occurrence of carbon seams on sedimentary discontinuity surfaces may reflect preferential emplacement of carbon along planes of weakness brittle extensional fracturing in pebbles indicates high fluid pressures which may relate to hydrocarbon generation bedding parallel minealization by quartz clays metamorphic chlorite pyrophyllite and gold which reflect a complex multistage paragenesis indicate repeated reactivation of bedding plane permeability previous arguments against a hydrocarbon origin for the carbon seams have been based upon the low intergranular permeability of the rocks however the present study shows that this condition actually caused focusing of hydrocarbon fluid flow along discontinuity surfaces and facies boundaries interaction with uranium could have caused fixation of fluid hydrocarbon by radiolytic polymerization the columnar nature of carbon in many of the seams interpreted by others as a biogenic structure could instead reflect polymer swelling normal to bedding in the direction of least confining stress

rare earth element mobility in alteration pipes below massive cuzn sulfide deposits abstract a convincing demonstration of trace element mobility should satisfy two criteria 1 the range of trace element concentrations in the rock before and after alteration must be known with confidence and 2 the absolute abundances and the ratios of the mobile trace elements should vary between altered and unaltered rocks these criteria have been applied to the host felsic volcanics and alteration pipes associated with several canadian massive cu zn sulfide deposits although there is no evidence of rare earth element ree mobility in the pervasive low grade alteration envelopes adjacent to the deposits the light and middle ree lree and mree plus y have been systematically mobilized relative to the heavy ree hree and other trace elements in the alteration pipes beneath the ore zones there is evidence of minor lree and mree depletion in the most altered rocks at the south bay mine uchi lake and of significant lree depletion in the alteration pipe beneath the mattabi mine sturgeon lake however the strongest mobility is found at the kidd creek mine where the lree were leached from the rocks in the footwall stringer zone and redeposited in a metasomatic chlorite zone directly beneath the massive ore horizon and the mree and y were preferentially lost from the system producing distinctive chondrite normalized ree patterns with tb yb n ratios less than unity eu2 was found to be more mobile than the trivalent ree especially at kidd creek where there is a marked increase in the magnitude of the eu anomaly in the most altered samples high field strength elements such as u th hf and zr have not decoupled and apparently remained relatively immobile during all but the most intense alteration i e kidd creek preliminary data suggest that the degree of ree mobility increases with the size of the deposit thus ree mobility is a potentially useful method of distinguishing between small and large massive sulfide deposits at an early stage in exploration however this hypothesis requires further testing

marine controls on modern sedimentation on the antarctic continental shelf abstract the extreme depth and rugged topography of the antarctic continental shelf its seasonal to perennial sea ice cover and its large slowly calving ice shelves exert major influences on the adjacent ocean and sediments regionally variable exchanges of heat and momentum between the atmosphere and ocean result in net off shelf transport of sea ice and net downward salt flux this buoyancy partitioning drives a vertical convection that is followed by westward and northward flows of ventilated dense water across the continental shelves shelf waters meet mix with and modify circumpolar deep water at a dynamic slope front near the continental shelf break locally or periodically forming antarctic bottom water nutrient rich deep water upwells along this front regionally intruding onto the continental shelf in long thermally attenuated filaments these intrusions are frequently linked to mass balance processes at the base of ice tongues and ice shelves melting freezing and tidal currents beneath glacial ice are related to the larger scale circulation and to the fate of basal debris in the ice sheet the ocean circulation near ice fronts and decadal variability in ice front positions may account for several features of the sediment record time series 1 yr current measurements in the ross and weddell seas have revealed moderate to strong currents that are seasonally variable vertically coherent and dominated by the diurnal tide and longer term events few long term current measurements exist in the shelf and slope bottom boundary layers that are most important to sediment deposition and erosion icebergs may frequent certain routes e g in association with currents and fronts near the edge of the continental shelf and are capable of scouring and resuspending sediments down to shelf break depths sea ice plays a role in shelf sedimentation by rafting eolian and bottom debris away from the nearshore regions in addition generally offshore winds produce numerous leads and polynyas along the coastline and within the sea ice fields higher biological productivity in these polynyas may be linked to higher local sedimentation rates biogenic marine sediments apparently comprise a major percentage of the regionally variable modern shelf deposits despite the impressive glacial setting and polar environment