geochemistry of vanadium in an epigenetic sandstone hosted vanadium uranium deposit henry basin utah the epigenetic tony m vanadium uranium orebody in south central utah is hosted in fluvial sandstones of the morrison formation upper jurassic although the deposit is mined for uranium vanadium has a higher average abundance in the ore thus the geochemistry of vanadium in the orebody was studied to characterize ore forming processes within the inferred ground water flow regime measurements of the relative amounts of v super 3 and v super 4 in ore minerals show that v super 3 is more abundant thermodynamic calculations show that vanadium was more likely transported to the site of mineralization as v super 4 the ore formed as v super 4 was reduced by hydrogen sulfide followed by hydrolysis and precipitation of v super 3 in oxide minerals e g montroseitc or paramontroseite or chlorite uranium was transported as uranyl ion u super 6 or some complex thereof and reduced by hydrogen sulfide forming coffinite detrital organic matter in the rocks served as the carbon source for sulfate reducing bacteria it was this bacteriogenic h 2 s that reduced the metals in the mineralization process possible sources for the v and u in this deposit have been identified previously vanadium most likely was derived from the dissolution of iron titanium oxides which liberated fe as well as v a zone of titanium rich remnants is observed updip and up the hydrologic gradient from the deposit m goldhaber and r l reynolds unpub data uranium probably was derived from the overlying brushy basin member of the morrison formation northrop 1982 a preliminary age date for the deposit of 115 ma k ludwig 1986 pers commun indicates that the ore formed after deposition of the brushy basin member previous studies have shown that the ore formed at the density stratified interface between a basinal brine and dilute meteoric water the mineralization processes described above occurred within the mixing zone between these two fluids stable isotope analyses of ore stage dolomite show a progressively heavier carbon and oxygen isotope signature with increasing depth through an ore horizon consistent with the two solution interface model

microbe clay interactions as a mechanism for the preservation of organic matter and trace metal biosignatures in black shales abstract organic rich fine grained sedimentary rocks such as black shales are important geochemical archives providing information on the evolution of seawater composition and biological activity over the past 3 billion years while biological productivity and sedimentation rates greatly affect the organic matter content in these rocks mechanisms linking these two processes remain poorly resolved here we examine the interactions of clay minerals with the marine planktonic cyanobacterium synechococcus sp pcc 7002 we suggest that clays settling through the water column could influence carbon and trace metal burial in three ways 1 the interaction of reactive clay surfaces with the bacterial cells increases organic matter deposition via mass increase in a seawater growth medium by several orders of magnitude 2 reactive bacterial cells become completely encased within a clay shroud enhancing the preservation potential of this organic matter and 3 the trace metal content of the biomass buried along with metals sorbed to the clay particles contributes to the trace metal concentrations of the black shale precursor sediments significantly our findings imply that the chemical composition of ancient organic rich fine grained deposits are not only archives of ancient seawater composition and redox state but they also provide a record of the degree of biological activity in the water column through geological time

depositional age and sediment source of the auriferous moeda formation quadrilatero ferrifero of minas gerais brazil new constraints from u pb hf isotopes in zircon and xenotime abstract the moeda formation is a metasedimentary clastic sequence of witwatersrand like metaconglomerate and quartzite at the base of the minas supergroup quadrilatero ferrifero of minas gerais brazil which is also known for its world class iron ore deposits the moeda formation can be subdivided into three units units i to iii from bottom to top with each unit consisting of metaconglomerates at their base mostly auriferous grading into quartzite and locally into phyllite this study presents the first combined u pb and lu hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from units ii and iii these data indicate that deposition of sediments occurred 2 68 ga for unit ii and 2 62 ga for unit iii as supported by the youngest magmatic zircon grains the data further provide evidence that the source area of sediments was affected by crust re working during two major magmatic metamorphic events at 2 89 2 87 and 2 72 2 69 ga and minor events between 3 3 and 3 0 and at 2 80 and 2 65 ga this interpretation is supported by mainly subchondritic hf t between 10 and 1 4 and large hf t variations including all magmatic events a few superchondritic hf t obtained for the magmatic events at 3 3 2 9 and 2 7 ga point to limited juvenile crust addition the new dataset indicates that the clastic sediments of the auriferous moeda formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources which are suggested for the mesoarchaean auriferous sediments of the witwatersrand basin of south africa and those of the eastern goldfield province of australia

a study on the regulation of the concentrations of some trace metals rb sr zn pb cu v cr ni mn and mo in saanich inlet sediments british columbia canada abstract bulk metal concentrations were measured in the sediments of saanich inlet in an attempt to establish the occurrence of trace metal enrichment in the anoxic central basin ba ni v cr zn pb cu and mo were found to be enriched in the anoxic ooze over the possible contributions from lithogenic sources spatial and seasonal variations in the chemical composition of the settling particulates collected with interceptor traps gave further indications of the mode of incorporation of these metals biogenic ba and cr appeared to be associated with opaline silica although alternative explanations are also possible particularly for ba zinc seemed to be added to the sediment essentially in association with planktonic materials while cu required an additional source directly linked to the anoxic environment similarly ni v and mo were added to the anoxic sediments by reactions occurring at the sediment water interface in the nearshore environment studied here these metals particularly ni and mo were not associated to any significant extent with planktonic materials of all the elements analyzed mo showed the largest enrichment in the anoxic sediments of saanich inlet

geochemical and isotopic evidence for paleoredox conditions during deposition of the devonian mississippian new albany shale southern indiana the upper part of the new albany shale is divided into three members in ascending order these are 1 the morgan trail member a laminated brownish black shale 2 the camp run member an interbedded brownish black and greenish gray shale and 3 the clegg creek member also a laminated brownish black shale the morgan trail and camp run members contain 5 to 6 total organic carbon toc and 2 sulfide sulfur isotopic composition of sulfide in these members ranges from 5 0 to 20 0 c s plots indicate linear relationships between abundances of these elements with a zero intercept characteristic of sediments deposited in a non euxinic marine environment formation of diagenetic pyrite was carbon limited in these members the clegg creek member contains 10 to 15 toc and 2 to 6 sulfide sulfur isotopic compositions of sulfide range from 5 0 to 40 the most negative values occur in the uppermost clegg creek member and are characteristic of syngenetic pyrite formed within an anoxic water column abundances of carbon and sulfur are greater and uncorrelated in this member consistent with deposition in as euxinic environment in addition dop degree of pyritization values suggest that formation of pyrite was generally iron limited throughout clegg creek deposition but sulfur isotopes indicate that syngenetic water column pyrite becomes an important component in the sediment only in the upper part of the member at the top of the clegg creek member a zone of phosphate nodules and trace metal enrichment coincides with maximal toc values during euxinic deposition phosphate and trace metals accumulated below the chemocline because of limited vertical circulation in the water column increased productivity would have resulted in an increased flux of particulate organic matter to the sediment providing an effective sink for trace metals in the water column phosphate and trace metals released from organic matter during early diagenesis resulted in precipitation of metal rich phosphate nodules

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

anoxic vs dysoxic events reflected in sediment geochemistry during the cenomanian turonian boundary event cretaceous in the umbria marche basin of central italy abstract one of the most spectacular expressions of global oceanic anoxic events oaes of the cretaceous is the livello bonarelli of central italy deposited during the cenomanian turonian boundary event ctbe or oae2 at around 93 5 ma at furlo in the umbria marche basin in italy the bonarelli consists of approximately 1 m of c org rich shales interbedded with radiolarian rich layers and contrasts sharply with the extensive over and underlying siliceous limestones several minor c org rich intervals herein termed black levels average thickness 4 5 cm are interbedded within the scaglia bianca beneath the bonarelli x ray fluorescence and icp ms analyses show that these sediments consist of different mixing systems with three end members 1 sio 2 caco 3 scaglia bianca 2 average shale caco 3 black levels and 3 average shale sio 2 bonarelli given the sharp lithologic contacts and the little geochemical mixing between the groups these are thought to represent rapid switching events between sedimentary modes major element concentrations are generally low except for si and p owing to dilution by silica up to 96 and carbonates as al normalised ratios for elements such as ti or zr taken as proxies for higher energy environments are close to average shale values excess sio 2 is most likely linked to biogenic processes rather than quartz element al ratios of ti mg k rb and zr are fairly constant pointing to a mostly homogeneous source area the black levels and bonarelli are characterised by lower mg al ratios compared to the scaglia bianca sediments suggesting variability in chlorite input which could be linked with short term sea level drops indicated by ti al ratios the bonarelli sediments are severely depleted in mn which is indicative of oxygen depleted bottom water conditions and an open marine environment while the black levels appear to have been deposited in a much more restricted setting a model involving partial physical isolation of the furlo site by a sill coupled to omz variations by extra basinal forcing mechanisms is proposed to account for these conflicting data based on re mo ratios the bonarelli sediments were deposited under varying oxygen depletion levels from suboxic to anoxic while re mo ratios in the black levels show deposition under less oxygen deficient conditions most minor elements many of them redox sensitive or sulphide residing are strongly enriched in the c org rich sediments further documenting the absence of oxygen and potential availability of hydrogen sulphide in the water column at times while high zn concentrations suggest elevated submarine hydrothermal activity during this time interval ba is also present in very high concentrations up to 1 4 with an average weighed concentration of 0 6 for the entire bonarelli and is probably indicative of high paleoproductivity in this area this is further supported by high p concentrations hinting at nutrient availability at this site

lack of organic matter accumulation on the upwelling influenced somalia margin in a glacial interglacial transition abstract monsoon induced upwelling has been operating off somalia for at least the last 160 000 years making the nw indian ocean one of the most productive oceanic areas in the world we have studied the sediments recovered during the indusom cruise of the r v marion dufresne from three different environments off this margin a core from the highly productive surface waters core md85 674 a core where the oxygen minimum intersects the slope core md85 664 and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production core md85 668 though the first two environments are a priori favourable to organic matter om accumulation no marine om enrichment can be observed in the cores to understand this anomaly emphasis is placed upon the transition from isotopic stage 6 to stage 5 because the end of the glacial stage 6 is characterised by intensified upwelling and primary productivity in this region whereas stage 5 experienced a marked decrease in productivity various palaeoenvironmental markers such as om v ni cu zn ba and p have been used to identify periods of high productivity and or bottom water oxygen depletion the results suggest that 1 the lack of marine om in two settings favourable to organic carbon storage may be due to the nature of primary productivity coccolithophorid production would not be prone to om accumulation in contrast to the contribution of om by diatoms or particularly naked phytoplankton dinoflagellates or other peridinians 2 ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of ba as a proxy indicator 3 the period of intensified upwelling in stage 6 as well as the stage 6 5 transition left no geochemical imprint on the sediments this means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments in other words the somalia upwelling does not participate to the so called biological pump and has no positive effect upon long term carbon storage

trace metal signatures of eastern mediterranean sapropels abstract sapropel samples of pliocene to pleistocene age recovered during odp leg 160 from sites 964 ionian basin 969 mediterranean ridge 966 967 top and northern slope of the eratosthenes seamount south of cyprus and 968 cyprus margin were analysed for major and trace elements eastern mediterranean sapropels are characterized by high organic carbon and sulphur contents and by significant enrichments in several redox sensitive and or sulphide forming trace metals bi cd co cr cu mo ni re sb tl u v enrichment factors relative to average shale are comparable to those found in ctbe black shales the re content is extremely high in the sapropels studied with maximum values up to 1000 ng g re mo ratios averaging 2 7 10 3 are close to the seawater value high trace metal enrichments and re mo ratios close to the seawater value point to an anoxic water column during sapropel formation additionally enhanced ba concentrations in the sapropels support the assumption that bioproductivity was also high during these episodes trace metal signatures of sapropels originating from different drill sites on a transect across the eastern mediterranean are not only influenced by water depth but also by their location in the eastern mediterranean metal enrichments seem to be higher at the western sites compared with the eastern sites this probably reflects a change in the circulation pattern during periods of sapropel formation pliocene sapropels exhibit elevated trace metal contents in comparison with pleistocene examples therefore different palaeoceanographic conditions may have developed during the pleistocene compared with the pliocene altering palaeoceanography was probably controlled by climatic change which may be related to the onset of glaciation of the northern hemisphere decreasing summer precipitation throughout the late pliocene and the pleistocene may also have influenced sapropel deposition

geochemistry and mineralogical residences of trace elements in oil shales from julia creek queensland australia abstract abundances and mineralogical residences have been determined for a comprehensive range of trace elements in julia creek oil shale many trace elements are well above normal abundances in shales of these elements v and mo are potentially useful by products whereas as se mo cd tl and u are of possible environmental or occupational health concern samples from a stratigraphic reference bore core through the toolebuc formation near julia creek in queensland were chemically analysed using the multi element techniques of neutron activation analysis and spark source mass spectrometry inter element correlation techniques selective leaching procedures and electron microprobe analyses were used to establish geochemical associations and specific mineralogical residences for a full range of elements the results provide a definitive chemical and mineralogical characterization of the oil shale and identify the major mineralogical residences of the important trace elements v occurs mainly in a mixed layer mica montmorillonite clay mineral and to a lesser extent in kerogen whereas most trace elements of environmental concern are resident in the sulphide minerals pyrite and sphalerite u is concentrated in the fluorapatite of fish debris a sound basis is established for understanding the inorganic chemistry involved in oil shale processing and waste disposal

organic rich layers in the metochia section gavdos greece evidence for a single mechanism of sapropel formation during the past 10 my abstract three well developed 10 ma old sapropels from the island of gavdos greece were selected for a detailed geochemical micropaleontological and sedimentological study the sapropels are characterized by a high organic carbon and a low carbonate content productivity was higher during sapropel formation as indicated by increased ba contents and higher percentages of the planktonic foraminiferal species neogloboquadrina acostaensis enrichment of mo and v in the sapropels indicates less oxygenated bottom water conditions which is supported by the flourishing of low oxygen tolerant benthic foraminifera variation in grainsize and the ti al ratio point to an alternating dominance of fluvial sapropels and eolian derived homogeneous intervals terrigenous clastics evidence for a local origin of the enhanced fluvial input comes from the gradual replacement of the sapropels by cyclic turbidite sequences which were fed by bed load type rivers the increased fluvial input during sapropel formation increased the sedimentation rate and caused the observed carbonate depletion the results obtained for the gavdos sapropels are consistent with those of previous studies of younger sapropels and related caco 3 cycles in the mediterranean the consistency in most of the geochemical and micropaleontological characteristics indicates that all sapropels and related layers are the result of a single mechanism for at least the past 10 my namely precession induced dry wet oscillations in the mediterranean climate still the effect of this mechanism on the intensity of surface water productivity dilution and diagenetic signals may differ from site to site depending on location depositional environment and intensity of the astronomical forcing

two billion years of evolution enclosed in hydrothermal rutile recycling of the sao francisco craton crust and constraints on gold remobilisation processes abstract the quadrilatero ferrifero in the sao francisco craton brazil is known for its world class gold deposits hosted by the rio das velhas greenstone belt rdv as in other greenstone belts gold is also concentrated in late archean early paleoproterozoic supracrustal units in this case within the moeda formation this unit has been compared to the witwatersrand basin where there has been a long term debate over the nature of the gold and whether it is detrital placer hydrothermal or hybrid modified placer presently links between gold in the moeda formation and rdv are tenuous to enlighten this area of ambiguity a new approach using chemical isotopic and mineral inclusion data from detrital rutile found in the gold bearing horizons of the moeda formation is presented grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile related to as fe cu sb pb w rich mineralising fluids fe nb and v indicate that the likely source of the gold related rutiles is the archean banded iron formations within the rdv where most of the lode gold occurs average nb ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and or competing mineral phases u pb data yield primary crystallization ages older than deposition ages 2 58 ga supporting the detrital nature of these rutiles rutiles record either resetting or new growth at 2 1 ga and at ca 500 ma during which gold was remobilised hence this study supports a modified placer model for the gold bearing horizons of the moeda formation these fluid assisted gold remobilisation events can be found in other correlated paleoproterozoic gold bearing horizon units that represent intracratonic basins related to the rifting of the kenorland supercontinent this study reinforces the suitability of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance

global nature of the paleoproterozoic lomagundi carbon isotope excursion a review of occurrences in brazil india and uruguay abstract positive carbon isotope excursion is reported from paleoproterozoic carbonates of the aravalli supergroup northwestern india the minas supergroup brazil and new sections of the paso severino formation uruguay the 2 42 ga gandarela formation minas gerais brazil contains red carbonate facies bif grading into dolostones and limestones and yielding 13 c values ranging from 1 6 to 0 4 v pdb the positive c isotope excursion up to 11 v pdb in marine shallow water carbonates in india and brazil jhamarkotra formation in northwestern india and cercadinho and fecho do funil formations in minas gerais state brazil is comparable to that observed in 2 22 2 1 ga carbonate successions worldwide that were deposited during the lomagundi excursion in uruguay 13 c values up to 11 6 v pdb in the deep water paso severino formation of the piedra alta terrane are compatible with deposition at ca 2 15 ga as indicated by the 2146 7 ma u pb age of dacites occurring at the top of the unit negative 13 c values are also present in carbonates of the paso severino formation but an origin related to organic matter remineralization cannot be ruled out thin carbonate beds in the rio itapicuru greenstone belt bahia state brazil are associated as in the paso severino formation with deep water black shales and have carbon isotope values up to 9 v pdb high metamorphic grade carbonates of the jacurici terrane in the medrado ipueira area bahia brazil have carbon isotope values up to 6 9 v pdb consistent with their minimum age of 2085 5 ma inferred from the intrusive contact with and the age of the medrado norite no evidence was found in india brazil or uruguay for paleoproterozoic glacial events recognized in the 2 45 2 22 ga sedimentary successions worldwide unconformities between the gandarela and cercadinho formations in brazil and the banded gneissic complex and the lower aravalli supergroup in india might explain the absence of glacial record compositional and isotopic data presented here for studied paleoproterozoic carbonate successions allow their integration into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age the study highlights the global nature of the lomagundi excursion furthermore it indicates that the lomagundi excursion is recorded in both shallow water aravalli and minas supergroups and deep water carbonates paso severino formation and rio itapicuru greenstone belt negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow water open marine and isolated basins

the inorganic geochemistry of cretaceous black shales dsdp leg 41 in comparison to modern upwelling sediments from the gulf of california

a three stage structural evolution of the quadrilatero ferriifero consequences for the neoproterozoic age and the formation of gold concentrations of the ouro preto area minas gerais brazil abstract the quadrilatero ferrifero occurs in the southern part of the sao francisco craton brazil where three major proterozoic supracrustal units the rio das velhas minas and itacolomi supergroups are exposed together with archaean to palaeoproterozoic granito gneissic basement microtectonic and kinematic analyses have been undertaken in the area close to ouro preto in order to precise the tectonic evolution and to distinguish the tectonic features related to the transamazonian and brasiliano orogenies respectively 2000 and 600 ma three main tectonic events have been recognized the earlier event de is clearly expressed within basement rocks of the bacao complex and surrounding supracrustal units a stretching lineation le lineation roughly oriented n s can be observed numerous shear criteria such as drag folds and asymmetric pressure shadows indicate a top to the south sense of movement along the southern border of the bacao complex this event was developed within amphibolite facies conditions fact confirmed by quartz c axis fabrics this earlier deformation is related to plutonic activity and reactivation of the archaean basement expressed by the occurrence of scarce leucocratic granitoid rocks intruded within the bacao gneisses it is here attributed to the transamazonian orogeny in agreement with recent u pb dating 2000 ma the large scale folds i e mariana anticline dom bosco syncline which define the dominant structural geometry of the quadrilatero ferrifero were formed during this event the second tectonic event dm has been defined as a major thrusting towards the wnw related to the brasiliano cycle a well expressed stretching and mineral lineation oriented e w to nw se lm lineation and associated nw verging shear criteria characterize this event occurrence of muscovite biotite and more scarcely sillimanite indicate that deformation developed in upper greenschist lower amphibolite facies quartz c axis fabrics confirm the sense of shearing but the large scattering of several fabrics traduces a weak intensity of strain all supracrustal and gneiss formations were affected by the thrusting which is developed on a folded surface previously formed during the de event the third event dm is characterized by hangingwall down movement of the nappe pile in response to relaxation of the brasiliano compressional dm forces this event developed in greenschist facies is marked by the occurrence of metre scale drag folds sometimes associated to the development of a new lineation lm gold mineralization which occurs along the main mineralized contact located between the minas and rio das velhas supergroups is found within late veins which comprise large recrystallized quartz carbonate sulphide chlorite and tourmaline microscopic analysis and veins geometry allow us to conclude that the veins were emplaced mostly after the ductile dm deformation it is proposed that the veins were formed by filling of open cavities created during dm tectonics this study highlights that the northwest verging thrusting which is the main tectonic event of the quadrilatero ferrifero is only relevant of the brasiliano orogeny a model of structural evolution which involves plutonic activity at 2000 ma within the archaean bacao complex followed by neoproterozoic thrust tectonics brasiliano is proposed and discussed the age of gold concentration assumed to be achieved during the relaxation that occurred in response to the thrusting event is re interpreted and related to the late stage of the brasiliano tectonics

post depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east atlantic abstract two cores from a ne atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age thickness and composition diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes the diagenetic process is thought to be driven by bacterially mediated organic oxidation and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process in one core a long turbidite section emplaced 330 000 years ago is overlain by clay organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a u concentration peak below the oxidation front marked by a colour change the u source is believed to be u initially associated with the organic matter in the oxidised section of the turbidite vanadium and cu behave similarly but upward migration of the redox sensitive metals mn and ni is also seen in the second core an 8 cm turbidite section was emplaced about 170 000 years ago in a clay column in this case organic consumption now appears complete but evidence for diagenetic effects is found in a 16 cm compositional alteration halo of the underlying pelagic clay fe ii has been enriched and the hydrogenous component of mn co ni and cu removed from the pelagic clay to form the halo

relationship between metal enrichment and organic composition in kupferschiefer hosting structure controlled mineralization from oberkatz schwelle germany base metal mineralization is observed in some kupferschiefer localities whererote faule oxidized front of hematite bearing fluids is not present and where contemporaneous mesozoic to tertiary tectonism controlled the vein type metal enrichment processes after diagenesis structure controlled mineralization one of these localities is the spessart rhon treischfelder oberkatz schwellen region in the hessian depression germany in order to clarify the mechanism of base metal precipitation under these particular conditions a closely sampled kupferschiefer sequence from the oberkatz schwelle was investigated using organic and inorganic geochemical methods the structure controlled mineralization is enriched in as mo and u in the bottom part of the kupferschiefer and in the weissliegendes sandstones further elements such as cu ag co sb zn and pb show a redox controlled pattern of zonation in the kupferschiefer section these elements were precipitated or redistributed in the final stage of kupferschiefer mineralization participation of organic matter in base metal deposition is suggested by the depletion of h equivalents in the organic matter a similar phenomenon has been observed inrote faule controlled base metal enrichment in the north sudetic syncline of sw poland however observed differences in the composition of extractable organic matter might be due to the oxidizing potential of the migrating metal bearing fluids in the north sudetic syncline the ascending fluids were more oxidizing compared with the oberkatz schwellen area additional s content and depletion of corg are observed in the bottom section of the profile the h index hi values from rock eval analyses provide further evidence for the depletion of h equivalents in the organic matter reconstruction of the thermal regime by the measurement of vitrinite reflectance and determination of molecular rank parameters indicate a maximum temperature in the kupferschiefer of around 150 c

trace metals in metalliferous sediments manop site m interfacial pore water profiles we present data showing enrichments of metals mn cu v cr cd and ni and dissolved organic matter dom in surficial pore waters depths 1 cm at a metalliferous sediment site in the pacific we also present a model of metal release during particulate organic carbon poc oxidation and opal dissolution at the sediment seawater interface a comparison of measured interfacial metal concentrations with those predicted from the model suggest that cd ni and cr are released during oxidation of poc at the sediment seawater interface however for v cu and mn additional source s and or processes are required to account for the measured interfacial concentrations for cu recycling via interfacial remineralization and release to bottom waters with subsequent scavenging out of bottom waters by particulates probably provides the major additional source for v we suggest that in addition to some contribution from a benthic flux bottom water scavenging scenario complexation with dom at the interface serves as a trap to accumulate v released from particulates in the zone of oxygen reduction for manganese we suggest that most of the interfacial pore water content is produced from reduction of sedimentary mn iv by reactions with dom at the sediment seawater interface the calculated benthic flux of manganese as a result of this reaction may account for a large fraction of the particulate manganese rain that is apparently not preserved in the sediment at this site

twentieth century tropical sea surface temperature trends revisited 1 this study compares the global distribution of 20th century sst and marine air temperature trends from a wide variety of data sets including un interpolated archives as well as globally complete reconstructions apart from the eastern equatorial pacific all datasets show consistency in their statistically significant trends with warming everywhere except the far northwestern atlantic the largest warming trends are found in the middle latitudes of both hemispheres two of the sst reconstructions exhibit statistically significant cooling trends over the eastern equatorial pacific in disagreement with the un interpolated sst and marine air temperature datasets which show statistically significant warming in this region twentieth century trends in tropical marine cloudiness precipitation and slp from independent data sets provide physically consistent evidence for a reduction in the strength of the atmospheric walker circulation accompanied by an eastward shift of deep convection from the western to the central equatorial pacific

late cretaceous cenozoic tectonic sedimentary evolution and u enrichment in the southern songliao basin abstract late cretaceous cenozoic tectonic sedimentary evolution is fundamental to u uranium migration and u enrichment in the southern songliao basin northeast china to establish the genetic connection between tectonic sedimentary evolution and u enrichment this paper presents a mineralogical study apatite fission track aft analysis and sem cl analysis of quartz grains two stages of mineralization were proposed including the u pre enrichment stage during the late cretaceous and the ore forming stage during the cenozoic based on the density plots of the aft ages paleoclimate studies and sem cl analysis of quartz grains it is inferred that the u pre enrichment was closely related to the rapid uplift of the great xing an range and the zhangguangcai range and the o2 rich atmosphere during 97 90 ma in the southern songliao basin the o2 rich atmosphere increased u mobility and led to the formation of red beds during the late cretaceous which widely developed u pre enrichment in the basins of northeast china the mineralogical study reveals that the qianjiadian uranium deposit developed in two stages the diagenetic process experienced the transition from early alkaline to late acidic conditions to promote the stage i mineralization u pre enrichment the dominant occurrence spaces for u were the dissolution pores of quartz tightly packed with calcite cement u enrichment during alkaline ambient conditions was mainly via adsorption with the increase of the buried depth the early alkaline diagenesis returned to acidic conditions due to the abundant organic material u vi was reduced to pitchblende due to the reducing capacity of the organic matter om stage ii ore forming stage mineralization was strictly related to the hydrocarbon re concentration according to the thermal history study of the cc1 and q2 boreholes the southern songliao basin experienced four episodes of rapid uplift during the cenozoic at 83 80 ma 68 52 ma 33 23 ma and 8 ma respectively the cenozoic uplifts led to hydrocarbon migration at 65 ma 23 ma and present the re concentration of hydrocarbon prompted the u mineralization in the qianjiadian uranium deposit in the southern songliao basin during the stable period of tectonic movements the reducing conditions caused by hydrocarbons promoted u mineralization the dominant spaces of coffinite were intergranular dissolved pores formed during the transformation from the feldspar to the kaolinite under acidic conditions the post rift phase and structural inversion phase directly controlled the u pre enrichment stage and the ore forming stage respectively overall the tectonic sedimentary evolution played an important role in controlling the u leaching from granites the dominant occurrence spaces and u recycling

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

environmental response to volcanic activity and its effect on organic matter enrichment in the permian lucaogou formation of the malang sag santanghu basin northwest china abstract lacustrine organic rich fine grained rocks are widespread in the second member of the permian lucaogou formation p2l2 in the malang sag santanghu basin with complex lithology and significant changes of lithofacies due to frequent volcanic activity in this study the mineralogical and petrological characteristics types of organic matter om and effect of volcanism on the sedimentary environment and om accumulation were analyzed using organic carbon and oxygen isotopes as well as major and trace element data the results revealed that the p2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals with four main rock types tuff dolomitic tuff tuffaceous dolomite and dolomite the fine grained rocks dominantly contain type i and ii1 kerogens which were in a low mature stage and exhibited excellent original hydrocarbon generative potential when volcanic activity was strong large amounts of nutrients were released by the volcanic ash which then entered the lake in turn promoting the blooming of phytoplankton and causing the 13c values in the tuff to decrease in addition the paleotemperature decreased for a short time and the suboxic euxinic environment was conducive to the preservation of om resulting in high total organic carbon toc content in the tuff in contrast during intermittent or weak periods of volcanic activity the climate was hotter and drier and evaporation resulted in a sustained increase of salinity consequently the growth of algae was restricted which brought about an increase in the 13c values and relatively low toc content in the dolomite the relationships among primary productivity redox conditions clastic influx proxies and toc content illustrate that primary productivity was the most vital control on the accumulation of om high primary productivity was mainly driven by volcanic ash falls meanwhile high salinity and suboxic euxinic conditions were beneficial to the preservation of om giving rise to a high degree of om enrichment

vanadium accumulation in carbonaceous rocks a review of geochemical controls during deposition and diagenesis abstract published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks reduction adsorption and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon dissolved vanadate v v species predominate in oxic seawater and are reduced to vanadyl ion v iv by organic compounds or h 2 s vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle the large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff vanadium rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate rich units a correspondence between vanadium content and organically bound sulfur is consistent with high activities of h 2 s during sediment deposition excess h 2 s exited the sediment into bottom waters and favored reduction of dissolved v v to v iv or possibly v iii the stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic phosphate concentrated to more reducing euxinic conditions that favor vanadium accumulation during diagenesis some vanadium organic complexes migrate with petroleum out of carbonaceous rocks but significant amounts of vanadium are retained in refractory organic matter or clay minerals as carbon in the rock evolves toward graphite during metamorphism vanadium is incorporated into silicate minerals

carbon isotopic composition and lattice bound carbonate of peru chile margin phosphorites new light stable carbonate carbon isotope and lattice bound co2 data from quaternary peru chile margin phosphatic nodules crusts and pelletal grains and from associated dolomicritic concretions are presented which provide constraints on the timing and mechanisms of growth of these phases in organic carbon rich sediments comparison of 13c values from carbonate fluorapatite cfa nodules and pelletal grains 4 8 to 0 0 and 2 9 to 1 0 pdb respectively with pore water total dissolved 13c values from these sediments suggests early authigenic cfa precipitation from pore waters within a few centimeters of the sediment water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter in contrast the dolomicritic cores of nodules recovered from about 12 s display both strongly negative to positive 13c values 10 8 to 6 1 characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes the amount of structural carbonate in cfa suggests that carbonate substitution generally increases as 13c in cfa decreases a probable consequence of increasing carbonate and accompanying charge balancing substitutions in the cfa lattice in response to increasing pore water carbonate ion concentrations with depth below the sediment water interface in one buried upward growing nodule decreasing cfa 13c and increasing structural co2 also correspond to decreasing cfa growth rates these data suggest that in addition to other constraints such as pore water phosphorus and fluoride availability the lower limit of cfa precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations in organic rich peru chile margin sediments this depth threshold appears to be at approximately 5 10 cm below the sediment water interface where maximum cfa co2 contents of about 6 wt occur in less organic rich settings greater depths of precipitation of cfa may be anticipated below this relatively shallow depth of cfa precipitation on the peru shelf high pore water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates

organic matter production and preservation and evolution of anoxia in the holocene black sea dating of sediments collected in gravity cores during leg 1 of the 1988 r v knorr expedition to the black sea suggests that the onset of water column anoxia at 7 5 ka was virtually synchronous across the basin over a depth range of 200 2250 m a finely laminated organic carbon oc rich sapropel unit ii was produced as a result of this anoxia the trigger for increased oc production and development of anoxia was the spillover of saline waters through the bosporus that probably began at 9 0 ka and peaked between 7 0 and 5 5 ka this spillover enhanced vertical mixing and nutrient cycling and caused a short term 2 3 kyr burst in surface water productivity during the early part of unit ii deposition continued incursion of saline waters enhanced vertical stability and inhibited mixing of nutrients into surface waters thus limiting primary production and decreasing the oc flux to sediments beginning 5 5 ka concentration accumulation rate and degree of preservation of organic matter all decreased in the upper part of unit ii as a result of decreasing productivity but anoxia persisted throughout most of the water column the end of unit ii sapropel deposition was synchronous across the black sea as the result of the first blooms of the coccolith emiliania huxleyi which presumably marked an increase in surface water salinity above 11 and the beginning of unit i deposition the high coccolith carbonate fluxes that occurred during deposition of unit i diluted the oc concentration in the sediments but oc accumulation rates are about the same as those in upper part of unit ii

tracking the onset of phanerozoic style redox sensitive trace metal enrichments new results from basal ediacaran post glacial strata in nw canada abstract a global rise in oxygen levels has been proposed to coincide with the ediacaran to cambrian radiation of animals yet the precise timing and nature of this change remains unresolved one hypothesis is that the ocean atmosphere system became temporarily well oxygenated in the earliest ediacaran directly following the marinoan snowball earth glaciation 635 ma the evidence for oxygenation is based on large enrichments of redox sensitive trace elements in black shale from south china presumably deposited under a euxinic water column these enrichments meet or exceed the bulk concentrations of redox sensitive trace elements found in phanerozoic shale deposited under euxinic water columns here we test the early ediacaran post snowball oxygenation hypothesis with new data from a high resolution multi proxy geochemical and sedimentological study of three stratigraphic sections in the earliest ediacaran sheepbed formation of the mackenzie and wernecke mountains nw canada iron speciation data from all sections suggest that the local water column was dominantly ferruginous with a notable exception of probable euxinic conditions recorded in part of one section redox sensitive elements show no appreciable enrichments in the basal sheepbed formation with maximum concentrations of molybdenum vanadium uranium and chromium only slightly above world average shale values the lack of substantial elemental enrichments within the sheepbed formation is consistent with results from coeval ferruginous strata in svalbard the dominance of local ferruginous conditions in nw canada cannot alone account for the muted redox sensitive element enrichments as v u and cr are thought to be enriched under these conditions and samples deposited under euxinic conditions also lack phanerozoic style enrichments these contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution

vanadium geochemistry along groundwater flow paths in contrasting aquifers of the united states carrizo sand texas and oasis valley nevada aquifers abstract vanadium v concentrations were measured along the flow paths in two contrasting aquifer systems to study how changing solution composition and redox conditions affect v cycling and transport in groundwater flow systems the oasis valley groundwater system in nevada is an unconfined system where groundwaters generally remain oxic along the flow path whereas the carrizo sand aquifer is a deep confined coastal plain aquifer in texas that contains well defined fe iii and so 4 2 reduction zones along the flow path speciation modeling predicts that in groundwaters from the oasis valley v chiefly occurs as the vanadate oxyanion e g h 2 vo 4 hvo 4 2 which is mobile along the entire flow path because of vanadate s solubility in oxic and alkaline waters furthermore the data indicate that v concentrations increase in groundwaters along the flow path suggesting that mineral dissolution and or desorption reactions progressively contribute v to groundwaters of the oasis valley system geochemical modeling is consistent with dissolution of mafic phenocrysts that are common in the local tuffaceous aquifer rocks as a probable source of v to oasis valley groundwaters within the carrizo sand aquifer groundwater v concentrations decrease with flow beyond the recharge zone and into the confined portion of the aquifer thereafter total v concentrations remain low and relatively constant along the remainder of the studied flow path exhibiting no systematic variations across the fe iii and so 4 2 reduction zones groundwaters from the carrizo sand aquifer are generally reducing and as such geochemical modeling suggests that both v v and v iv species occur in carrizo sand groundwaters and furthermore that the proportion of v iv species increases concurrent with the decrease in total v concentrations along the flow path results from sequential extraction experiments on carrizo sand aquifer sediment samples suggest that v adsorption onto fe iii mn iv oxides oxyhydroxides acts to decrease the aqueous v concentrations in carrizo sand groundwater furthermore geochemical modeling is consistent with complexation of v iv with dissolved organic matter dom within the carrizo sand aquifer groundwaters which owing to the low dom concentrations appears to buffer v at low but relatively constant concentrations within the confined and reducing carrizo sand groundwaters these results support previous reports of v iv dom binding and imply that aquifer material maintains an essential role in not only determining the availability of v to enter aqueous solution but also in influencing partitioning between solid and aqueous phases

sequential extraction of iron manganese and related elements in s1 sapropel sediments eastern mediterranean a detailed sequential extraction scheme mesex was developed to distinguish fe and mn phases in eastern mediterranean sediments after calibration and validation using single natural minerals and a mixed mineral standard mesex was applied to holocene eastern mediterranean sediments from 1 a former brine basin and 2 a normal setting including the organic rich sapropel s1 in both sediments the mn ii mn iii iv redox boundary is clearly distinguished with carbonate related mn being enriched below and mn oxides occurring only above it a clear fe ii fe iii redox boundary is present in the normal eastern mediterranean sediments and is located just below the mn ii mn iii iv redox boundary the fe ii fe iii boundary closely coincides with the visible boundary between the lower unoxidised and the upper oxidised burned down part of the original sapropel s1 interval pyrite is only present within the unoxidised interval whereas fe oxides are abundant above it the fe ii fe iii redox boundary in the former brine sediments resides on top of the organic enriched sediments as deduced from the distribution of fe oxides and pyrite as a result of burn down of the sapropel s1 unit in the normal sediments a vanadium peak has been formed below the oxidation front whereas no such v peak was observed in the former brine sediments this together with the location of the mn ii mn iii iv redox boundary well above the organic enriched sediments suggests that no burn down has taken place into the sapropelic sediments of the former brine basin resulting in the preservation of the relationships of trace metal to organic carbon or pyrite in these latter sediments mn and co appear to be associated with pyrite whereas ni cu and zn as well as v are more related to organic matter on the basis of mesex variations have also been detected in the terrigenous component i e aluminosilicates fe al clay mg al clay and several trace metal to al ratios are higher in sapropelic than in non sapropelic sediments these results indicate that sources of terrestrial material have been different during sapropel times compared to the period immediately after it on the other hand the fairly constant k alclay ratio suggests that no major changes may have occurred in the relative illite contribution

estimating specific surface area of fine stream bed sediments from geochemistry specific surface area ssa of headwater stream bed sediments is a fundamental property which determines the nature of sediment surface reactions and influences ecosystem level biological processes measurements of ssa commonly undertaken by bet nitrogen adsorption are relatively costly in terms of instrumentation and operator time a novel approach is presented for estimating fine 2 5 mg kg 1 four elements were identified as significant predictors of ssa ordered by decreasing predictive power v ca al rb the optimum model from these four elements accounted for 73 of the variation in bed sediment ssa range 6 46 m2 g 1 with a root mean squared error of prediction based on leave one out cross validation of 6 3 m2 g 1 it is believed that v is the most significant predictor because its concentration is strongly correlated both with the quantity of fe oxides and clay minerals in the stream bed sediments which dominate sediment ssa sample heterogeneity in ssa based on triplicate measurements of sub samples was a substantial source of variation standard error 2 2 m2 g 1 which cannot be accounted for in the regression model the model was used to estimate bed sediment ssa at the other 1792 sites and at 30 duplicate sites where an extra sediment sample had been collected 25 m from the original site by delineating sub catchments for the headwater sediment sites only those sub catchments were selected with a dominant 50 of the sub catchment area bedrock formation and land use type the bedrock and land use classes accounted for 39 and 7 of the variation in bed sediment ssa respectively variation in estimated fine bed sediment ssa from the paired duplicate sediment sites was small 2 7 m2 g 1 showing that local variation in ssa at stream sites is modest when compared to that between catchments how the approach might be applied in other environments and its potential limitations are discussed

metal accumulation during and after deposition of the kupferschiefer from the sangerhausen basin germany abstract a densely sampled profile 58 cm in thickness composed of 13 samples of the kupferschiefer and overlying zechstein carbonates from the sangerhausen basin germany has been analysed by various geochemical and microscopic methods in order to clarify the mechanism of base metal accumulation in this location the kupferschiefer is only slightly influenced by the hematite bearing oxidized fluids called rote faule the determination of facies dependent parameters along the profile indicates that kupferschiefer from the sangerhausen basin was largely deposited in a marine environment only at the beginning of kupferschiefer sedimentation did euxinic conditions prevail the bottom part of the profile is significantly enriched in trace elements such as cu ph zn as co ag and u the cu concentration amounts to 19 88 wt post depositional oxidation of the organic matter is observed only in the transition zone between the kupferschiefer and the zechstein conglomerate indicating the influence of ascending oxidizing brines microscopic analyses show that only fe sulfides form framboidal textures cu minerals are present along the total profile preferentially in fractures and as patchy structures composed of chalcocite chalcopyrite and bornite in the highly mineralized bottom section cu sulfides are associated with pyrobitumen sparry calcite and arsenopyrite results from maturation studies of organic matter suggest that the maximum temperature affecting the kupferschiefer was approximately 130 c a 3 step process of metal accumulation is proposed during deposition of the sediment framboidal pyrite and pyrite precursors were precipitated by bacterial so 4 2 reduction bsr during diagenesis the pyrite and pyrite precursors were largely replaced by mixed cu fe minerals and by chalcocite pr in the section with very high cu contents 8 reduced sulfur from fe sulfides was not sufficient for precipitation of cu and other trace metals from ascending solutions in this part of the profile thermochemical so 4 2 reduction tsr occurred after pyrite replacement as indicated by the presence of pyrobitumen and sparry calcite

dwindling vanadium in seawater during the early cambrian south china abstract elemental vanadium v an essentially redox sensitive metal in seawater has had a significant impact on the understanding of the evolution of the atmosphere ocean system throughout the history of the earth in fact the geochemical cycle of v in early cambrian seawater may have had an influence on the chengjiang biota in south china however it has not yet been well established given the authigenic vanadium accumulation is sensitive to the redox conditions of seawater here to constrain the geochemical cycle of v in seawater during the early cambrian the mo u and total organic carbon toc distributions with high resolution samples from both the outer shelf and slope facies e g the duoding and longbizui sections are applied to evaluate the redox conditions of ambient seawater the mo u relationships indicate that the redox conditions of the mid depth seawater evolved in a systematic way in south china transitioning from an fe mn reduction zone to anoxic intermittently euxinic states and then to oxic conditions during the early cambrian as a consequence the authigenic v enrichment constrained by the marine redox conditions was mainly controlled by the fe mn particulate shuttle and the reduction and adsorption of organic matter in anoxic euxinic conditions however the decoupling among v mo u and toc under anoxic euxinic conditions suggests a dwindling vanadium concentration in the early cambrian seawater of south china the scavenging efficiency of v from seawater is much higher than those of mo and u under anoxic euxinic conditions ultimately these trace elements e g mo u and especially v in seawater could effectively be regulated and adjusted to a reasonable level under the widespread anoxic euxinic conditions the drawdown of trace elements in seawater might provide an early stage preparation of the marine environment for the subsequent chengjiang biota

mineralogical characterization and origin of uranium mineralization in witwatersrand like metaconglomerate of the moeda formation quadrilatero ferrifero brazil abstract the moeda formation is the basal unit of the minas supergroup in the quadrilatero ferrifero brazil and has quartz pebble metaconglomerate that hosts au u mineralization commonly assumed to be detrital in origin the au u mineralization occurs immediately above an unconformity with the underlying archean rio das velhas greenstone sequence in this work we focus on the moeda metaconglomerate in its u bearing portions in the gandarela syncline the southern sector near the syncline axis and the western limb near the palmital gold deposit the metaconglomerate has mica seams that host zircon xenotime rutile nodules of organic matter and sulfide minerals the sulfide assemblage includes four generations of pyrite along with fe ni cu co zn pb mo sulfides a sb as sulfosalt assemblage of gersdorffite stibnite and ullmannite is late in relation to pyrite and other sulfides gold is argentiferous with minor hg uranium minerals include uraninite coffinite brannerite and uranyl arsenates an early generation of uraninite and brannerite occurs as inclusions in organic matter nodules mainly in the southern sector a late generation of uraninite formed due to a low temperature hydrothermal overprint coffinite and late stage brannerite are products of hydrothermal alteration of zircon and rutile respectively uranyl arsenates have ni as the prevalent cation they formed as replacement of arsenic minerals such as gersdorffite by oxidized fluids containing u microstructural data and mineral chemistry favor a hydrothermal hypothesis analogous to an unconformity related setting rather than a detrital placer deposit model for the occurrence of u in the moeda metaconglomerate

alteration halos in the tordillos sediment hosted copper deposit of the neuquen basin argentina abstract the tordillos sediment hosted cu deposit is located in the north central sector of the huincul high structure in the neuquen basin argentina it is hosted in sedimentary rocks of the huincul formation an upper cretaceous redbed sequence deposited in a fluvial environment field mapping petrography and chemical analysis of the authigenic clay sulfate carbonate and sulfide minerals at tordillos have identified systematic variation in alteration and paragenesis with time and with respect to the main feeder zones the alteration associated with cu mineralization provides evidence of a multistage infiltration of fluids spatially associated with paleo hydrocarbon migration driven by tertiary tectonism early diagenesis formed hematite kaolinite and calcite1 that have been preserved in the reddish brown mudstones later influx of hydrocarbons and warm reduced formation waters from deeper reservoir rocks is associated with dissolution of early cements and clasts and precipitation of new minerals in the thickest paleo channels of the huincul formation dissolution of early cements hematite kaolinite calcite1 and barite1 and clasts locally increased the porosity of sandstones up to 24 and destroyed hematite that changed sandstones and conglomerates from red to yellowish grey bitumen impregnations are abundant along with calcite2 3 and 4 pyrite minor montmorillonite quartz overgrowth and coarse barite2 crystals with primary organic fluid inclusions at the contact with hydrocarbon tubes the mno enrichment of calcite in stages 3 and 4 reflects their precipitation under reduced conditions grey and greenish grey sandstones with montmorillonite v hematite smectite chlorite group minerals and pyrite respectively are exposed between the reddish brown mudstones and the yellowish grey sandstones and conglomerates the grey and greenish grey horizons are representative of abrupt changes in eh ph conditions between regional early oxidizing solutions responsible for the formation of early diagenetic redbeds and later infiltration of reducing fluids responsible for alteration of sedimentary rocks copper mineralization is located inside the altered sandstones and conglomerates zones forming halos surrounding vertical tubes containing bitumen these tubes are interpreted as the conduits through which migrated hydrocarbons formation waters and subsequent chloride and metal rich fluids in these places pyrite calcite and barite cements of the altered rocks have been dissolved and replaced by copper rich sulfides and then by late supergene cu minerals interconnection among stacked permeable paleo channels in the mid section of the huincul formation facilitated the migration of several generations of fluids during early diagenesis hydrocarbon up flow and exhumation of the neuquen group resulting in the development of alteration halos and cu mineralization that can be used both in outcrop and drill core during exploration for sediment hosted cu mineralization

mineralogy and geochemistry of palaeozoic black shales from peninsular malaysia implications for their origin and maturation abstract palaeozoic black shale bearing formations comprise 25 of the sedimentary cover in peninsular malaysia however nothing has been published regarding their geology composition and maturation representative samples from different ages and localities of the black shales were subjected to detailed mineralogical and geochemical investigations during this study to determine their origin these investigations also provide an opportunity to trace the changes in depositional environments and paleoclimate during the palaeozoic as well as the nature of unknown and hidden basement rocks in malaysia this paper reports nineteen palaeozoic formations in malaysia that contain black shales that are classified into seven categories based on their age these are the cambrian ordovician ordovician devonian silurian devonian devonian carboniferous carboniferous permian and permian black shales although illite was identified in all black shales other clay minerals including kaolinite and smectite illite mixed layers show significant variations among the studied samples the al 2 o 3 tio 2 ratios 13 33 sio 2 contents of probable magmatic source rocks 54 69 wt and zr tio 2 bivariate diagram suggest a mixture of felsic and intermediate source rocks for the studied black shales this indicates that the hidden basement rocks in malaysia are dominated by felsic igneous rocks variations in clay minerals sio 2 and al 2 o 3 tio 2 ratios and upper continental crust ucc normalized ree patterns suggest a local provenance for each locality rather than regional and common source rocks for all black shales al 2 o 3 sio 2 ratios vary from one age to the other ranging between 0 09 and 0 37 this indicates changes in the degree of weathering of the sediment source rocks of these shales and suggests climate changes in the peninsular malaysia during the palaeozoic from relatively wet conditions during the cambrian to relatively dry conditions during the ordovician followed by relatively wet conditions extends from the silurian to the carboniferous which followed by relatively dry conditions during the formation of carboniferous permian black shales and finally to relatively wet conditions during the permian the v ni v v ni and v v cr ratios as well as the positive ce anomalies indicate that these black shales were deposited under reducing conditions the high v ni ratios 3 also suggest a purely marine source input for the organic matter devonian black shales show relatively high concentrations of redox sensitive elements which may suggest the possible existence of devonian anoxia as well as the devonian carboniferous d c boundary in malaysia based on the abundance of illite 85 and illite crystallinity ic values 27 46 2 the majority of the palaeozoic black shales in peninsular malaysia are probably anchimetamorphic and over matured

anoxia and fluctuating climate recorded from the devonian carboniferous black shales tethys himalaya india a multi proxy approach due to inaccessibility and tough conditions high altitude low oxygen region very less information is available about the indian tethys himalayan sequences particularly geochemical data are scarce an attempt is made in the present study to evaluate the sediment geochemistry and paleo redox conditions of devonian carboniferous black shales from the spiti region tethys himalaya using multi proxy approach besides black shales this paper also provides the first in depth study of the geochemistry of phosphatic nodules hosted by the black shales that will allow primary and secondary processes to be understood geochemical investigations particularly redox sensitive trace element ratios for e g v v ni 0 5 ni co 4 3 7 2 v cr 2 and positive ce anomalies suggest deposition of these black shales under sub oxic to anoxic marine conditions thus recording an ocean anoxic event in the region these ocean anoxic events of devonian carboniferous spiti black shales which possess distinctive geochemical signatures can very well be correlated with global events such as hangenberg events and the lower alum shale event the span of the chemical index of alteration cia values 54 78 variation of 4si parameter 82 3 and c values 0 1 0 6 recorded in the black shales strongly indicate varying degrees of weathering conditions in the source area which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in devonian carboniferous period in the spiti region the distinct geochemical characteristics such as y sr and rare earth element ree enrichments particularly middle ree enrichments with a hat shaped pattern with high p2o5 contents up to 14 wt distinguish the nodules from the host rocks black shales sem scanning electron microscope results supplemented by xrd x ray diffraction studies of the nodules show the dominance of calcium silica and phosphatic phases the diversity of clay minerals significant variations in mineralogical composition and lack of any continuous vertical trend in the nodules suggest a relatively low impact on the primary environmental signal by burial diagenesis

geologic framework of phosphate resources in onslow bay north carolina continental shelf high resolution uniboom sparker and 3 5 khz subbottom seismic profiles in combination with 9 m vibracores from onslow bay delineate a belt of neogene sediments which crop out on the north carolina continental shelf around the east flank of the mid carolina platform high this broad southeast trending basement high has controlled the general distribution of cenzoic sediment sequences within the mid atlantic continental margin the neogene section includes the miocene pungo river formation which thickens downdip to the east and southeast off the mid carolina platform high the pungo river formation consists of three third order and at least 21 fourth order seismic depositional sequences many of which are abruptly truncated by erosional surfaces and associated channels these seismic sequences reflect third and fourth order eustatic sea level cycles within the miocene second order transgressive sea level cycle the fourth order seismic sequences appear to be equivalent to cyclic lithostratigraphic units characterized by interbedded phosphorite sands phosphatic foraminiferal muds calcareous quartz sands calcarenites and dolosilts some phosphorite sands also occur in the holocene sand sheet which forms a thin and discontinuous cover on the pungo river sediments in the frying pan phosphate district preliminary evaluation of phosphate resources in onslow bay has delineated several potential economic prospects the pungo river formation crops out in a northeast southwest trending belt which is 150 km long by 25 to 50 km wide and extends into the subsurface to the east and southeast in two different areas within onslow bay vibracores have penetrated five beds which contain anomalous concentrations of phosphate two beds within the frying pan phosphate district are estimated to contain 3 75 billion metric tons of phosphate concentrate with an average content of 29 2 percent p 2 o 5 the other three beds in the northeast onslow bay phosphate district are extensive units with low to intermediate phosphate content within the total sediment up to 9 8 p 2 o 5 these three lean deposits contain at least 780 million metric tons of phosphate concentrate with average p 2 o 5 contents which range from 29 7 to 31 percent phosphate resources in the frying pan phosphate district have a moderate to high potential for economic development whereas those in the northeast onslow bay phosphate district have a low potential for economic development this newly discovered continental shelf phosphate province could become economically feasible to mine in the near future when rapidly expanding demand for fertilizers and agricultural products combined with ever increasing land use pressures produces an unacceptable escalation in the cost of mining land based reserves

plume dispersed hydrothermal particles a time series record of settling flux from the endeavour ridge using moored sensors abstract settling particles collected with moored sediment traps near an area of active hydrothermal venting in the northeast pacific ocean provide information about the deep sea distribution of hydrothermal material results of an 8 day near field single trap experiment and a 340 day multi instrument 5 trap deployment two kilometers from the endeavour ridge vent field show variability in the quantity and composition of the plume to sediment particle flux over time with depth and with distance from the vent field hydrothermal elemental fluxes vary by a factor of two among the four sampling intervals of the longer experiment a factor of three variation in fe s among these periods reflects temporal changes in phase composition changes in flux with depth for example a 200 fold increase in cu and zn between above and below plume traps are evidence of hydrothermal input comparison of fluxes determined in the near and far field experiments reveals the spatial influence of vent effluent near field particle flux exceeds that measured two kilometers away by a factor of 10 to 20 and is relatively enriched in fe cu and organic carbon but depleted in mn calculations of the residence time of the plume with respect to settling indicate that more than 90 percent of the particulate material is transported farther than two kilometers from the vent field interpretation of elemental mineralogical and grain size analyses suggest that chemical precipitation kinetics other physical and biological particle forming processes changes in vent output and lateral transport of hydrothermal material by abyssal currents are tenable explanations for our observations our findings suggest that these rate interdependent processes be included in realistic models of hydrothermal particle flux

large scale bleaching of red beds related to upward migration of hydrocarbons los chihuidos high neuquen basin argentina abstract the striking arrangement of colorful facies in the sandstones outcrops of the los chihuidos high argentina has been attributed to the surface expression of hydrocarbon induced alteration the huincul formation late cenomanian early turonian is part of the thick continental red bed dominated neuquen group which consists of meandering fluvial deposits developed in a foreland basin visual distinctions reveal four diagenetic facies red white gray and minor brown sandstones differences in the diagenetic mineralogy among sandstones facies are interpreted to depend on the degree of interaction with hydrocarbon bearing solutions and the relative position of the redox roll front developed during this process the red facies represents the original oxidized sandstone with hematite kaolinite quartz and albite overgrowths and calcite cement precipitated under oxidizing conditions during the regional burial diagenesis the white facies formed during the interaction of red beds with reduced fluids in presence of hydrocarbons or organic acids where clasts and cement were partially dissolved and iron was reduced to fe2 due to redox reactions and removed resulting in the sandstone decoloration and in a marked development of the secondary porosity minor montmorillonite replaced kaolinite and detrital feldspars and a poikilotopic calcite precipitated filling the pores the gray and the brown sandstone bound the redox front which is characterized by the presence of abundant clay minerals and oxide minerals enriched in vanadium and copper montmorillonite and secondary hematite are dominant in the more oxidized gray sandstones whereas interstratified chlorite montmorillonite is more abundant in the more reduced brown sandstones the coexistence of these minerals indicates metastable reducing oxidizing conditions at the redox front based on diagenetic characteristics tectonic framework maturation timing of source rocks and petroleum emplacement history hydrocarbons could have migrated into the huincul formation during the tertiary uplift but later erosion could have released the hydrocarbons and exhumed the reservoir this multi scale investigation of diagenetic features related to burial history and to structure controlled upflow of hydrocarbons in a succession of siliciclastic rocks enhances modeling of a regional scale redox system worldwide our study offers new insight into the application of authigenic minerals to trace hydrocarbon pathways along red beds and to evaluate the reservoir quality in the understanding the importance of fluid compositions mixing and fluid rock interaction along a major redox system

early proterozoic gold placers of the moeda formation within the gandarela syncline minas gerais brazil gold bearing placer conglomerates dated between 2 2 and 2 8 ga have been identified at the base of the moeda formation in the gandarela syncline intensive investigation of outcrop and underground exposures both old and new and borehole intersections around the western and southern margin of the syncline has defined a paleoenvironment consisting of an upland fluvial drainage system in the north leading southward to an alluvial fan and braided plain depository heavy mineral concentrates composed predominantly of coarse rounded pyrite are concentrated in the matrix of cobble conglomerates that occur in the form of discrete bars and sheets the mineralogy and geochemistry of the ore closely resembles that of witwatersrand placers as does the composition of the gold gold concentrations are equal to those of the average witwatersrand gold deposit and therefore these deposits may make a significant contribution to brazil9s future gold production

chemostratigraphy of carbonates from the minas supergroup quadrilatero ferryifero iron quadrangle brazil a stratigraphic record of early proterozoic atmospheric biogeochemical and climactic change the ca 2 42 ga old gandarela formation overlies the finely laminated caue banded iron formation bif of the itabira group minas supergroup brazil and consists of red carbonate facies bif grading upsection into buff dolomites and limestones which are locally stromatolitic carbonates of the gandarela formation have 1 3 c values ranging narrowly from 1 6 to 0 4 permil versus pdb n 93 the cercadinho formation at the base of the overlying piracicaba group is a thick arenaceous succession with rare thin carbonates that have 1 3 c values ranging from 3 3 to 5 4 permil n 12 stromatolitic dolomites of the overlying fecho do funil formation have pb pb carbonate age 2 11 0 11 ga babinski and others 1995 that provide the minimum age for this unit and 1 3 c values ranging from 5 6 to 7 4 permil n 69 at present we find no evidence in brazil for the any of the three early paleoproterozoic glacial events recognized in 2 45 to 2 22 ga sedimentary successions in north america given the radiometric constraints however it is possible that geological and geochemical traces of these ice ages are missing along the unconformity between the gandarela and cercadinho formations if correct carbonates of the gandarela formation may provide a record of the carbon isotope composition of the seawater prior to any of the paleoproterozoic ice ages combined with other available 1 3 c data results from the gandarela formation suggest that seawater preceding the ice ages was not significantly enriched in 1 3 c on the other hand the carbon isotopic signature of the cercadinho carbonates and chemical composition of the cercadinho arenites are consistent with deposition after the end of the glacial epoch and likely during the early stage of ca 2 22 to 2 1 ga carbon isotope excursion the 1 3 c values of marine carbonates of the fecho do funil formation are consistent with carbon isotope record of contemporaneous carbonate successions worldwide providing additional evidence for a global biogeochemical anomaly at ca 2 22 to 2 1 ga these compositional and isotopic changes allow integration of the minas supergroup into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age

a re assessment of elemental proxies for paleoredox analysis abstract paleoredox conditions are commonly evaluated based on elemental proxies but despite their frequency of use most of these proxies have received little comparative evaluation or assessment of their range of applicability to paleomarine systems here we evaluate 21 elemental proxies including six proxies based on the c s fe p system toc s toc s dopt fe al corg p nine proxies based on trace metal enrichment factors coef cref cuef moef nief pbef uef vef znef and six additional proxies from jones and manning 1994 u th uauth v cr ni co ni v cu mo zn in 55 phanerozoic marine formations we used principal components analysis pca to determine relationships among these 21 proxies in each formation and then sought to identify patterns across the full database the first principal component pc1 accounted for 40 1 of total dataset variance on average with the highest median loadings on trace metal enrichment factors nief 0 82 moef 0 76 all nine 0 50 the next highest median loadings are on c s fe p proxies toc 0 58 dopt 0 30 corg p 0 28 with bimetal proxies yielding uniformly lower loadings ni co 0 18 v cr 0 13 pca of the factor loadings for the 55 study formations demonstrated associations among the 21 elemental proxies linked to specific sediment host phases 1 an organic cluster associated with toc mo v and zn 2 a uranium cluster associated with all u based proxies and 3 a sulfide cluster associated with s and fe as well as the trace metals co cu ni and pb i e the major and typical minor constituents of diagenetic pyrite the findings of the present study have important ramifications for use of elemental proxies for paleoredox analysis first all of the proxies examined here are influenced by environmental redox conditions to some degree although the degree of redox influence on any given proxy can vary considerably from one formation to the next second sedimentary enrichment of most proxies depends on the presence of specific mineral and organic host phases and evaluation of elemental redox proxy data requires an understanding of how elements are partitioned among those phases third no single proxy is a universally reliable redox indicator although some are more consistently useful than others notably toc and trace metal efs fourth because of this inherent variability in proxy response adoption of redox proxy thresholds established in earlier published studies is discouraged instead we recommend that future redox studies establish redox thresholds on a formation specific basis through internal cross calibration of multiple redox proxies

geochemistry and isotopic study of southern bay of bengal sediments implications for provenance and paleoenvironment during the middle miocene abstract the bay of bengal bob the largest bay in the world is known to receive large amounts of fresh water and suspended particulate matter from different sources the sediments deposited in the bay serve as a repository for paleoenviromental reconstruction studies the miocene epoch is an important interval in geologic time as it is marked by major climatic events that controlled ocean productivity redox conditions and provenance of sediments however limited work has been carried out to establish paleoenviromental changes and provenance of sediments during the miocene here we present detailed geochemical and radiogenic sr nd isotopic results from predominantly middle miocene sediments of ocean drilling program odp site 758a located in the southern bob lat 5 23 n long 90 21 e water depth 2925 m to understand paleoredox conditions paleoproductivity and provenance the results suggest that these sediments are strongly linked to climatically modulated surface processes that affected the region the distribution of redox sensitive element proxies u v cr at interval 17 8 ma points to an oxic depositional environment the major trace and rare earth elements distribution supports prevalence of the indian summer monsoon ism at 12 10 ma productivity proxies babio porg show a decreasing trend during colder events east antarctic ice sheet expansion eais and advent of late miocene cooling lmc events the trace and rare earth element concentrations are generally low except at intervals where glass shards were found the 87sr 86sr and end 0 values indicate that the sediments belong to the high himalayan sedimentary series hhss throughout the middle miocene however prior to initiation of the ism these sediments underwent meteoric alteration in the lowlands before deposition in the bob our records suggest that sediments at odp site 758a may have been introduced into the bob through the irrawaddy yarlung tsangpo drainage system during the entire period of this study the irrawaddy yarlung tsangpo system drained the indus tsangpo suture zone southern tibet the eastern himalayas and the arakan coast before debouching in the bob

origins of metals and organic matter in the mecca quarry shale member and stratigraphically equivalent beds across the midwest samples of the middle pennsylvanian age mecca quarry shale member and stratigraphically equivalent beds from indiana illinois missouri and kansas have been analyzed for both inorganic and organic constituents trends for mo values reported by coveney and martin 1983 who found the highest tenors to be associated with the ancient shoreline in indiana have been confirmed regionally across the midwest similarly it has been found that cd v u se and sb are most concentrated near the shoreline whereas constituents such as ca and sr are richest offshore e g kansas organic matter analyses including pyrolysis gas chromatography show the eastern samples to be richest in total organic carbon and also in terrestrial organic matter unbuffered aqueous solutions containing powdered organic rich shales from indiana equilibrate to acid ph values ph 4 6 where mo is readily retained by organic matter laboratory experiments show the shales retain a capacity to fix mo from dilute solutions correlations between organic matter type and metal values such as those existing between mo and v and terrestrial organic matter in shale samples generally fit with the syngenetic model for metal fixation in near shore brackish waters proposed by coveney and martin 1983 however other evidence including the remnant activity of organic matter rich samples to fix metals suggests that nonsyngenetic agents such as basinal brines or meteoric fluids have probably augmented the metal contents of the deposits subsequent to sedimentation and diagenesis

vanadyl ions in ancient marine carbonaceous sediments abstract vanadyl ions in ancient shaly type sedimentary rocks of marine origin from a variety of world sources and geological periods have been investigated using electron spin resonance these and other results provide evidence that there are two types of vanadyl ions the first is inorganically bound in the clay silicate fraction of these rocks and the second type is associated with vanadyl porphyrin compounds

dissolved vanadium in rivers and estuaries abstract new measurements of dissolved vanadium in a variety of rivers and estuaries are presented here the data indicate that the average concentration of dissolved vanadium in major rivers entering the ocean is 15 nmol kg weathering rate and type of source rock rather than solution chemistry or anthropogenic influences appear to be the important factors in determining fluvial dissolved vanadium concentrations laboratory experiments suggest that in oxic waters vanadium is found predominantly in its most oxidized anionic form v v complexation with organic matter and formation of large colloidal species appear to be unimportant adsorption also appears to be a less important influence for vanadium than for some other trace elements such as zinc in estuaries vanadium behaves as a bioactive element showing a close correspondence with the distribution of phosphate the extent of estuarine vanadium removal is presently uncertain but may be low due to rapid remineralization of this element based on the river flux the oceanic residence time for dissolved vanadium is estimated to be 100 000 years

mineral formation and redox sensitive trace elements in a near surface hydrothermal alteration system a recent hydrothermal mudpool at the southwestern slope of the rincon de la vieja volcano in northwest costa rica exhibits an argillic alteration system formed by intense interaction of sulfuric acidic fluids with wall rock materials detailed mineralogical analysis revealed an assemblage with kaolinite alunite and opal c as the major mineral phases electron paramagnetic resonance spectroscopy epr showed 3 different redox sensitive cations associated with the mineral phases cu sup is structure bound in opal c whereas vo sup 2 and fe sup 3 are located in the kaolinite structure the location of the redox sensitive cations in different minerals of the assemblage is indicative of different chemical conditions the formation of the alteration products can be described schematically as a 2 step process in a first step alunite and opal c were precipitated in a fluid with slightly reducing conditions and a low chloride availability the second step is characterized by a decrease in k sup activity and subsequent formation of kaolinite under weakly oxidizing to oxidizing redox conditions as indicated by structure bound vo sup 2 and fe sup 3 the detection of paramagnetic trace elements structure bound in mineral phases by epr provide direct information about the prevailing redox conditions during alteration and can therefore be used as additionalmore insight into the genesis of the hydrothermal near surface system less

differences in the depositional environment of basal zechstein in southwest poland implication for base metal mineralization abstract base metal enrichment in the permian basal zechstein sediments occurs in two major areas in southwest poland the north sudetic syncline and the fore sudetic monocline two profiles of kupferschiefer and adjacent limestones from the konrad mine and from the rudna mine were studied by petrographic and geochemical methods in order to recognize similarities and differences in the comparative depositional environment of both areas in the north sudetic syncline basal zechstein was deposited in a shallow marine environment under slightly oxidizing sedimentary conditions it is composed of 1 2m thick mergelkalk which might be the equivalent of werrakalk ca1 in the fore sudetic monocline the basal zechstein is a black shale the so called kupferschiefer t1 with a thickness of 0 2 0 5 m microscopical investigations have shown that a substantial contribution of higher continental plant material is observed only in the north sudetic syncline because of its location near the zechstein sea shore pyrite is present in small amounts only in the fore sudetic monocline apart from the former shoreline of the zechstein sea the organic matter is preferentially composed of degraded algae due to its deposition under deeper anoxic bottom waters abundant pyrite is also present differences in the chemical composition of the respective kerogens are reflected in the results obtained from rock eval analyses hydrogen indices hi are significantly higher in the t1 section of the profile from the rudna mine in comparison to the basal zechstein of the konrad mine profile a depletion of hydrogen is only observed in the highly mineralized section and in rote faule of the konrad mine this indicates that hydrogen equivalents from organic matter participated in metal sulfide precipitation by thermochemical sulfate reduction tsr this is further confirmed by the occurrence of pyrobitumen saddle dolomite and calcite spars in contrast in the mineralized section of the rudna mine profile a depletion of hydrogen is not observed pyrobitumen is missing and saddle dolomite and calcite spars are very rare instead abundant pyrite originating from bacterial sulfate reduction bsr could have been the source of reduced sulfur species available for base metal precipitation after deposition of the sediment pyrite replacement the results allow one to conclude that in both areas base metals were precipitated from ascending solutions due to the interaction with the reducing zechstein barrier several observations argue for a post depositional mineralization process at a temperature of less than 130 c in both areas

oxygen isotopic compositions of dsdp site 480 diatoms implications and applications abstract the oxygen isotope composition of marine diatoms from hydraulic piston cored samples of dsdp site 480 vary widely reflecting the great temperature variations and large isotopic changes of surface water in the gulf of california nevertheless the results can be correlated with standard oxygen isotope records for the late quaternary based on carbonates such as the record of core v19 29 the diatom based results also parallel oxygen isotope data on benthic foraminifera from site 480 thus confirming that signals of environmental changes have been retained in the diatom oxygen isotope record throughout the section the data therefore allow us to assign an oxygen isotope stratigraphy and to define the chronology of site 480

trace element characteristics of different pyrite types in mesoarchaean to palaeoproterozoic placer deposits a comparative in situ la icp ms trace element study on pyrite from three different variably auriferous archaean to palaeoproterozoic palaeoplacer deposits in the ouro fino syncline quadrilatero ferrifero brazil the elliot lake area north of lake huron se canada and several deposits within the witwatersrand basin south africa revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits especially the ni and au concentrations as well as co ni and mo ni ratios were found to be systematically different in detrital compact detrital porous and post sedimentary hydrothermal pyrite grains from different source areas high co ni ratios and low au concentrations are typical of post sedimentary pyrite which is hydrothermal in origin in contrast relatively low co ni ratios and high au contents characterise detrital porous banded and concentric pyrite grains au 1 ppm which are syn sedimentary in origin in the elliot lake area and the witwatersrand basin detrital compact rounded pyrite is characterised by high co ni ratios which is in agreement with derivation from a hydrothermal source low au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks in contrast derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the ouro fino syncline because of low to intermediate co ni ratios high au contents may indicate a genetic relationship between pyrite and gold there systematic differences exist between the three areas with respect to au ni co mo and cu distributions in detrital pyrite which reflects differences in the provenance a predominantly mafic ultramafic source is indicated for the ouro fino a felsic source for the elliot lake and a mixed felsic mafic provenance for the witwatersrand pyrite populations independently of pyrite type the higher au endowment of the studied witwatersrand and ouro fino conglomerates are also reflected by an overall higher au concentration in the respective pyrite grains compared to the relatively au poor samples from elliot lake in general a strong positive correlation between au and pb levels in the various pyrite grains is noted analogous to pb which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete pbs phases au is considered to be present mainly in the form of discrete au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

sedimentary environments and mechanism of organic matter enrichment of dark shales with low toc in the mesoproterozoic cuizhuang formation of the ordos basin evidence from petrology organic geochemistry and major and trace elements abstract proterozoic strata have become a new target for hydrocarbon exploration worldwide they exhibit the potential to become a resource replacement target to provide a reference for hydrocarbon source rock evaluation the sedimentary environment and mechanism of organic matter enrichment of dark shales in the mesoproterozoic cuizhuang fm were investigated for the first time by combining organic and inorganic research concepts although the total organic carbon toc content in the cuizhuang fm is relatively low mean value of 0 59 it still can be considered as a potential source rock thin section observations main and trace element contents and biomarker compounds revealed the following 1 the cuizhuang fm was deposited in a neritic shelf sedimentary environment at a passive continental margin and the climate was warm and humid the hydrocarbon generating organisms mainly comprised prokaryotes such as cyanobacteria and other bacteria all these characteristics meet the conditions of source rock formation 2 the low oxygen level felsic source provenance and absence of submarine hydrothermal activity led to low productivity and the low productivity and low grade hydrocarbon generating organisms led to low toc content in the cuizhuang fm 3 the enrichment of v in the cuizhuang fm is apparent but does not reach the hyper enrichment threshold of 500 ppm associated with euxinic conditions revealing anoxic ferruginous conditions moreover the v content is positively correlated with toc content indicating that organic matter enrichment was mainly controlled by the redox conditions of the bottom water therefore the sedimentary areas with high productivity were related to basic source provenance and submarine hydrothermal activity and anoxic bottom water may have promoted the development of better source rocks this findings may represent the main direction for identifying effective source rocks in subsequent work

episodic negative anomalies of cerium at the depositional onset of the 2 65 ga itabira iron formation quadrilatero ferrifero of minas gerais brazil abstract magnetite bands in sericitic phyllite of the batatal formation mark the transition to the overlying 2 65 ga itabira iron formation or caue itabirite in the quadrilatero ferrifero of minas gerais brazil the magnetite bands have low non crustal th u ratios suprachondritic y ho ratios and sporadic anomalies of ce that are characterised as truly negative the data point to mild oxygenation of sea water at the depositional onset of the itabira iron formation some 200 ma before the great oxidation event

redox sensitivity of p cycling during marine black shale formation dynamics of sulfidic and anoxic non sulfidic bottom waters abstract a high resolution geochemical record of a 120 cm black shale interval deposited during the coniacian santonian oceanic anoxic event 3 odp leg 207 site 1261 demerara rise has been constructed to provide detailed insight into rapid changes in deep ocean and sediment paleo redox conditions high contents of organic matter sulfur and redox sensitive trace metals cd mo v zn as well as continuous lamination point to deposition under consistently oxygen free and largely sulfidic bottom water conditions however rapid and cyclic changes in deep ocean redox are documented by short term 15 20 ka intervals with decreased total organic carbon toc s and redox sensitive trace metal contents and in particular pronounced phosphorus peaks up to 2 5 wt p associated with elevated fe oxide contents sequential iron and phosphate extractions confirm that p is dominantly bound to iron oxides and incorporated into authigenic apatite preservation of this fe p coupling in an otherwise sulfidic depositional environment as indicated by fe speciation and high amounts of sulfurized organic matter may be unexpected and provides evidence for temporarily non sulfidic bottom waters however there is no evidence for deposition under oxic conditions instead sulfidic conditions were punctuated by periods of anoxic non sulfidic bottom waters during these periods phosphate was effectively scavenged during precipitation of iron oxyhydr oxides in the upper water column and was subsequently deposited and largely preserved at the sea floor after 15 25 ka sulfidic bottom water conditions were re established leading to the initial precipitation of cds zns and pyrite subsequently increasing concentrations of h2s in the water column led to extensive formation of sulfurized organic matter which effectively scavenged particle reactive mo complexes thiomolybdates at site 1261 sulfidic bottom waters lasted for 90 100 ka followed by another period of anoxic non sulfidic conditions lasting for 15 20 ka the observed cyclicity at the lower end of the redox scale may have been triggered by repeated incursions of more oxygenated surface to mid waters from the south atlantic resulting in a lowering of the oxic anoxic chemocline in the water column alternatively sea water sulfate might have been stripped by long lasting high rates of sulfate reduction removing the ultimate source for hs production

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

episodic concentration of gold to ore grade through earth s history abstract concentration of gold to economic ore grades has been highly episodic through earth s history with different sedimentary magmatic and hydrothermal processes having operated as principal driving forces at different times comparison of the various types of gold deposits their secular and spatial distribution and the amounts of gold contained in them combined with thermodynamic considerations reveals that the formation of gold deposits reflects fundamental changes in environmental conditions biological evolution and styles of large scale tectonism over the past four billion years here i suggest that most of the gold in the earth s crust was first concentrated at around 2 9 billion years ga ago by early photosynthesising microbes under an oxygen deficient atmosphere sedimentary reworking of auriferous microbial mats provided the source for the richest known gold palaeoplacers between 2 9 and 2 7 ga and an overall gold enrichment of coeval marine sediments only after modern style plate tectonics had begun to operate sediments enriched in gold had been accreted along active plate margins subducted and subcontinental lithospheric mantle had been correspondingly metasomatised did the formation of larger orogenic porphyry type and epithermal and to a lesser extent also other hydrothermal gold deposits become possible thus the principal secular peak in orogenic gold formation at 2 75 2 55 ga can be explained

evidence for episodic oxygenation in a weakly redox buffered deep mid proterozoic ocean abstract over the last two decades popular opinion about prevailing conditions in the mid proterozoic deep ocean has evolved from fully oxygenated to globally euxinic sulfidic to a more heterogeneous stratified water column with localized pockets of euxinia existing in predominantly iron rich ferruginous deep waters the animikie basin in the lake superior region has been essential in shaping our view of marine redox evolution over this time period in this study we present a multi proxy paleoredox investigation of previously unanalyzed strata of the late paleoproterozoic animikie basin using drill cores through the 1 85 ga stambaugh formation paint river group in the iron river crystal falls district of the upper peninsula of michigan usa based on previous tectonic reconstructions and analysis of sedimentary regimes the iron river crystal falls section captures strata from among the deepest water facies of the animikie basin in contrast to previous work on sedimentary rocks in this basin we find evidence from iron speciation trace metal and mo isotope data for episodes of at least local deep water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions while trace metal enrichments and iron speciation data suggest predominantly anoxic conditions the occurrence of mn rich intervals up to 12 3 wt mno containing abundant mn fe carbonate and a wide range of mo isotope data with extremely negative values 98 95mo 1 0 to 1 1 record the shuttling of mn oxides from surface waters through oxic or suboxic waters to the sediment water interface we propose that such conditions are analogous to those of locally restricted modern and holocene basins in the baltic sea which receive episodic inflow of oxygenated water producing similar geochemical signatures to those observed for the animikie basin we argue that the mid proterozoic was characterized by a lack of a strong redox buffer low sulfide ferrous iron and oxygen contents and thus was vulnerable to dramatic and at least local redox shifts including briefly oxygenated bottom waters a refined view of the mid proterozoic ocean is emerging one that was still predominantly anoxic but marked by regional heterogeneities and short term redox variability that may in part reflect a transitional state between prevailingly anoxic archean and predominantly oxic phanerozoic oceans

geochemical environment of the coniacian santonian western tropical atlantic at demerara rise abstract organic carbon rich shales deposited during the coniacian santonian oceanic anoxic event 3 were drilled during odp leg 207 at demerara rise we present integrated high resolution geochemical records of core intervals from odp sites 1259 and 1261 both from nannofossil biozone cc14 our results reveal systematic variations in marine and detrital sediment contribution depositional processes and bottom water redox conditions during black shale formation at two locations on demerara rise in different paleo water depths a combination of redox proxies fe s p al c p redox sensitive sulfide forming trace metals mn cd mo ni v zn and other analytical approaches bulk sediment composition p speciation electron microscopy x ray diffraction evidence anoxic to sulfidic bottom water and sediment conditions throughout the deposition of black shale these extreme redox conditions persisted and were periodically punctuated by short termed periods with less reducing bottom waters irrespective of paleo water depth sediment supply at both sites was generally dominated by marine material carbonate organic matter opal although relationships of detrital proxies as well as glauconitic horizons support some influence of turbidites winnowing bottom currents and or variable detritus sources along with less reducing bottom water at the proposed shallower location odp site 1259 at site 1261 located at greater paleo depth redox fluctuations were more regular and steady hemipelagic sedimentation sustained the development of mostly undisturbed lamination in the sedimentary record strong similarities of the studied deposits exist with the stratigraphic older cenomanian turonian oae2 black shale sections at demerara rise suggesting that the primary mechanisms controlling continental supply and ocean redox state were time invariant and kept the western equatorial atlantic margin widely anoxic over millions of years

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

deep water changes the near synchronous disappearance of a group of benthic foraminifera from the late miocene mediterranean abstract benthic foraminiferal and geochemical data from the monte del casino section in northern italy are employed to reconstruct the sequence of events preceding the late miocene mediterranean salinity crisis we evaluate the effects of eustatic sea level changes and tectonic events affecting the atlantic mediterranean connections changing benthic foraminiferal assemblages record changes in the deep water environment that can be explained by progressive isolation of the mediterranean basin the results of the analyses of stable oxygen and carbon isotopes and redox sensitive elements are in line with the benthic faunal trends at 7 16 ma a first major step in isolation of the basin is indicated by the nearly simultaneous disappearance of a group of deeper water benthic species usually found in middle to lower slope environments at the same time stable isotopes and redox sensitive elements indicate cooling and decreasing bottom water oxygenation after 6 8 ma gradual development of water mass stratification probably accompanied by increasing bottom water salinity is indicated by all proxies the cyclic pattern of homogeneous and sapropelitic sediments in the section is related to astronomical parameters and allows a detailed correlation with mediterranean sections at considerable geographic distances we compared the benthic foraminiferal faunas with those from the metochia section gavdos greece deposited at a similar water depth 1000 m to find that the benthic foraminifera at both sites indicate a similar synchronous paleoenvironmental development all proxies indicate that the onset of the late miocene mediterranean salinity crisis dates back to the earliest messinian the development of the mediterranean basin toward an evaporite trap is a result of interactions between a 400 kyr eccentricity related climatic effect superimposed upon gateway dynamics at the atlantic mediterranean connections benthic foraminifera accurately record the steps in increasing isolation of the mediterranean basin

the relative mobility of trace elements from short term weathering of a black shale abstract black shales contain high concentrations of trace elements which may be liberated to the environment as a result of natural weathering and potentially during shale gas development and future mining of black shales for extraction of shale oil the sunbury shale is one of the youngest units making up the thick devonian and mississippian black shale sequence in the appalachian basin this study compares the trace element geochemistry of samples collected from two exposures of sunbury shale located

paleoenvironmental changes across the carnian norian boundary in the black bear ridge section british columbia canada abstract the black bear ridge section in northeastern british columbia canada consists of a continuously exposed sequence of upper carnian through lower norian upper triassic continental margin strata the section has been proposed as a candidate global stratotype section and point gssp for the carnian norian boundary cnb in order to assess late carnian to early norian environmental changes recorded in the section we examined stratigraphic variations in 87sr 86sr 13c and 18o values and also values of redox sensitive elements v ni and cr in the cnb interval the study section is located along the north shore of williston lake northeastern british columbia the black bear ridge section was deposited in a distal ramp environment on the passive western margin of the north american craton the strata across the cnb display a positive shift in 13c values and a corresponding increase in the redox indices v v ni and v cr the synchronous increase in 13c values and redox indices suggests that burial rates of marine organic carbon increased in response to the development of anoxic conditions in the water column an increase in 13c values in carbonate rocks across the cnb has also been reported from upper triassic sections in the western tethys e g in the pizzo mondello section sicily which suggests that the development of anoxic conditions within the cnb interval was widespread affecting both the panthalassan ocean and tethyan sea the geochemical data from this study as well as from research into conodont biostratigraphy in the black bear ridge section show that the onset of oceanic anoxic conditions may have been responsible for the faunal turnover event at the cnb the cause of this anoxic event is unknown but the 87sr 86sr and 13c isotope data largely exclude the possibility that the event was triggered by dissociation of methane hydrates and degassing related to large scale volcanic activity

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

hydrothermal biogenic and seawater components in metalliferous black shales of the brooks range alaska synsedimentary metal enrichment in a carbonate ramp setting trace element and os isotope data for lisburne group metalliferous black shales of middle mississippian early chesterian age in the brooks range of northern alaska suggest that metals were sourced chiefly from local seawater including biogenic detritus but also from externally derived hydrothermal fluids these black shales are interbedded with phosphorites and limestones in sequences 3 to 35 m thick deposition occurred mainly on a carbonate ramp during intermittent upwelling under varying redox conditions from suboxic to anoxic to sulfidic deposition of the black shales at 335 ma was broadly contemporaneous with sulfide mineralization in the red dog and drenchwater zn pb ag deposits which formed in a distal marginal basin relative to the composition of average black shale the metalliferous black shales n 29 display large average enrichment factors 10 for zn 10 1 cd 11 0 and ag 20 1 small enrichments 2 10 are shown by v cr ni cu mo pd pt u se y and all rare earth elements except ce nd and sm a detailed stratigraphic profile over 23 m in the skimo creek area central brooks range indicates that samples from at and near the top of the section which accumulated during a period of major upwelling and is broadly correlative with the stratigraphic levels of the red dog and drenchwater zn pb ag deposits have the highest zn toc total organic carbon cu toc and tl toc ratios for calculated marine fractions no detrital component of these three metals average authigenic detrital free contents of mo v u ni cu cd pb ge re se as sb tl pd and au show enrichment factors of 4 3 103 to 1 2 106 relative to modern seawater such moderate enrichments which are common in other metalliferous black shales suggest wholly marine sources seawater and biogenic material for these metals given similar trends for enrichment factors in organic rich sediments of modern upwelling zones on the namibian peruvian and chilean shelves the largest enrichment factors for zn and ag are much higher 1 4 107 and 2 9 107 respectively consistent with an appreciable hydrothermal component other metals such as cu pb and tl that are concentrated in several black shale samples and are locally abundant in the red dog and drenchwater zn pb ag deposits may have a partly hydrothermal origin but this cannot be fully established with the available data enrichments in cr up to 7 8 106 are attributed to marine and not hydrothermal processes the presence in some samples of large enrichments in eu up to 6 1 107 relative to modern seawater and of small positive eu anomalies eu eu up to 1 12 are considered unrelated to hydrothermal activity instead being linked to early diagenetic processes within sulfidic pore fluids initial os isotope ratios 187os 188os calculated for a paleontologically based depositional age of 335 ma reveal moderately unradiogenic values of 0 24 to 0 88 for four samples of metalliferous black shale a proxy for the ratio of coeval early chesterian seawater is provided by initial 187os 188os 335 ma ratios of four unaltered black shales of the coeval kuna formation that average 1 08 nearly identical to the initial ratio of 1 06 for modern seawater evaluation of possible sources of unradiogenic os in the metalliferous black shales suggests that the most likely source was mafic igneous rocks that were leached by externally derived hydrothermal fluids this unradiogenic os is interpreted to have been leached by deeply circulating hydrothermal fluids in the kuna basin followed by venting of the fluids into overlying seawater we propose that metal bearing hydrothermal fluids that formed zn pb ag deposits such as red dog or drenchwater vented into seawater in a marginal basin were carried by upwelling currents onto the margins of a shallow water carbonate platform and were then deposited in organic rich muds together with seawater and biogenically derived components by syngenetic sedimentary processes metal concentration in the black shales was promoted by high biologic productivity sorption onto organic matter diffusion across redox boundaries a low sedimentation rate and availability of h2s in bottom waters and pore fluids

a major archean gold and crust forming event in the kaapvaal craton south africa the 2 89 to 2 76 gigayear old conglomerates of the central rand group of south africa host an immense concentration of gold the gold and rounded pyrites from the conglomerates yield a rhenium osmium isochron age of 3 03 0 02 gigayears and an initial 187 os 188 os ratio of 0 1079 0 0001 this age is older than that of the conglomerates thus the gold is detrital and was not deposited by later hydrothermal fluids this middle archean gold mineralization event corresponds to a period of rapid crustal growth in which much of the kaapvaal craton was formed and is evidence for a significant noble metal flux from the mantle

stable isotope and fluid inclusion study of sediment hosted stratiform copper deposits from the neuquen basin argentina the los chihuidos and el porvenir deposits of the neuquen basin are examples of sediment hosted stratiform cu deposits generated by interaction of hydrocarbons and formation water with host red beds and metal charged basinal brines during early diagenesis of red beds precipitated hematite kaolinite at 60 c from meteoric water followed by calcite 1 13c 8 to 3 3 and barite 34s 4 1 with increasing temperature 75 85 c from an evolved surface fluid 18ofluids 0 9 to 2 7 during the tertiary andean orogeny hydrocarbons and formation water migrated and reacted with the red beds resulting in bleaching of the sandstone smectite fluid 18o 2 2 and d 73 7 chlorite smectite mixed layer minerals fluid 18o 6 9 and d 84 pyrite 34s 10 2 and calcite 2 13c 12 9 to 6 8 formed as by product of the redox reactions calcite 2 formed from low salinity fluids 0 4 to 5 9 wt nacl equiv at slightly higher temperatures 125 145 c at el porvenir and at los chihuidos deposit 80 105 c during subsequent uplift metal charged basinal brines flowed into the bleached sandstone and precipitated chalcopyrite bornite 34s 12 3 followed by chalcocite spionkopite 34s 64 to 4 1 calcites 3a 13c 19 to 10 1 and 3b 13c 31 4 to 9 5 that accompanied fe cu and cu sulfides respectively formed from saline fluids up to 21 3 wt nacl equiv at temperatures of 159 70 c the 18o values 5 6 to 11 4 of fluids in equilibrium with calcite 3a are similar to oilfield and basinal brines fluids in equilibrium with calcite 3 in los chihuidos and calcite 3b in el porvenir show much lighter 18o values 7 3 to 4 6 and 0 6 to 2 3 respectively which suggests the involvement of methane as the main reductant and possibly meteoric water interaction

history and geochemistry of a metalliferous sediment core from the mid atlantic ridge at 26 n fourteen thousand years of hydrothermal deposition are recorded in a metalliferous sediment core recovered from the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 n an estimated 26 to 96 of the sediment at this core site is hydrothermally derived as determined from the caco3 al and fe data layers of essentially pure vent precipitates contain 3 caco3 and 0 5 al with high concentrations of fe 43 cu 4 1 zn 1 2 mn 1 1 v 480 ppm pb 175 ppm cd 32 ppm and hg 3 8 ppm sediment accumulation rates vary from 1 to 30 g cm2 1000 y throughout the core a function of the intensity of hydrothermal inputs distinct hydrothermal events are recorded at 6000 and 8500 y b p in layers containing 90 vent derived material vertical metal profiles and interelement relationships in the core result from variable deposition of oxides and sulfides oxidation and dissolution of sulfide phases and scavenging of metals from seawater

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

xenotime hematite aggregates on opaline filaments evidence for biomineralization in weathered siliciclastic rocks capanema quadrilatero ferrifero of minas gerais brazil dissolution cavities in weathered pebbly quartzite of the 2 5 ga moeda formation at capanema quadrilatero ferrifero of minas gerais brazil are decorated with suspended filaments of opaline silica the filaments sustain xenotime hematite aggregates in the open space xenotime occurs as inclusions in buds and botryoidal aggregates of hematite the filamentous structures consist of strand forming buds hypha like extensions and thin strands that compose mat like arrangements they resemble microbial filaments that were replaced by opaline silica and fossilized the occurrence of spherical hematite as protuberances on hematite free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare earth elements from groundwater to the iron accreting polymers these would have resulted in botryoidal aggregates of hematite with xenotime inclusions the presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology

o2 constraints from paleoproterozoic detrital pyrite and uraninite redox sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the archean and early paleoproterozoic atmosphere today these minerals are rapidly chemically weathered within short transport distances prior to the rise of oxygen low o2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes after the rise of oxygen redox sensitive detrital grains effectively vanish from the sedimentary record to get a better understanding of the timing of this transition we examined sandstones recorded in a scientifi c drill core from the south african 2 415 ga koegas subgroup a mixed siliciclastic and iron formation bearing unit deposited on the western deltaic margin of the kaapvaal craton in early paleoproterozoic time we observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section indicating the rise of oxygen is younger than 2 415 ga to better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain earth surface redox conditions we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental o2 concentrations even conservative model calculations for deltaic depositional systems with suffi cient transport distances approximately hundreds of kilometers show that redox sensitive detrital grains are remarkably sensitive to environmental o2 concentrations and they constrain the archean and early paleoproterozoic atmosphere to have 3 2 10 5 atm of molecular o2 these levels are lower than previously hypothesized for redox sensitive detrital grains but they are consistent with estimates made from other redox proxy data including the anomalous fractionation of sulfur isotopes the binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric o2 concentrations rose substantially at this time and were never again suffi ciently low 0 01 atm to enable survival and preservation of these grains in short transport systems

variations in organic geochemistry of anoxic oxic black shale carbonate sequences in the pennsylvanian of the midcontinent u s a abstract two cores containing thin organic rich phosphatic black shale members the excello and little osage shale and transitional lithologies from pennsylvanian cyclothems of southeast kansas and northeast oklahoma were characterized by geochemical and organic petrographic methods by very detailed sampling geochemical analyses included organic carbon determination rock eval pyrolysis bitumen extraction and chromatographic separation into saturated aromatic and nonhydrocarbon fractions gas chromatography of saturated hydrocarbons carbon isotope analysis of coexisting saturated and aromatic fractions and whole rock v and ni analyses organic petrography included maceral identification on polished whole rock fragments and vitrinite reflection and thermal alteration index tai determinations of kerogen concetrates the analytical results demonstrate large and systematic variations in geochemical properties within the black shale units in both cases an abrupt increase in toc 20 25 level and hi 400 level at the initiation of phosphatic black shale deposition is followed by a gradual decline to values of less than 1 toc and 100 hi organic constituents include both liptinitic algal derived and humic terrestrial components variations in their relative amounts control the nature and quality of the organic profile of the little osage shale level of anoxia varied within this sequence and influenced several geochemical parameters pr ph ratios decrease nonhydrocarbons increase relative to hydrocarbons saturated hydrocarbons decline relative to aromatics carbon isotopic composition of coexisting saturated and aromatic hydrocarbons converge and v and ni and the v ni v ratio increase with increased anoxia eustatic rise of sea level and rapid marine transgression over the continental craton with consequent flooding of adjacent extensive brackish marine peat swamps and the resultant influx of a large supply of nutrients and humic detritus appear to be the key geologic control of black shale deposition

trace metal signatures of jurassic cretaceous black shales from the norwegian shelf and the barents sea abstract black shale samples of jurassic to cretaceous age recovered during the norwegian shelf drilling program between 1987 and 1991 from sites 7430 10 u 01 barents sea 6814 04 u 02 norwegian shelf near the lofoten and 6307 07 u 02 norwegian shelf near trondheim were analyzed for major and trace elements these laminated black shales are characterized by high total organic carbon toc and total sulfur ts contents as well as by significant enrichments in several redox sensitive and or sulfide forming trace metals ag bi cd co cr cu mo ni re sb tl u v and zn enrichment factors relative to average shale are comparable to those found in cenomanian turonian boundary event ctbe black shales and mediterranean sapropels the re content is high in the studied black shales with maximum values up to 1221 ng g re mo ratios averaging 2 3 10 3 are close to the seawater value high trace metal enrichments and re mo ratios close to the seawater value point to a dominantly anoxic and sulfidic water column during black shale formation interbeds with higher re mo ratios especially in high resolution sampled core sections point to brief periods of suboxic conditions additionally enhanced zn concentrations in the black shales from the barents sea support the assumption that hydrothermal activity was also high during black shale deposition trace metal signatures of black shales at different drill sites on a transect along the norwegian shelf are not only influenced by water depth but also by their location in the boreal realm metal enrichments are higher in the northern compared to the southern sites volgian tithonian 151 144 ma bp black shales exhibit elevated trace metal contents in comparison to their berriasian 144 137 ma bp counterparts this probably reflects a change in the circulation pattern during periods of black shale formation therefore different paleoceanographic conditions probably controlled by climatic change linked to the transgression of the paleo sealevel and the north atlantic opening may have developed from the volgian to the berriasian

vanadium nickel and sulfur in crude oils and source rocks and their relationship with biomarkers implications for the origin of crude oils in venezuelan basins abstract this work presents a study of vanadium nickel and sulfur concentrations and biomarkers in a suite of crude oils and source rocks from three venezuelan basins maracaibo and eastern basins and barinas sub basin crude oils range from unaltered to altered by biodegradation and source rocks are characterized by having different kerogen types type ii iii or iv and maturity levels early mature to post mature vanadium nickel and sulfur concentrations v ni or v v ni and saturate biomarkers were used to classify the oils and source rocks based on inferred paleo redox environment lithology and maturity of the source rock oils are classified into five groups based on v ni ratios they appear to be related to variations in the paleo redox environment from suboxic dysoxic to anoxic suboxic of source rocks with variable lithological composition and organic matter input regardless of their maturity or biodegradation level these five groups are also classified based on biomarkers related to maturity organic matter type paleo redox environment and source rock lithology in source rocks vanadium nickel and sulfur concentrations together with v ni or v v ni ratios provide information about the paleo redox environment and are related to lithology regardless of their maturity these results also indicate variable paleo redox conditions during the sedimentation of venezuelan source rocks all this suggests that while the main source rocks in venezuelan basins are well established there are still uncertainties regarding their lateral and vertical variations in organic and inorganic facies and paleo redox conditions the presence of other source rocks contributing to the accumulation of hydrocarbons in the venezuela basins is also a possibility